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Title: History of Botany (1530-1860)
Author: Sachs, Julius von
Language: English
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HISTORY OF BOTANY
_SACHS_
London
HENRY FROWDE
[Illustration: Shield]
OXFORD UNIVERSITY PRESS WAREHOUSE
AMEN CORNER, E.C.
HISTORY OF BOTANY
(1530-1860)
BY
JULIUS VON SACHS
PROFESSOR OF BOTANY IN THE UNIVERSITY OF WÜRZBURG
_AUTHORISED TRANSLATION_
BY
HENRY E. F. GARNSEY, M.A.
_Fellow of Magdalen College, Oxford_
REVISED BY
ISAAC BAYLEY BALFOUR, M.A., M.D., F.R.S.
_Professor of Botany in the University
And Keeper of the Royal Botanic Garden, Edinburgh_
=Oxford=
AT THE CLARENDON PRESS
1890
[_All rights reserved_]
=Oxford=
PRINTED AT THE CLARENDON PRESS
BY HORACE HART, PRINTER TO THE UNIVERSITY
PREFACE.
Botanical Science is made up of three distinct branches of knowledge,
Classification founded on Morphology, Phytotomy, and Vegetable
Physiology. All these strive towards a common end, a perfect
understanding of the vegetable kingdom, but they differ entirely from
one another in their methods of research, and therefore presuppose
essentially different intellectual endowments. That this is the case
is abundantly shown by the history of the science, from which we learn
that up to quite recent times morphology and classification have
developed in almost entire independence of the other two branches.
Phytotomy has indeed always maintained a certain connection with
physiology, but where principles peculiar to each of them, fundamental
questions, had to be dealt with, there they also went their way in
almost entire independence of one another. It is only in the present
day that a deeper conception of the problems of vegetable life has led
to a closer union between the three. I have sought to do justice to
this historical fact by treating the parts of my subject separately;
but in this case, if the present work was to be kept within suitable
limits, it became necessary to devote a strictly limited space only
to each of the three historical delineations. It is obvious that the
weightiest and most important matter only could find a place in so
narrow a frame, but this I do not exactly regard as a misfortune,
and in the interests of the reader it is rather an advantage; for, in
accordance with the objects of the ‘General History of the Sciences,’
this History of Botany is not intended for professional persons only,
but for a wider circle of readers, and to these perhaps even the
details presented in it may here and there seem wearisome.
The style of the narrative might have been freer, and greater space
might have been allotted to reflections on the inner connection of
the whole subject, if I had had before me better preliminary studies
in the history of botany; but as things are, I have found myself
especially occupied in ascertaining questions of historical fact, in
distinguishing true merit from undeserved reputation, in searching
out the first beginnings of fruitful thoughts and observing their
development, and in more than one case in producing lengthy refutations
of wide-spread errors. These things could not be done within the
allotted space without a certain dryness of style and manner, and I
have often been obliged to content myself with passing allusions where
detailed explanation might have been desired.
As regards the choice of topics, I have given prominence to discoveries
of facts only when they could be shown to have promoted the development
of the science; on the other hand, I have made it my chief object to
discover the first dawning of scientific ideas and to follow them as
they developed into comprehensive theories, for in this lies, to my
mind, the true history of a science. But the task of the historian of
Botany, as thus conceived, is a very difficult one, for it is only with
great labour that he succeeds in picking the real thread of scientific
thought out of an incredible chaos of empirical material. It has
always been the chief hindrance to a more rapid advance in botany, that
the majority of writers simply collected facts, or if they attempted
to apply them to theoretical purposes, did so very imperfectly. I have
therefore singled out those men as the true heroes of our story who
not only established new facts, but gave birth to fruitful thoughts
and made a speculative use of empirical material. From this point of
view I have taken ideas only incidentally thrown out for nothing more
than they were originally; for scientific merit belongs only to the
man who clearly recognises the theoretical importance of an idea, and
endeavours to make use of it for the promotion of his science. For this
reason I ascribe little value, for instance, to certain utterances of
earlier writers, whom it is the fashion at present to put forward as
the first founders of the theory of descent; for it is an indubitable
fact that the theory of descent had no scientific value before the
appearance of Darwin’s book in 1859, and that it was Darwin who gave
it that value. Here, as in other cases, it appears to me only true
and just to abstain from assigning to earlier writers merits to which
probably, if they were alive, they would themselves lay no claim.
J. SACHS.
WÜRZBURG, _July 22, 1875_.
THE AUTHOR’S PREFACE
_To the English translation of the History of Botany of Julius von
Sachs._
I am gratefully sensible of the honourable distinction implied in
the determination of the Delegates of the Clarendon Press to have
my History of Botany translated into the world-wide language of the
British Empire. Fourteen years have elapsed since the first appearance
of the work in Germany, from fifteen to eighteen years since it
was composed,—a period of time usually long enough in our age of
rapid progress for a scientific work to become obsolete. But if the
preparation of an English translation shows that competent judges do
not regard the book as obsolete, I should be inclined to refer this to
two causes. First of all, no other work of a similar kind has appeared,
as far as I know, since 1875, so that mine may still be considered to
be, in spite of its age, the latest history of Botany; secondly, it
has been my endeavour to ascertain the historical facts by careful and
critical study of the older botanical literature in the original works,
at the cost indeed of some years of working-power and of considerable
detriment to my health, and facts never lose their value,—a truth which
England especially has always recognised.
But the present work is not a simple enumeration of the names of
botanists and of their writings, no mere list of the dates of botanical
discoveries and theories; such was not at all my plan when I designed
it. On the contrary I purposed to present to the reader a picture
of the way in which the first beginnings of scientific study of the
vegetable world in the sixteenth century made their appearance in
alliance with the culture prevailing at the time, and how gradually
by the intellectual efforts of gifted men, who at first did not even
bear the name of botanists, an ever deepening insight was obtained into
the relationship of all plants one to another, into their outer form
and inner organisation, and into the vital phenomena or physiological
processes dependent on these conditions.
For the attainment of this end it was above all things necessary for
me to form a clear judgment respecting the influence of the views
and principles enunciated by the different authors on the further
development of botanical science. This is to the historian of science
the central point round which all beside should be disposed, and
without which the entire work breaks up into a collection of unmeaning
details, and it is one which demands knowledge of the subject, and
capacity and impartiality of judgment. On questions connected with
times long gone by the decision of the experts has in most cases been
already given, though I myself found to my surprise that older authors
had for centuries been regarded as the founders of views which they
had distinctly repudiated as absurd, showing how necessary it is that
the works of our predecessors should from time to time be carefully
read and compared together. But in the majority of cases there is no
dispute at the present day respecting the historical value, that is the
operative influence on posterity, of works written three hundred or
even one hundred years ago.
But it is a very different matter when the author of a book like mine
ventures, as I have done for sufficient reasons but at the same time
with regret, to sit in judgment on the works of men of research and
experts, who belong to our own time and who exert a lively influence
on their generation. In this case the author can no longer appeal to
the consentient opinion of his contemporaries; he finds them divided
into parties, and involuntarily belongs to a party himself. But it is
a still more weighty consideration that he may subsequently change his
own point of view, and may arrive at a more profound insight into the
value of the works which he has criticised; continued study and maturer
years may teach him that he overestimated some things fifteen or twenty
years ago and perhaps undervalued others, and facts, once assumed to be
well established, may now be acknowledged to be incorrect.
Thus it has happened in my own case also in some but not in many
instances, in which I have had to express an opinion respecting the
character of works which appeared after 1860, and which to some extent
influenced my judgment on the years immediately preceding them. But
this was from fifteen to eighteen years ago when I was working at my
History. It might perhaps be expected that I should remove all such
expressions of opinion from the work before it is translated. In some
few cases, in which this could be effected by simply drawing the
pen through a few lines, I have so done; but it appeared to me that
to alter with anxious care every sentence which I should put into a
different form at the present day would serve no good purpose, for
I came to the conclusion that my book itself may be regarded as a
historical fact, and that the kindly and indulgent reader may even be
glad to know what one, who has lived wholly in the science and taken
an interest in everything in it old and new, thought from fifteen to
eighteen years ago of the then reigning theories, representing as he
did the view of the majority of his fellow-botanists.
However, these remarks relate only to two famous writers on the
subjects with which this History is concerned. If the work had been
brought to a close with the year 1850 instead of 1860, I should hardly
have found it necessary to give them so prominent a position in it.
Their names are Charles Darwin and Karl Nägeli. I would desire that
whoever reads what I have written on Charles Darwin in the present
work should consider that it contains a large infusion of youthful
enthusiasm still remaining from the year 1859, when the ‘Origin of
Species’ delivered us from the unlucky dogma of constancy. Darwin’s
later writings have not inspired me with the like feeling. So it
has been with regard to Nägeli. He, like Hugo von Mohl, was one
of the first among German botanists who introduced into the study
that strict method of thought which had long prevailed in physics,
chemistry, and astronomy; but the researches of the last ten or
twelve years have unfortunately shown that Nägeli’s method has been
applied to facts which, as facts, were inaccurately observed. Darwin
collected innumerable facts from the literature in support of an idea,
Nägeli applied his strict logic to observations which were in part
untrustworthy. The services which each of these men rendered to the
science are still acknowledged; but my estimate of their importance
for its advance would differ materially at the present moment from
that contained in my History of Botany. At the same time I rejoice in
being able to say that I may sometimes have overrated the merits of
distinguished men, but have never knowingly underestimated them.
DR. J. VON SACHS,
_Foreign Fellow of the Royal Society_.
WÜRZBURG, _March 24, 1889_.
NOTE BY THE TRANSLATOR.
No History of Botany in English has ever been published, and it is
to supply in some measure this want, long felt by English-speaking
students, that this translation of Professor Sachs’ masterly sketch has
been prepared.
H. E. F. G.
CONTENTS.
FIRST BOOK.
HISTORY OF MORPHOLOGY AND CLASSIFICATION.
1530-1860.
PAGE
Introduction 3
CHAPTER I.
The Botanists of Germany and the Netherlands from Brunfels to
Caspar Bauhin, 1530-1623 13
CHAPTER II.
Artificial Systems and Terminology of Organs from Cesalpino to
Linnaeus, 1583-1760 37
CHAPTER III.
Development of the Natural System under the Influence of the
Dogma of the Constancy of Species, 1759-1850 108
CHAPTER IV.
Morphology under the Influence of the Doctrine of Metamorphosis
and of the Spiral Theory, 1790-1850 155
CHAPTER V.
Morphology and Systematic Botany under the Influence of the
History of Development and the knowledge of the Cryptogams,
1840-1860 182
SECOND BOOK.
HISTORY OF VEGETABLE ANATOMY.
1671-1860.
Introduction 219
CHAPTER I.
Phytotomy founded by Malpighi and Grew, 1671-1682 229
CHAPTER II.
Phytotomy in the Eighteenth Century 246
CHAPTER III.
Examination of the Matured Framework of Cell-Membrane in Plants,
1800-1840 256
CHAPTER IV.
History of Development of the Cell, Formation of Tissues, Molecular
Structure of Organised Forms, 1840-1860 311
THIRD BOOK.
HISTORY OF VEGETABLE PHYSIOLOGY.
1583-1860.
Introduction 359
CHAPTER I.
History of the Sexual Theory
1. From Aristotle to R. J. Camerarius 376
2. Establishment of the Doctrine of Sexuality in Plants by R. J.
Camerarius, 1691-1694 385
3. Dissemination of the New Doctrine; its Adherents and
Opponents, 1700-1760 390
4. The Theory of Evolution and Epigenesis 402
5. Further Development of the Sexual Theory by J. G. Koelreuter
and Konrad Sprengel, 1761-1793 406
6. New opponents of Sexuality and their refutation by Experiments,
1785-1849 422
7. Microscopic Investigation into the Processes of Fertilisation
in the Phanerogams, the Pollen-Tube and Eggs, 1830-1850 431
8. Discovery of Sexuality in the Cryptogams, 1837-1860 436
CHAPTER II.
History of the Theory of Nutrition of Plants, 1583-1860 445
1. Cesalpino, 1583 450
2. First Inductive Experiments and Opening of New Points of
View in the History of the Theory of the Nutrition of
Plants, to 1730 453
3. Fruitless Attempts to Explain the Movement of the Sap in
Plants, 1730-1780 482
4. The Modern Theory of Nutrition Founded by Ingen-Houss
and Theodore de Saussure, 1779-1804 491
5. Vital Force. Respiration and Heat of Plants. Endosmose,
1804-1840 504
6. Settlement of the Question of Food-Material of Plants,
1840-1860 524
CHAPTER III.
History of Phytodynamics
From end of 17th century to about 1860 535
INDEX 565
ERRATA.
Page 18, line 3 from bottom, _for_ Chini _read_ Ghini
" 20, " 7, _for_ Schmiedel _read_ Schmidel
" 160, " 2 from bottom, _for_ many _read_ some
" 160, note, _for_ Robert _read_ Louis Marie Aubert
" 201, line 11, _for_ asexually _read_ sexually.
FIRST BOOK
HISTORY OF MORPHOLOGY AND CLASSIFICATION
(1530-1860)
INTRODUCTION.
The authors of the oldest herbals of the 16th century, Brunfels, Fuchs,
Bock, Mattioli and others, regarded plants mainly as the vehicles of
medicinal virtues; to them plants were the ingredients in compound
medicines, and were therefore by preference termed ‘simplicia,’ simple
constituents of medicaments. Their chief object was to discover the
plants employed by the physicians of antiquity, the knowledge of which
had been lost in later times. The corrupt texts of Theophrastus,
Dioscorides, Pliny and Galen had been in many respects improved and
illustrated by the critical labours of the Italian commentators of
the 15th and of the early part of the 16th century; but there was one
imperfection which no criticism could remove,—the highly unsatisfactory
descriptions of the old authors or the entire absence of descriptions.
It was moreover at first assumed that the plants described by the Greek
physicians must grow wild in Germany also, and generally in the rest
of Europe; each author identified a different native plant with some
one mentioned by Dioscorides or Theophrastus or others, and thus there
arose as early as the 16th century a confusion of nomenclature which it
was scarcely possible to clear away. As compared with the efforts of
the philological commentators, who knew little of plants from their own
observation, a great advance was made by the first German composers of
herbals, who went straight to nature, described the wild plants growing
around them and had figures of them carefully executed in wood. Thus
was made the first beginning of a really scientific examination of
plants, though the aims pursued were not yet truly scientific, for no
questions were proposed as to the nature of plants, their organisation
or mutual relations; the only point of interest was the knowledge of
individual forms and of their medicinal virtues.
The descriptions were at first extremely inartistic and unmethodical;
but the effort to make them as exact and clear as was possible led
from time to time to perceptions of truth, that came unsought and lay
far removed from the object originally in view. It was remarked that
many of the plants which Dioscorides had described in his Materia
Medica do not grow wild in Germany, France, Spain, and England, and
that conversely very many plants grow in these countries, which
were evidently unknown to the ancient writers; it became apparent
at the same time that many plants have points of resemblance to one
another, which have nothing to do with their medicinal powers or
with their importance to agriculture and the arts. In the effort to
promote the knowledge of plants for practical purposes by careful
description of individual forms, the impression forced itself on
the mind of the observer, that there are various natural groups of
plants which have a distinct resemblance to one another in form and
in other characteristics. It was seen that there were other natural
alliances in the vegetable world, beside the three great divisions
of trees, shrubs, and herbs adopted by Aristotle and Theophrastus.
The first perception of natural groups is to be found in Bock, and
later herbals show that the natural connection between such plants
as occur together in the groups of Fungi, Mosses, Ferns, Coniferae,
Umbelliferae, Compositae, Labiatae, Papilionaceae was distinctly felt,
though it was by no means clearly understood how this connection was
actually expressed; the fact of natural affinity presented itself
unsought as an incidental and indefinite impression, to which no great
value was at first attached. The recognition of these groups required
no antecedent philosophic reflection or conscious attempt to classify
the objects in the vegetable world; they present themselves to the
unprejudiced eye as naturally as do the groups of mammals, birds,
reptiles, fishes and worms in the animal kingdom. The real resemblance
of the organisms in such groups is unconsciously accepted by the mind
through the association of ideas, and it is not till this involuntary
mental act, which in itself requires no effort of the understanding, is
accomplished, that any necessity is felt for obtaining a clearer idea
of the phenomenon, and the sense of this necessity is the first step to
intentional systematic enquiry. The series of botanical works published
in Germany and the Netherlands from 1530 to 1623, from Brunfels to
Kaspar Bauhin, shows very plainly how this perception of a grouping by
affinity in the vegetable kingdom grew more and more distinct; but it
also shows how these men merely followed an instinctive feeling in the
matter, and made no enquiry into the cause of the relationship which
they perceived.
Nevertheless a great step in advance was thus taken; all the
foreign matter introduced into the description of plants by medical
superstition and practical considerations was seen to be of secondary
importance, and was indeed altogether thrown aside by Kaspar Bauhin;
the fact of natural affinity, the vivifying principle of all botanical
research, came to the front in its place, and awakened the desire to
distinguish more exactly whatever was different, and to bring together
more carefully all that was like in kind. Thus the idea of natural
affinity in plants is not a discovery of any single botanist, but is a
product, and to some extent an incidental product, of the practice of
describing plants.
But before the exhibition of the natural affinity gave birth to the
first efforts at classification on the part of de l’Obel (Lobelius)
and afterwards of Kaspar Bauhin, the Italian botanist Cesalpino (1583)
had already attempted a system of the vegetable kingdom on a very
different plan. He was led to distribute all vegetable forms into
definite groups not by the fact of natural affinity, which impressed
itself on the minds of the botanists of Germany and the Netherlands
through involuntary association of ideas, but by philosophical
reflection. Trained in the philosophy which flourished in Italy in
the 16th century, deeply imbued with the doctrines of Aristotle, and
practised in all subtleties of the schools, Cesalpino was not the
man to surrender himself quietly to the influence of nature on the
unconscious powers of the mind; on the contrary, he sought from the
first to bring all that he learnt from the writings of others and from
his own acute observation of the forms of plants into subjection to
his own understanding. Hence he approached the task of the scientific
botanist in an entirely different way from that of de l’Obel and Kaspar
Bauhin. It was by philosophical reflections on the nature of the plant
and on the substantial and accidental value of its parts, according to
Aristotelian conceptions, that he was led to distribute the vegetable
kingdom into groups and sub-groups founded on definite marks.
This difference in the origin of the systematic efforts of Cesalpino on
the one hand and of de l’Obel and Bauhin on the other is unmistakably
apparent; the Germans were instinctively led by the resemblances to
the conception of natural groups, Cesalpino on the contrary framed his
groups on the sharp distinctions which resulted from the application
of predetermined marks; all the faults in Bauhin’s system are due to
incorrect judgment of resemblances, those of Cesalpino to incorrectness
in distinguishing.
But the main point of difference lies in the fact, that the system
is presented by de l’Obel and Bauhin without any statement of the
principles on which it rests; in their account of it the association
of ideas is left to perfect itself in the mind of the reader, as it
grew up before in the authors themselves. De l’Obel and Bauhin are
like artists, who convey their own impressions to others not by words
and descriptions, but by pictorial representations; Cesalpino, on
the other hand, addresses himself at once to the understanding of
his reader and shows him on philosophic grounds that there must be a
classification, and states the principles of this classification; it
was on philosophic grounds also that he made the characters of the seed
and the fruit the basis of his arrangement, while the German botanists,
paying little attention to the organs of fructification, were chiefly
influenced by the general impression produced by the plant, by its
habit as the phrase now is.
The historians of botany have overlooked the real state of the case
as here presented, or have not described it with sufficient emphasis;
due attention has not been paid to the fact, that systematic botany,
as it began to develope in the 17th century, contained within itself
from the first two opposing elements; on the one hand the fact of
a natural affinity indistinctly felt, which was brought out by the
botanists of Germany and the Netherlands, and on the other the desire,
to which Cesalpino first gave expression, of arriving by the path of
clear perception at a classification of the vegetable kingdom which
should satisfy the understanding. These two elements of systematic
investigation were entirely incommensurable; it was not possible by
the use of arbitrary principles of classification which satisfied
the understanding to do justice at the same time to the instinctive
feeling for natural affinity which would not be argued away. This
incommensurability between natural affinity and _a priori_ grounds of
classification is everywhere expressed in the systems embracing the
whole vegetable kingdom, which were proposed up to 1736, and which
including those of Cesalpino and Linnaeus were not less in number than
fifteen. It is the custom to describe these systems, of which those
of Cesalpino, Morison, Ray, Bachmann (Rivinus), and Tournefort are
the most important, by the one word ‘artificial’[1]; but it was by no
means the intention of those men to propose classifications of the
vegetable kingdom which should be merely artificial, and do no more
than offer an arrangement adapted for ready reference. It is true that
the botanists of the 17th century and Linnaeus himself often spoke of
facility of use as a great object to be kept in view in constructing
a system; but every one who brought out a new system did so really
because he believed that his own was a better expression of natural
affinities than those of his predecessors. If some like Ray and Morison
were more influenced by the wish to exhibit natural affinities by
means of a system, and others as Tournefort and Magnol thought more
of framing a perspicuous and handy arrangement of plants, yet it is
plain from the objections which every succeeding systematist makes to
his predecessors, that the exhibition of natural affinities was more
or less clearly in the minds of all as the main object of the system;
only they all employed the same wrong means for securing this end, for
they fancied that natural affinities could be brought out by the use of
a few easily recognised marks, whose value for systematic purposes had
been arbitrarily determined. This opposition between means and end runs
through all systematic botany from Cesalpino in 1583 to Linnaeus in
1736.
But a new departure dates from Linnaeus himself, since he was the first
who clearly perceived the existence of this discord. He was the first
who said distinctly, that there is a natural system of plants, which
could not be established by the use of predetermined marks, as had
been previously attempted, and that even the rules for framing it were
still undiscovered. In his Fragments of the date of 1738, he gave a
list of sixty-five groups or orders, which he regarded provisionally
as cycles of natural affinity, but he did not venture to give their
characteristic marks. These groups, though better separated and more
naturally arranged than those of Kaspar Bauhin, were like his founded
solely on a refined feeling for the relative resemblances and graduated
differences that were observed in comparing plants with one another,
and this is no less true of the enumeration of natural families
attempted by Bernard de Jussieu in 1759. To such of these small
groups of related forms as had not been already named both Linnaeus
and Jussieu gave names, which they took not from certain marks, but
from the name of a genus in each group. But this mode of naming plainly
expresses the idea which from that time forward prevailed in systematic
botany, that there is a common type lying at the foundation of each
natural group, from which all its forms though specifically distinct
can be derived, as the forms of a crystal may all be derived from
one fundamental form,—an idea which was also expressed by Pyrame de
Candolle in 1819.
But botanists could not rest content with merely naming natural
groups; it was necessary to translate the indistinct feeling, which
had suggested the groups of Linnaeus and Bernard de Jussieu, into the
language of science by assigning clearly recognised marks; and this was
from this time forward the task of systematists from Antoine Laurent
de Jussieu and de Candolle to Endlicher and Lindley. But it cannot
be denied, that later systematists repeatedly committed the fault of
splitting up natural groups of affinity by artificial divisions and of
bringing together the unlike, as Cesalpino and the botanists of the
17th century had done before them, though continued practice was always
leading to a more perfect exhibition of natural affinities.
But while natural relationship was thus becoming more and more the
guiding idea in the minds of systematists, and the experience of
centuries was enforcing the lesson, that predetermined grounds of
classification could not do justice to natural affinities, the fact of
affinity became itself more unintelligible and mysterious. It seemed
impossible to give a clear and precise definition of the conception,
the exhibition of which was felt to be the proper object of all efforts
to discover the natural system, and which continued to be known by
the name of affinity. A sense of this mystery is expressed in the
sentence of Linnaeus: ‘It is not the character (the marks used to
characterise the genus) which makes the genus, but the genus which
makes the character;’ but the very man, who first distinctly recognised
this difficulty in the natural system, helped to increase it by his
doctrine of the constancy of species. This doctrine appears in Linnaeus
in an unobtrusive form, rather as resulting from daily experience and
liable to be modified by further investigation; but it became with
his successors an article of faith, a dogma, which no botanist could
even doubt without losing his scientific reputation; and thus during
more than a hundred years the belief, that every organic form owes its
existence to a separate act of creation and is therefore absolutely
distinct from all other forms, subsisted side by side with the fact
of experience, that there is an intimate tie of relationship between
these forms, which can only be imperfectly indicated by definite marks.
Every systematist knew that this relationship was something more than
mere resemblance perceivable by the senses, while thinking men saw
the contradiction between the assumption of an absolute difference
of origin in species (for that is what is meant by their constancy)
and the fact of their affinity. Linnaeus in his later years made some
strange attempts to explain away this contradiction; his successors
adopted a way of their own; various scholastic notions from the 16th
century still survived among the systematists, especially after
Linnaeus had assumed the lead among them, and it was thought that the
dogma of the constancy of species might find especially in Plato’s
misinterpreted doctrine of ideas a philosophical justification, which
was the more acceptable because it harmonised well with the tenets
of the Church. If, as Elias Fries said in 1835, there is ‘quoddam
supranaturale’ in the natural system, namely the affinity of organisms,
so much the better for the system; in the opinion of the same writer
each division of the system expresses an idea (‘singula sphaera
(sectio) ideam quandam exponit’), and all these ideas might easily
be explained in their ideal connection as representing the plan of
creation. If observation and theoretical considerations occasionally
suggested objections to such views, these objections were usually
little regarded, and in fact reflections of this kind on the real
meaning of the natural system did not often make their appearance;
the most intelligent men turned away with an uncomfortable feeling
from these doubts and difficulties, and preferred to devote their
time and powers to the discovery of affinities in individual forms.
At the same time it was well understood that the question was one
which lay at the foundation of the science. At a later period the
researches of Nägeli and others in morphology resulted in discoveries
of the greatest importance to systematic botany, and disclosed facts
which were necessarily fatal to the hypothesis, that every group
in the system represents an idea in the Platonic sense; such for
instance were the remarkable embryological relations, which Hofmeister
discovered in 1851, between Angiosperms, Gymnosperms, Vascular
Cryptogams and Muscineae; nor was it easy to reconcile the fact, that
the physiologico-biological peculiarities on the one hand and the
morphological and systematic characters on the other are commonly quite
independent of one another, with the plan of creation as conceived by
the systematists. Thus an opposition between true scientific research
and the theoretical views of the systematists became more and more
apparent, and no one who paid attention to both could avoid a painful
feeling of uncertainty with respect to this portion of the science.
This feeling was due to the dogma of the constancy of species, and to
the consequent impossibility of giving a scientific definition of the
idea of affinity.
This state of things finally ceased with the appearance of Darwin’s
first and best book on the origin of species in 1859; from a multitude
of facts, some new, but most of them long well-known, he showed that
the constancy of species was no longer an open question; that the
doctrine was no result of exact observation, but an article of faith
opposed to observation. The establishment of this truth was followed
almost as a matter of course by the true conception of that which had
been hitherto figuratively called affinity; the degrees of affinity
expressed in the natural system indicated the different degrees of
derivation of the varying progeny of common parents; out of affinity
taken in a figurative sense arose a real blood-relationship, and the
natural system became a table of the pedigree of the vegetable kingdom.
Here was the solution of the ancient problem.
Darwin’s theory has this special interest in the history of the
science, that it established clearness in the place of obscurity, a
scientific principle in place of a scholastic mode of thought, in the
domain of systematic botany and morphology. Yet Darwin did not effect
this change in opposition to the historical development of our science
or independently of it; on the contrary his great merit is that he has
correctly appreciated the problems long existing in systematic botany
and morphology from the point of view of modern research, and has
solved them.
That the constancy of species is incompatible with the idea of
affinity, that the morphological (genetic) nature of organs does not
proceed on parallel lines with their physiological and functional
significance, are facts which were known in botany and zoology before
the time of Darwin; but he was the first to show, that variation and
natural selection in the struggle for existence solve these problems,
and enable us to conceive of these facts as the necessary effects
of known causes; it is at the same time explained, why the natural
affinity first recognised by de l’Obel and Kaspar Bauhin cannot be
exhibited by the use of predetermined principles of classification, as
was attempted by Cesalpino.
CHAPTER I.
THE BOTANISTS OF GERMANY AND THE NETHERLANDS FROM BRUNFELS TO KASPAR
BAUHIN[2].
1530-1623.
When those who are accustomed to modern botanical literature take up
for the first time the works of Otto Brunfels (1530), Leonhard Fuchs
(1542), Hieronymus Bock (Tragus), or of the later authors Rembert
Dodoens (Dodonäus), Charles de l’Écluse (Carolus Clusius), Matthias
de l’Obel (Lobelius, 1576), or even those of Kaspar Bauhin from the
beginning of the 17th century, they are surprised not only by the
strange form, the curious and unfamiliar accessories from which what
is really useful must be laboriously extracted, but still more by the
extraordinary poverty of thought which characterises these composers of
usually very thick folios. If however instead of travelling backwards
from the present time they pursue the opposite direction; if they have
previously occupied themselves with the botanical views of Aristotle
and the comprehensive botanical works of his disciple Theophrastus
of Eresus, with Pliny’s Natural History and the medical science
of Dioscorides; if they have made themselves acquainted with the
botanical literature of the middle ages and noted how it continually
grows less and less valuable, and have proceeded through the works
of Albertus Magnus, as prolix as they are deficient in ideas, to the
‘Hortus Sanitatis’ (Garden of Health), the popular work on natural
history before and after 1500, and similar productions, then certainly
they receive a very different and almost imposing impression even from
the first herbals, those of Brunfels, Bock, and Fuchs. These books
will appear to them almost modern in comparison with the last-named
productions of medieval superstition, nor will they fail to perceive
that a new epoch of natural science commenced with these men, and above
all that they laid the foundations of modern botany. They give us, it
is true, nothing but separate descriptions of the wild and cultivated
plants of Germany, and these for the most part of common occurrence,
arranged by Fuchs alphabetically, by Bock grouped under the heads of
herbs, shrubs, and trees, and following one another under each head
in the most motley order; it is true that these descriptions are so
naive and inartistic as hardly to offer points of comparison with
modern scientifically correct diagnoses; but the great point is, that
they are taken from the plants as they lay before the writers, who had
often seen and carefully examined them. Woodcuts are added to supply
any defects in the description, and to give a clear idea of the plant
intended by the name; and these figures, which always give the whole
plant and were drawn immediately from nature by the hands of practised
artists, are so true to nature that a botanist’s eye at once recognises
in every case the object meant to be represented. These figures and
descriptions (the latter are wanting in Brunfels[3], 1530) would have
rendered a great service to the science, even if they had not been
as good as they are; for botanical literature had sunk so low, that
not only were the figures embellished with fabulous additions, as in
the ‘Hortus Sanitatis,’ and sometimes drawn purely from fancy, but the
meagre descriptions of quite common plants were not taken from nature,
but borrowed from earlier authorities and eked out with superstitious
fictions. The powers of independent judgment were oppressed and stunted
in the middle ages, till at last the very activity of the senses,
resting as it does to a great extent on unconscious operations of
the understanding, became weak and sickly; natural objects presented
themselves to the eye even of those who made them their study in
grotesquely distorted forms; every sensuous impression was corrupted
and deformed by the influence of a superstitious fancy. In comparison
with these perversions the artless descriptions of Bock appear suitable
and true, and are refreshing from their immediate contact with nature;
while in the more learned Fuchs criticism of other writers is already
seen united with actual examination of natural objects. Great was the
gain when men began once more to look at plants with open eyes, to
take pleasure in their variety and beauty. It was not necessary for a
while that they should speculate on the nature of plants, or the cause
of plant-life; time enough for that when sufficient practice had been
gained in the perception of their resemblances and differences.
The German fathers of botany connected their labours with the
botanical literature of classical antiquity only so far as they
sought to recognise in the plants of their own country those named by
Theophrastus, Dioscorides, Pliny and Galen. The attempt to do this
indeed led to many mistakes, for the descriptions of the ancient
botanists were very imperfect and often quite unserviceable for the
recognition of the plants described. In this point therefore the
compilers of herbals found no models worthy of imitation in the old
writers. But in seeking to recover a knowledge of the medicinal plants
of the Greek physicians[4], they were compelled to compare together a
great variety of native plants, and thus to exercise and perfect the
faculty of apprehending differences of form. This mode of proceeding,
arising out of medical requirements, directed the attention entirely
to the individual form, which was also the chief thing required in the
interest of pure science, and much more was thus gained than if these
men had only followed the philosophical writings of Aristotle[5] and
Theophrastus[6]. The Greek authors built their views on the philosophy
of botany on very weak foundations; scarcely a plant was known to them
exactly in all its parts; they derived much of their knowledge from
the accounts of others, often from dealers in herbs. From this scanty
material and from various popular superstitions had Aristotle formed
his views on the nature of plants, and if Theophrastus possessed more
experimental knowledge, he still saw facts in the light of his master’s
philosophical doctrines. If we succeed in the present day in extracting
much that is accurate from the writings of Aristotle and Theophrastus,
it was nevertheless well that the first compilers of herbals ceased
to pay attention to them, and occupied themselves with accumulating
descriptions of individual plants worked out by themselves with all
possible exactness. History shows that in this way a new science
arose in the course of a few years, while the philosophical botany of
Aristotle and Theophrastus has led to no important result. Moreover
we shall see how even in the hands of a philosophically gifted and
scholarly man like Cesalpino the teaching of Aristotle had only a
mischievous effect on the study of plants.
If the compilers of herbals did not aim at deducing general conclusions
from their observations, yet the continually accumulating descriptions
of individual forms gradually gave rise of themselves to perceptions
of an abstract and more comprehensive character. The feeling for
resemblance and difference of form especially was developed, and
finally the idea of natural relationship; and though this idea was
as yet by no means worked out with scientific precision, it was
nevertheless, even in the indistinct form in which it appears in de
l’Obel in 1576 and more clearly in Kaspar Bauhin in 1623, a result
of the highest value, and one of which neither learned antiquity
nor the middle ages had ever caught a glimpse. The perception of
a natural affinity among plants could only be obtained from exact
description a thousand times repeated, never from the abstractions
of the Aristotelian school, which rested essentially on superficial
observation. It appears then that the scientific value of the herbals
of the 16th century lay mostly in the description of such plants as
every botanist found in a somewhat limited portion of his native land,
and considered worth his notice; at the same time the later compilers
endeavoured to give a universal character to each herbal by admitting
plants which had not been actually seen by the writer; each as far as
possible gathered from his predecessors all that they had seen, and
added what he had himself seen that was new; but in contrast with the
previous centuries the peculiar merit of each new herbal was held to
depend not on what the compiler had borrowed from his predecessors,
but on what he had added from his own observation. Hence every one was
anxious to introduce into his work as many plants unknown till that
time or unnoticed as he possibly could, and the number of descriptions
of individual forms mounted rapidly up; in Fuchs in 1542 we find
about five hundred species described and figured, but in 1623 the
number of species as enumerated by Kaspar Bauhin had risen to six
thousand. As the botanists were spread over a large part of Germany,
Fuchs in Bavaria and afterwards at Tübingen, Bock on the middle Rhine,
Konrad Gesner at Zürich, Dodoens and de l’Obel in the Netherlands, a
territory of considerable extent was thus examined; it was enlarged
by the contributions which travellers brought or transmitted to the
botanists, and de l’Écluse especially traversed a large part of Germany
and Hungary and even of Spain, and eagerly collected and described
the plants of those countries. During this period also the number of
known plants was increased from Italy, partly by the exertions of
Italian botanists, such as Mattioli, and partly by travelling Germans.
The first flora of the Thüringer-Wald was written by Thal, but not
published till after his death in 1588. Botanical gardens even, though
in more modest form than in our day, were already helping in the 16th
century to add to the knowledge of plants; the first were formed in
Italy, as at Padua in 1545, at Pisa in 1547, at Bologna in 1567 under
Aldrovandi, afterwards under Cesalpino. Soon similar collections of
living plants were made in the north; in 1577 a botanic garden was
founded at Leyden, over which de l’Éluse long presided, in 1593 at
Heidelberg and at Montpellier; in the course of the next century the
number of these gardens was considerably increased.
The preserving of dried plants, the formation of the collections
which we now call herbaria, dates from the 16th century; at that time
however the word herbarium meant a book of plants. In this matter also
the Italians led the way. According to Ernst Meyer, Luca Ghini seems
to have been the first who made use of dried plants for scientific
purposes, and his two pupils Aldrovandi and Cesalpino are said to have
formed the first herbaria in our sense of the word; one of the first
collections of the kind, perhaps of the date of 1559, was the herbarium
formed by Ratzenberger, which was discovered in the museum at Cassel a
few years since and described by Kessler.
These are matters somewhat external to our immediate subject, but
they show how lively an interest was taken in botany in the latter
half of the sixteenth century; this is still more shown by the great
number of books of plants, published with numerous and expensive
plates and in some cases going through several editions. But the
artistic and scientific value of the drawings, which were appended
to the descriptions and in later herbals were reckoned by thousands,
did not keep equal pace with their number; Fuchs’ splendid figures
remained unapproached, and gradually, as the distance from Dürer’s time
increased, the wood-cuts grew smaller and poorer[7], and sometimes even
quite indistinct. The art of describing on the contrary continually
improved; the descriptions became fuller, and gradually a certain
method appeared in assigning marks and in estimating their value;
critical remarks on the identity or non-identity of species, the
separation of forms previously considered to be alike, and similar
matters occur more frequently. The descriptions in de l’Éluse may in
fact claim to be called scientific; in Kaspar Bauhin they appear in the
form of terse and methodical diagnoses.
The most remarkable thing to us in these descriptions from Fuchs and
Bock to Bauhin is the striking neglect of the flowers and fruit. The
earliest descriptions, especially those of Bock, endeavour to depict
the form of the plant in words, to render directly the impression on
the senses; special attention was paid to the shape of the leaves, the
nature of the ramification, the character of the roots, the size and
colour of the flowers. KONRAD GESNER[8] was the only one who bestowed
a closer attention on the flowers and parts of the fruits; he figured
them repeatedly, and recognised their great value for the determination
of affinity, as we learn from his expressions in his letters; but the
much occupied and much harassed man died before he could complete the
work on plants which he had long been preparing, and when in the 18th
century Schmidel published Gesner’s figures, which meanwhile had passed
through various hands, the work too long delayed remained useless to a
science which had already outstripped it.
It will be gathered from the above remarks, that we find in these
authors no approach to a system of morphology founded on a comparative
examination of the parts of plants, and therefore no regular technical
language. Still the more learned among them felt the necessity of
connecting the words they used in describing a plant with a fixed
sense, of defining their conceptions; and though their first efforts
in this direction were weak, they deserve notice, because they show
more than anything else how great has been the advance in the study of
nature from the 16th century to the present day.
The first attempt to establish a botanical terminology is to be found
as early as 1542 in the ‘Historia Stirpium’ of LEONHARD FUCHS[9]. Four
pages at the beginning of the work are thus occupied. A considerable
number of words are explained in alphabetical order—the mode of
arrangement which he followed also in describing his plants. It is
difficult to give a clear idea of this the first botanic terminology
by selected examples; yet the attempt must be made, because it is in
this way only that we learn to see from what feeble beginnings the
later scientific terminology and morphology has been developed. Thus
we read: ‘Acinus’ denotes not merely, as many believe, the grains
inside the grape, but the whole fruit, which consists of juice, of a
fleshy portion with the stones (‘vinaceis’), and of the outer skin.
Galen is quoted as authority for the following explanation: ‘Alae’
are said to be the hollows (angles) between the stem and its branches
(the leaves), from which new sprouts (‘proles’) proceed. ‘Asparragi,’
the germs of herbs which appear before the leaves and the first
edible shoots are developed. ‘Baccae’ are smaller ‘foetus’ of herbs,
shrubs, and trees, which appear separate and isolated on the plant, as
for example laurel-berries (‘partus lauri’), and differ from acini,
inasmuch as these are more crowded together. ‘Internodium’ is that
which lies between the articulations or knees. ‘Racemus’ is used for
the bunch of grapes, but does not belong to the vine only, but also to
the ivy and other herbs and shrubs which bear clusters of any kind. The
majority of such explanations of names concern the forms of the stem
and the branches, but the most remarkable thing about the whole list
is, that it does not include the words flower and root; yet under the
word ‘julus’ occurs the statement, that it is that which in the hazel
‘compactili callo racematim cohaeret,’ and may be described as a long
worm borne on a special pendent stalk and coming before the fruit.
Though the word flower is not explained, yet some parts of the flower
are mentioned; thus it is said, ‘stamina sunt, qui in medio calycis
erumpunt apices, sic dicta quod veluti filamenta intimo floris sinu
prosiliant.’ The explanation of the word fruit may be added: ‘Fructus,
quod carne et semine compactum est; frequenter tamen pro co, quod
involucro perinde quasi carne et semine coactum est, accipi solet.’
Progress in this direction was slow but still recognisable. In the
last edition of the ‘Pemptades’ of DODOENS[10] of the year 1616, a
folio volume of 872 pages, only one page and a third are devoted to
the explanation of the parts of plants; but the selection of the words
explained and the substance of the explanations hit the essential
points better than in Fuchs. We find for instance: Root (‘radix, ῥίζα’)
is the name given in the tree and in every other plant to the lower
part, by which it penetrates into the earth and cleaves to it, and by
which it draws its nourishment. This part, unlike the leaves which
are usually deciduous, is common to all plants, a few only excepted
which live and grow without roots, such as Cassytha, Viscum, and
the plant called ‘Hyphear,’ Fungi, Mosses, and Fuci, all which are
however usually reckoned among φῦτα. ‘Caudex’ is in trees and shrubs
that which springs from the root and rises above the ground, and by
which the nourishment is carried upwards; the same part is called in
herbs caulis or cauliculus. Leaf (‘folium’) is in every plant that
which clothes and adorns it, and without which trees and other plants
appear naked. The definition of a flower would lose in a translation:
‘flos, ἄνθος, arborem et herbarum gaudium dicitur, futurique fructus
spes est; unaquaeque etenim stirps pro natura sua post florem partus
ac fructus gignit.’ The parts of the flower are with him the calyx
(‘calyx’), in which the blossom is at first enclosed and with which
the ‘foetus’ is soon surrounded, stamens (‘stamina’) which arise like
threads from the depth of the blossom and from the calyx, and ‘apices’
(anthers), certain thickish appendages on the summit of the stamens.
‘Julus’ (catkin) is that which hangs down round and long in place of
the flower, as in the walnut, hazel, mulberry, beech, and other trees.
‘Fructus’ is that in which the seed is formed, but frequently it is
itself the seed, as where the latter is not enclosed in anything else
and is formed naked. We must not be led by these words to think of our
Gymnosperms, but must understand that here, as with all botanists till
the time of A. L. de Jussieu and Joseph Gärtner (1788), naked seeds
mean dry indehiscent fruits.
De l’Obel, from whom especially we might have looked for similar
explanations, has given none.
The absence of more profound comparative examination of the parts of
plants, as shown in the examples of terminology here adduced, may
serve as an additional support of the assertion, that natural affinity
was not inferred from exact comparison of the form of organs, but was
the result of a feeling arising from the likeness of habit directly
apprehended by the senses, that is by the collective impression
produced by the whole plant.
Passing to the consideration of the attempts in systematic botany made
by the Germans in this period, the chief thing to notice is, that
the division into the main groups of trees, shrubs, undershrubs, and
herbs was the one generally adopted; these groups were borrowed from
antiquity and were maintained even by the special systematists, from
Cesalpino to the beginning of the 18th century; nor was any change made
in principle when these four groups were reduced to three or two (trees
and herbs). It was moreover considered to be self-evident that trees
were the most perfect plants. Hence when relationship is spoken of in
subsequent remarks, it must be understood that this holds good only
within the groups just mentioned. The classifications of the German
and Dutch botanists not only sprang from the describing of individual
plants, but they were originally in a certain sense identical with
it. In undertaking to describe individual forms, the first task
was to separate those which closely resembled one another, for the
resemblance of systematically-allied plants is often so great, that
to distinguish them specifically requires consideration and careful
comparison. The resemblance is more obvious than the difference. There
are moreover many plants which are entirely distinct from one another
in their inner nature, but which appear strikingly alike if we regard
the impression produced immediately on the senses, and the converse
of this statement is equally true. Hence the attempt to circumscribe
and fix individual forms in the act of describing was at once found
to involve difficulties, the solution of which leads directly to the
conception of some kind of arrangement. A comparison of the herbals
of Fuchs and Bock up to Kaspar Bauhin shows very plainly how these
difficulties were gradually overcome, how the describing of single
species led necessarily, and without the intention of the describer,
to considerations of a distinctly systematic character. Where the
species in a group of forms, which we now designate as a genus or
family, closely resemble each other in habit, there arose of itself the
instinctive feeling that such forms belong to one another. This feeling
asserted itself in words when, as was done from the first, a number of
such forms were without conscious reflection designated by the same
name; thus, to mention one of many examples, we find Bock applying
the name Wolfsmilk, Euphorbia, not to one species of the genus, but
to several, which he then distinguishes by epithets (common, least,
cypress, sweet). The customary mode of expression in the herbals is
very instructive on this point; there are, they say, two or more of
this or that plant which have not been hitherto distinguished. But this
feeling of connection and similarity of kind was produced not only by
forms that were closely allied, but also by such as belong to extensive
groups of the system; thus the words moss, lichen, fungus, alga, fern,
had long served to include a great number of distinct forms, though the
separation of these groups had nowhere in truth been carried out with
logical precision.
These remarks are important as serving to show in the most decisive
manner the incorrectness of the assertion, that the study of organisms
sprang from the recognition of individual species; that it is this
which is directly given, and that without it no advance in the science
is possible. The historical fact rather is, that descriptive botany
began often, perhaps most often, not with species but with genera
and families, that very often at first whole groups of forms were
conceived of as unities, which had to be divided later and of set
purpose into separate forms; and up to the present day one part of
the task of the systematist is to undertake the splitting up of forms
previously regarded as identical. The notion that the species is the
object originally presented to the observer, and that certain species
were afterwards united into genera, is one that was invented in
post-Linnaean times under the dominion of the dogma of the constancy of
species; it happened so sometimes, but just as often the genus was the
object first presented, and the task of the describer was to resolve it
into a number of species. In the 16th century the conception neither
of genus or species had yet been defined; for the botanists of that
period genera and species had the same objective reality. But, in the
process of continually making the descriptions of individual plants
more exact, forms once separated were united, and those before assumed
to be identical were separated, till it gradually became apparent that
both operations must be pursued with system and method. It cannot
therefore be exactly said that somebody first established the species,
another the genus, and a third person again the larger groups. It is
more correct to say that the botanists of the 16th century carried out
this process of separation up to a certain point without intending it,
and in the effort to give the greatest possible preciseness to their
descriptions of individual forms. It lay therefore in the nature of
the case, that those groups which we call genera and species should
first be cleared up, and we find in fact at the end of this period in
Kaspar Bauhin the genera already distinguished by names, if not by
characters; the species by names and characters. Together with these
smaller groups, many more comprehensive ones, which we now designate
families, were also marked off and supplied with names, which are still
in use. The 16th century established the groups and names of Coniferae,
Umbelliferae, Verticillatae (Labiatae), Capillares (Ferns), and others.
It is true that the determination of the limits of these groups by
distinct marks was not yet attempted, but the plants belonging to these
groups were again and again treated of in special chapters or ranged
in due succession one after another. But as long as this was done to
some extent without design, and the real meaning of this relationship
was not yet recognised, other considerations of very various kinds
influenced the composition of the books and disturbed the natural
arrangement. The feeling for natural affinity supplants all other
considerations in de l’Obel first, and after him much more completely
in Kaspar Bauhin.
Enough perhaps has now been said to render the main result of the
botanical efforts of the period, which we are considering, intelligible
to the reader; but a clear view of the method of describing plants
at that time, and of the way in which systematic botany came into
being, can only be shown by examples; and if we proceed to give some
here, it is with the purpose with which figures copied as exactly as
possible from nature are added to treatises on natural history, because
a real understanding is only to be gained in this way. The botanical
literature of the 16th century is so different from that of the 19th,
that a very indistinct idea of it could be obtained from a statement of
results expressed in modern terms.
Fuchs, Historia Stirpium, 1542.
The common plant now known as Convolvulus arvensis is there called
Helxine cissampelos, and is described in the following manner:
‘_Nomina._—Ἑλξινὴ κισσάμπελος Graecis, Helxine cissampelus et
Convolvulus Latinis nominatur. Vulgus herbariorum et officinae
Volubilem mediam et vitealem appellant, Germani Mittelwinden oder
Weingartenwinden. Recte autem Cissampelos dicitur, in vineis enim
potissimum nascitur et folio hederaceo. Convolvulus vero quod crebra
revolutione vicinos frutices et herbas implicet.
_Forma._—Folia habet hederae similia, minora tamen, ramulos exiguos
circumplectentes quodcumque contigerint. Folia denique ejus scansili
ordine alterna subeunt. Flores primum candidos lilii effigie, dein in
puniceum vergentes, profert. Semen angulosum in folliculis acinorum
specie.
_Locus._—In vineis nascitur, unde etiam ei appellatio cissampeli, ut
diximus, indita est.
_Tempus._—Aestate, potissimum autem Julio et Augusto mensibus, floret.‘
HIERONYMUS BOCK[11], at page 299 of his ‘Herbal,’ published at
Strassburg in 1560, describes the same plant and Convolvulus sepium as
follows:
‘Of the white wind-bell.
‘Two common wind-plants grow everywhere in our land with white
bell-flowers. The larger prefers to dwell by hedges, and creeps over
itself, twists and twines, etc. The little wind- or bell-flower
(Convolvulus arvensis) is like the large one with its roots, round
stems, leaves and bell-flowers, in all things smaller, thinner,
and shorter. Some flowers on this plant are quite white, some of a
beautiful flesh colour, painted with reddish brown streaks. It grows in
dry meadows, in herb-and onion-gardens, and does harm therein, because
with its creeping and twining it oppresses other garden herbs, and is
also bad to exstirpate, because the thin white rootlets make their way
deep downwards, spread very widely, and are continually putting forth
new and young clusters like hops.’
Then follows a long paragraph on the names, that is, a critical
review of the opinions of different writers on the question, which
of Dioscorides’ or Pliny’s names should be applied to the plant
described. ‘I must think,’ says Bock, ‘that this flower is a wild sort,
Scammonia Dioscoridis (but harmless), which herb Dioscorides also calls
colophonia, dactylion, apopleumenon, sanilum, and colophonium,’ and
so on. Then follows a chapter on its virtue and effect externally and
internally.
As regards the arrangement of the 567 species described by Bock, he
divides his book into three parts, the first and second containing the
smaller herbs, the third the shrubs and trees. In each part closely
allied plants are generally described in larger or smaller numbers one
immediately after another, though the compiler is all the time under
the influence of very various considerations, and follows no general
principle. For instance, our Convolvulus stands in the midst of a
number of other very different plants, which either climb as the ivy,
or twine with tendrils as Smilax; then follows Lysimachia Nummularia,
which simply runs along the ground, then the hop, Solanum Dulcamara,
Clematis, Bryonia, Lonicera, and different Cucurbitaceae; immediately
after come the Burdocks, Teasels, and Thistles, and these are followed
by some Umbelliferae. The whole work is conceived in a similar spirit;
the feeling for relationship is clearly to be traced within very
narrow circles, but it finds imperfect expression and is frequently
disturbed by reference to biological habit; this appears especially
in the beginning of the third part, which treats of shrubs generally,
shrubs which form hedges, and trees, ‘as they grow in our German land’;
the first chapter is on the fungi which grow on trees, the second on
some mosses, and these are followed immediately by the mistletoe. Then
come the heather and some smaller shrubs, and finally larger and
the largest trees. The chapter on Fungi under the section ‘Of names’
contains a statement of views on the nature of fungi, such as are often
repeated even into the 17th century: ‘Mushrooms are neither herbs nor
roots, neither flowers nor seeds, but merely the superfluous moisture
of the earth and trees, of rotten wood and other rotten things. From
such moisture grow all tubera and fungi. This is plain from the fact
that all the above-mentioned mushrooms, those especially which are used
for eating, grow most when it will thunder or rain, as Aquinas Ponta
says. For this reason the ancients paid peculiar regard to them, and
were of opinion that tubera, since they come up from no seed, have some
connection with the sky; Porphyrius speaks also in this manner, and
says that fungi and tubera are called children of the gods, because
they are born without seeds and not as other kinds.’
We pass over Valerius Cordus, Conrad Gesner, Mattioli[12], and some
other unimportant writers, and turn to Dodoens, de l’Écluse, and
Dalechamps, in whom a marked tendency to orderly arrangement appears,
though the principle of arrangement in all three lies essentially in
points external and accidental, and above all in the relations of the
plant-world to mankind. Within the divisions thus artificially formed a
constantly increasing attention is paid to natural affinities, but at
the same time allied forms are separated without scruple in deference
to the artificial principle of classification. It can also be plainly
seen, that these writers think more of giving some order to their
matter than of discovering the arrangement that will be in conformity
with nature. It is impossible to give the reader a good idea of these
classifications in our scientific language; it would be necessary to
transcribe them. For brevity’s sake we will here quote DE L’ÉCLUSE
only[13], the best of the three writers named above. In his ‘Rariorum
plantarum historia,’ which appeared as early as 1576, but which lies
before the writer of these pages in the edition of 1601, the first book
treats of trees, shrubs, and undershrubs; the second of bulbous plants;
the third of sweet-smelling flowers; the fourth of those without
smell; the fifth of poisonous, narcotic, and acrid plants; the sixth
of those that have a milky juice, and of Umbelliferae, Ferns, Grasses,
Leguminosae, and some Cryptogams.
A similar arrangement is found in Dalechamps[14]; that of Dodoens in
his ‘Pemptades’ is more perplexed and unnatural; but the design in
both of them is evidently much the same as that of de l’Écluse. This
design is best seen from the introductory observations to each book;
de l’Écluse, for instance, says at page 127, ‘Having treated of the
history of trees, shrubs, and undershrubs, and put these together in
the preceding book, we will now in this second book describe such
plants as have a bulbous or tuberous root, many of which attract and
delight the eyes of all persons in an extraordinary degree by the
elegance and variety of their flowers, and which therefore ought not
to have the lowest place assigned to them among garland-plants (‘inter
coronarias’). We will begin with the plants of the lily kind, on
account of their size and the beauty of their flowers, etc. etc.‘ The
introductions to the several books of the ‘Pemptades’ of Dodoens are
more learned and more diffuse. It is plain that the composers of these
works had no thought of arranging their matter on the principles of a
true natural system, but were only anxious to give some kind of order
to their descriptions of individual plants. Hence their divisions do
not appear under the names of classes and subdivisions (‘genera majora
et minora,’ as they would have been called at that time), but they
are sections of the whole work kept as symmetrical as was possible.
If we would discover in these works whatever may really lay claim
to systematic value, we must not rely on the sections as they are
typographically distinguished, but must observe within each of them
the order in which the plants are given, and then it becomes apparent
that within the frame once established forms naturally allied are, as
far as may be, grouped together. For instance, we find in the second
book of de l’Écluse’s work first of all a long list of true Liliaceae
and Asphodeleae, Melanthaceae, and Irideae described in unbroken
succession; then comes Calamus, and then without any explanation a
number of the Ranunculaceae, among which the genera Ranunculus and
Anemone are very well distinguished; but then follows the genus
Cyclamen with several species, and next a number of Orchideae, in the
middle of which appear Orobanche and Corydalis, followed by Helleborus
niger, Veratrum album, Polygonatum, and others. So it is in the other
sections, though in general the species of a genus stand together,
and even the genera of a family are not unfrequently united; but with
all this there are no proper breaks, because other considerations
are perpetually disturbing the feeling for natural relationship. The
descriptions of de l’Écluse are generally commended, and they deserve
to be commended for their fulness of detail and their attention to
the structure of the flowers, though he, like de l’Obel and Dodoens,
describes the leaves more minutely than any other part of the plant.
With DE L’OBELL[15], as has been already observed, the feeling for
natural affinity declares itself for the first time so decidedly as to
outweigh if not entirely to set aside all other considerations. The
fact is disclosed to us in the preface to his ‘Stirpium adversaria
nova’ of 1576, where these words occur: ‘proinde adversariorum voce
novas veteribus additas plantas et novum ordinem quadantenus innuimus.
Qui ordo utique sibi similis et unus progreditur ducitque a sensui
propinquioribus et magis familiaribus ad ignotiora et compositiora,
modumque sive progressum similitudinis sequitur et familiaritatis, quo
et universim et particulatim, quantum licuit per rerum varietatem et
vastitatem, sibi responderet. Sic enim ordine, quo nihil pulchrius in
coelo aut in sapientis animo, quae longe lateque disparata sunt unum
quasi fiunt, magno verborum memoriae et cognitionis compendio, ut
Aristoteli et Theophrasto placet.’
We must not indeed expect to find that de l’Obel really produced a
natural system of plants; but his ‘Observationes’ still more than his
‘Adversaria’ attest his efforts to arrange plants according to their
resemblances in form; and in these efforts he is guided not by instinct
merely and the general habit, but mainly and with evident purpose by
the form of the leaves; thus beginning with Grasses, which have narrow,
long, and simple leaves, he proceeds to the broader-leaved Liliaceae
and Orchideae; then passing on to the Dicotyledons he exhibits the
main groups in fairly well limited masses. Still the Ferns appear in
the middle of the Dicotyledons on account of the form of their leaves,
while on the other hand, the Cruciferae, Umbelliferae, Papilionaceae
and Labiatae remain but little disturbed in their continuity by
secondary considerations.
The progress of botanical science in the period which we have been
considering reaches its highest point in the labours of Kaspar
Bauhin[16], as regards both the naming and describing of individual
plants and their classification according to likeness of habit. In
Bauhin all secondary considerations have disappeared; his works may be
called botanical in the strict scientific meaning of the word, and they
show how far it is possible to advance in a descriptive science without
the aid of a general system of comparative morphology, and how far the
mere perception of likeness of habit is a sufficient foundation for
a natural classification of plants; it was scarcely possible to make
greater advances on the path pursued by the botanists of Germany and
the Netherlands.
The descriptions of species in the ‘Prodromus Theatri Botanici’ of
KASPAR BAUHIN (1620) notice all obvious parts of the plant with all
possible brevity and in a fixed order; the form of the root, height and
form of the stem, characters of the leaves, flowers, fruit, and seed
are given in concise sentences seldom occupying more than twenty short
lines; the description of a single species is here in fact developed
into an art and becomes a diagnosis.
A still higher value must be set on the fact, that in Kaspar Bauhin the
distinction between species and genus is fully and consciously carried
out; every plant has with him a generic and a specific name, and this
binary nomenclature, which Linnaeus is usually thought to have founded,
is almost perfectly maintained by Bauhin, especially in the ‘Pinax’; it
is true that a third and fourth word is not unfrequently appended to
the second, the specific name, but this additional word is evidently
only an auxiliary. It is remarkable on the other hand, that he has
added no characters to the names of the genera; it is only from the
name that we know that several species belong to one genus; we might
almost believe that the characters of the genus are intended to be
supplied by the strange etymological explanation appended in italics
to the generic name. These fanciful etymologies maintained themselves
to the end of the 17th century, when Tournefort did battle with them;
they were an evil which sprang in a great measure from Aristotelian and
scholastic modes of thought, and from the belief that it was possible
to conceive of the nature of a thing from the original meaning of its
name.
Nothing shows better the earnestness of Bauhin’s research than the
fact, that he devoted the labour of forty years to his ‘Pinax,’ in
order to show how each one of the species given by him was named by
earlier botanists. The example already given from Fuchs shows how many
names a plant had received by the middle of the 16th century; even in
Dioscorides and Pliny we find a whole row of names given for a single
plant, and the botanists of Fuchs’ time used their utmost endeavours
to attach the names in Dioscorides and other ancient writers to
particular plants found in central Europe. Dioscorides, Theophrastus,
and Pliny either add no descriptions to the names of their plants, or
they describe them in so unsatisfactory a manner, that it was a very
difficult task for the science of that day, as it is still for us,
to recognise the plants of the ancient writers; hence arose such a
confusion of names that the reader of a botanical work can never be
sure whether the plant of one author is the same as that of another
with the same name. A description of a plant is therefore usually
accompanied in the 16th century by a critical enquiry how far the name
used agrees with that of other authors. Kaspar Bauhin sought to put an
end to this condition of uncertainty by his ‘Pinax,’ in which he showed
in the case of all species known to him what were the names given to
them by the earlier writers, and he has thus enabled us to see our
way through the nomenclature of the period of which we are speaking;
the ‘Pinax’ is in a word the first and for that time a completely
exhaustive book of synonyms, and is still indispensable for the history
of individual species—no small praise to be given to a work that is
more than 250 years old.
It would not have been unsuitable to the purpose of the author of the
‘Pinax,’ if he had allowed himself to give the plants in alphabetical
order, but instead of this we find a careful arrangement according
to natural affinities. This directly proves what is also confirmed
by the ‘Prodromus,’ that Bauhin regarded such an arrangement as of
the greatest importance. In this point, as in others, he goes far
beyond his predecessors; he pursues the same method as de l’Obel had
pursued forty years before, but he carries it out more thoroughly. At
the same time he shares with his predecessors the peculiarity of not
distinguishing the larger groups, which with some exceptions answer to
our present families, by special names or by descriptions; it is only
from the order in which the species follow one another that we can
gather his views on natural relationship. It follows therefore that
the natural families, so far as they are distinguishable in Bauhin’s
works, have no sharp bounding lines; we might almost conclude that he
purposely avoided assigning such limits, that he might be able to pass
without interruption from one chain of relationship to another.
Like de l’Obel, Bauhin proceeds in his enumeration from the supposed
most imperfect to the more perfect forms, beginning with the Grasses
and the majority of Liliaceae and Zingiberaceae, passing on to
dicotyledonous herbs, and ending with shrubs and trees.
The Cryptogams that were known to him stand in the middle of the series
of dicotyledonous herbs, between the Papilionaceae and the Thistles,
the Equisetaceae being reckoned among the Grasses. On the great
distinction between Cryptogams and Phanerogams the views of Bauhin
were evidently less clear than those of many of his predecessors; but
it will not seem strange that he should place some Phanerogams, as
for instance the Duckweeds, among the Cryptogams and the Salviniaceae
among the Mosses, and unite the Corals, Alcionieae, and Sponges with
the Seaweeds, when we consider that it was not till the middle of the
18th century that more correct views arose in respect to these forms,
and that Linnaeus himself could not decide whether the Zoophytes
should be excluded from the vegetable kingdom and ranked with animals.
The knowledge of plants in the scientific sense of the word was till
the beginning of the 19th century limited to the Phanerogams; and
in speaking of principles and methods in descriptive botany before
that time we must think only of the Phanerogams, or at most of the
Phanerogams and the Ferns. The methodical examination of the Cryptogams
belongs to quite recent botanical research. The matter is here alluded
to only in connection with the fact, that it is from the works of
Kaspar Bauhin, a writer of ability, in whom the first period of
scientific botany culminates, that we most clearly see how great the
advance has been since his time.
CHAPTER II.
ARTIFICIAL SYSTEMS AND TERMINOLOGY OF ORGANS FROM CESALPINO TO LINNAEUS.
1583-1760.
While botany was being developed in Germany and the Netherlands in the
manner described in the previous chapter, and long before this process
of development reached its furthest point in Kaspar Bauhin, ANDREA
CESALPINO in Italy was laying down the general plan, on which the
further advance of descriptive botany was to proceed in the 17th and
till far into the 18th century; all that was done in the 17th century
in Germany, England, and France towards furthering morphology and
systematic botany was done with a reference to Cesalpino’s principles,
whether these were accepted and made use of, or whether it was sought
to refute them. This connection with Cesalpino became gradually less
close and less obvious, being concealed by new points of view and by
the increase of material for observation; but Cesalpino’s ideas on the
theoretical principles of systematic botany and the nature of plants
appear so plainly, even in the views of Linnaeus, that no one can
read both authors without lighting not unfrequently upon passages in
Linnaeus’ ‘Fundamenta’ or in his ‘Philosophia Botanica,’ which remind
him of Cesalpino, and even upon sentences borrowed from him. As we
saw in Kaspar Bauhin the close of the course of development commenced
by Fuchs and Bock, so we may regard Linnaeus as having built up and
completed the edifice of doctrine founded by Cesalpino.
Cesalpino comes before us, in strong contrast with the simple-minded
empiricism of the German fathers of botany, as the thinker in presence
of the vegetable world. Their main task was the amassing descriptions
of individual plants. Cesalpino made the material gathered by
experience the subject of earnest reflection; he sought especially to
obtain universals from particulars, important principles from sensuous
perceptions; but as his forms of thought were entirely Aristotelian, it
was inevitable that his interpretation of the facts should introduce
into them much that would have to be got rid of subsequently by the
inductive method. Cesalpino differs also from the German botanists in
another respect; he did not rest satisfied with the general impression
produced by the plants, but carefully examined the separate parts and
paid attention to the small and concealed organs; he was the first
who converted observation into real scientific research; and thus
we find in him a remarkable union of inductive natural science and
Aristotelian philosophy, a mixture which gives a peculiar character to
the theoretical efforts of his successors down to Linnaeus.
Cesalpino was moreover much before his time in his mode of
contemplating the vegetable kingdom, seeking always for philosophical
combinations and comprehensive points of view. His work which appeared
in 1583 exercised no perceptible influence on his contemporaries; a
trace of such influence only may be seen in Kaspar Bauhin thirty or
forty years later, while the work of the botanists who followed Bauhin
down to 1670 was confined everywhere to increasing the knowledge of
individual plants. With this object travels were undertaken after 1600
to all parts of the world; many new botanic gardens were added to the
few which had been founded in the 16th century—as at Giessen in 1617,
at Paris in 1620, at Jena in 1629, at Oxford in 1632, at Amsterdam in
1646, at Utrecht in 1650. Instead of endeavouring to embrace with their
labours the whole vegetable kingdom, botanists preferred to devote
themselves to the examination of single districts. This gave rise to
the first local floras (the word flora, however, was first introduced
by Linnaeus in the next century), and of these Germany especially
soon produced a considerable number; a flora of Altorf was published
by Ludwig Jungermann in 1615, of Ingolstadt by Albert Menzel in 1618,
of Giessen by Jungermann in 1623, of Dantzic by Nicolaus Oelhafen in
1643, of Halle by Carl Scheffer in 1662, of the Palatinate by Frank von
Frankenau in 1680, of Leipsic by Paul Ammann in 1675, of Nuremberg by
J. Z. Volkamer in 1700.
But though travel, catalogues in local floras, and the cultivation of
plants in botanic gardens promote knowledge of very varied kind, yet
this remains scattered about among descriptions of plants, until at
last a writer with powers of combination and wider and deeper glance
endeavours to gain some general conclusions from them. Such attempts
we first meet with late in the second half of the 17th century in
Morison, Ray, Bachmann (Rivinus), Tournefort, and others, who took
up Cesalpino’s principles after they had lain neglected for almost a
hundred years, and indeed were almost forgotten by botanists.
In the dearth of higher scientific efforts during this period, the
describing of plants and cataloguing of species prolonged a somewhat
pitiful existence. This describing, a work of great usefulness in the
fathers of German botany, was now become by perpetual repetition a
mechanical labour; all that was to be gained in this way had already
been gained by de l’Obel and Bauhin. This sterility which followed
upon the fruitful beginnings of the 16th century was general; neither
in Germany nor Italy, neither in France nor England, did the botanists
produce anything of importance. The representatives of the science did
not count among the more highly gifted or among the thinkers of their
time; and so content with the minor work of collecting and cataloguing
plants, and with endeavouring to know all plants as far as possible by
name, they lost whatever capacity they may have possessed for more
difficult operations of the mind simply by not attempting them.
There was one man indeed in Germany who studied the vegetable kingdom
in the first half of the 17th century in the spirit of Cesalpino before
him, but who, like Cesalpino, found no honour among contemporary
botanists. This man was the well-known philosopher Joachim Jung,
who invented a comparative terminology for the parts of plants, and
occupied himself with critical enquiries into the theory of the system,
the naming of species and other subjects, embodying their results in a
long array of aphorisms. Free from the genius-stifling burden which the
knowledge of individual species had become, a man possessed of varied
accomplishments and a well-trained mind, Jung was better qualified
than the professed botanists to see what was wanted in botany and
would advance it—a phenomenon more than once repeated in the history
of the science. But his results remained unknown to all except his
immediate pupils, till Ray admitted them into his great work on plants
in 1693, and made them the foundation of his own theoretical botany.
Enriched by Ray’s good morphological remarks, Jung’s terminology passed
to Linnaeus, who adopted it as he adopted every thing useful that
literature offered him, improving it here and there, but impairing its
spirit by his dry systematising manner.
The labours of the botanists of Germany and the Netherlands during
the 17th century, which culminated in Kaspar Bauhin, were not without
important influence upon the development of systematic botany which
began with Cesalpino. When Cesalpino wrote the work which forms an
epoch in the science, he was perhaps unacquainted with the natural
classification of de l’Obel (1576); at least there is nothing in his
book which shows that he had seen it; it appears even as though he had
made the discovery independently, that there is an actual connection of
relationship among plants expressed in their organisation as a whole;
it is at any rate certain that this fact assumed from the first an
entirely different expression in his system from that which it received
at the hands of de l’Obel and Bauhin, inasmuch as he was not guided
by an indistinct feeling for resemblances, but believed that he could
establish on predetermined grounds a system of marks, by which the
objective relationship must be recognised. If Cesalpino was thus in
advance of the German botanists, since he endeavoured to express with
clearness and on principle that which they only felt indistinctly,
he was at the same time treading a dangerous path, and one which led
succeeding botanists astray till the time of Linnaeus,—the path which
must always lead to artificial classifications, since the natural
system can never be laid down upon _a priori_ principles of division.
Through this labyrinth, in which botanists down to Linnaeus wandered
fruitlessly hither and thither, there remained one guide consistently
pointing to the goal to be attained, namely, the feeling for natural
affinity first vividly apprehended by the German botanists, and
expressed by them to some extent in their classifications. And when at
last Linnaeus and Bernard de Jussieu made the first feeble attempts
at a natural arrangement, it was the same indistinct perception which
asserted itself in them as in de l’Obel and Bauhin, and enabled them to
see that the path hitherto trodden could only lead astray.
The period in the development of descriptive botany which begins with
Cesalpino and reaches to Linnaeus may accordingly be perhaps best
characterised by saying, that botanists sought to do justice to natural
affinities by means of artificial classifications, till at length
Linnaeus clearly perceived the contradiction involved in this method
of proceeding. But inasmuch as Linnaeus left it to the future to work
out the natural system, and arranged the plants which he described in
a confessedly artificial manner, he so far marks rather the close of a
previous condition of the science than the beginning of modern botany.
These introductory observations will have supplied the reader with the
thread which will guide him through the following account of the more
prominent points in the history of botanical science from Cesalpino to
Linnaeus.
The often-quoted work of Andrea Cesalpino[17], ‘De plantis libri XVI,’
appeared in Florence in the year 1583. If the value of the contemporary
German botanists lies pre-eminently in the accumulation of descriptions
of individual plants, and these, it is true, occupy fifteen books of
Cesalpino’s work, it is on the contrary the introduction in the first
book, a discussion of the general theory of the subject, which in
his case is of much the higher importance for the history of botany.
This contains in thirty pages a full and connected exposition of the
whole of theoretical botany, and though based on broad and general
views is at the same time extremely rich in matter conveyed in a very
concise form. The different branches into which the subject has since
been divided are here united into an inseparable whole; morphology,
anatomy, biology, physiology, systematic botany, terminology are
so closely combined, that it is difficult to explain Cesalpino’s
views on any one more general question without at the same time
touching on a variety of other matters. Three things more especially
characterise this introductory book; first, a great number of new and
delicate observations; secondly, the great importance which Cesalpino
assigns to the organs of fructification as objects of morphological
investigation; lastly, the way in which he philosophises in strictly
Aristotelian fashion on the material thus gained from experience. If
this treatment has produced a work beautiful in style and fascinating
to the reader, if the whole subject is vivified by it while each
separate fact gains a more general value, it is on the other hand
apparent that the writer is often led astray by the well-known elements
of the Aristotelian philosophy, which are opposed to the interests of
scientific investigation. Mere creations of thought, the abstractions
of the understanding, are treated as really existent substances,
as active forces, under the name of principles; final causes appear
side by side with efficient; the organs and functions of the organism
exist either _alicujus gratiâ_ or merely _ob necessitatem_; the whole
account is controlled by a teleology, the influence of which is the
more pernicious because the purposes assumed are supposed to be
acknowledged and self-evident, plants and vegetation being conceived of
as in every respect an imperfect imitation of the animal kingdom. It
was moreover a necessary consequence of the treatment of his material
adopted by Cesalpino, that his ignorance of the sexuality of plants
and of the use of leaves as organs of nutrition led him to false and
mischievous conclusions; this defect of knowledge would have been of
less importance in a purely morphological consideration of plants, as
we shall see presently in Jung; but with Cesalpino morphological and
physiological considerations are so mixed up together, that a mistake
in the one direction necessarily involved mistakes in the other.
These remarks on Cesalpino’s method may be illustrated by some examples
tending to show how closely he attaches himself to Aristotle, and
how certain Aristotelian conceptions, the origin of which has not
been sufficiently regarded, passed through him into later botanical
speculation. We shall recur in the History of Physiology to Cesalpino’s
views on nutrition, and to his rejection of the doctrine of sexuality
in plants.
‘As the nature of plants,’ so begins Cesalpino’s book, ‘possesses
only that kind of soul by which they are nourished, grow, and produce
their like, and they are therefore without sensation and motion in
which the nature of animals consists, plants have accordingly need of
a much smaller apparatus of organs than animals.’ This idea reappears
again and again in the history of botany, and the anatomists and
physiologists of the 18th century were never weary of dilating on
the simplicity of the structure of plants and of the functions of
their organs. ‘But since,’ continues Cesalpino, ‘the function of
the nutritive soul consists in producing something like itself, and
this like has its origin in the food for maintaining the life of the
individual, or in the seed for continuing the species, perfect plants
have at most two parts, which are however of the highest necessity; one
part called the root by which they procure food; the other by which
they bear the fruit, a kind of foetus for the continuation of the
species; and this part is named the stem (‘caulis’) in smaller plants,
the trunk (‘caudex’) in trees.‘
This in the main correct conception of the upright stem as the
seed-bearer of the plant was also long maintained in botany. We should
observe also that the production of the seed is spoken of as merely
another kind of nutrition, a notion which afterwards prevented Malpighi
from correctly explaining the flower and fruit, and in a modified
form led Kaspar Friedrich Wolff in 1759 to a very wrong conception
of the nature of the sexual function. The next sentence in Cesalpino
takes us into the heart of the Aristotelian misinterpretation of the
plant, according to which the root answers to the mouth or stomach,
and must therefore be regarded in idea as the upper part although it
is the lower in position, and the plant would have to be compared with
an animal set on its head, and the upper and lower parts determined
accordingly: ‘this part (the root) is the nobler (‘superior’) because
it is prior in origin and sunk in the ground; for many plants live by
the roots only after the stem with the ripe seeds has disappeared; the
stem is of less importance (‘inferior’) although it rises above the
ground; for the excreta, if there are any, are given off by means of
this part; it is, therefore, with plants as with animals as regards the
expressions ‘pars superior’ and ‘inferior.’ When indeed we take into
consideration the mode of nourishment, we must define the upper and
the lower in another way; since in plants and animals the food mounts
upward (for that which nourishes is light because it is carried upwards
by the heat), it was necessary to place the roots below and to make the
stem go straight upwards, for in animals also the veins are rooted in
the lower part of the stomach, while their main trunk ascends to the
heart and the head.’ Here, in genuine Aristotelian fashion, the facts
are forced into a previously constructed scheme.
Cesalpino’s discussion of the seat of the soul in plants is of special
interest in connection with certain views of later botanists. ‘Whether
any one part in plants can be assigned as the seat of the soul, such
as the heart in animals, is a matter for consideration—for since the
soul is the active principle (‘actus’) of the organic body, it can
neither be ‘tota in toto’ nor ‘tota in singulis partibus,’ but entirely
in some one and chief part, from which life is distributed to the
other dependent parts. If the function of the root is to draw food
from the earth, and of the stem to bear the seeds, and the two cannot
exchange functions, so that the root should bear seeds and the shoot
penetrate into the earth, there must either be two souls different in
kind and separate in place, the one residing in the root, the other in
the shoot, or there must be only one, which supplies both with their
peculiar capabilities. But that there are not two souls of different
kinds and in a different part in each plant may be argued thus; we
often see a root cut off from a plant send forth a shoot, and in like
manner a branch cut off send a root into the ground, as though there
were a soul indivisible in its kind present in both parts. But this
would seem to show that the whole soul is present in both parts, and
that it is wholly in the whole plant, if there were not this objection
that, as we find in many cases, the capabilities are distributed
between the two parts in such a way that the shoot, though buried in
the ground, never sends out roots, for example in Pinus and Abies, in
which plants also the roots that are cut off perish.’ This, he thinks,
proves that there is only one soul residing in root and stem, but that
it is not present in all the parts; in a further discussion he seeks
to discover the true seat of the soul. He points out an anatomical
distinction between the shoot and the root; the root consists of the
rind and an inner substance which in some cases is hard and woody,
in others soft and fleshy. In the stem on the other hand there are
three constituent parts; outside the rind, inside the pith, between
the two a body which in trees is called the wood. This, on the whole,
correct distinction between stem and root is followed by a thoroughly
Aristotelian deduction.
‘Since then in all creatures’ (we must remark, that this is assuming
a point which has yet to be proved in the case of the half of living
creatures) ‘nature conceals the principle of life in the innermost
parts, as the entrails in animals, it is reasonable to conclude that
the principle of life in plants is not in the rind, but is more deeply
hidden in the inner parts, that is, in the pith, which is found in the
stem and not in the root. That this was the opinion of the ancients
we may gather from the name, for they called this part in plants the
heart (‘cor’), or brain (‘cerebrum’ or ‘matrix’), because from this
part in some degree the principle of foetification (the formation of
the seed) is derived.’ Here we see why the seed must, according to
Cesalpino, have its origin in the pith; the idea was loyally repeated
after him by Linnaeus, as we shall see hereafter. The argument, which
is a long one, ends with the sentence: ‘There are then two chief
parts in plants, the root and the ascending part; therefore the most
suitable spot for the heart of plants seems to be in the central part,
namely, where the shoot joins on to the root. There appears also at
this spot a certain substance differing both from the shoot and from
the root, softer and more fleshy than either, for which reason it is
usually called the cerebrum; it is edible in many plants while they
are young.’ We shall see below how important a part this seat of the
soul of the plant, brought to light with such difficulty and with
all appliances of scholasticism, is intended to play in Cesalpino’s
system, and how by this _a priori_ path he was led to the use of the
position of the embryo in the seed as his principle of division. It
may be remarked here that the point of union between the root and the
stem, in which Cesalpino placed the seat of the plant-soul, afterwards
received the name of root-neck (collet); and though the Linnaean
botanists of the 19th century were unaware of what Cesalpino had proved
in the 16th, and did not even believe in a soul of plants, they still
entertained a superstitious respect for this part of the plant, which
is really no part at all; and this, it would seem, explains the fact,
that an importance scarcely intelligible without reference to history
was once attributed to it, especially by some French botanists. To
return once more to Cesalpino’s ‘cor,’ he is not much troubled by the
circumstance that plants can be reproduced from severed portions; in
true Aristotelian manner he says that although the principle of life
is actually only one, yet potentially it is manifold. Ultimately a
‘cor’ is found in the axil of every leaf, by which the axillary shoot
is united with the pith of the mother-shoot, and finally, in direct
contradiction to the previous proof that the crown of the root is the
seat of the plant-soul, it is distinctly affirmed in Chapter V that the
soul of plants is in some sense diffused through all their parts.
The theoretical introduction to his excellent and copious remarks on
the parts of fructification may supply another example of Cesalpino’s
peripatetic method: ‘As the final cause (‘finis’) of plants consists
in that propagation which is effected by the seed, while propagation
from a shoot is of a more imperfect nature, in so far as plants do
exist in a divided state, so the beauty of plants is best shown in the
production of seed; for in the number of the parts, and the forms and
varieties of the seed-vessels, the fructification shows a much greater
amount of adornment than the unfolding of a shoot; this wonderful
beauty proves the delight (‘delitias’) of generating nature in the
bringing forth of seeds. Consequently as in animals the seed is an
excretion of the most highly refined food-substance in the heart, by
the vital warmth and spirit of which it is made fruitful, so also
in plants it is necessary that the substance of the seeds should be
secreted from the part in which the principle of the natural heat lies,
and this part is the pith. For this reason, therefore, the pith of the
seed (that is, the substance of the cotyledons and of the endosperm)
springs from the moister and purer part of the food, while the husk
which surrounds the seed for protection springs from the coarser part.
It was unnecessary to separate a special fertilising substance from the
rest of the matter in plants, as it is separated in animals which are
thus distinguished as male and female.‘
This last remark and some lengthy deductions which follow are intended
to prove, after the example of Aristotle, the absence and indeed the
impossibility of sexuality in plants, and accordingly Cesalpino goes
on to compare the parts of the flower, which he knew better than his
contemporaries, with the envelopes of the ova in the foetus of animals,
which he regards as organs of protection. Calyx, corolla, stamens,
and carpels are in his view only protecting envelopes of the young
seed, as the leaves are only a means of protecting the young shoots.
Moreover by the word flower (‘flos’) Cesalpino understands only those
parts of the flower which do not directly belong to the rudiment of
the fruit, namely, the calyx, the corolla, and the stamens. This must
be borne in mind if we would understand his theory of fructification,
and especially his doctrine of metamorphosis. We must also note, that
by the expression pericarp he understands exclusively juicy edible
fruit-envelopes, though at the same time pulpy seed-envelopes inside
the fruit pass with him for pericarps. The parts of his flower are
the ‘folium,’ which evidently means the corolla, but in certain
cases includes also the calyx; the ‘stamen,’ which is our style; and
the ‘flocci,’ our stamens. We see that Cesalpino uses the same word
‘folium’ without distinction for calyx, corolla, and ordinary leaves;
just as he, and Malpighi a hundred years later, unhesitatingly regarded
the cotyledons as metamorphosed leaves. In fact the envelopes of the
flower and the cotyledons approach so nearly to the character of
leaves, that every unprejudiced eye must instinctively perceive the
resemblance; and if doubts arose on this point in post-Linnaean times,
it was only a consequence of the Linnaean terminology, which neglected
all comparative examination.
Moreover the doctrine of metamorphosis appears in a more consistent
and necessary form in Cesalpino than in the botanists of the 19th
century before Darwin; it flows more immediately from his philosophical
views on the nature of plants, and appears therefore up to a certain
point thoroughly intelligible. We may also consider as part of this
doctrine in Cesalpino the view that the substance of the seed (embryo
and endosperm) arises from the pith, because the pith contains the
vital principle[18], and as the pith in the shoot is surrounded for
protection by the wood and the bark, so the substance of the seed
is surrounded by the woody shell, and by the bark-like pericarp or
by a fruit-envelope answering to a pericarp. According to Cesalpino
therefore the substance of the seed with its capability of development
springs from the pith, the woody shell from the wood, the pericarp
from the rind of the shoot. The difficulty which arises from this
interpretation, namely, that in accordance with his theory the parts
of the flower also, the calyx, the corolla, and the stamens ought to
spring from the outer tissues of the shoot, he puts aside with the
remark (p. 19) that these parts of the flower are formed when the
pericarp is still in a rudimentary state; that the pericarp is only
fully developed after these parts have fallen off, and that they are so
thin that there is nothing surprising in this view of the matter. We
see in Cesalpino’s doctrine of metamorphosis without doubt the theory
of the flower afterwards adopted by Linnaeus, though in a somewhat
different form. That Linnaeus himself regarded the theory ascribed to
him on the nature of the flower as the opinion of Cesalpino also,
is shown in his ‘Classes Plantarum,’ where in describing Cesalpino’s
system he says: ‘He regarded the flower as the interior portions of
the plant, which emerge from the bursting rind; the calyx as a thicker
portion of the rind of the shoot; the corolla as an inner and thinner
rind; the stamens as the interior fibres of the wood, and the pistil
as the pith of the plant.’ It may be observed however that this was
not exactly what Cesalpino says; but it is nevertheless certain that
Linnaeus’ own view as given in these words was intended to reproduce
that of Cesalpino; and if it does not do this exactly, there is no
essential difference in principle between the two, Linnaeus’ conception
being perhaps a more logical statement of Cesalpino’s meaning.
Cesalpino’s doctrine of metamorphosis appears plainly on another
occasion also; he says, that we do not find envelopes, stamens, and
styles in all flowers; the flowers change in some cases into another
substance, as in the hazel, the edible chestnut, and all plants that
bear catkins; the catkin is in place of a flower, and is a longish
body arising from the seat of the fruit, and in this way fruits appear
without flowers, for the styles (‘stamina’) form the longer axis of
the catkin (‘in amenti longitudinem transeunt’), while the leafy parts
and the stamens are changed into its scales. All this shows that the
notion of a metamorphosis, of which we find intimations as early
as Theophrastus, was a familiar one to Cesalpino, and it fitted in
perfectly with his Aristotelian philosophy, while Goethe’s doctrine on
the same subject is equally scholastic in its character, and therefore
looks strange and foreign in modern science. It has already been
observed that Cesalpino includes only the envelopes and stamens under
the word flower, and distinguishes the rudiments of the fruit from
them; therefore he says that there are plants which produce something
in the shape of a catkin, without any hope of fruit, for they are
entirely unfruitful; but those which bear fruit have no flowers, as
Oxycedrus, Taxus, and among herbs Mercurialis, Urtica, Cannabis, in
which the sterile plants are termed male, the fruitful female. Thus he
distinguished the cases which we now call dioecious from the previously
mentioned monoecious plants, among which he reckons the maize.
All this may serve to give the reader some idea, though a very
incomplete one, of Cesalpino’s theory; to do him justice, it would be
necessary to give a full account of his very numerous, accurate, and
often acute observations on the position of leaves, the formation of
fruit, the distribution of seeds and their position in the fruit, of
his comparative observations on the parts of the fruit in different
plants, and above all of his very excellent description of plants with
tendrils and climbing plants, of those that are armed with thorns
and the like. Though there is naturally much that is erroneous and
inexact in his accounts, yet we have before us in the chapters on
these subjects the first beginning of a comparative morphology, which
quite casts into the shade all that Aristotle and Theophrastus have
said on the subject. But the most brilliant portions of his general
botany are contained in the 12th, 13th, and 14th chapters, in which
he gives the outlines of his views on the systematic arrangement of
plants; to prepare the way for what is to follow, he shows first
that it is better to give up the four old divisions of the vegetable
kingdom, and to unite the shrubs with the trees and the undershrubs
with the herbs. But how these genera are to be distinguished into
species is, he says, hard to conceive, for the multitude of plants is
almost innumerable; there must be many intermediate genera containing
the ‘ultimae species,’ but few are as yet known. He then turns to the
divisions founded on the relations of plants to men. Such groups, he
says, as vegetables and kinds of grain, which are put together under
the name of ‘fruges’ and kitchen-herbs (‘olera’), are formed more
from the use made of them than from the resemblance of form, which we
require; and he shows this by good examples. The discerning of plants,
he continues, is very difficult, for so long as the genera (larger
groups) are undetermined, the species must necessarily be mixed up
together[19]; the difficulty arises from our uncertainty as to the
rules by which we should determine the resemblances of the genera.
While there are two chief parts in plants, the root and the shoot, we
cannot, as it seems, determine the genera and species from the likeness
or unlikeness either of the one or of the other; for if we make a genus
of those plants which have a round root, as the turnip, Aristolochia,
Cyclamen, Arum, we separate generically things which agree together in
a high degree, as rape and radish which agree with the turnip, and the
long Aristolochia which agrees with the round, while at the same time
we unite things most dissimilar, for the Cyclamen and the turnip are
in every other respect of a quite different nature; the same is the
case with divisions which rest merely on differences in the leaves and
flowers.
In pursuing these reflections, which have the conception of species
chiefly in view, he arrives at the following proposition: That
according to the law of nature like always produces like, and that
which is of the same species with itself.
All that Cesalpino says on systematic arrangement shows that he was
perfectly clear in his own mind with regard to the distinction between
a division on subjective grounds, and one that respects the inner
nature of plants themselves, and that he accepted the latter as the
only true one. He says, for instance, in the next chapter: ‘We seek
out similarities and dissimilarities of form, in which the essence
(‘substantia’) of plants consists, but not of things which are merely
accidents of them (‘quae accidunt ipsis’).’ Medicinal virtues and other
useful qualities are, he says, just such accidents. Here we see the
path opened, along which all scientific arrangement must proceed, if
it is to exhibit real natural affinities; but at the same time there
is a warning already of the error which beset systematic botany up to
Darwin’s time; if in the above sentence we substitute the word idea
for that of substance, and the two expressions have much the same
meaning in the Aristotelian and Platonic view of nature, we recognise
the modern predarwinian doctrine, that species, genera, and families
represent ‘ideam quandam’ and ‘quoddam supranaturale.’
Pursuing his deductions, Cesalpino next shows, that the most important
divisions, those of woody plants and herbs, must be maintained in
accordance with the most important function of vegetation, that of
drawing up the food through root and shoot; this division passed from
the first and later on up to the time of Jung for an unassailable
dogma, to which science simply had to conform. The second great
function of plants is the producing their like, and this is effected
by the parts of fructification. Though these parts are only found in
the more perfect forms, yet the subdivisions (‘posteriora genera’) must
be derived in both trees and herbs from likeness and unlikeness in the
fructification. And thus Cesalpino was led, not by induction but by the
deductive path of pure Aristotelian philosophy, to the conclusion, that
the principles of a natural classification are to be drawn from the
organs of fructification; for which conclusion Linnaeus declared him
to be the first of systematists, while he thought de l’Obel and Kaspar
Bauhin, who founded their arrangements on the habit only, scarcely
deserving of notice.
It appears, then, that Cesalpino obtained the subdivisions which he
founded on the organs of fructification from _a priori_ views of the
comparative value of organs, such as run through all Aristotelian
philosophy. Of much interesting matter in the remainder of his
introduction we must mention only that he makes the highest product
of plants to be the fructification, of animals sense and movement,
of man the intellect; and because the latter stands in need of no
special bodily instruments, there is no specific difference in men, and
therefore only one species of man.
In his 14th chapter he gives in broad outline a view of the system
of plants which he founded on the fructification, beginning with the
least perfect; no one who knows the botanical writers of the 17th and
18th centuries will be surprised to find that Cesalpino admits the
doctrine of ‘generatio spontanea’ in the case of the lower plants, and
in a somewhat crude form; this came from the teaching of Aristotle, and
even a hundred years later Mariotte endeavoured to set up a plausible
defence of spontaneous generation on physical grounds even in highly
developed plants.
‘Some plants,’ says Cesalpino, ‘have no seed; these are the most
imperfect, and spring from decaying substances; they have only
therefore to feed themselves and grow, and are unable to produce their
like; they are a sort of intermediate existences between plants and
inanimate nature. In this respect Fungi resemble Zoophytes, which are
intermediate between plants and animals, and of the same nature are the
Lemnae, Lichenes, and many plants which grow in the sea.’
Some on the other hand produce seed, which they form after their
peculiar nature in an imperfect condition, as the mule among animals;
these are of the same nature as mere monstrosities or diseased growths
of other plants, and many occur in the class of grain and bear empty
ears. Cesalpino is evidently speaking of the Ustilagineae, but he
includes also the Orobancheae and Hypocystis, which instead of seed
contain only a powder; and he adds that some of the more perfect plants
are sterile, but they do not belong to this division, because the
peculiarity is confined in their case to individuals.
Some plants bear a substance, a kind of wool, on the leaves, which to
some extent answers to seed, because it serves to propagate the plant;
such plants have neither stem, flower, nor true seed, and the Ferns
are of this kind. We should notice this conclusion from Cesalpino’s
morphology, that plants without true seeds have also no stem; the view
that ferns have no stems continued to be held by later botanists,
though the original reason for it was gradually lost; and those who in
the middle of the 19th century argued still in favour of this opinion,
little suspected that they were endeavouring to establish a dogma of
the Aristotelian philosophy. It is a similar case to that of the crown
of the root mentioned above. But other plants, continues Cesalpino,
produce true seeds; and he proceeds to treat of this division first,
on account of its great extent as comprising all perfect plants. Three
things, he says, contribute especially to the constitution of organs,
the number, position, and shape of the parts; the play of nature in the
composition of fruits varies according to their differences, and hence
arise the different divisions of plants. He then shows how he proposes
to apply these relations to the framing of his system, but his various
points of view may be omitted here, as they can be better and more
shortly gathered from the table below.
Other marks to be derived from roots, stems, and leaves, may be used,
he says, for forming the smaller divisions. Lastly, some marks which
contribute to the constitution neither of the whole plant nor of the
fruit, such as colour, smell, taste, are mere accidents and are due to
cultivation, place of growth, climate, and other causes.
The first of Cesalpino’s sixteen books ends with this general view of
his system. The remaining fifteen books contain about 600 pages of
descriptions of individual plants arranged in fifteen classes; some of
the descriptions are exceedingly minute; the trees come first, and are
followed by the shrubs on account of their affinity (‘ob affinitatem’).
Two things have interfered with the recognition and acceptance of this
system; the omission of a general view to precede the text, and its
appearance in the traditional form of books and chapters, such as we
find in de l’Écluse, Dodoens, and Bauhin, instead of in classes and
orders, though it is true that the headings and introductions to the
several books contain the designations and general characteristics
of the classes described in them. Linnaeus has done good service by
giving in his ‘Classes Plantarum’ a general view of all the systems
proposed before his time, among which he gives the first rank to that
of Cesalpino; he has also pointed out the peculiar characteristics of
each system, and has appended to the old names of the genera those with
which he has himself made us familiar. This invaluable work, which is
a key to the understanding of the efforts that were made in systematic
botany from Cesalpino to Linnaeus himself, will often be referred to
in later pages of this history; it will supply us here with a tabular
view of Cesalpino’s main divisions as precisely formulated by Linnaeus,
which is well worth the space it will occupy, as presenting the first
plan proposed for a systematic arrangement of the vegetable kingdom,
with characters for each division. For the better understanding of
these diagnoses it should be remembered that the ‘cor’ (heart) is the
important point in the seed with Cesalpino, and that it is the place in
the embryo where the radicle and the plumule unite, as has been said
in a former page; Cesalpino himself says somewhat inexactly, the place
from which the cotyledons spring.
The characters of the classes are given, for brevity’s sake, in Latin.
ARBOREAE
(Arbores et frutices).
I. Corde ex apice seminis. Seminibus saepius solitariis (e.g. Quercus,
Fagus, Ulmus, Tilia, Laurus, Prunus).
II. Corde e basi seminis, seminibus pluribus (e.g. Ficus, Cactus,
Morus, Rosa, Vitis, Salix, Coniferae, etc.).
HERBACEAE
(Suffrutices et herbae).
III. Solitariis seminibus. Semine in fructibus unico (e.g. Valeriana,
Daphne, Urtica, Cyperus, Gramineae).
IV. Solitariis pericarpiis. Seminibus in fructu pluribus, quibus est
conceptaculum carnosum, bacca aut pomum (e.g. Cucurbitaceae, Solaneae,
Asparagus, Ruscus, Arum).
V. Solitariis vasculis. Seminibus in fructu pluribus quibus est
conceptaculum e siccâ materiâ (e.g. various Leguminosae, Caryophylleae,
Gentianeae, etc.).
VI. Binis seminibus. Semina sub singulo flosculo invicem conjuncta, ut
unicum videantur ante maturitatem; cor in parte superiore, quâ flos
insidet. Flores in umbellâ (Umbelliferae).
VII. Binis conceptaculis (e.g. Mercurialis, Poterium, Galium,
Orobanche, Hyoscyamus, Nicotiana, Cruciferae).
VIII. Triplici principio (ovary) non bulbosae. Semina trifariam
distributa; corde infra sito, radix non bulbosa (e.g. Thalictrum,
Euphorbia, Convolvulus, Viola).
IX. Triplici principio bulbosae. Semina trifariam distributa; corde
infra sito, radix bulbosa (Large-flowered Monocotyledons).
X. Quaternis seminibus. Semina quatuor nuda in communi sede (Boragineae
and Labiatae).
XI. Pluribus seminibus, anthemides. Semina nuda plurima, cor seminis
interius vergens; flos communis distributus per partes in apicibus
singuli seminis (Compositae only).
XII. Pluribus seminibus, cichoraceae aut acanaceae. Semina nuda
plurima, cor seminis inferius vergens, flos communis distributus per
partes in apicibus singuli seminis (Compositae, Eryngium, and Scabiosa).
XIII. Pluribus seminibus, flore communi. Semina solitaria plurima,
corde interius; flos communis, non distributus, inferius circa fructum
(e.g. Ranunculus, Alisma, Sanicula, Geranium, Linum).
XIV. Pluribus folliculis. Semina plura in singulo folliculo (e.g.
Oxalis, Gossypium, Aristolochia, Capparis, Nymphaea, Veratrum, etc.).
XV. Flore fructuque carentes (Filices, Equiseta, Musci including
Corals, Fungi).
The examples appended by me to the diagnoses show that with the
exception of the sixth, tenth, and fifteenth classes, no one perfectly
represents a natural group of the vegetable kingdom. Most of them
are a collection of heterogeneous objects, and the distinction of
Dicotyledons and Monocotyledons, almost perfectly carried out by
de l’Obel and Bauhin, is to a great extent effaced; the ninth class
certainly contains only Monocotyledons, but not all of them. This
result of great efforts on the part of a mind so well trained as
Cesalpino’s is highly unsatisfactory. Not a single new group founded
on natural affinities is established, which does not appear already in
the herbals of Germany and the Netherlands. It is characteristic of
the natural system to reveal itself to a certain extent more readily
to instinctive perception than to the critical understanding. We have
seen that Cesalpino intended as far as possible to give expression in
his system to natural affinities, and the final result was a series
of highly unnatural groups, almost every one of which is a collection
of the most heterogeneous forms. The cause of this apparently so
remarkable fact is this, that he believed that he could establish on
predetermined grounds the marks which indicate natural affinities. The
uninterrupted labour of nearly 300 years, starting again and again
from the same principle or practically under its influence, has given
us inductive proof that the path taken by Cesalpino is the wrong one.
And if, while this path was pursued even into the middle of the 18th
century, we see natural groups emerge with increasing distinctness,
it is because the botanist, though on the wrong track, was still
continually gaining better acquaintance with the ground over which he
was wandering, and attained at length to an anticipation of the truer
way.
JOACHIM JUNG[20] was born in Lübeck in the year 1587, and died after an
eventful life in 1657. He was a contemporary of Kepler, Galileo, Vesal,
Bacon, Gassendi, and Descartes. After having been already a professor
in Giessen, he applied himself to the study of medicine in Rostock,
was in Padua in 1618 and 1619, and there, as we may confidently
believe, became acquainted with the botanical doctrines of Cesalpino,
who had died fifteen years before. Returning to Germany, he held
various professorships during the succeeding ten years in Lübeck and
Helmstädt, and became Rector of the Johanneum in Hamburg in 1629. He
occupied himself with the philosophy of the day, in which he appeared
as an opponent of scholasticism and of Aristotle, and also with various
branches of science, mathematics, physics, mineralogy, zoology, and
botany. In all these subjects he displayed high powers as a student
and a teacher, and especially as a critical observer; in botany at
least he was a successful investigator. He was the first in Germany, as
Cesalpino had been in Italy, who combined a philosophically educated
intellect with exact observation of plants.
His pupils were at first the only persons who profited by his botanical
studies, for with his many occupations and a perpetual desire to make
his investigations more and more complete he himself published nothing.
In 1662 his pupil Martin Fogel printed the ‘Doxoscopiae Physicae
Minores,’ a work of enormous compass left in manuscript at the master’s
death, and another pupil, Johann Vagetius, the ‘Isagoge Phytoscopica,’
in 1678. Ray however tells us that a copy of notes on botanical
subjects had already reached England in 1660. The ‘Doxoscopiae’
contains a great number of detached remarks on single plants and on
their distinguishing marks, and propositions concerning the methods and
principles of botanical research,—all in the form of aphorisms which
he had from time to time committed to paper. The number and contents
of these aphorisms show the earnest attention which he bestowed on
the determination of species; he is displeased that so many botanists
devote more time and labour to the discovery of new plants, than to
referring them carefully and logically to their true genera by means
of their specific differences. He was the first who objected to the
traditional division of plants into trees and herbs, as not founded
on their true nature. But how firmly this old dogma was established
is well shown by the fact, that Ray at the end of the century still
retained this division, though he founded his botanical theories on
the ‘Isagoge’ of Jung. Jung was in advance of Cesalpino and his own
contemporaries in repeatedly expressing his doubt of the existence of
spontaneous generation.
The ‘Isagoge Phytoscopica,’ a system of theoretical botany, very
concisely written and in the form of propositions arranged in strict
logical sequence, was a more important work and had more lasting
effects upon the history of botany. We must look more closely into
the contents of this volume, because it contains the foundation of
the terminology of the parts of plants subsequently established by
Linnaeus. Since the matter of the ‘Isagoge’ is produced in Ray’s
‘Historia Plantarum’ in italics, with special mention of the source
from which it is derived, it cannot be doubted that Linnaeus had made
acquaintance with the teaching of Jung as a young man, in any case
before 1738. It is as important as a matter of history to know that
Linnaeus’ terminology is founded on Jung, as it is to learn that his
most general philosophical propositions on botanical subjects are to
be traced to Cesalpino. It will moreover be fully shown in the account
of the doctrine of sexuality that his knowledge of that subject was
derived from Rudolf Jacob Camerarius.
The first chapter of the ‘Isagoge’ discusses the distinction between
plants and animals. A plant is, according to Jung, a living but not
a sentient body; or it is a body attached to a fixed spot or a fixed
substratum, from which it can obtain immediate nourishment, grow and
propagate itself. A plant feeds when it transforms the nourishment
which it takes up into the substance of its parts, in order to replace
what has been dissipated by its natural heat and interior fire. A
plant grows when it adds more substance than has been dissipated,
and thus becomes larger and forms new parts. The growth of plants is
distinguished from that of animals by the circumstance that their
parts are not all growing at the same time, for leaves and shoots
cease to grow as soon as they arrive at maturity; but then new leaves,
shoots, and flowers are produced. A plant is said to propagate itself
when it produces another specifically like itself; this is the idea in
its broader acceptation. We see that here, as in Cesalpino, the idea of
the species is connected with that of propagation. The second chapter,
headed ‘Plantae Partitio,’ treats of the most important morphological
relations in the external differentiation of plants; here Jung adheres
essentially to Cesalpino’s view, that the whole body in all plants,
except the lowest forms, is composed of two chief parts, the root
as the organ which takes up the food, and the stem above the ground
which bears the fructification. Jung, too, draws attention to the
meeting-point of the two parts, Cesalpino’s ‘cor,’ but under the name
of ‘fundus plantae.’
The upper part, or a portion of the plant, is either a stem, a leaf, a
flower, a fruit, or a structure of secondary importance, such as hairs
and thorns. His definition of the stalk and the leaf is noteworthy;
the stalk, he says, is that upper part which stretches upwards in
such a manner, that a back and front, a right and left side, are not
distinguished in it. A leaf is that which is extended from its point
of origin in height, or in length and breadth, in such a manner, that
the bounding surfaces of the third dimension are different from one
another, and therefore the outer and inner surfaces of the leaf are
differently organised. The inner side of the leaf, which is also called
the upper, is that which looks towards the stem, and is therefore
concave or less convex than the other side. One conclusion he draws,
which is a striking one for that time, that the compound leaf is taken
for a branch by inexperienced or negligent observers, but that it may
easily be determined by having an inner and an outer surface, like the
simple leaf, and by falling off as a whole in autumn. He calls a plant
‘difformiter foliata,’ whose lower leaves are strikingly different
from the upper, an idea which Goethe, in the fragment in Guhrauer,
seems to have altogether misunderstood.
In connection with these general definitions, the different forms of
the stem and of the ramification, and the varieties of leaves are
pointed out and supplied with distinctive names, which are for the
most part still in use. The fourth chapter treats of the division
of the stem into internodes; if the stem or branch, says Jung, is
regarded as a prismatic body, the articulations, that is, the spots
where a branch or a leaf-stalk arises, are to be conceived of as
cross-sections parallel to the base of the prism. These spots when they
are protuberant are called knees or nodes, and that which lies between
such spots is an internode.
It is not possible to quote all the many excellent details which follow
these definitions; but some notice must be taken of Jung’s theory of
the flower, which he gives at some length from the 13th to the 27th
chapters. It suffers, as in Cesalpino, from his entire ignorance of
the difference of sexes in plants, which is sufficient to render any
satisfactory definition of the idea of a flower impossible. Like
Cesalpino too he distinguishes the pistil from the flower, instead
of making it a part of the flower. He regards the flower as a more
delicate part of the plant, distinguished by colour or form, or by
both, and connected with the young pistil. Like all botanists up to the
end of the 18th century, he follows Cesalpino in including under the
term fruit both the dry indehiscent fruits which were supposed to be
naked seeds, and any seed-vessel. He differs from him in calling the
stamens ‘stamina,’ and the style ‘stilus,’ but like Cesalpino he uses
the word ‘folium’ for the corolla. He calls a flower perfect only when
it has all these three parts. He afterwards describes the relations of
form and number in the parts of the flower, and among other things he
enunciates the first correct view of the nature of the capitulum in
the Compositae, which Cesalpino quite misunderstood; and he examined
inflorescences and superior and inferior flowers, which Cesalpino
had already distinguished, with more care than they had previously
received. In his theory of the seed he follows Cesalpino, and adds
nothing to him.
There is nothing which more essentially distinguishes the theoretical
botany of Jung, and marks the advance which he made upon Cesalpino’s
views, than the way in which he discusses morphology in as entire
independence as was possible of all physiological questions, and
therefore abstains from teleological explanations. His eye is fixed
on relations of form only, while his mode of treating them is
essentially comparative, and embraces the whole of the vegetable
kingdom that was known to him. Jung certainly learnt much from
Cesalpino; but in rejecting at least the grosser aberrations of the
Aristotelian philosophy and of scholasticism, he freed himself from
the prepossessions of his master, and succeeded in arriving at more
correct conceptions of the morphology of plants. That his mathematical
gifts assisted him in this respect is easy to be gathered from his
definitions as given above, which bring into relief the symmetry
apparent in the forms of stems and leaves. No more profound or apt
definitions were supplied till Schleiden and Nägeli introduced the
history of development into the study of morphology.
While Cesalpino, Kaspar Bauhin, and Jung stand as solitary forms each
in his own generation, the last thirty years of the 17th century are
marked by the stirring activity of a number of contemporary botanists.
While during this period physics were making rapid advances in the
hands of Newton, philosophy in those of Locke and Leibnitz, and the
anatomy and physiology of plants by the labours of Malpighi and Grew,
systematic botany was also being developed, though by no means to
the same extent or with equally profound results, by Morison, Ray,
Bachmann (Rivinus), and Tournefort. The works of these men and of their
less gifted adherents, following rapidly upon or partly synchronous
with each other, led to an exchange of opinions and sometimes to
polemical discussion, such as had not before arisen on botanical
subjects; this abundance of literature, with the increased animation
of its style, excited a more permanent interest, which spread beyond
the narrow circle of the professional adepts. The systematists
above-named endeavoured to perfect the morphology and the terminology
of the parts of plants, and they found ready to their hands in the
works of their predecessors a considerable store of observations and
ideas, upon which they set themselves to work. A very great number
of descriptions of individual plants had been accumulated since the
time of Fuchs and Bock, and the fact of natural affinity had been
recognised in the ‘Pinax’ of Kaspar Bauhin as the foundation of a
natural system; Cesalpino had pointed to the organs of fructification
as the most important for such a system, and Jung had supplied the
first steps to a comparative morphology in place of a mere explanation
of names. The botanists of the last thirty years of the 17th century
could not fail to perceive that the series of affinities as arranged
by de l’Obel and Bauhin could not be defined by predetermined marks
in the way pursued by Cesalpino, nor fashioned in this way into a
well-articulated system. Nevertheless they held fast in principle to
Cesalpino’s mode of proceeding, though they endeavoured to amend it
by obtaining their grounds of division, not as he had done, chiefly
from the organisation of the seed and fruit, but from other parts of
the flower; variations in the corolla, the calyx, and the general
habit were employed to found systems, which were intended to exhibit
natural affinities. And while the true means were thus missed, the end
itself was not clearly and decidedly adhered to; a system was desired
for the purpose of facilitating the acquisition of a knowledge of
the greatest possible number of individual forms; the weight of the
burden caused by the foolish demand that every botanist should know
all described plants, was continually increasing, and naturally led to
seeking some alleviation in systematic arrangement. Excessive devotion
to the describing of plants stood in the way of such a profound study
of the principles of systematic botany as might have led to enduring
results, and even destroyed the very capacity for those difficult
intellectual operations, which were absolutely necessary to build
up a truly natural system on scientific foundations; the wood could
not be seen for the trees. Above all the morphology founded by Jung,
though acknowledged and employed, was not sufficiently developed by the
labours of others to form the foundation of the system in its grander
features,—a reproach which must be made against the systematists of the
succeeding hundred years with few exceptions. How could the botanists
of the 17th century succeed in acquiring a true conception of the
larger groups indicated by natural affinity, when they still held to
the old division into trees and herbs, which Jung had already set
aside and which is opposed to all consistent morphology, and when they
paid so little attention to the structure of the seed and the fruit,
that they commonly treated dry indehiscent fruits as naked seeds, and
were guilty of other and similar mistakes? But if nothing new and good
in principle found its way into systematic botany, much service was
rendered to it in matters of detail. The working out of various systems
helped to show what marks are not admissible in fixing the limits of
the natural groups; the contradiction between the method and aim of the
systematists became in this empirical way continually more apparent,
till at length Linnaeus was able to recognise it distinctly; and this
was beyond doubt a great gain.
To attempt to give an account of all the systematists of England,
France, Italy, Germany, and the Netherlands during this period
would serve only to obscure the subject; all that is historically
important will be brought out more clearly by mentioning those only
who have really enriched systematic botany. Whoever wishes for a more
complete knowledge of all the systems which made their appearance
before Linnaeus will find a masterly account of them in his ‘Classes
Plantarum,’ and another worth consulting in Michel Adanson’s ‘Histoire
de la Botanique’ (Paris, 1864). It is sufficient for our present
purpose to consider more particularly the labours of the four men whose
names have recently been mentioned.
ROBERT MORISON[21], who was born in Aberdeen in 1620 and died in
London in 1683, was the first after Cesalpino and Bauhin who devoted
himself to systematic botany, that is, to founding and perfecting the
classification of plants. He was reproached by his contemporaries
and successors with having borrowed without acknowledgment from
Cesalpino; this was an exaggeration. Morison commenced his efforts as
a systematist with a careful examination of Kaspar Bauhin’s ‘Pinax’;
there he obtained his conceptions of natural relationship in plants;
and if he afterwards founded his own system more peculiarly on the
forms of the fruit, it was in a very different way from that adopted
by Cesalpino. Linnaeus answers the reproach above-mentioned by the
pertinent remark, that Morison departs as far from Cesalpino in this
point as he is inferior to him in the purity of his method. In the year
1669 appeared a work with the characteristic title, ‘Hallucinationes
Kaspari Bauhini in Pinace tum in digerendis quam denominandis plantis,’
which Haller justly calls an ‘invidiosum opus’; for as there are
writers at all times who ungratefully accept all that is good and
weighty in their predecessors as self-evident, while they point with
malicious pleasure to every little mistake which the originator of a
great idea may commit, so Morison has no word of recognition for the
great and obvious merits of the ‘Pinax,’ though such a recognition
was specially due from one whose design was to point out the numerous
mistakes in that work on the subject of affinities. Kurt Sprengel in
his ‘Geschichte,’ ii. p. 30, also suspects with reason that Jung’s
manuscript, which was communicated by Hartlieb to Ray in 1661, was
not unknown to Morison, and in this paper he might certainly have
found much that suited his purposes. Sprengel says well, that the
‘Hallucinationes’ are a well-grounded criticism of the arrangement of
plants, which the Bauhins had chosen; that the writer goes through the
‘Pinax’ page by page, and shows what plants occupy a false position,
and that it is certain that Morison laid the first foundation of a
better arrangement and a more correct discrimination of genera and
species.
His ‘Plantarum umbelliferarum distributio nova,’ Oxford, 1672, shows
considerable advance; it is the first monograph which was intended to
carry out systematic principles strictly within the limits of a single
large family. The very complex arrangement is founded exclusively on
the external form of the fruit, which he naturally terms the seed. It
is the first work in which the system is no longer veiled by the old
arrangement in books and chapters, perspicuity being provided for by
typographical management,—an improvement which de l’Obel, it is true,
made a feeble attempt to introduce a hundred years before. Morison
also endeavours to give a clear idea of the systematic relations
within the family by the aid of linear arrangement, to some extent
the first hint of what we now call a genealogical tree, and a proof
at any rate of the lively conception which he had formed of affinity,
not drawn indeed only ‘ex libro naturae,’ as the title of his book
states, but in principle from Bauhin. Morison’s inability to appreciate
the merits of his predecessors, and to believe that when he made a
step in advance the way had ever been trodden before, may be seen in
this work also. One of its merits is, that it contains for the first
time careful representations of separate parts of plants, executed in
copper plate[22]. In 1680 appeared the first volumes of his ‘Historia
plantarum universalis Oxoniensis,’ the third portion of which was
published after his death by Bobart in 1699,—a collection of most of
the plants then known and a large number of new ones with descriptions;
the systematic arrangement in this work is to be seen in Linnaeus’
‘Classes Plantarum.’ If Morison in his criticism of Bauhin displayed
considerable acuteness within narrow circles of affinity, his universal
system on the contrary shows extremely small feeling for affinities
on the large scale; the most different forms are brought together
even in the smaller divisions; the last class of his Bacciferae, for
example, contains genera like Solanum, Paris, Podophyllum, Sambucus,
Convallaria, Cyclamen, a result which is the more surprising as Morison
does not, like Cesalpino, confine himself to single fixed marks, but
has regard also to the habit. On the whole his arrangement as an
expression of natural affinities must be ranked after those of de
l’Obel and Bauhin.
Morison’s merit lay in truth less in the quality of what he did,
than in the fact that he was the first to renew the cultivation of
systematic botany on a comprehensive scale. The number of his adherents
was always small; in Germany Paul Ammann, Professor in Leipsic, adopted
Morison’s views in his ‘Character Plantarum Naturalis’ (1685), and
Paul Hermann, Professor in Leyden from 1679 to 1695, after collecting
plants in Ceylon for eight years, proposed a system founded on that of
Morison, but which can scarcely be called an improvement upon it.
In contrast to Morison, JOHN RAY[23] (1628 to 1705) not only knew how
to adopt all that was good and true in the works of his predecessors,
and to criticise and complete them from his own observations, but could
also joyfully acknowledge the services of others, and combine their
results and his own into a harmonious whole. He wrote many botanical
works; but none display his character as a man and a naturalist better
than his comprehensive ‘Historia Plantarum,’ published in three large
folio volumes without plates in the period from 1686 to 1704. This
work contains a series of descriptions of all plants then known; but
the first volume commences with a general account of the science in
fifty-eight pages, which, printed in ordinary size, would itself make a
small volume, and which treats of the whole of theoretic botany in the
style of a modern text-book. If morphology, anatomy, and physiology,
in which latter subject he relies on the authority of Malpighi and
Grew, are not kept strictly apart in his exposition, yet it is easy to
separate the morphological part, and his theory of systematic botany
is in fact given separately. Jung’s definitions of the subject-matter
of each of the chapters on morphology are first given, and Ray then
adds his own remarks, in which he criticises, expands, and supplements
those of his predecessor. Omitting all that is not his own, and the
anatomical and physiological portions, we will describe some of the
more important results of his studies on system. First and foremost Ray
adopted the idea which Grew had conceived, but in a very clumsy form,
that difference of sex prevails in the vegetable kingdom, and hence
the flower had a different meaning and importance for him from what
it had had for his predecessors, though his views on the subject were
still indistinct. Ray perceived more clearly than Cesalpino that many
seeds contain not only an embryo but also a substance, which he calls
‘pulpa’ or ‘medulla,’ and which is now known as the endosperm, and
that the embryo has not always two cotyledons, but sometimes only one
or none; and though he was not quite clear as regards the distinction,
which we now express by the words dicotyledonous and monocotyledonous
embryo, yet he may claim the great merit of having founded the
natural system in part upon this difference in the formation of the
embryo. He displays more conspicuously than any systematist before
Jussieu the power of perceiving the larger groups of relationship in
the vegetable kingdom, and of defining them by certain marks; these
marks moreover he determines not on _a priori_ grounds, but from
acknowledged affinities; but it is only in the great divisions of
his system that he is thus true to the right course; in the details
he commits many and grievous offences against his own method, as we
shall see below when we come to an enumeration of his classes. Modern
writers have often attributed to Ray the merit of having first taught
the transmutation of species, and of being thus one of the founders
of the theory of descent. Let us see how much truth there is in this
assertion. Though plants, says Ray, which spring from the same seed
and produce their species again through seed, belong to the same
species, yet cases may occur in which the specific character is not
perpetual and infallible. Seeds may sometimes degenerate and produce
plants specifically distinct from the mother-plant, though this may
not often happen, and so there would be a transmutation of species, as
experience teaches. It is true that he considered the statements of
various writers, that Triticum may change into Lolium, Sisymbrium into
Mentha, Zea into Triticum, etc., to be very doubtful, yet there were,
he thought, other cases which were well ascertained; it was in evidence
in a court of law that a gardener in London had sold cauliflower seed
which had produced only common cabbage. It is to be observed, he says,
that such transmutations only occur between nearly allied species and
such as belong to the same genus, and some perhaps would not allow that
such plants are specifically distinct. These words, especially when
judged by Ray’s general views, appear only to express the opinion that
certain inconsiderable variations are possible within a narrow circle
of affinity, especially in cultivated plants. Ray does not speak of
the appearance of new forms, but says that a known form changes into
another already existing and known form, which is the reverse of that
which the theory of descent requires.
In his development of the principles of his system, among other
errors we encounter one that leads to very important consequences in
his application of the dictum, ‘natura non facit saltus,’ which he
interprets as though all affinities must present themselves in a series
that would be represented by a straight line,—an error which has misled
systematists even in recent times, and was first recognised as an
error by Pyrame de Candolle. Ray overlooked the fact that the dictum
holds good even when the affinities arrange themselves in the form of
branching series, that is, after the manner of a genealogical tree.
Much more sound is his remark, that the framing of the true system had
previously been impossible, because the differences and agreements of
forms were not sufficiently known; and another saying of his, that
nature refuses to be forced into the fetters of a precise system, shows
the dawn of the knowledge which afterwards led in Linnaeus to a strict
separation of the natural and artificial systems.
It excites no small astonishment after all Ray’s judicious and
clear-sighted utterances on the nature and method of the natural system
to find him adopting the division into woody plants and herbs; nor is
the matter improved by his making the distinctive mark of trees and
shrubs to be the forming of buds, that is, distinct winter buds, which
is a mistake into the bargain. Yet we feel ourselves in some degree
compensated for this serious error by his dividing trees and herbs into
those with a two-leaved and those with a one-leaved or leafless embryo,
in modern language into Dicotyledons and Monocotyledons. Ray’s system
is undoubtedly the one which in the time preceding Linnaeus does most
justice to natural affinities. The following synopsis of his Classes
will serve to show the progress made since Cesalpino. The names in
brackets are the Linnaean names for some of the genera in particular
classes.
A. PLANTAE GEMMIS CARENTES (HERBAE).
(a) _Imperfectae._
I. Plantae submarinae (chiefly Polypes, Fucus).
II. Fungi.
III. Musci (Confervae, Mosses, Lycopods).
IV. Capillares (Ferns, Lemna, Equisetum).
(b) _Perfectae._
Dicotyledones (binis cotyledonibus).
V. Apetalae.
VI. Planipetalae lactescentes.
VII. Discoideae semine papposo.
VIII. Corymbiferae.
IX. Capitalae (vi-ix are Compositae).
X. Semine nudo solitario (Valerianeae, Mirabilis, Thesium, etc.).
XI. Umbelliferae.
XII. Stellatae.
XIII. Asperifoliae.
XIV. Verticillatae (Labiatae).
XV. Semine nudo polyspermo (Ranunculus, Rosa, Alisma!).
XVI. Pomiferae (Cucurbitaceae).
XVII. Bacciferae (Rubus, Smilax, Bryonia, Solanum, Menyanthes).
XVIII. Multisiliquae (Sedum, Helleboreae, Butomus, Asclepias).
XIX. Vasculiferae monopetalae (various).
XX. Vasculiferae dipetalae (various).
XXI. Tetrapetalae siliquosae (Cruciferae, Ruta, Monotropa).
XXII. Leguminosae.
XXIII. Pentapetalae vasculiferae enangiospermae (various).
Monocotyledones (singulis aut nullis cotyledonibus).
XXIV. Graminifoliae floriferae vasculo tricapsulari (Liliaceae,
Orchideae, Zingiberaceae).
XXV. Stamineae (Grasses).
XXVI. Anomalae incertae sedis.
B. PLANTAE GEMMIFERAE (ARBORES).
(a) _Monocotyledones._
XXVII. Arbores arundinaceae (Palms, Dracaena).
(b) _Dicotyledones._
XXVIII. Arbores fructu a flore remoto seu apetalae (Coniferae and
various others).
XXIX. Arbores fructu umbilicato (various).
XXX. Arbores fructu non umbilicato (various).
XXXI. Arbores fructu sicco (various).
XXXII. Arbores siliquosae (woody Papilionaceae).
XXXIII. Arbores anomalae (Ficus).
Of these classes only the Fungi, Capillares, Stellatae, Labiatae,
Pomiferae, Tetrapetalae, Siliquosae, Leguminosae, Floriferae, and
Stamineae can pass as wholly or approximately natural groups, and
there are mistakes even in these; moreover the majority of them had
long been recognised. The examples annexed in brackets show how open
the others are to objection. If it must be allowed on the one side
that Ray, like Jung, doubts whether the Cryptogams are propagated
without seeds, it is on the other side obvious that he makes as little
objection as his predecessors, contemporaries, and immediate successors
to the idea that Polypes and Sponges are vegetables. But worse than
this is the extremely faulty subordination and coordination in his
system; while the class of Mosses contains the Confervae, Lichens,
Liverworts, Mosses, and Clubmosses, and therefore objects as distinct
from one another as Infusoria, Worms, Crabs, and Mollusks, we find on
the contrary the one family of Compositae split up into four classes
founded on quite petty and unimportant differences. Finally, if Ray
recognised the general importance to the system of the leaf-formation
in the embryo, he was still far from strictly separating all
Monocotyledons and Dicotyledons.
Ray’s chief merit is that he to some extent recognised natural
affinities in their broader features; the systematic separation of the
smaller groups was but little advanced by him. He too, like Morison,
found two adherents in Germany in the persons of Christopher Knaut
(1638-1694), who published a flora of Halle in 1687 arranged after
Ray’s method, and Christian Schellhammer (1649-1716), professor at
Helmstädt and afterwards at Jena.
AUGUSTUS QUIRINUS BACHMANN (RIVINUS)[24] (1652-1725) was for Germany
what Morison and Ray were for England, and Tournefort for France.
From the year 1691 he was Professor of botany, physiology, materia
medica, and chemistry in Leipsic; he applied himself with such ardour
to astronomy that he injured his eyesight by observing spots in the
sun. With such a variety of occupations it is not surprising that his
special knowledge of plants was inconsiderable when compared with
that of the three just named; but he was better able than they to
appreciate the principles of morphology laid down by Jung, and to use
them for deciding questions of systematic botany. He did most service
by his severe strictures on the more prominent errors which botanists
up to his time had persisted in, his own positive contributions, at
least as far as the recognition of affinities is concerned, being
inconsiderable. His ‘Introductio universalis in rem herbariam,’ which
appeared in 1690, and contains 39 pages of the largest size, is the
most interesting for us; in it he declines the great quantity of
unnecessary work with which botanists occupied themselves, and declares
the scientific study of plants to be the only end and aim of botany.
He first treats of naming, and lays down with respect to generic and
specific names the principles which Linnaeus afterwards consistently
applied, whereas Bachmann himself did not follow his own precepts,
but injured his reputation as a botanist by a tasteless nomenclature.
Nevertheless he declared distinctly that the best plan is to designate
each plant by two words, one of which should be the name of the genus,
the other that of the species, and he ingeniously pointed out the great
convenience of this binary nomenclature in dealing with medicinal
plants, and in the writing of prescriptions. He refused to regard
cultivated varieties as species, though Tournefort and others continued
to do so.
In his system he rejected the division into trees, shrubs, and herbs,
showing by good examples that there is no real distinction of the kind
in nature. From many of his remarks in his critical dissertations
we might infer that he possessed a very fine feeling for natural
relationship, but at the same time expressions occur which seem to
show that he did not at all appreciate its importance in the system;
we notice this in Tournefort also. Because flowers come before the
fruit he jumps with curious logic to the conclusion that the main
divisions in the system should be derived from the flower, and in
following this rule he makes use of exactly that mark in the corolla
which has the least value for classification, namely, regularity or
irregularity of form. It is strange, moreover, that Bachmann, who spent
a considerable fortune on the production of copper-plate figures of
plants without any special object, though he founded his system on
the form of the flower, should yet have devoted only a superficial
study to its construction; his account of it is very inferior to that
of any one before or since his time. His classification thus founded
cannot be said to be an advance in systematic botany; nevertheless, he
had no lack of adherents, and among them in Germany, Heucher, Knaut,
Ruppius, Hebenstreit, and Ludwig; in England, Hill and others, who made
alterations here and there in his system, but any real development of
it was from its nature an impossibility; he endeavoured to defend it
against the assaults of Ray and Dillen; Rudbeck also declared against
him.
JOSEPH PITTON DE TOURNEFORT[25] (1656-1708) founded his system also
on the form of the corolla, but his views are to some extent opposed
to those of Bachmann. While the latter was pre-eminently critical and
deficient in knowledge of species, Tournefort was more inclined to
dogmatise, and atoned in the eyes of his contemporaries for want of
morphological insight by his extensive acquaintance with individual
plants. He is commonly regarded as the founder of genera in the
vegetable kingdom; but it has been already shown that the conceptions
of genera and species had been framed as early as the 16th century
from the describing of plants, and that Kaspar Bauhin also, in naming
his plants, consistently distinguished genera and species; moreover
Bachmann in 1690 had supported the claims of the binary nomenclature
as the most suitable for the designation of plants, though he did
not himself adopt it; Tournefort did adopt it, but in an entirely
different way from that of Bauhin. Bauhin gave only the name of the
genus, and supplied the species with characters; Tournefort, on the
other hand, provided his genera with names and characters, and added
the species and varieties without special description. Tournefort
therefore was not the first who established genera; he merely
transferred the centre of gravity, so to speak, in descriptive botany
to the definition of the genera; but in doing so he committed the great
fault of treating specific differences within the genus as a matter
of secondary importance. How little depth there was in his botanical
ideas may be seen not only from his very poor theory of the flower,
the imperfections in which, as in the case of Bachmann, are the more
remarkable, since he founded his system on the outward form of the
flower, but still more from the expression which he uses at the end of
his history of botany, a work otherwise of considerable merit; he says
there that the science of botany has been so far advanced since the
age of Hippocrates, that hardly anything is still wanting except an
exact establishing of genera. His general propositions on the subject
of systematic botany, together with much that is good, but which is
generally not new and is better expressed in the works of Morison, Ray,
and Bachmann, contain strange misconceptions; for instance, he classes
plants which have no flower and fruit with those in which these parts
are to be seen only with the microscope, that is, the smallness of
the organs is equivalent to their absence. It may seem strange that
his theory of the flower should be so imperfect, when the excellent
investigations of Malpighi and Grew into the structure of flowers,
fruit, and seed were already before the world (1700), and Rudolph Jacob
Camerarius had made known his discovery of sexuality in the vegetable
kingdom. This doctrine, however, Tournefort expressly refused to admit.
But the reproach of neglecting the labours of Malpighi and Grew is
equally applicable to Bachmann and the systematists up to A. L. de
Jussieu; we have here only the first example of the fact since so often
confirmed, that professed systematists shrank with a certain timidity
from the results of more delicate morphological research, and rested
their classifications as far as possible on obvious external features
in plants,—a proceeding which more than anything else delayed the
construction of the natural system.
Tournefort’s system is thoroughly artificial, if possible, more
artificial than that of Bachmann, and certainly inferior to Ray’s.
If we meet with single groups that are really natural, it is simply
because in some families the genera so agree together in all their
marks, that they necessarily remain united, whatever mark we select for
the systematic purpose. We do not find in Tournefort the distinction
between Phanerogams and Cryptogams already established by Ray, nor
the division of woody plants and herbs into Monocotyledons and
Dicotyledons; if his chief work, to which we confine ourselves here,
the ‘Institutiones rei herbariae,’ did not bear the date of 1700, we
might conclude that it was written before the ‘Historia Plantarum’
of Ray, and the chief work of Bachmann. Yet it has one merit of a
purely formal kind; it is pervaded by a rigorous spirit of system;
every class is divided into sections, these into genera, and these
again into species; figures of the leaves and of the parts of the
flower, very beautifully engraved on copper-plate and filling a whole
volume, are perspicuously arranged; the whole work therefore is easy
to consult and understand. But to form an idea of the confusion as
regards natural affinities that reigns in his system, we need only
examine the first three sections of his first class, when we shall find
Atropa and Mandragora together in the first section, Polygonatum and
Ruscus in the second, Cerinthe, Gentiana, Soldanella, Euphorbia, and
Oxalis in the third. The handiness of the book, the little interest
taken by most of the botanists of the time in the question of natural
relationship, and the continually increasing eagerness for a knowledge
of individual plants, are evidently the reasons why Tournefort gained
over to his side most of the botanists not only of France, but also
of England, Italy, and Germany; and why later attempts in systematic
botany during the first thirty or forty years of the 18th century were
almost exclusively founded on his system, as they were afterwards on
the sexual system of Linnaeus. Boerhaave, among others, proposed a
system in 1710, which may be regarded as a combination of those of
Ray, Hermann, and Tournefort, but it met with no support on any other
grounds.
We here take our leave of the systematists of the 17th century, and,
passing over the mere plant-collectors of the first thirty years of the
18th, turn at once to Linnaeus.
CARL LINNAEUS[26], called Carl von Linné after 1757, was born in 1707
at Rashult in Sweden, where his father was preacher. He began the study
of theology, but was soon drawn away from it by his preference for
botany, and in this pursuit he was encouraged by Dr. Rothmann, who sent
him to the works of Tournefort. In Lund, where he now studied medicine,
he became acquainted with Vaillant’s treatise, ‘De sexu plantarum,’ and
had his attention drawn by it to the sexual organs. In 1730, when he
was only twenty-three years old, the aged Professor Rudbeck gave up to
him his botanical lectures and the management of the botanic gardens,
and here Linnaeus began the composition of the ‘Bibliotheca Botanica,’
the ‘Classes Plantarum,’ and the ‘Genera Plantarum.’ In the year 1732
he made a botanical journey to Lapland, and in 1734 to Dalecarlia; in
1735 he went to Holland, where he obtained a degree; in that country
he remained three years, and printed the works above-named, together
with the ‘Systema Naturae,’ the ‘Fundamenta Botanica,’ and other
treatises. From Holland he visited England and France. In the year 1738
he returned to Stockholm and was compelled to gain a livelihood as a
physician, till in 1741 he became Professor of Botany in Upsala, where
he died in the year 1778.
Linnaeus is commonly regarded as the reformer of the natural sciences
which are distinguished by the term descriptive, and it is usual to say
that a new epoch in the history of our science begins with him, as a
new astronomy began with Copernicus, and new physics with Galileo. This
conception of Linnaeus’ historical position, as far at least as his
chief subject, botany, is concerned, can only be entertained by one who
is not acquainted with the works of Cesalpino, Jung, Ray, and Bachmann,
or who disregards the numerous quotations from them in Linnaeus’
theoretical writings. On the contrary, Linnaeus is pre-eminently the
last link in the chain of development represented by the above-named
writers; the field of view and the ideas of Linnaeus are substantially
the same as theirs; he shares with them in the fundamental errors of
the time, and indeed essentially contributed to transmit them to the
19th century. But to maintain that Linnaeus marks not the beginning
of a new epoch, but the conclusion of an old one, does not at all
imply that his labours had no influence upon the time that followed
him. Linnaeus stands in the same relation to the systematists of the
period we are considering that Kaspar Bauhin does to the botanists
of the 16th century; as Bauhin gathered up all that was serviceable
in his predecessors, Cesalpino only excepted, while the botanists of
our second period drew again from him, though they set out from other
points of view than his; so Linnaeus adopted all that the systematists
of the 17th century had built upon the foundation of Cesalpino’s ideas,
gave it unity and fashioned it into a system without introducing into
it anything that was fundamentally and essentially new; all that had
been developed in systematic botany from Cesalpino to Tournefort
culminated in him, and the results, which he put together in a very
original form and with the power of a master, were no more unfruitful
for the further development of botany than the contents of Kaspar
Bauhin’s works for the successors of Cesalpino.
Whoever carefully compares the works of Cesalpino, Jung, Morison, Ray,
Bachmann, and Tournefort with Linnaeus, ‘Fundamenta Botanica’ (1736),
his ‘Classes Plantarum’ (1738), and his ‘Philosophia Botanica’ (1751),
must be thoroughly convinced that the ideas on which his theories
are based are to be found scattered up and down in the works of his
predecessors; further, whoever has traced the history of the sexual
theory from the time of Camerarius (1694), must allow that Linnaeus
added nothing new to it, though he contributed essentially to its
recognition, and that even after Koelreuter’s labours he continued to
entertain some highly obscure and even mystical notions on the subject.
But that which gave Linnaeus so overwhelming an importance for his own
time was the skilful way in which he gathered up all that had been done
before him; this fusing together of the scattered acquisitions of the
past is the great and characteristic merit of Linnaeus.
Cesalpino was the first who introduced Aristotelian modes of thought
into botany; his system was intended to be a natural one, but it was
in reality extremely unnatural; Linnaeus, in whose works the profound
impression which he had received from Cesalpino is everywhere to be
traced, retained all that was important in his predecessor’s views, but
perceived at the same time what no one before him had perceived, that
the method pursued by Cesalpino, Morison, Ray, Tournefort, and Bachmann
could never do justice to those natural affinities which it was their
object to discover, and that in this way only an artificial though very
serviceable arrangement could be attained, while the exhibition of
natural affinities must be sought by other means.
As regards the terminology of the parts of plants, which was all that
the morphology of the day attempted, Linnaeus simply adopted all that
was contained in the Isagoge of Jung, but gave it a more perspicuous
form, and advanced the theory of the flower by accepting without
hesitation the sexual importance of the stamens, which was still but
little attended to; he thus arrived at a better general conception of
the flower, and this bore fruit again in a terminology which is as
clear as it is convenient; the terms monoecious, dioecious, triandrous,
monogynous, etc., still used in the science, and the later-invented
expressions dichogamous, protandrous, protogynous, etc., owe their
origin to this correct conception of the sexual relations in plants.
But there was one great misconception in the matter, which has not a
little contributed to increase Linnaeus’ reputation. He called his
artificial system, founded on the number, union, and grouping of the
stamens and carpels, the sexual system of plants, because he rested
its supposed superiority on the fact, that it was founded upon organs
the function of which lays claim to the very highest importance. But
it is obvious that the sexual system of Linnaeus would have the same
value for the purposes of classification, if the stamens had nothing
whatever to do with propagation, or if their sexual significance were
quite unknown. For it is exactly those characters of the stamens which
Linnaeus employs for purposes of classification, their number and mode
of union, which are matter of entire indifference as regards the sexual
function.
But though the notion that this artificial system has any important
connection with the doctrine of the sexuality of plants is evidently
due to a confusion of ideas, yet the progress of the science has shown,
that Linnaeus’ sexual system did often and necessarily lead to the
establishing of natural groups for the very reason, that the characters
of the stamens which he employed are entirely independent of their
function; for we must regard it as an important result of the labours
of systematists, that those characters of organisms are shown to be of
the greatest value for classification, which are entirely or in a very
great measure independent of the functions of the organs. The error,
which led Cesalpino to make the functional importance of the parts of
fructification the principle of his division, reappears therefore in
Linnaeus in another form; to find a principle of division, he turns to
those organs, whose function appears to him the most important, but he
takes his characters not from differences of function, but from the
number and mode of union, which are of no importance for the sexual
function. We meet with this error in Leibnitz and Burckhard, who are
mentioned here merely to defend Linnaeus from the charge repeatedly
brought against him by his contemporaries that he was indebted to these
two writers for the idea of his sexual system. They erroneously found
in the great physiological importance of the sexual organs a reason
for deriving from their differences the principles of division that
were to found a system; this error in theory Linnaeus shared with them,
but they did not correct it in practice, as Linnaeus did, by confining
himself to purely morphological features in working out his system.
What the renowned philosopher[27] incidentally uttered in the year 1701
on the matter in question is moreover so unimportant and so indistinct,
that Linnaeus could not gain much from it; what Burckhard[28] says
on the subject in his often-quoted letter to Leibnitz (1702) is
indeed much better, and comes near to Linnaeus’ idea; but it is a
very long way from the hints there given to the completion of the
well-articulated and highly practical system which Linnaeus constructed.
The botanists of the 16th century, and in the main even Morison
and Ray, had in one-sided fashion devoted their chief attention to
distinguishing species, Bachmann and Tournefort to the establishment
of generic characters, while they neglected species; Linnaeus, on the
contrary, applied equal care and much greater skill to describing both
genera and species. He reduced to practical shape the suggestion which
Bachmann had left to his successors, and so must be regarded, if not as
the inventor, at least as the real founder of the binary nomenclature
of organisms.
It is only fulfilling the duty of a historian to state the sources
from which Linnaeus drew, but it would be a misapprehension to see
in this any depreciation of a great man; it were to be desired that
all naturalists would, like Linnaeus, adopt all that is good in the
contributions of their predecessors, and improve or adapt it as
he did. Linnaeus himself has repeatedly quoted the sources of his
knowledge as far as they were known to him, and has in many cases
estimated the services of his predecessors with a candour which never
betrays a trace of jealousy, but often displays a warm respect, as
may be seen especially in the short introductions to the several
systems given in the ‘Classes Plantarum.’ Linnaeus could not only
recognise what was good in his predecessors and occasionally make
use of it, but he imparted life and fruitfulness to the thoughts of
others by applying them as he applied his own thoughts, and bringing
out whatever theoretical value they possessed. It was evidently this
freshness of life that often misled his successors into believing that
Linnaeus thought out and discovered everything for himself. We learn
to appreciate the contributions of Cesalpino and his successors in the
17th century, and even of Kaspar Bauhin for the first time in the works
of Linnaeus; we are astonished to see the long-known thoughts of these
writers, which in their own place look unimportant and incomplete,
fashioned by Linnaeus into a living whole; thus he was at once and in
the best sense both receptive and productive, and he might perhaps have
done more for the theory of the science if he had not been entangled
in one grave error, which was more sharply pronounced in him than in
his predecessors and contemporaries, that, namely, of supposing that
the highest and only worthy task of a botanist is to know all species
of the vegetable kingdom exactly by name. Linnaeus distinctly declared
that this was his view, and his school in Germany and England adhered
to it so firmly that it established itself with the general public, who
to the present day consider it as a self-evident proposition that a
botanist exists essentially for the purpose of at once designating any
and every plant by a name. Like his predecessors, Linnaeus regarded
morphology and general theoretical botany only as means to be used for
discovering the principles of terminology and definition, with a view
to the improvement of the art of describing plants.
We have hitherto spoken chiefly of the manner in which Linnaeus dealt
with his subject in matters of detail; in his inner nature he was a
schoolman, and that in a higher degree than even Cesalpino himself,
who should rather be called an Aristotelian in the strict sense of
the word. But to say that Linnaeus’ mode of thought is thoroughly
scholastic is virtually saying that he was not an investigator of
nature in the modern meaning of the word; we might point to the fact
that Linnaeus never made a single important discovery throwing light on
the nature of the vegetable world; but that would still not prove that
he was a schoolman.
True investigation of nature consists not only in deducing rules from
exact and comparative observation of the phenomena of nature, but
in discovering the genetic forces from which the causal connexion,
cause and effect may be derived. In the pursuit of these objects,
it is compelled to be constantly correcting existing conceptions
and theories, producing new conceptions and new theories, and thus
adjusting our own ideas more and more to the nature of things. The
understanding does not prescribe to the objects, but the objects to
the understanding. The Aristotelian philosophy and its medieval form,
scholasticism, proceeds in exactly the contrary way; it is not properly
concerned with acquiring new conceptions and new theories by means of
investigation, for conceptions and theories have been once for all
established; experience must conform itself to the ready-made system of
thought; whatever does not so conform must be dialectically twisted and
explained till it apparently fits in with the whole. From this point
of view the intellectual task consists essentially in this twisting
and turning of facts, for the general idea of the whole is already
made and needs not to be altered. Experience in the higher sense of
investigation of nature is rendered impossible by the fact, that we
are supposed to know all the ultimate principles of things; but these
ultimate principles of scholasticism are at bottom only words with
extremely indefinite meaning, abstractions obtained by a series of
jumps from every-day experience, which has not been tried and refined
in the crucible of science, and is therefore worthless; and the higher
the abstraction is raised, the farther it withdraws from the guiding
hand of experience, the more venerable and more important do these
‘abstracta’ appear, and we can finally come to a mutual understanding
about them, though again only through figures and metaphors[29].
Science, according to the scholastic method, is a playing with abstract
conceptions; the best player is he who can so combine them together,
that the real contradictions are skilfully concealed. On the contrary,
the object of true investigation, whether in philosophy or in natural
science, is to make unsparing discovery of existing contradictions and
to question the facts until our conceptions are cleared up, and if
necessary the whole theory and general view is replaced by a better.
In the Aristotelian philosophy and in scholasticism facts are merely
examples for the illustration of fixed abstract conceptions, but in the
real investigation of nature they are the fruitful soil from which new
conceptions, new combinations of thought, new theories, and general
views spring and grow. The most pernicious feature in scholasticism
and the Aristotelian philosophy is the confounding of mere conceptions
and words with the objective reality of the things denoted by them;
men took a special pleasure in deducing the nature of things from the
original meaning of the words, and even the question of the existence
or non-existence of a thing was answered from the idea of it. This
way of thinking is found everywhere in Linnaeus, not only where he
is busy as systematist and describer, but where he wishes to give
information on the nature of plants and the phenomena of their life,
as in his ‘Fundamenta,’ his ‘Philosophia Botanica,’ and especially in
his ‘Amoenitates Academicae.’ From among many instances we may select
his mode of proving sexuality in plants. Linnaeus knew and lauded the
services rendered to botany by Rudolph Jacob Camerarius, who as a
genuine investigator of nature had demonstrated the sexuality of plants
in the only possible way, namely, that of experiment. But Linnaeus
cares little for this experimental proof; he just notices it in
passing, and expends all his art on a genuine scholastic demonstration
intended to prove the existence of sexuality as arising necessarily
from the nature of the plant. He connects his demonstration with the
dictum ‘omne vivum ex ovo,’ which Harvey had founded on an imperfect
induction, and which he evidently takes for an _a priori_ principle,
and concludes from it that plants also must proceed from an ‘ovum,’
overlooking the fact that in ‘omne vivum ex ovo’ plants already form
a half of the ‘omne vivum’; then he continues, ‘reason and experience
teach us that plants proceed from an ‘ovum,’ and the cotyledons
confirm it’; reason, experience, and cotyledons! Surely a remarkable
assemblage of proofs. In the next sentence he confines himself at first
to the cotyledons, which according to him spring in animals from the
yolk of the egg, in which the life-point is found; consequently, he
says, the seed-leaves of plants, which envelope the ‘corculum,’ are
the same thing; but that the progeny is formed not simply from the
‘ovum,’ nor from the fertilising matter in the male organs, but from
the two combined, is shown by animals, hybrids, reason, and anatomy. By
reason in this and the previous sentence he understands the necessity,
concluded from the nature, that is, the conception of the thing, that
it must be so; animals supply him with the analogy, and anatomy can
prove nothing, as long as it is not known what is the design of the
anatomical arrangements. But the weakest side of this proof lies in the
hybrids, for Linnaeus, when he wrote the ‘Fundamenta,’ knew of none
except the mule; hybrids in plants were first described by Koelreuter
in 1761, and these Linnaeus nowhere mentions; and what amount of proof
can be drawn from the vegetable hybrids, which Linnaeus afterwards
supposed himself to have observed, but which were no hybrids, we
shall see in the history of the sexual theory; here we need only
remark that he arrives at the existence of these hybrids from the
idea of sexuality exactly as he arrived at that of sexuality from the
idea of hybridisation. Then he goes on with his demonstration; ‘that
an egg germinates without fecundation is denied by experience, and
this must hold good therefore of the eggs[30] of plants—every plant
is provided with flower and fruit, even where these are not visible
to the eye’; with Linnaeus, of course, this is logically concluded
from the conception of the plant or of the ‘ovum’; he alleges indeed
certain observations as well, but they are incorrect. He continues,
‘The fructification consists of the sexual organs of the flowers; that
the anthers are the male organs, the pollen the fertilising matter, is
proved by their nature, further by the fact that the flower precedes
the fruit, as also by their position, the time, the loculaments
(anthers), by castration, and by the structure of the pollen.’ Here too
the main point with Linnaeus is the nature of the male organs, and that
we may know what this nature is he refers to a former paragraph, where
we learn that the essence of the flower is in the anthers and stigma.
Almost all his demonstrations consist of such reasonings in a circle
and in arguing from the thing to be proved. And while the passages
quoted show how much he did for the doctrine of sexuality, we find
this sophistical style of reasoning still more copiously displayed in
the essay entitled ‘Sponsalia Plantarum’ in the ‘Amoenitates’ (i. p.
77), and in a worse form still in the essay, ‘Plantae Hybridae’ (Amoen.
iii. p. 29). That Linnaeus had not the remotest conception of the way
in which the truth of a hypothetical fact is proved on the principles
of strict inductive investigation is shown by these and many other
examples, and by his enquiry into the seeds of mosses (Amoen. ii. p.
266), upon which he prided himself not a little, but which is really
inconceivably bad even for that time (1750). It was not Linnaeus’ habit
to occupy himself with what we should call an enquiry; whatever escaped
the first critical glance he left quietly alone; it did not occur to
him to examine into the causes of the phenomena that interested him; he
classified them and had done with them; as for instance in his ‘Somnus
Plantarum,’ as he called the periodical movements of plants. We cannot
read much of the ‘Philosophia Botanica’ or the ‘Amoenitates’ without
feeling that we are transported into the literature of the middle ages
by the kind of scholastic sophistry which is all that his argumentation
amounts to; and yet these works of Linnaeus date from the middle of
the last century, from a time when Malpighi, Grew, Camerarius, and
Hales had already carried out their model investigations, and his
contemporaries Duhamel, Koelreuter, and others were experimenting in
true scientific manner. This peculiarity in Linnaeus explains why
men like Buffon, Albert Haller, and Koelreuter treated him with a
certain contempt; and also why his strict adherents in Germany, who
lived on his writings and were unable to separate what was really
good in him from his mode of reasoning, came to make their own botany
like anything rather than a science of nature. Linnaeus was in fact a
dangerous guide for weak minds, for his curious logic, among the worst
to be met with in the scholastic writers, was combined with the most
brilliant powers of description; the enormous extent of his knowledge
of particulars, and above all the pre-eminent firmness and certainty
which distinguished his mode of dealing with systematic botany, could
not fail to make the profoundest impression on those who judged of
the powers of an investigator of nature by these qualities alone. One
of his greatest gifts was without doubt the power which he possessed
of framing precise and striking descriptions of species and genera in
the animal and vegetable kingdoms by means of a few marks contained in
the smallest possible number of words; in this point he was a model of
unrivalled excellence to all succeeding botanists.
On the whole the superiority of Linnaeus lay in his natural gift for
discriminating and classifying the objects which engaged his attention;
he might almost be said to have been a classifying, co-ordinating,
and subordinating machine. He dealt with everything about which he
wrote in the way in which he dealt with objects of natural history.
The systematic botanists whom he mentions in the ‘Classes Plantarum’
are classified then and there as fructists, corollists, and calycists.
All who occupy themselves in any way with botany are divided into two
great classes, the true botanists and mere botanophils, and it is
very characteristic of his way of thinking that he places anatomists,
gardeners, and physicians in the latter class. True botanists again
are either mere collectors or systematists. To the collectors
belong all who add to the number of known plants, also authors
of monographs and floras, and the botanical explorers of foreign
countries, whom we should now more courteously call systematists. By
systematists Linnaeus understands those who occupy themselves with
the classification and naming of plants, and he divides them into
philosophers, systematists proper, and nomenclators; the philosophers
are those who study the theory of the science on principles founded on
reason and observation, and are subdivided into orators, institutors,
erystics, and physiologists; the latter are those who discovered the
mystery of sexuality in plants, and hence Malpighi, Hales, and such
men are not physiologists in Linnaeus’ sense. The second class of
systematists, the systematists proper, he distinguishes into orthodox
and heterodox, the former taking the grounds of division exclusively
from the organs of fructification, while the latter use other marks
as well. In this manner Linnaeus treats every subject of which he has
to speak, and wherever he can in short, numbered sentences, which
look like descriptions of genera and species. His mind and character
were fully formed in 1736 when he wrote his ‘Fundamenta,’ and he
preserved his peculiarities of style from that time forward; we find
the same modes of expression in the ‘Nemesis Divina,’ a treatise on
religion and morals addressed as a legacy to his son. Where these
peculiarities of manner and expression are suitable they make a
favourable impression on the reader, as for instance in the short
accounts he gives of the various systems in the ‘Classes Plantarum,’
a work in which Linnaeus was quite in his element; there he traces
with a fine instinct the guiding principles of each system, pronounces
upon its merits and defects, and sets it before the reader in numbered
sentences of epigrammatic brevity. This manner is strictly adhered to
in the ‘Philosophia’ also, and it has certainly helped not a little
to withdraw the attention of his reader from his many fallacies in
argument, especially his oft-recurring reasonings in a circle.
This remarkable combination of an unscientific philosophy with mastery
over the classification of things and conceptions, this mixture of
consistency in carrying out his scholastic principles with gross
inaccuracies of thought, impart to his style an originality, which is
rendered still more striking by the native freshness and directness,
and not unfrequently by the poetic feeling, which animate his periods.
In any attempt to estimate the advance which the science owes to the
labours of Linnaeus, the chief prominence must be assigned to two
points; first to his success in carrying out the binary nomenclature in
connection with the careful and methodical study which he bestowed on
the distinguishing of genera and species; this system of nomenclature
he endeavoured to extend to the whole of the then known vegetable
world, and thus descriptive botany in its narrower sense assumed
through his instrumentality an entirely new form, which, serving as
a model for the naming and defining of the larger groups, could be
applied without modification to the founding and completing the natural
system. When at a later time Jussieu and De Candolle marked out their
families and groups of families, their mode of proceeding was in the
main that of Linnaeus when distinguishing his genera by abstraction of
specific differences. This merit has been always assigned to Linnaeus
without reserve. The second merit has been less recognised, and yet it
is at least of equal importance; it is that of having first perceived
that the attempt made by Cesalpino and his successors to found a
system, that shall do justice to natural affinities, on predetermined
marks can never succeed. Linnaeus framed his artificial sexual system,
but he exhibited a fragment of a natural system by its side, while he
repeatedly declared that the chief task of botanists is to discover
the natural system. Thus he cleared the ground for systematic botany.
He made use of his own system, because it was extremely convenient for
describing individual plants, but he ascribed all true scientific value
exclusively to the natural system; and with what success he laboured
to advance it may be gathered from the fact, that Bernard de Jussieu
founded his improved series of families on the fragment of Linnaeus,
and that his nephew, A. L. de Jussieu, by simply adopting Linnaeus’
conception of the principle which lies at the foundation of the natural
system, succeeded in carrying it on to a further stage of development.
The main features of Linnaeus’ theoretical botany can best be learned
from the ‘Philosophia Botanica,’ which may be regarded as a text-book
of that which Linnaeus called botany, and which far surpasses all
earlier compositions of the kind in perspicuity and precision, and
in copiousness of material; and indeed it would be difficult to find
in the ninety years after 1781 a text-book of botany which treats
what was known on the subject at each period with equal clearness
and completeness. In giving the reader some idea of the way in which
Linnaeus deals with his subject, it will be well to pass over the
first two chapters, which discuss the literature and the various
systems which had been proposed, and turn to the third, which under
the heading ‘Plantae’ treats of the general nature of plants, and
specially of the organs of vegetation. The vegetable world, says
Linnaeus, comprises seven families, Fungi, Algae, Mosses, Ferns,
Grasses, Palms, and Plants. All are composed of three kinds of vessels,
sap-vessels which convey the fluids, tubes which store up the sap
in their cavities, and tracheae which take in air; these statements
Linnaeus adopts from Malpighi and Grew. He gives no characteristic
marks for the Fungi; of the Algae he says that in them root, leaf,
and stem are all fused together; to the Mosses he ascribes an anther
without a filament, and separate from the female flower which has no
pistil; the seeds of the Mosses have no integument or cotyledons;
this characteristic of the Mosses is explained in his paper entitled
‘Semina Muscorum’ in the ‘Amoenitates Academicae,’ ii. The Ferns are
marked by the fructification on the under side of the fronds, which
are therefore not conceived of as leaves. The very simple leaves, the
jointed stalk, the ‘calyx glumosus,’ and the single seed mark the
Grasses. The simple stem, the rosette of leaves at the summit, and
the spathe of the inflorescence are characteristic of the Palms. All
vegetable forms which do not belong to any of the previous families he
names Plants. He rejects the customary division into herbs, shrubs,
and trees as unscientific. This arrangement of the vegetable kingdom
must not be confounded with Linnaeus’ fragment of a natural system,
in which he adopts sixty-seven families (orders), the Fungi, Algae,
Mosses, and Ferns forming each a family. He evidently introduces the
divisions in the ‘Philosophia,’ in order that it may be seen how far
the statements that follow are applicable to all the Vegetabilia or
only to certain sections of them. The parts in the individual plant
which the beginner must distinguish are three; the root, the herb[31],
and the parts of fructification, in which enumeration Linnaeus departs
from his predecessors, by whom the fructification and the herb together
are opposed to the root. In the central part of the plant is the
pith, enclosed by the wood which is formed from the bast; the bast
is distinct from the rind, which again is covered by the epidermis;
these anatomical facts are from Malpighi; the statement that the pith
grows by extending itself and its envelopes is borrowed from Mariotte.
Cesalpino’s view on the formation of the bud is expressed by Linnaeus
in the statement, that the end of a thread of the pith passing through
the rind is resolved into a bud, etc. The bud is a compressed stem,
capable of unlimited extension till fructification puts a term to
vegetation. The fructification is formed by the leaves uniting into a
calyx, from which the apex of a branch issues as a flower about one
year in advance, while the fruit arising from the substance of the
pith cannot begin a new life till the woody substance of the stamens
has been absorbed by the fluids of the pistil. In this way Linnaeus
corrected Cesalpino’s theory of the flower, that he might take into
account the sexual importance of the stamens discovered by Camerarius.
He concludes by saying that there is no new creation but only a
continuous generation, for which he gives the remarkable and thoroughly
Cesalpinian reason, ‘cum corculum seminis constat parte radicis
medullari.’
The root, which takes up the food, and produces the stem and the
fructification, consists of pith, wood, bast, and rind, and is divided
into the two parts, ‘caudex’ and ‘radicula.’ The ‘caudex’ answers
pretty nearly to our primary root and rhizomes, the ‘radicula’ to what
we now call secondary roots.
The herb springs from the root, and is terminated by the
fructification; it consists of the stem, leaves, leaf-supports
(‘fulcrum’), and the organs of hibernation (‘hibernaculum’). Then
follow the further distinctions of stem and leaves; the terminology,
still partly in use and resting essentially on the definitions of
Jung, is here set forth in great detail. Linnaeus however does not
mention the remarkable distinction between stem and leaf which Jung
founded on relations of symmetry, and in general he shows less depth of
conception than Jung, confining himself more to the direct impression
on the senses, and so distinguishing sometimes where there is no real
difference. Examples of this are furnished by the paragraph devoted
to ‘fulcra.’ By this term he designates the subsidiary organs of
plants, among which he reckons stipules, bracts, spines, thorns,
tendrils, glands, and hairs. It appears from this, that Linnaeus did
not extend the idea of the leaf (‘folium’) to stipules and bracts, and
the examples he gives of tendrils show at the same time that he was
ignorant of the different morphological character of the organ in Vitis
and Pisum. The putting the seven organs above-named together under the
idea of ‘fulcrum’ shows plainly enough that Linnaeus, in framing his
terminology, aimed only at distinguishing what was different to the
sense by fixed words, in order to obtain means for short diagnoses
of species and genera. He had no thought of arriving at more general
propositions from a comparison of forms in plants, in order to attain
to a deeper insight into their nature. The same thing appears from his
notion of ‘hibernaculum,’ by which he understands a part of the plant
which envelopes the stem in its embryonal state and protects it from
harm from without; he here distinguishes bulbs from the winter buds of
woody plants. In this course of mixing up morphological and biological
relations of organs he was followed by botanists till late into our own
century.
Linnaeus goes far beyond his predecessors in distinguishing and naming
the organs of fructification, the subject of the fourth chapter of the
‘Philosophia Botanica.’ The fructification, he says, is a temporary
part in plants devoted to propagation, terminating the old and
beginning the new. He distinguishes the following seven parts: (1) the
calyx, which represents the rind, including in this term the involucre
of the Umbelliferae, the spathe, the calyptra of Mosses, and even the
volva of certain Fungi,—another instance of the way in which Linnaeus
was guided by external appearance in his terminology of the parts of
plants; (2) the corolla, which represents the inner rind (bast) of the
plant; (3) the stamen, which produces the pollen; (4) the pistil, which
is attached to the fruit and receives the pollen; here for the first
time the ovary, style, and stigma are clearly distinguished. But next
comes as a special organ (5) the pericarp, the ovary which contains
the seed. As bulbs and buds were treated not simply as young shoots,
but as separate organs, so here too the ripe fruit is regarded not
merely as the developed ovary, but as a special organ. Nevertheless,
Linnaeus distinguishes the different forms of fruit much better than
his predecessors had done. (6) The seed is a part of the plant that
falls off from it, the rudiment of a new plant, and it is excited to
active life by the pollen. The treatment of the seed and its parts is
the feeblest of all Linnaeus’ efforts; he follows Cesalpino, but his
account of the parts of the seed is much more imperfect than that of
Cesalpino and his successors. The embryo is called the ‘corculum,’ and
two parts are distinguished in it, the ‘plumula’ and the ‘rostellum’
(radicle). The cotyledon is co-ordinated with the ‘corculum,’ and is
regarded therefore not as part of the embryo but as a distinct organ of
the seed; it is defined as ‘corpus laterale seminis bibulum caducum.’
Nothing could be worse, and it seems almost incredible that so bad a
definition and distinction could be given in 1751, and again in 1770,
by the first botanist of his time, when Malpighi and Grew, nearly
a hundred years earlier, had illustrated the parts of the seed and
even the history of its development and its germination by numerous
figures. He does not mention the endosperm, evidently confounding it
with the cotyledon, though Ray had already distinguished it clearly
from the other parts of the seed. Linnaeus’ terminology of the seed
supplies more than sufficient corroboration of our previous remark,
that he shows incapacity for the careful investigation of any object
at all difficult to observe, and it will now seem a small matter that
he, like most of the earlier botanists, treats one-seeded indehiscent
fruits as seeds, and hence makes the pappus a part of the seed. (7) By
the word ‘receptaculum’ he understands everything by which the parts
of the fructification are connected together, both the ‘receptaculum
proprium,’ which unites the parts of the single flower, and the
‘receptaculum commune,’ under which term he comprises the most diverse
forms of inflorescence (umbel, cyme, spadix).
He concludes with the remark that the essence of the flower consists in
the anther and the stigma, that of the fruit in the seed, that of the
fructification in the flower and the fruit, and that of all vegetable
forms in the fructification, and he adds a long list of distinctions
between the organs of fructification with their names; among these
organs appear the nectaries, which he was the first to distinguish.
In the fifth chapter he discusses the question of difference of sex
in plants. His views on this subject have been already mentioned in
order to show that they were entirely founded on worthless scholastic
deductions; here we may quote a few of the propositions which were
famous in after times. We assume, he says, that two individuals of
different sexes were created in the beginning of things in every kind
of living creatures. Plants, though they are without sensation, yet
live as do animals, for they have a beginning and an advance in age
(aetas), and are liable to disease and death; they have also a power
of movement, a natural appetency (propulsio), an anatomy, and an
organic structure (organismus). Simple explanations are given of these
words, but they prove nothing about the matter. He then expounds the
whole theory of sexuality, which is made to rest entirely on scholastic
arguments, and in doing this he spins out to excessive length the
parallel which he draws between the conditions of sexuality in
animals and plants. It is manifestly this chapter of the ‘Philosophia
Botanica,’ together with the treatise ‘Sponsalia Plantarum,’ which led
the adherents of Linnaeus, who were ignorant of the older literature
of the subject and were much impressed by his scholastic dexterity, to
celebrate him as the founder of the sexual theory of plants; whereas
a more careful study of history shows incontrovertibly that Linnaeus
helped in this way to disseminate the doctrine, but did absolutely
nothing to establish it.
The writings of Linnaeus which we have hitherto examined are occupied
with the nature of plants, and of this he knew nothing more than he
gathered from the investigations and reflections of his predecessors;
and it is here especially that his peculiar scholasticism is exhibited
in contrast with the facts obtained by induction which he communicated
to his readers. But the strong side of his intellect appears with
splendid effect in the succeeding chapters of the ‘Philosophia,’ which
treat of the principles of systematic botany; here, where he has
no longer to establish facts, but to arrange ideas, to dispose and
summarise, we find Linnaeus thoroughly in his element.
The groundwork of botanical science, he begins, is twofold,
classification and naming. The constituting of classes, orders, and
genera he calls theoretical classification; the constituting of species
and varieties is practical classification. The work of classification
carried out by Cesalpino, Morison, Tournefort, and others leads to the
establishing of a system; the mere practice of describing species may
be carried on by those who know nothing of systematic botany. These
expressions of Linnaeus are interesting, because like other remarks of
his they show that he placed the establishment and arrangement of the
larger groups above the mere distinguishing of individual forms; his
disciples to a great extent forgot their master’s teaching, and fancied
that the collecting and distinguishing of species was systematic
botany. He opposes the system itself, which deals with the relative
conceptions of classes, orders, genera, species, and varieties, to a
mere synoptical view, serving with its dichotomy only to practical
ends. Then comes the often-quoted sentence, ‘We reckon so many species
as there were distinct forms created “in principio.”’ In a former
place he had said ‘ab initio’ instead of ‘in principio’; instead
therefore of a beginning in time he here posits an ideal, theoretical
beginning, which is more in accordance with his philosophical views.
That new species can arise is, he continues, disproved by continuous
generation and propagation, and by daily observation, and by the
cotyledons. It is hard to understand how the Linnaean school till far
into our own century could have remained firm in a doctrine resting
on such arguments as these. Linnaeus’ definition of varieties shows
that he understood by the word species fundamentally distinct forms;
there are, he says, as many varieties as there are different plants
growing from the seed of the same species; and he adds that a variety
owes its origin to an accidental cause, such as climate, soil, warmth,
the wind; but this is evidently mere arbitrary assumption. Judging by
all he says, his view is that species differ in their inner nature,
varieties only in outward form. Here, where we find the dogma of the
constancy of species for the first time expressed in precise terms,—a
dogma generally accepted till the appearance of the theory of descent,
we should be justified in demanding proof; but since dogmas as a rule
do not admit of proof, Linnaeus simply states his view[32], unless we
are to take the sentence, ‘negat generatio continuata, propagatio,
observationes quotidianae, cotyledones,’ as proving the assertion that
new species never appear. We shall see further on to what surprising
conclusions Linnaeus was himself led by his dogma, when he had to take
into account the relations of affinity in genera and larger groups. The
species and the genus, he continues, are always the work of nature,
the variety is often that of cultivation; the class and the order
depend both on nature and on art, which must mean that the larger
groups of the vegetable kingdom have not the same objective reality as
the species and the genus, but rest partly on opinion. That Linnaeus
estimated the labours of the systematists after Cesalpino and the
contributions of the German fathers of botany up to Bauhin, as they
have been judged of in the present work, is shown by paragraph 163,
where he explains the word habit, and adds that Kaspar Bauhin and the
older writers had excellently divined (divinarunt) the affinities of
plants from their habit, and even real systematists had often erred,
where the habit pointed out to them the right way. But he says that the
natural arrangement, which is the ultimate aim of botany, is founded,
as the moderns have discovered, on the fructification, though even
this will not determine all the classes. It is interesting therefore
to observe how Linnaeus further on (paragraph 168) directs, that in
forming genera, though they must rest on the fructification, yet it is
needful to attend to the habit also, lest an incorrect genus should
be established on some insignificant mark (levi de causâ): but this
attention to the habit must be managed with reserve, so as not to
disturb the scientific diagnosis.
Linnaeus next lays down with great detail each several rule, which
must be observed in establishing species, genera, orders, and
classes, and it is here that he displays his unrivalled skill as a
systematist. These rules were strictly observed by himself in his
numerous descriptive works, and thus a spirit of order and clearness
was introduced into the art of describing plants, which gave it at once
a different appearance from that which it had received at the hands of
his predecessors. Whoever therefore compares the ‘Genera Plantarum,’
the ‘Systema Naturae,’ and other descriptive works of Linnaeus with
those of Morison, Ray, Bachmann, or Tournefort, finds so great a
revolution effected by them, that he is impressed with the persuasion
that botany first became a science in the hands of Linnaeus; all
former efforts seem to be so unskilful and without order in comparison
with his method. Without doubt the greatest and most lasting service
which Linnaeus rendered both to botany and to zoology lies in the
certainty and precision which he introduced into the art of describing.
But if a reformation was thus effected in botany, as Linnaeus
himself took pleasure in saying, it must not be overlooked that the
knowledge of the nature of plants was rather hindered than advanced
by him. Ray, Bachmann, and in part also Morison and Tournefort, had
already liberated themselves to a great extent from the influence of
scholasticism, and they still give us the impression of having been
genuine investigators of nature; but Linnaeus fell back again into the
scholastic modes of thought, and these were so intimately combined with
his brilliant performances in systematic botany, that his successors
were unable to separate the one from the other.
The feeling for order and perspicuity, which made Linnaeus a reformer
of the art of describing, combined with his scholasticism, was
evidently the cause of his not bestowing more energetic labour on
the natural system. It has been repeatedly mentioned that it was
he who first established sixty-five truly natural groups in his
fragment of the early date of 1738; and a certain feeling for natural
affinity is shown in the establishment of his seven families, Fungi,
Algae, Mosses, Ferns, Grasses, Palms, and Plants properly so-called.
Moreover in paragraph 163 of the ‘Philosophia Botanica,’ he carries
out the division of the whole vegetable kingdom into Acotyledons,
Monocotyledons, and Polycotyledons with their subdivisions very
admirably; and thus we see him continually impelled towards a natural
arrangement, but never bestowing upon it the necessary labour and
thought.
And so two different conceptions of a system of plants continued to
subsist side by side with each other in the mind of Linnaeus; one more
superficial, and adapted for practical use, expressed in his artificial
sexual system, and one more profound and scientifically valuable,
embodied in his fragment and in the natural groups above-mentioned.
The same may be said also of Linnaeus’ morphological views; here,
too, a more superficial pursued its way along with a more profound
conception. He formed his terminology of the parts of plants for
practical use in describing them, and convenient as it is, it seems
nevertheless shallow or superficial, because its foundations are not
more deeply laid in the comparative study of forms. But we discover
from very various passages in his writings that he felt the need of
a more profound conception of plant-form, and what he was able to
say on the subject he put together under the head of ‘metamorphosis
plantarum.’ His doctrine of metamorphosis is entirely based on the
views of Cesalpino, with which we have already become acquainted,
though he did not adopt them in their original form, but endeavoured
to develop them in true Cesalpinian fashion; for on the one hand he
derived leaves and parts of flowers from the tissues of the stem, and
on the other conceived of the parts of the flower as only altered
leaves. This doctrine of metamorphosis appears in somewhat confused
form in the last page of his ‘Philosophia Botanica.’ There he says that
the whole of the herb is a continuation of the medullary substance of
the root; the principle of the flowers and leaves is the same, because
both spring from the tissue-layers surrounding the pith, as Cesalpino
had taught. The statement which follows, that the principle of the bud
and the leaves is identical, would be a departure from Cesalpino, and
in any case inconsistent, without the explanation that the bud consists
of rudimentary leaves; but this again puts the axial portion of the bud
out of sight. The perianth, he says, comes from concrescent rudiments
of leaves. How closely Linnaeus adhered to Cesalpino in his later years
appears in his explanation of the catkin, which comes next and which
is taken entirely from Cesalpino’s theory. That a more superficial and
a more profound conception pursue their way together unadjusted in
Linnaeus’ speculations on form is specially shown by the fact, that in
the text of the ‘Philosophia Botanica,’ paragraph 84, he places the
‘stipulae’ under the idea of ‘fulcra’ and not under that of ‘folia,’
while on the contrary at the end of the same work, where he brings
together the different paragraphs respecting metamorphosis, he speaks
of the ‘stipulae’ as appendages of the leaves.
The idea of Cesalpino, that the parts of the flower which surround the
fruit arise like the ordinary leaves from the tissues that enclose
the pith, is further developed by Linnaeus in his ‘Metamorphosis
Plantarum,’ in the fourth volume of the ‘Amoenitates Academicae’
(1759), in a very strange manner. He compares the formation of the
flower with the metamorphosis of animals, and especially of insects,
and after describing the changes that take place in animals, he says at
page 370 that plants are subject to similar change. The metamorphosis
of insects consists in the putting off different skins, so that
they finally come forth naked in their true and perfect form. This
metamorphosis we also find in most plants, for they consist, at least
in the truly living part of the root, of rind, bast, wood, and pith.
The rind is to the plant what the skin is to the larva of an insect,
and after putting this skin off there remains a naked insect. When
the flower is produced in the plant the rind opens and forms the calyx
(exactly Cesalpino’s view), and from out of this the inner parts of the
plant issue to form the flower, so that the bast, the wood, and the
pith issue forth naked in the form of corolla, stamens, and stigma. So
long as the plant lies concealed within the rind and clothed only with
leaves, it appears to us as unrecognisable and obscure as a butterfly,
which in its larva-condition is covered with skin and spines.
In this doctrine of metamorphosis, which Linnaeus founded on Cesalpino,
the chief point to observe is, that the ordinary leaves are identical
with the exterior parts of the flower, because both originate in the
outer tissues of the stem. The pertinent fact, which may easily be
observed without a microscope, that the concentric arrangement of
outer and inner rind, wood, and pith occurs only in some flowering
plants, that the case is quite different with Monocotyledons, and
that Cesalpino’s theory of the flower cannot properly be applied to
them,—these are things which we must not expect to find Linnaeus with
his peculiar modes of thought taking into consideration.
The want of firm standing-ground in experience is shown also by the
fact, that with his own and Cesalpino’s theory of the flower he
combined another view of its nature, which under the name of ‘prolepsis
plantarum’ was set forth in two dissertations in 1760 and 1763, but
the two theories are scarcely compatible with one another. While the
last paragraph in the ‘Philosophia Botanica’ says, ‘Flos ex gemmâ
annuo spatio foliis praecocior est,’ the dissertations contain the
doctrine[33], that the flower is nothing but the synchronous appearance
of leaves, which properly belong to the bud-formations of six
consecutive years, in such a way that the leaves of the bud destined to
be unfolded in the second year of the plants become bracts, the leaves
of the third year the calyx, those of the fourth the corolla, those of
the fifth the stamens, those of the sixth the pistil. Here we see once
more how Linnaeus moves in the sphere of arbitrary assumptions with no
thought of exact observation, for this whole theory of prolepsis rests
on nothing that can be called a well-ascertained fact.
Yet a third time we find in Linnaeus the juxtaposition of a superficial
view resting on every-day perception, and a more profound and to some
extent a philosophical view; this is the case where he is concerned on
the one hand with the dogma of the constancy of species, and on the
other hand has to explain the fact of natural relationship and its
gradations. Apart from some insignificant verbal explanations, Linnaeus
adduced nothing in support of the dogma but the every-day perception of
the unchangeableness of species, and to this he held fast to the end
of his life; but it was important to find an explanation of the fact,
to which he himself repeatedly drew attention, that genera, orders,
and classes do not merely rest on opinion but indicate really existing
affinities. His mode of solving the difficulty was a very remarkable
one; not only does the scholastic manner of thought appear here again
quite unalloyed by modern science, but he grounds his explanation once
more on the old _a priori_ notion that the pith is the vital principle
in the plant, and also on his own assumption, that in the sexual act
the woody substance of the anthers combines with the pith-substance of
the pistil. Hugo Mohl has given a clear account of the matter in No. 46
of the ‘Botanische Zeitung’ for 1870, although neither he nor Wigand
nor most of Linnaeus’ biographers seem to know, that his theories are
all to be traced to Cesalpino. Linnaeus’ theory of natural affinities,
as he gave it in 1762 in the ‘Fundamentum Fructificationis,’ and in
1764 in the sixth edition of the ‘Genera Plantarum,’ is as follows: At
the creation of plants (in ipsa creatione) one species was made as the
representative of each natural order, and these plants so corresponding
to the natural orders were distinct from one another in habit and
fructification, that is, absolutely distinct. In the communication of
1764 the following words occur:—
1. Creator T.O. in primordio vestiit vegetabile medullare principiis
constitutivis diversi corticalis, unde tot difformia individua, quot
ordines naturales, prognata.
2. Classicas has plantas Omnipotens miscuit inter se, unde tot genera
ordinum, quot inde plantae.
3. Genericas has miscuit natura, unde tot species congeneres, quot
hodie existunt.
4. Species has miscuit casus, unde totidem quot passim occurrunt
varietates.
Hugo Mohl was right in rejecting Heufler’s assumption that a view
resembling the modern theory of descent was contained in these
paragraphs. It must be plain to any one who knows the ideas of
Aristotle, Theophrastus, and Cesalpino, within the sphere of which
Linnaeus is here moving, what he understands by his ‘vegetabile
medullare’ and ‘corticale’; that he does not for a moment mean a plant
of simplest organisation, but that both expressions indicate only
the original elements of vegetation which the Creator, according to
Linnaeus, united to one another at the first. He assumed that plants of
the highest and of the lowest grades of organisation were originally
created at the same time and alongside of one another; no new
class-plants were afterwards created, but from the mingling together
of the existing ones by the act of the Creator generically distinct
forms were produced, and the natural mingling of these gave birth to
species, while varieties were mere chance deviations from species. But
it is to be noticed that in these minglings or hybridisations the woody
substance of the one form which supplies the pollen is united with the
pith-substance of the other form, whose pistil is thus fertilised; and
so in these supposed crossings it is always the two original elements
of the plant, the medullary and the cortical, which are mingled
together.
No further proof is wanting that this theory of Linnaeus is no
precursor of our theory of descent, but is most distinctly opposed to
it; it is utterly and entirely the fruit of scholasticism, while the
essential feature in Darwin’s theory of descent is that scholasticism
finds no place in it.
CHAPTER III.
DEVELOPMENT OF THE NATURAL SYSTEM UNDER THE INFLUENCE OF THE DOGMA OF
THE CONSTANCY OF SPECIES.
1759-1850.
From the year 1750 Linnaeus’ terminology of the organs of plants
and his binary method of naming species came into general use; the
opposition which his doctrines had till then encountered by degrees
died away, and if all that he taught was not universally accepted,
his treatment of the art of describing plants soon became the common
property of all botanists.
But in course of time two very different tendencies were developed;
most of the German, English, and Swedish botanists adhered strictly to
Linnaeus’ dictum, that the merit of a botanist was to be judged by the
number of species with which he was acquainted; they accepted Linnaeus’
sexual system as one that completed the science in every respect; they
thought that botany had reached its culminating point in Linnaeus, and
that any improvement or addition could only be made in details, by
continuing to smooth over some unevennesses in the system, to collect
new species and describe them. The inevitable result was that botany
ceased to be a science; even the describing of plants which Linnaeus
had raised to an art became once more loose and negligent in the hands
of such successors; in place of the morphological examination of the
parts of plants there was an endless accumulating of technical terms
devoid of depth of scientific meaning, till at length a text-book of
botany came to look more like a Latin dictionary than a scientific
treatise. In proof of this we may appeal to Bernhardi’s ‘Handbuch
der Botanik,’ published at Erfurt in 1804, and Bernhardi was one of
the best representatives of German botany of the time. How botany,
especially in Germany, gradually degenerated under the influence of
Linnaeus’ authority into an easy-going insipid dilettantism may very
well be seen from the botanical periodical, entitled ‘Flora,’ the first
volumes of which cover the greater part of the first fifty years of the
19th century; it is scarcely conceivable how men of some cultivation
could occupy themselves with such worthless matter. It would be quite
lost labour to give any detailed account of this kind of scientific
life, if it can be so called, this dull occupation of plant-collectors,
who called themselves systematists, in entire contravention of the
meaning of the word. It is true indeed that these adherents of Linnaeus
did some service to botany by searching the floras of Europe and of
other quarters of the globe, but they left it to others to turn to
scientific account the material which they collected.
But before this evil had spread very widely, a new direction to the
study of systematic botany and morphology was given in France, where
the sexual system had never met with great acceptance. Bernard de
Jussieu and his nephew, Antoine Laurent de Jussieu, taking up Linnaeus’
profounder and properly scientific efforts, made the working out of the
natural system, in Linnaeus’ own opinion the highest aim of botany, the
task of their lives. Here more was needed than a perpetual repetition
of descriptions of single plants after a fixed pattern; more exact
inquiries into the organisation of plants, and especially of the parts
of the fructification, must supply the foundation of larger natural
groups. It was a question therefore of new inductive investigation,
of real physical science, of penetrating into the secrets of organic
form, whereas the botanists who confined themselves to Linnaeus’ art of
description made no new discoveries respecting the nature of plants.
And if these men held to the dictum just quoted from Linnaeus, and
therefore regarded themselves as his genuine disciples, the founders
of the natural system had as good a right to the title, not because
they followed his nomenclature and method of diagnosis, but because
they strove after exactly that object which he had placed first in the
science, the construction of the natural system; they were really the
men whom he had meant when he spoke of ‘methodici’ and ‘systematici.’
The German, English, and Swedish collectors of plants adhered to the
less profound, every-day, practical precepts of their master; the
founders of the natural system followed the deeper traces of his
knowledge. This direction proved to be the only one endowed with living
power, the true possessor of the future.
The efforts of Jussieu, Joseph Gärtner, De Candolle, Robert Brown, and
their successors up to Endlicher and Lindley, are not marked only by
the fact that they did truly seek to exhibit the gradations of natural
affinities by means of the natural system; equally characteristic
of these men is their firm belief in the dogma of the constancy of
species as defined by Linnaeus. Here at once was a hindrance to their
efforts; the idea of natural relationship, on which the natural system
exclusively rests, necessarily remained a mystery to all who believed
in the constancy of species; no scientific meaning could be connected
with this mysterious conception; and yet the farther the inquiry
into affinities proceeded, the more clearly were all the relations
brought out, which connect together species, genera, and families.
Pyrame de Candolle developed with great clearness a long series of
such affinities as revealed to us by comparative morphology, but how
were these to be understood, so long as the dogma of the constancy
of species severed every real objective connection between two
related organisms? Little indeed could be made of these acknowledged
affinities; still, in order to be able to speak of them and describe
them, recourse was had to indefinite expressions, to which arbitrary
and figurative meanings could be assigned. Where Linnaeus had spoken
of a class-plant or generic plant, the expression ‘plan of symmetry’ or
‘type’ was used, meaning an ideal original form, from which numerous
related forms might be derived. It was left undecided, whether this
ideal form ever really existed, or whether it was merely the result
of intellectual abstraction; and thus the forms of thought of the
old philosophy soon began to reappear. The Platonic ideas, though
mere abstractions and therefore only products of the understanding,
had been regarded not only by the school of Plato, but also by the
so-called Realists among the schoolmen, as really existing things. The
systematists obtained the idea of a type by abstraction, and the next
step was easy, to ascribe with the Platonists an objective existence to
this creature of thought, and to conceive of the type in the sense of a
Platonic idea. This was the only view that was possible in combination
with the dogma of the constancy of species, and so Elias Fries, in
his ‘Corpus Florarum,’ 1835, in speaking of the natural system, could
consistently say, ‘est quoddam supranaturale,’ and maintain that each
division of it ‘ideam quandam exponit.’ So long as the constancy of
species is maintained, there is no escaping from the conclusion drawn
by Fries, but it is equally certain that systematic botany at the
same time ceases to be a scientific account of nature. Systematists,
adopting this conclusion as necessarily following from the dogma,
might consider themselves as seeking to express in the natural system
the plan of creation, the thought of the Creator himself; but in this
way systematic botany became mixed up with theological notions, and
it is easy to understand why the first feeble attempts at a theory of
descent encountered such obstinate, nay, fanatical opposition from
professed systematists, who looked upon the system as something above
nature, a component part of their religion. And if we look back we find
that these views are based on the dogma of the constancy of species,
while Linnaeus’ ‘Philosophia Botanica’ teaches us on what grounds this
dogma rests, where it says, ‘Novas species dari in vegetabilibus
negat generatio continuata, propagatio, observationes quotidianae,
cotyledones.’
In spite of all this one important advance was made by the successors
of Jussieu; the larger groups of genera, the families, were defined
with the certainty and precision, with which Linnaeus had fixed the
boundaries of species and genera, and were supplied with characteristic
marks. They succeeded also in clearly distinguishing various still
larger groups founded on natural affinity, such as the Monocotyledons
and Dicotyledons; the distinction between Cryptogams and Phanerogams
was by degrees better appreciated, though this point could not be
finally settled, so long as it was attempted to reduce the Cryptogams
entirely to the scheme of the Phanerogams. The chief hindrance however
to the advance of systematic botany, at least at the beginning of
this period, lay in the defective morphology enshrined in Linnaeus’
terminology and in his doctrine of metamorphosis. A great improvement
certainly was effected in the early part of the 19th century by De
Candolle’s doctrine of the symmetry of plants,—a doctrine which has
been much undervalued, and that merely on account of its name; it is
really a comparative morphology, and the first serious attempt of the
kind since the time of Jung that has produced any great results; a
series of the most important morphological truths, with which every
botanist is now conversant, were taught for the first time in De
Candolle’s doctrine of symmetry in 1813. But one thing was wanting
not only in Jussieu and De Candolle, but in all the systematists of
this period, with the single exception of Robert Brown, and this
was the history of development. The history of the morphology and
systematic botany of this period shows indeed, that the comparison of
mature forms leads to the recognition of many and highly important
morphological facts; but as long as matured organisms only are
compared, the morphological consideration of them is always disturbed
by the circumstance that the organs to be compared are already adapted
to definite physiological functions, and thus their true morphological
character is often entirely obscured; on the other hand, the younger
the organs are, the less is this difficulty experienced, and this is
the real reason why the history of development is of so great service
to morphology. It was then one of the characteristic features of the
period we are describing, that its morphology was formed upon the study
of matured forms; the history of development, or at all events of very
early stages of development, could not be turned to account till after
1840, for skill in the use of the microscope, here indispensable, was
not sufficiently advanced before that time to make it possible to
follow the growth of organs from their first beginnings.
The establishment of natural affinities combined with the assumption of
the constancy of species, the growth of comparative morphology without
the history of development, lastly, the very subordinate attention
still paid to the Cryptogams,—these are the special characteristics of
the period which has now to be described at greater length.
* * * * *
Here we must once more call attention to the fact, that Linnaeus was
the first to perceive that a system which was to be the expression
of natural affinities could not be attained in the way pursued by
Cesalpino and his immediate successors. All who have attentively
studied the writings of Linnaeus which appeared after the ‘Classes
Plantarum’ (1738) must have seen the difference between that way and
the one recommended by him—a difference which is the more obvious
because Linnaeus himself, like his predecessors, constructed an
artificial system on predetermined principles of classification, and
always employed it for practical purposes, while he published at the
same time in the above-named work his fragment of a natural system,
and in the preface set forth the peculiar features of the natural and
artificial systems in striking contrast with one another. The first
thing and the last, he says in his prefatory remarks to his fragment,
which is demanded in systematic botany, is the natural method, which
slighted by less learned botanists has always been highly regarded by
the more sagacious, and has not yet been discovered. If, he continues,
we collect the natural orders from all existing systems (up to 1738),
we shall get but a small list of really allied plants, though so many
systems have claimed to be natural. He had himself long laboured
to discover the natural method and had found out some things that
were new; but though he had not succeeded in carrying it through to
a perfect work, he would continue his efforts as long as his life
lasted. He makes the very important remark, that a key, that is, _a
priori_ principles of classification, cannot be given for the natural
method, till all plants have been reduced to orders; that for this
no _a priori_ rule is of value, neither this nor that part of the
fructification, but the simple symmetry alone (simplex symmetria) of
all the parts, which is often indicated by special marks. He suggests
to those who are bent on trying to find a key to the natural system,
that nothing has more general value than relative position, especially
in the seed, and in the seed especially the ‘punctum vegetans,’—a
distinct reference to Cesalpino. He says that he establishes no classes
himself, but only orders; if these are once obtained, it will be easy
to discover the classes. The essence of the natural system could not
have been more clearly expounded in Linnaeus’ time, than it is in
these sentences. He established as early as 1738 sixty-five natural
orders, which he at first simply numbered; but in the first edition
of the ‘Philosophia Botanica’ in 1751, where the list is increased to
sixty-seven, he gave a special name to each group; and he showed his
judgment by either taking his names from really characteristic marks,
or what was still better, by selecting a genus and so modifying its
name as to make it serve as a general term for a whole group. Many of
these designations are still in use, though the extent and content
of the groups have been greatly changed. This mode of naming is an
important point, because it expresses the idea, that the different
genera of such a group are to some extent regarded as forms derived
from the one selected to supply the name. Many of Linnaeus’ orders
do in fact indicate cycles of natural affinity, though single genera
are not unfrequently found to occupy a false position; at all events,
Linnaeus’ fragment is much the most natural system proposed up to 1738,
or even to 1751. It is distinguished from Kaspar Bauhin’s enumeration
in this, that its groups do not run into one another, but are defined
by strict boundaries and fixed by names.
The Linnaean list is distinctly marked by the endeavour to make first
the Monocotyledons, then the Dicotyledons, and finally the Cryptogams
follow one another; that the old division into trees and herbs already
rejected by Jung and Bachmann, but still maintained by Tournefort and
Ray, disappears in Linnaeus’ natural system will be taken for granted
after what has been already said of it, and from this time forward this
ancient mistake is banished for ever.
In BERNARD DE JUSSIEU’S[34] arrangement of 1759 we find some
improvements in the naming, the grouping, and the succession, but at
the same time some striking offences against natural affinity. He
published no theoretical remarks on the system, but gave expression
to his views on relations of affinity in the vegetable kingdom in
laying out the plants in the royal garden of Trianon, and in the
garden-catalogue. His nephew published his uncle’s enumeration in the
year 1789 in his ‘Genera Plantarum,’ affixing the date of 1759 given
above. The difference between it and the Linnaean fragment does not
seem sufficiently marked to make it necessary to reproduce it here.
It should be noticed however that Jussieu begins with the Cryptogams,
passes through the Monocotyledons to the Dicotyledons, and ends with
the Conifers. Adanson’s claims of priority over Bernard de Jussieu
(see the ‘Histoire de la Botanique’ de Michel Adanson, Paris, 1864,
p. 36) may be passed over as unimportant. The natural system was not
advanced by Adanson to any noticeable extent; how little he saw into
its real nature and into the true method of research in this department
of botany is sufficiently shown by the fact, that he framed no less
than sixty-five different artificial systems founded on single marks,
supposing that natural affinities would come out of themselves as
an ultimate product,—an effort all the more superfluous, because a
consideration of the systems proposed since Cesalpino’s time would have
been enough to show the uselessness of such a proceeding.
The first great advance in the natural system is due to ANTOINE LAURENT
DE JUSSIEU[35] (1748-1836). After all that has been said no further
proof is needed that he was no more the discoverer or founder of the
natural system than his uncle before him. His real merit consists in
this, that he was the first who assigned characters to the smaller
groups, which we should now call families, but which he called orders.
It is not uninteresting to note here how Bauhin first provided the
species with characters, and named the genera but did not characterise
them, how Tournefort next defined the limits of the genera, how
Linnaeus grouped the genera together, and simply named these groups
without assigning to them characteristic marks, and how finally Antoine
Laurent de Jussieu supplied characters to the families which were now
fairly recognised. Thus botanists learnt by degrees to abstract the
common marks from like forms; the groups thus constituted were being
constantly enlarged, and an inductive process was thus completed which
proceeded from the individual to the more general.
It might appear that the merit of Antoine de Jussieu is rated too low,
when we praise him chiefly and simply for providing the families with
characters; but this praise will not seem small to those who know the
difficulty of such a task; very careful and long-continued researches
were necessary to discover what marks are the common property of a
natural group. Jussieu’s numerous monographs show with what earnestness
he addressed himself to the task; and it must be added, that he was
not content simply to adopt the families established by Linnaeus and
by his uncle and the limits which they had assigned to them, but
that he corrected their boundaries and in so doing established many
new families, and was the first who attempted to distribute these
into larger groups, which he named classes. But in this he was not
successful. His attempt to exhibit the whole vegetable kingdom in all
its main divisions, to unite the classes themselves into higher groups,
was also unsuccessful, for these larger divisions remained evidently
artificial. The three largest groups on the contrary, into which he
first divides the world of plants, the Acotyledons, Monocotyledons, and
Dicotyledons are natural; but they had been already partly marked out
by Ray, afterwards by Linnaeus, and finally in Bernard de Jussieu’s
enumerations. Still it is the younger Jussieu’s great and abiding
merit, to have first attempted to substitute a real division of the
whole vegetable kingdom into larger and gradually subordinate groups
for mere enumerations of smaller co-ordinated groups,—an undertaking
which Linnaeus expressly declared to be beyond his powers. If then
Jussieu’s system was far from giving a satisfactory insight into
the affinities of the great divisions of the vegetable kingdom, yet
it opened out many important points of view, from which they could
afterwards be discovered, and it certainly became the foundation for
all further advance in the natural method of classification; for this
reason it is necessary to give a view of it in the following table:—
A. L. de Jussieu’s System of 1789.
CLASS.
Acotyledones I.
{ Stamina hypogyna II.
Monocotyledones { perigyna III.
{ epigyna IV.
{ { Stamina epigyna V.
{ Apetalae { perigyna VI.
{ { hypogyna VII.
{ { Corolla hypogyna VIII.
{ Monopetalae{ perigyna IX.
Dicotyledones.{ { epigyna { antheris connatis X.
{ { { distinctis XI.
{ { Stamina epigyna XII.
{ Polypetalae{ hypogyna XIII.
{ { perigyna XIV.
{ Diclines irregulares XV.
This table shows that Jussieu did not oppose the Cryptogams, which
he calls Acotyledones, to the whole body of Phanerogams, as Ray did
under the name of Imperfectae; he rather regards the Acotyledones as
a class co-ordinate with the Monocotyledones and Dicotyledones; but
this mistake or similar mistaken views run through all systematic
botany up to 1840; the morphology founded by Nägeli and by Hofmeister’s
embryological investigations first showed that the Cryptogams separate
into several divisions, which co-ordinate with the Monocotyledons
and Dicotyledons. At the same time the use of the word Acotyledones
for Linnaeus’ Cryptogams shows that Jussieu overrated the systematic
value of the cotyledons, because, as is seen from the introduction
to his ‘Genera Plantarum,’ he was quite in the dark on the subject
of the great difference between the spores of Cryptogamic plants and
the seeds of Phanerogams. His conception of the organs of generation
was essentially that of Linnaeus; hence he judged of the Cryptogams
according to the scheme of the Phanerogams, and, not perceiving their
peculiarities, he virtually characterised them by negative marks.
If we notice in the above table how the Phanerogams are separated
into classes, it strikes us that the triple division into hypogynous,
perigynous, and epigynous is repeated no less than four times; this
shows that Jussieu had mistaken ideas of the value of these marks for
classification, whereas the recurrence of them so often should of
itself have suggested a doubt on this point. To judge of his system
more exactly we must here give his series of the families, which he had
already raised to the number of a hundred.
Class I.
1. Fungi.
2. Algae.
3. Hepaticae.
4. Musci.
5. Filices.
6. Naiades.
Class II.
7. Aroideae.
8. Typhae.
9. Cyperoideae.
10. Gramineae.
Class III.
11. Palmae.
12. Asparagi.
13. Junci.
14. Lilia.
15. Bromeliae.
16. Asphodeli.
17. Narcissi.
18. Irides.
Class IV.
19. Musae.
20. Cannae.
21. Orchides.
22. Hydrocharides.
Class V.
23. Aristolochiae.
Class VI.
24. Elaeagni.
25. Thymeleae.
26. Proteae.
27. Lauri.
28. Polygoneae.
29. Atriplices.
Class VII.
30. Amaranthi.
31. Plantagines.
32. Nyctagines.
33. Plumbagines.
Class VIII.
34. Lysimachiae.
35. Pediculares.
36. Acanthi.
37. Jasmineae.
38. Vitices.
39. Labiatae.
40. Scrophulariae.
41. Solaneae.
42. Borragineae.
43. Convolvuli.
44. Polemonia.
45. Bignoniae.
46. Gentianeae.
47. Apocyneae.
48. Sapotae.
Class IX.
49. Guajacanae.
50. Rhododendra.
51. Ericae.
52. Campanulaceae.
Class X.
53. Cichoraceae.
54. Cinarocephalae.
55. Corymbiferae.
Class XI.
56. Dipsaceae.
57. Rubiaceae.
58. Caprifolia.
Class XII.
59. Araliae.
60. Umbelliferae.
Class XIII.
61. Ranunculaceae.
62. Papaveraceae.
63. Cruciferae.
64. Capparides.
65. Sapindi.
66. Acera.
67. Malpighiae.
68. Hyperica.
69. Guttiferae.
70. Aurantia.
71. Meliae.
72. Vites.
73. Gerania.
74. Malvaceae.
75. Magnoliae.
76. Anonae.
77. Menisperma.
78. Berberides.
79. Tiliaceae.
80. Cisti.
81. Rutaceae.
82. Caryophylleae.
Class XIV.
83. Sempervivae.
84. Saxifragae.
85. Cacti.
86. Portulaceae.
87. Ficoideae.
88. Onagrae.
89. Myrti.
90. Melastomae.
91. Salicariae.
92. Rosaceae.
93. Leguminosae.
94. Terebinthaceae.
95. Rhamni.
Class XV.
96. Euphorbiae.
97. Cucurbitaceae.
98. Urticae.
99. Amentaceae.
100. Coniferae.
Jussieu’s division of the Cryptogams and Monocotyledons offers much
that is satisfactory, if we put the position of the Naiades out of
sight. The grouping of the Dicotyledons on the contrary is to a great
extent unsuccessful, chiefly owing to the too great importance which
he attached to the insertion of the parts of the flowers, that is, to
the hypogynous, perigynous, and epigynous arrangement. It is in this
grouping of families into classes that the weak side of the system
lies; it is utterly artificial, and the task of his successors has been
to arrange the families of the Phanerogams, which were most of them
well-established, and especially those of the Dicotyledons, in larger
natural groups. But this could not be effected, till morphology opened
new points of view for systematic botany; Jussieu, as has been already
remarked, accepted Linnaeus’ views of the morphology of the organs of
fructification in Phanerogams, though he introduced many improvements
in details. He laid greater stress on the number and relative positions
of the different parts of the flower; attention to their insertion on
the flowering axis, which he designated as hypogynous, perigynous,
and epigynous, would have been a great step in advance, if he had not
overrated its systematic value. The morphology of the fruit is very
superficial in Jussieu; even the designation of dry indehiscent fruits
as naked seeds recurs in his definitions, though as it happens this
misconception does not cause any great disturbance. How inexact was his
investigation of the organs of fructification, when they were somewhat
small and obscure, is best shown by the fact that the Naiades, which
are made to include Hippuris, Chara, and Callitriche, appear among the
Acotyledons, and that Lemna and the Cycads are placed with the Ferns.
Jussieu explained the dictum, ‘Natura non facit saltus,’ to mean that
the whole body of plants in its natural arrangement must exhibit a
lineal series ascending from the most imperfect to the highest forms;
but he does not say whether Linnaeus’ comparison of the natural system
to a geographical map, the countries in which answer to orders and
classes, is also admissible.
His theoretical observations on the value to be given to certain marks
in a systematic point of view are not attractive, and for the most
part not very correct; he speaks as though some marks must have a
more extensive, others a less extensive value; the perception of the
fact, so far as it is true, rests entirely upon induction; that is,
after the natural affinities have been already recognised to a certain
extent, it becomes apparent that certain marks remain constant in
larger or smaller groups; the systematist can now go on to try whether
such constant marks occur in other plants also, which he had hitherto
assigned to other groups, and thus put it to the test whether those
marks may not be accompanied by others, which would serve to establish
the affinities; that Jussieu did so proceed in defining his families
admits of no doubt, but he was not himself thoroughly conscious of the
fact; at all events, he did not extend this mode of proceeding, the
seeking after leading marks, to the establishing of larger groups or
classes, for these he founded on predetermined principles.
Jussieu’s labours as a systematist were not confined to the publication
of his ‘Genera Plantarum’; on the contrary, his most fruitful
researches began after 1802, and were continued to the year 1820, and
their results appeared in a long series of monographs on different
families in the Mémoires du Museum. He felt with De Candolle, Robert
Brown, and later systematists, that the perfecting of the natural
system depended mainly on the careful establishing and defining of
families. His efforts received a new impulse from the work of a German
writer, whose first volume had appeared in 1788, a year therefore
before the ‘Genera Plantarum,’ a second following it in 1791, and a
supplementary volume in 1805.
This work was JOSEPH GÄRTNER’S[36] ‘De fructibus et seminibus
plantarum,’ in which the fruits and seeds of more than a thousand
species are described and carefully figured. But almost more important
than these numerous descriptions, though they offered rich material
to the professed systematists, were the introductions to the first
two volumes, and especially to those of 1788. They contain valuable
reflections on sexuality in plants,—a subject which had remained in
the condition in which it was left by Camerarius (1694) till it was
greatly developed by Koelreuter after 1761, and had since then been
little studied,—and an account of the morphology of fruits and seeds,
the knowledge of which had gone back rather than advanced since the
days of Malpighi and Grew. Gärtner was well qualified for this work by
his unparalleled knowledge of the forms of fruits, and still more by
the character of his mind. Free from Linnaeus’ scholastic bias, he
addressed himself to the examination of the most difficult organs of
plants with as great freedom from prepossessions as exact acquaintance
with the writings of others; he gives us the impression of a modern man
of science more than any other botanist of the 18th century, with the
exception of Koelreuter. He knew how to communicate with clearness of
language and perspicuity of arrangement whatever he gathered of general
importance from each investigation. Though it is easy to see that the
founding of the natural system was ever before his mind as the final
object of his protracted labours, he was in no eager haste to reach it;
he contented himself with arranging his fruits, saying expressly that
the natural system would never be founded by these means alone, though
the exact knowledge of fruits and seeds supplied the most important
means for decision. Thus his great work was at once an inexhaustible
mine of single well-ascertained facts, and a guide to the morphology
of the organs of fructification and to its application to systematic
botany. The imperfections, which are to be found even in this work,
are due to the circumstances of the time; in spite of Schmeidel’s and
Hedwig’s researches into the Mosses there was still the old obscurity
with regard to the organs of propagation in the Cryptogams, and this
rendered a right definition of the ideas, seed and fruit, extremely
difficult. But Gärtner made one great step in advance on this very
point when he showed that the spores of the Cryptogams were essentially
different from the seeds of Phanerogams, with which they had been
hitherto compared, because they contain no embryo; he called them
therefore not seeds, but gemmae. The second great hindrance to a true
conception of certain characters in fruits and seeds on the part of
Gärtner was the entire ignorance of the history of development which
then reigned; yet even here we see an advance, if only a small one,
made by him in his repeatedly going back to the young state for a more
correct idea of the organs.
Above all, Gärtner put an end to the blunder of regarding dry
indehiscent fruits as naked seeds, by rightly defining the pericarp
as in all cases the ripened wall of the ovary, and by considering its
strong or weak construction, its dry or pulpy condition, as a secondary
matter. It is obvious that the whole theory of the flower was thus
placed upon a better basis, since dry indehiscent fruits may come from
inferior or superior ovaries. But Gärtner’s theory of the seed is
one of his most valuable contributions to the science. After careful
consideration of the seed-envelopes, he submitted the inner portion
(nucleus) enclosed by them to a searching comparative examination;
he correctly distinguished the endosperm from the cotyledons, and
described the variations in its form and position. This was the more
needful, since Linnaeus had denied the existence of an ‘albumen’ in
plants, which Grew had already recognised and so named; to Linnaeus
it appeared to be of no use to the seed. Though Gärtner speaks of the
cotyledons as uniting with the embryo to form the nucleus of the seed,
yet his account shows that he regarded them as outgrowths of the embryo
itself. The uncertainty which still existed in the interpretation of
the parts of the seed is shown even in Gärtner by his curious notion of
a ‘vitellus,’ which in fact takes in everything that he was unable to
explain aright inside the seed; for instance, he makes the scutellum
in grasses, and even the cotyledonary bodies of Zamia a vitellus, and
applies the same name to the whole contents of the spores of Seaweeds,
Mosses, and Ferns. In spite of the striking defects connected with
this mistaken notion in his theory of the seed, his views far surpass
in clearness and consistency all that had hitherto been taught on the
subject. His giving the term embryo to that part of the seed which is
capable of development was also an advance in respect of logic and
morphology, in spite of his mistake in not admitting the cotyledons
which are attached to the embryo into the conception; this, however,
could easily be corrected at a later time. What Gärtner now named
the embryo, had been up to his time called the ‘corculum seminis,’
especially by Linnaeus and Jussieu; it was evidently thought that
Cesalpino’s phraseology was thus retained; but he, as we have seen,
understood by the words ‘cor seminis’ the spot where the cotyledons
spring from the germ, which spot he wrongly took for the meeting-point
of root and stem and the seat of the soul of the plant. And so at last
after two hundred years the word disappeared from use, which might
have reminded the botanist of Cesalpino’s views respecting the soul of
plants.
A work such as Gärtner’s could scarcely find a fruitful soil in
Germany, where some thirty years before even Koelreuter’s brilliant
investigations had met with little sympathy, and Conrad Sprengel’s
remarkable enquiries into the relations of the structure of the flower
to the insect-world in 1793 failed to be understood; Gärtner complains
in the second part, published in 1791, that not two hundred copies of
the first volume were sold in three years. But the work, which forms an
epoch in the history of botany, was better received in France, where
the Academy placed it as second in the list of the productions which
in later times had been most profitable to science; there lived the
man who was able to measure the whole value of such a work—Antoine
Laurent de Jussieu. But even in Germany, where plant-describing was
comfortably flourishing, there were not altogether wanting men who
knew how to estimate both the services of Gärtner and the importance
of the natural system. First among these was August Johann Georg Karl
Batsch, Professor in Jena from 1761 to 1802, who published in the
latter year a ‘Tabula affinitatum regni vegetabilis,’ with characters
of the groups and families. Kurt Sprengel, who was born in 1766, and
died as Professor of Botany in Halle in 1833, contributed still more
to the spread of clearer views respecting the real character of the
natural system and the task of scientific botany generally by numerous
works, and especially by his ‘Geschichte der Botanik,’ which appeared
in 1817 and 1818. But even this highly gifted and accomplished man
agreed with the Linnaean botanists in attributing an excessive value
to the describing of plants, as is shown in his history, where to exalt
the merits of the old botanists he gives figures of the plants first
described by them.
Meanwhile the meritorious efforts of these men were not in themselves
capable of directly advancing the natural system, or of greatly
increasing the number of its adherents in Germany, nor did it find
general acceptance in that country till it had made considerable
progress in the hands of the two foremost botanists of the time, De
Candolle and Robert Brown.
AUGUSTIN PYRAME DE CANDOLLE[37] (1778-1841) belongs to the number of
those distinguished investigators of nature, who at the end of the last
and the beginning of our own century made their native city Geneva a
brilliant centre of natural science. De Candolle was the contemporary
and fellow-countryman of Vaucher, Theodore de Saussure, and Senebier.
Physics and physiology especially were being successfully cultivated
at that time in Geneva, and Pyrame de Candolle was attracted to these
studies; among his youthful efforts are some important investigations
into the effect of light on vegetation, and the contributions which he
made to vegetable physiology in his great work on that subject will
be noticed in a later portion of this history. De Candolle turned his
attention to all parts of theoretical and applied botany, but his
importance for the history of the science lies chiefly in the direction
of morphology and systematic botany, and it is this which we will now
proceed to describe.
The amount and compass of De Candolle’s labours as a systematic and
descriptive botanist exceed those of any writer before or after him.
He wrote a series of comprehensive monographs of large families of
plants, and published a new edition of De Lamarck’s large ‘Flore
Française’ substantially altered and enlarged; and in addition to these
and many similar works and treatises on the geographical distribution
of plants, he set on foot the grandest work of descriptive botany that
is as yet in existence, the ‘Prodromus Systematis Naturalis,’ in which
all known plants were to be arranged according to his natural system
and described at length,—a work not yet fully completed, and in which
many other descriptive botanists of the last century participated, but
none to so large an extent as De Candolle, who alone completed more
than a hundred families. It is not possible to give an account in few
words of the service rendered to botany by such labours as these; they
form the real empirical basis of general botany, and the better and
more carefully this is laid, the greater the security obtained for the
foundations of the whole science.
But a still higher merit perhaps can be claimed for De Candolle,
inasmuch as he not only like Jussieu elaborated the system and its
fundamental principles in his descriptive works, but developed the
theory, the laws of natural classification, with a clearness and
depth such as no one before him had displayed. To this purpose he
applied morphological researches, which in profundity and wealth of
thought and in the fruitfulness of their results for the whole domain
of systematic botany far surpassed all that Linnaeus and Jussieu had
accomplished, and show us that while engaged in his splendid labours
in descriptive botany he had caught during his ten years’ residence
in Paris the true spirit of modern investigation of nature, as it had
been developed by the French naturalists of the end of the previous
century. Scarcely a trace is to be found in De Candolle of the
scholasticism of Cesalpino and Linnaeus, which occasionally makes its
appearance even in Jussieu. For instance, he dealt with morphology as
essentially the doctrine of the symmetry of form in plants, that is, he
found the basis of morphological examination in the relative position
and numbers of the organs, disregarding their physico-physiological
properties as of no account from the morphological point of view. He
was therefore the first who recognised the remarkable discordance
between the morphological characters of organs, which are of value
for systematic purposes, and their physiological adaptations to the
conditions of life, though it must at the same time be acknowledged,
that he did not consistently carry out this principle, but committed
grave offences against it in laying down his own system. It is a point
of the highest interest in De Candolle’s morphological speculations,
that he was the first who endeavoured to refer certain relations of
number and form to definite causes, and thus to distinguish what is
primary and important in the symmetry of plants from merely secondary
variations, as is seen in his doctrine of the abortion and adherence
of organs. In these distinctions De Candolle laid the foundation of
morphological views, which, though now modified to some extent, do
still contain the chief elements of morphology and the natural system;
but his morphological speculations were confined to the domain of
the Phanerogams, and chiefly advanced the theory of the flower; a
morphology of the Cryptogams was as little to be thought of in the
condition of microscopy before 1820, as the application of the history
of development to the establishment of morphological theories.
De Candolle published his morphology or doctrine of symmetry and his
theory of classification together in a book which appeared first
in 1813, with the title, ‘Théorie Élémentaire de la botanique ou
exposition des principes de la classification naturelle et de l’art de
décrire et d’étudier les végétaux,’ and again in 1819 in an improved
and enlarged edition. The second edition will be the one referred to in
the further account of his views. The second chapter of the second book
concerns us most at present. After alluding to the fact, that anatomy
and physiology are concerned with the structure of the individual
organ only so far as the power to fulfil its proper function depends
on the structure, he points out that the physiological point of view
is no longer sufficient when we are engaged in comparing the organs of
different plants. Though it is true that the function of the organs
is the most important for the life and permanence of the individual,
yet we find these functions modified in the case of homologous organs
in different plants; for the natural classification we must take into
consideration only the entire system of organisation, that is, the
symmetry of the organs. All organisms of a kingdom, he continues, have
the same functions with slight modifications; the immense amount of
variation in systematically different species depends therefore only
on the way in which the general symmetry of structure varies. This
symmetry of the parts, the discovery of which is the great object
in the investigation of nature, is nothing more than the sum total
(l’ensemble) of the positional relations of the parts. Whenever these
relations (disposition) are regulated according to the same plan,
the organisms exhibit a certain general resemblance to one another,
independently of the form of the organs in detail; when this general
resemblance is perceived, without any attempt to give any account of
it in the detail, we have what has been called habitual relationship;
but it is the task of the doctrine of symmetry to resolve this likeness
of habit into its elements, and to explain its causes. Without this
study of symmetry it may easily happen that two different kinds of
symmetry may be supposed to be alike, because they seem outwardly
alike to our senses, just as forms of crystals of different systems
may be confounded together for want of careful examination; the chief
thing is to know the plan of symmetry in every class of plants,
and the study of this is the foundation of every theory of natural
affinities. But success in this study depends on the certainty with
which organs are distinguished, and the distinguishing them must be
independent of changes of form, size, and function. He then shows that
the difficulties in the morphological comparison of organs, or, as we
should now say, in the establishing the homology, are due to three
causes; abortion, degeneration, and adherence (adhérence). These three
causes, by which the original symmetry of a class is changed and may
even be utterly obscured, are then fully illustrated by examples.
In respect to abortion he distinguishes that which is produced by
internal causes from that which is due to accidental and external ones;
he refers especially to the abortion of two loculaments in the fruit of
the horse-chestnut and the oak, to the suppression of the terminal bud
in some shrubs by the adjoining axillary buds, and to the fact that all
organs of plants may become abortive in a similar manner; for instance,
the sexual organs disappear entirely in the disk-flowers of Viburnum
Opulus, and one of the two sexes in the flower of Lychnis dioica. He
goes on to answer the question, how it is possible to discover the
symmetry in such cases; one method he finds supplied by monstrosities,
among which there are even some that may be regarded as a return to the
original symmetry, the cases known as peloria. Analogy or ‘induction’
is, he says, less certain, but of much more extensive application;
this is founded exclusively on the knowledge of the relative position
of organs. Armed with this, we find that the flower of Albuca, which
corresponds to a flower of Liliaceae in everything except in having
only three stamens, is to be considered one of the Liliaceae, because
it has three filaments placed between the three stamens exactly in
the position of the three other stamens in the Liliaceae; it must
be concluded therefore that they are abortive stamens. Similar
conclusions from analogy must be carried from species to species,
from organ to organ, and the great systematists have in fact done so.
In certain cases abortion is produced by defect, in others by excess
of nourishment, of which he gives examples. An important sentence
occurs in this place; everything in nature, he says, leads us to
believe that all organisms in their inner nature are regular, and that
different forms of abortion differently combined are the cause of all
irregularity; from this point of view the smallest irregularities
are important, because they lead us to expect greater ones in nearly
allied plants; and wherever in a given system of organisation there
are inequalities between organs of the same name, the inequality will
possibly reach a maximum, that is, end by annihilating the smallest
part. Thus in the Labiatae with two stamens, it is the two which in
other cases also are the smaller, which are here completely aborted.
When in Crassulaceae there are twice as many stamens as petals, those
that alternate with the petals are larger and earlier developed, and
we may therefore expect that those which are opposite the petals may
become abortive; and therefore we may place a genus like Sedum, in
which the latter are sometimes wanting, with Crassulaceae; but we could
not do so, if we found only the stamens that are superposed upon the
petals. It occurs sometimes, he continues, that an organ is prevented
from fulfilling its function by partial abortion. In this case it may
assume another function, as the abortive leaves of the vetch and the
abortive inflorescences of the vine are employed as tendrils. In other
cases the abortive organ appears to be quite useless, as for instance
many rudimentary leaves. All such useless organs, says De Candolle,
exist only in consequence of the primitive symmetry of all organs.
Finally the abortion may be so complete that no trace of the organ
remains, of which case there are however two kinds, one where the organ
is at first perceptible and afterwards quite disappears, as in the
abortive loculaments in the fruit of the oak; in other instances no
trace is to be seen from the first of the abortive organs, as happens
with the fifth stamen of Antirrhinum.
All that has here been said might be alleged word for word in proof
of the theory of descent, but our author is an adherent of the dogma
of the constancy of species; what from his point of view he really
means by abortion is difficult to say, for the object which is aborted
is wanting. If species are constant, and therefore of absolutely
distinct origin, we must not speak of abortion; we can only say
that an organ which is present or large in one species is small or
wanting in another. In introducing the idea of abortion De Candolle
at once goes beyond the dogma of the constancy of species, without
being clear in his own mind with regard to this important step.
His proceeding shows that facts lead even a defender of constancy
against his will to theories which run counter to that dogma. This
is confirmed by his perception of the correlation of growth, which
is connected with abortion; he points to the fact that owing to the
disappearance of sexual organs in the disk-flowers of Viburnum Opulus
the corollas become larger, as do the bracts of the abortive flowers
of Salvia Horminum; similarly he regards the disappearance of the
seeds in Ananas, Banana, and the Bread-fruit tree as the cause of the
enlargement of the pericarps; it does not escape him, that the fertile
flower-stalks in Rhus Cotinus remain naked, while an elegant pubescence
forms on the barren ones; the leaf-like expansion of the leaf-stalks
of Acacia heterophylla, which do not develop their laminae, he refers
also to this correlation of growth. He finds the most remarkable
example of the kind in the doubling of flowers, where according to his
view the disappearance of the anthers is a condition of the corolline
expansion of the filaments; in the same way sometimes the carpel is
changed into a petal through the disappearance of the stigma. Though in
many of these cases it is quite possible to conceive of the relations
of cause and effect in the reverse way, yet De Candolle’s principle of
correlation will be equally applicable.
The second cause by which the symmetry may be obliterated, namely
degeneration, asserts itself in the formation of thorns, of threadlike
prolongations of membranous expansions, and in the production of fleshy
parts, or of parts with dry membranes.
The third kind of departure from the symmetrical plan is the adherence
of parts, the theory of which he grounds first and chiefly on the
phenomena of grafting, and then passes to more difficult cases. The
close packing of the ovaries in some species of honeysuckle, is, he
says, the primary cause of their adherence. This therefore does not
depend on the plan of symmetry, but upon an accident, which however is
constant in its appearance, owing to the specific constitution of such
plants. In connection with the phenomena of adherence he next considers
the question whether a structure composed of several parts, as for
instance a compound ovary, should be considered as originally simple
and afterwards divided into parts, or whether the converse is the true
account, and he says that we must examine each particular case and
decide which is the correct conception. Thus it may be shown that the
perfoliate leaves of honeysuckles, as well as the involucres of many
Umbelliferae, and monosepalous calyces and monopetalous corollas are
due to adherence, and he proceeds to prove that ovaries with several
loculaments and several parts have in like manner been formed by
adherence of two or more carpellary leaves, and concludes by pointing
out the systematic importance of such considerations. Further on he
takes occasion to speak of the significance of the relative number
of the parts of the flower, on which head he says much that is good,
but does not thoroughly investigate the matter; it was not till a
later time that Schimper’s doctrine of phyllotaxis made it possible to
express these relations of number and position more precisely.
He concludes his rules for the application of his morphology to the
determination of relations of affinity with the declaration, that the
whole art of natural classification consists in discerning the plan
of symmetry, and in making abstraction of all the deviations from it
which he has described,—much in the same way as the mineralogist seeks
to discover the fundamental forms of crystals from the many derivative
forms. It is obvious that all this teaching was a great step in advance
upon the right path, that De Candolle has here given utterance for
the first time to an important principle of morphology and systematic
botany; nevertheless he did not succeed in always consistently carrying
out his own principle; he was true to himself only in the determination
of small groups of relationship; in framing the largest divisions of
the vegetable kingdom he entirely lost sight of the rule which he had
himself laid down, that the morphological character of organs and the
extent to which it can be turned to account for systematic purposes is
entirely independent of their physiological character, and that the
most important physiological characters are just those which are of
quite subordinate importance in the determination of affinities. In
spite of this strange inconsistency, to De Candolle belongs the merit
of being the first to point emphatically to the distinction between
morphological and physiological marks, and to bring clearly to light
the discordance between morphological affinity and physiological habit;
but in this discordance lurks a problem, which could only be solved
forty years later by Darwin’s theory of selection. A genuine inductive
process alone could reveal these remarkable relations between the
morphological and physiological characters of organs. But it is at the
same time true that De Candolle could not have made this discovery,
if his predecessors had not already established a large number of
affinities. It was while he was engaged in an exact comparison of forms
already recognised as undoubtedly related to one another, that that
which he called the plan of symmetry, and which was afterwards named
a type, revealed itself to him; and as he examined it more closely,
and compared it with peculiarities of habit in different plants formed
on the same plan, he discovered certain causes, by means of which the
deviations were to be explained; these were abortion, degeneration,
and adherence. By attending to these he succeeded in discovering
affinities that had been hitherto doubtful or unknown; this was at all
events the true inductive way of advancing the system, and whatever
the earlier systematists had effected that was really valuable had
been effected virtually in the same way, only they never arrived at a
clear understanding of their own mode of proceeding; they had followed
unconsciously the method which De Candolle clearly understood and
consciously pursued.
The majority of De Candolle’s successors were far from fully
appreciating the entire significance of his theory, its importance as
a matter of method and principle; on the contrary in the search for
affinities they continued to surrender themselves to a blind feeling
rather than to a clearly recognised method, and the same must be
said unhappily of De Candolle himself, when he was dealing with the
establishment of the large divisions of the vegetable kingdom. With
equal surprise we find him in the book before us, in which he has
developed the true method in systematic botany, expressing the opinion
that the most important physiological characters must be employed for
the primary divisions of the system, and this idea is not improved
by the fact that he ascribes to the organs physiological characters
which they do not really possess; thus he regards the vessels as the
most important organs of nutrition, which they are not in fact, and
upon this double error he builds his primary division of the whole
vegetable kingdom into vascular and cellular plants, and then by a
third mistake believes that this division coincides with the division
of plants into those which have and those which have not cotyledons.
The already established division into Monocotyledons and Dicotyledons,
which rests upon a leading and purely morphological mark, is spoilt
by De Candolle through his following Desfontaines in ascribing to
the Dicotyledons a different mode of growth in thickness from that
of the Monocotyledons, and characterising the one as exogenous, the
other as endogenous. But this notion is utterly incorrect, as von Mohl
showed twelve years later; and if it were correct, it would still be
unimportant in a systematic point of view, because it appeals to a mark
which is morphologically of quite subordinate importance. The worst
consequence of these mistakes was, that the Vascular Cryptogams were
introduced into the same class with the Monocotyledons,—a decided step
backwards, if we compare De Candolle’s system with that of Jussieu.
In spite of these grave defects in the primary divisions of the whole
vegetable kingdom De Candolle’s system deserved the fame which it
acquired and long maintained; it had this advantage over Jussieu’s
system that in the class of Dicotyledons, the largest division of the
whole kingdom, larger subdivisions appeared, and these served to unite
families that were in many points essentially related; the Dicotyledons
were in fact divided first of all into two artificial groups according
to the presence of two floral envelopes or one; the first and much
the larger of these was again broken up into a series of subordinate
groups, which pointed in many ways to natural affinities. That these
groups, which have only quite recently been materially altered, did to
a very considerable extent take account of natural affinities, is due
to the fact that De Candolle in framing them really followed his own
rules, whereas the superior divisions, which are artificial, owe their
existence to his disregard of them.
De Candolle declared emphatically against the old notion, that the
vegetable system answers to a linear series,—a notion which sprang
from a misunderstanding of the saying, ‘Natura non facit saltus,’—and
demonstrated its impossibility by examples; but he allowed himself
to be too much influenced by the idea which had been thrown out by
Linnaeus, and taken up by Giseke, Batsch, Bernardin de St. Pierre,
L’Heritier, Du Petit-Thouars and others, that the vegetable kingdom
might be compared as respects its grouping to a geographical map, in
which the quarters of the globe answer to the classes, the kingdoms
to the families, and so on. If the theory of descent is to a certain
degree compatible with the idea of a linear sequence from the most
imperfect to the highest forms of plants, it is quite incompatible
with the above comparison; and systematic investigation, led astray
from the right path, is in danger of ascribing the importance of
real affinities to mere resemblances of habit, incidental analogies,
by which a group of plants appears to be connected with five or six
others. In exhibiting his system on paper De Candolle allowed the use
of the linear sequence as a convenience, for here it was not, he said,
a matter of any importance, since the true task of the science is to
study the relations of symmetry in each family and the mutual relations
of families to one another; yet in a linear presentation of the system
for didactic purposes the sequence ought not to begin with the most
simple plants, for these are the least known, but with the most highly
developed. Thus De Candolle was the means of removing from the system
the last trace of anything which harmonised with an ascending and
uninterrupted development of forms. Resting on the doctrine of the
constancy of species, and assuming that every group of relationship is
founded on a plan of symmetry round which individual forms are grouped
as crystals round their parent form, De Candolle was quite consistent
in his views. The mode of representation came to prevail in the
vegetable kingdom which De Candolle’s contemporary, Cuvier, an equally
sturdy defender of the dogma of constancy, had introduced in the animal
kingdom as the type-theory. Thus the most splendid results obtained by
induction were united in the case of De Candolle with the barren dogma
of the constancy of species, which, as Lange wittily remarks, comes
direct from Noah’s ark, to form an intimate mixture of truth and error;
nor did De Candolle’s many adherents succeed in unravelling the coil,
though they removed the chief errors from his system and introduced
many improvements.
To these remarks may be appended a table of the main divisions of De
Candolle’s system of 1819, which so far as it is presented in linear
arrangement he calls expressly an artificial system.
I. Vascular plants or plants with cotyledons.
1. Exogens or Dicotyledons.
A. With calyx and corolla:
Thalamiflorals (polypetalous hypogynous),
Calyciflorals (polypetalous perigynous),
Corolliflorals (gamopetalous).
B. Monochlamydeous plants (with a single floral envelope).
2. Endogens or Monocotyledons.
A. Phanerogams (true Monocotyledons),
B. Cryptogams (vascular Cryptogams including Naiadeae).
II. Cellular plants or Acotyledons.
A. With leaves (Muscineae),
B. Without leaves (Thallophytes).
The number of families, with Linnaeus 67, with A. L. de Jussieu 100,
was increased by De Candolle to 161.
If the principles of comparative morphology laid down by De Candolle
were at first prevented from being rapidly disseminated in Germany by
the philosophical tendencies then reigning among its botanists, and
especially by the obscurities of Goethe’s doctrine of metamorphosis,
yet these principles and his views also on the natural system won
their way by degrees to acknowledgment and acceptance; and after the
year 1830 the study of the system was prosecuted by the botanists of
Germany, as well as by those of England and France, as the proper
object of the science. We may even say that the impulse given by De
Candolle worked more powerfully from that time forward in Germany
than in France. It may be said too of De Candolle’s contemporary, the
Englishman ROBERT BROWN[38] (1773-1858), whose chief labours fall
in the period between 1820 and 1840, that he, like De Candolle, was
better appreciated during that time in Germany than in any other
country. Robert Brown, who spent the five years from 1801 to 1805
in Australia, studied the flora of that quarter of the world, and
discussed in numerous essays the botanical results of various journeys
made by other naturalists in polar regions and in the tropics. In this
way he found opportunity to leaven the ideas, which through Humboldt’s
influence had become predominant respecting the geography of plants,
with the spirit of the natural system; he also made the morphology and
systematic position of a number of families the subject of critical
investigation.
Robert Brown’s literary efforts were limited to these monographs; he
nowhere attempted to give a connected account of the principles which
he follows in them, an exposition of his morphology or a theory of
classification, nor did he frame a new system. The results of his
studies which were really fruitful and served to advance the science
are to be found in the more general remarks, which he managed to insert
quite incidentally in his monographs. In this way he succeeded in
clearing up the morphology of the flower and with it the systematic
position of some difficult families of plants, such as the Grasses,
Orchids, Asclepiads, the newly-discovered Rafflesiaceae and others, and
to throw new light at the same time on wider portions of the system;
in his considerations on the structure and affinities of the most
remarkable plants, which had been collected in Africa by different
travellers in the years immediately following 1820, he discussed
difficult and remarkable morphological relations in the structure
of the flower. He referred especially in this essay (1826) to the
relations between the numbers of the stamens and carpels, and those of
the floral envelopes in the Monocotyledons and Dicotyledons, and showed
how these typical, or as he calls them in De Candolle’s phraseology,
symmetrical relations were changed by abortion, while he entered at
the same time into a more exact determination of the position of the
aborted and of the perfect organs, in order to discover new relations
of affinity. His most valuable work in this direction is a paper on
a genus Kingia, discovered in New Holland in 1825; the structure of
the seeds in this genus led him to seek more accurate knowledge of the
unfertilised ovule in the Phanerogams generally, and especially in
the Cycads and Conifers. In spite of the labours of Gärtner and the
later researches of Treviranus, there was still considerable obscurity
attaching to the theory of the seed, for no one had yet succeeded in
referring the position of the embryo in the ripe seed to a general law.
For this it was necessary to submit the ovule before fertilisation
to careful examination, and Robert Brown carried out this first step
to a history of development with great success; he was the first to
distinguish the integuments and the nucleus in the ovule, and the
embryo-sac in the nucleus, parts which Malpighi and Grew had indeed
observed but had not brought out with perfect clearness. The micropyle
and the hilum of the seed had not yet been properly distinguished, but
had been to some extent even confounded with one another. Robert Brown
showed that the hilum answers to the point of attachment of the ovule,
while the micropyle is a canal formed by the integuments of the ovule
and leading to the summit of the nucleus; that in anatropous ovules
the micropyle lies beside the hilum, in orthotropous ovules opposite
to it; that the embryo in the embryo-sac (amnion) is always formed at
the spot which lies nearest the micropyle, and that the radicle of the
embryo is always turned towards the micropyle,—facts which at once
established the general rule by which to determine the position of the
embryo in the seed and in the fruit. He also gave the first correct
explanation of the endosperm as a nourishing substance formed inside
the embryo-sac after fertilisation, and more than this, he was the
first to distinguish the perisperm as a substance formed outside the
embryo-sac in the tissue of the nucleus.
In this way Robert Brown established morphological relations in the
organisation of the seed of the Monocotyledons and Dicotyledons, which
count among the most important principles of classification in these
classes; he was still more happy in being the first to detect the
peculiar structure of the flower of Conifers and Cycads, as compared
with that of other flowering plants; it was he who perceived that
what had been hitherto called a female flower in these plants was
really a naked ovule, a view which Trew of Nüremberg had, it is true,
suggested in the year 1767. He also called attention to the agreement
in structure of the male and female organs in these families. Thus
one of the most remarkable facts in vegetation, the gymnospermy of
the Conifers and Cycads, was for the first time established, and this
led afterwards through Hofmeister’s investigations to the important
result, that the Gymnosperms, which had been up to that time classed
with Dicotyledons, are to be regarded as co-ordinate with Dicotyledons
and Monocotyledons, forming a third class through which remarkable
homologies were brought to light in the propagation of the higher
Cryptogams and the formation of seeds in Phanerogams. No more important
discovery was ever made in the domain of comparative morphology and
systematic botany. The first steps towards this result, which was
clearly brought out by Hofmeister twenty-five years later, were secured
by Robert Brown’s researches, and he was incidentally led to these
researches by some difficulties in the construction of the seed of an
Australian genus. He discussed in a similar manner, if not always with
such important results, a great variety of questions in morphology
and systematic botany; even purely physiological problems were raised
by him in this peculiar way, and especially the question how the
fertilising matter of the pollen-grains is conveyed to the ovule;
he had already concluded from the position of the embryo that it is
conveyed through the micropyle and not through the raphe and the hilum,
as was then supposed, and he was the first also to follow the passage
of the pollen-tubes in the ovary of Orchids up to the ovules; but this
is a point which will be more properly considered in the history of the
sexual theory.
The peculiar character of the natural system as compared with every
artificial arrangement is brought out into higher relief by Robert
Brown than by Jussieu and De Candolle, and he succeeded better
than any of his predecessors in separating purely morphological
and systematically valuable relations of organisation from the
physiological adaptations of organs. While the majority of systematists
surrendered themselves to the guidance of a blind feeling in the
discovery of affinities, their correct determinations being the
accidental result of instinct and unconscious operations of the
understanding, Brown endeavoured to give an account to himself in every
case of the reasons why he took this or that view of the relationships
which he determined; from what was already established and indubitable
he gathered the value of certain marks, in order to obtain rules for
the determination of unknown relationships. In this way he discovered
also, that marks, which are of great value for classification within
the limits of certain groups of affinity, may possibly prove to be
valueless in other divisions. Thus Robert Brown in his numerous
monographs supplied the model, by which others might be guided in
further applying and completing the method of the natural system; and
in this respect he was met by the botanists of Germany in the spirit
of the best good-will and most profound appreciation, as is shown
by the fact that a collection of his botanical works, translated by
different German botanists, was edited in five volumes by Nees von
Esenbeck as early as the period between 1825 and 1834. The natural
system established itself in Germany through the labours of Brown and
De Candolle; and the more correct appreciation of it as compared with
the sexual system of Linnaeus was promoted by a work of Carl Fuhlrott
which appeared in 1829, in which the systems of Jussieu and De Candolle
are compared with those of Agardh, Batsch, and Linnaeus, and the
superiority of the natural system is clearly set forth. A still greater
effect in this direction was produced by the appearance in 1830 of the
‘Ordines naturales plantarum’ of Bartling, an independent contribution
to this department of botany, and a distinct advance upon what had
hitherto been effected. The contemporary monographs of Roeper on the
Euphorbiaceae and Balsamineae and his treatise ‘De organis plantarum’
(1828), are an able, independent, and logical application of the
principles of the morphology of the flower laid down by De Candolle and
Brown to the elucidation of morphological and systematic conceptions.
But the new methods of investigation introduced by De Candolle and
Robert Brown had to encounter in Germany, and to some extent in France
also, not only the antiquated views of Linnaeus, but, what was still
worse, the erroneous notions of the nature-philosophy founded by
Schelling. The misty tenets of this philosophy could scarcely find
a more fruitful soil than the natural system with its mysterious
affinities, and Goethe’s doctrine of metamorphosis contributed not
a little to increase the confusion. These historical phenomena will
be further considered in the following chapter; at present we are
more concerned to show how the professed systematists pursued the
path opened by De Candolle and Brown. And here it must be noticed
that from about the year 1830, in Germany especially, morphological
enquiry became separated as a special subject from systematic botany;
it became more and more the fashion to treat the latter as independent
of morphology, and thus to forsake the source of deeper insight which
comparative and genetic morphology alone can open to the systematist;
morphology on the other hand took a new flight, and as it thus
developed itself apart from pure systematic botany, its progress must
be described by itself in a later portion of this history.
If advance in systematic botany depended on the number of systems that
were proposed from 1825 to 1845, that period must be looked upon as
its golden age; no less than twenty-four systems made their appearance
during these twenty years, without counting those which were inspired
by the views of the nature-philosophy. There was great and spreading
growth, but no corresponding depth; no really new points of view were
opened for classification, and as regards the true principles of the
natural system there were symptoms of evident decline rather than of
advance, as will be shown below. Improvements were effected certainly
in the details of the system, since botanists generally adhered to
the principles laid down by De Candolle, Jussieu, and Brown. Families
were cleared up and better defined, and groups of families were
proposed which assumed more and more the appearance of natural cycles
of relationship. The class more especially treated was the extensive
one of the Dicotyledons, in which the families, continually growing
more and more numerous, were in Jussieu’s arrangement a chaos, but had
been united into larger groups in a somewhat artificial manner by De
Candolle. Here we see once more how the formation of the system rises
step by step from the particular to the more general; at an earlier
period genera were constructed out of species, and families out of
genera, and during the years from 1820 to 1845 the families were united
into more comprehensive groups; but these orders or classes were not
yet grouped together in such a manner as to ensure the separation of
the largest divisions of the vegetable kingdom in a natural manner.
The great class of Dicotyledons is not even yet so arranged that the
smaller aggregates of families connect satisfactorily one with another.
Nevertheless a considerable advance was made by the establishment of a
large number of smaller groups of families, and Bartling and Endlicher
were especially successful in founding such groups and supplying them
with names and characters.
If on the other hand we turn to the primary divisions of the vegetable
kingdom, we find that certain large and natural groups came to be most
generally recognised and placed in the front rank in every scheme; such
were the groups of the Thallophytes, Muscineae, Vascular Cryptogams,
Gymnosperms, Dicotyledons and Monocotyledons. But the coordination
of these great divisions of the whole vegetable kingdom was far from
being rightly understood. It was usage rather than anything else, which
gradually put them forward as primary types; in the systems themselves
some received too great, others too little prominence, or other groups
of doubtful character were admitted alongside of them. Bartling, for
instance, whose system up to 1850 or even longer may rank as one of
the most natural, adheres to De Candolle’s division of the vegetable
kingdom into cellular and vascular plants, and rightly divides the
former into two main groups, Thallophytes and Muscineae (Homonemeae and
Heteronemeae), while he separates the latter into Vascular Cryptogams
and Phanerogams; but the Phanerogams are divided into Monocotyledons
and Dicotyledons, which again are distributed into four groups, one
of these being characterised by the presence of a vitellus, that is,
of an endosperm surrounded by a perisperm,—a thoroughly artificial
division. The three other divisions are named apetalous, monopetalous,
and polypetalous, but the Coniferae and Cycadeae are placed in
the apetalous division. Less satisfactory is the primary division
into Thallophytes and Cormophytes proposed by ENDLICHER[39], the
latter separating into the divisions Acrobrya (Muscineae, Vascular
Cryptogams, and Cycads), Amphibrya (Monocotyledons), and Acramphibrya
(Dicotyledons and Conifers); the names of the three latter groups,
the first of which is utterly unnatural, are founded on erroneous
assumptions respecting growth in length and thickness, which Endlicher
borrowed from Unger. While Endlicher’s great work has continued down
to our own time to be indispensable to the botanist as a book of
reference on account of the fulness of its descriptions of families
and genera, the system projected by Brongniart in 1843 has acquired a
sort of official authority in France. The whole vegetable kingdom is
here distributed into two divisions, Cryptogams and Phanerogams, and
the former are incorrectly characterised as asexual, the latter as
having distinction of sex. The Phanerogams, divided into Monocotyledons
and Dicotyledons, are distributed into groups in a manner that is
not satisfactory; but the system has one merit, that it keeps the
Gymnosperms together in one body; and if they are incorrectly classed
with the Dicotyledons, it was still a sign of progress, that Robert
Brown’s discovery of gymnospermy was to some extent practically
recognised. The system devised by JOHN LINDLEY[40] attained to about
the same importance in England as attached to those of Bartling
and Endlicher in Germany, and that of Brongniart in France. After
various earlier attempts he proposed a system in 1845, in which, as
in Brongniart’s arrangement, the Cryptogams are characterised as
asexual or flowerless plants, the Phanerogams as sexual or flowering
plants; the former are divided into Thallogens and Acrogens, the
Phanerogams into five classes; (1) Rhizogens (Rafflesiaceae, Cytineae,
Balanophorae); (2) Endogens (parallel-nerved Monocotyledons); (3)
Dictyogens (net-veined Monocotyledons); (4) Gymnogens (Gymnosperms);
(5) Exogens (Dicotyledons). This classification is one of the most
unfortunate that were ever attempted; the systematic value of the
Rhizogens is much overrated on account of their striking habit; the
Monocotyledons are separated into two classes on the strength of an
unimportant mark. The characters assigned to all these groups are on
the whole thoroughly faulty.
These systems have been selected for notice from among many others,
because they attained an extended notoriety and importance from the
circumstance that their authors, Brongniart excepted, made them the
occasions of comprehensive descriptions of the whole vegetable kingdom,
and again because it would be superfluous for our present purpose to
bestow a closer consideration on the systems of less eminent men.
Whoever desires further information on the matter will find it in the
introduction to Lindley’s ‘Vegetable Kingdom’ of 1853.
If we consider the principles and points of view adopted in these
systems, one thing especially strikes us, that, except in the case
of Bartling, physiologico-anatomical marks were employed along with
morphological ones to characterise the primary divisions; their authors
fell into the mistake committed by De Candolle, and unfortunately
these very marks rested in part or wholly on misapprehensions, as in
Endlicher’s division into Acrobrya, etc., and Lindley’s classes of
Rhizogens and Dictyogens. It was still more unfortunate that individual
systematists obstinately refused to accept well authenticated facts,
which it is true had not been discovered by systematists, but were
nevertheless of the highest value for the system. It is scarcely
credible that Lindley in 1845, and again in 1853, maintained the
distinction between endogenous and exogenous growth in stems, though
Hugo von Mohl had in 1831 produced decisive proof that this distinction
laid down by Desfontaines and adopted by De Candolle had no real
existence. The same was the case with the characters of the Cryptogams,
in which the mark of having no sexual organs was repeatedly adopted
as running through the whole class, although various instances of
sexuality in Cryptogams were known before 1845; Schmidel had described
the sexual organs of the Liverworts about the middle of the previous
century, Hedwig those of the Mosses in 1782, and Vaucher in 1803 had
suggested that the conjugation of Spirogyra among the Algae should be
regarded as a sexual act; the systematists in fact did not know what to
make of these intimations.
It was again a misfortune that the systematists in their labours
often neglected to distinguish between the search for marks and the
use to be made of them; the examination of all possible marks should
lead to the establishing the systematic importance of certain fixed
marks or their value for classification. When research has done its
work, then it is sufficient in exhibiting the system to put forward
only the prominent marks; and frequently a single one suffices to
unite a natural group. Such a leading mark is like the standard of a
regiment; its significance is not great in itself, but it serves the
great practical purpose of indicating a whole group of marks which are
connected with it. It was a still greater misfortune that scarcely any
systematist after De Candolle endeavoured to form a clear conception
in his own mind of the principles on which the natural system must be
elaborated, and to set them forth in a connected form as the theory
of the system. The student had to accept the arrangement offered
him as a fact simply without understanding it, and the systematists
themselves usually followed only a blind feeling in the framing of
their groups, and never unfolded the grounds of their proceeding
with logical distinctness. In this respect John Lindley forms an
honourable exception, inasmuch as he did, on several occasions after
1830, give full expositions of his views on the principles of natural
classification, and like De Candolle endeavoured to develop a theory
of the system[41]. But he deserves credit only for the endeavour,
for the principles themselves which he laid down are not only to a
great extent incorrect, but they are opposed to his own and to every
other natural system. We find this opposition between theory and
practice much more strongly marked in Lindley than in De Candolle; the
cases only are so far different, that De Candolle laid down correct
principles for the determination of affinities, but in some cases
did not follow them, whereas Lindley deduced quite incorrect rules
of system from existing and long-established natural affinities. The
consideration of all the systems framed up to the year 1853 shows
clearly that the characters of truly natural groups are to be found
only in morphological marks; yet Lindley enunciates the principle that
a mark, or, as he incorrectly says, an organ, is more important for
classification in proportion as it possesses a higher physiological
value for the preservation and propagation of the individual. If this
were true, nothing would be easier than to frame a natural system of
plants; it would suffice to divide plants first of all into those
without and those with chlorophyll, for the presence of chlorophyll
is more essential than that of any other substance to the nourishment
of plants, and its physiological importance is therefore pre-eminent;
in that case of course such Orchideae as have no chlorophyll, the
Orobancheae, Cuscuta, Rafflesia, etc., would form one class with the
Fungi, and all other plants the other. It is very important for the
existence of a plant whether its organisation is adapted to its growing
in water, or on dry land, or underground, and if we took Lindley
at his word, he would be obliged to bring the Algae, Rhizocarps,
Vallisnerias, water Ranunculuses, Lemna, etc., into one group. It is
very important for the existence of a plant whether it grows upright
of itself, or climbs upwards by the aid of tendrils or of a twining
stem or otherwise, and accordingly we might on Lindley’s principle
collect certain ferns, the vine, the passion-flower, many of the pea
kind, etc., into one order. It is obvious that Lindley’s main axiom
of systematic botany appears in this way utterly unreasonable; yet
by this principle he judges of the systematic value of anatomical
characters, those of the embryo and endosperm, of the corolla and the
stamens, everywhere laying stress on their physiological importance,
which in these parts has really little systematic value. This mode of
proceeding on the part of Lindley, compared with his own system, which
with all its grave faults is still always a morphologically natural
system, proves that like many other systematists, he did not literally
and habitually follow the rules he himself laid down, for if he had,
something very different from a natural system must have been the
result. The success which was really obtained in the determination of
affinities was due chiefly to a correctness of feeling, formed and
continually being perfected by constant consideration of the forms of
plants. It was still therefore virtually the same association of ideas
as in de l’Obel and Bauhin, operating to a great extent unconsciously,
by which natural affinities were by degrees brought to light; and men
like Lindley, of pre-eminent importance as systematists, were, as the
above examples show, never clear about the very rules by which they
worked. And yet in this way the natural system was greatly advanced in
the space of fifty years. The number of affinities actually recognised
increased with wonderful rapidity, as appears from a comparison of the
systems of Bartling, Endlicher, Brongniart, and Lindley, with those
of De Candolle and Jussieu. Nothing shows the value of the systems
thus produced before 1850 as classifications of the vegetable kingdom
more forcibly than the fact that a clear and methodical thinker like
Darwin was able to draw from them the chief supports of the theory
of descent. For it is quite certain that Darwin has not framed his
theory in opposition to morphology and system, and drawn it from any
hitherto unknown principles; on the contrary, he has deduced his most
important and most incontestable propositions directly from the facts
of morphology and of the natural system, as it had been developed up to
his time. He is always pointing expressly to the fact that the natural
system in the form in which it has come to him, which he accepts in the
main as the true one, is not built upon the physiological, but upon
the morphological value of organs; it may, he says, be laid down as a
rule, that the less any portion of the organisation is bound up with
special habits of life, the more important it is for classification.
Like Robert Brown and De Candolle, he insists upon the high importance
for purposes of classification of aborted and physiologically useless
organs; he points to cases in which very distant affinities are brought
to light by numerous transition-forms or intermediate stages, of which
the class of the Crustaceae offers a specially striking example in
the animal kingdom, while certain series of forms of Thallophytes,
the Muscineae, the Aroideae and others, may be adduced as instances
of the same kind in the vegetable world; in such cases the most
distant members of a series of affinities have sometimes no one common
mark, which they do not share with all other plants of a much larger
division. From these and other similar statements of Darwin we see
plainly, that he actually did gather from existing natural systems
of plants and animals the rules by which systematists had worked,
but which they themselves observed only more or less unconsciously,
and never with a full and clear recognition of them. He says quite
rightly, when the investigators of nature are practically engaged with
their task, they do not trouble themselves about the physiological
value of the characters which they employ for the limiting a group
or the establishment of a single species. Darwin clearly perceived
and consistently kept in view the discordance between the systematic
affinity of organisms and their adaptation to the conditions of life,
which De Candolle had already but imperfectly recognised. The clear
perception of this discordance was in fact the one thing needed to
mark the true character of the natural system, and to make the theory
of descent appear as the only possible explanation of it. The fact
which morphologists and systematists had painfully brought to light,
but had not sufficiently recognised in its full importance, that
two entirely different principles are united in the nature of every
individual organism, that on the one hand the number, the arrangement,
and the history of the development of the organs of a species point to
corresponding relations in many other species, while on the other hand
the manner of life and the consequent adaptation of the same organs
may be quite different in these allied species. This fact admits of no
explanation but the one given by the theory of descent; it is therefore
the historical cause and the strongest logical support of that theory,
and the theory itself is directly deduced from the results which the
efforts of the systematists have established. That the majority of
systematists did at first distinctly declare against the theory of
descent can surprise no one who observes that they were so little able
to give an account of their own mode of procedure, as appears in so
striking a manner from Lindley’s theoretical speculations.
One consequence of this want of clearness in combination with the
dogma of the constancy of species has been already mentioned in the
introduction; namely, the notion professedly adopted by Lindley,
Elias Fries, and others, that an idea lies at the foundation of every
group of affinities, that the natural system is a representation of
the plan of creation. But the question, how such a plan of creation
could explain the strange fact that the physiological adaptations
of organs to the conditions of life have nothing at all to do with
their systematic connection, was quietly disregarded; and in fact the
notion, founded on Platonic and Aristotelian philosophy, of a plan
of creation and of ideal forms underlying systematic groups, could
not explain this discordance between morphological and physiological
characters. It would be easy to maintain the view of the systematists,
that the natural system represents a plan of creation, if physiological
and morphological characters went always truly hand in hand, if the
adaptation of the organs to the conditions of life in the species were
perfect; but facts show that the adaptation is in the best of cases
comparatively imperfect, and that it is in all cases brought about by
the accommodation to new requirements of organs which originally served
to other functions.
CHAPTER IV.
MORPHOLOGY UNDER THE INFLUENCE OF THE DOCTRINE OF METAMORPHOSIS AND OF
THE SPIRAL THEORY.
1790-1850.
The efforts of Jussieu, De Candolle, and Robert Brown were directed to
the discovery of the relationship between different species of plants
by comparing them together; the doctrine of metamorphosis founded
by Goethe set itself from the first to bring to light the hidden
relationship between the different organs of one and the same plant.
As De Candolle’s doctrine of symmetry derived the different species
of plants from an ideal plan of symmetry or type, so the doctrine
of metamorphosis assumed an ideal fundamental organ, from which the
different leaf-forms in a plant could be derived. The stem came into
consideration only as carrying the leaves, the root was almost entirely
disregarded. As the resemblance of nearly allied species of plants
suggests itself naturally and unsought to the mind of the unbiassed
observer, so also does the connection between different organs of a
leafy nature in one and the same plant. Cesalpino called the corolla
simply a ‘folium’ (leaf); he and Malpighi regarded the cotyledons also
as leaves; Jung called attention to the variety of the leaf-forms,
which are found in many plants at different heights on the same stem;
Caspar Friedrich Wolff, the first who bestowed systematic consideration
on the subject, declared in 1766, that he saw nothing ultimately in
the plant but leaves and stem, including the root in the stem[42].
Long before Goethe’s time speculation had busied itself with attempts
to explain these observations; we saw how Cesalpino and Linnaeus,
starting from the old view that the pith is the seat of the soul in
plants, regarded the seeds as metamorphosed pith, the floral envelopes
with the stamens and the true leaves as metamorphosed layers of the
rind and wood of the stem. The word metamorphosis from their point
of view had a very plain meaning; it was really the cylindrical pith
whose upper end changed into seeds, it was the actual substance of the
cortex which produced both the ordinary leaves and the parts of the
flower. Wolff on the other hand from a point of view of his own gave
an apparently intelligible physical explanation of the proposition,
that all appendages of the stem are leaves, but the explanation had
the fault of not being true; he attributed the metamorphosis of leaves
to altered nourishment, the flowers especially to his ‘vegetatio
languescens.’
GOETHE’S conception of the matter was from the first much less
clear, and chiefly because he was never able to bring the abnormal
into its true connection with the normal or ascending metamorphosis.
In the first sentence of his ‘Doctrine of metamorphosis’ (1790) he
says, ‘that it is open to observation that certain exterior parts of
plants sometimes change and pass into the form of adjacent parts,
either wholly or in a greater or less degree.’ In the cases of
which Goethe is here thinking a distinct meaning can be affixed to
the word metamorphosis; if, for example, the seeds of a plant with
normal flowers produce a plant which has petals in place of stamens,
or in which the ovaries are resolved into green expanded leaves, it
is actually the case that a plant of a known form has given rise
to another plant of a different form, in other words, a change or
metamorphosis has really taken place. But we cannot reason in this
way in the case of that which Goethe calls normal or ascending
metamorphosis. When in a given species, which has remained constant
with all its marks for countless generations, the cotyledons, the
leaves, the bracts, and the parts of the flower are called leaves,
this must be merely the result of abstraction, which has led to the
generalising of the idea of a leaf; if we make abstraction of the
physiological characters of the carpels, stamens, floral envelopes,
and cotyledons, and regard only the way in which they originate on
the stem, we are justified in including them in one general idea
with ordinary leaves, and to this idea we quite arbitrarily give
the name leaf. But this does not justify us in speaking of a change
of these organs, so long as we consider the whole plant in question
as a hereditary and constant form. For the plant therefore taken as
constant the idea of metamorphosis has only a figurative meaning; the
abstraction performed by the mind is transferred to the object itself,
if we ascribe to it a metamorphosis which has really taken place only
in our conception. The case would be different, if here as well as in
the abnormal instances above-mentioned we could assume that the stamens
and other organs of the plants lying before us were ordinary leaves in
their progenitors. So long as this assumption of an actual change is
not even hypothetically made, the expression change or metamorphosis is
purely figurative, the metamorphosis is a mere ‘idea.’ This distinction
Goethe has not made; he did not clearly see that his normal ascending
metamorphosis can only have the meaning of a scientific fact, if a
real change is assumed to take place in the course of propagation
in this case, as in that of abnormal metamorphosis or misformation.
A comparison of his various expressions shows that he took the word
metamorphosis sometimes in its literal, sometimes in its ideal and
figurative sense; for instance, he says expressly, ‘We may say that
a stamen is a folded petal, just as we may say that a petal is a
stamen in a state of expansion.’ This sentence shows that Goethe did
not regard a particular leaf-form as first in time, and that others
proceeded from it by change; he uses the word metamorphosis in a purely
ideal sense. At other times his remarks may be interpreted as though
he really considered the normal ascending metamorphosis to be a real
change in the organs, arising from a transmutation of the species.
With this confusion of notion and thing, idea and reality, subjective
conception and objective existence, Goethe took up exactly the position
of the so-called nature-philosophy.
Goethe’s doctrine could only make its way to logical consistency and
clearness of thought by deciding for the one or the other way; he must
either assume that the different leaf-forms, which were regarded as
alike only in the idea, were really produced by change of a previous
form,—a conception that at once presupposes a change of species
in time; or he must entirely adopt the position of the idealistic
philosophy, in which idea and reality coincide. In this case the
assumption of a change in time was not necessary; the metamorphosis
remained an ideal one, a mere mode of view; the word leaf then
signifies only an ideal fundamental form from which the different forms
of leaves actually observed may be derived, as De Candolle’s constant
species from an ideal type.
If now we read Goethe’s further remarks on the doctrine of
metamorphosis attentively[43], we perceive that he really arrived at
neither of these conclusions, but perpetually vacillated between the
two; a number of his sayings might be collected, which might be taken
for precursors of a theory of descent, as they have been taken by some
modern writers; but it is quite as easy to make a selection which would
carry us back to the position of the ideal philosophy and the constancy
of species. In the later years of his life the idea of a physical
metamorphosis accomplished in time, and involving a change of species,
does appear more distinctly in Goethe’s writings. This explains
the lively, nay passionate, interest which he took in the dispute
between Cuvier and Geoffrey de St. Hilaire in 1830[44]. We gather from
it that Goethe, in spite of all his wanderings in the mists of the
nature-philosophy of the time, felt a growing need for some clearer
insight into the nature of metamorphosis, both in plants and animals,
without ever being able to make his way into the clear light.
But these better motions remained without importance for the history of
botany; the adherents of his doctrine of metamorphosis all apprehended
it in the sense of the nature-philosophy, and Goethe himself did
not remonstrate against the frightful way in which it was distorted
by them. Its further development therefore was in accordance with
the principles of that philosophy, which was accustomed to apply
the results of purely idealistic views in an uncritical way to
imperfectly observed facts. Above all the difficulty remained unsolved,
how the dogma of the constancy of species was to be brought into
logical connection with the idea of the metamorphosis of organs. The
supranatural, which Elias Fries found in the natural system, subsisted
still in the doctrine of metamorphosis in comparing the organs of a
plant.
Still more obscure and entirely the product of the nature-philosophy is
Goethe’s view of the spiral tendency in vegetation. At p. 194 of his
essay entitled ‘Spiraltendenz der Vegetation’ (1831) he says: ‘Having
fully grasped the idea of metamorphosis we next turn our attention to
the vertical tendency, in order to gain a nearer acquaintance with
the development of the plant. This tendency must be looked upon as an
immaterial staff, which supports the existence.... This principle of
life (!) manifests itself in the longitudinal fibres which we use as
flexible threads for many purposes; it is this which forms the wood in
trees, which keeps annual and biennial plants erect, and even produces
the extension from node to node in climbing and creeping plants. Next
we have to observe the spiral direction which winds round the other.’
This spiral direction which passes at once with Goethe into a ‘spiral
tendency,’ is seen in various phenomena of vegetation, as in spiral
vessels, in twining stems, and sometimes in the position of leaves.
The closing remarks of this short essay, in which he explains the
vertical tendency as the male, the spiral as the female principle in
the plant, show how far Goethe lost himself in the profundities of the
nature-philosophy. Thus he introduced his readers into the deepest
depths of mysticism.
It would be as useless as it would be wearisome to follow out in detail
to its extremest point of absurdity the progressive transformation
which the doctrine of metamorphosis underwent in the hands of the
botanists of the nature-philosophy school, and to see how its
catchwords, polarity, contraction and expansion, the stem-like and
the fistular, anaphytosis and life-nodes, and others, were compounded
with the results of the most every-day observation into meaningless
conglomerates; rough obscure impressions of the sense, as well as
incidental fancies, were regarded as ideas and principles. A full
account of these inconceivable aberrations is to be found in Wigand’s
‘Geschichte und Kritik der Metamorphose.’ Our own countrymen certainly,
Voigt, Kieser, Nees von Esenbeck, C. H. Schulz, and Ernst Meyer
(the historian of botany) bear off the palm of absurdity, but there
were others also, among them the Swedish botanist Agardh, and some
Frenchmen, Turpin, for instance, and Du Petit-Thouars[45], who were not
altogether free from this weakness. Even the best German botanists
of the time, such as Ludolph Treviranus, Link, G. W. Bischoff, and
others, managed to escape the influence of this philosophy of nature,
only where they confined themselves to the most barren empiricism.
Strange phenomenon! that as soon as gifted and understanding men began
to talk of the metamorphosis of plants, they fell into senseless
phrase-mongering; Ernst Meyer, for instance, was it is true no great
botanist, but he shows in his ‘Geschichte der Botanik’ that he
possessed a clever and cultivated intellect. The painful impression,
which the treatment of the doctrine of metamorphosis by these writers
makes upon us, is due partly to the fact that the deeper meaning
of the idealistic philosophy never attained to logical expression
in their hands, and still more to their indulgence in an unmeaning
play of phrases, combining the highest abstractions with the most
negligent and rudest empiricism, and sometimes with utterly incorrect
observations. Oken can claim the merit of more correct observation and
greater philosophical consistency, and if we reject his views, yet
his mode of presenting them has at least the pleasing appearance of
more consequential reasoning. We perceive for the first time the full
greatness of the debt which modern botany owes to men like Pyrame de
Candolle, Robert Brown, von Mohl, Schleiden, Nägeli, and Unger, the
latter of whom only slowly worked his way out of the trammels of the
nature-philosophy, when we compare the literature of the doctrine of
metamorphosis before the year 1840 with the present condition of our
science, for which they paved the way.
In spite of the real and apparent differences between Goethe’s
doctrine of metamorphosis and De Candolle’s doctrine of a plan of
symmetry, these writers agreed in this, that they set out alike from
the doctrine of the constancy of species, and led up equally to the
result, that alongside of manifold physiological differences in the
organs of plants certain points of formal agreement can be discovered,
which are expressed chiefly in the order of their succession and in
their relative positions. In this distinction lay the good kernel
of the doctrine of metamorphosis in Goethe, and Wolff, and even in
Linnaeus and Cesalpino: it was only necessary to set this free from
the dross with which the nature-philosophy had surrounded it, and
to make the relations of position in organs the subject of earnest
investigation, in order to secure important results in this branch
of morphology. The first step in this direction was taken by Carl
Friedrich Schimper, who was followed by Alexander Braun; both adopted
the main idea of the doctrine of metamorphosis in the form in which it
can be reconciled with the doctrine of constancy, that is, in a purely
idealistic sense. Both liberated themselves from the gross errors of
the nature-philosophers, and thus gave a more logical expression to the
purely idealistic morphological consideration of form in plants.
KARL FRIEDRICH SCHIMPER[46] founded before the year 1830 the theory
of the arrangement of leaves which is named after him, and which
he expounded to the naturalists assembled at Stuttgart in 1834 as
a complete and perfected system. Alexander Braun, in a review of
Schimper’s exposition in ‘Flora’ of 1835, gave a clear and simple
account of the theory, having already himself published an excellent
and comprehensive treatise on the same subject. The doctrine of
phyllotaxis appeared in these publications with a formal completeness
which could not fail to attract the attention of the botanical world
and indeed of a larger audience; and justly so, for, as unfortunately
so very seldom happens in botanical subjects, a scientific idea was in
this case not merely incidentally suggested, but was worked out in all
its consequences as a complete structure, and this structure gained in
external splendour from the circumstance that its propositions, dealing
with geometrical constructions, could be expressed in numbers and
formulae,—a thing hitherto unknown in botanical science.
That the leaves are arranged on the stems that produce them according
to fixed geometrical rules had been noticed by Cesalpino and by
Bonnet in the middle of the eighteenth century; but nothing more
resulted than weak attempts at mere description of different cases.
Schimper’s theory is marked by that which is at once its greatest
merit and its fundamental error, the referring of all relations of
position to a single principle. This principle lies in the idea
that growth in a stem has an upward direction in a spiral line, and
that the formation of leaves is a local exaggeration of this spiral
growth. The direction of the spiral line may change in the same
species, or in the same axis, and may even change from leaf to leaf.
The important variations in the arrangement of leaves are not shown
in their longitudinal distances, but in the measure of their lateral
deviations on the stem. The characteristic point in this theory is the
mode of considering these lateral deviations or divergences of the
leaves as they follow one another on an axis, the referring them to a
more general law of position. Means were at the same time skilfully
supplied for discovering the true conditions of arrangement, the
genetic spiral, in cases where the genetic succession of the leaves,
and consequently their divergence, could not be immediately recognised.
After innumerable observations, it appeared that there is a wonderful
variety in the disposition of leaves, but that at the same time a
comparatively small number of these variations commonly occur, and
that these ordinary divergences 1/2, 2/3, 3/8, 8/13, 13/21, etc. have
this remarkable relation to one another, that both the numerator and
denominator of each successive fraction are obtained by adding together
the numerators and denominators of the two preceding fractions, or
the individual fractions named are the successive convergents of a
continuous fraction:—
1
1 + 1
——-
1 + 1
——-
1....
By change of single cyphers in this, the simplest of all continuous
fractions, the expressions were also obtained for all measures of
position that deviate from the usual main series. The common occurrence
of so-called leaf-whorls seemed at once to be opposed to the principle
of special growth and to the doctrine of position founded upon it,
especially in the cases in which it was supposed that all the leaves
of a whorl arise simultaneously. But the founders of the doctrine,
relying on their geometrical constructions, declared that every
theory is incorrect, which sets out from the whorl as a simultaneous
formation. But the way in which the different leaf-whorls of a stem are
arranged among themselves, and are connected with continuous spiral
positions, required new geometrical constructions; it was necessary
to assume a supplementary relation (prosenthesis), which the measure
of the phyllotaxis adopts in the transition from the last leaf of
one cycle to the first of the next. Artificial as this construction
appears, it has the advantage of saving the spiral principle, and the
prosenthetic relation itself admits of being again expressed in highly
simple fractions,—a great advantage for the formal consideration of
the relative positions of the parts of the flower, and their relation
to the preceding positions of the leaves. The great skill shown by the
founders of the doctrine in the morphological consideration of the
whole plant-form appears equally in the establishment of the rules,
according to which the relations of position of the leaves of a
side-shoot connect with those of the mother-axis, and which made it
possible to represent the nature of inflorescences especially with
extreme clearness by means of geometrical figures. An expressive and
elegant terminology not only made the whole theory attractive, but
fitted it in a high degree to supply a suitable, plain, and precise
phraseology for describing the most varied forms of plants. That the
theory possesses such advantages as these may be gathered from the
fact, that since 1835 the morphological examination and comparison
not only of flowers and inflorescences, but also of vegetative shoots
and their ramification, has reached great formal completeness. A
thorough acquaintance with the principle of this doctrine has made it
possible to explain to reader or hearer the most intricate forms of
plants so clearly, that they may be said to reveal the law of their
formation themselves, and to grow before the eye of the observer,
while at the same time the most recondite relations of the organs of
the same or of different plants were brought out distinctly and in
elegant phraseology. When this mode of description was combined with
De Candolle’s views on abortion, degeneration, and adherence, and at
the same time took into consideration the chief physiological forms
of leaf-structures, according as these were developed as scales,
foliage-leaves, bracts, floral envelopes, staminal and carpellary
leaves, it was possible to give such an artistic account of every form
of plant, as made it visible to sense in its entirety, and at the same
time brought out the morphological law of its construction. Whoever
reads the writings of Alexander Braun and Wydler, and especially of
Thilo Irmisch (after 1873), who knew how to combine his descriptions in
a variety of ways with remarks on the biological relations of plants,
cannot fail to admire the extraordinary skill displayed by these men in
describing plants. Compared with the dry diagnoses of the systematists,
their descriptions attain to the dignity of an art, and present the
commonest forms to the reader in a new and attractive light. But the
theory had a further advantage; it seemed not only to present the form
of the plant in its matured state, but to treat it genetically; and
in fact it did possess an element of historical development, inasmuch
as it made the genetic succession of the leaves and of their axillary
shoots, which is at the same time the succession from the base to the
summit, the foundation of all consideration of the plant-form. But it
is also true that in this lay one of the weak sides of the theory; as
long as it was a question only of continuous spirals, the succession of
matured leaves does also represent the succession of their formation
in time; but this was not actually proved in the case of leaf-whorls,
and here, to save the theory, genetic relations had to be presupposed
for which no further proof was forthcoming, while fresh researches
have repeatedly shown that a strict application of Schimper’s theory
is found frequently to contradict the facts of development as directly
observed[47]. Moreover, regard was had only to those measurements
of divergence on the continuous genetic spiral which were taken on
the matured stem, while there was always the possibility that the
divergences might have been different at the first, and been afterwards
modified, as Nägeli subsequently suggested[48]. And again, the theory
had a dangerous adversary to encounter in the frequent occurrence
of leaves that are strictly alternate or crossed in pairs, and to
conceive of this as a spiral arrangement must at once appear to be
an arbitrary proceeding both from the mathematical point of view and
from that of historical development; the assumption of a return of the
genetic spiral from leaf to leaf, as for instance in the Grasses, like
the prosenthesis in the change of divergence, afforded, it is true, a
construction which was geometrically correct, but which could hardly
be made to agree with the history of development and the mechanical
forces concerned. Again, it was a great and essential defect in the
theory, that in assuming the spiral arrangement it entirely neglected
the relations of symmetry of the plant-form, which are in many cases
clearly expressed, and their connection with the outer world, on
which Hugo von Mohl had already published some excellent remarks in
1836,—a defect, which unhappily is not yet sufficiently appreciated. A
due consideration of these objections, and of the cases in which the
history of development is opposed to the constructions of the theory,
must have led to the conviction that the idea of a spiral tendency in
the growth of plants is at least not borne out in all cases, and more
profound reflexion would show, that a scientific principle, really
explaining the phenomena, is no more to be found in the assumption
of such a general tendency, than in a like assumption with regard to
the heavenly bodies, that they have a tendency to elliptic movement
because they commonly move in ellipses. Hence Hofmeister, the latest
investigator of the doctrine of phyllotaxis on the basis of the
history of development, comes to the conclusion that the notion of a
screw-shaped or spiral course of evolution of lateral members of plants
is not merely an unsuitable hypothesis, but an error. Its unreserved
abandonment is, he considers, the first condition for attaining an
insight into the proximate causes of the varieties of relative position
in the vegetable kingdom. But this judgment, correct as it is, was
pronounced thirty years after the appearance of Schimper’s theory;
history, which speaks from another point of view, and not only enquires
into the correctness of a theory but has to appraise its historical
importance, speaks in a less unfavourable manner. The chief point here
is not whether the theory was right, but how far it contributed to the
advance of the science. It was distinctly fruitful in results, for it
brought the important question of the relative positions of organs
for the first time into the front rank in the study of morphology; we
may even say that a large part of the results of the study of the
history of development were first brought into the true light by the
consistent application of the theory, or in the effort to disprove it.
With all its fundamental errors, Schimper’s theory remains one of the
most interesting phenomena in the history of morphology, because it was
carried out with thorough logical consistency. We should as little wish
to omit it from our literature, as modern astronomy would wish to see
the old theory of epicycles disappear from its history. Both theories
served to connect together the facts that were known in their time.
The fundamental error of the theory lies much deeper than appears at
first sight. Here too we have the idealistic conception of nature,
which refuses to know anything of the causal nexus, because it takes
organic forms for the ever-recurring copies of eternal ideas, and
in accordance with this platonic sphere of thought confounds the
abstractions of the mind with the objective existence of things. This
confusion shows itself in Schimper’s doctrine, inasmuch as he takes
the geometrical constructions, which he transfers to his plants and
which, though they may be highly suitable from his point of view, are
nevertheless purely arbitrary, for actual characters of the plants
themselves, in other words, takes the subjective connection of the
leaves by a spiral line for a tendency inherent in the nature of the
plant. Schimper in making his constructions overlooked the fact that,
because a circle can be described by turning a radius round one of its
extremities, it does not follow that circular surfaces in nature must
really have been formed in this way; in other words, he did not see
that the geometrical consideration of arrangements in space, useful as
it may otherwise be, gives no account of the causes to which they are
due. But this was not properly an oversight in Schimper’s case, for he
would have scarcely admitted efficient causes in the true scientific
sense into his explanations of the form of plants. How far Schimper
was from regarding plants as something coming into being in time and
according to natural laws, how profoundly he despised the principles of
modern natural science is shown in his judgment of Darwin’s theory of
descent and of the modern atomic theory, the coarseness of which is the
more surprising, because Schimper was a man of refined and even poetic
feeling. ‘Darwin’s doctrine of breeding,’ he says, ‘is, as I discovered
at once and could not help perceiving more and more after repeated and
careful perusal, the most shortsighted possible, most stupidly mean and
brutal, much more paltry even than that of the tesselated atoms with
which a modern buffoon and hired forger has tried to entertain us.’
Here is the old platonic view of nature flying at modern science; the
sternest ‘opposites’ that culture has ever produced.
The theory of Schimper, which should rather be called the theory of
Schimper and Braun, considering the active part which Braun took
from the first in framing and applying it, was capable of further
development only in the mathematical and formal direction, as was shown
especially in Naumann’s essay, ‘Ueber den Quincunx als Grundgesetz der
Blattstellung vieler Pflanzen’ (1845). The defects above described, but
not the merits of the theory were shared by the doctrine of phyllotaxis
laid down about ten years later by the brothers Louis and Auguste
Bravais. Their theory makes use of mathematical formulae to even a
greater extent than that of Schimper without paying any attention to
genetic conditions, and yet it is less consistent with itself, for it
assumes two thoroughly different kinds of phyllotaxis, the positions
in which are arranged in a straight and in a curved line; for the
latter without any apparent reason a purely ideal original divergence
is assumed which stands in irrational relation to the circumference of
the stem, and from it all other divergences should be derivable; and
this ultimately degenerates into mere playing with figures which in
this form afford no deeper insight into the causes of the relations of
position. As regards serviceableness in the methodic description of
plants the theory of the brothers Bravais is much inferior to that of
Schimper[49].
The genetic morphology founded about the year 1840 had to make the best
terms it could with the doctrine of phyllotaxis, which was constructed
on a totally different principle; the two went their way on the whole
side by side without disturbance from one another till the year 1868,
when Hofmeister in his general morphology attacked the principle
of Schimper’s theory, and endeavoured to substitute a genetic and
mechanical explanation of the relative positions for the purely formal
account of them; this attempt however, which from the nature of the
case has not yet led to a finished theory but nevertheless contains the
germ of a further development of this important doctrine, does not come
within the scope of this history.
The doctrine of phyllotaxis of Schimper and Braun, as it appeared
after 1830, had clearly presented only one side of the theory of
metamorphosis; what other elements there were in it capable of being
turned to speculative account were further cultivated by ALEXANDER
BRAUN between the years 1840 and 1860. In this period fresh points of
view were asserting themselves in botanical research; the founding of
the doctrine of cells, the study of the more delicate anatomy of plants
and of the history of development, and increased methodical knowledge
of the Cryptogams were enlarging the repertory of botanical facts,
while the physico-mechanical method of investigation was being more and
more adopted. Braun, who took an active part by his own researches in
this revolution in morphological botany, remained true nevertheless to
idealistic views; and in his frequent and comprehensive discussions of
the general results of the new investigations in accordance with these
views he has shown how far the idealistic platonising contemplation
of nature is in a condition to do justice to the results of exact
inductive enquiry. The opposition between his point of view and that
of the most eminent representatives of the inductive method became
more and more pronounced as years went on, and must be treated here
as a historical fact. But if the new tendency in botany pursued
especially by von Mohl, Schleiden, Nägeli, Unger, and Hofmeister may
be called inductive in the absence of a better term, and be contrasted
with the idealistic tendency represented by Braun and his school, it
must not be supposed that the latter did not equally contribute in
matters of detail to the enriching of the science by the method of
induction; on the contrary, Braun himself was the author of a series
of important works conceived in this spirit. When the new method is
here called inductive, it should be understood that the word is used
in a higher than the usual sense, and some explanation of this point
will not be superfluous in this place. Idealistic views of nature of
all times, whether they present themselves as Platonism, Aristotelian
logic, Scholasticism or modern Idealism, have all of them this in
common, that they regard the highest knowledge attainable by man
as something already won and established; the highest axioms, the
most comprehensive truths are supposed to be already known, and the
task of inductive enquiry is essentially that of verifying them; the
results of observation serve to elucidate already received views,
to illustrate already known truths; inductive enquiry has only to
establish individual facts. But in the sense in which inductive enquiry
was understood by Bacon, Locke, Hume, Kant, and Lange, its task is one
that goes essentially farther than this; it must not be content with
establishing individual facts, but it must employ them in the critical
examination of the most general notions that have come down to us,
and do its best to deduce new and comprehensive theories from them,
even where these may be entirely opposed to traditional views. But it
is part of the very nature of this method of investigation, that its
general results are subject to constant modification and improvement;
each more general truth has only a temporary value, and endures as
long as no new facts militate against it. The distinction therefore
between idealism and the inductive method in the domain of natural
science comes to this, that the former fits new facts into a scheme of
old conceptions, the latter deduces new conceptions from new facts;
the one is in its nature dogmatic and intolerant, the other eminently
critical; the one is conservative, the other always pressing forwards;
the one inclines to philosophic contemplation, the other to vigorous
and productive investigation. To this must be added one point of
great importance; the idealistic view of nature, rejecting causality,
explains nature from notions of design, and is teleological; ethical
and even theological elements are thus introduced into natural science.
It is in this form that the distinction between the idealistic view
represented by Braun and the modern inductive morphology presents
itself to us. If it were the task of this history only to record the
discovery of new facts, it would be superfluous to allude to these
differences here; but then it would also be impossible to estimate
rightly that portion of Braun’s long scientific labours which is at
once the most original and the most interesting from the historical
point of view, and which is to be found not so much in his many
descriptive and monographic works, as in his philosophic efforts in the
domain of morphology; these moreover deserve our consideration, because
they carry out Goethe’s half-explained conceptions to their remotest
consequences, and express in purer form the idealism which lies at the
foundation of the older nature-philosophy. No botanist since Cesalpino
has so thoroughly endeavoured to leaven the entire results of inductive
investigation with the principles of an idealistic philosophy, and to
explain them in its light.
Braun’s philosophical views not only accompany his knowledge of
facts, but everywhere permeate and colour it; in his writings,
contributions, and monographs on the most various subjects, facts
are regarded from the point of view of his philosophy. He has given
a general view of his philosophical principles and illustrated them
by a vast variety of facts in his famous book, ‘Betrachtungen über
die Erscheinung der Verjüngung in der Natur, insbesondere in der
Lebens-und Bildungsgeschichte der Pflanze’ (1849-50). He himself
directs attention to the opposition between his own stand-point and the
modern induction in the tenth page of the preface, where he replies to
the obvious objection, that his ideas may be regarded as antiquated,
in the words, ‘A more living contemplation of nature, such as is here
attempted, which seeks in natural bodies not merely the operation of
dead forces, but the expression of a living fact, does not lead, as
is supposed, to airy structures of fancy, for it does not pretend to
gain a knowledge of life in nature in any other way than as it is
revealed in phenomena,’ etc. This thought is still more distinctly
uttered in page 13 of the text; ‘As external nature without mankind
presents to us only the spectacle of a labyrinth without a guide,
so too scientific contemplation, which denies the inner spiritual
principle in nature and the intimate connection of nature with the
informing spirit[50], leads to a chaos of substances and forces, which
are unknown because divorced from spirit, or, to speak more precisely,
to a chaos of nothing but unknown causes, which work together in an
inexplicable manner.’ In a note to this passage he points expressly
to ‘the comfortless character of such an unreal mode of viewing
nature, which must necessarily endeavour to root out everything in
the conceptions and language of science which appears from its own
point of view to be anthropopathic,’ and he requires a tender, ethical
element as essential to botanical investigation. The chief object of
the volume is to prove that everything in organic life may be resolved
into rejuvenescence, of which idea no definition is actually given,
though the whole contents of the book are a search after a definition.
We may regard the idea of rejuvenescence, as presented by Braun, as an
extension of the idea of metamorphosis, in which extended form it is
adapted to take in even the results of the cell-theory, of the history
of development, and of the modern knowledge of the Cryptogams from the
idealistic point of view. One peculiarity of his mode of expounding
his views is observed here, as on other occasions, namely, that he
gives no precise and arbitrary definition to a word, for instance, like
rejuvenescence in the present place, and in a later work to the word
individual, but looks behind the word for a profound or even mysterious
meaning, which is to be perceived and brought to light by contemplation
of the phenomena. In page 5 he says, ‘Thus we see youth and age appear
alternately in one and the same history of development; we see youth
burst through age, and by growth or transformation step into the middle
of the development. This is the phenomenon of rejuvenescence, which is
repeated in endless multiplicity in every province of life, but nowhere
appears more clearly expressed or more accessible to investigation than
in the vegetable kingdom. Without rejuvenescence there is no history
of development.’—‘If then we ask for the causes of the phenomena of
rejuvenescence (page 7), we shall indeed allow that nature, into which
special life enters in its various manifestations, excites, awakes,
and works by the influences which the years and even the days bring
with them; but the true and inner cause can only be found in the
desire after perfection which belongs to every being in its kind, and
urges it to bring the outer world, which is strange to it, more and
more into complete subjection to itself, and to fashion itself in it
as independently as its specific nature admits.’ Further on he says
(page 17), ‘The impulse or tendency to development in each creature
is likewise no direction of activity impressed from without, but one
given from within and working as an inner determination and force
from the depth of the inner nature.’ A passage also from page 111 of
his treatise on polyembryony, published in 1860, may be quoted here;
‘Though the organism, in the process of realising itself, is subject
to physical conditions, yet the proper causes of its morphological and
biological characteristics do not lie in these conditions; its laws
belong to a higher stage of development of its being, to a sphere in
which the faculty of self-determination is distinctly manifested. If
this is so, the laws of an organic being appear as tasks imposed, the
fulfilling of which is not absolutely necessary but only in relation
to the attainment of a definite end, as precepts, to which strict
obedience may possibly not be paid.’ To return once more to the idea
of rejuvenescence, we find at page 18 the words, ‘As regards the idea
of rejuvenescence, from the foregoing considerations we draw the
conclusion, that the surrender of growths already accomplished and
the going back to new beginnings, the commencement of rejuvenescence,
indicate only the outer side of the proceeding, while the essential
part of it is an inner gathering up of forces, a new creating, as it
were, out of the individual principle of life, a fresh reflecting upon
the specific task or the gaining renewed hold upon the type which is
to be presented in the outer organism. By this means rejuvenescence
maintains its fixed relation to development, which can and ought to
present in gradually attained perfection that only which lies in the
nature of the creature, and is most intimately its own.’ And at the
conclusion of the work (page 347) he says, ‘The way in which the inner
spiritual nature of life is specially manifested in the phenomenon of
rejuvenescence may be defined as reminiscence in the true sense of
the word, as the power of grasping anew in the phenomenon the inner
destination of life as contrasted with its daily alienation and decay,
and applying it with renewed strength towards that which is without,’
etc.
This conception of rejuvenescence is, then, applied to all the
phenomena of life in plants; not only the metamorphosis of leaves, the
formation of shoots and their ramification, and the different modes of
cell-formation, but even palaeontological facts are manifestations of
rejuvenescence, which in the sequel puts off the form of an abstract
idea, and becomes personified into an active personality, as is seen in
page 8 in the expression, ‘activity of rejuvenescence.’
The relation of Braun’s views to the question of the constancy of
species may to some extent appear doubtful; some utterances of his may
be interpreted to admit a transmutation of species accomplished in
the course of ages, while others are opposed to this, and it is the
latter which appear to be consistent with the idealistic position. We
read, for instance, at page 9, ‘The appearance, as though the like was
always repeating itself in nature, is suggested when we glance back
from our station in time upon the succession of former epochs. Here
we find the real first beginnings of species and genera, and even of
orders and classes in the vegetable and animal kingdoms; we see at the
same time that more or less thorough transformations are connected
with the appearance of the higher grades in the organic kingdom, so
that genera and species of the old world disappear, and new ones step
into their place. All this change expresses not the mere accident of
convulsions, which, while they destroy, at the same time prepare new
ground for the prosperity of organic nature, but rather definite laws
whose action pervades all the individual detail of the development
of organic life.’ On the other hand we find at the conclusion of the
treatise on polyembryony, written a short time before the appearance
of Darwin’s memorable work, a sentence which makes the assumption of
a transmutation of species appear very doubtful; it says (page 257),
‘If we are justified in assuming a general organic connection in the
history of development in plant-forms, can we imagine that the type of
the Mosses and of the Ferns has come from the Algae, or _vice versa_,
that the Alga-form owes its origin to the Mosses and Ferns?’
The sentences here quoted to show Braun’s philosophical position
still give no idea of the way in which the principles embodied in
them influence the whole manner of presenting the facts in the
arrangement of his empirical material, but to give a clear idea of
this is impossible in so brief a notice as the present. His conception
of his subject is shown still more distinctly in a treatise which
appeared three years later, entitled ‘Das Individuum der Pflanze in
seinem Verhältniss zur Species, Generationsfolge, Generationswechsel
und Generationstheilung der Pflanze’ (1852-3). The definition of
the word individual is here sought, as that of rejuvenescence was
in the previous work,—a really difficult task, if we consider how
many meanings have been assigned to this word in the course of time;
in the individuals or atoms of Epicurus, the individuals or monads
of Leibnitz, the atoms of modern chemistry, the speculations of the
schoolmen on the ‘principium individuationis’ as opposed to the reality
which they assigned to universal conceptions, and in the customary
application of the word in every-day language, in which a man or a
single tree is called an individual, we have the general views of
various centuries, showing how the sense and meaning of old words
become changed, not unfrequently into their exact opposites. From
the nominalist position of modern natural science this is of little
importance, because this treats words and ideas as mere instruments
for mutual understanding, and seeks no meaning in either which has
not been previously and purposely assigned to them. Braun’s mode of
proceeding is quite different; by comparison of very various phenomena
of vegetation, and by examining former views on the subject of the
individual plant, he seeks to demonstrate a deeper meaning which must
be connected with the word.
Moreover, he makes the enquiry into the individual only a thread on
which to string his own reflections, in the course of which he once
more explains the principles of the teleological nature-philosophy,
and points out its opposition to modern science, the latter being
grievously misrepresented as materialistic, its atoms qualified as
dead, its forces as blind. It would scarcely be guessed from Braun’s
account that the history of philosophy could point to Bacon, Locke, and
Kant, as well as to Aristotle, that even the question of the individual
had been already handled by the schoolmen. A consideration of the
other point of view would have been all the more profitable, since the
author in the beginning of his treatise expresses the opinion that the
doctrine of the individual belongs to the elements of botany; it might
certainly be maintained that it is altogether superfluous.
His train of thought in search of that which must be called an
individual in the vegetable kingdom is briefly as follows: In forming a
conception of the plant-individual as the unity of a cycle of formation
or a morphological whole, our chief difficulty lies in the division
into parts and the divisibility (Getheiltheit und Theilbarkeit) which
are present in the very different stages of the organic structure of
plants. It is requisite therefore to find the middle way between the
morphological consideration of the individual plant which breaks up
the whole from above downwards, and the physiological which extends it
in the upward direction beyond all limits. Neither the leaf-bearing
shoots, though they are capable of developing into independent plants,
nor the parts of them, which have the same power, neither the single
cells, nor the granules they contain, and least of all the atoms of
dead matter which are the sport of blind forces, would answer to
the idea of the individual in plants. We have therefore to decide
which member of this many-graded series of potences in the cycle of
development subordinated to the species deserves by preference the name
of individual (p. 48). A compromise is then made; it is sufficient to
find a part of the plant which answers above all others to the idea
of the individual, for in this idea there must be two genetic forces,
multiplicity and unity. He then decides for the shoot or bud. ‘In
contemplating the plant-stem which is usually branched, especially
a tree with its many branches, mere instinctive feeling awakens the
suspicion that it is not a single being, a single life, to be classed
with the individual animal or individual man, but that it is a world
of united individuals which spring from one another in a succession of
generations,’ etc. He proceeds to show that this conception, arising as
it does from a sound, natural feeling, is also confirmed by scientific
examination. It appears, however, that many phenomena in the growth
of plants will not fall in well with this instinctive feeling, and
so he says at page 69, ‘We cut the Gordian knot in this way, that
if we have other and sufficient grounds for regarding branches as
individuals, we come to the determination to let every branch pass for
an individual, however strongly the appearance may be against it.’
The shoot is therefore the morphological individual in the plant, and
is analogous to the individual animal. It may certainly be objected,
that we may cut the knot in another way and maintain with Schleiden
that the cells are the individuals in the vegetable kingdom, if we do
not actually arrive by the same path at calling each atom, or at the
other end of the scale the whole self-nourishing plant, an individual,
for about equally strong reasons might be adduced for both one and the
other of these views. It all depends on the point of view we adopt in
such speculations, and on the weight we allow to instinctive feeling
in establishing scientific ideas. Braun declares very decidedly in
page 39 against the notion that the invisible ‘individua’ or atoms
of dead matter can be introduced into the consideration of the
plant-individual, as though the plant were a mere concrete of mutually
attracting and repelling atoms. If, he says, we will understand by the
term individual something absolutely indivisible, this is certainly the
last resort, but then we shall have no plant-individual. Moreover, no
eye has ever seen these atoms; their assumption is a mere hypothesis,
which we may confront with the other hypothesis of the continuity and
permeability of matter. The question therefore, he says, at page 39,
is whether we can speak of individuals in plants at all, and this
coincides with the other question, whether the plant is a mere product
of the activity of matter, and so an unsubstantial appearance in the
general circulation of nature, the offspring of blind agencies, or
whether it possesses a peculiar and independent existence. The views
of the physiologists, who reject the vital force and explain the
phenomena of life by physical and chemical laws, have robbed life of
its mysterious and most directly operative principle, and pulled down
the strong wall of separation between organic and inorganic nature.
‘Because physical forces appear to be everywhere confined to matter and
show in their operation a strict subjection to law, men have ventured
to regard the sum total of natural phenomena as the result of original
matter working in conjunction with definite powers according to the
laws of blind necessity, as a natural mechanism moving in endless
circulation.’ But he objects that the eternally necessary can only be
conceived of as accomplished from all eternity, and thus this physical
view would make all eventuality inconceivable. Further, the purpose of
the movement of nature must remain an insoluble enigma in this scheme
of blind necessity. ‘The inadequateness of the so-called physical view
of nature as compared with the teleological is therefore most felt in
the domain of organic nature, where special purpose in the phenomena
of life appears everywhere in greatest distinctness.’ The last remark
is indisputable so long as we maintain either the constancy of species
or a merely internal law of development; the solution of the enigma
was discovered a few years later in Darwin’s hypothesis, that all
adaptations of organisms are to be explained by the maintenance or
suppression of varieties, according as they are well or ill provided
with the means of sustaining the struggle for existence. No other
refutation or rather explanation of teleology in the science of organic
life has hitherto been attempted. It has been already pointed out that
systematic botany, by establishing the fact of affinity, saw itself
compelled at last to give up the constancy of specific forms in order
to make this fact intelligible, and here we see how the idea of the
adaptation of organisms is found to conflict with causality, unless
we assume that the forms which arise through variation only maintain
themselves, if they are sufficiently adapted to the surrounding
conditions.
The movement which began with Goethe and the nature-philosophy assumed
a clearer form, found its purest expression, and revealed its most
hidden treasures in the writings of Schimper and Alexander Braun; it
would be superfluous to submit to a detailed review the numerous works
of less important representatives of these views.
We turn from this realm of idealistic philosophy and imagination, from
rejuvenescence, the wave-pulse of metamorphosis, the spiral tendency
of growth, and the individuality of plants, to the last chapter of
our history of systematic botany and morphology, where there is less
dogmatism and less poetry, but a firmer ground on which will spring an
unexpected wealth of new discoveries and of deeper insight into the
nature of the vegetable world.
CHAPTER V.
MORPHOLOGY AND SYSTEMATIC BOTANY UNDER THE INFLUENCE OF THE HISTORY OF
DEVELOPMENT AND THE KNOWLEDGE OF THE CRYPTOGAMS.
1840-1860.
In the years immediately before and after 1840 a new life began to
stir in all parts of botanical research, in anatomy, physiology,
and morphology. Morphology was now specially connected with renewed
investigations into the sexuality of plants and into embryology, and
attention was no longer confined to the Phanerogams but was extended to
the higher and later on to the lower Cryptogams. These researches into
the history of development first became possible, when von Mohl had
restored the study of anatomy, and Nägeli had founded and elaborated
the theory of cell-formation about the year 1845. The success of both
these enquirers was due to the previous development of the art of
microscopy; it was the microscope which revealed the facts on which
the foundations of the new research were laid, while its promoters at
the same time started from other philosophical principles than those
which had hitherto prevailed among botanists. Investigation by means
of the microscope enforces on the observer the very highest strain
of attention and its concentration on a definite object, while at
the same time a definite question to be decided by the observation
has always to be kept before the mind; there are sources of error on
all sides to be avoided, and possible deceptions to be taken into
consideration; the securing of the facts demands all the powers
which specially display the individual character of the observer.
Thus serious attention to microscopy was one of the causes which
introduced the best observers to the practice of inductive enquiry,
and gave them an insight into its nature; and in a few years’ time
when the actual results of these investigations began to appear, and
when a wholly new world disclosed itself to botanists, especially in
the Cryptogams, then questions arose on which the dogmatic philosophy
had not essayed its ancient strength; the facts and the questions
were new and untouched, and presented themselves to unprejudiced
observation in a purer form than those, which during the first three
centuries had been so mixed up with the old philosophy and with the
principles of scholasticism. Von Mohl, who only occasionally occupied
himself with morphological subjects, was a firm adherent of the
inductive method, and was bent on the establishment of individual facts
rather than of general principles; but the founders also of the new
morphology, Schleiden and Nägeli, started from philosophical points
of view, which, different as they were in the two men, had yet two
things in common, a demand for severely inductive investigation as
the foundation of all science, and the rejection of all teleological
modes of explaining phenomena, in which latter point their opposition
to the idealistic nature-philosophy school was most distinctly
manifested. They had indeed one very important point of contact with
this school, the belief in the constancy of organic forms: but this
belief, not being connected with the Platonic doctrine of ideas,
was with them only a recognition of every-day observations, and was
therefore of less fundamental importance, being felt merely as an
inconvenient element in the science. Treating the question in this
way, and influenced by the results of the new researches, they either
inclined to entertain the idea of descent before the appearance of
Darwin’s great work, or gave a ready assent to the principle of the
new doctrine, though they expressed some doubts respecting matters
of detail. Hofmeister’s researches in morphology and embryology
(‘Vergleichende Untersuchungen,’ 1851) threw an entirely new light on
the relations of affinity between the great groups in the vegetable
kingdom, and were leading more and more to the view, that there must be
some special peculiarity in the question of the constancy of organic
forms. But the idea of evolution in the vegetable kingdom was brought
more distinctly home to men’s minds by palaeontological researches;
Sternberg (1820-1838), Brongniart (1828-1837), Goeppert (1837-1845),
and Corda (1845) made the flora of former ages the subject of careful
study, and compared fossil plants with living allied forms. Unger
especially, while advancing the knowledge of the structure of cells and
of vegetable anatomy and physiology, and generally taking a prominent
part in the development of the new botany, applied the results of its
investigations to the examination of primeval vegetation, and showed
the morphological and systematic relations between past and existing
floras. After twenty years of preliminary study he declared distinctly
in 1852, that the immutability of species is an illusion, that the new
species which have made their appearance in geological periods are
organically connected, the younger having arisen from the elder[51]. It
was shown in the former chapter, how about the same time the leading
representative of idealistic views, Alexander Braun, was driven to the
hypothesis, though in a more indefinite form, of an evolution of the
vegetable kingdom: and in the year that Darwin’s book on the origin
of species appeared, Nägeli (‘Beiträge,’ ii. p. 34) wrote:—‘External
reasons, supplied by the comparison of the floras of successive
geological periods, and internal reasons given in physiological and
morphological laws of development and in the variability of the
species, leave scarcely a doubt that species have proceeded one from
another.’
Though these words might not contain a theory of descent capable
at once of scientific application, yet they show that the latest
researches and candid appreciation of facts were compelling the most
eminent representatives of the botany of the day to give up the
constancy of forms. At the same time in the genetic morphology which
had developed itself mainly under Nägeli’s guidance since 1844, and
still more in embryology, which in Hofmeister’s hands was leading to
results of the greatest systematic importance, there lay a fruitful
element destined to correct and enrich Darwin’s doctrine of descent
in one essential point. That doctrine in its original form sought
to show that selection, the result of the struggle for existence,
combined with perpetual variation was the sole cause of progressive
improvement in organic forms[52]; but Nägeli, relying on the results
of German morphology, was able as early as 1865 to point out that this
explanation was not satisfactory, because it leaves unnoticed certain
morphological relations, especially between the large divisions of the
vegetable kingdom, which scarcely seem explainable by mere selection
in breeding. While Nägeli allowed that Darwin’s principle of selection
was well adapted to explain fully the adaptation of organisms to their
environment and the suitableness and physiological peculiarities
of their structure, he pointed out that in the nature of plants
themselves there are intimations of laws of variation, which lead to a
perfecting of organic forms and to their progressive differentiation,
independently of the struggle for existence and of natural selection;
the importance of this result of morphological research has since been
recognised by Darwin. Thus Nägeli supplied what was wanting in the
theory of descent and gave it the form, in which it is adequate to
explain the problem already recognised by the systematists of the old
persuasion, namely, how it is possible for the morphological affinity
of species in the system to be in so high a degree independent of their
physiological adaptation to their environment.
The modern teaching on vegetable cells, modern anatomy, and morphology,
and the improved form of the theory of selection are the product of
inductive enquiry since 1840, a product, the full importance of which
will be described in the following portions of our history. At present
we have to deal only with morphological and systematic results, and
therefore with a part only of the abundant labours of the botanists
who will be noticed in this chapter; the remainder will be reserved
for succeeding books, which contain the history of the anatomy and
physiology of plants.
It is one of the characteristic features of this period of botany, that
morphology enters into the closest connection with the doctrine of the
cell, with anatomy and embryology, and that researches, especially
into the process of fecundation and the formation of the embryo, form
to some extent the central point of morphological and systematic
investigations. A strict separation of these various enquiries, which
are all ultimately applicable to the purposes of systematic botany, can
therefore scarcely be maintained, and least of all in dealing with the
lower Cryptogams.
* * * * *
The condition of botanical literature about the year 1840 was highly
unsatisfactory; it is true that eminent service was rendered in
the several domains of systematic botany, morphology, anatomy, and
physiology, and a number of von Mohl’s best works were produced in
this period; Meyen also, Dutrochet, Ludolph Treviranus and others were
cultivating vegetable anatomy and physiology, and it has been already
stated that good and noticeable work was done in the previous years in
morphology and systematic botany. But there was no one to put together,
to criticise and apply the knowledge which had been accumulated in
all parts of the science; no one really knew what a wealth there was
at that time of important facts; least of all was it possible to form
a judgment on the matter from the text-books of the period, which
were deficient in ideas and facts, and crammed with a superfluous
terminology; their mode of treating their subject was trivial and
tasteless, and whatever was specially worth knowing and important to
the student they did not contain. Those who undertook really scientific
enquiries separated themselves from those who dealt with botany after
the old schematism of the Linnaean school; but botanical instruction,
the propagation of knowledge, was almost everywhere in the hands of
this school, though it was the one least fitted for the task; and thus
a mass of lifeless phrases was the instruction offered to the majority
of students under the name of botany, with the inevitable effect of
repelling the more gifted natures from the study. This was the evil
result of the old and foolish notion, that the sole or chief business
of every botanist is to trifle away time in plant-collecting in wood
and meadow and in rummaging in herbaria,—proceedings which could
do no good to systematic botany even as understood by the Linnaean
school. Even the better sort lost the sense for higher knowledge
while occupying themselves in this way with the vegetable world; the
powers of the mind could not fail after a time to deteriorate, and
every text-book of the period on every page supplies proof of this
deterioration.
But such a condition of things is dangerous for every science; of what
profit is it, that single men of superior merit advance this or that
part of the science when a connected view of the whole is wanting,
and the beginner has no opportunity of studying the best things in
their mutual relations. However, the right man was found at the right
moment to rouse easy indolence from its torpor, and to show his
contemporaries, not in Germany only but in all countries where botany
was studied, that no progress was possible in this way. This man was
MATTHIAS JACOB SCHLEIDEN, born at Hamburg in 1804, and for many years
Professor in Jena. Endowed with somewhat too great love of combat, and
armed with a pen regardless of the wounds it inflicted, ready to strike
at any moment, and very prone to exaggeration, Schleiden was just the
man needed in the state in which botany then was. His first appearance
on the scene was greeted with joy by the most eminent among those who
afterwards contributed to the real advance of the science, though their
paths it is true diverged considerably at a later period, when the time
of reconstruction was come. If we were to estimate Schleiden’s merit
only by the facts which he discovered, we should scarcely place him
above the level of ordinarily good botanists; we should have to reckon
up a list of good monographs, numerous refutations of ancient errors
and the like; the most important of the theories which he proposed, and
over which vigorous war was waged among botanists during many years,
have long since been set aside. His true historical importance has
been already intimated; his great merit as a botanist is due not to
what he did as an original investigator, but to the impulse he gave
to investigation, to the aim and object which he set up for himself
and others, and opposed in its greatness to the petty character of the
text-books. He smoothed the way for those who could and would do really
great service; he created, so to speak, for the first time an audience
for scientific botany capable of distinguishing scientific work from
frivolous dilettanteism. Whoever wished from this time forward to take
part in the discussion of botanical subjects must address all his
powers to the task, for he would be judged by another standard than had
hitherto prevailed.
Schleiden, who had commenced his botanical labours with some important
researches in anatomy and the history of development, the most valuable
of which in matter and form was an enquiry into the development of
the ovule before fertilisation (1837), composed also a comprehensive
text-book of general botany, which appeared first in 1842-3, and
in much improved editions in 1845 and 1846, and in two subsequent
years. The difference between this and all previous text-books is the
difference between day and night; in the one, an indolent carelessness
and an absence of ideas; in the other, a fulness of life and thought,
calculated to influence young minds all the more, because it was in
many respects incomplete and still in a state of fermentation. On
every page of this remarkable work, by the side of facts really worth
knowing, the student found interesting reflections, a lively and
generally coarse polemic, and praise and blame of others. It was not
a book to be studied quietly and comfortably, but one that excited
the reader everywhere to take a side for or against, and to seek for
further instruction.
The work is generally quoted as ‘Grundzüge der wissenschaftlichen
Botanik,’ but its chief title is ‘Die Botanik als inductive
Wissenschaft,’ which indicates the point on which Schleiden laid most
stress. His great object was to place the study, which had been so
disfigured in the text-books as scarcely to wear the semblance of
a natural science, on the same footing with physics and chemistry,
in which the spirit of genuine inductive enquiry into nature had
already asserted itself in opposition to the nature-philosophy of
the immediately preceding years. It may seem strange to us now to
see a text-book of botany introduced by a formal essay, 131 pages
long, on the inductive method of investigation as opposed to dogmatic
philosophy, and to find the principles of induction set forth again
and again in connection with a great variety of subjects in the
book itself. Many objections may be raised to the contents of this
introduction; it may be said that many philosophical dicta are
misunderstood in it; that Schleiden himself has frequently offended
against the rules there laid down, for instance, when he substitutes
a formative impulse (nisus formativus) for the vital force which he
rejects, which is only introducing vital force again under another
name; that it is superfluous to present the history of development
as a ‘maxim’ in Kant’s use of the word, instead of showing that the
history of development enters naturally and of itself into inductive
investigation, and so on. All this will not lessen the historical
importance of this philosophic introduction; the traditional way in
which descriptive botany was at that time presented to the student was
so thoroughly dogmatic and scholastic, trivial and uncritical, that it
was necessary to impress upon him in many words, that this is not the
method of true investigation of nature.
Passing on to the more special problems of botanical enquiry, Schleiden
next dwells on the history of development as the foundation of all
insight into morphology, though he overshot the mark when he rejected
as unfruitful the simple comparative method, which had produced
considerable results in the hands of De Candolle, and was virtually the
fruitful element in the doctrine of phyllotaxis of Schimper and Braun.
Still he took an active part himself in the study of development in
plants, and gave special prominence to embryology; he also discussed
the doctrine of metamorphosis from the point of view of the history
of development, and pointed to Caspar Friedrich Wolff’s treatment of
that subject as much clearer than that which had been introduced by
Goethe. Finally, Schleiden’s mode of dealing with the natural system
must be reckoned among the good services which he rendered to method;
not because his classification of the vegetable kingdom presents any
specially interesting features or brought to light any new affinities,
but because we see an attempt made for the first time to give detailed
characters drawn from morphology and the history of development to the
primary divisions, and because by this means the positive and distinct
nature of the Cryptogams was from the first clearly brought out. The
old way of treating morphology, as though there were only Phanerogams
in the world, and then having recourse to unmeaning negatives in
dealing with the Cryptogams, was thus set aside, much to the profit
of the immediate future, which directed its attention specially to the
Cryptogams.
Schleiden however did not succeed in securing firm ground for the
morphology of the Cryptogams as founded on the history of their
development. His investigations into the morphology of the Phanerogams
were more successful. His theory of the flower and fruit is an
admirable performance for the time, even though we abandon his view of
the stalk nature of placentas and some other notions, as we obviously
must. As Robert Brown founded the history of the development of the
ovule, so Schleiden founded that of the flower, and his example
influenced other botanists. Soon investigations into the genesis of
the flower was one of the chief occupations of morphologists, and the
results of enquiry into development proved to be of great value for the
systematic arrangement of the Phanerogams, especially when more exact
attention was paid to the sequence of development in the organs of an
inflorescence, to abortion, doubling and branching of the stamens, and
to the like matters. Duchartre, Wigand, Gelesnoff and many others,
were soon working in the same direction with great success. Payer
deserves special mention for his enormous perseverance in examining the
development of the flower in all the more important families in his
‘Organogénie de la fleur,’ 1857, and thus producing a standard work,
distinguished alike for the certainty of the observations, the simple
unbiassed interpretation of the things observed, and the beauty and
abundance of the figures—a work which became more important every year
for the morphology of the flower.
Schleiden’s text-book was the first of its kind that supplied the
student with really good figures based on careful observations. With
all its many and obvious defects it had one merit which cannot be
rated too highly; its appearance at once put botany on the footing
of a natural science in the modern sense of the word, and placed it
upon a higher platform, extending its horizon by raising its point of
view. Botany appeared all at once as a science rich in subject-matter;
Schleiden had not only himself made many investigations and broached
new theories, but he everywhere drew attention to what was already
before the world and was important; for it is not sufficient as regards
the literature of a science that there should be good investigators; it
is as necessary that the scientific public, and especially the rising
generation of professed students, should be well and sufficiently
instructed in the art of distinguishing important from unimportant
contributions. It must be distinctly affirmed in this place, that if
Schleiden’s theory of cell-formation, his strange notion about the
embryology of Phanerogams and the like were very quickly shown to be
untenable, this does not in the least affect the great historical
importance which his writings possess in the sense here indicated.
That others besides Schleiden in the period following 1840 felt
strongly, that botany must thenceforward give up its complacent resting
in the old ideas, was shown among other things by the addition at this
time of new periodicals to the old journal ‘Flora.’ The ‘Botanische
Zeitung’ was founded by von Mohl and Schlechtendal in 1843, and the
‘Zeitschrift für wissenschaftliche Botanik’ by Schleiden and Nägeli.
The latter, however, only lived three years, from 1844 to 1846, and was
filled almost entirely with Nägeli’s contributions. Both publications
expressly set themselves the task of representing the new aims in the
science. The immediate consequence was that ‘Flora’ braced up its
energies, and endeavoured to do more justice to the modern spirit;
excellent notices of botanical works now appeared in it under the
exclusive management of Fürnrohr.
Schleiden’s productivity in the higher sense of the word expended
itself in his labours on the elements of scientific botany. His
later somewhat discursive writings exerted no great influence on the
further development of the science. The ideal which he had set up for
scientific botany and had sketched in its larger outlines, could only
be realised by the most persevering labour not of one man only, but of
whole generations of observers and thinkers, nor did he apply himself
with painful unremitting industry to the attainment of this exalted aim.
Soon after Schleiden’s ‘Grundzüge’ first stirred the scientific world,
a man of a very different character of mind began to address himself
to the great task. This was CARL NÄGELI, whose researches from this
time onwards laid the foundations of knowledge in every department of
botany. He showed what points were the most immediately attainable, and
aided in perfecting the inductive method of enquiry and in advancing
the study of the history of development. He did not make discoveries
here and there by desultory efforts, but worked with earnest endurance
at every question which he took up till he had arrived at a positive
result; and this was almost always an enlargement of previous
knowledge, and a new foundation on which others might build, and a
copious literature be developed.
Nägeli like others felt the necessity of first determining his position
with respect to the philosophical principles of the investigation of
nature, but he did not proceed to give a general exposition of the
inductive method as opposed to the dogmatism of the idealistic school.
He went straight to the application of the laws of induction to the
most general problems of organic nature, and specially of vegetation.
It is easy to say that the task of natural science is simply to deduce
conceptions and laws from the facts of experience by aid of exact
observation. Many considerations present themselves as soon as the
attempt is made to satisfy this demand; for it is not enough merely to
accumulate individual facts, the point to which the inductive enquiry
is to lead must be kept constantly and clearly before the mind. Nägeli
insisted that it is only in this way that facts and observations
have any scientific value; that the one important thing is to make
every single conception obtained by induction find its place in the
scheme of all the rest of our knowledge. With greater consistency of
reasoning than Schleiden, and in entire accordance with the nominalist
view of genuine investigation of nature in its sternest opposition to
the idealistic school, Nägeli’s first principle is not only to deduce
conceptions from the observation of phenomena, to classify them and
establish their subordination, but to treat these conceptions as mere
subjective products of the understanding and employ them as instruments
of thought and communication, and to be always ready to modify them
as soon as inductive enquiry renders such modification necessary.
Till this happens, the conception once laid down and connected with
a word is to be strictly adhered to, and every arbitrary change or
confusion with another conception is strictly forbidden. Since in
nature everything is in movement, and every phenomenon is transitory,
presenting itself to us in organic life especially as the history of
development, all due regard must be paid to this condition of constant
motility in forming scientific conceptions. The history of development
is not merely to be treated generally as one of various means of
investigation, but as identical with investigation into organic nature.
These views are expressed in Nägeli’s detailed observations on method
in the first and second volume of the journal which he brought out in
conjunction with Schleiden in 1844 and 1855, where the chief hindrance
to his carrying them out fully and consistently is also to be found;
for, like all his contemporaries, Nägeli believed at that time in the
constancy of species, and consistently with this view he looked upon
the natural system as a framework of conceptions, though these do not
take the form of Platonic ideas with him as with the systematists of
the idealistic school. It is equally consistent with his philosophical
position, which refused to regard a change in our conceptions as a
change in things themselves, that ‘the idea of metamorphosis’ in the
sense of Goethe and Alexander Braun disappears in Nägeli from the field
of scientific observation. It has been shown in the previous chapter
that what Goethe called the normal or ascending metamorphosis has
no scientific meaning unless species are supposed to be variable. It
appeared moreover that if the Cryptogams are made the chief subjects
of investigation, as Nägeli made them, the so-called metamorphosis of
the leaves is a phenomenon of secondary importance, and only attains
to its full importance in the Phanerogams. If Schleiden, illogically
from his point of view, conceived of metamorphosis as the principle
of development, Nägeli on the contrary scarcely employed the word. He
regarded the history of development as the law of growth of the organs,
and, in accordance with the theory of the constancy of species, the
law of growth of every species and every organ was invariable in the
same sense in which we apply the term to natural laws in physics and
chemistry. In a word, Nägeli’s considerations on the ‘present task of
natural history’ in the work above cited, are not only logically and
entirely consistent on the principles of the inductive method, but they
are also consistent where others have been misled by the theory of the
constancy of species into illogical conclusions.
Nägeli set himself in earnest to meet the demands of inductive
enquiry, such as he had himself described them. It will be shown
more in detail in the history of phytotomy, how he satisfied these
demands in his refutation of Schleiden’s doctrine of the cell, and
in the establishment of his own, and at a later time in the framing
of his theory of molecular structure and of the growth of organised
bodies, and how he made these investigations true models of genuine
inductive enquiry. Here we are concerned only with what he effected
in this way for morphology and systematic botany. In this field of
research he introduced two innovations of the profoundest importance,
which affected both the aim and method of enquiry for some years. He
connected his own morphological investigations, as far as possible,
with the lower Cryptogams, extending them afterwards to the higher
Cryptogams and to the Phanerogams; that is, he proceeded from simple
and plain facts to the more difficult, thus not only introducing the
Cryptogams into the field of systematic investigation, but making
them its starting-point. In this way morphology not only secured a
foundation in exact historical development, but it assumed a different
aspect, inasmuch as the morphological ideas hitherto drawn from
the Phanerogams were now examined by the light of the history of
development in the Cryptogams. This was one innovation; the second,
closely connected with it, was the way in which Nägeli made the new
doctrine of the cell the starting-point of morphology. Both the first
commencement of organs and their further growth were carried back to
the formation of the separate cells; and the remarkable result was to
show, that in the Cryptogams especially, whose growth is intimately
connected with cell-division, precise conformity to law obtains in the
succession and direction of the dividing walls, and that the origin and
further growth of every organ is effected by cells of an absolutely
fixed derivation. The most remarkable thing was, that every stem and
branch, every leaf or other organ has a single cell at its apex, and
that all succeeding cells are formed by division of this one cell
according to fixed laws, so that the origin of all cell-tissue can be
traced back to an apical cell; and as early as the years 1845 and 1846
Nägeli described in the ‘Zeitschrift für wissenschaftliche Botanik’
the three main forms, according to which the segmentation of an apical
cell proceeds, namely, in one, two, and three rows (Delesseria,
Echinomitrium, Phascum, Jungermannia, Moss-leaves). In this way the
separate points in the history of growth in the Cryptogams were brought
out with unusual clearness and decision; but on the other hand, Nägeli
showed in 1844 in the case of a genus of Algae (Caulerpa) that the
growth of a plant may show the usual morphological differentiation
into axis, leaf, and root, when the propagative cell undergoes no
cell-divisions in the process of development and further growth,
and similar conditions were for the first time demonstrated in 1847
in Valonia, Udotea, and Acetabularia. Beside other results it was
established by these facts, that morphological differentiation during
growth must not be regarded as an effect of cell-divisions, and from
such cases as these the conception of the cell experienced a notable
expansion.
Moreover, Nägeli was not satisfied with seeking instructive examples
for general morphological axioms in the lower Cryptogams; he devoted
special study to the Algae for systematic and descriptive purposes;
and his ‘Neuen Algensysteme,’ which appeared in 1847, and ‘Gattungen
einzelliger Algen,’ of 1849, were the first successful attempts to
substitute serious investigation for the mere zeal of the collector in
this part of the vegetable kingdom, which had not indeed been hitherto
neglected, but had not been systematically worked since the time of
Vaucher. In the same spirit Alexander Braun also in his ‘Verjüngung’
contributed a rich material of new observations on the mode of life
of the Algae and the morphological conditions connected with it, and
his labours were followed in the succeeding years by the important
researches of Thuret, Pringsheim, De Bary, and others, to which we
shall recur in a later portion of this history.
But before the examination of the Algae, and soon after of the Fungi
also, led to such great results, the systematic botany of the higher
plants underwent important changes through the methodical study of
the embryology of the Muscineae and Vascular Cryptogams. These groups
had been frequently and carefully examined by good observers since
the last century, and the systematists, without penetrating deeply
into the peculiarities of their organisation, had brought the species
and genera, the families and even the higher divisions into tolerable
order. Comprehensive and methodically arranged catalogues of these
plants had been formed, and attempts had been made to explain their
morphology by that of the Phanerogams; Schmidel[53] published valuable
observations on the Liverworts in the year 1750, Hedwig especially
on the Mosses in 1782; these works were followed by Mirbel’s thorough
examination of Marchantia in 1835, by Bischoff’s of Marchantieae and
Riccieae, by Schimper’s study of the Mosses in 1850, and by Lantzius
Beninga’s[54] contributions to the knowledge of the structure of
the moss-capsule in 1847. The organisation, and to some extent the
germination, of the Vascular Cryptogams had become better known since
1828 through Bischoff’s[55] researches; Unger had as early as 1837
described the spermatozoids in the antheridia of various Mosses, Nägeli
had discovered them on an organ of the Ferns which had up to that time
been taken for the cotyledonary leaf of these plants, and on the same
part of the plant Suminski in 1848 observed the female sexual organs
and the entrance of the spermatozoids into them. The history of the
germination of the Rhizocarps, from which Schleiden thought that he
had proved his erroneous theory of fertilisation with more than usual
certainty, had been examined some years before by Nägeli, and also by
Mettenius, in great detail; here too Nägeli detected the spermatozoids.
Thus important fragments of the life and organisation of these plants
had been described up to the year 1848, but until they were more fully
understood and connected together they had but little scientific value,
the one fact perhaps excepted, that fertilisation in the Cryptogams
as in animals was effected by spermatozoids. A perfect insight into
the embryological conditions in question could only be obtained when
the embryology of the Phanerogams especially had been cleared up, for
according to Schleiden’s theory, which made the pollen-tube enter the
embryo-sac in the ovule and develop into the embryo, the ovule was no
longer to be regarded as a female sexual organ, but only as a place of
incubation for the embryo, which was thus really produced asexually.
This important question was set at rest by WILHELM HOFMEISTER’S work,
‘Die Entstehung des Embryos der Phanerogamen,’ which appeared in 1849.
In this work, and in a series of subsequent treatises, he showed that
the egg-cell is formed in the embryo-sac before fertilisation, and that
it is this which is excited to further development by the appearance
of the pollen-tube, and produces the embryo. Hofmeister had observed
the organisation of the ovule, the nature of the embryo-sac and of
the pollen-grain, and the formation of the embryo from the fertilised
egg-cell step by step and cell by cell, and his account of these
processes was aided by the light which Nägeli’s theory of the cell,
and his reference of all processes of development to the processes of
cell-formation, had thrown upon the history of development. He went
on to apply the same method to the study of the embryology of the
Muscineae and the Vascular Cryptogams, and followed the development
of the sexual organs cell by cell in a large number of species; he
observed the origination of the egg-cell which was to be subsequently
fertilised, and the formation of spermatozoids, and above all he
showed the divisions which take place in the fertilised egg-cell,
and the relation of its segments to the further growth of the sexual
product in course of formation. The whole course of development in the
Muscineae and Vascular Cryptogams displayed a return twice repeated to
the single cell as the starting-point in each case of a new phase of
development; the true relation between the asexually produced spore and
its germ-product on the one side, and the sexually generated embryo on
the other, and their significance in the history of development, were
brought out clearly by Hofmeister’s investigation, while the exactness
of his method rendered lengthy discussions on the subject unnecessary.
With these embryological processes, especially those of the Rhizocarps
and Selaginellae, in which the presence of two kinds of spores was
now for the first time correctly interpreted, Hofmeister compared the
embryology of the Conifers, and by their aid that of the Angiosperms
also.
The results of the investigations published in the ‘Vergleichende
Untersuchungen’ in 1849 and 1851 were magnificent beyond all that has
been achieved before or since in the domain of descriptive botany;
the merit of the many valuable particulars, shedding new light on
the most diverse problems of the cell-theory and of morphology, was
lost in the splendour of the total result, which the perspicuity of
each separate description revealed to the reader before he came to
the conclusion of the work, and there a few words in plain and simple
style gave a summary of the whole. Briefly to describe this result
in all its importance for botanical science is a difficult task; the
idea of what is meant by the development of a plant was suddenly and
completely changed; the intimate connection between such different
organisms as the Liverworts, the Mosses, the Ferns, the Equisetaceae,
the Rhizocarps, the Selaginellae, the Conifers, the Monocotyledons,
and Dicotyledons could now be surveyed in all its relations with
a distinctness never before attained. Alternation of generations,
lately shown to exist though in quite different forms in the animal
kingdom, was proved to be the highest law of development, and to
reign according to a simple scheme throughout the whole long series
of these extremely different plants. It appeared most clearly in the
Ferns and Mosses, though at the same time with a certain difference in
each; in the Ferns and allied Cryptogams a small inconspicuous body
grows out of the asexually produced spore, and immediately produces
the sexual organs; from the fertilisation of these organs proceeds
the root-bearing and leafy stem of the Fern, which in its turn again
produces only asexual spores. In the Muscineae, on the other hand, a
much differentiated and usually long-lived plant is developed from the
spore, and this plant proceeds again after some time to form sexual
organs, the product of which is the so-called Moss-plant. The first
generation that arose from the spore, the sexual, is in the Muscineae
the vegetative plant, while in the Ferns and their allies the whole
fulness of vital activity and of morphological differentiation is
unfolded in the second generation which is sexually produced. Here
all was at once clear and obvious: but Hofmeister’s researches also
showed that the same scheme of development holds good in the Rhizocarps
and Selaginellae where two kinds of spores are formed; and it appeared
plainly from their case that the recognition of the true relation
between the production of spores and sexual organs is the guide to the
morphological interpretation. When the processes in the large female
spore of the most perfect of the Cryptogams was known, the formation
of the seeds in the Conifers was at once understood; the embryo-sac in
these answered to this large spore, while the endosperm represented
the prothallium, and the pollen-grain the microspore; the last trace
of alternation of generations, so obvious in the Ferns and Mosses, was
seen in the formation of the seed in the Phanerogams. The changes,
which the alternation of generations passes through from the Muscineae
upwards to the Phanerogams, were, if possible, still more surprising
than the alternation of generations itself.
The reader of Hofmeister’s ‘Vergleichende Untersuchungen’ was presented
with a picture of genetic affinity between Cryptogams and Phanerogams,
which could not be reconciled with the then reigning belief in the
constancy of species. He was invited to recognise a connection
of development which made the most different things appear to be
closely united together, the simplest Moss with Palms, Conifers, and
angiospermous trees, and which was incompatible with the theory of
original types. The assumption that every natural group represents an
idea was here quite out of place; the notion entertained up to that
time of what was really meant by the natural system had to be entirely
altered; it could as little pass for a body of Platonic ideas as for a
mere framework of conceptions. But the effect of the work was great in
respect to the system also; the Cryptogams were now the most important
objects in the study of morphology; the Muscineae were the standard by
which the lower Cryptogams must be tried, the Ferns were the measure
for the Phanerogams. Embryology was the thread which guided the
observer through the labyrinth of comparative and genetic morphology;
metamorphosis now received its true meaning, when every organ could
be referred back to its parent-form, the staminal and carpellary
leaves of the Phanerogams, for example, to the spore-bearing leaves
of the Vascular Cryptogams. That which Häckel, after the appearance
of Darwin’s book, called the phylogenetic method, Hofmeister had
long before actually carried out, and with magnificent success. When
Darwin’s theory was given to the world eight years after Hofmeister’s
investigations, the relations of affinity between the great divisions
of the vegetable kingdom were so well established and so patent, that
the theory of descent had only to accept what genetic morphology had
actually brought to view.
So gorgeous a picture as Hofmeister had designed of the genetic
connection of the vegetable kingdom, except the Thallophytes, could
not possibly be completely perfect and correct in all its separate
features; there were still many gaps to fill up and particular
observations to correct. Hofmeister himself continued his labours; the
remarkable genera Isoetes and Botrychium were in the following years
more carefully studied by himself, the fertilisation and embryology of
the Equisetaceae by himself and Milde, and those of Ophioglossum by
Mettenius, and all were fitted into their place in the system. To the
present day it is always a profitable task to submit the different
forms of the Muscineae, the Vascular Cryptogams, and the Gymnosperms
to exact investigation in order to ascertain all the details in the
process of development in these plants, the formation of the embryo,
the succession of cells at the apex, the first appearance and further
growth of the lateral organs; and the more careful the observation, the
more clearly even to its farthest results does the correctness of the
alternation of generations asserted by Hofmeister everywhere appear. It
does not fall within the limit of this history to pursue the subject
further, and to show how the doctrine of alternation of generations
and the knowledge of the morphology of the Cryptogams were further
advanced by later and distinguished researches, such as those of Cramer
on the Equisetaceae, of Pringsheim on Salvinia (1862), of Nägeli and
Leitgeb on the formation of roots in the Cryptogams, of Hanstein on the
germination of the Rhizocarps, and of others.
THALLOPHYTES.
The method of investigation which starts from the first steps towards
the formation of the embryo before and after fertilisation, and
follows the advancing segmentation and growth through all the stages
of development up to the final completion of the embryo-plant, has
led since 1850 in the case of the Muscineae, Vascular Cryptogams, and
Phanerogams to great certainty in the morphological explanation of
the organs, while the determination of affinities has ceased to be
arbitrary and insecure; the way was now known which would lead to the
desired end, whenever it was sought to establish the affinities of a
genus of Cryptogams or of the larger groups of Phanerogams; the day
of ingenious guessing and trying was over; the only plan was patient
investigation, and this always yielded a result of lasting value.
The case was quite different with the Thallophytes still in 1850;
what was certainly known about them only showed how uncertain the
rest was; the Algae, Fungi, and Lichens presented a chaotic mass of
obscure forms in contrast with the well-ordered knowledge of the
Muscineae and Vascular plants. In the Mosses and Ferns the series of
developments within the limits of the species was so set forth in its
several stages, that all the important points in the advancing growth
were clearly ascertained, while the alternation of generations at once
sharply distinguished and connected together the chief sections in the
development; on the other hand the development of the Algae and Fungi
seemed to break up into a disorderly and motley throng of forms that
appeared and disappeared, and it seemed scarcely possible to discover
their regular genetic connection. Here the important point was to
determine which of the known forms belonged to one and the same cycle
of development, for these plants go back at the most various stages of
development to the segregation of single cells, which are the beginning
of a new development either repeating or carrying on the old one. The
beginnings of the most different species of Algae lay mixed up together
in the same drop of water, those of quite different Fungi grew together
and even upon one another on the same substratum; in the Lichens,
Fungus and Alga were united together. Such was the case with the small
and microscopic species; the large Seaweeds, the Mushrooms, and the
large Lichens were easier to distinguish specifically, but less if
possible was known of their development than of that of the microscopic
Thallophytes.
Nevertheless the knowledge of individual forms in these organisms had
been considerably extended before 1850. Collectors and amateurs, intent
only on determining what is immediately presented to the eye and making
little enquiry into origin and affinities, were indefatigable in adding
to their collections, and made catalogues and proposed various systems
founded on external marks taken at pleasure. The names of species were
counted by thousands, their characters filled thick volumes and the
figures large folios; the abundance of forms in the Thallophytes proved
to be so great that many botanists devoted their whole attention to
them, many collected and described only the Algae, others only the
Fungi and Lichens. It is true that a deeper insight into the connection
of these forms of life with one another and with other plants was not
to be obtained in this way; still an empirical basis was formed for
a knowledge of the Cryptogams, such as had been established for the
Phanerogams by the herbals of the 17th century. All forms open to
observation were named and arranged in one way or another; and there
was no difficulty in understanding what form was meant, when names, or
tables and figures, were cited from the various books. Of such works,
those of Agardh[56], Harvey, and Kützing on the Algae, those of Nees
von Esenbeck[57], Elias Fries, Léveillé, and Berkeley on the Fungi, and
especially Corda’s elaborate work on the latter plants are the most
valuable.
The views entertained on the subject of the development and propagation
of the lower Cryptogams down to the year 1850 were very uncertain
and fluctuating. In some Algae, Fungi, and Lichens certain organs of
multiplication and propagation were known, in others they were quite
unknown; some forms appeared in places and under circumstances which
seemed to necessitate the assumption of spontaneous generation; in 1827
Meyen declared that the small Algae, known as ‘Priestley’s matter,’
which are formed in stagnant water and even in closed vessels, are
produced by free generation, and Kützing endeavoured to show this by
experiment in 1833; some Fungi were regarded as diseased growths from
other organisms, many were supposed to spring up spontaneously, though
they might be capable at the same time of propagating themselves by
spores; this view was shared by even the best botanists with regard to
the most simple Fungi up to 1850. But the systematic investigation of
the Algae and Fungi was as little hindered by the notion of spontaneous
generation after 1850 as that of Phanerogams had been in the 17th
century by the same notion; it was however at first affected by the
view put forth by Hornschuch in 1821 and by Kützing in 1833, that the
simplest of all Alga-cells (Protococcus and Palmella), once produced
spontaneously, could develop according to circumstances into a variety
of Algae, and even of Lichens and Mosses; as some observers even
now consider Penicillium and Micrococcus to be the starting-points
of very different Fungi. There was a difficulty also in drawing the
boundary-line between the lower animals and plants; the difficulty was
solved by classing all objects capable of independent movement with
animals; thus whole families of Algae (the Volvocineae, Bacillariaceae,
and others) were claimed by the zoologists, and when the swarmspores
of a genuine Alga were seen for the first time in the act of escaping,
the phenomenon was described as the changing of the plant into an
animal. Trentepohl in 1807, and Unger in 1830, explained in this way
the escape of the zoospores of Vaucheria. The remarkable thing is, not
that such views were entertained, but that the majority of botanists
combined with them a belief in the constancy of species. But this
dogma rendered good service to the science in this instance, for the
botanists, who at a later time applied themselves to the systematic
examination of the Algae and Fungi, confided in the constancy of the
processes of development in each species, which they expected would
assert itself in these forms as in the Mosses and higher plants.
With much that was obscure and doubtful, the result of occasional
observation accompanied by uncritical interpretation, the literature
of the subject had contained for some time a certain number of single
well-established facts of real importance, which were well adapted
to serve as starting-points for earnest and exact investigation.
Among the Algae the genera Spirogyra and Vaucheria especially had
supplied remarkable phenomena; Joseph Gärtner observed the formation
of zygospores in Spirogyra in 1788, Hedwig saw in the mode of their
production at least a suggestion of sexuality (1798), and Vaucher[58],
in his ‘Histoire de Conferves d’eau douce,’ which appeared in 1803
and was far in advance of its time, called conjugation distinctly a
sexual process; the optical means at his disposal did not enable him
to observe the fertilisation in Vaucheria (Ectosperma), which was
named after him, though he described the sexual organs accurately;
the movement also of the zoospores in this genus escaped him, and
Trentepohl first observed their escape and swarming in 1807[59].
Vaucher had also observed the formation of new nets in the old cells
of Hydrodictyon, and Areschoug repeated the observation in 1842, when
he saw the swarming of young cells in the old ones. Bischoff, as early
as 1828, saw the spermatozoids of Chara, though without understanding
them. Observations on conjugating Algae were multiplied; Ehrenberg in
1834 saw corresponding phenomena in Closterium, and Morren described
them more exactly in 1836. The formation of swarmspores in fresh-water
and salt-water Algae was frequently observed between 1820 and 1830,
and in his ‘Neues System,’ iii, which appeared in 1839, Meyen gave a
summary of all that was known up to that time of the propagation of
the Algae. But a new aspect was given to the knowledge of the Algae by
those researches of Nägeli between the years 1844 and 1849, which have
been already mentioned, and which are the first since Vaucher’s time
that can be regarded as systematic. Nägeli studied especially the laws
of cell-division in sexual multiplication and growth, but he considered
the Florideae to be the only Algae that were sexually differentiated,
and distinguished the rest as being without sexuality. Braun in his
‘Verjüngung’ (1850) made numerous contributions to the biology of the
fresh-water Algae, affording many and most interesting glimpses into
a connection still little understood between these forms; and in 1852
he gave an account of the history of growth in the Characeae, a work
conceived in Nägeli’s spirit and a model of scientific research, in
which the mode of derivation of every cell from the apical cell of
the stem was shown, the sexual organs were minutely examined, and
the relation established between the direction of the ‘streaming’ of
the cell-contents and the morphology of the organs. Gustav Thuret
had already made the zoospores of the Algae the subject of detailed
examination.
Such was the condition of affairs with respect to the Algae about the
year 1850, when Hofmeister made the formation of the embryo in the
Phanerogams, the Vascular Cryptogams, and the Muscineae the central
point of investigation in morphology and systematic botany. He made
it clear that a perfect insight into the whole cycle of development
in the plant and into its affinities can only be obtained, if we
succeed in making its sexual propagation, the first commencement of
the embryo, the starting-point of the investigation. It was natural
to expect as happy results from the embryology of the Algae, as had
been obtained in the case of the higher plants; it was important
therefore, that the observer should no longer rest satisfied with a
knowledge of the sexual multiplication of the Algae; he must enquire
into their asexual propagation, and by its aid discover the complete
history of their development. Former observations suggested the
probability that here too sexual propagation is the prevailing rule;
but it was easy to foresee that it would be a task of great labour
to make out a connected history of development, a task of which the
collectors who liked to call themselves systematists had never formed
a conception; but Nägeli’s and Hofmeister’s researches had made
botanists familiar with the highest demands of this kind, and the
men who were to gain new conquests for genuine science were already
engaged in the work in 1850. A splendid result appeared in 1853, in
Thuret’s account of the fertilisation of the genus Fucus; this was a
simple process as a matter of embryology; but the sexual act was so
clear, and even open to experimental treatment, that it threw light
at once upon other cases more difficult to observe. Then followed
discoveries of sexual processes in rapid succession; Pringsheim solved
the old enigma in Vaucheria in 1855, and between 1856 and 1858 in the
Oedogonieae, Saprolegnieae and Coleochaetae; in 1855 Cohn observed
the sexual formation of spores in Sphaeroplea. Pringsheim however was
not content with carefully observing the sexual act; he gave detailed
descriptions of growth in the same families in its progress cell by
cell, of the formation of the sexual organs, and the development of
the sexual product. The asexual propagations which are intercalated
into the vegetation and embryology were shown in their true connection.
Processes were recognised which often recalled the alternation of
generations in the Muscineae; it was shown that very different
forms of sexuality and of general development occur in the Algae,
and these led to the formation of systematic groups, quite different
from those founded on the superficial observation of collectors. It
soon appeared in the Algae, and later in the Fungi and Lichens, that
special investigation must lay new foundations for the system. From
the confused mass of forms not before understood, Pringsheim brought
out a series of characteristic groups, which, thoroughly examined and
skilfully described in words and by figures, stood out as islands in
the chaotic sea of still unexamined forms, and threw light in many ways
on all around them. In like manner the morphology of the Conjugatae
was thoroughly examined by De Bary before 1860; fragments of the
history of development in the Algae were added by Thuret, and he and
Bornet cleared up the remarkable embryology of the Florideae in 1867,
while Pringsheim established the pairing of the swarmspores in the
Volvocineae in 1869. The Algae offer at present a greater variety in
the processes of development than any other class of plants; sexual and
asexual propagation and growth work one into the other in a way which
opens entirely new glimpses into the nature of the vegetable world.
The old conceptions of the nature of plants had been greatly modified
by Hofmeister’s discovery of the alternation of generations, and the
reduction to it of the formation of the seed in Phanerogams; in like
manner the first beginnings of plant-life, the simplest forms of
Algae, exhibit phenomena, which compel us to revise our fundamental
conceptions of morphology, if we are ever to be able to give a
systematic view of the whole vegetable kingdom.
The methodical examination of the Fungi after 1850 led to similar but
still more comprehensive results. From earliest times the Fungi had
been objects of wonder and superstition; what Hieronymus Bock said of
them has been told in the first chapter; this was repeated by Kaspar
Bauhin, and similar notions existed till late into our own century;
about the middle of the 17th century Otto Von Münchausen thought that
mushrooms were the habitations of Polypes, and Linnaeus assented to
that view. What the nature-philosophers, as Nees von Esenbeck for
instance, had to say on the nature of Fungi, need not be reproduced
here.
Still some useful observations had been accumulating for some time on
this subject; as early as 1729 Micheli[60] had collected the spores
of numerous Fungi, had sown them and obtained not only mycelia but
also sporophores (fructifications), and Gleditsch confirmed these
observations in 1753; Jacob Christian Schaeffer[61] about the year
1762 published very good figures of all the Fungi of Bavaria and the
Palatinate, and collected the spores of many species. Yet Rudolphi
and Link at the beginning of the present century ventured to deny the
germination of the spores of Fungi; Persoon in 1818 thought that some
Fungi grow from spores, others from spontaneous generation. A decided
improvement appears after 1820 in the views of botanists with respect
to Fungi, and to this Ehrenberg’s elaborate essay, ‘De Mycetogenesi,’
published in that year in the Leopoldina, contributed greatly. In that
work he collected together all that was then known on the nature and
propagation of the Fungi, and communicated observations of his own
on spores and their germination; he gave figures also of the course
of the hyphae in large sporophores and in other parts, but his most
important service was a description of the first observed case of
sexuality in a Mould, namely, the conjugation of the branches of
Syzygites. In the same year Nees von Esenbeck sowed Mucor stolonifer
on bread, and obtained ripe sporangia in three days (Flora, 1820, p.
528); Dutrochet proved in 1834 (Mém. ii. p. 173) that the larger Fungi
are only the sporophores of a filiform branching plant, which spreads
usually under ground or in the interstices of organic substances, and
had been till that time regarded as a peculiar form of Fungus under
the name of Byssus. Soon after, Trog (Flora, 1837, p. 609) carried
these observations further; he distinguished the mycelium from the
sporophore, and pointed out that the former is often perennial and is
the first product of the germinating spores. He made an attempt to
examine the morphology of the larger sporophores, and showed that it
was possible to collect the spores of mushrooms on paper, and that
those of Peziza and Helvella are forcibly ejected in little clouds
of dust; he also produced new proofs of Gleditsch’s statement, that
the spores of Fungi are disseminated everywhere by the air. Schmitz
published in ‘Linnaea,’ between the years 1842 and 1845 excellent
observations on the growth and mode of life of several of the larger
Fungi. It was not unnecessary at that time to make it clearly
understood that the spores of Fungi reproduce their species exactly.
But the lower, the small and simple Fungi, those especially which are
parasitic on plants and animals, were the most attractive objects in
the whole field of mycology. Here were difficulties in abundance, here
were the darkest enigmas with which botany has ever had to deal, here
was new ground to be slowly won by extreme scientific circumspection
and foresight. In these forms, as in the Algae, the first thing to
be done was to make out the complete history of development in a few
species; but it was much more difficult in the Fungi than in the Algae
to discover what properly belonged to one cycle of development, and
to separate it from casual phases of development of other associated
Fungi. The merit of first breaking ground in this direction belongs
to the brothers Tulasne, who published before 1850 the first more
exact researches into the Smuts and Rusts; these were followed by a
long series of excellent works on different forms of Fungi, especially
the subterranean, whose mode of life and anatomy were described and
illustrated by splendid figures; but their account of the development
of Ergot of rye (1853), their further investigations into the
formation of the spores and the germination of Cystopus, Puccinia,
Tilletia, and Ustilago, and their discovery of the sexual organs in
Peronospora before 1861, were of greater theoretical importance.
The ‘Selecta Fungorum Carpologia,’ which appeared in three volumes
from 1861 to 1865 with fine figures, some of which represented the
process of development, contributed greatly to the reformation of
mycology. Meanwhile, Cessati had published investigations into the
Muscardine-fungus of the silkworm-caterpillar, and Cohn into a
remarkable Mould, the Pilobolus.
But mycology owes its present form to none more than to ANTON DE BARY,
whose writings, the fruit of twenty years’ labour, it would take too
much space to enumerate one by one. With a correct understanding of
the only means which can lead to sure results in this difficult branch
of study, De Bary made it his first endeavour to perfect the methods
of observation, and not only sought for the stages of development of
the lower Fungi in their natural places of growth, but cultivated them
himself with all possible precautions, and thus obtained complete and
uninterrupted series of developments. By these means he succeeded in
proving that parasitic Fungi make their way into the inside of healthy
plants and animals, and that this is the explanation of the remarkable
fact, that Fungi live in the apparently uninjured tissue of other
organisms, a fact which formerly had led to the supposition that such
Fungi owe their origin to spontaneous generation, or to the living
contents of the cells of their entertainers. Pringsheim had already
observed these occurrences in 1858 in the case of an unusually simple
water-fungus (Pythium). De Bary showed that the intrusive parasite
vegetates inside the plant or animal which is its host, and afterwards
sends out its organs of propagation into the open air, and that at
a given time the organism attacked by the fungus sickens or dies.
These investigations were not only of high scientific interest to
the biologist, but they produced a series of results of the greatest
importance to agriculture and forestry, and even to medicine.
With the Fungi, even more than with the Algae, the chief difficulty in
making out a complete series of developments in the history of each
species arose from the frequent intercalation of the asexual mode of
multiplication into the course of its development, and in the further
peculiarity, that the several stages of development in some cases could
only be completed on different substrata. One of the most important
tasks was to find the sexual organs, the existence of which was
rendered probable by various analogies, and after De Bary had observed
the sexual organs in the Peronosporeae in 1861, he succeeded in 1863
proving for the first time that the whole fruit-body of an Ascomycete
is itself the product of a sexual act, which takes place on the threads
of the mycelium.
The literature of mycology based on De Bary’s methods of observation
and its actual results has been enriched by others also in various
directions since 1860; in the case of the Fungi, as in that of the
Algae, it is not possible yet to see to what results investigation
will ultimately lead; but it is one of the fairest fruits of strictly
inductive method, that it has succeeded in smoothing this thorny
and indeed perilous route, where the enquirer is constantly in
danger of being misled, and in satisfying the severest demands of
science. Conclusions have been already reached that are important for
morphology and systematic botany, and among these the establishment
of the nature of the large sporophores, and of processes similar to
the alternation of generations in the higher Cryptogams should be
especially mentioned. But the most important result remains to be
told; it is, that the two classes of Algae and Fungi, hitherto kept
strictly separate, must obviously be now united, and an entirely
new classification adopted, in which Algae and Fungi recur as forms
differing only in habit in various divisions founded on their
morphology[62].
A few words must be given here to the Lichens. They are the division
of the Thallophytes, whose true nature was last recognised, and
that only in modern times; till after 1850 scarcely more was known
of their organisation than Wallroth had discovered in 1825[63],
namely, that green cells, known as gonidia are scattered through the
fungus-like hyphal tissue of the thallus. After Mohl’s investigations
in 1833, it was known that free spores were formed in the tubes of
the fructifications (apothecia), and that a dust collected from the
thallus and consisting of a mixture of gonidia and hyphae was in a
condition to propagate the species. The genetic relation between
the chlorophyll-containing gonidia and the fungus-like hyphae long
continued to be obscure, till at last, after 1868, it was shown that
the gonidia are true Algae, and the hyphal tissue a genuine Fungus, and
that therefore the Lichens are not a class co-ordinating with the Algae
and Fungi, but a division of Ascomycetes, which have this peculiarity,
that they spin their threads round the plants on which they feed, and
take them up into their tissue. De Bary suggested this explanation, but
it was Schwendener who adopted it without reserve and openly declared
it, as much to the surprise as the annoyance of Lichenologists. It may
be foreseen that their opposition will yield to the weight of facts,
which already leave no doubt in the minds of the unprejudiced.
Thus researches in the domain of the Thallophytes have led during the
last twenty years to a complete revolution in the views entertained
with respect to the nature of these organisms, and enriched botany with
a series of surprising achievements; and the movement there is still
far from having come to an end. But we must regard it as one of the
great results for the whole science that through the examination of the
lower and higher Cryptogams, morphology and systematic botany have been
rescued from many ancient prejudices, that the survey has become freer,
the methods of investigation surer, the questions more clearly seen and
put in more definite form.
SECOND BOOK
HISTORY OF VEGETABLE ANATOMY
(1671-1860)
INTRODUCTION.
That the substance of the more perfect plants consists of layers of
different constitution was a fact that could not escape the most
untutored observation in primitive times; ancient languages had still
words to designate the most obvious anatomical components of plants,
rind, wood and pith. It was also easy to perceive that the pith
consists of an apparently homogeneous succulent mass, the wood of a
fibrous substance, while the rind of woody plants is composed partly
of membranous layers, partly of fibrous and pith-like tissue. The
obtaining of threads for spinning from the rind of the flax-plant,
for instance, must have suggested some idea, if only a vague one, in
the earliest ages of the way, in which the fibrous could be separated
from the pulpy part of the bark by decay and mechanical treatment.
Neither Aristotle nor Theophrastus failed to compare these components
of vegetable substance with corresponding ones in animal bodies, and
it has been already shown in the first book how Cesalpino, following
his masters, took the pith for the truly living part of the plant and
the seat of the vegetable soul, and applied this idea in his morphology
and physiology. He remarked that the root generally has no pith, and
that the part of the root which answers to the wood of the stem is
often soft and fleshy; the composition of the leaves from a green and
succulent substance and strands of fibres at once suggested a certain
resemblance to the green rind of the stem; and it was evidently this
which led him to consider that not only the leaves, but also the
leaf-forms of the flower-envelopes had their origin in the rind of
the stem, while the soft, pulpy, succulent condition of the unripe
seeds and seed-vessels seemed to point to their identity with the pith.
That not only are juices contained in plants, but that they must move
in them, could not escape the simplest reflection; and further, the
bleeding of the vine, the flow of gum from resiniferous trees, the
gushing of a milky juice from the wounds of certain plants, exhibited
so striking a resemblance to the bleeding of a wound in the body of
an animal, that the idea of canals inside the plant, which, like the
veins in animals, contain those juices and set them in motion, appeared
quite natural, as we see plainly from Cesalpino’s reflections on these
structural conditions. If we add that it was known that the seeds are
enclosed in the fruits, and that the embryo, together with a pulpy mass
(cotyledons and endosperm), are in their turn enclosed in the seed,
we have pretty well the whole inventory of phytotomic knowledge up to
about the middle of the seventeenth century.
With careful preparation and skilful dissection of suitable parts of
plants, and attentive consideration of the changes produced by decay
and corruption, anatomical knowledge might have been considerably
enlarged at an earlier time; but seeing is an art that must be learnt
and cultivated; a definite aim and end must stimulate the observer into
willingness to see exactly, and to distinguish and connect together
correctly what he sees. But this art of seeing was not far advanced in
the middle of the 17th century. All that was achieved in this direction
did not go beyond the distinguishing the outer organs of leaf-forms and
stem-forms, and we have seen in the first book how unsuccessful was the
attempt to distinguish the minuter parts of the flower and fruit.
The invention of the microscope made small things seem large, and
revealed to sight what was too small to be seen without it; but the
use of magnifying glasses brought an advantage with it of a different
kind—it taught those who used them to see scientifically and exactly.
In arming the eye with these increased powers the attention was
concentrated on definite points in the object; what was seen was to
some extent indistinct, and always only a small part of the whole
object; perception by means of the optic nerve had to be accompanied
by conscious and intense reflection, in order to make the object,
which is observed in part only with the magnifying glass, clear to
the mental eye in all the relations of the parts to one another and
to the whole. Thus the eye armed with the microscope became itself a
scientific instrument, which no longer hurried lightly over the object,
but was subjected to severe discipline by the mind of the observer
and kept to methodical work. The philosopher Christian Wolff observed
very truly in 1721, that an object once seen with the microscope can
often be distinguished afterwards with the naked eye; and this, which
is the experience of every microscopist, is sufficient evidence of
the effect of the instrument in educating and training the eye. This
remarkable fact appears also in another way. We saw in the history
of morphology and systematic botany that botanists for a hundred
years scarcely attempted to make themselves masters in a scientific
sense of the external and obvious relations of form in plants, and to
consider them from more general points of view. Jung was the first who
applied systematic reflection to the morphological relations of plants
which lay open before his eyes, and it was not till late in our own
century that this part of botany was again handled in a scientific
and methodical manner. This extremely slow progress in obtaining a
mental mastery over external form in plants on the part of those
who are continually occupied with them appears to be due chiefly to
the fact, that the unassisted eye glances too impatiently over the
form of the object, and the attention of the observer is disturbed
by its hasty movements. In direct contrast to this customary want
of thoughtfulness in contemplating the external form of plants, we
find the first observers with the microscope, Robert Hooke, Malpighi,
Grew, and Leeuwenhoek in the latter half of the seventeenth century,
endeavouring by earnest reflection to apply the powers of the mind
to the objects seen with the assisted eye, to clear up the true
nature of microscopic objects, and to explain the secrets of their
constitution. If we compare the works of these men with the utterances
of the systematists of the same period on the relations of form in
plants, we cannot fail to see how superior the matter of the former is
in intellectual value. This appears most strikingly when we put what
Malpighi and Grew tell us of the construction of the flower and fruit
side by side with the knowledge of Tournefort, Bachmann, and Linnaeus
on the same subject.
This enhancement of the mental capacity of the observer by the
microscope is however the result of long practice; the best microscope
in unpractised hands is apt soon to become a tiresome toy. It would be
a great mistake to suppose that progress in the study of the anatomy
of plants has simply depended on the perfecting of the microscope. It
is obvious that the perception of anatomical objects must grow more
distinct as the magnifying power of the instrument is increased, and
the field of sight is made brighter and clearer, but these things by
themselves would not add much to real knowledge. In examining the
structure of plants, as in every science, it is necessary to work with
the mind upon the object seen with the eye of sense, to separate the
important from the unimportant, to discover the logical connection
between the several perceptions, and to have a special aim in the
examination; but the aim of the phytotomist can only be to obtain so
clear an idea of the whole inner structure of the plant in all its
connections, that it can be reproduced by the imagination at any moment
in full detail with the perfect distinctness of sense-perception. It is
not easy to attain this end because the more the microscope magnifies,
the smaller is the part of the whole object which it shows; skilful
and well-considered preparation is required, careful combination of
different objects and long practice. The history of phytotomy shows
how difficult a task it is to combine the separate observations and
to fashion what has been seen bit by bit into a clear and connected
representation.
It appears then that progressive improvement of the microscope was
not in itself sufficient to ensure the advance of phytotomy. It would
not indeed be too much to say, that the progress which microscopic
anatomy made step by step with the aid of imperfect instruments
repeatedly gave the impulse to energetic efforts to improve them. Only
practical microscopists could tell where the real defects of existing
instruments lay; it was their anxiety to make them more manageable,
their constant complaints of the poor performance of the optical
part—complaints loudly expressed, especially at the end of the previous
and the beginning of the present century, which urged the opticians
to turn their attention to the microscope and to endeavour to make it
more perfect. Moreover, essential improvements in the instrument were
made by microscopists themselves. Thus Robert Hooke was the first who
in 1760 gave the compound microscope a form convenient for scientific
observation, and Leeuwenhoek developed the powers of the simple
microscope to their highest point. The modern microscope is greatly
indebted for its perfectness to Amici; nor ought the name of von Mohl
to be omitted here, who invented improved methods for microscopic
measurement, and in his work ‘Mikrographie’ (1846) on the construction
of the microscope gave many practical hints to the opticians.
We shall not then make the most important advances in the anatomy of
plants depend as a matter of course and quite passively on the history
of the microscope; they were determined here as in other parts of
botany by a logical necessity of their own; here as elsewhere we have
to fix our eye on the objects pursued by successive enquirers. If for
this purpose we cast a glance over the history of the subject, it
will appear that its founders in the latter half of the seventeenth
century, Malpighi and Grew, were chiefly bent on determining the
connection between the cellular and fibrous elements in the structure
of plants. Two fundamental forms of tissue were assumed from the first,
the succulent cellular tissue composed of chambers or tubes, and, in
contrast to this, the elongated usually fibrous or tubular elementary
organs, the distinction of which into open canals or vessels and
fibres with closed ends continued to be doubtful. The characteristic
feature of this period is, that the investigation of the more delicate
structure is everywhere closely interwoven with reflections on the
function of the elementary organs, and that thus anatomy and physiology
support each other, but not without mutual injury through the
imperfections of both. But the physiological interest far outweighed
the anatomical with the first phytotomists, who used anatomical
research for the purposes of physiology.
The imperfectness of the microscope during the whole of the eighteenth
century produced a certain disinclination to anatomical studies, which
were after all only regarded as auxiliary to physiology. The latter
had made very important progress without the help of anatomy in the
hands of Hales, and later on towards the end of the 18th century in
those of Ingen-Houss and Senebier, and thus the interest in phytotomy
was almost extinguished. Not only was very little addition made to the
contributions of Malpighi and Grew during the 18th century, but they
had to some extent ceased to be understood.
However towards the end of that time the microscope came again into
fashion; in the compound form it had become somewhat more convenient
and manageable; Hedwig showed how it revealed the organisation of the
smallest plants, and especially of the Mosses, and he examined also
the construction of cell-tissue and vascular bundles in the higher
plants. But with the beginning of the present century the interest in
phytotomy suddenly rose high again; Mirbel in France, Kurt Sprengel in
Germany made the microscopic structure of plants once more the subject
of serious investigation. The performances of both men were at first
extremely weak and contradicted one another; a lively dispute on the
nature of cells, fibres, and vessels grew up during the succeeding
years, and many German botanists soon took part in it; life was once
more infused into the whole subject, especially when the academy of
Göttingen in 1804 offered a prize for the best essay on the disputed
points, for which Link, Rudolphi and Treviranus contended, while
Bernhardi occupied himself with private researches into the nature
of vessels in plants. It was not much that was attained in this way;
men began once more from the beginning, and after 130 years Malpighi
and Grew were still the authorities to whom everybody appealed. Yet
the questions now discussed were in the main different from the old
ones; Malpighi, Grew and Leeuwenhoek had chiefly set themselves the
task of studying the different tissues in their mutual connection; the
moderns were chiefly concerned to get a clearer understanding of the
more delicate construction of the various tissues themselves, to know
what was the true account of cell-structure in parenchymatous tissue,
and the real nature of vessels and fibres. That very slow progress was
made at first in this direction was due partly to the imperfectness
of the microscope, and still more to very unskilful preparations, to
the influence of various prejudices, and to too slight exertion of the
mind. But a comprehensive work by the younger Moldenhawer in 1812 was
a considerable step in advance. It is marked by careful and suitable
preparation of the objects, and by critical examination of what was
observed by the writer himself and of what had been written by others;
in fact it is a fresh commencement of a strict scientific treatment
of phytotomy. Hugo von Mohl continued Moldenhawer’s work after 1828,
and Meyen was a contemporary and a zealous student of phytotomy; but
the period in the study of vegetable anatomy which reaches to 1840 may
be said to have been brought to a conclusion chiefly by von Mohl’s
contributions. Weak as the beginnings were at the commencement of this
period (1800-1840), and important as was the advance made by von Mohl
towards the end of it, yet we may include all that was done during that
time in one view, since the questions examined were essentially the
same; like Mirbel and Treviranus, Moldenhawer and Meyen, von Mohl was
chiefly occupied up to the year 1840 in deciding the questions, what
is the nature of the solid framework of cellulose in the plant in its
matured state, whether a single or double wall of membrane lies between
two cell-spaces, what is the true account of pits and pores, and of the
various forms of fibres and vessels; one great result of these efforts
must be mentioned, namely, the establishment of the fact that all the
elementary organs of plants may be referred to one fundamental form,
the closed cell; that the fibres are only elongated cells, but that
true vessels are formed by cells which are arranged in rows, and have
entered into free communication with one another.
Phytotomists before 1840, and von Mohl especially, had occasionally
paid attention among other things to circumstances connected with the
history of development, and single cases of the formation of various
cells had been described by von Mohl and Mirbel between 1830 and
1840, but greater interest was taken in the right understanding of
the structure of mature tissues; physiological questions also, though
no longer of the first importance in anatomical investigations, were
still of weight, so far as the enquiry was influenced by the relation
of anatomical structure to the functions of elementary organs. But
with Schleiden and Nägeli the question of historical development and
the purely morphological examination of interior structure assumed
an exclusive prominence in phytotomy. The first commencement of
vegetable cells especially and their growth were the subjects now
discussed. Schleiden had proposed a theory of cell-formation before
1840, which, resting on too few and inexact observations, referred
all processes of cell-formation in the vegetable kingdom to a single
form; it attracted great attention in the botanical world, but could
not easily be reconciled with what was already known; and in 1846 it
was completely refuted by Nägeli, who substituted for it the history
of the formation of the various kinds of vegetable cells in their
main features, based on profound and extensive investigations. It was
natural that these researches into the formation of cells should turn
the attention of observers, which had hitherto been almost exclusively
devoted to the solid framework of cell-tissue, to the juicy contents of
cells. Robert Brown had already discovered the cell-nucleus; Schleiden
recognised its more constant presence, but misunderstood its relation
to cell-formation; Nägeli and von Mohl next demonstrated the peculiar
nature of protoplasm, the most important component of vegetable cells,
and especially the weighty part which it plays in their origination.
Unger in 1855 called attention to the great resemblance which exists
between the protoplasm of the vegetable cell and the sarkode of the
more simple animals,—a discovery which was subsequently brought into
prominence by observations on the behaviour of the Myxomycetes,
and after 1860 finally led zootomists as well as phytotomists to
the conclusion, that protoplasm is the foundation of all organic
development, vegetable and animal. But there is yet another direction
in which the study of the history of development by the phytotomist led
to new points of view and to new results; we have already pointed in
the end of the first book to the way in which Nägeli after 1844 made
the sequences of cell-division in the growth of organs the basis for
his morphology, and how in this way the Cryptogams especially revealed
their inner structure; we also noticed the splendid results which
Hofmeister achieved by his study of the development of the embryo; here
we have further to show, how after 1850 the various forms of tissue,
especially the vascular bundles, were examined by observation of the
history of their development, and how in this way botanical science
has succeeded in explaining the inner histological connection between
leaves and axes, shoots and parent-shoots, primary and secondary roots,
and above all in gaining a correct insight into subsequent growth in
thickness and so learning to understand the true mode of formation of a
woody body and of the secondary rind.
It is then the task of the following chapter to give a more detailed
account of the history of phytotomy, the salient points in which have
now been indicated.
CHAPTER I.
PHYTOTOMY FOUNDED BY MALPIGHI AND GREW.
1671-1682.
The foundation of vegetable anatomy, indeed of all insight into the
structure of the substance of plants, is the knowledge of their
cellular structure. We find the first perception of this truth in
a comprehensive work of ROBERT HOOKE[64], which appeared in London
in 1667 under the title of ‘Micrographia or some physiological
descriptions of minute bodies made by magnifying glasses.’ The author
of this remarkable book was not a botanist, but an investigator of
nature of the kind more especially to be found in the seventeenth
century; he was mathematician, chemist, physicist, a great mechanician,
and later an architect, and moreover a philosopher of the new school
then rising. Beside many discoveries in various subjects he succeeded
in 1660 in so far improving the compound microscope, that with
considerable increase in magnifying power it had tolerably clear
definition. With this instrument Henshaw in 1661 is stated to have
discovered the vessels in walnut-wood, a fact not of importance for our
history. Hooke himself was anxious to show the world how much could be
seen with his improved instrument; as an adherent of the inductive
method he desired to aid in perfecting the perceptions of sense which
are the foundation of all human knowledge; with this feeling he
submitted all sorts of objects to his glass, that it might be known how
much the unassisted eye fails to perceive. He made what he saw texts
for discussions on a multiplicity of questions of the day. The book
therefore was not devoted to phytotomy; the structure of the substance
of plants is noticed in the same incidental manner, as the discovery
of parasitic fungi on leaves, or other similar matters. And what Hooke
saw of the structure of plants was not much, but it was new, and on
the whole fairly judged. It appears that he discovered the cellular
structure in plants by examining charcoal with his glass, and that he
then tried cork and other tissues. He says that a thin section of cork
on a black ground (by direct light therefore) looks like honey-comb; he
distinguishes between the hollow spaces (pores) and the dividing walls,
and to the former he gives the name which they yet bear; he calls
them cells. The arrangement of the cork-cells in rows misleads him
into taking them for divisions of elongated hollow spaces, separated
by diaphragms. These, he says, are the first microscopic pores, which
he or any one else had ever seen, and he regards the cell-spaces of
plants as examples of the porousness of matter, as do the text books
of physics up to modern times. Hooke employed his discovery especially
to explain the physical qualities of cork; he estimates the number
of pores in a cubic inch at about twelve hundred millions. He draws
another botanical conclusion; he gathers from the structure of the
cork that it must be an outgrowth from the bark of a tree, and appeals
to the statements of one Johnston in proof of this hypothesis. The
fact, that cork is the bark of a tree, was therefore not yet known
to all educated people in England. Hooke afterwards says that this
kind of texture is not confined to cork; for as he examined the pith
of elder and other trees with his microscope and the pulp of hollow
stems, such as those of fennel, teasel and reed, he found a similar
kind of structure with the difference only, that in the latter the
pores (cells) are arranged lengthwise, in cork in transverse rows. He
says that he has never seen any passages for communication between the
cells, but that they must exist, because the nourishing juice passes
from one to another; for he has seen how in fresh plants the cells are
filled with sap, as are the long pores in the wood; but these he found
empty of sap in the carbonised wood, and filled with air.
It is plain that it was not much that Hooke saw with his improved
microscope; thin cross-sections of the stem of balsam or gourd, two
plants that grew at that time in every garden, would have shown the
naked eye as much or even more of vegetable structure. At the same time
there is proof here of what was said above on the influence of the
microscope on the use of the eye; the pleasure in the performance of
the new instrument must first direct attention to things which can be
seen without it, but were never seen.
About the time of the appearance of Hooke’s ‘Micrographia’ Malpighi
and Grew had already made the structure of the plant the subject of
detailed and systematic investigations, the results of which they
laid before the Royal Society in London almost at the same time in
1671. The question to which of the two the priority belongs has been
repeatedly discussed, though the facts to be considered are undoubted.
The first part of Malpighi’s large work, the ‘Anatomes plantarum idea,’
which appeared at a later time, is dated Bologna, November 1, 1671;
and Grew, who from 1677 was Secretary to the Royal Society, informs
us in the preface to his anatomical work of 1682, that Malpighi laid
his work before the Society on December 7, 1671, the same day on
which Grew presented his treatise, ‘The Anatomy of plantes begun,’ in
print, having already tendered it in manuscript on the eleventh of
May in the same year. But it must be observed that these are not the
dates of the larger works of the two men, but only of the preliminary
communications, in which they gave a brief summary of the researches
they had then made; the fuller and more complete treatises appeared
afterwards; the preliminary communications formed the first part of the
later works and to some extent the introduction to them. Malpighi’s
longer account was laid before the Society in 1674, while Grew produced
a series of essays on different parts of vegetable anatomy between 1672
and 1682; and these appeared together with his first communication in
a large folio volume under the title, ‘The anatomie of plantes,’ in
1682. Thus Grew had opportunity to use Malpighi’s ideas in his later
compositions; he actually did so, and the important point as regards
the question of priority is, that where he makes use of Malpighi he
distinctly quotes from him. No more is necessary to remove the serious
imputation which Schleiden has made against Grew in the ‘Grundzüge’
(1845), i. p. 207.
Whoever has not himself read the elaborate works of Malpighi and Grew,
but knows them only from the quotations in later phytotomists, may
easily imagine that these fathers of phytotomy had found their way
to a theory of the cell, such as we now possess. But it is not so;
their works have very little resemblance to modern descriptions of
vegetable anatomy; the difference lies chiefly in this, that modern
writers in their accounts of the structure of plants start with the
idea of the cell, and afterwards treat of the connection of cells
into masses of tissue. The founders of phytotomy on the contrary, as
might naturally be expected, consider first and foremost the coarser
anatomical circumstances; they describe the rind, bast, wood, and
pith chiefly of woody dicotyledons, and the histological distinctions
between root, stem, leaf, and fruit in their broader relations, and
examine the detail of the structure of buds, flowers, fruits, and seeds
for the most part only so far as it can be seen with the naked eye.
The more delicate structural conditions are afterwards discussed as a
supplement to this less minute anatomy and always in close connection
with it. The chief emphasis is laid on the consideration of the way
in which the fibrous tissue connects with the succulent parenchyma,
while such questions as the nature of the cell, the fibre, and the
vessel are only incidentally touched upon or discussed at greater
length in the course of the exposition. The mode of investigation and
exposition is therefore chiefly analytic, while in modern compendiums
of phytotomy it is essentially synthetic. It need scarcely be said
that with this mode of treatment the questions which are now regarded
as fundamentally important are either treated as of secondary moment,
or are disregarded; we must not therefore, in judging of the merit of
these men, approach their works with the demands upon them which our
more advanced science would lead us to make. It would be quite wrong
even to think of measuring the value of their books by the extent to
which their contents agree with the modern cell-theory. Both of them
had enough to do to find their way at all in the new world which the
microscope had revealed; many questions which have become trivial for
us had then to be solved for the first time, and the chief merit of
both lies in this very effort to understand first of all the coarser
relations of the anatomical structure of plants; in this respect the
study of their works may yet be recommended to beginners, because
modern phytotomical books are generally very imperfect on these
points. And yet we must not undervalue what Malpighi and Grew had to
say on the more delicate anatomy, and especially on the nature of the
solid framework of cell-membrane in the plant; imperfect and crude as
their views on such points may be, yet they continued for more than a
hundred years to be the foundation of all that was known about cellular
structure; and when phytotomy took a new flight at the beginning of the
present century, Malpighi’s and Grew’s scattered remarks on the union
of cells with one another, and on the structure of fibres and vessels,
were adopted by the later phytotomists and connected with their own
investigations.
If the views of Malpighi and Grew agreed in the main on the points here
mentioned, yet the style and manner of the two were very different.
Malpighi kept more closely to that which could be directly seen;
Grew delighted in tacking on a variety of theoretical discussions to
his observations, and especially endeavoured to follow the path of
speculation beyond the limits of what was visible with the microscope.
Malpighi’s account reads like a masterly sketch, Grew’s like an
elaborate production of great and almost pedantic carefulness; Malpighi
displays a greater formal cultivation, and deals with the questions
with light touches, allusively, and almost in the tone of conversation.
Grew on the other hand is at pains to reduce the new science to a
learned and well-studied system, and to bring it into connection with
chemistry, physics, and above all with the Cartesian philosophy.
Malpighi was one of the most famous physicians and zootomists of his
time, and treated phytotomy from the points of view already opened
in zootomy; Grew too occupied himself occasionally with zootomy, but
he was a vegetable anatomist by profession, and gave himself up,
especially after 1688, almost exclusively to the study of the structure
of plants with a devotion hardly to be paralleled till we come down to
Mirbel and von Mohl.
As in medicine in the 17th century human anatomy was intimately
connected with physiology, and the latter was not yet treated as
a distinct study, so the founders of phytotomy naturally combined
the physiological consideration of the functions of organs with the
examination of their structure. Considerations on the movement of sap
and on food appear in the front of every anatomical enquiry; relations
of structure, which the microscope could not reach, were assumed
hypothetically on physiological grounds, although little positive was
known at the time about the functions of the organs of plants; hence
recourse was had to analogies between vegetable and animal life, and
it is true that vegetable physiology received its first great impulse
by this means, but occasion was given at the same time to many
errors, which in their turn often misled the anatomist. At present,
when vegetable anatomy has separated itself more than is desirable
from physiology, that is, from the investigation of the functions of
organs, it is difficult, nay impossible, to give the reader a brief
account of the contents of these two books which form an epoch in the
science. I must confine myself to noticing a few chief points, which
are historically connected with the further development of phytotomy,
though some of these are just the questions to which Malpighi and Grew
only gave occasional attention, and which it is therefore a little
unjust to them to bring into prominent notice. I shall recur to the
physiological portion of their writings in the third book of this
history, confining myself here to that which concerns the structural
relations of plants.
The phytotomical work of MARCELLO MALPIGHI[65] appeared under the title
‘Anatome Plantarum,’ and to it was added a treatise on hens’ eggs
during the process of incubation (1675). The phytotomical portion of
the book separates into two main divisions, the first of which, the
‘Anatomes Plantarum idea,’ was, as was stated above, completed in 1671,
and contains a general abstract and survey of Malpighi’s views on the
structure and functions of vegetable organs in fourteen-and-a-half
folio pages; the second and much larger portion illustrates in detail
by numerous examples and with the help of many copper-plates the views
expressed in the first part; it will answer our purpose best to turn
principally to the connected expression of the author’s views in the
first part.
He begins his remarks with the anatomy of the stem, and as the rind
first attracts the eye, he takes it first. The outer part of it, he
says, the cuticle, consists of utricles or little sacs arranged in
horizontal rows; these die in time and decay, sometimes forming a dry
epidermis. On the removal of the epidermis, layer after layer of woody
fibre is disclosed, and these layers, usually forming reticulations and
lying one on another, follow the longitudinal direction of the stem.
These fibrous bundles are composed of numerous fibres, and each single
fibre of tubes which open into one another (‘quaelibet fibra insignis
fistulis invicem hiantibus constat’) and so on. The interspaces of the
network are filled with roundish tubes, which usually have a horizontal
direction towards the wood. If the rind is removed the wood appears,
chiefly composed of elongated fibres and tubes, and consisting of rings
or vesicles open towards one another and arranged in longitudinal rows.
The fibres also of the wood do not run parallel to one another, but
allow a network of angular anastomosing spaces to be formed between
them, the larger of which are filled with bundles of tubes, which run
from the rind through these interspaces to the pith, etc., etc. Between
the fibrous and fistulose bundles of the wood lie the spiral tubes
(‘spirales fistulae’), smaller in number but of larger size, so that in
cross sections of the stem they appear with open orifices. They lie in
different positions, but the majority in concentric circles. He says
that in the course of ten years’ examination (from 1661 therefore) he
found these spiral tubes in all plants, and it may be added here that
Grew in the introduction to his book expressly concedes the priority
in this discovery to Malpighi; but Malpighi’s ideas on the subject
of these tubes are extremely indistinct[66], and this gave occasion
to much misinterpretation and to gross errors on the part of later
writers. Malpighi thought he observed a peristaltic movement in these
vessels, a delusion to which many of the nature-philosophers were
particularly fond of surrendering themselves at the beginning of the
present century.
In addition to the bundles of fibres and the tracheae, Malpighi
observed a number of tubes in Ficus, Cupressus, and other plants, which
allowed the escape of a milky juice, and he concludes that similar
special tubes might be present also in the wood of stems from which
milk, turpentine, gum, and the like exude.
Such are the elementary organs of plants, as far as they were known to
Malpighi; in the subsequent part of his book we find them applied to a
histology of the stem, and here a mistake at once makes its appearance,
which, resting on his authority, was reproduced by the phytotomists of
the 18th and even of the early part of the 19th century,—the theory,
namely, that the young layers of wood in the stem originate in the
periodic transformation of the innermost layers of bark (secondary
bast-layers); Malpighi was led into this mistake, as it appears, partly
by the softness and light colour of the alburnum, partly by its fibrous
character. In this substance the spiral tubes are gradually formed, and
as the mass becomes more solid and compact, it subsequently forms the
true wood.
The pith lies in the centre of the stem, and, according to Malpighi,
consists of numerous rows of spheres (‘multiplici globulorum ordine’)
arranged longitudinally one after another, and composed of membranous
tubes, as may be clearly seen in walnut, elder, and other trees. In
this place also he mentions the milk-vessels in the pith of the elder.
Passing over many and various matters, it may be mentioned next that
Malpighi recognises the connection of the layers of tissue in young
shoots with those of the parent-stem, and very expressly notices the
same continuity of structure between the leaf and the axis of the
shoot. He then briefly touches on the anatomical relations of the
fruit and the seed, the existence of the embryo in the seed and its
structure, and then goes on to the roots. ‘The roots of trees are a
part of the stem, which divides into branches and ultimately ends
in capillary threads (‘capillamenta’); so that, in fact, trees are
simply fine tubes, which run separate from one another underground but
gradually collect into bundles; these bundles unite further on with
other and larger bundles, and all together ultimately join to form a
single cylinder, the stem, which then by separation of the tubes at the
opposite extremity stretches out its branches, and by continued gradual
separation of the larger into smaller finally expands into leaves, and
so reaches its furthest limits.’ The conclusion of the whole account is
chiefly concerned with the part played by the various kinds of tissue
in the nourishment of the plant.
In the second part published in 1674, the different kinds of tissue in
the stem are discussed at greater length; here there is much that is
really good, but at the same time much that is imperfect to an extent
which cannot be attributed solely to the inferiority of his microscope.
Very excellent is the way in which he endeavours to make out the more
obvious anatomical relations of the rind, the wood, and the pith, and
in the texture of the rind and the wood connects the longitudinal
course of the vessels and woody fibre with the horizontal course of the
medullary rays and the ‘silver-grain.’ The magnifying powers which he
used must, to judge from his figures, have been very considerable; how
much of what is imperfect in them is due to the indistinctness of the
field of view, and how much to inaccurate observation, we cannot say.
For instance, he sees the bordered pits in the wood of Conifers without
perceiving the central pore, and represents them as coarse grains lying
on the outside of the wood-cells; it was unfortunate for Malpighi, as
for his successors, that the large vessels in the wood of dicotyledons,
to which they gave most of their attention, are often filled with
secondary tissue (thylosis), which Malpighi figures Tab. vi, fig. 21,
but the true nature of which was not understood till 150 years later.
Malpighi, like succeeding phytotomists till as late as 1830, lays great
stress on the structure of the spiral vessels or tracheae, and mentions
particularly that they are surrounded by a sheath of woody fibre;
but he did not fall into the strange notions which Grew and other
phytotomists entertained with regard to the nature of these vessels.
We may at present omit the numerous remarks on assimilation and the
movement of the sap; the descriptions and figures of the parts of
buds and of the course of the bundles of vessels in different parts
of plants, and especially the analyses of the flower and fruit and
the examination of the seed and embryo, conducted with a carefulness
remarkable for that time, deserve a fuller notice, but this would
detain us too long from our main subject.
If Malpighi’s work reads like a masterly sketch in which the author
is bent only on giving the outlines of the architecture of plants,
the much more comprehensive work of NEHEMIAH GREW[67], ‘The anatomy
of plantes’ (1682), has the appearance of a text-book of the subject
thoroughly worked out in all its details; the tasteful elegance of
Malpighi is here replaced by a copiousness of minute detail that is
often too diffuse; while in Malpighi we only occasionally encounter
the philosophical prejudices of his time, which usually lead him
into mistakes, Grew’s treatise is everywhere interwoven with the
philosophical and theological notions of the England of that day;
but we are compensated for this by the more systematic way in which
he pursues the train of thought, and especially by the constant
effort to give as clear a representation as possible of what he sees.
Though he too everywhere introduces physiological considerations
into his anatomical investigation, yet he keeps himself free from
many preconceptions which his successors imported in this way into
phytotomy. To mention one point by anticipation, he avoided the
erroneous notion so common at a later time, and first definitively
removed by von Mohl in 1828, that the cell-walls must have visible
openings to serve for the movement of the sap.
Grew’s work, as has been said, separates into two main divisions;
the first, ‘The anatomy of plants begun, with a general account of
vegetation founded thereupon,’ was printed in 1671, and contains a
brief and rapid account of the general anatomy and physiology of
plants in forty-nine folio pages. Then the anatomy of roots, stems,
leaves, flowers, fruits and seeds appeared as separate treatises in the
following years up to 1682. We may pass over the chemical researches
embodied in this work and the enquiries into the colours, taste and
smell of plants, as well as the previously issued treatise, ‘An idea of
a philosophical history of plants,’ which, as it was first laid before
the Royal Society in 1672, we may imagine to have been intended as a
counterpart to Malpighi’s ‘Anatomes plantarum idea,’ though it is very
different in character and admits much that is foreign to vegetable
anatomy and physiology.
With Grew as with Malpighi the main point of enquiry is not the
individual cell, but the histology; after distinguishing, like
Malpighi, between the parenchymatous tissue and the longitudinally
elongated fibrous forms, the true vessels and the sap-conducting
canals, he is chiefly bent on explaining the combination of these
tissues in the different organs of the plant; and in this point he is
superior to Malpighi both in carefulness of description and in the
beauty of his delineations. Grew’s numerous figures on copper plates,
more carefully executed than Malpighi’s, give in fact so clear an idea
especially of the structure of the root and stem that a beginner may
still use them with advantage; such figures as those on plates 36 and
40 and elsewhere show that he knew how to fashion his observations by
aid of much reflection into a clear representation of the thing seen;
there are, as might be expected, many errors in the details of the more
delicate structure of the various forms of vessels and cells.
Malpighi had not said, whether he considered the cells of the
parenchyma (the term parenchyma comes from Grew) to be perfectly closed
or porous, nor how they cohere; Grew leaves no doubt on this point; he
says distinctly on page 61 that the cells or vesicles of the parenchyma
are closed, that their walls are not traversed by any visible pores,
so that the parenchyma may be compared to the foam of beer. He quotes
Malpighi’s view respecting the vessels of the wood, and supplements
it by saying that the spiral band is not always single, but that two
or more bands entirely separate from one another may form the wall of
the vessel, and also that the spiral thread is not flat but roundish
like a wire, and its turns are more or less close together according to
the part of the plant. He also notices that the spiral tubes are never
branched, and that when they run straight, as in Arundo Donax, they can
be seen throughout considerable distances. The view of the structure of
spiral vessels, which began with Malpighi and was maintained through
the whole of the 18th century, Grew (p. 117) expresses still more
distinctly than Malpighi; but it is to be observed that neither of
them clearly distinguished true spiral vessels with separable spiral
threads from vessels of the kind which occurs in secondary wood, and
only shows a spiral structure on being torn. From the way, says Grew,
in which the threads are woven, it comes to pass that the vessels often
unroll into a flat surface, as we may imagine a narrow ribbon wound in
a spiral about a round staff so that edge meets edge; and if the staff
is drawn out, the ribbon so wound will remain behind in the form of a
tube, and this would answer to an air-vessel in the plant. We should
notice specially that Grew, better taught than the phytotomists of the
18th century, considers the vessels of the wood as air-passages, though
they sometimes convey water. But he goes on with his description of
the wall of the vessel; the flat surface disclosed by the unwinding of
a vessel is, he says, itself composed of many parallel threads, as in
an artificial ribbon, and the threads that are spirally wound answer
to the warp in an artificial tissue, being held together by transverse
threads, which correspond to the woof. To realise to ourselves this
very strange idea of the structure of a spiral vessel as it appeared to
Grew, we ought to know that he thinks that all cell-walls, even those
of the parenchyma, are composed of an extremely fine web; his previous
comparison of cell-tissue with foam was only intended to make the more
obvious circumstances clear to the reader; his real idea is, that the
substance of the walls of vessels and cells consists of an artificial
web of the finest threads. He hints at this on pages 76 and 77, and on
page 120 he returns once more to this conception and dwells upon it at
great length. The most exact comparison, he says, which we can make of
the whole body of a plant is with a piece of fine lace-tissue, such as
women make upon a cushion; for the pith, the medullary rays, and the
parenchyma of the rind are an extremely delicate and perfect tissue of
thread. The threads of the pith run horizontally like the threads in a
piece of woven stuff, and form the boundaries of the numerous vesicles
of the pith and the rind, as the threads in a web bound the interstices
in it. But the woody fibres and the air-vessels are perpendicular to
this tissue, and therefore at right angles to the horizontal threads
of the parenchyma, just as the needles in a piece of lace work that
lies on the cushion are perpendicular to the threads. To complete the
comparison we ought to suppose the needles to be hollow and the tissue
of thread-lace in a thousand layers one above another. Grew himself
states incidentally, that he lit upon this notion from looking at
shrivelled masses of tissue, when he naturally saw wrinkles and folds,
which he took for threads. Besides he seems to have used blunt knives,
which might easily tear the cell-walls into threads; so we might gather
from the figure in Plate 40, where what he supposes to have been a
tissue of thread from the walls of a cell is depicted quite plainly.
Lastly the observation of vessels with reticulated thickening, and
parenchyma-cells with crossed striation may have contributed to his
view.
It will hardly be superfluous to remark here, that Grew’s idea of this
very delicate structure of cell-walls has evidently given rise to the
common expression cell-tissue (contextus cellulosus) when speaking
of plants and animals, an expression which has become naturalised in
microscopy, and is still retained though we no longer think of Grew’s
comparison of cell-structure with artificial lace. But the word tissue
has often misled later writers, as words are apt to do, and made them
found their conception of vegetable structure on the resemblance to an
artificial tissue of membranes and threads.
Grew, like Malpighi, derives the young layers of wood in the stem from
the innermost layers of the rind. The true wood, he says on page 114,
is entirely composed of old lymph-vessels, that is of fibres, which lay
originally in the inner circumference of the rind. But by true woody
substance he understands the fibrous components of the wood, excluding
the air-vessels; his lymph-vessels are the bast-fibres and similar
forms; for, he goes on, the air-vessels with the medullary rays and
the true wood form what is commonly called the wood of a tree; he uses
the term air-vessels, not because these forms never contain sap, but
because they only contain a vegetable air during the proper period of
vegetation, when the vessels of the rind are filled with sap.
The above is certainly a very imperfect account of Grew’s services to
phytotomy; for the points here made prominent were treated by him as
accessories only to the coarser histological relations with which he
chiefly occupied himself.
These two works of Malpighi and Grew, so important not only for botany
but for the whole range of natural science, were not followed during
the course of the next hundred and twenty years by a single production,
which can claim in any respect to be of equal rank with them; that
long time was a period not of progress but of steady retrogression,
as we shall see in the next chapter. But before the beginning of the
18th century ANTON VON LEEUWENHOEK[68] made some contributions to
the knowledge of the details of vegetable anatomy, if not exactly
to the settling of very important points in it; he communicated his
observations on animal and vegetable anatomy in numerous letters to the
Royal Society of London, and these appeared for the first time in a
collected form in Delft in 1695 under the title of ‘Arcana naturae.’ It
is not easy to gain a clear idea of Leeuwenhoek’s phytotomic knowledge
from his scattered statements. He too discussed the less minute anatomy
of fruits, seeds and embryos, and among other things he made occasional
observations on germination, and many on the structure of different
woods. But all bears the stamp of only occasional study of plants; he
was led to his observations by questions of the nature-philosophy then
in vogue, and especially by such as were connected with the theory of
evolution, not unfrequently by mere curiosity and pleasure in things
obscure and inaccessible to ordinary people, but he did not gain from
them a general idea of the structure of plants. In the course of
these observations he did unquestionable service in perfecting simple
magnifying glasses; he made a large number with his own hands, and
these possessed magnifying powers evidently not at the command either
of Malpighi or Grew. By aid of such glasses he discovered the vessels
of secondary wood which are not spirally thickened but beset with
pits, the true character of which however he did not investigate. He
was the first moreover who perceived the crystals in vegetable tissue,
namely in the rhizome of Iris florentina and in species of Smilax,
and this could only be done with strong magnifying powers. In other
matters he repeats the histological views of Malpighi and Grew, and on
the whole his numerous communications seem painfully fragmentary and
unscientific in presence of Malpighi’s elegance and perspicuity, and
Grew’s systematic thoroughness. His figures too, which were not drawn
by himself, are with some exceptions inferior to those of his great
contemporaries.
CHAPTER II.
PHYTOTOMY IN THE EIGHTEENTH CENTURY.
Malpighi had no successor of note in Italy; in England the new light
was extinguished with Hooke and Grew, and has so remained, we may
almost say, till the present day; in Holland also Leeuwenhoek found
none to follow him of equal rank with himself, and the work done in
Germany up to the year 1770 is more wretched than can well be imagined.
There was in fact no original phytotomic research in the first fifty
or sixty years of the last century; the accounts which were given of
the structure of plants were taken from Malpighi, Grew, and Leeuwenhoek
by persons, who, unable to observe themselves, did not understand
their authors and stated things not to be found in their writings.
The feebler and obscurer notions of the older writers were preserved
with a particular preference, and thus it was Grew’s complicated idea
of the web-like structure of cell-walls that made most impression on
those who reported him. This state of decline must not be ascribed to
imperfect microscopes only; these certainly were not good, and still
less conveniently fitted up; but no one saw and described clearly even
what can be seen with the naked eye or with very small magnifying
power; the worst part of the case was that no one tried fully to
understand either the little he saw himself or the observations to be
found in older works, but contented himself from want of reflection
with most misty notions of the inner structure of plants. It is not
easy to discover the causes of this decline in phytotomy in the first
half of the 18th century; but one of the most important appears to lie
in the circumstance, that botanists, following in this the example of
Malpighi and Grew, did not make the knowledge of structure the sole
aim in their anatomical investigations, but sought it chiefly for the
purpose of explaining physiological processes. The food and circulation
of the sap of plants were more and more the prominent questions, and
Hales showed how much may be done in this direction even without the
microscope; the interest therefore of the few, who like Bonnet and Du
Hamel occupied themselves almost entirely with vegetable physiology,
was concentrated on experiment.
Others who knew how to use the microscope, as the Baron von
Gleichen-Russworm and Koelreuter, were drawn away from the examination
of the structure of vegetable organs by their attention to the
processes of fertilisation and especially of propagation. The real
botanists, according to the ideas of the time, and specially those
who belonged to the Linnaean school, considered physiological and
anatomical researches generally to be of secondary importance, if
not mere trifling, with which an earnest collector had no need to
concern himself. That Linnaeus himself thought little of microscopical
phytotomy is sufficiently shown by what has been said of him in the
first book.
It is not worth while to notice each of the few small treatises on the
subject which appeared towards 1760, for they contain nothing new; a
few examples will show the truth of the opinion here expressed on the
general condition of phytotomy at this time.
We first of all encounter a writer, whom few would expect to find
among the phytotomists, the well-known philosopher Christian Baron von
Wolff, who in his two works, ‘Vernünftige Gedanken von den Wirkungen
der Natur,’ Magdeburg (1723) and ‘Allerhand nützliche Versuche,’ Halle
(1721) gives here and there descriptions of microscopes and discusses
subjects connected with phytotomy. This he does more particularly
in the latter work, where he describes a compound microscope with a
focussing lens between the objective and the ocular but without a
mirror, an instrument which must have served therefore for observing
with the light from above on an opaque surface; the objective was a
simple lens. But to magnify objects more strongly, he says that he
used a simple instead of this compound instrument, as was more the
custom at the time. Like a true amateur Wolff submitted all sorts of
small and delicate objects to his glass, without examining any of
them thoroughly and persistently. His phytotomic gains were small; he
observed for instance that starch-flour (powder) consists of grains,
but believed from the way in which they refracted light that they were
small vesicles filled with a fluid; yet he satisfied himself that these
grains are already in the grains of rye and therefore not produced in
the grinding. He laid thin sections of portions of plants on glass
which was too imperfectly polished to allow of his seeing anything
distinctly. His pupil Thümmig in his ‘Meletemata’ (1736) addressed
himself to the subject with still less skill. By the case of these
two men we may see plainly that want of success was due much less to
the imperfectness of the microscope than to unskilful management and
unsuitable preparation. But Wolff and Thümmig at least endeavoured
to see something for themselves of the structure of plants; a famous
botanist of the time, Ludwig, plainly never made a similar attempt,
for in his ‘Institutiones regni vegetabilis’ (1742) he speaks of the
inner structure of the plant in the following manner; ‘Laminae or
membranous pellicles, so connected together that they form little
cavities or small cells and often reticulated by the intervention of
fine threads, form the cell-tissue which we see pervading all parts
of plants. These are what Malpighi and others call tubes, since they
appear in different parts in the form of rows of connected vesicles!’
Boehmer’s ‘Dissertatio de celluloso contextu’ (1785) is still worse;
‘White elastic thicker or thinner fibres and threads woven together
of differing shape and size form cavities or cells or caverns, and
are usually known by the name of cell-tissue.’ We see what mischief
Grew did with his theory of the fibrous structure of the cell-walls,
and how the expression cell-tissue literally taken led the botanists
here named and others into utterly incorrect ideas. The works of Du
Hamel, Comparetti, and Senebier show that such misconceptions were not
confined to Germany, and Hill, a countryman of Grew, according to von
Mohl’s account pictured to himself cells as cups standing one above
another, closed below and open above.
Baron von Gleichen-Russworm (1717-1783), privy counsellor to the
Margrave of Anspach, gave much attention to the perfecting of the
mechanical arrangements of the microscope, but his plates themselves
show how strangely unsuitable these arrangements were. With these
instruments he made many observations, which are recorded in two
voluminous works, ‘Das Neueste aus dem Reich der Pflanzen’ (1764)
and ‘Auserlesene mikroskopische Entdeckungen’ (1777-1781). But these
works contain little or nothing about microscopic anatomy or the
structure of vegetable cells. His observations with the microscope
are chiefly devoted to processes of fertilisation and to proving that
spermatozoa are contained in the pollen[69], and in connection with
these subjects he gives magnified figures of many small flowers, some
of them beautifully executed; these figures must have made his works
very instructive to many in their time. He saw the stomata, which Grew
had already discovered, on the leaves of ferns, but took them for the
male organs of fertilisation, which at the same time showed that he was
still unacquainted with the existence of stomata in phanerogams.
CASPAR FRIEDRICH WOLFF[70] in his efforts in phytotomy stands a
solitary figure among his contemporaries, not only because he was the
first since Malpighi and Grew who devoted labour and perseverance to
the study of the anatomy of plants, but still more because at a time,
when the structure even of matured vegetable organs was almost a
forgotten subject, he endeavoured to penetrate into the history of the
development of this structure and the formation of cellular tissue.
Unfortunately he was not directed to this by an exclusive interest in
phytotomy, but by a more general question which he endeavoured to set
at rest in this manner; he wished to refute the prevailing theory of
evolution by demonstrating the development of the organs of plants,
and to obtain an inductive basis for his doctrine of epigenesis.
Though he was often diverted by these means from the pursuit of purely
phytotomic questions, yet his famous work, ‘Theoria Generationis’
(1759) is nevertheless important in the history of phytotomy; for
though it was disregarded by botanists during the succeeding forty
years, or at any rate exercised no noticeable influence, yet it was
Wolff’s doctrine of the formation of cellular structure in plants
which was in the main adopted by Mirbel at the beginning of the
present century, and the opposition which it encountered contributed
essentially to the further advance of phytotomy. This late but
lasting influence of Caspar Friedrich Wolff’s work was due not to the
actual correctness but to the thoughtfulness of his observations, and
to the earnest desire which inspired them to search out the true
nature of vegetable cell-structure and to explain it on physical and
philosophical grounds. The observations themselves on this point are
highly inexact, and influenced by preconceived opinions, and his
account of them is rendered obscure and often quite intolerable by
his eagerness to give an immediate philosophic explanation of objects
which he had only imperfectly examined. His efforts to follow the
course of development in the first beginnings of the formation of
cell-tissue were evidently not seconded by sufficient knowledge of
the structure of matured organs, and, to judge by his figures and by
his theoretical reflections, his microscope was of insufficient power
and its definition imperfect. Notwithstanding all these deficiencies,
Wolff’s treatise is doubtless the most important work on phytotomy that
appeared in the period between Grew and Mirbel, not, as has been said,
on account of any particular excellence of observation, but because its
author was able to make some use of what he saw, and to found a theory
upon it.
According to that theory all the youngest parts of plants, the punctum
vegetationis in the stem, which Wolff first distinguished, the youngest
leaves and parts of the flower, consist of a transparent gelatinous
substance; this is saturated with nutrient sap, which is secreted at
first in very small drops (we might say vacuoles), and these, as they
gradually gain in circumference, expand the intermediate substance and
so present enlarged cell-spaces. The intermediate substance therefore
answers to what we should now call the cell-walls, only these are at
first much thicker, and are constantly becoming thinner with the growth
of the cell-spaces. We may compare young vegetable tissue, formed as
Wolff imagines, with the porosity of fermenting dough, except that the
pores are not filled with gas but with a fluid. It is plain from the
above description that the vesicles or pores, as Wolff names the cells,
are connected together from the first by the intermediate substance,
and that one lamina or cell-membrane only lies between each of two
adjoining cells, a point which succeeding phytotomists were a long
time in determining. As cells are formed by the secretion of drops of
sap in the fundamental substance which is at first homogeneous, so
vessels, according to Wolff, are produced by longitudinal extension of
a drop in the mucilage and formation of a canal; consequently adjoining
vessels must be separated from one another by a single lamina of the
fundamental substance. Though Wolff expressly mentions the movement of
the sap within the firm mucilaginous substance between the cellular
cavities and the vascular canals, a movement of diffusion as it might
now be termed, he inconsistently enough thinks it necessary to assume
the existence of perforations in the bounding-walls of cells and
vessels to serve for the movement of sap from cell to cell and vessel
to vessel; yet in the single case in which he succeeded in obtaining
isolated cells, namely in ripe fruits, he was obliged to allow that the
walls were closed.
The growth of the parts of plants, according to Wolff, is effected by
expansion of existing cells and vessels, and by the formation of new
ones between them in the same way as the first vacuoles were formed
in the mucilaginous substance of very young organs; that is to say,
the sap which saturates the solid substance between the passages and
cavities of the tissue separates in the form of drops, which increase
in size and then appear as cells and vessels introduced between the
older ones. The substance between the passages and cavities, at first
soft and extensible, becomes firmer and harder with increasing age,
and at the same time a hardening substance may be deposited on it from
the sap which is stagnant in the cell-cavities and in movement in the
vascular passages, and this substance in many cases appears as their
proper membrane.
This is in all essential points Wolff’s theory. We may omit his
statements on the subject of the first formation of leaves at the
growing point and of the development of the parts of the flower, as
well as his physiological views on food and sexuality, which remained
for a long time without influence on the growth of opinion, and mention
only his doctrine of the growth of thickness of the stem. The stem is
originally the prolongation of all the leaf-stalks united together.
As many bundles of vessels are formed in the developed stem as there
are leaves springing from the vegetative axis; each leaf has a single
vascular bundle belonging to it in the stem, in modern phraseology
an inner leaf-trace. The union of these bundles from the different
leaves forms the rind of the stem; but if the leaves are very numerous,
their descending bundles form a closed cylinder, and if the stem is
perennial, the fresh production of leaves every year produces new zones
of wood of this kind every year, which are the yearly rings. This view
of Wolff’s on the growth of the stem in thickness bears an unmistakable
resemblance to the theory afterwards suggested by Du Petit-Thouars,
according to which the roots which descend from the buds are supposed
to effect the thickening of the stem.
The contests between Mirbel and his German antagonists at the beginning
of the present century will bring us back again to the more important
points in Wolff’s theory of the cell. Contemporary botanists paid less
attention to the ‘Theoria Generationis’ than they did to Hedwig’s[71]
phytotomic views, not on the formation of cells, but on the structure
of mature tissue. HEDWIG had given various figures and descriptions of
phytotomic subjects in his ‘Fundamentum Historiae Muscorum’ (1782) and
afterwards in his ‘Theoria Generationis’ (1784); but he treats these
topics at greater length in his treatise ‘De fibrae vegetabilis et
animalis ortu,’ published in 1789, and known to the author of this work
only imperfectly from quotations in later writers. Hedwig’s figures
of histological objects appear to be better than those of any of his
predecessors; they show that he used strong magnifying powers, and that
his glass had a clear field of sight. His defect lay in preconceived
opinions and hasty interpretation of what he observed. In order to
refute Gleichen’s view of the stomata in ferns, he demonstrated the
existence of these organs in many phanerogams, and observed the opening
of the slits, which he named ‘spiracula.’ On the epidermis which he
had removed for the purpose of these observations he saw plainly the
double contour lines bounding the epidermis-cells, and therefore the
cell-walls, which are at right angles to the surface. These he took
for a particular form of vessel, and called them ‘vasa reducentia’ or
‘lymphatica,’ and afterwards ‘vasa exhalantia,’ and he thought that
he had found them again in the interior of parenchymatous tissue,
evidently taking the places where three wall-surfaces meet for vessels;
such vessels he also saw in the milk-cells of Asclepias, described in
1779 by the elder Moldenhawer, who seems himself to have regarded even
the intercellular spaces in the pith of the rose as equivalent to these
milk-cells. The word vessel even in the 18th century was used in such
an indefinite manner, that the broad air-tubes of the wood and the
finest fibres were called vessels. Hedwig’s idea of the construction
of spiral vessels was strange enough; he took the spiral band itself
for the vessel, and supposed it to be hollow because it is coloured by
absorption of coloured fluids; in those spiral vessels in which the
turns of the spiral band are distant he saw, it is true, the delicate
original membrane which lies between the turns, but he supposed that it
lay inside the spiral band, which was wound round it on the outside. On
the second plate of the first part of the ‘Historia Muscorum’ he even
figures the network of ridges which the adjoining cells have left on
the wall of the spiral vessel, but explains it as wrinkles caused by
desiccation.
Hedwig was without doubt a very practised microscopist, and he
constantly recommended the extremest care in the interpretation of
all that the instrument reveals; but if an observer so careful and
practised, who moreover was provided with a glass of tolerably strong
magnifying power, fell into such gross mistakes, it cannot surprise us
if others, as P. Schrank, Medicus, Brunn, and Senebier, accomplished
still less. These highly unimportant achievements are all that mark the
close of the 18th century.
CHAPTER III.
EXAMINATION OF THE MATURED FRAMEWORK OF CELL-MEMBRANE IN PLANTS.
1800-1840.
There is no sharp line of division between the 18th and the 19th
centuries; the phytotomists who appear on the scene during the first
years of the new century are scarcely more successful than Hedwig and
Wolff; careful and judicious interpretation of their own and others’
observations is still rare, and they are often misled by preconceived
opinions.
In one respect indeed a very great improvement appeared with the
commencement of the 19th century; the number of phytotomists working
contemporaneously, checking and criticising one another, became all at
once much larger. Hitherto ten or twenty years had intervened between
every two works on phytotomy; but in the course of the twelve years
after 1800 nearly as many publications followed one another, and
scientific discussion enlivened enquiry. Now we meet with a Frenchman
for the first time in the field of phytotomy, Brisseau Mirbel, who
brought out his ‘Traité d’Anatomie et de Physiologie Végétale’ in 1802,
and raised a series of questions in the discussion of which several
German botanists, Kurt Sprengel (1802), Bernhardi (1805), Treviranus
(1806), Link and Rudolphi (1807), at once took part. It was a step
in advance and one affecting all botanical studies, that with the
exception of Rudolphi all these men, like Hedwig before them, were
botanists by profession; it was at last felt that the examination
of the internal structure of plants, as well as the describing them
according to Linnaean patterns, was a part of botanical enquiry; it is
at the same time true that the botanical knowledge of these observers
was often of service to them in their phytotomical investigations, and
directed their work decidedly and from the first towards that which
was worth knowing, and towards the objects which claimed the first
attention. This remark applies to the younger Moldenhawer even more
than to the botanists above-named; his ‘Beiträge,’ published in 1812,
may be taken as closing the first section of this century, during
which time he improved the methods of observation, compared his own
observations and those of others with great acuteness of judgment, and
did all that could be expected with the microscopes of the time.
The period of sixteen years after Moldenhawer, from 1812 to 1828, has
nothing of material importance to show in phytotomy. On the other
hand, it produced a series of the most important improvements that the
compound microscope has undergone since its invention.
As early as 1784 Aepinus had produced objectives of flint and crown
glass, and in 1807 Van Deyl[72] made similar ones with two achromatic
lenses, and still the phytotomists complained of the condition of their
instruments. Their figures show that they could not see clearly with
them, though the magnifying powers were not high; Link says expressly
in the preface to his prize-essay of 1807, that he usually observed
with a lens that magnified a hundred and eighty times. Moldenhawer
in 1812 gives the preference over all the microscopes he had used
to one by Wright, which was serviceable with a magnifying power of
four hundred times, while the German instruments, especially those by
Weickert, could not be used with higher powers than from one hundred
and seventy to three hundred.
A certain interval elapsed each time between an improvement in the
instrument and the appearance of the advantages which phytotomy derived
from it; thus in 1824, Selligue exhibited to the Academy of Paris an
excellent microscope with double lenses, several of which could be
screwed on one over the other, and which could be used with ordinary
daylight and a magnifying power of five hundred times; in 1827 Amici
made the first achromatic and aplanatic objectives with three double
lenses screwed on one over the other, the flat sides being turned
to the object. And yet still in 1836 a practised phytotomist like
Meyen spoke with disapproval of the instruments of his time, and gave
the preference to an old English microscope by James Man, though he
allowed that the newest instruments by Ploessl were a little better.
In his work on phytotomy, which appeared in 1830, all the figures were
magnified two hundred and twenty times, as were the very beautiful
figures in his prize essay of 1836; but in his ‘Neues System’ (1837),
he had already adopted powers that magnified to over five hundred
times. How rapid the progress was in the years before and after 1830 is
shown by comparing von Mohl’s work on climbing plants of 1827 and its
antiquated illustrations, with his publications of 1831 and 1833, where
the figures have a thoroughly modern appearance.
The art also of preparing anatomical objects rose by degrees with the
improvement of the microscope. It was not in a very advanced state at
the beginning of the century, if we judge by the language of writers
and by their figures. It was a great step in advance when the younger
Moldenhawer in 1812 isolated cells by maceration and decay in water,
and was thus enabled to view cells and vessels on every side and in a
perfect condition, to see their real shape, and to survey the manner of
their combination more exactly than had hitherto been done. But even
Moldenhawer still made the mistake of submitting delicate microscopic
objects to observation in a dry state, though Rudolphi and Link in 1807
had urged the advisability of keeping every part of the preparations
moist, especially the surface towards the object-glass, which shows
that they did not then use covering glasses. Nor was sufficient
attention shown to the use of sharp knives of suitable form, such as
the razor, which is now almost exclusively employed, or to practice
in making transverse and longitudinal sections of the utmost possible
delicacy,—two things which, through the example of Meyen’s and von
Mohl’s practice, were afterwards recognised as indispensable helps to
phytotomy; even in their time observers were satisfied with crushing
and picking their preparations to pieces.
Drawing from the microscope kept even pace on the whole with increasing
skill in making preparations, and with the improvement of the
instrument. If we compare together the drawings of Mirbel and Kurt
Sprengel in the beginning of the century, those of Link and Treviranus
in 1807, Moldenhawer’s in 1812, and Meyen’s and von Mohl’s from
1827 to 1840, we shall obtain a rapid and instructive survey of the
history of phytotomy during this period of forty years. The figures
testify at once to constant increase in the magnifying powers, to
the greater clearness of the field of sight, and still more to the
constant improvement in the arts of preparing and observing objects.
But a curious misconception crept in among the phytotomists at this
time; they believed that more correct and trustworthy figures would be
obtained, if the observer and writer did not himself make them, but
employed other eyes and other hands for that purpose; they imagined
that in this way every kind of prejudice, of preconceived opinion would
be eliminated from the drawings. Thus both Mirbel and Moldenhawer had
their figures drawn by a woman, and many later phytotomists entrusted
the execution of their drawings to hired draughtsmen, as Leeuwenhoek
had done before them. A drawing from the microscope, like every
other copy of an object in natural history, cannot pretend to take
the place of the object itself, but is intended to give an exact and
clear rendering of what the observer has perceived, and by so doing
illustrate the verbal description. The drawing will be perfect in
proportion to the practised skill of the eye that observes and of the
mind that interprets the forms. The copy should only show to another
person what has passed through the mind of the observer, for then
only can it serve the purpose of a mutual understanding. There is
also another point to be considered; it is exactly in the process of
drawing a microscopic object that the eye is compelled to dwell on
the individual lines and points and to grasp their true connection in
all dimensions of space; it will often happen that in this process
relations will be perceived, which previous careful observation
had disregarded, and which may be decisive of the question under
examination or even open up new ones. As the microscope trains the
eye to scientific sight, so the careful drawing of objects makes the
educated eye become the watchful adviser of the investigating mind;
but this advantage is lost to the observer who has his drawings made
by another hand. It is not one of the least of von Mohl’s merits, that
he practised microscopic drawing under the influence of the views here
indicated, and sought to make his figures no mere undigested copies of
the objects, but an expression of his own opinions about them.
Enough has been said to show that an important portion of the history
of phytotomy lies between the beginning and the end of the period under
consideration. The distance between the knowledge of the structure of
vegetable tissue which existed at the beginning of the century, and
that of Meyen and von Mohl on the same subject in 1840, is wonderfully
great; in the one case an uncertain groping about among obscure
ideas, in the other a complete exposition of the inner architecture
of the mature plant. But in spite of this great difference between
beginning and end, it is better to review the efforts of this period
of forty years as a connected process of historical development, and,
notwithstanding the interval between the appearance of Moldenhawer’s
contributions in 1812 and Meyen’s and von Mohl’s labours about 1840, to
consider the latter as the settlement of the questions taken up at the
commencement of the century. Moreover after 1840, with the appearance
of Schleiden and Nägeli on the scene, new points of view were suddenly
disclosed, and new aims were proposed in phytotomic investigation; it
is no objection to this view of the subject, that the most productive
portion of von Mohl’s labours falls in the succeeding twenty years, and
that during this later period his position is one of equal authority
with the new tendency and of participation in it. Up to 1845 his
discoveries were the culminating point of the older phytotomy; they put
the finishing stroke to the work which Mirbel, Link, Treviranus, and
Moldenhawer had begun. The object almost exclusively pursued during
all this period was to frame as true a scheme as possible of the inner
structure of the mature organs of the plant; it was requisite to
gain a right understanding of the diversities of cells and forms of
tissues, to classify them and supply them with names, and to secure
well-conceived definitions of these names. Hence almost exclusive
attention was paid to the configuration of the solid framework of
cell-membrane, and of this chiefly in the matured state, to the form
of the several elementary organs and their combination in the tissue,
to the sculpture of the wall-surfaces, and to the connection of
cell-spaces by pores or their separation by closed walls. There was
much discussion, especially at first, on the contents of vessels and
cells, and on supposed movements of sap in connection with anatomical
research, but there was no careful connected investigation of the
cell-contents; it was not yet recognised that the true living body of
the vegetable cell is only a definite part of the contents inclosed by
the cell-wall; the solid walls, the framework of the whole building,
were regarded as of primary importance in the structure of the cell.
It was not till the following period that in the light of historical
development another view asserted itself, namely, that the solid
framework of vegetable tissue with all its importance is yet in the
genetic sense only a secondary product of the phenomena of vegetative
life, that the true cell body, the cell-protoplasm is prior in time and
in conception, and can claim the higher position.
* * * * *
Mirbel, to whom we now return, had in 1801 laid down a theory of
cell-formation which agreed in the main with that of Caspar Friedrich
Wolff; he supposed with Wolff that each cell-space was separated from
its neighbour by a single wall, and relying on fresh observations
asserted the existence of visible pores in the dividing walls of
parenchyma and of vessels, and also maintained some new views on
the nature and formation of vessels. The essential points of this
theory found an opponent in Germany in the person of KURT SPRENGEL,
the well-known historian of botany, and one of the most variously
accomplished botanists of his time, who had published in 1802 a work
written in diffuse and familiar style under the title of ‘Anleitung
zur Kenntniss der Gewächse.’ He relied on his own observations, but
these were evidently made with small magnifying powers, an obscure
field of sight, and indifferent preparations. The cell-tissue, says
Sprengel, consists of cavities of very various shape communicating with
one another, the dividing walls being in some places broken through
and in others wanting. He took the starch-granules which he saw in the
seed-leaves of beans and other plants for vesicles, which increase in
size by absorption of water and so form new tissue; but he did not
explain how we are to conceive of the growth of organs with such a mode
of cell-formation. His account of the vessels is extremely obscure,
even more obscure than Hedwig’s, though he has the merit of refuting
the latter’s strange theory of reconducting vessels in the epidermis;
he also suggested, though only incidentally, the happy idea that spiral
passages and even vessels might arise from cell-tissue, since the
youngest parts of plants have only the latter; but he did not attempt
to explain how or where the process takes place. Like Malpighi and
Grew he supposed that the spiral vessels had no wall of their own,
but thought that the closely-rolled spiral threads formed a wall; the
constrictions in broad short-membered vessels he regarded as real
contractions in their substance, caused by the increased tightening of
the spiral threads through a sort of peristaltic movement,—a mistaken
notion often entertained at the beginning of the century, by Goethe
among others, and connected with ideas of vital power prevalent at the
time. In the stomata, to which he gave the name still in use, Sprengel
like Grew, Gleichen, and Hedwig, saw a circular cushion instead of
the two guard-cells; but he notices the observation first made by
Comparetti, that the orifice closes and opens alternately, being wide
open in the morning and closed in the evening. But he considered the
stomata to be organs of absorption.
Sprengel in enunciating his own theory of cell-formation accused
Mirbel of mistaking the starch-grains in the cells for the pores of
the cell-walls. On this point, so important in the doctrine of the
cell and in physiology, he was followed by the three candidates for
the Göttingen prize, though Bernhardi had in 1805 defended Mirbel’s
view, and had pointed out how little likely it was, that so skilful
an observer as Mirbel should fall into so gross an error. BERNHARDI’S
short treatise, ‘Beobachtungen über Pflanzengefässe,’ Erfurt
(1805[73]), was in general distinguished by a variety of new and
correct observations, and was the work of a simple and straightforward
understanding, which takes things as they are presented to the eye
without allowing itself to be led astray by preconceived opinions. His
observations are certainly the best in the whole period from Malpighi
and Grew to the younger Moldenhawer; his method of dealing with
questions of phytotomy is much more to the purpose than that of the
three rivals for the Göttingen prize.
In the work just mentioned Bernhardi treats of other forms of tissue as
well as vessels, and endeavours to distinguish and classify them more
exactly than had hitherto been done. He contrasts favourably with his
contemporaries in the fact, that he sought to define the histological
terms employed as precisely as possible,—a great step in advance at a
time when phytotomic conceptions were in a very misty condition. He
distinguishes three chief forms of vegetable tissue, pith, bast, and
vessels.
By pith he means the tissue which Grew had named parenchyma, and
which is still so called; it remained a question with him whether the
cells of the pith are pierced by visible pores. By the word bast he
understood not only the fibrous elements of the rind, but those of the
wood also, and in general what is now known as prosenchyma; this agrees
very well with Malpighi’s view, which was adopted by Bernhardi and by
all his contemporaries, that the inner layers of the bast are changed
into the exterior layers of wood to make the increase in thickness of
the woody stem; but he did not admit the same origin in the case of the
innermost portion of the wood, for this is formed from the first in the
young shoots, which alone contain true spiral vessels with threads that
may be wound off.
Bernhardi distinguishes vessels into two main groups, air-vessels and
vessels properly so called. He calls the first group air-vessels for
the same reason that led Grew to give them that name, namely, that they
are filled with air during a part at least of the period of vegetation;
they are found in the wood, and, where there is no closed woody body,
there the woody bundles are formed both of vessels and also of bast
strands which enclose vascular canals. These latter he next divides
into three chief kinds; annular vessels, which he was the first to
discover, true spiral vessels with a band which can be unwound, and
scalariform vessels, by which term he understood not only those with
broad slits, such as are found in Ferns, but also the pitted vessels in
secondary wood. His idea of annular and spiral vessels was perfectly
correct, and he mentions Hedwig’s notion already described, and shows
that its exact opposite is true, namely, that the spiral band is
surrounded by a membrane on the outside,—a fact which was afterwards
denied by Link, Sprengel, and Moldenhawer. On the other hand he did
not understand the sculpturing on the scalariform vessels; he took the
pits in the dotted vessels for thickenings of the wall, such as are
seen in the transverse ridges between the slits in true scalariform
vessels, and the slits he thought were closed. If there was much that
was erroneous in these views, yet Bernhardi contributed essentially
to the clearing up of the subject by his effort to distinguish the
different forms of air-vessels, and especially by pointing attention to
the fact that neither spiral nor annular vessels are found in secondary
wood. The resemblance between different forms of vessels misled many of
his contemporaries into supposing that they are due to metamorphosis
of true spiral vessels. Bernhardi showed that different forms of wall
are found inside one vascular tube, but that this does not depend on
modification with age; observation rather teaches that every kind of
vessel receives its character in its young state, and especially that
the youngest scalariform vessels do not present the form of spiral
vessels.
Under the head of vessels proper he reckoned all tubular forms filled
with a peculiar juice, milk-cells and true milk-vessels, and also
resin-ducts and the like, and he made many good and still valuable
observations on their distribution and sap-contents. He could not see
the differences of structure in these various fluid-conveying vessels
with the low magnifying power of his glass, and therefore attended
chiefly to the structure of the large resin-ducts, which on the whole
he rightly understood.
The question whether there are any other forms of vessels in the plant
beside those already named gave him occasion to define a vessel better
than it had yet been defined, namely as an uninterrupted tube or canal,
and at the same time he found himself obliged to consider whether his
bast-threads are vessels; but he did not give a decided answer to the
question. He declared however distinctly against Hedwig’s reconducting
vessels in the epidermis, as Sprengel had done, and it is worthy of
recognition that he understood the true nature of the corners where
three longitudinal walls of the parenchyma meet, while later observers
found difficulties in them.
Before the appearance of Bernhardi’s work the Royal scientific Society
of Göttingen proposed a subject for a prize in the year 1804, which
shows very plainly what uncertainty was felt at that time on all
points of phytotomy. For this reason it will be well to give it at
length from the preface to Rudolphi’s ‘Anatomie der Pflanzen’ (1807):
‘Since some modern physiologists deny the peculiar construction of
vessels in plants which is attributed to them by other and especially
the older observers, it would be well to institute new microscopical
investigations, which shall either confirm the observations of
Malpighi, Grew, Du Hamel, Mustel, and Hedwig, or prove that plants
have a special organisation of their own which is more simple than
that of animals, whether that organisation is supposed to originate
in simple peculiar fibres and threads (Medicus) or with cellular and
tubular tissue (tissu tubulaire of Mirbel). Attention should also be
given to the following subordinate questions: 1. How many kinds of
vessels may certainly be distinguished from the first period of their
development? The existence of certain forms having been established; 2.
Are the twisted fibres which are called spiral vessels (vasa spiralia)
themselves hollow, and do they therefore form vessels, or do they
serve by their convolutions for the formation of closed cavities,
and how? 3. Do fluids as well as gases move in these cavities? 4. Do
the scalariform ducts arise from adherence of the twisted threads
(Sprengel), or do the threads owe their origin to the ducts (Mirbel)?
Do alburnum and woody fibres originate in the scalariform ducts, or in
true vessels, or in tubular tissue?’
We see in this case as in many similar ones, that the subject was
proposed by persons who understood little of it, and who were unable
to judge of what had been written about it; how else could they have
placed the opinions of a Mustel and a Medicus side by side with those
of Malpighi and Grew? Had Bernhardi or Mirbel set the question, it
would certainly have been better conceived. It was in keeping that
the three essays sent in, all inferior to Bernhardi’s work already
mentioned, though they contradicted one another on the most important
points, were nevertheless all accepted; not less so that Treviranus’
essay obtained only the second place, though it was decidedly better
than the other two, and very much better than Rudolphi’s. The best
result of the whole affair was that it stirred up the phytotomists
of the day, and led Mirbel to submit the three prize treatises to a
searching criticism, especially that of Treviranus, which Mirbel with
professional acumen recognised as the best. Link’s essay appeared
in 1807 under the title ‘Grundlehren der Anatomie und Physiologie
der Pflanzen,’ that of Rudolphi as ‘Anatomie der Pflanzen,’ also in
1807, each forming a handsome octavo volume. The work of Treviranus
had already appeared in 1806 with the title, ‘Vom inwendigen Bau der
Gewächse.’
If we compare the works of Link and Rudolphi[74], which both received
a prize, and which had all the appearance of text-books of general
vegetable phytotomy and physiology, we miss in both any clear
exposition of the conceptions connected with the words used, and the
train of thought therefore is constantly obscure and vacillating.
Yet it is easy to see that they are opposed to one another in all
essential points, Link[75] generally hitting on the correct, or at
least the correcter view. For instance, Rudolphi denies altogether the
vegetable nature of Fungi and Lichens, because he finds no resemblance
between their hyphae and vegetable cell-tissue, and he supposes them
to arise by spontaneous generation; even of the Confervae he says that
the microscope has shown him nothing that agrees with the structure
of plants,—evidently a sign of bad observation or of incapacity to
understand what he saw. Link on the other hand regards all Thallophytes
as plants, and sees that the filaments of Lichens and Fungi consist of
cells, and that cells occur at least in many Algae. Rudolphi praises
in the same breath the views of Wolff and Sprengel on cell-tissue,
although they are directly opposed to one another, and although he
adopts Sprengel’s strange theory of cell-formation without alteration.
Link on the contrary declares against Sprengel’s theory, and on good
grounds, and shows that the vesicles which Sprengel took for young
cells are starch-grains; at the same time he makes new cells be formed
between the old ones. Rudolphi is of opinion that cells open into one
another, as is plainly shown by the passage of coloured fluids. Link
maintains that cells are closed bodies, and proves it well by the
occurrence of cells with coloured juice in the middle of colourless
tissue. Rudolphi represents the orifices of the stomata as encircled
by a roundish rim, which he takes without hesitation for a closing
muscle because the apertures enlarge and diminish. Link is more happy
in taking the part that surrounds the aperture for a cell, or a group
of cells. Rudolphi considers the great cavities in hollow stems and in
the tissue of water-plants as the only air-passages in plants; Link
explains these cavities as gaps caused by the irregular growth of
cellular tissue. With Rudolphi the word vessel means not only vascular
forms in wood, but milk-vessels and resin-ducts also, and to the former
he even transfers Malpighi’s view of the structure of spiral vessels.
Link designates the tubes of the wood only as vessels, combining the
most various forms of them under the term spiral vessels; he excludes
milk-vessels, resin-ducts, and the like from the conception of a
vessel, and in this he is somewhat inconsistent, since he assumes with
Rudolphi that a vessel, in plants as in animals, is a canal for the
conveyance of nutrient sap.
With all these contradictions, the two essays agree in adopting the
old Malpighian view of the growth in thickness of stems, according to
which the new layers of wood are formed from the inner layers of bast,
while between the bast-cells, which are here taken to be identical with
woody fibre, new spiral vessels arise contemporaneously, and, as Link
expressly says, from juices which pour out between the bast-cells.
It is hard to understand how two treatises, so contradictory as
they have been shown to be, could have both received a prize at the
same time, or how the great difference could have been overlooked
between Link’s sensible and well-arranged account of his subject, and
Rudolphi’s uncritical statements, which everywhere rely more on old
authority than on his own observation. It is however certain that
Link’s better production is inferior to Bernhardi’s treatise, unless
we choose to consider the greater copiousness of detail in Link, the
number of his observations, and his acquaintance with the literature
of the subject, as giving him the advantage. His figures, as well as
Rudolphi’s, are not so good as those of Bernhardi.
The work of TREVIRANUS[76], to which the judges at Göttingen awarded
the second place, is much less comprehensive than those of his
competitors; the style is inferior to Link’s, and may even be called
clumsy. But the much better figures show at once that Treviranus was
the more accurate observer, and his work, in spite of the inferiority
of its style, is of far higher value on account of the attention paid
in it to the history of development; Treviranus laid greater stress on
this method than either Link or Rudolphi, and it led him to form views
on some of the fundamental questions of phytotomy, in which we see the
germs of theories afterwards perfected by von Mohl. His account of the
formation of cell-tissue is mainly that of Sprengel, and therefore an
unfortunate one; but nevertheless his observations on the composition
of wood and the nature of vessels were as good and correct as could
be expected from the condition of the microscope at the time. He made
one discovery of considerable value, that of the intercellular spaces
in parenchyma, but he lessened its merit by filling these passages
with sap, and even describing its movement. Woody fibres are due, he
thinks, to strong longitudinal extension of vesicles. He supported
Bernhardi’s view of the nature of vessels, that the separable spiral
threads of spiral vessels are not wound round a membranous tube but
are surrounded by one. He maintains against Bernhardi the distinctness
of punctated vessels or porous woody tubes from false tracheae or
scalariform vessels, while he gave a more correct description of the
latter as they occur in Ferns. He rejected Mirbel’s view that the pits
in dotted vessels are depressions surrounded by a raised glandular
edge, and explained them as grains or little spheres. Against this
mistake we may set off the very important step which he made in
advance, when he not only conjectured that the pitted vessels of the
wood are formed from cells previously divided off from one another,
but proved by observation that the members composing such vessels are
at first actually separated by oblique cross-walls, which afterwards
disappear. But this correct observation was impaired by the mistaken
idea, which Treviranus shared with his predecessors, that the wood
is the result of transformation of the bast, and consequently that
the vessels of the wood are bast-fibres, which elongate considerably
after they are arranged in a direct chain one after the other; the
unevennesses caused by the oblique junctions of the tissue gradually
disappear, the boundaries of each member of a vessel being still for
some time indicated by oblique transverse markings. The dividing
walls originally existing at these points disappear by widening of
the cavities, so that the different parts come to form a continuous
canal. To illustrate the disappearance of a parting wall between two
adjoining cells Treviranus aptly points, somewhat to our surprise, to
the formation of the conjugating tube in Spirogyra. He rejects with
Bernhardi the view represented by Sprengel, Link, and Rudolphi, that
the different kinds of vessels are formed from true spiral vessels;
he says that he had found the scalariform ducts in Ferns so formed in
their earliest stage and not as spiral vessels; he thinks it highly
probable that the distinct transverse bands on false spiral vessels
(scalariform ducts) and the pits of dotted vessels are formed on
the walls of membranous fibre-tubes; in like manner he derives true
spiral vessels from long thin-walled cells, on whose inner surface the
spiral band is formed, and well compares the members of young spiral
vessels with the elaters of the Jungermannieae. Here then we find the
first more definite indications of a theory of growth in thickness
of cell-walls, which, like the theory of the origin of vessels from
rows of cells, was afterwards developed by von Mohl and laid on better
foundations. At the close of the essay the histology of the Cryptogams,
Monocotyledons and Dicotyledons is compared, and the subject is better
and more perspicuously handled than in the corresponding chapters of
his competitors.
Though Treviranus’ account of vegetable tissues was on the whole
weak as far as concerns the history of development, yet MIRBEL[77]
recognised in him the most dangerous opponent of his own theory,
and addressed a public letter to him and not to his other German
antagonists, Sprengel, Link and Rudolphi, in defence of the views he
had formerly expressed. This letter is the first part of a larger
work which appeared in 1808, ‘Exposition et défense de ma théorie
de l’organisation végétale,’ in which Mirbel endeavours to meet the
objections of his opponents with great adroitness of style and with
the results of varied rather than profound observation, and to find
new arguments for his theory of vegetable tissue; he admits that his
former treatises were in many respects faulty, but demands that his
critics should discuss his system as a whole and not take offence at
single expressions. Mirbel’s idea of the inner structure of plants
is essentially the same as that broached by Caspar Friedrich Wolff.
The first and fundamental idea is that all vegetable organisation
is formed from one and the same tissue differently modified. The
cell-cavities are only hollow spaces of varying form and extension in
homogeneous original matter, and have no need therefore of a system of
filaments, as Grew supposed, to hold them together. The tracheae only
are an exception, which Mirbel, in striking opposition to the much
more correct view of Treviranus, considers to be narrow spirally wound
laminae, inserted into the tissue and connected with it only at the
two ends. If it is asked how interchange of sap is effected in such
a cellular tissue as this, it becomes necessary to assume that the
membranous substance of plants is pierced by countless invisible pores,
through which fluids find their way. But nature has a speedier and more
powerful instrument in the larger pores, which the microscope reveals.
Mirbel does not discuss the question how the fluids are set in motion,
easily disregarding such mechanical difficulties, as was usual in those
days, when vital power was always in reserve to be the moving agent. He
warmly repels the imputation, which Sprengel had made against him, of
having confounded pores and granules, by appealing to his figures; he
says that he has depicted prominences on the outside of the walls of
the dotted vessels, and an orifice in each of them, which his opponents
simply never saw. The question whether these prominences lie on the
inside or the outside of the walls of the vessel has no meaning, if
we suppose with Mirbel that the dividing wall is single; he is only
concerned to enquire whether the perforated projections lie on the one
or on the other side of the wall. He refers Treviranus, who had denied
the presence of the pores, to his description of scalariform vessels,
in which he had himself seen the slits which correspond to the pores.
In comparison with these fundamental questions Mirbel’s further account
of matters of detail does not concern us here. He gave a connected
view of the whole of his doctrine of tissues in the form of aphorisms,
which occupy the second part of his book. Of all that he says on the
five kinds into which he distinguishes vessels the most interesting
is the statement, that diaphragms pierced like a sieve separate the
different members of his ‘beaded’ vessels. We find that the weakest
part of Mirbel’s phytotomy, as of that of his opponents, is his
description of the true vessels (vasa propria), with which he classes
the milk-cells of the Euphorbiae and the resin-ducts of Coniferae, but
he saw clearly enough that the latter were canals inclosed in a layer
of tissue of their own. The third part of the book is devoted to these
forms of tissue, and we learn that he classes not only many kinds of
sieve-cell-bundles, but also true bast-fibres, as those of nettle and
hemp, with his bundles of true vessels. Like his opponents he makes
the growth in thickness in woody stems to be due to transformation of
the inner layer of bast; but he gives a new turn to this view, which
brings it nearer to the modern theory; during the period of vegetation
a delicate tissue with large vessels is developed in Dicotyledons on
the confines of the wood and the bark, and these augment the mass of
the woody body, while a loose cellular tissue is formed on the other
side, destined to replace the constant losses of the outer rind. To
later phytotomists, who understood by the word cambium a thin layer
of tissue constantly engaged in producing wood and rind, Mirbel’s
otherwise indistinct conception of growth in thickness must have become
more indistinct from his using the word cambium not for the layer of
tissue afterwards so called, but for a highly ‘elaborated and purified
sap’ which is intended for the food of the plant and makes its way
through all membranes; we see this cambium-sap appear at the spots
where it produces new tubes and cells after the manner of the Wolffian
theory. The cells appear at first as minute spheres, the tubes are very
fine lines; both enlarge and gradually show pores, clefts, etc. This
is essentially Wolff’s doctrine, which Mirbel afterwards endeavoured
to confirm against his German opponents from the germination of the
date-palm with the help of a more powerful microscope.
Mirbel insisted more than the German phytotomists of his day on the
idea, that all forms of vegetable tissue are developed originally
from young cell-tissue, an idea suggested by Sprengel and following
naturally with Mirbel from Wolff’s theory. Both Mirbel and Wolff were
hasty in observation and too much under the influence of theory in
giving reasons for what they observed, and therefore too ready with
far-reaching explanations of phenomena which only long-continued
observation could decide.
Treviranus replied, though after some delay, to Mirbel’s polemics
by incorporating into his ‘Beiträge zur Pflanzenphysiologie,’
Göttingen (1811), an essay entitled ‘Beobachtungen im Betreff einiger
streitigen Puncte der Pflanzenphysiologie,’ in which he again took up
the questions in dispute between himself, Mirbel, Link and others,
and supported his own views by fresh investigations. It cannot be
denied that in this short treatise Treviranus brought some important
questions nearer to a decision; he added materially to the knowledge
of bordered pits, on which subject his views now approximated more
nearly to those of Mirbel; he drew attention to the vesicular nature
of vegetable cells, which are often separable from one another, and
to the occurrence of true spiral vessels in the neighbourhood of the
pith in Conifers also, and among other things discovered the stomata on
the capsule of Mosses. On the subject of the theory of cell-formation
which he had borrowed from Sprengel, he endeavoured to extricate
himself from his difficulty by ingeniously pointing out that though
the starch-grains in the seed-leaves of the bean disappear without
producing new cells in them, they are dissolved and then serve as fluid
material for new cell-formation in other parts of the germinating
plant, which however was giving up Sprengel’s theory; yet he cited as a
direct proof of that theory the origination of gonidia in the cells of
Hydrodictyon, and their development into new nets.
Mirbel and his German opponents moved for the most part in a circle
of ideas which had been formed by the speculations of Malpighi, Grew,
Hedwig and Wolff, though it must be allowed that the observations
of Treviranus did open new points of view. But JOHANN JAKOB PAUL
MOLDENHAWER[78] travelled far beyond these older views as early as
1812 in his important work, ‘Beiträge zur Anatomie der Pflanzen.’ He
took up from the first a more independent position as regards former
opinions than either of the writers hitherto considered. He relied
on very detailed, varied, and systematic observations evidently made
with a better instrument, abided by what he himself saw, and chose his
point of view in accordance with it, while he criticised the views
of his predecessors in detail with an unmistakable superiority, and
in so doing displayed minute acquaintance with the literature of the
subject and varied phytotomical experience. He fixed his eye firmly
on the points in question, and made each one the subject of earnest
investigation and copious and perspicuous discussion. His figures prove
the carefulness of his examination and the greater excellence of his
instrument; they are undoubtedly the best that were produced up to
1812. His mode of dealing with his subject and his figures, though they
were not executed by himself, remind us in many respects of von Mohl,
though it would be more correct to say that von Mohl’s manner reminds
us of Moldenhawer, for from the great respect which von Mohl displays
for him, especially in his earlier writings, it can scarcely be doubted
that he formed himself on Moldenhawer’s ‘Beiträge,’ and first learnt
from them the earnestness and carefulness demanded by phytotomic work.
It has been already mentioned that the study of vegetable physiology
is indebted to Moldenhawer for one important practical improvement.
He was the first who isolated cells and vessels by allowing parts of
plants to decay in water and afterwards crushing and dissecting them, a
process not much used in modern times, though it may still be applied
with advantage in conjunction with what is known as Schulze’s solution,
especially if it is carried out with Moldenhawer’s carefulness and
circumspection. The isolation of the elementary organs of plants
by maceration in water necessarily brought Moldenhawer into direct
antagonism with Mirbel, who with Wolff assumed that the partition
between any two cells was a single wall; whereas Moldenhawer found
that the cells and vessels were closed tubes and sacs after isolation,
and must necessarily, as it would seem, so lie one against another
in the living plant, that the wall between every two cell-spaces is
formed of two membranous laminae, and he expressly says that this is
the case even in very thin-walled parenchyma. This result remained
unassailable, so long as no one was in a position to conclude from
the history of the development of cell-tissue that the partitions
are originally single, or by aid of strong magnifying power to prove
the true structure of the walls and their later separation, and the
differentiation of the once single wall into two separable laminae.
If the view based on the results of maceration was still not the
true view, yet it was nearer the truth as regards the matured state
of the cell-wall than that of Wolff and Mirbel, and the important
advantage was gained of being able to study the form of elementary
organs and the sculpture on their walls more accurately than before.
It is true that Link had occasionally isolated cells by boiling in
1809, and Treviranus had drawn attention in 1811 to the fact that it
was possible to isolate parenchyma-cells in their natural condition;
but neither of them made systematic use of these observations, and
to Moldenhawer belongs the exclusive merit of having first isolated
vessels and woody cells; but as usually happens, he did not himself
obtain all the possible results from his method of preparation. In his
work which indeed embraces the whole of phytotomy, he is continually
recurring to one species, maize. This supplies the starting-point in
every question to be discussed. The results obtained there are the
firm supports on which he leans in the examination of a great variety
of plants, which he then compares together at length. This mode of
treatment was well chosen both for investigation and instruction in the
existing state of the science; it was a particularly happy idea that
of choosing the maize-plant for his purpose; former phytotomists had
generally had recourse to dicotyledonous stems, and preferred those
that had compact wood and complex rind, but the examination of these
plants presents difficulties at the present day to a practised observer
with a good microscope. Occasionally observers had taken the stem of
the gourd, where the large cells and vessels suited small magnifying
power, but where many abnormal conditions occurred to interfere with
their conclusions. The Monocotyledons, like the Vascular Cryptogams,
had hitherto been comparatively neglected. When then Moldenhawer made
a monocotyledonous and rapidly growing plant, with very large-celled
tissue and comparatively very simple structure, the chief subject of
his investigations, he was sure to succeed in making out many things
more clearly than his predecessors. It was an important point that
he found the fibrous elementary organs in this plant united with
the vessels into bundles, which are separated by a strict line of
demarcation from the large-celled parenchyma that surrounds them. Thus
the peculiar character, the idea, of the vascular bundle was brought
prominently into contrast with that of other forms of tissue. This took
the place of the distinction between rind, wood, and pith, which had
served former phytotomists as the basis of their histological survey,
but which is in itself only a secondary result of the later elaboration
of certain parts of the plant. Moldenhawer, in laying the chief stress
from the first on the contrast between vascular bundles and parenchyma,
hit upon a histological fact of more fundamental importance, the right
appreciation of which has since enabled the phytotomist to find his way
through the histology of the higher plants. For while the construction
of Monocotyledons and Ferns must seem abnormal and quite peculiar to
any one who starts with examining the rind, wood, and pith of old
dicotyledonous stems, those on the contrary who, with Moldenhawer, have
recognised a special histological system in the vascular bundles of
Monocotyledons, have the way opened to them to seek for a similar one
in the Dicotyledons, and to refer the secondary phenomenon of wood and
rind to the primary existence of vascular bundles. Moldenhawer did in
fact open this way, when he showed how the growth of a dicotyledonous
stem may be understood from the structure and position of the
originally isolated vascular bundles (Beiträge, p. 49, etc.). But he
was thus of necessity led to the rejection of Malpighi’s theory of the
growth in thickness of woody stems, which all vegetable anatomists from
Grew to Mirbel had adopted. Though Bernhardi and Treviranus made weak
attempts to discredit it, Moldenhawer was the first who distinctly
rejected the origin of the external layers of wood from the inner bast,
and proposed the first really practical basis for the later and correct
theory of secondary growth in thickness (p. 35). The removal of this
ancient error is in itself a very important result, and one which,
apart from all other services, must secure him an honourable place in
the history of botany.
But the light must have its attendant shadow, and all his carefulness
in observation and cautiousness in judgment did not protect him from
one prejudice and its evil consequences. After Moldenhawer had isolated
the elementary organs by maceration, he had to answer the question how
we are to conceive of their firm coherence in the living plant. He came
to the conclusion, as did von Mohl, Schacht, and others after him,
that there must be some special connecting medium; but he did not hit
upon their idea of a matrix, in which the cells are imbedded, or of a
cement which holds them together, but on a much stranger theory, which
reminds us at once of Grew’s thread-tissue, and like that rests partly
on incorrect observations. These were too hastily accepted as the basis
of a theory which in its turn interfered with after observations. He
thought that the cells and vessels were surrounded and held together
by an extremely delicate network of fine fibres; in some cases he
really believed that he saw these fibres, and interpreted in this way
the thickened bands in the well-known cells of Sphagnum, and still
more strangely he appears to have taken the thickened longitudinal
and transverse edges of cells and vessels for such threads. The
unfavourable impression produced by this theory is necessarily
heightened by the fact that he gave the name of cell-tissue, a term
long used in a different sense, to his fancy-structure of reticulated
threads which were to hold the cells and vessels together, while he
called the parenchyma itself cellular substance, an expression which
fortunately no one copied, and which certainly contributed at a later
time to discredit the great services which Moldenhawer rendered to
phytotomy.
His ‘Beiträge zur Anatomie der Pflanzen’ are divided into two portions;
the first treats of the parts surrounding the spiral vessels; the
second of the spiral vessels themselves.
The position and collective form of the component parts of the vascular
bundle in the stem of the maize-plant are well described in the first
section of the work. It is correctly stated that there is a sheath to
the whole bundle composed of strongly thickened fibrous cells, that
each of these cells has its own membrane and is entirely closed, and
that they resemble the bast and the fibrous elements of the wood of
Dicotyledons. The segmented wood-cells and the parenchyma-cells of
the wood arranged in rows are incidentally noticed. Under the name of
fibrous tubes he included the cells of the sclerenchyma-sheath of many
vascular bundles and the true bast and wood-fibres, which latter he
says are wanting in the Coniferae. He explained the secondary growth in
thickness of rind and bast by the example of the shoot of the vine, in
which he correctly distinguished the medullary sheath and the spiral
vessels. In herbaceous Dicotyledons he found the bundles of vessels
to consist of a bast portion and a woody portion, and he attributed
the formation of the compact wood of true woody plants to the blending
together of the woody portions of these separate bundles.
In discussing the parenchymatous cell-tissue he rejects emphatically
and on good grounds the origin of new cells from the granular contents
of older ones, which had been the view of Sprengel and Treviranus, as
also the theory of Wolff and Mirbel, while he maintains against Mirbel
especially, that the separation of fibrous tubes is possible even
where no dividing line can be seen between them in the cross section.
He considers that both in thin-walled and thick-walled parenchyma
the dividing wall is double and the cell-membrane entirely closed.
‘It appears,’ he continues on p. 86, ‘from these observations that
cellular substance consists of separate closed tubes, which may be
round or oval, or more or less elongated, or almost cylindrical in
shape, and these by mutual pressure assume an angular and flattened
form, which is either regular like the cells of the comb of bees or
more or less irregular. Such an aggregate of separate cells (and here
he is certainly quite right) has nothing in common with a tissue,
and the word cell-tissue seems therefore less suitable than the term
cellular substance, composed of cell-like tubes.’ Further on he
rejects Mirbel’s idea of the existence of visible holes in the walls
of cells, and points out that they are not necessary for the movement
of sap. The dispute between Mirbel and his opponents respecting the
porousness of cell-walls was extended at the same time to the stomata
of the epidermis[79], the slits in them being supposed to be apertures
in the epidermis regarded as a simple membrane. Moldenhawer took
occasion to examine the anatomy of stomata more closely, and produced
the first accurate descriptions and figures of these organs, showing
especially that the apertures are not surrounded by a simple border,
as most previous observers believed, but lie between two cells, and
that therefore they are not examples of the existence of pores in
cell-walls, as Mirbel imagined. It may be observed here by the way,
that Mirbel afterwards considered stomata to be short broad hairs;
Amici in 1824, Treviranus in 1821, demonstrated their true structure
by cross sections, and von Mohl at a later period investigated it
thoroughly. Moldenhawer on the present occasion also enquired into
the faculty attributed to stomata of opening and closing alternately,
which, first observed by Comparetti, was then much discussed by
the German phytotomists, and has been made the subject of repeated
investigation in modern times. The whole of this discussion was in
connection with the question of the pitting of cell-walls, the true
nature of which Moldenhawer however never clearly understood.
The peculiar vessels, known as ‘vasa propria,’ were a stone of
stumbling to Moldenhawer, as they were to his predecessors and to many
of his successors, because misled by the resemblance in their contents
he included under this name forms of very different kinds. A very good
description of the soft bast in the vascular bundle of the maize-plant
is followed by a notice of the milk-tubes of Musa, the milk-cells
of Asclepias which he explains incorrectly, and the milk-vessels of
Chelidonium which he understood better. All these ‘vasa propria’ he
took for cellular vessels, formed of tubes opening into one another;
but he clearly distinguished the turpentine-ducts from them, and has
given a correct figure of such a duct from the pine, though he assumes
the existence of a special membrane lying inside the cell-rows which
surround it, and lining the passage. Finally he passes on to the
intercellular spaces, which he considers to be gaps in the cellular
substance, and illustrates by Musa and Nymphaea. He does not notice
particularly the narrow interstices which Treviranus had observed
traversing the parenchyma.
In the second portion of his work he includes all the vessels found in
the vascular bundle of the maize-plant under the term spiral vessels,
but he distinguishes the different forms of them well, and especially
points out that rings and spirals appear on one and the same vascular
tube in different parts of its course, as Bernhardi had already
shown. The isolating of the vessels gave him a better opportunity of
seeing how they are made up of portions of different lengths than his
predecessors had enjoyed, and he proves at some length the existence of
a thin closed membrane forming the vessel, but like Hedwig he places
the thickenings on the outside. He as little overcame the difficulties
of bordered pits as did von Mohl and Schleiden after him. In this case
as in others, it was the history of development which first taught the
true nature of these formations (Schacht, 1860).
It was mentioned in the Introduction that Moldenhawer may be said to
close the first portion of the period from 1800 to 1840, not only
because the majority of the questions ventilated up to that time were
to a certain extent settled by him, but also because there is no
material advance in phytotomy to be recorded for several years after
the publication of his work in 1812. It is true that Kieser in his
‘Grundzüge der Anatomie der Pflanzen’ (1815) attempted a connected
exposition of the whole subject, but his book offers nothing really
new, being merely a playing with the unmeaning phrases of the current
nature-philosophy, while it revived gross errors like Hedwig’s doctrine
of the presence of lymphatic vessels in the tissue of the epidermis,
and made the Mosses consist of conferva-threads. Phytotomy was on the
contrary really enriched by the miscellaneous works of Treviranus
published in 1821, especially in respect to questions connected with
the epidermis, and by Amici’s discovery in 1823, that the intercellular
spaces in plants contain not sap but air, and that the vessels too
chiefly convey air. We may quietly pass over the later writings of
Mirbel, Schulze, Link, Turpin and others, which appeared after 1812
and before 1830, as our business is not so much with an account of the
literature of the subject as with evidence of real advance.
Meyen and von Mohl may be said to have commenced their labours with
1830, and in the course of the succeeding ten years they became the
chief authorities on phytotomy, though a highly meritorious work of
Mirbel’s on Marchantia polymorpha and the formation of pollen in
Cucurbita falls as late as 1835. We may even pass over so elaborate a
work as the ‘Physiologie der Gewächse’ of Treviranus (1835-1838), which
embraces also the whole of phytotomy, because though its treatment of
some of the details is good, it presents its subject virtually from
the points of view opened before 1812. This work, though it neglects
no part of its subject and contains much useful reference to the works
of other observers, was unfortunately out of date at the time of its
appearance, for owing to von Mohl’s labours an entirely new spirit had
entered since 1828 into the treatment of phytotomy.
Though Meyen and von Mohl must be regarded as the chief representatives
of phytotomy from 1830 to 1840, yet they are men of very different
importance in the science. The essential difference between them cannot
perhaps be better shown than by pointing to the fact, that Meyen’s
labours cannot at present claim more than a historical interest, while
von Mohl’s earliest investigations between 1828 and 1840, so far from
being obsolete, are the sources of our present knowledge, and from
them every one must still draw who proposes to cultivate any portion
of phytotomy. Meyen’s views, in spite of the many investigations which
he made himself, are entirely confined within the circle of thought
represented by the Göttingen essayists, though in his observations he
went beyond them, and even beyond Moldenhawer; but the phytotomical
views of these men were from the first no law to von Mohl; he took
up an entirely independent position at once with respect even to
Moldenhawer and Treviranus, though a longer time certainly elapsed,
before he succeeded in freeing himself wholly from Mirbel’s authority.
For these reasons, and because Meyen’s work was interrupted by his
death so early as 1840, while von Mohl aided to advance phytotomy for
another thirty years, we will speak first of Meyen’s labours in that
department.
MEYEN[80] is remarkable for the extraordinary number of his written
productions. In 1826, at the early age of twenty-two, he wrote his
treatise ‘De primis vitae phenomenis in fluidis’; two years later he
published researches anatomical and physiological into the contents of
vegetable cells, and in 1830 appeared his ‘Lehrbuch der Phytotomie,’
founded on his own investigations in every branch of the subject, with
many figures on thirteen copper plates very beautifully executed for
the time. His industry as a writer was then interrupted by a voyage
round the world made in the years 1830-1832, but was again marvellously
productive during the last four years of his life (1836-1840); it
is difficult to conceive how he found time even for the mechanical
part of his work, for in 1836 he published his treatise on the latest
advances in vegetable anatomy and physiology, a quarto volume of 319
pages with twenty-two plates, which gained the prize from the Teyler
society in Haarlem; the figures are well drawn, the style is that of a
practised writer, but the matter of the work is somewhat superficially
handled. A year later (1837) appeared the first volume of his ‘Neues
System der Pflanzenphysiologie,’ and two more volumes by the year
1839,—a work also rich in new observations and figures. In the course
of the same years (1836-39) he wrote detailed annual reports of the
results of investigations in the field of physiological botany, which
fill a portly volume, and published in 1837 a prize-essay on the
organs of secretion, and in 1836 a sketch of the geography of plants;
in 1840 appeared a treatise on fructification and polyembryony, and a
posthumous work on vegetable pathology in 1841. The number of works
thus given to the world between the years 1836 and 1840, though partly
prepared before that period, is so unprecedented, that it is impossible
for the composer to have maturely meditated his facts or their inner
connection, and the study of his writings shows that he was often
too hasty in propounding new views, and in rejecting or accepting
the statements of others. The style is perspicuous and flowing, and
animated by a genuine scientific spirit; but the expressions are often
inexact, the ideas not unfrequently immature, and points of fundamental
importance are sometimes neglected for unimportant and secondary
matters. These faults are the result of hasty production; we must
set against them conspicuous merits; Meyen had an eye open to every
question in phytotomy and left nothing unnoticed, while he made it his
constant aim to give clear general views of his subject as a connected
whole, and enable his reader to see his way in every direction, in
order to make phytotomy and vegetable physiology accessible to wider
circles of scientific men; the like praise is due to his drawings from
the microscope which are beautifully executed; they present to the
reader not the small fragments of earlier phytotomic works but whole
masses of tissue so connected together, that it is possible to gain
some insight into the disposition of the different systems of tissue
and their mutual relations. The superiority of Meyen’s drawings of
1836 as compared with those of 1830 is very striking, though he used
the same microscope in both cases and the same magnifying power of two
hundred and twenty times.
To learn what were Meyen’s independent contributions to the advance
of phytotomy, we must turn to his ‘Phytotomie’ of 1830; for in his
later works and especially in the ‘Neues System der Physiologie’ of
1837 he was able to avail himself of von Mohl’s earliest and searching
investigations; these necessarily influenced his views, though he
always assumed the character of a rival and opponent of von Mohl,
and treated not only Treviranus and Link, but even Kieser and men of
his stamp, as entitled to equal rank with him. And as in his later
writings he was reluctant to acknowledge von Mohl’s services to science
and overlooked their fundamental importance, so in his earlier work
in 1830 he often appears as an assailant of Moldenhawer and tries to
set up Link’s authority against him; we find to our astonishment in
the first volume of the ‘Neues System’ a dedication to Link as the
‘founder of German vegetable physiology.’ The position of a scientific
man in relation to his science as a whole is certainly most simply and
clearly defined by his judgment on the merits of his contemporaries
and predecessors, and we may conclude from what has now been said that
Meyen moved within the circle of ideas of the Göttingen prize-essays,
and did not clearly see the importance of the points of view opened by
Moldenhawer and von Mohl; though it must always be allowed that Meyen
working independently far outstripped Link on his own path.
If it was our purpose to write a biography of Meyen, we should have to
go through his works, and show the steps by which his views arrived at
clearness and precision; it is sufficient in this history to show what
was peculiar and original in his general conception of the problems of
phytotomy. This appears most plainly in the ‘Phytotomie’ of 1830; and
we may base our historical survey on that work because its views are in
the main those of the first volume of the ‘Neues System’ which appeared
seven years later, and still more because a detailed examination of the
later publication would involve us in a lengthy discussion on Meyen’s
scientific relation to von Mohl. It is less important in this place
to give an estimate of Meyen’s character as a man of science than to
show, how in the year 1830, when Mohl was beginning to apply himself
to phytotomy but as yet exercised no important influence on opinion,
views on the structure of plants were formed by one who gave himself up
to its study with decided ability and great zeal; in this way we shall
gain a standard by which to judge of the advance made chiefly by von
Mohl and in part by Mirbel during the succeeding ten years. In judging
of Meyen’s book, we must not forget that it was written when he was
only twenty-five or twenty-six years old, and that it is under any view
of it a remarkable performance for so young a man.
Meyen adopted three fundamental forms of elementary organs in plants;
cells, spiral tubes, and sap-vessels; systems, he says, are formed by
union of similar elementary organs; hence there is a cell-system, a
spiral tube-system, and a system of sap-vessels (vascular system). We
see at once by this classification how closely he follows the ideas
formed before Moldenhawer. The establishment of these three systems is
a retrograde step, since Moldenhawer had already clearly distinguished
between vascular bundles and cell-tissue. Meyen then discusses each
system at length and shows how they are grouped together. He lays
great stress, as he did also at a later period, on the difference in
the characteristic forms of cell-tissue, for which he introduced the
names merenchyma, parenchyma, prosenchyma and pleurenchyma. These
he calls regular cell-tissue, the shapes of the cells being like
geometrical bodies, in opposition to the irregular tissue of Fuci,
Lichens and Fungi. It is a decided improvement on former practice,
and one that marks his later works also, that in connection with the
structure of the solid cell-fabric he discusses the contents of cells
in a special chapter, in which first the matter in solution, then the
granular bodies with organized structure are considered, though with
the latter he classes not only starch-grains, chlorophyll-corpuscles
and the like, but also the spermatozoa in pollen-grains and layers
of thickening matter projecting on the inside of cell-walls, such
as the spiral bands in the elaters of Jungermannieae and several
similar formations. He describes the crystals in vegetable cells at
some length, and finally discusses the movement of the cell-contents
(‘sap’), not omitting that of rotation in the Characeae as observed
by Corti, and in other water-plants. The chapter on intercellular
spaces also shows considerable advance on the views which obtained in
1812; Meyen calls it an account of the spaces produced in cell-tissue
by the union of the cells; the true intercellular passages filled
with air are here distinguished from receptacles of secretions,
resin-passages, gum-passages, oil-passages, and secretion-receptacles
of the nature of cavities. The large air-passages and gaps, such as
occur in water-plants, are a third form of intercellular space; his
air-canals in the wood of oak filled with cell-tissue are obviously
vessels filled with the substance known as thylosis. The form of the
cells in the tissue he thinks is not due to mutual pressure, and he
rejects Kieser’s view that the ideal fundamental form of cells must be
a rhombododecahedron; but he thinks there is a significant resemblance
between the shape of cells and that of basaltic columns.
In dealing with the spiral tube-system he first discusses the spiral
fibre, which appears, he says, either detached between the cells or
inside them as well,—an account of the matter decidedly inferior to
those of Bernhardi and Treviranus. The spiral tubes are, he says on
page 225, cylindrical or conical bodies formed of spiral fibres which
are afterwards surrounded by a delicate membrane. He puts annular,
reticulated, and pitted vessels together as metamorphosed spiral tubes.
His explanation of these forms cannot well be understood except by
supposing that he assumed an actual metamorphosis in time in accordance
with the view of Rudolphi and Link; but he afterwards in his ‘Neues
System,’ i. p. 140 declares this to be a misunderstanding, though his
real meaning is still doubtful; the obscurity attending the doctrine
of metamorphosis did not fail to cause misunderstandings in phytotomy,
as it did in the morphology of organs. Meyen makes only the striated
and pitted vessels in the wood convey air, the true spiral vessels sap.
That vessels are formed from cells, as Mirbel had already maintained
and Treviranus had partly observed, Meyen intimates indeed, but not
with an air of entire conviction.
The different forms of laticiferous organs are examined under the head
of the ‘system of circulation in plants.’ Meyen sees in this system the
highest product of the plant, being fully persuaded with Schulz, that
the latex (milk), or as he also terms it the life-sap, is in constant
circulation, like the blood in the veins. He gives a more summary
account than is his wont of the course of the laticiferous organs, but
bestows more care on the nature of the latex, and on the structure of
the receptacles that contain it. That some of these are produced by
cell-fusion, that others represent intercellular spaces, while others
again are long branched cells, was not known to Meyen or even to later
phytotomists before 1860.
This condensed account of the contents of Meyen’s ‘Phytotomie’ shows a
striking mixture of advance and retrogression, when compared with what
had been achieved before his time; by the side of the fact established
by Treviranus that the epidermis does not consist of a single membrane
but of a layer of cells, to which Meyen assents, we find the gross
mistake of taking the guard-cells of stomata for cuticular glands,
the apertures in which he considers as of secondary importance. It is
still more striking that Meyen expressly rejects on page 120 the fact
established two years before by von Mohl that the pits of parenchyma
are thinner spots, and treats the various pit-formations of the
cell-wall as raised portions of the surface.
In the first volume of his later work the ‘Neues System,’ Meyen gives
a detailed account of phytotomy, which accords on the whole with the
scheme developed in the book we have been examining, and as might
be expected he corrects many errors, adduces many new observations,
and introduces us to many steps in advance of former knowledge; we
shall recur to some of his later views in ensuing portions of this
history with which they are more in connection, remarking only here,
that Meyen paid more attention to the contents of the cell than his
contemporaries, and especially made a number of observations on the
streaming movement, without however recognising the peculiar nature
of the protoplasm which is its substratum. The cell-wall, which he
had once considered to be homogeneous, he afterwards believed to be
composed of fine fibres, a view resting on correct but insufficient
observation and afterwards set right by von Mohl and Nägeli.
It is scarcely possible to imagine a more striking contrast between
two men pursuing the same science than that between Meyen and his much
more important contemporary Hugo von Mohl; Meyen was more a writer
than an investigator; von Mohl wrote comparatively little in a long
time, and only after most careful investigation; Meyen attended more
to the habit, the collective impression produced by objects seen with
the microscope, von Mohl troubled himself little about this, and
always went back to the foundation and true inner connection of the
structural relations; Meyen quickly formed his judgment, von Mohl often
delayed his even after long investigation; Meyen was not critical,
though always prone to opposition, in von Mohl the critical power
much overweighed that of constructive thought. Meyen has not so much
contributed to the definitive settlement of important questions,
as brought to light manifold phenomena, and so to speak accumulated
the raw material; von Mohl on the other hand aimed from the first at
penetrating as deeply as possible into vegetable cell-structure, and
employing all the anatomical facts in framing a coherent scheme.
We have already called attention to HUGO VON MOHL’S[81] pre-eminent
position in the history both of this and also of the succeeding period.
Occupying himself for the most part with phytotomical questions which
had been already investigated, he made the solid framework of cellulose
the object of special and searching examination, and completed the work
of his predecessors on this subject; he thus laid a firm foundation for
the researches into the history of development afterwards undertaken
by Nägeli. Von Mohl, like former phytotomists, generally connected his
researches into structural relations with physiological questions;
but there was one great and unmistakable difference; he never forgot
that the interpretation of visible structure must not be disturbed
by physiological views; he used therefore his thorough physiological
knowledge chiefly to give a more definite direction to his anatomical
researches, and to illustrate the connection between structure and
function in organs. By scarcely any other phytotomist was the true
relation between physiological and anatomical research so well
understood and turned to such practical account as by von Mohl, who was
equally averse to the entire separation of phytotomy from physiology,
and to the undue mixing up of the one with the other, which has led his
predecessors, Meyen especially, into misconceptions.
His anatomical researches profited by his extraordinary technical
knowledge of the microscope; he could himself polish and set lenses,
which would bear comparison with the best of their time. As the
majority of botanists from 1830 to 1850 had little knowledge of
the kind, there was no one so well qualified as von Mohl to give
instruction in short treatises on the practical advantages of a
particular instrument, to remove prejudices and finally as in his
‘Mikrographie’ (1846) to give detailed directions for the management of
the instrument.
But his mental endowments were of course of the higher importance,
and it is difficult to imagine any more happily suited to the
requirements of vegetable anatomy during the period from 1830 to
1850. At a time when men were building fanciful theories on inexact
observations, when Gaudichaud was once more explaining the growth in
thickness of the woody portions of the plant after the manner of Wolff
and Du Petit-Thouars, when Desfontaines’ account of the endogenous
and exogenous growth of stems was still accepted, when Mirbel was
endeavouring to support his old theory of the formation of cells by new
observations and beautiful figures, when Schulz Schulzenstein’s wildest
notions respecting laticiferous vessels were being rewarded with a
prize by the Paris Academy, when Schleiden’s hastily adopted views
respecting cells and fertilisation appeared on the scene with great
external success, von Mohl, for ever going back to exact observation,
was cutting away the ground from under ill-considered theories in
careful monographs, and at the same time bringing to light a mass of
well-established facts leading to further and serious investigation.
These theories have now only a certain historical interest, while von
Mohl’s contemporaneous works are still a rich repertory of useful
observations, and true models of clear exposition.
His written productions were preceded by a careful study of all
branches of botanical knowledge and the auxiliary sciences. That he not
merely acquired knowledge in this way, but trained the powers of his
understanding also, is shown by the striking precision and clearness
of his account of his first investigations. At a time when the
nature-philosophy and Goethe’s doctrine of metamorphosis in a distorted
form were still flourishing, von Mohl in spite of his youth approached
the subjects of his investigation with a calmness and a freedom from
prepossessions, which are the more remarkable when we observe that his
friend Unger was at first quite carried away by the stream, and only
slowly managed to reach the firm ground of genuine inductive enquiry.
Owing to the extravagances and aberrations with which he made
acquaintance as a young man in the nature-philosophy, von Mohl
contracted an aversion to all philosophy, evidently taking the
formless outgrowths from the doctrines of Schelling and Hegel for
something inseparable from it, as we may gather from his address at
the opening of the faculty of natural history in Tübingen, which had
been separated at his instance from that of philosophy. His dislike
to the abstractions of philosophy was evidently connected with his
distaste for far-reaching combinations and comprehensive theories,
even where they are the result of careful conclusions from exact
observations. Von Mohl was usually satisfied with the establishment of
separate facts, and in his speculative conclusions he kept as closely
as possible to what he had actually seen, for instance, in his theory
of the thickening of cell-walls; and where new views opened before
him as a result of his exact observation, he cautiously restrained
himself and was generally content to hint at matters which bolder
thinkers afterwards proceeded to investigate; such a case occurred in
his examination of cell-membranes by polarised light. Hence we miss
to some extent the freer flight of imaginative genius in von Mohl’s
scientific labours; but there is more than sufficient compensation for
this want in the sure and firm footing which he offers to the reader of
his works; if we pass from the study of the writings of phytotomists
before 1844 to those of von Mohl, we are sensible of one predominant
impression, that of security; we have the feeling that the observer
must have seen correctly because the account which he gives of the
matter before us seems so thoroughly natural and almost necessarily
true, and all the more because he himself notices all possible doubts,
and lets those which he cannot remove remain as doubts. In these points
von Mohl’s style resembles that of Moldenhawer, but in von Mohl it
attains to a mastery which is wanting in the other.
There is an evident connection between von Mohl’s dislike of
far-reaching abstractions and philosophic speculation on the results of
observation and the fact, that in the course of more than forty years’
unintermitted application to phytotomy he never composed a connected
general account of his subject. His efforts as a writer were confined
to monographs usually connected with questions of the day or suggested
by the state of the literature. In these he collected all that had been
published on some point, examined it critically, and ended by getting
at the heart of the question, which he then endeavoured to answer from
his own observations.
For the purpose of these observations he looked about in every case
for the most suitable objects for examination, a point to which
former phytotomists, with the exception of Moldenhawer, had paid
little attention; he then studied these objects thoroughly, and thus
prepared the way for the examination of others, which presented greater
difficulties. Every monograph of this kind was a nucleus, round which
a larger number of observations might afterwards gather. In a long
series of such solid productions he treated conclusively all the more
important questions of phytotomy.
Von Mohl’s extraordinary carefulness was not however able to guard
him, calm observer though he was, from some serious mistakes, at least
in his earlier years, such as those which occur in his first theory
of intercellular substance (1836), and in his earliest views on the
nature of the cell-membrane of the pollen-grain (1834). These and some
other errors on the part of a gifted and truly inductive enquirer are
instructive, since they show that observation without any groundwork
of theory is psychologically impossible; it is a delusion to suppose
that an observer can take the phenomena into himself as photographic
paper takes the picture; the sense-perception encounters views
already formed by the observer, preconceived opinions with which the
perception involuntarily associates itself. The only means of escaping
errors thus produced lies in having a distinct consciousness of these
prepossessions, testing their logical applicability and distinctly
defining them. When von Mohl laid down his theory of intercellular
substance, there evidently floated before his mind indistinct,
half-conscious ideas of the kind that Wolff and Mirbel entertained
of the structure of the vegetable cell; and as he considered the
cell-membrane of the pollen-grain to consist of a cell-layer, he
summarised its obscure structural relations under the then very
obscure conception of the cell. As a true investigator of nature, who
adheres always and firmly to the results of further observation, and
endeavours to clear his ideas by their aid, conceding only a relative
value to every view, von Mohl soon escaped from these errors, and
himself supplied proofs of the incorrectness of his former opinion. The
number of really erroneous statements in his works is wonderfully small
considering the very large number of investigations in which he engaged.
In examining the part which von Mohl played in the general development
of phytotomy we can distinguish satisfactorily two periods in his
scientific career, the first of which extends from 1827 to about 1845.
Before 1845 he was acknowledged to be the first of phytotomists,
decidedly superior to all rivals; his authority, though often attacked
by unimportant persons, grew from year to year. This period may be said
to close with the publication of his ‘Vermischte Schriften’ in 1845.
Up to that time investigations into the form of the solid framework of
cell-membrane had chiefly attracted the interest of phytotomists, and
in this subject there was no one who could measure himself with von
Mohl. Yet he began soon after 1830 to study the history of development
in plants; in 1833 he described the development of spores in a great
variety of Cryptogams, in 1835 the multiplication of cells by division
in an alga, and the cell-division in the formation of stomata in 1838;
in this period appeared Mirbel’s first observations on the formation
of pollen-cells (1833). Von Mohl too was the first, if we disregard
Treviranus’ somewhat imperfect notices of the origin of vessels in 1806
and 1811, who explained the history of the development of those organs;
and his theory of the thickening of cell-membranes, the principles of
which are to be found in his treatise on the pores in cellular tissue
(1828), may also be regarded as a mode of conceiving the sculpture of
the cell-membrane from the point of view of the history of development.
Ever since 1838 Schleiden had raised the history of development to
the first rank in botanical investigation, but he had proposed a
thoroughly faulty theory of cell-formation, to which von Mohl at
first at least did not withhold his assent in spite of previous and
much better observations; but after 1842 Nägeli devoted himself still
more thoroughly and with more lasting results to the study of the
development both of vegetable cells and tissue-systems, and of the
external organs. He introduced new elements into phytotomic research,
and it soon became apparent that even the questions hitherto examined
must be grappled with in a different fashion. Von Mohl did not hold
aloof from the new direction, but completed a series of excellent
investigations connected with the new questions in the theory of
cell-formation. The most important of these was his enquiry into the
nature of protoplasm, to which he gave the name still in use. In
his treatise, ‘Die Vegetabilische Zelle,’ which came out in 1851 in
Wagner’s Dictionary of Physiology, he even gave an excellent account
of the modern theory of cell-formation; but notwithstanding all this,
and the great authority which he rightly continued to enjoy, he was no
longer the guide who led the way in the domain of phytotomy, as he had
been before 1845.
His zeal as an observer had at all times been chiefly attracted to the
solid framework of vegetable structure in its matured condition, though
a number of his most important works were devoted to the study of
cell-contents.
Except in his ‘Anatomic der Palmen’ (1831), where he expended much
and to some extent even unnecessary labour on figures representing
the general appearance of the tissue (histologic habit), von Mohl’s
microscopic drawings do not aim at giving the collective impression,
but at facilitating the understanding of the delicate structure of
single cells and their combinations by aid of the simplest possible
lines. He always despised pictures from the microscope, such as were
introduced at a later time by Schacht,—a kind of artistic restoration
of the originals and to some extent a playing with science; and in his
later publications he was more sparing of illustrations or omitted them
altogether, in proportion as he acquired the power of giving clear
verbal explanations of even difficult structural conditions.
Von Mohl’s scientific activity was so wonderfully productive that it
is not easy to present the reader with a clear account of it; but we
must endeavour at least to furnish such a summary of its chief results
as may serve to give a general idea of his importance in the history
of our science. We may here pass over such of his treatises as do not
bear on the main questions of phytotomy, and notice only those that
relate to the structure of the solid framework of plants, because
the historical significance of his investigations into the history of
development can only be understood in connection with the questions to
be treated in the following chapter. But we shall not limit ourselves
to publications which appeared before 1845, though we may be thus
compelled to notice researches which in succession of time belong to
the next period, and indeed almost to the present moment.
1. The view that the cell is the sole and fundamental element in
vegetable structure had been already maintained by Sprengel and Mirbel,
but not supported by exact observations. Treviranus too had shown
that the vessels in wood are formed by the union of rows of cell-like
tubes, but he had never arrived at a thoroughly clear conception of the
matter. On the one side was the theory that the plant consists entirely
of cells, on the other, and for long the old and strange view, that
the spiral thread was an independent elementary organ of vegetable
structure,—a view which Meyen still maintained in 1830. Von Mohl must
be regarded as the first who took up the all-important position, that
not only the fibrous elements of bast and wood, which had long been
considered to be elongated cells, but the vessels of the wood also
are formed from cells; and we may on this point give great weight to
his own assertion that he was the first who observed the formation
of vessels from rows of closed cells. This discovery happened in the
year 1831, and he describes distinctly, though briefly, the decisive
observations in his treatise on the structure of the palm-stem. At the
points of constriction in the vessels he saw the dividing walls, the
existence of which had been denied by all former phytotomists; ‘these
dividing walls,’ he says, ‘are entirely different from the rest of
the membranes of the plant, being formed of a network of thick fibres
with openings between them.’ He studied the history of the development
of these vessels both in palms and in dicotyledonous plants. ‘In the
young shoot,’ he says, ‘are found at the spots, where afterwards
there are large vessels, perfectly closed large cylindrical tubes
with a transparent and very delicate membrane.’ He then shows how by
degrees the sculpture peculiar to the walls of vessels is formed on the
inside of these tubes, and he takes the opportunity of saying that a
metamorphosis in time from one form of vessel into another is entirely
out of the question, as Treviranus also and Bernhardi had maintained.
‘The dividing walls (transverse septa),’ he continues, ‘are formed
in a precisely similar manner to the side-walls of vessels; only the
original tender membrane of the septa is usually lost in the meshes
of the network of fibres.’ From that time no phytotomist capable of
an independent judgment has had any doubt with regard to this view of
the formation of the vessels in wood. It is however striking enough
that von Mohl, who thought it so important to show that the cell is
the sole foundation of vegetable structure, never extended the proof
to milk-vessels and other secretion-canals in order to show whether
and how these also are formed from the cells. In his treatise on the
vegetable cell (1851) he still expressed doubt about Unger’s assertion,
that the milk-vessels are also formed from rows of cells that coalesce
with one another, and held rather to the view of an anonymous writer
in the ‘Botanische Zeitung’ of 1846, page 833, that these vessels are
membranous linings of gaps in the cell-tissue. He might well lose his
taste for the examination of these and similar organs after Schultz
Schultzenstein had by his various treatises, written after 1824, on the
so-called vital sap and the circulation which he attributed to it, made
this part of phytotomy a very quagmire of error, and had not refrained
from replying in an unbecoming manner to von Mohl, who repeatedly
opposed his views; moreover Schultz’s essay ‘Ueber die Circulation des
Lebenssafter’ (1833), which teems with absurdities, had received a
prize from the Academy of Paris.
2. The growth in thickness of the cell-membrane, and the sculpture
caused by it was a subject that is more or less connected with most
of von Mohl’s writings. He developed the chief features of his
view in 1828 in his first work, ‘Die Poren des Pflanzengewebes!’
The way in which he represented to himself the growth in thickness
of cell-membranes at a later time may be expressed as follows. All
elementary organs of a plant are originally very thin-walled perfectly
closed cells, which in the tissue are separated by walls formed of two
laminae[82]; on the inside of these primary cell-membranes, after they
have ceased to increase in circumference, new layers of membranous
substance are formed, which lying one upon another adhere closely
together, and represent the whole amount of secondary thickening
layers; on the inner side of the membrane thus thickened by apposition
there may usually[83] be perceived a tertiary layer of a different
character.
But there are certain sharply defined spots on the original cell-wall,
where this thickening does not take place; in such spots the cell is
still bounded only by the primary membrane; it is these thin spots
which bear the name of pits, and which Mirbel, and in some cases
Moldenhawer, took for holes, but von Mohl considered that it was only
in very exceptional cases that they were really changed into holes by
resorption of the thin primary wall. In accordance with this theory,
the spiral, annular, and reticulated vessels are produced by deposition
of thickening matter in the form suitable to each case on the inside
of the originally smooth thin cell-wall. But like Schleiden and other
phytotomists, von Mohl was not quite clear in his views either of the
origin or mode of formation of matured bordered pits; it was supposed
that the two laminae of the dividing wall parted from one another
at certain spots in such a manner that a lenticular hollow space
was formed between them, and that this space answered to the outer
border of the pit, while the inner border was the result of ordinary
pit-formation. This view, which could be shown to be incorrect by the
history of development, arose in fact from inexact observation,—a rare
case with von Mohl; the true course of events in the formation of
bordered pits was first described by Schacht in 1860.
It was mentioned above, that Meyen in his ‘Neues System der
Physiologie’ of 1837, i. p. 45, made cell-membranes consist of
spirally wound fibres; von Mohl had described in 1836 the structural
relations of certain long fibrous cells of Vinca and Nerium, which
might be provisionally explained in this way; he was led by Meyen’s
ideas on the subject to a renewed and minute examination of the more
delicate structure of the cell-membrane in 1837; he first of all
cleared the ground round the question, by distinguishing the cases in
which real spiral thickenings lie on the inner side of the membrane,
from those in which the membrane is smooth on the outside, but shows
an inner structure of fine spiral lines; in these cases he assumed a
peculiar arrangement of the molecules of cellulose, and endeavoured
to illustrate the possibility of such a disposition by the phenomena
of cleavage in crystals (‘Vermischte Schriften,’ p. 329); but he did
not succeed in explaining these very delicate conditions of structure,
which we now call the striation of the cell-membrane, so clearly as
Nägeli afterwards did in connection with his molecular theory.
3. The question of the substance and chemical nature of cell-membranes
was intimately connected with von Mohl’s theory of its growth in
thickness; he was engaged in 1840 in minutely studying the reactions
which various cell-membranes exhibit with iodine solution under
different conditions,—a question on which Schleiden and Meyen had
recently disagreed; von Mohl arrived at the result, that iodine imparts
very various colours to vegetable cell-membrane, according to the
quantity in which it is absorbed; a small amount produces a yellow or
brown, a larger a violet, a still larger a blue tint; this depends
partly on the extent to which the membrane is capable of distention;
the blue colour especially depends on the absorption of a sufficient
quantity of iodine. Greater interest, excited at first by a very
important work by PAYEN[84] in 1844, was taken in the question of
the chemical nature of the solid framework of the vegetable body, in
which it was shown that the substance of all cell-membranes exhibits a
similar chemical composition when freed from foreign elements. Payen
considers that this material, cellulose, is present in a tolerably
pure form in the membranes of young cells, but is rendered less pure
in older ones by ‘incrusting substances,’ whose presence changes the
physical and chemical characters of cell-membranes in various ways.
These incrusting substances may be more or less removed by treating
the membranes with acids, alkalies, alcohol, and ether, while other
inorganic matters remain behind after combustion as an ash-skeleton.
This theory, which has been more perfectly worked out in modern times,
was soon afterwards met by Mulder with the assertion, that a large part
of the layers composing the walls of cells consist from the first of
other combinations and not of cellulose; he at the same time deduced
from this view certain conclusions respecting the growth in thickness
of cell-walls. He and Halting, relying on microscopic examination,
maintained that the innermost tertiary layer in thickened membranes
is the oldest, and that the other layers are deposited on the outside
of this, and are not composed of cellulose. Von Mohl opposed this
view decidedly and successfully in the Botanische Zeitung of 1847; he
likewise in his work on the vegetable cell (p. 192), refuted the view
of the varying substance of cell-membrane, which Schleiden had founded
on some obscure chemical grounds.
It would carry us much too far to enter into the details of this
scientific dispute; Payen’s view of the chemical nature of the
vegetable cell-wall, which von Mohl adopted and elaborated, has
maintained itself to the present day, and is generally considered to be
the true one; on the other hand, the foundations of von Mold’s theory
of growth in thickness were shaken in 1858 by Nägeli’s observations,
and we may say that on the whole it has been for ever superseded. It
has been nevertheless of great service in the development of our views
on cell-structure in plants; keeping closely to the facts directly
observed, it served to bring almost all the conditions of the sculpture
of cell-walls under one point of view, and to refer their formation to
one general and very simple scheme; every such theory helps to advance
science, because it facilitates mutual understanding; in this case,
when Nägeli proposed his more profound theory of intussusception,
the understanding of it was essentially assisted by a previous exact
knowledge of von Mohl’s theory in its principles and results. In
conclusion it may be mentioned here that von Mohl afterwards in his
investigation into the occurrence of silica in cell-membranes made a
large and important addition to the knowledge of their more delicate
structure, and of the way in which incrusting substances are deposited
in them (Botanische Zeitung, 1861).
4. The views of phytotomists on the so-called intercellular substance
during the twenty years from 1836 to 1856 were closely connected with
the older theories of cell-formation, but were opposed to the modern
doctrine of the cell founded by Nägeli in 1846. Von Mohl himself had
introduced this idea for the first time into the science in 1836 in
one of his earlier and inferior essays, ‘Erläuterung meiner Ansicht
von der Structur der Pflanzensubstanz,’ rather in opposition to than
in connection with his own theory of the growth and structure of
cell-walls. Setting out from modes of formation of cell-membranes
in some Algae, difficult to understand and in some respects quite
peculiar, von Mohl believed that he saw in many cases in the higher
plants also between the sharply-defined membranes, which bound the
cell-spaces and which he regarded as the entire cell-membranes, a
substance in which the cells are imbedded, for such is its appearance
when it is largely developed; when it lies in small quantity only
between cells in close apposition, it looks like a thin layer of
cement. After Meyen in his ‘Neues System,’ pp. 162, 174 had declared
against this view in 1837, von Mohl too abandoned it more and more,
and afterwards limited the occurrence of intercellular substance to
certain cases, being convinced that much that he had before taken for
it consisted only of layers of secondary thickening, between which
he still saw the primary lamina of the cell-membrane. The theory of
intercellular substance was taken up and further developed by other
phytotomists, by Unger especially in the Botanische Zeitung for 1847,
p. 289, and afterwards chiefly by Schacht; Wigand came forward as an
opponent of it in 1854 in his ‘Botanische Untersuchungen,’ p. 65, and
logically following out von Mohl’s theory of the cell-membrane declared
the thin layers of intercellular substance as well as the cuticle,
which had been first correctly distinguished by von Mohl, to be laminae
of primary cell-membrane, the substance of which had undergone profound
chemical change. These ideas also of the intercellular substance and
the cuticle assumed an entirely different aspect when Nägeli introduced
his theory of intussusception.
The limits imposed on this history render it necessary to be content
with these indications of von Mohl’s share in the working out of the
theory of cells in its connection with the structure of the solid
framework of cell-membrane; we shall return again to his observations
on the formation of individual cells.
5. Forms of tissue and comparative anatomy. Phytotomy up to 1830
had been weak in its classification of tissues, in its ideas as to
their arrangement, and consequently in its histological terminology;
the inconvenience arising from this state of things was most
distinctly felt when it became necessary to compare the structure of
different classes of plants, Cryptogams, Conifers, Monocotyledons
and Dicotyledons, and to establish their true differences and actual
agreements. How little phytotomy had advanced in this respect is
shown plainly in the account of tissues given by Meyen in his ‘Neues
System’ in 1837. To von Mohl belongs the merit of having perceived at
an early period in his scientific career, and more clearly than his
contemporaries, the value of a natural and sufficient discrimination
of the various forms of tissue, and the necessity of obtaining a
correct view of their relative disposition; he thus showed the way to
an understanding of the general structure of the higher plants, and
rendered it possible to make a scientific comparison of the structure
of different classes of plants.
Von Mohl, like Moldenhawer long before, showed from the first a correct
apprehension of the peculiar character of the vascular bundles as
compared with other masses of tissue. He, too, examined them first in
Monocotyledons, and gave an account of them in his treatise on the
structure of Palms (1831), and also in his later essays on the stems
of Tree-ferns, Cycads, and Conifers and on the peculiar form of stem
in Isoëtes and Tamus elephantipes, to be found in his ‘Vermischte
Schriften’ of 1845. His just conception of them as special systems
composed of various forms of tissue has made his account clear and
intelligible, and his whole treatment of the subject appears new in
comparison with that of every previous writer except Moldenhawer. If
these labours of von Mohl are surpassed in value by later studies of
the history of development, they served for the time as a nucleus
for further investigations, especially into the nature of stems. It
contributed in a high degree to a correct insight into the structure
of the stem, that von Mohl, agreeing in this with Moldenhawer,
distinguished the portion belonging to the wood from the portion
belonging to the bast in the vascular bundles, and regarded both as
essential constituents of a true vascular bundle. Not less important
were his enquiries into the longitudinal course of the vascular bundle
in the stem and leaf, which showed that in the Phanerogams the bundles
in the stem are only the lower extremities of the bundles, the upper
extremities of which bend outwards into the leaves, and that the
Monocotyledons and Dicotyledons agree in this particular, though the
course of the bundle differs considerably in the two cases. He obtained
an important result in this respect in his researches on palm-stems
in 1831, when he proved the incorrectness of the distinction between
endogenous and exogenous growth in thickness, which had been laid
down by Desfontaines, and even employed by De Candolle in framing his
system. According to Desfontaines, the wood of Monocotyledons appears
as a collection of scattered bundles, of which those that run out
above into the leaves come from the centre of the stem. From this very
imperfect observation he deduced the view, that the bundles of vessels
in Monocotyledons originate in the centre of the stem, and that they
continue to be formed there, until the older hardened bundles in the
circumference form so solid a sheath that they withstand the pressure
of the younger; then all further growth in thickness must cease, and
hence the columnar form of the monocotyledonous stem. This doctrine
found general acceptance, and was employed by De Candolle to divide
vascular plants into Endogens and Exogens, in accordance with the very
general inclination felt in the first half of the present century to
distinguish the great groups of the vegetable kingdom by anatomical
characters. It is true that Du Petit-Thouars had already shown that
some monocotyledonous stems have unlimited growth in thickness; neither
his nor Mirbel’s later observations succeeded in shaking the theory,
the adherents of which met such cases by assuming a peripherical as
well as a central growth. Then von Mohl in the treatise above-mentioned
demonstrated the true course of the vascular bundles in the stem
of Monocotyledons, and at once did away with the whole theory of
endogenous growth in the opinion of all who were capable of judging,
though some even eminent systematists for a long time maintained the
old error. The results which von Mohl obtained from his study of the
comparative anatomy of the stem, rested mainly on careful observation
of the mature tissue-masses, and when he studied the history of
development, he was not in the habit of going back to the very earliest
and most instructive stages. Hence he failed to explain fully the real
points of agreement and difference of structure between Tree-ferns
and other Vascular Cryptogams and Phanerogams, and in like manner he
stopped halfway when engaged in explaining the secondary growth in
thickness of dicotyledonous stems from the nature of their vascular
bundles, and the formation of cambium. The account of growth in
thickness which he still gave in 1845 (‘Vermischte Schriften,’ p. 153),
and which rests less on observation than on an ideal scheme, is highly
obscure, and even in the treatise which he published in the Botanische
Zeitung in 1858 on the cambium-layer of the stem of Phanerogams, and
in which he criticises the newer doctrines of Schleiden and Schacht,
the subject is far from being fully cleared up, though the views there
advocated are decidedly superior to his former ones. A satisfactory
conclusion with respect to growth in thickness of the woody body and of
the rind was not reached till the history of development in vegetable
histology began to be more thoroughly studied.
As von Mohl had from the first laid special stress on the peculiar
character of the vascular bundles as compared with other tissue-masses,
so he perceived that the structure of the epidermis and of the
different forms of exterior tissue was thoroughly characteristic, and
he succeeded in arriving at a clearer understanding of the matter
in this case than in the other. Very confused ideas had prevailed
on the subject before he took it up, and we owe to him the best and
most important knowledge which we at present possess. Especially
important were his researches into the origination and true form of
stomata (1838 and 1856), and into the cuticle and its relation to the
epidermis (1842 and 1845). He brought entirely new facts to light
by his study of the development of cork and the outer bark in 1836;
these tissues had scarcely been examined with care till then, and
their formation and relation to the epidermis and the cortical tissue
were quite unknown. In the latter treatise, one of his best, the
difference between the suberous periderm and the true epidermis was
first shown, the various forms of the periderm were described, and the
remarkable fact established that the scaling of the bark was due to
the formation of fine laminae of cork, which, penetrating gradually
into the substance of the cortex, withdraw more and more of it from
its connection with the rest of the living tissue, and as they die off
form by their accumulation a rugged crust, which is the outer bark
surrounding most thick-stemmed trees. The investigation was so thorough
and comprehensive, that later observers, Sanio especially in 1860,
could only add to it some more delicate features in the history of the
process. In the same year appeared his enquiry into the lenticels,
where von Mohl however overlooked what Unger discovered at the same
time (‘Flora,’ 1836), namely, that these forms arise beneath the
stomata; but he at once corrected Unger’s hazardous supposition that
the lenticels are similar forms to the heaps of gemmae on the leaves of
the Jungermannieae. Unger, for his part, was not long in adopting von
Mohl’s explanation of the lenticels as local cork-formations.
Since von Mohl thus distinctly brought out the special character of
the vascular bundles and of the different forms of epidermal tissues,
it must excite surprise that he, like former phytotomists, did not
find himself under the necessity of framing some conception of the
rest of the tissue-masses in their peculiar grouping as a whole, as a
special system, and of classifying and suitably naming the different
forms that compose them, though his examination of Tree-ferns would
seem to have offered him an occasion for doing so. Von Mohl, like his
contemporaries, was satisfied with calling everything that is neither
epidermis, cork or vascular bundle, parenchyma, without distinctly
defining the expression.
Here we leave von Mohl and his labours for the present, to return once
more in the following chapter to the share which he took in the further
progress of phytotomy. We shall perhaps best realise his importance
in the history of the science, if we try to think of all that we have
now seen him doing for it as still undone. There would then be a huge
gap in modern phytotomic literature, which must have been filled up by
others before there could be any further addition to the knowledge of
cells and tissues founded on the history of their development; for it
can hardly be conceived that the advance to which we owe the present
condition of vegetable anatomy, could have been based upon ideas such
as those of Meyen, Link, and Treviranus, without von Mohl’s preliminary
discoveries.
CHAPTER IV.
HISTORY OF DEVELOPMENT OF THE CELL, FORMATION OF TISSUES, MOLECULAR
STRUCTURE OF ORGANISED FORMS.
1840-1860.
In the period between 1830 and 1840 it had come to be understood, that
the old theories of cell-formation of Wolff, Sprengel, Mirbel, and
others, resting on indistinct perceptions and not on direct and exact
observation, could only give an approximate idea of the formation
of cells. But in the course of that time really different cases of
formation of new cells were accurately observed by Mirbel, and more
especially by von Mohl, who described different modes of formation
of spores, and in 1835 the first case of vegetative cell-division.
Unfortunately these observations, excellent in themselves, applied
to cases of cell-formation which do not occur in the ordinary
multiplication of cells in growing organs, and von Mohl guarded himself
from founding a general theory of cell-formation on his observations
on cells of reproduction and on a growing filamentous Alga. Mirbel
also cautiously regarded the formation of pollen-cells and that which
he supposed to be the process in the germination of spores as cases of
a peculiar kind, adhering to his old theory of the origin of ordinary
tissue-cells.
Schleiden’s behaviour was different. Having somewhat hastily observed
the free cell-formation in the embryo-sac of Phanerogams in 1838, he
proceeded at once to frame a theory upon it which was to apply to all
cases of cell-formation, and especially to that in growing organs.
The very positive way in which he announced this theory and set
aside every objection that was made to it, combined with his great
reputation at the time, at once procured for it the consideration
of botanists generally; and the most important representatives of
phytotomy, von Mohl himself at first not excepted, allowed that there
was a certain amount of justification for it. It was a question in
which theoretical considerations were not of primary importance; direct
and varied observation of careful preparations with strong magnifying
powers could alone form the basis for further investigation. Unger
showed in this way that the processes at the growing point of the stem
could scarcely be reconciled with Schleiden’s theory, and in this
view he was supported by the English botanist Henfrey; but Nägeli
was the first who addressed himself with energy and sound reasoning
to the important and difficult question, how cells are formed in
reproductive and growing vegetative organs, and how far the processes
are the same in the lower Cryptogams and in the Phanerogams. He set
out by assuming that Schleiden’s theory was in the main correct, but
his long-continued investigations led him finally to the conviction
that it must be entirely abandoned, and he proposed the outlines
of the theory of cell-formation which is accepted at the present
time. In this case, as before in questions of morphology, he applied
himself first, and with great success, to the investigation of the
lower Cryptogams, while Alexander Braun’s observations on some very
simple Algae contributed materially to the further development of the
cell-theory, and especially to extending and correcting the idea of
the cell; Hofmeister’s researches also in embryology not only produced
great results for morphology, but at the same time supplied a variety
of facts which served to complete Nägeli’s view. The further this was
worked out, the more apparent it became that the external circumstances
in the processes of cell-formation might be very various, and that von
Mohl’s earlier observations especially gave a correct representation
of individual and typical cases; but more important than this result
was the fact declared by Nägeli in 1846, that in all these different
kinds of cell-formation it was only the external and secondary matters
that varied, while the essential part of the process was in all cases
the same, and it was soon perceived that cell-formation in the animal
kingdom, which was now being more thoroughly examined, agreed in the
main with that of the vegetable kingdom, as Schwann and Kölliker had
intimated in 1839 and 1845.
It is unnecessary to give any account here of the totally different
theories which Theodor Hartig and Karsten proposed about the same time.
They do not rest on careful observation, and we may omit them not
merely because they are rejected by the unanimous judgment of better
observers, but because they had no influence upon the development of
the doctrine of cell-formation, and are therefore without historical
interest.
It lies in the nature of the case, that investigations into the origin
and multiplication of cells should turn the attention of observers
more and more to their living contents, for these are actively and
immediately concerned with the formation of new cells. The various
granular, crystalline, and mucilaginous portions of the contents of
cells had been repeatedly observed before 1840, and Schleiden and
Meyen had specially studied the ‘movements of cell-sap’; but it was
in the course of observations on the history of development between
1840 and 1850 that attention was first called to a substance which
plays a regular part in the formation of new cells, which envelopes
the cell-nucleus discovered by Robert Brown, which undergoes the most
important changes as the cell grows, which forms the entire substance
of swarmspores, and the disappearance of which leaves behind it a
dead framework of cell-membrane. This substance, which is much more
immediately concerned with sustaining the processes of life than is
the cell-wall, was seen by Schleiden in 1838 and taken for gum. It was
more carefully studied by Nägeli between 1842 and 1846, and perceived
by him to be nitrogenous matter. Von Mohl described it in 1844 and
1846 from new points of view, gave it the name of protoplasm which it
still bears, and showed that it is this substance, and not the proper
cell-sap, which carries out the movement of rotation and circulation in
cells discovered by Corti in the previous century, and again observed
by Treviranus in 1811. The Algae proved highly instructive in the
study of this remarkable substance also. The swarmspores of Algae and
Fungi observed by Alexander Braun, Thuret, Nägeli, Pringsheim, and De
Bary showed that protoplasm is not dependent on the cell-membrane for
its vitality, that by virtue of its own internal powers it can alter
its form, and even move in space. In 1855 Unger in his ‘Lehrbuch’
pointed out the resemblance of this substance to the matter known as
sarcode in the lower forms of animals, a resemblance brought out more
plainly in 1859, when De Bary’s studies of the Myxomycetes proved that
the substance of these forms was protoplasm, which continues to live
for a considerable time, and often in large masses, before it forms
cell-membranes. Zootomists now began to take an interest in these
results of botanical research; Max Schulze (1863), Brücke, and Kühne
studied animal and vegetable protoplasm, and the conviction gained
ground more and more in the years from 1860 to 1870 that protoplasm is
the immediate principle of vegetable and animal life. This discovery is
one of the most important results of research in modern natural science.
Not less important were the results obtained from the study of
the rest of the organised contents of cells; von Mohl proved that
chlorophyll-corpuscles, the most considerable organs of nutrition
in the plant, are formed of protoplasm, and Theodor Hartig, though
his cell-theory was a mistake, did good service by his discovery of
aleurone-grains in seeds and of the crystalloids which sometimes occur
in the grains, and which are also formed of protoplasm and renewed
from protoplasm. Radikofer, Nägeli, and others added to our knowledge
of the form and chemical composition of these aleurone-grains. To
starch-grains, which had been frequently examined, by Payen especially,
Nägeli devoted an investigation at once comprehensive and profound,
and obtained results of extraordinary value; these were given to the
world in an exhaustive work published in 1858 under the title ‘Die
Stärkekörner,’ and form an epoch not in phytotomy only, but in the
general knowledge of organised bodies. By the application of methods of
research unknown before in microscopy, Nägeli arrived at clear ideas
of the molecular structure of the grains, and of their growth by the
introduction of new molecules between the old ones. This theory of
intussusception founded on the observation of starch-grains derives
its great importance from the fact that it served directly to explain
the growth of cell-membrane, could be applied generally to molecular
processes in the formation and alteration of organic structures, and
accounted for a long series of remarkable phenomena, especially the
behaviour of organised bodies in polarised light. Nägeli’s molecular
theory is the first successful attempt to apply mechanico-physical
considerations to the explanation of the phenomena of organic life.
While men of the highest powers of mind were devoting themselves to
the solution of these difficult problems, the study of tissues was
not neglected in the years after 1840, and here too it was Nägeli who
gave the chief impulse and the direction to further development. In
the periodical which he published in conjunction with Schleiden he had
already (1844-46) given an account of some searching enquiries which he
had made into the first processes in the formation of vascular bundles
from uniform fundamental tissue; in the Cryptogams he observed the
production of the tissue of the whole plant from the apical cell of the
growing stem, and this discovery, still further pursued by Hofmeister
especially, has given rise during the last twenty years to a copious
literature, which has been of service to the theory of the formation of
tissues, to morphology, and consequently also to systematic botany.
The researches of Hofmeister, Nägeli, Hanstein, Sanio, and others into
the first formation of vascular bundles from the fundamental tissue of
young organs led to important results for morphology, in so far as it
was now for the first time possible to judge of the morphological value
of anatomical and histological relations. The growth in thickness of
woody plants, a question of primary importance to vegetable physiology,
was first made intelligible by the discovery of the mode of formation
of vascular bundles and their true relation to cambium; Hanstein and
Nägeli, and afterwards Sanio especially, cleared up the questions
connected with growth in thickness in their main features before and
after 1860.
* * * * *
When we pass on to show how the great results above-mentioned were
attained, we encounter some difficulties. After 1840 botanical
literature multiplied to an extent before unknown; it is from elaborate
monographs on single subjects in phytotomy, from some text-books, and
especially from smaller essays in botanical periodicals that we must
gather an account of the further development of scientific thought.
Much as the founding of scientific periodicals has facilitated
communication between professed botanists, yet this form of literature
makes it more difficult to see the way clearly through the work of
earlier periods and to discover the historical connection in the
science, not to speak of the harm that usually results from it to young
and inexperienced students.
Such being the nature of the sources from which we must draw our
information, we shall obtain a better general view of the whole subject
if we depart from the practice of former chapters, and follow out the
more important questions in their historical development instead of
connecting them directly with leading persons. Such a treatment of the
subject is indeed suggested by the fact that we are now no longer on
pure historic ground; for the majority of the men who have developed
modern doctrines since 1840 are still alive, and it must be uncertain
whether the account here attempted may not be impugned on some ground
or other. Owing to the extraordinary diversity of opinion that exists
among botanists even on the most general questions in the science, it
is extremely difficult to ascertain what can be considered as a common
possession,—an unfortunate condition of things, from which no science
perhaps suffers so much as botany.
The extent to which individual botanists have contributed to the
advance of phytotomy during the period under consideration will
appear of itself from the following narrative; and if we speak almost
exclusively of Germans, it is for the simple reason that Englishmen
from Grew’s time till now can scarcely be said to have added anything
to our knowledge of phytotomy; the Italians also, once so gloriously
represented by Malpighi, scarcely come under consideration in the
questions now to be dealt with, while French botanists, represented by
Mirbel in the preceding period, though they have produced many works on
phytotomy since his time, have had no important share in deciding the
fundamental questions of modern science.
In the preceding period it was necessary to take into consideration
the increasing improvement of the microscope, in order to understand
the development of opinion on vegetable structure; but it is scarcely
needful to do so after 1840. Since that time good and serviceable
instruments with strong magnifying powers and clear definition have
been within the reach of every phytotomist; and though improvements
are still being constantly made, yet the microscopes that were in the
hands of skilful observers between 1840 and 1860 were fully adequate
to deciding the new questions proposed to them. The chief improvement
effected in the microscope during this period was the fitting it with
apparatus for the polarisation of light, and for the more convenient
measurement of objects; we shall see presently what influence the
former improvement had on the perfecting of Nägeli’s molecular
theory. As microscopes improved and the questions to be solved grew
more difficult, it became necessary to bestow increased care on the
preparation of objects; it was no longer sufficient to cut or dissect
neatly, and so learn the form of the solid portions of vegetable
structure; measures of precaution and auxiliary measures of the most
various kinds were needed to obtain a clear view of the soft contents
of cells, and to observe the protoplasm as far as possible in a living
state and protected from prejudicial influences; all sorts of chemical
reagents were applied to make the objects more transparent, or to show
their physical and chemical characters. The method invented by Franz
Schulze before 1851 deserves to be specially mentioned; it consisted
in isolating the cells in a few minutes’ time by boiling them in a
mixture of nitric acid and potassium-chlorate, and thus shortening
Moldenhawer’s process of maceration or superseding it altogether. In a
word, the technicalities of the microscope were perfected in a variety
of ways by Schleiden, von Mohl, Nägeli, Unger, Schacht, Hofmeister,
Pringsheim, De Bary, Sanio, and others, and raised to an art which must
be learnt and practised like any other art. Young microscopists were
able after 1850 to learn this art in the laboratories of their elders,
and to profit by their technical experience and scientific counsels;
schools of phytotomy were formed at least in the German universities;
elsewhere, it is true, the old condition of things remained in which
everyone had to trust to himself from the beginning.
The general dissemination of good microscopes was accompanied by a
higher standard of requirement in the execution of drawings from
the instrument, especially after von Mohl had shown the way; and
the invention of lithography and the revival of wood-engraving
ministered to the needs of science, supplying the place of the old
costly copper-plate printing. Hence we find an increasing number of
beautiful drawings in scientific monographs; the text-books also
could now be supplied with an abundance of figures, and this greatly
promoted the general understanding of things which could otherwise
be seen only under the glass of each observer. From the close of the
16th century wood-cuts had fallen more and more into disuse, and had
been replaced by copper-plates; after 1840 wood-engraving was restored
to its old rights and was found to be a more convenient method of
pictorial illustration, especially for text-books; thus Schleiden’s
‘Grundzüge’ of 1842, von Mohl’s ‘Vegetabilische Zelle’ of 1851,
Unger’s and Schacht’s text-books were enriched with many and sometimes
very beautiful wood-cuts. Lithographs were generally preferred for
periodicals and monographs; the ‘Botanische Zeitung,’ founded by Mohl
and Schlechtendal in 1843, and till after 1860 the chief organ for
shorter phytotomic communications, was illustrated by a large number
of beautiful prints from the establishment of the Berlin lithographer,
Schmidt.
1. DEVELOPMENT OF THE THEORY OF CELL-FORMATION FROM 1838 TO 1851.
Since we are here dealing with questions of fundamental importance
not only to one branch of botanical study but to the whole science
of botany, and even to the rest of the natural sciences, it seems
imperative that we should follow step by step the founding and
perfecting of the theory of the cell, as far as is possible in the
limited space at our command; we shall deal with the sexual theory
further on in a similar manner.
As usually happens in the inductive sciences, the period of strict
inductive investigation into cell-formation was preceded by a still
longer time, during which botanists ventured to put forward general
theories in reliance on highly imperfect observations. We have already
seen how Caspar Friedrich Wolff in 1759 made cells originate as
vacuoles in a homogeneous jelly, and how this view was adopted in
all essential points by Mirbel at a late period in the 18th century;
how Kurt Sprengel, and with him a number of phytotomists, among them
Treviranus as late as 1830, supposed cells to be formed from granules
and vesicles in the cell-contents, an idea which Link it is true
opposed in 1807, but afterwards accepted to a great extent. Though
Moldenhawer as early as 1812 (‘Beiträge,’ p. 70) distinctly rejected
these theories, and published observations which if followed up would
have led to the right path, yet the botanists above-named and others
with them, long continued to adhere to the earlier views. Kieser,
for example (‘Mémoire sur l’organisation des plantes,’ 1812) further
developed Treviranus’ theory, that the fine granules in the latex of
plants are cell-germs which are afterwards hatched in the intercellular
spaces. Schultz-Schultzenstein in his work ‘Die Natur der lebenden
Pflanze,’ 1823-28, i, p. 607 rejected this view and adopted that
of Wolff and Mirbel. Scarcely better than the notion of cell-germs
represented by Sprengel, Treviranus, and Kieser was the theory
propounded by Karsten soon after 1840; that of the French botanists
Raspail and Turpin[85] (1820-1830), though conveyed in a different
terminology, corresponded in its main points with the views of Sprengel.
It had been the good fortune of Mirbel at the beginning of the century,
and again thirty years later, to promote the advance of phytotomy by
means of important observations, though he may have interpreted some of
them incorrectly; the same thing happened again thirty years later, and
it was a German enquirer, von Mohl, who corrected his observations and
views on both occasions.
In his famous treatise on Marchantia polymorpha, which appeared in 1835
in the Memoirs of the French Institute, the first part having been
laid before the Paris Academy in 1831-32, Mirbel distinguished three
modes of cell-formation; in the germination of the spores of Marchantia
new cells are formed from the germ-tube and new cells again from these
by a similar process, much in the same way therefore as that which
actually occurs in the germination of Yeast-fungi; he found a second
kind of cell-formation in the production of the gemmae of Marchantia,
where he distinctly observed the successive appearance of the dividing
walls, but formed an erroneous idea of the proceeding on the whole; in
the further development of the gemmae and in other cases of growth he
considered that new cells are formed between those that are already
present in the manner supposed in his earlier theory.
Von Mohl’s dissertation on the multiplication of vegetable cells
by division, published in 1835 and reprinted in ‘Flora’ of 1837,
shows how strange these processes even then appeared; in this work
he expresses some doubts about Mirbel’s statements, but he accepts
them on the whole, and only makes incidental mention of his own more
numerous and better observations on the development of spores (‘Flora,’
1833), though he had there seen several cases of cell-division and
free cell-formation with tolerable distinctness. Adolph Brongniart
(‘Annales des sciences naturelles,’ 1827) also had observed, though
imperfectly, the formation of pollen-grains in their mother-cells in
Cobaea scandens, and Mirbel, in the appendix to the work mentioned
above, had given a correct description and good figures of the
formation of pollen-cells; and yet von Mohl neglected to compare
these important observations of cases of cell-division with his own;
even in 1845, when he published the latter in a revised form in his
‘Vermischte Schriften,’ he overlooked the close relation between the
formation of those pollen-grains and spores, and the cell-division in
Cladophora. Still this treatise of von Mohl’s is of great importance
in the history of the theory of cell-formation, because it described a
case of cell-division for the first time step by step and brought all
the salient points into relief. Dumortier had observed the division
of cells as early as 1832[86], and Morren had seen it in Closterium in
1836, but had not given the needful details. Finally, von Mohl applied
the experience which he had gained from Cladophora to other filamentous
Algae, and pointed out the similarity between these processes and
the division of Diatoms, which he consequently claimed as plants in
opposition to Ehrenberg, who considered them to be animals (‘Flora,’
1836, p. 492).
Meyen next, relying on von Mohl’s observations on Cladophora, declared
in the second volume of his ‘Neues System’ that cell-division was a
very common occurrence in Algae, Filamentous Fungi and the Characeae,
but he neglected any closer investigation of the processes by which
the division is introduced and completed. His comparison of these
cases of cell-formation with the formation of spores, pollen-grains,
and endosperm-cells is moreover noticeable as the first attempt
to distinguish what is now known as free cell-formation from
cell-division; it was obviously the want of this distinction which long
prevented clearer views on the whole of this field of observation.
The due separation of these two modes of cell-formation was a short
step after the observations that had been already made; and if that
step had been taken, Schleiden’s theory would have been impossible,
and the development of the cell-theory would not have been prejudiced
by the mistake, introduced by Schleiden after 1838, of applying the
mode of free cell-formation, which he believed he had observed in the
embryo-sac of Phanerogams, to the multiplication of vegetative cells in
growing organs, and regarding it as the only mode of cell-formation.
This would have been the more impossible, since von Mohl in the same
year gave an excellent description of the development of stomata by
division of a young epidermis-cell and the later separation of the
dividing wall into two laminae. But von Mohl in the years immediately
following was over-cautious in refraining from all speculative
consideration of cases that lay clearly before him, and his views
were still undecided in 1845, when Unger and Nägeli had already made
good observations on the formation of tissue-cells in growing organs
(‘Vermischte Schriften,’ 1845, p. 336).
Schleiden’s theory of cell-formation arose out of a curious mixing
together of obscure observations and preconceived opinions, and reminds
us indeed strongly of the old notions of Sprengel and Treviranus; it is
true that he distinctly rejected their views, but he too made new cells
arise from very minute granules, and his theory like theirs did not
rest on any thorough course of observation.
Robert Brown, (see his Miscellaneous Writings, edited by T. T. Bennett,
I.) had discovered the nucleus in the cells of the epidermis of
Orchidaceous plants in 1831, and had shown that it was very generally
present in the tissue-cells of Phanerogams, but had obtained no results
from his discovery. The cell-nucleus lay undisturbed, till Schleiden
suddenly made it the soul of his theory and the starting-point of all
cell-formation. He considered that the nucleus was formed from the
mucilaginous content of the cell, which he assumed on insufficient
grounds to be of the nature of gum; this he called the cytoblastem, and
the nucleus itself the cytoblast. As he states that his cytoblastem
becomes yellow and granular in solutions of iodine, we may recognise in
it our own protoplasm.
We make acquaintance with Schleiden’s theory of cell-formation in its
original form, if we turn to his treatise, ‘Beiträge zur Phytogenesis,’
(in the Archiv für Anatomie, Physiologie, etc. von Johannes Müller,
1838). The work begins with some remarks on the general and fundamental
laws of human reason, etc., discusses the literature of cell-formation
in a few lines without mentioning von Mohl’s numerous observations,
goes on to mention the general occurrence of the nucleus which here
receives its new name, then occupies itself with gum, sugar, and
starch, and at last comes to the main subject. There are two spots,
says Schleiden, in the plant, where the formation of new organisation
may be most easily and most certainly observed, the embryo-sac and
the end of the pollen-tube, in the latter of which, according to his
theory of fertilisation, the first cells of the embryo are supposed to
be formed, but where in fact no cells are formed. At both spots small
granules soon arise in the gum-mucilage, which, before homogeneous,
now becomes turbid, and then single larger and more sharply defined
granules, the nucleoli, appear. Soon after, the cytoblasts are seen as
granular coagulations from the granular mass; they grow considerably in
this free condition, but as soon as they have reached their full size,
a delicate transparent vesicle is formed upon them; this is the young
cell, which at first presents the appearance of a very flat segment of
a sphere, whose plane side is formed by the cytoblast, the convex by
the young cell (the cell-membrane), which rests upon the cytoblast as
a watch-glass on a watch. Gradually the vesicle becomes larger and of
firmer consistence, and now the whole of the wall, except where the
cytoblast forms part of it, consists of a jelly. By-and-bye the cell
grows beyond the edge of the cytoblast and rapidly becomes so large
that the latter appears only as a small body inclosed in one of the
side walls. The shape of the cell becomes more regular with advancing
growth and under the pressure of adjoining cells, and often passes
into that of a rhombododecahedron, which Kieser for reasons drawn from
the nature-philosophy assumed to be the fundamental form. It is only
after the resorption of the cytoblast that the formation of secondary
deposits on the inner surface of the cell-wall commences, though some
exceptional cases are adduced. Schleiden thinks (p. 148) that he may
assume that the process here described is the general law of formation
of vegetative cell-tissue in Phanerogams. He adds particularly that the
cytoblast can never lie free inside the cell, but is always enclosed in
a duplication of the cell-wall, and he thinks that it is an absolute
law that every cell, except perhaps in cambium, begins as a minute
vesicle, and grows to the size which it reaches in its matured state.
The resemblance of this view to that of Sprengel and Treviranus is
increased by what we find further on, where we read that from the
cell-germs in the spores of Marchantia usually only from two to four
serve to form cells, the rest becoming overlaid with chlorophyll,
and being consequently withdrawn from the vital process. He who is
acquainted with the modern view of the processes of free cell-formation
founded on the numerous and careful investigations of later times will
scarcely discover in the above account of Schleiden’s theory a single
correct observation.
Soon after, von Mohl published in ‘Linnaea,’ 1839, p. 272, his
observations on the division of the mother-cells of the spores of
Anthoceros; these were carefully made and were correct in all the main
points; and in opposition to Mirbel’s former statements they establish
the fact, that the division is effected by the mucilaginous contents
of the cell, and consequently that it is not a passive division of
the contents of the mother-cell produced by the growth inwards of
projections of the cell-wall.
Unger[87] was the first to declare distinctly against Schleiden’s
doctrine, and his observations on the punctum vegetationis appeared
in the ‘Linnaea’ of 1841, p. 389; from the size and position of the
cells he concluded that the tissue-cells in this case are formed by
division, and not in the manner alleged by Schleiden. Soon after Nägeli
also (‘Linnaea,’ 1842, p. 252) observed the processes of cell-formation
in the extremities of roots, but he did not conceive them to be cases
of division; he saw two nuclei form in each mother-cell, and a new
cell form round each nucleus, and explained the origin of the dividing
wall as due to the meeting together of the two new cells; he thought
that a similar process takes place in stomata and in the mother-cells
of pollen; this conception was not absolutely incompatible with
Schleiden’s theory, but there was this difference, that in Nägeli’s
case essential processes were correctly observed, but were to some
extent incorrectly interpreted. In the same year appeared the first
edition of Schleiden’s ‘Grundzüge der wissenschaftlichen Botanik,’
in which his theory of cell-formation was repeated in a more precise
form. That he was thoroughly in earnest to maintain it is shown by the
fact that he gave still another exposition of it in his ‘Beiträge zur
Botanik’ in 1844, where he insists that his method of cell-formation
is the general one, though it has been distinctly ascertained in the
Phanerogams only. But how completely an observer may be led captive
by a preconceived opinion may be learnt from Schleiden’s suggestion,
that the formation of zygospores in Spirogyra is in accordance with his
views, though it is impossible to conceive of a case of cell-formation
more easy to observe, or less reconcilable with Schleiden’s theory.
It was mentioned in the first book, that Hedwig and Vaucher were
acquainted with the remarkable process of the formation of zygospores
in the alga-genus Spirogyra; but this as late as Schleiden’s time
was not regarded as an example of cell-formation, and his view was
really a step in advance, since it brought a process, so highly
peculiar according to existing ideas, under the general conception of
cell-formation.
The systematic elaboration of the theory of cells, founded on careful
observation and mature reflection, began with the year 1844. Almost
at the same time in this year appeared Nägeli’s detailed enquiries
into the occurrence of the cell-nucleus and into cell-division, von
Mohl’s observations on the primordial utricle and its behaviour in
the process of cell-division in young tissue, and lastly those of
Unger on merismatic cell-formation (cell-division) as a general
mode of proceeding in the growth of organs. As these observers were
chiefly concerned to test the correctness and general applicability
of Schleiden’s theory, they necessarily paid special attention to
the general occurrence of the cell-nucleus and to its position on
the side of the cell-wall, for these were the points most accessible
to observation and criticism. The discussion of these observations
disclosed a defect in the current phraseology, in which the word
cell was commonly understood to mean only the cell-membrane, but
sometimes included everything belonging to and contained in the cell;
hitherto moreover the protoplasm of the cell had not been sufficiently
distinguished from the rest of the cell-contents.
Nägeli and von Mohl arrived simultaneously at a clearer understanding
of these points; von Mohl recognised the primordial utricle (1844) as a
component part of the cell-contents and not belonging to the cell-wall,
and explained the part which it plays in cell-division; in 1846 he
arrived at a clear conception of the protoplasm as a peculiar substance
distinct from the other contents of the cell and gave it the name it
still bears. Meanwhile Nägeli had also distinguished the protoplasm
from everything else in the cell, and noticed its pre-eminent
importance in cell-formation and its nitrogenous character.
We must not omit to mention here, that investigations into the
processes of cell-formation compelled observers to search for the
spots where cell-formation actually takes place, and thus the fact was
ascertained, that cells in statu nascendi are not to be found in all
parts, not even in all growing parts of the plant, but that we must
look for them in the so-called puncta vegetationis in the stem and
root, in the youngest lateral organs, and between the bark and the wood
in woody plants. About this time a new idea began to be attached to
the word cambium, which Mirbel had used in the sense of a nourishing
juice saturating the plant; it was now applied to the tissue-masses in
which the formation of new cells takes place, and specially to the very
thin layer of tissue lying between the wood and the rind, from which
new layers of wood and rind in woody plants are formed—a layer, which
according to Mirbel’s theory had been a mass of sappy matter, in which
new cells arise as vacuoles.
Unger in an enquiry into the growth of internodes (‘Botanische
Zeitung,’ 1844) again declared himself as an opponent of Schleiden’s
theory. He maintained first of all and erroneously that the
cell-nucleus is not of general occurrence in tissue where division is
taking place, but he argued rightly from the position of the cells,
from the difference of thickness in their walls, and from their
relative size, in favour of their multiplication by the formation of
dividing walls; he noticed the part played by the cell-contents in
the multiplication of cells in hairs, and asserted that merismatic
cell-formation (cell-division) is the general rule in the growth
of organs of vegetation, while he distinctly declared that it was
not possible to bring all that is actually seen at the spots where
formation of cellular tissue is taking place into agreement with
Schleiden’s theory. But Unger did not observe the processes that take
place in cell-division step by step; his observations sufficed to make
Schleiden’s theory very improbable without offering enough foundation
for a new one, and Schleiden did not fail to reply to Unger’s
objections in the second edition of his ‘Grundzüge’ in 1845.
Earlier in the same year, von Mohl published in the ‘Botanische
Zeitung’ the treatise on the primordial utricle which has been already
mentioned; by the term primordial utricle he meant partly the very thin
layer of protoplasm, which in large cells full of sap lines the inside
of the cell-wall, and partly an outer layer of the protoplasm in young
cells, which are still rich in that substance. It is true that the
distinguishing the primordial utricle was not a very important matter;
but von Mohl applied it with his usual thoroughness to obtaining a
better insight into cell-formation by calling attention (p. 289) to
the circumstance, that the cells of the cambium-layer between the rind
and the wood fit into one another and leave no intercellular spaces;
from this he concluded that there are only two possible modifications
of cell-multiplication, either division of cells by formation of a
dividing wall or formation of cells within cells; in each of these
young cells is a primordial utricle, the origin of which must at
least be contemporary with that of the cell (cell-membrane). ‘Could
it then be distinctly shown, that two primordial utricles exist side
by side in cells, which are in the act of multiplying, before a
partition-wall is formed between them, it would be evident that in the
cambium layer and at the points of the stem and root the formation of
the primordial utricle precedes that of the cell.’ Von Mohl believed
that he had seen this process, but was not perfectly satisfied as to
the correctness of his observation; but he continues: ‘Since every
young cell contains a primordial utricle, this must either be absorbed
before a multiplication of the cell commences in order to make way
for two new ones formed in its stead, or the old primordial utricle
must separate into two.’ He considered the first supposition to be the
probable one, rejecting Unger’s statement that the nuclei are formed
after the division. It is surprising that after these considerations
von Mohl thought that his own observations necessarily confirmed
Schleiden’s theory of cell-formation, although he noticed beside that
the nucleus never forms a part of the cell-wall, an essential feature
in that theory; but in fact von Mohl took the membrane which according
to Schleiden separates from the nucleus for the primordial utricle.
But these mistakes are soon followed by the right conjecture, that
the substance of the primordial utricle may be identical with the
mucilaginous mass, which commonly encloses the nucleus, and so with
that which von Mohl two years later named protoplasm. In this later
treatise (‘Botanische Zeitung,’ 1846), in which he proves that the
well-known movements in the interior of cells are made not by the
watery cell-sap but by the protoplasm, he states (p. 75) that it is
the protoplasm which produces the nucleus, that the organisation of
the nucleus ushers in the formation of the new cell, and that contrary
to Schleiden’s theory the protoplasm completely envelopes the nucleus,
which always occupies the centre of very young cells, as is the case
especially in the endosperm-cells observed by Schleiden. He then shows
how the protoplasm of young cells at first solid afterwards forms
sap-cavities and stretches between them in walls, bands or threads, the
substance of which exhibits the streaming movement. Von Mohl strangely
neglected on this occasion to compare carefully his former observations
on the origin of spores and the division of Alga-cells with his new
results, and to seek for the essential resemblances between them; on
the contrary he said emphatically that the cell-division in Cladophora
is probably a quite different process from the multiplication of
tissue-cells in higher plants.
The discoveries of Unger and von Mohl up to the year 1846 were quite
sufficient to refute Schleiden’s theory, but not to give a clear
and general view of the processes in the formation of cells; the
different kinds of cell-formation were neither carefully distinguished
from one another, nor could they be referred to a common principle.
Both observers had endeavoured to conjecture the course of events
from certain data, supplying by inference what they had not directly
observed.
Nägeli about the same time took up a different position as an opponent
of Schleiden’s theory. In an exhaustive treatise on the cell-nucleus,
cell-formation, and cell-growth in plants, the first part of which
appeared in 1844 in the periodical founded by himself and Schleiden,
he collected together all that had hitherto been observed by himself
and others from various points of view. All sections of the vegetable
kingdom were once more systematically examined with reference to
the occurrence of the cell-nucleus and the different kinds of
cell-formation; all cases of the latter were carefully compared
together in their resemblances and differences, in order to deduce
from the observed phenomena that which was essential and universal.
The first result was, that Schleiden found himself obliged, in the
second edition of his ‘Grundzüge’ in 1845, to accept the cell-division
established by Nägeli in Algae and the mother-cells of pollen as a
second kind of cell-formation; thus began the movement in retreat
which was destined to end in the following year with the overthrow of
Schleiden’s theory. This was effected by the continuation of Nägeli’s
treatise in the third volume of the periodical for 1846. In the first
part of his work Nägeli had set out by assuming the correctness of
Schleiden’s assertions, though he was even then compelled to modify
them considerably. In the second part, however, in consequence of
further observations Schleiden’s theory was declared in plain terms
to be utterly incorrect, and was refuted point by point. But Nägeli
was not obliged to confine himself to this negative result; his
comprehensive investigations supplied material at the same time for
constructing a new theory of cell-formation, which not only took in all
the various cases, but declared the principle which lay at the root
of all. If we compare this second part of Nägeli’s treatise with von
Mohl’s publications from 1833 to 1846, we shall see that von Mohl had
observed with accuracy a number of important facts, but that Nägeli
added largely to them, and, which is the main point, elaborated them
into a comprehensive theory embracing all kinds of cell-formation.
How important the correct distinction of the protoplasm from the rest
of the cell-contents was for the perfecting of the theory of cells is
seen from Nägeli’s declaration, that he retracts his former view which
rested on the authority of Schleiden, because it sprang from a time
when he was ignorant of the significance of the mucilage-layer (the
protoplasm), though it is true that he indicates at the same time other
points and new considerations which definitively set aside Schleiden’s
theory. After investigating the different modes of free cell-formation
and finding the processes there quite different from Schleiden’s
account of them, he proceeded to search for free cell-formation where
Schleiden had affirmed that it invariably occurs, namely in growing
vegetative organs in the higher plants. But this investigation led
him to the conclusion that all vegetative cell-formation is true
cell-division, and that even the reproductive cell-formation in some
Algae and Fungi is effected by division; the reproductive cells of
most plants are the result of free cell-formation, but it should be
observed that the term free cell-formation is here used not exactly in
the modern sense, inasmuch as Nägeli included in it the formation of
four-fold grains (tetrads) in spores and pollen. If the distinction
between cell-division and free cell-formation had often been suggested
by former observers, Nägeli was the first who distinctly defined
it, though not exactly as it is now defined. ‘In cell-division the
contents of the mother-cell separate into two or more portions; a
perfect membrane forms round each of these portions, which at the
moment of its appearance rests partly on the wall of the mother-cell
and partly on the adjacent walls of the sister-cells. In free
cell-formation a smaller or larger part of the contents of a cell, or
even the whole of them becomes isolated. On its surface is formed a
perfect membrane, which is everywhere free on its outer face. There are
two processes in the formation of a cell; the first is the isolation
or individualising of a part of the contents of the mother-cell,
the second the formation of a membrane round the individualised
portion.’ He then proceeds to show that the cell-wall is formed by
the separation of non-nitrogenous molecules from the nitrogenous
mucilage (protoplasm). These sentences contain all that is general
and essential in vegetative cell-formation. Further on he notices the
peculiarities in the various processes in cell-formation; he says that
the individualising of the cell-contents assumes four forms; first,
single small portions of the contents separate themselves inside the
rest, as occurs in the formation of free germ-cells in Algae, Fungi,
and Lichens, and of endosperm-cells in Phanerogams; secondly, the whole
contents of one cell, or of two by conjugation of associated cells,
collect into a free spherical or ellipsoidal mass, as in the formation
of germ-cells in the Conjugatae; thirdly, the whole contents of a cell
separate into two or more portions, which is now called cell-division;
from this Nägeli distinguishes as his fourth form, the process known as
abscision (Abschnürung), which occurs in the formation of germ-cells in
many Algae and Fungi.
Schleiden had declared it to be a general law in plants, that cells are
only formed inside mother-cells. Meyen however, Endlicher, and Unger,
had recently assumed the formation of new cells between the older
ones; Nägeli maintained that all normal cell-formation, vegetative and
reproductive, takes place only within mother-cells.
In opposition to the long-cherished notion that there must be one
general and fundamental form of cell, Nägeli pointed to the fact that
cells have very different forms at the moment of their production.
Those which arise by free cell-formation are, he says, at first always
spherical or ellipsoidal; those produced by cell-division have a shape
necessarily conditioned by the form of the mother-cell and the manner
of division. He showed further that changes in the shape of cells with
advancing growth depend materially on whether they enlarge equally
in all parts of their circumference or not. These considerations,
obvious as they are, were now for the first time pointed out and fully
appreciated.
The reader who is already familiar with our subject will recognise
in the passages adduced from Nägeli without further explanation the
essential principles of the modern theory of cells, especially if he
compares them with the views propounded at the same time and previously
by Schleiden, Unger, and von Mohl. But, as might be expected, the
further investigations, which were pursued with zeal during the
succeeding twenty years and produced a considerable literature, did
much to enlarge and perfect Nägeli’s theory in many of its details
and to correct it in some minor points; the theory itself facilitated
this process by supplying a scheme to which the investigation of
special questions could readily be referred. Whether the nucleus is
a solid body or a vesicle, whether in the division of a mother-cell
into compartments the wall of partition always grows from without
inwards or is formed simultaneously over its whole surface, whether it
is originally composed of two laminae or of one which is afterwards
differentiated,—these and many other questions were decided in course
of time.
Schleiden’s theory was now definitively set aside, a deeper insight was
obtained into the nature of the cell, and the ideas connected with the
word became broader and more profound. The knowledge of the formation
of cells showed that the cell-walls, which had been hitherto regarded
as the important part, are only secondary products, that the true
living body of the cell is represented by its contents and especially
by the protoplasm. Alexander Braun, relying on numerous researches
into the lower Algae, expressed himself in 1850 (‘Verjüngung,’ p.
244) to the effect that it is an inconvenience that the word cell is
used at one time to designate the cell with its wall, at another time
the cell without its wall, or again the wall without the cell. Since
the contents are the essential part of the cell and form a separate
and individual whole which has its own membrane-like boundary, the
primordial utricle, before the secretion of the membrane of cellulose,
we must either confine the term cell to the enveloping membrane or
to the chamber which it forms and find another name for the body
of the contents, or else call this the true and proper cell. This,
which presents itself at once as the correct mode of conception to
anyone who observes the formation of swarmspores in Algae and Fungi
and many other cases of cell-formation, was from this time forward
a vital point in the doctrine of the cell. Braun contributed also
to the clearing up of the ideas of botanists on this subject by
bringing together under one systematic view and classifying all the
varieties of cell-formation which were known to him up to the year
1850, and especially by a more searching investigation into modes of
conjugation. Henfrey’s contributions (‘Flora’ of 1846 and 1847) rested
entirely on the observations of German botanists, and brought to light
nothing that was independently and essentially new. On the other hand
Hofmeister’s new observations on the development of pollen (1848), and
his many remarks on cell-formation in his epoch-making researches into
embryology in 1851, contributed repeatedly to the deciding of doubtful
points, especially in the behaviour of the nucleus in cell-formation
and the production of the dividing walls. Von Mohl, who in spite
of his own excellent observations maintained up to 1846 a somewhat
undecided attitude of mind in respect to Schleiden’s theory, which was
at that time still in vogue, published in 1851, in his treatise ‘Die
vegetabilische Zelle,’ an excellent summary of the results which had
been so far achieved. In describing cell-division he notices specially
that the new nuclei occupy the centres of the future daughter-cells
before the division of the contents commences; but he still clung to
his old view, that in every instance of cell-division the parting-wall
must form progressively from without inwards, as in Cladophora,
contrary to Nägeli’s and Hofmeister’s correct statements, that cases
also occur of simultaneous formation at every point of the surface of
the partition-wall. As usual, however, von Mohl rested his opposition
on a good observation, and showed that in the case of the formation of
pollen in dicotyledonous plants it is possible to burst the membrane
of a mother-cell in the act of dividing, and set free the protoplasm
when it is already deeply divided into the four parts, and so to see
the half-formed partition-walls; but this only proved that such was the
process in the cases observed, the formation of the partition-walls
being simultaneous in others. It may be mentioned in this place, that
the idea of special mother-cells in the formation of pollen introduced
by Nägeli in 1842 was in entire accordance with the condition of the
science at the time, since he meant by the term the laminae of membrane
formed during the successive divisions of the mother-cell. To call
these still special mother-cells, as some modern phytotomists do,
is quite unjustifiable, because since 1846, when Nägeli propounded
his theory, the word cell, as we have seen, no longer designated the
mere membrane but the whole body of the cell, while the expression
special mother-cell rests on the older phraseology, in which cell and
cell-membrane are identical.
The additions made to the doctrine of cell-formation during the greater
part of the twenty years after 1851 were unimportant in comparison with
the mighty development which it had experienced during the preceding
ten years. These years had indeed been marked by the greatest possible
activity and fruitfulness in results in all parts of botanical study.
By the labours of Unger, von Mohl, Nägeli, Braun, and Hofmeister,
not only were the foundations laid for a true theory of cells, but
the details were worked out, and the conceptions connected with them
finally cleared up. Textbooks could now disseminate the new teaching
through wider circles, and with these works may be classed von Mohl’s
treatise already mentioned on the vegetable cell, since it came much
into use in a later and special edition, and was made by many teachers
of botany the foundation and guide in their lectures. It was now become
the fashion to compose not general text-books of botany, but compendia
of anatomy and physiology, and thus morphology and systematic botany
were neglected, as anatomy and physiology had been in the period
immediately preceding Schleiden’s time. Whoever therefore wished to
consult a complete manual of general botany was for some time obliged
to be content with Schleiden’s ‘Grundzüge’; and this had a great
deal to do with keeping alive his erroneous doctrines on cells and
fertilisation among general readers, while the professed botanists
had long given in their adherence to more modern and more correct
views. It is a misfortune in our science to be singularly poor in good
text-books, which might have given a general account from time to time
of the existing condition of research; this is one of the reasons why
for some time past even official representatives of botanical science
often differ so much from one another in their fundamental views on
method, and on the question of how much has been actually established
and how much still remains doubtful in the main divisions of the
subject, that a mutual understanding seems often impossible. That a
better state of things in this respect prevails in zoology, physics,
and chemistry, is certainly not a little due to the many good compendia
and text-books, which endeavour to give some account of the progress of
those sciences from year to year.
However, during the period from 1850 to 1870 Schacht and Unger
attempted to make the results of modern phytotomic investigation
accessible to general readers by means of text-books. Such was the
nature of Schacht’s[88] work, ‘Die Pflanzenzelle,’ published in
1852, a book which claimed to expound all parts of phytotomy by the
aid of the author’s own observations, with occasional reference only
to the writings of others; the attempt was so far impossible, as the
essential points had already been fully cleared up by the labours of
other botanists. The work had however the advantage of attracting the
attention of the reader by numerous good original drawings, and the
style was enlivened by the constant appeal to original observation;
at the same time, through insufficient use of the available
literature, the author’s views not unfrequently fell short of the
existing standard of knowledge. Worse than this however was a certain
defect of education, which led the writer into self-contradiction
and to incorrect classification of his facts; things fundamentally
important were sometimes neglected for unimportant details, and a
certain unreflecting empiricism was apparent in the whole work, in
marked contrast with the logical exactness of von Mohl, Nägeli, and
Hofmeister. In the second edition of the work, published in 1856 under
the title, ‘Lehrbuch der Anatomie und Physiologie der Gewächse,’ we
find many improvements in the details, but still on the whole the same
formal defects. It is not unimportant in a historical point of view to
notice this character of Schacht’s writings, because during this period
most young botanists and other persons also derived their knowledge
of phytotomy and of the nature of cells chiefly from him; his books
did not truly represent the condition of the science; their defective
reasoning had an injurious effect on the minds of younger readers,
and they introduced into phytotomy and vegetable physiology a habit
of accumulating a mass of undigested facts, such as has for some time
marked the condition of morphology and systematic botany.
Unger’s text-book ‘Anatomie und Physiologie der Pflanzen’ (1855) was
superior in conception and execution. It introduced the beginner to
the doctrine of cells with careful attention to all that was known
on the subject, if sometimes with some hastiness of decision, while
it brought the really important points everywhere into prominence
and employed individual facts to explain the general propositions,
as should always be done in a work intended for learners. But in
addition to this Unger’s book contained much that was really new and
valuable, and among other things some very important remarks on the
physiological characteristics of protoplasm; and it pointed out for
the first time the similarity between vegetable protoplasm and the
sarcode in Rhizopods, which Max Schulze had before carefully described.
In this year Nägeli also published investigations into the primordial
utricle and the formation of swarmspores in his ‘Pflanzenphysiologische
Untersuchungen,’ Heft I, which gave a new insight into the physical
and physiological characteristics of protoplasm. It has been
mentioned above that De Bary’s investigations into the Myxomycetes
in 1859 had thrown new light on the subject of protoplasm, and had
called attention to vital phenomena connected with it, which, though
analogous to what had been before observed, were rendered very striking
from the circumstance that in this case the protoplasm was not in
microscopically small portions enclosed by firm cell-walls, but moved
about and showed changes of shape in large, sometimes in very large,
masses, that were entirely free and unconfined. Here was the best
opportunity for making a nearer acquaintance with protoplasm and for
learning to recognise it as the immediate support of all vegetable
and animal life; in succeeding years the zootomists and physiologists
Max Schulze, Brücke, Kühne, and others established the fact that the
substance which lies at the foundation of cell-formation in animals
agrees in its most important characteristics with the protoplasm of
vegetable cells. A more detailed account of modern researches on this
subject, which would moreover involve the examination of Hofmeister’s
work of 1867, ‘Die Lehre von der Pflanzenzelle,’ does not fall within
the limits of our history.
2. FURTHER DEVELOPMENT OF OPINION ON THE NATURE OF THE SOLID FRAMEWORK
OF CELL-MEMBRANE IN PLANTS AFTER 1845.
Between 1840 and 1850 the most eminent representatives of phytotomy
were chiefly engaged, as we have seen, in observing the formation of
vegetable cells, and in framing the true theory of the subject by
process of induction. It was not to be expected that, while these
labours were bringing year by year new things to light and keeping
opinion on the formation of cells in a constant state of fluctuation,
their results would lead to very important changes in the theory of
the solid framework of cell-membrane founded by von Mohl. On the
contrary, it was at this time that his views such as we have seen them
on the connection of cells one with another, on the configuration of
their partition-walls and on their growth in thickness, attained their
greatest influence. His theory seemed to stand firm and complete when
contrasted with the unsettled state of opinion respecting the origin
of cells, and the question, how far it could be made to agree with the
new observations on the history of cell-formation, was hardly raised.
In the midst of the strife of opinion on the latter subject appeared
von Mohl’s ‘Vermischte Schriften’ in 1845, in which his views on the
structure of mature vegetable tissue were produced in a series of
monographs as the apparently irrefragable result of his observations.
And in fact phytotomic research up to 1860 followed the train of
thought initiated by von Mohl, till at last the inadequacy of his views
was rendered apparent between 1858 and 1863 by Nägeli’s new theory of
growth by intussusception, and by the profounder insight obtained into
the nature of cell-formation.
A sufficient proof of the correctness of these remarks is to be
found in the further development of the views of botanists on the
intercellular substance and the cuticle, which might have adapted
themselves before 1850 to the new theory of cells, but instead of
doing so were moulded by the ideas current before 1845. It has been
shown in the preceding chapter how von Mohl gradually restricted the
theory of intercellular substance which he had proposed in 1836, and
had come in 1850 to regard this substance as only a cement which might
in many cases be perceived between the cell-walls. It should be added
here, that Schleiden in connection with his theory of cells considered
both the intercellular substance and the cuticle to be supplementary
secretions from the cells, and made the former fill the intercellular
spaces, just as laticiferous and resiniferous passages are filled
with secretions from the adjacent cells (1845). Unger too in 1855
(‘Anatomie und Physiologie der Pflanzen’) thought the existence of a
cement between the cells necessary to prevent their falling asunder.
Schacht, who in his ‘Pflanzenzelle’ of 1852 had followed Schleiden in
explaining the intercellular substance and the cuticle as secretions or
excreta from the cells of the plant, still kept on the whole to this
view in 1858, though he modified it in some important points. This
theory of Schleiden and Schacht was first opposed by Wigand in a series
of essays (1850-1861), in which in strict adherence to von Mohl’s
theory of apposition he sought to prove, that the layers which are
visible in wood-cells as intermediate laminae in the partition-walls,
and which till then had been regarded as a cement between contiguous
cells, an intercellular substance, were nothing else than the thin
primary membranous laminae formed in the process of cell-division,
and subjected to subsequent chemical change, while the secondary
layers of thickening in von Mohl’s sense lie on both sides of them.
The cuticle on the epidermis was explained in a corresponding manner.
Though Sanio in 1863 raised a variety of objections to Wigand’s view,
he still adhered to it in principle, and found a strong confirmation
of it in the fact, that he succeeded in producing the well-known
cellulose-reaction in the intercellular substance of wood-cells when
freed from foreign admixtures.
The researches of Wigand and Sanio were sufficient to overthrow von
Mohl’s account of the intercellular substance and the cuticle, but
they had not proved that the intermediate laminae are in fact the
primary partition-walls on which von Mold’s secondary thickening-layers
had been deposited, on both sides in the case of the intercellular
substance, on one side in that of the cuticle. The structure of the
partition-walls and the existence of the cuticle could be explained
in a totally different way from the point of view now opened by
Nägeli’s theory of intussusception; there was no need now to see
either a secretion or a primary cell-wall in the intermediate lamina
of thickened cells or in the cuticle, for it was possible that
this lamination might be due to subsequent chemical and physical
differentiation of membranes thickened by intussusception. As
phytotomists are not yet quite agreed as to the correctness of this
view, we must be content with observing here that in the matter of the
cuticle and the intercellular substance lies one of the points, the
determination of which will involve the question of von Mohl’s earlier
theory of apposition. It is not the purpose of this history to give the
more modern views that have asserted themselves since 1860, especially
where the question is still in debate.
It was a part of von Mohl’s idea of the cell-tissue and one to which
he had firmly adhered since 1828, that except in the cross walls of
genuine wood-vessels and some very isolated cases the partition-walls
in cellular tissue are never perforated; that both simple and bordered
pits always remain closed by the very thin primary lamina of cellulose.
But between 1850 and 1860 several cases were discovered which were
at once exceptions to von Mohl’s rule, and of great importance to
physiology. Theodor Hartig, in his ‘Naturgeschichte der forstlichen
Kulturpflanzen Deutschlands’ (1851), described peculiar rows of
cells in the bast-system, in which the transverse and sometimes the
longitudinal walls appear to be pierced like a sieve by numerous
minute holes, and to these cells he gave the name of sieve-tubes.
Von Mohl (1855), while in other points confirming and extending
Hartig’s discovery, declared against the perforation of the walls,
believing that the appearances were due to lattice-like thickenings
of the cell-walls; he proposed therefore to call Hartig’s sieve-tubes
latticed cells. Then Nägeli showed in 1861 that in some cases at least
there can be no doubt that the walls are actually perforated, and
that the sieve-plates serve for the passage of mucilaginous matter
in bast-tissue, and the author of this history, it may be remarked
in passing in 1863, and Hanstein in 1864, suggested means by which
it may be ascertained with certainty that Hartig’s sieve-plates
are perforated. Meanwhile a number of laticiferous organs had been
recognised as forms of vessels in von Mohl’s sense, and it was found
that such canals are produced by dissolution of the septa of adjacent
cells. But the knowledge of the laticiferous organs continued till
towards 1865 to be very unsettled and defective, and the examination
of resin-passages, and the discovery that they are formed by simple
parting of cells from one another, belong to modern phytotomy;
Hanstein, Dippel, N. J. C. Müller, Frank, and others have since
1860 enlarged our knowledge of these forms of tissue. Schacht in
1860 established one of the most important exceptions to von Mohl’s
view above-mentioned, by demonstrating the formation and true form
of bordered pits in the wood of Conifers and in dotted vessels in
Angiosperms from the history of their development, and by showing
moreover that in all cases where bordered pits are formed on both sides
of a partition-wall and the adjacent cells afterwards convey air, there
the original very thin partition-wall in the bordered pit disappears,
and that consequently in such cases the bordered pits represent so many
open holes, through which adjacent cells and vessels communicate. At
the same time another hitherto inexplicable phenomenon received its
explanation. Malpighi, and after him the phytotomists at the beginning
of the present century had remarked, that the large vessels in the wood
are not unfrequently filled with parenchymatous cell-tissue, for the
origin of which no one could account. The phenomenon, however, could
now be explained quite simply after Schacht’s discovery; the formation
of thylosis in vessels only takes place when these border on closed
parenchyma-cells in the wood; when this is the case, the very thin
membrane which separates the bordered pits from the contiguous cells is
not absorbed, but it bulges inwards into the cavity of the vessel under
the pressure of the sap of the neighbouring parenchyma-cell, there
swells up like a bladder, and may by the formation of partition-walls
give rise to parenchymatous tissue; this, if proceeding from a number
of pits, fills up the cavity of the vessel.
3. HISTORY OF DEVELOPMENT AND CLASSIFICATION OF TISSUES.
It has been already stated, that the first step to a real understanding
of the structure as a whole of the higher plants was made by
Moldenhawer, who beginning with the study of the Monocotyledons,
first formed an idea of the vascular bundles as a distinct whole, a
system composed of various forms of tissue, and applied this idea
to explain the construction of the stems of Dicotyledons, upsetting
thereby Malpighi’s earlier theory of the growth in thickness of stems.
It was also observed, that von Mohl, advancing further in the same
direction, gave a more exact description of the epidermis and of the
tissues connected with it, and classified them, that is, introduced
a terminology founded on real investigation, but did not succeed in
bringing the subject to an entirely satisfactory conclusion; this could
in fact be reached only by the study of the history of development,
the only decisive method of investigation, whether the object be
to determine the true nature of cells and their subordinate forms,
or the solid fabric of vegetable structure, or as in the present
case to distinguish and classify forms of tissue; it is this method
which supplies the morphological points of view necessary for the
understanding of the inner structure of the plant by investigating
tissues in those states of development, in which they are not yet
adapted to subsequent physiological functions. The combination of
morphological and physiological points of view in the determination of
facts has maintained itself longer in this part of botanical study than
in any other; but here too ideas and opinions were gradually sifted and
cleared up under the influence of the modern study of the history of
development, though it was not till after 1850 that the determination
of the chief points in the theory of cell-formation left the leading
phytotomists at liberty to devote themselves to histological questions.
How little advance had been made towards the true understanding of
the varieties of forms of tissue in the higher plants before 1850 is
shown, for instance, by Schleiden’s account of tissues on page 232 of
his ‘Grundzüge’ of 1845, where parenchyma, intercellular substance,
vessels, vascular bundles, bast-tissue, bast-cells in Apocyneae and
Asclepiadeae, laticiferous vessels, felted tissue, epidermal tissue,
are discussed in this succession in co-ordinated sections of the text.
It is obvious that no well-ordered view of the whole cellular structure
of a plant of the higher order could be obtained in this way. Further
on in the same work, where Schleiden attempts a classification of
vascular bundles, which he distinguishes into closed and open, and
assigns the latter to Dicotyledons, we find the cambium-layer named as
the outer boundary of these open vascular bundles; the bast which lies
outside the cambium was therefore not considered to be a part of the
open vascular bundles, and this necessarily excluded any profitable
comparison of the circumstances in Monocotyledons and Dicotyledons.
The case is still worse in many respects in Schacht’s work already
mentioned, ‘Die Pflanzenzelle’ of 1852, where under the heading ‘Kinds
of vegetable cells’ the histology is discussed in the following
co-ordinated sections; the swarm-filaments of Cryptogams, the spores of
the same, pollen-grains, cells and tissue of Fungi and Lichens, cells
and tissue of Algae, parenchyma and its cells, vessels of the plant,
wood and its cells, bast-cells, stomata, appendicular organs of the
epidermis, cork; then follows a paragraph on the thickening-ring, and
then to the no small astonishment of the reader comes an account of the
vascular bundles, after the vessels, the wood, and the bast-cells have
been already dismissed. That such a mode of presenting the subject is
due to the little insight possessed by the writer into the structure of
the plant as a whole is apparent from simply reading the book, and a
similar confusion of ideas is found in his text-book of 1856.
We find a much better classification of tissues in 1855 in Unger’s
‘Anatomie und Physiologie der Pflanzen’; an account of cells is
followed by a description of cell-complexes, as one of the chief
divisions of the book, and herein of cell-families, cell-tissues, and
cell-fusions. Another chief section is occupied with cell-groups,
and here epidermal formations, air-spaces, sap-receptacles, glands
and vascular bundles are noticed; here certainly the fact has been
overlooked that vascular bundles may be co-ordinated with epidermal
formations, but not air-spaces, sap-receptacles and glands. His last
chief division gives an account of tissue-systems and of the way in
which the vascular bundles are united together in different plants, and
secondary growth in thickness and the activity of the cambium-layer are
described quite in the right connection.
In this branch of the science, as in every case where it is a question
of establishing fundamental conceptions, of surveying facts from
extensive points of view, and of seeking the requisite principles
by means of the history of development, we find that it is Nägeli
who opens the way and lays the foundation. In his ‘Beiträge zur
wissenschaftlichen Botanik’ of 1858, he proposed a classification
of tissues from purely morphological points of view. His first
division was into generating and permanent tissue; in each section
he distinguished two forms, prosenchymatous and parenchymatous
tissue. Parenchymatous generating tissue, the original component of
every young organ, he named primary meristem as distinguished from
prosenchymatous generating tissue, which is differentiated in the
form of strands and layers, and received from him the general name of
cambium; this was certainly not a happy distinction, because Nägeli’s
cambium by no means consists entirely of prosenchymatous tissue. By
the term secondary meristem Nägeli designated the tissue-strands and
tissue-layers which are formed between the permanent tissue of older
parts. The cambium he regards as the first product of the primary
meristem. The second chief form, permanent tissue, he divides into
two classes, not according to the form of the cells or physiological
relations, but according to its origin; all permanent tissue, which
is derived immediately from primary meristem, is protenchyma, all
that comes directly or indirectly from cambium is epenchyma. And
since the tissue-strands, till then known as vascular bundles, do not
contain vessels only but always fibrous elements also, as Bernhardi
had shown in 1805, Nägeli thought that they should therefore be
called fibrovascular strands. If it cannot be denied that the obvious
distinction between epidermal and other tissue did not find suitable
expression in this classification, and though other points of view
may at the present day be proposed for the genetic arrangement of
tissues, yet Nägeli’s classification and terminology have the merit of
having for the first time exhibited the general histology of plants
on comprehensive and genetic principles. It contributed materially to
impart a better understanding of the collective structure of plants.
The vascular bundles or fibrovascular strands especially demanded
further investigation of the genetic and morphological kind; for a
correct insight into the origin and subsequent transformation of this
tissue-system is as important for phytotomy as a similar knowledge with
respect to the bony system in vertebrate animals is for zootomy. But
a knowledge of the vascular bundles and their course in the stem has
a special importance in phytotomy, because it is the only way to the
understanding of secondary growth in thickness in true woody plants.
It was noticed above, that von Mohl had proved in 1831 the separate
character of the bundles which begin in the stem and bend outwards into
the leaves where they end, so that the entire system of bundles in a
plant consists of single bundles isolated when formed and subsequently
brought into connection with one another. Nägeli had already examined
the corresponding circumstances in the vascular Cryptogams in 1846,
when Schacht took the retrograde step of making the vascular system in
the plant originate in repeated branching, instead of in subsequent
blending of isolated strands; Mohl declared unhesitatingly against this
mistake in 1858, but it was refuted at greater length and still more
clearly by Johannes Hanstein in 1857, and by Nägeli in 1858. Hanstein
in a treatise on the structure of the ring of wood in Dicotyledons
confirmed Nägeli’s previous statements, and proved in the case of
Dicotyledons and Conifers that the first woody circle in the stem is
formed from a number of vascular bundles, which are identical with
those of the leaves and originate in the primary meristem of the bud.
These primordial bundles pass downwards through a certain number of
internodes in the stem independent and separate, and either retain
their isolation to the point where they end below or unite with
adjacent bundles which originated lower down. Hanstein happily termed
the portions of the vascular bundles, which enter the stem from the
base of the leaf and traverse a certain portion of it in a downward
direction, leaf-traces, so that it may be stated briefly, that the
primary wood-cylinder in Dicotyledons and Conifers consists of the sum
of the leaf-traces. Nägeli’s observations were of a more comprehensive
character, and supplied, as we have seen, a terminology for tissues.
He distinguished three kinds of vascular bundles according to their
course; the common bundles, which represent Hanstein’s leaf-traces in
the stem, and whose upper ends bend outwards into the leaves; the
cauline bundles, which extend above to the punctum vegetationis of the
stem without bending outwards into leaves; and leaf-bundles, which
belong to the leaves only. He laid it down as a general rule as regards
the common bundles in Dicotyledons and Conifers that they begin to form
where their ascending and descending halves meet, at the spot therefore
where they bend outwards into the leaf, and continue to form as they
descend into the stem and ascend into the leaf by differentiation of
suitable tissue. It follows from the nature of these common bundles,
that a more thorough understanding of their course and origin
presupposes a more accurate knowledge of the order of formation of the
leaves at the end of the stem and of the changes in the phyllotaxis
during growth; these relations Nägeli took into detailed consideration,
and even derived from them new points of view for the examination of
the genetic arrangement of leaves, pointing out at the same time the
unsatisfactory nature of the principles of the doctrine propounded
by Schimper and Braun. Nägeli was also the first who compared the
anatomical structure of roots with that of stems, and drew attention to
the peculiar character of the fibrovascular body in these organs. As
his previous discovery of the apical cell and its segmentation promoted
further research, so now his treatise on fibrovascular strands called
forth many others from various quarters; among them that of Carl Sanio
on the composition of the wood (‘Botanische Zeitung,’ 1863) must be
mentioned as one of the first and most important, and as serving in
conjunction with the works of Hanstein and Nägeli to throw light upon
the processes of growth in thickness of stems. It has been already
said that neither von Mohl nor Schleiden, neither Schacht nor Unger
succeeded in finding the true explanation of growth in thickness. It
was impossible that they should do so, for they were insufficiently
acquainted with the origin, true course, and composition of the
vascular bundles before growth in thickness commences; the study of
the subject was greatly perplexed by the confounding together in
thought and language of totally different things which came under
consideration, the so-called thickening-ring, in which the first
vascular bundles were supposed to originate close under the summit
of the stem, being confounded with the cambium of true woody plants
which is formed at a much later period, and both of them again with
the very late-formed meristem-layer in arborescent Liliaceae, in
which new vascular bundles are continually being produced and cause a
peculiar enlargement of the stem[89]. Sanio’s treatise first removed
this confusion of ideas, which appears in von Mohl himself to some
extent even in 1858, by sharply distinguishing the thickening-ring
beneath the point of the stem, in which the vascular bundles begin to
be formed, from the true cambium, which is formed at a later time in
and between the vascular bundles, and produces the secondary layers of
wood and rind; Sanio also occupied himself with submitting the various
elements of the wood to a more careful examination, and with giving
them a better classification and terminology. The peculiar instance
of secondary growth in thickness in the arborescent Liliaceae, which
had long been known and had helped to mislead von Mohl and Schacht,
was fully explained for the first time by A. Millardet in 1865. The
later works of Nägeli, Radlkofer, Eichler and others on abnormal
wood-formations contributed materially to enlarge the knowledge of
normal growth also; but these coming after 1860, and Hanstein’s later
investigations into the differentiation of tissues at the end of the
stem in Phanerogams, do not fall within the limits of our history.
4. NÄGELI’S THEORY OF MOLECULAR STRUCTURE AND OF GROWTH BY
INTUSSUSCEPTION.
This theory, the importance of which to the further development of
phytotomy and vegetable physiology has been already pointed out,
will form the conclusion of our history of the anatomy of plants. It
was a remarkable coincidence that this molecular theory of organic
forms, which is not without results for zootomy also, was brought
to completion at about the same time, namely, the year 1860, that
Darwin first published his theory of descent. At the first glance
the two theories seem to have no connection with one another, and so
the coincidence in time appears to be quite accidental. But if we
go deeper into the matter, we find a resemblance between them which
is of great historical importance; they both of them exchange the
purely formal consideration of organic bodies, which had prevailed
up to that time, for a consideration of causes; as Darwin’s doctrine
endeavours to account for the specific forms of animals and plants from
the principles of inheritance and variability under the disturbing
or favouring influence of external circumstances, so the object of
Nägeli’s theory is to refer the growth and inner structure of organised
bodies to chemical and mechanical processes. The future will show,
whether the views which we owe to Nägeli will not contribute to the
laying a deeper foundation for the theory of descent, since it is
not improbable that a more thorough understanding of the molecular
structure of organisms may add light and certainty to the still obscure
conceptions of inheritance and variation.
The first beginnings were, as is usual in similar cases, small
and inappreciable, and no one could have foreseen from the first
observations of the facts in question what the ultimate development
would be. We have said above, that von Mohl observed as early as
1836 the striation of certain cell-walls, and that this led Meyen,
on the ground of further but to some extent inaccurate observations,
to conceive of vegetable cell-walls as composed of spirally twisted
threads. It was also noticed that von Mohl next distinguished true
striation from spiral thickenings (1837), the two having been confused
together by Meyen, and advanced so far as to form some idea of the
molecular structure of cell-walls, without arriving however at any
satisfactory conclusion. Agardh, who discovered some new instances
of cell-striation, was still less successful in his speculations.
Von Mohl resumed the subject in 1853 in the ‘Botanische Zeitung,’
and insisted on the fact that it was not possible to separate the
striae or apparent fibres by mechanical or chemical means, but he
left it still undecided whether the lines which cross each other
in the surface-view belong to the same or to different layers of
membrane. The communications of Crüger and Schacht, made shortly
after, did not help to advance the question; Wigand also took part
in the discussion in 1856, but wandered at once from the right path
by supposing the cross-striations to belong to different layers of
membrane. As long as botanists adhered to von Mohl’s theory, that
the concentric stratification of cell-walls was due to deposition of
new layers, it was scarcely possible for them to arrive at a correct
decision with respect to striation; it became possible, when Nägeli
proved in his great work ‘Die Stärkekörner’ (1858) that the concentric
stratification of starch-grains and of cell-membranes generally does
not mean, that similar layers lie simply one on another, but that
denser and less watery layers alternate with layers that are less
dense and contain more water; and that it is not possible to explain
this mode of stratification by deposition as understood by von Mohl,
but that it may be explained by intercalation of new molecules between
the old ones and by corresponding differentiation of the amount of
water. That surface-growth in cell-walls does take place by this kind
of intussusception had been incidentally suggested by Unger, and
the appearance, known as the striation of the cell-wall might now
be referred to the same principle as the concentric stratification,
namely to the intercalation of more and less watery matter in regular
alternation. But Nägeli pointed out a fact which had escaped other
observers, namely, that the difference of structure which usually
appears on the surface-view as double cross-striation, passes through
the whole thickness of a stratified cell-wall. Thus Nägeli arrived
at a differentiation in three directions in space of the substance
of every minute portion of cell-membrane, and made better use than
von Mohl himself had made of the comparison which he had suggested,
namely, that the structure of a cell-wall with cross-striation and
at the same time with concentric stratification resembles that of a
crystal cleaving in three directions. He first gave expression to this
conception of the structure of the cell-wall in 1862 in his ‘Botanische
Untersuchungen,’ I. p. 187, and further developed it in the second
volume of the same work at p. 147.
But the true starting-point of Nägeli’s theory of molecular structure
is to be found in his searching investigations in 1858, into the
constitution of starch-grains. From the way in which they resist the
effects of pressure, drying, distention, and withdrawal of a part of
their substance, he arrived at the conclusion that the whole substance
of a starch-grain is composed of molecules, whose shape must be not
spherical but polyhedral, that these are separated from one another
in their normal condition by envelopes of water, and that the amount
of water in the stratified substance depends on the size of these
molecules, the water being less when the molecules are larger; this
view could at once be applied to the structure of the cell-wall,
the growth of which may be explained as the increase in size of the
molecules already present, and the intercalation of new small molecules
between the old ones. These molecules of Nägeli are themselves very
compound bodies, for the smallest of them would consist of numerous
atoms of carbon, hydrogen and oxygen, and ordinarily a molecule would
be composed of thousands of those aggregates of atoms, which the
chemists call molecules.
In examining starch-grains Nägeli came to the conclusion that molecules
of different chemical character are grouped together at every visible
point; the material which colours blue with iodine, the granulose,
could be removed from the grains, and then there remained behind a
skeleton of the starch-grain very poor in substance, but showing
exactly the original stratification and giving no blue colour with
iodine; this Nägeli named starch-cellulose. It followed from this
behaviour, that two chemically different molecules lie everywhere side
by side in the grain of starch, much as if red and yellow bricks had
been so employed to build a house, that when all the yellow bricks
were afterwards removed, the red alone would still represent the
wall in its original form as a whole though in a looser condition.
He arrived at similar results in the case of the crystalloid proteid
bodies, which Theodor Hartig discovered, and Radlkofer had examined
crystallographically, Maschke chemically. Since it is possible in
the same manner to extract the so-called incrusting matters from
cell-membranes without essentially altering their form, and to obtain
ash-skeletons of them which imitate all the delicacies of their
structure, the comparison adopted above may also be applied in still
more complex manner to the molecular structure of these membranes;
and indeed many considerations lead to the belief, that the ideas
which Nägeli obtained from starch-grains may be applied with some
modifications to the structure of protoplasm also.
We said that the appearances in the starch-grains led Nägeli to suppose
that their molecules are not spherical but polyhedral, and the question
naturally arose whether they are really crystalline. The point could
be settled by the use of polarised light, to which different observers
had already turned their attention. Erlach in 1847, Ehrenberg in 1849,
had employed polarised light for the determination of microscopic
objects, without however arriving at any conclusions on the subject
of molecular structure; Schacht indeed at a later time declared
such observations to be a pretty amusement, but without scientific
value. But soon we have once more one of von Mohl’s careful and solid
investigations (‘Botanische Zeitung,’ 1858), in which with the aid
of technical improvements in the apparatus he arrived at conclusions
respecting the nature and substance of cell-membranes, starch-grains,
&c., which proved that in the hands of a reflecting observer perfectly
familiar with the physics of polarised light the instrument is no
toy, but a means for penetrating deeply into nature’s secrets. Yet on
this occasion also appeared that peculiarity in von Mohl which twenty
years before had prevented him from founding a conclusive theory upon
his profound and extended observations on cell-formation; he was
content once more to observe thoroughly and correctly, to describe
what he observed carefully, and to connect it with proximate physical
principles in such a manner as to supply rather a classification of
phenomena, than a new and deeper insight into the essence of the
matter. He wanted the creative thought, the intense mental effort,
to arrive by analysis at the ultimate elements in the results of his
investigations and to frame for himself a clear representation of
the inner structure of the organised parts. Von Mohl in this case
also stopped short at induction and did not pass on to deductive and
constructive elaboration of the question before him; this was left to
Nägeli, as we shall see.
Meanwhile a more exhaustive work appeared in 1861 from the pen of
Valentin on the investigation of vegetable and animal tissue in
polarised light, in which the author, equipped with great knowledge of
the subject itself and its literature, examined in detail the phenomena
of polarisation, gave a good account of the instrument and the mode
of using it, and explained generally the theory and practice of
investigations of the kind. But he overlooked one fact noticed by von
Mohl, that vegetable cell-membranes, through which rays of polarised
light pass perpendicularly to their surface, show interference-colours,
and this was sure to lead him to an incorrect explanation of their
inner structure.
Nägeli from 1859 onwards made the phenomena of polarisation the
subject of protracted study, practical and theoretical; the results
were published in 1863 in his ‘Beiträge,’ Heft 3, but he had in the
previous year made known that portion of them which bore on the
molecular structure of cell-walls and starch-grains (‘Botanische
Mittheilungen,’ 1862). The phenomena of polarisation led him once
more and by a different path to the view that the organised parts
of the vegetable cell consist of isolated molecules surrounded by a
fluid, and his renewed investigations of these phenomena resulted in
more definite conceptions of the nature of these molecules, which
from the optical behaviour of the objects examined he concluded were
not only polyhedral but crystalline; in effect, the molecules of
the substance of the organised parts of plants behave, according to
Nägeli, as crystals with two optic axes, which therefore possess three
different axes of elasticity; in starch-grains and cell-membranes these
crystalline molecules are so arranged that one of these axes is always
perpendicular to the stratification, while the two others lie in its
plane. The effect of the organised parts of the cells on polarised
light is the sum of the effects of the single molecules, whereas the
fluid that lies between them is optically inactive, and only comes into
consideration because according to its quantity the molecules separate
more or less far from or approach one another.
THIRD BOOK
HISTORY OF VEGETABLE PHYSIOLOGY
(1583-1860)
INTRODUCTION.
All that was known in the 16th and at the beginning of the 17th
centuries of the phenomena of life in plants was scarcely more than
had been learnt in the earliest times of human civilisation from
agriculture, gardening, and other practical dealing with plants. It was
known, for instance, that the roots serve to fix plants in the soil and
to supply them with food; that certain kinds of manure, such as ashes
and, under certain conditions, salt, strengthen vegetation; that buds
develope into shoots; and that the blossom precedes the production
of seeds and fruits. These and a variety of minor physiological
phenomena were disclosed by the art of gardening. On the other hand,
the physiological importance of leaves in the nourishment of plants
was quite unknown, nor can we discover more than a very indistinct
perception of the connection between the stamens and the production of
fruitful seeds. That the food-material taken up from the soil must move
inside the plant in order to nourish the upper parts was an obvious
conclusion, which it was attempted to explain by comparing it with the
movement of the blood in animals. Writers on the subject up to the end
of the 17th century make very slight mention of the influence of light
and warmth on the sustentation and growth of plants, though doubtless
the operation of these agencies in the cultivation of plants, as in
other matters, must have been early recognised.
So scanty was the stock of knowledge which the founders of vegetable
physiology in the latter half of the 17th century found ready to their
hand. While the physiological significance of the different organs of
the human body and of most animals were known to every one, at least
in their more obvious features, the study of vegetable life had to
begin with laborious enquiries, whether the different parts of plants
are generally necessary to their maintenance and propagation, and
what functions must be ascribed to individual parts for the good of
the whole. It was no easy matter to make the first step in advance in
this subject; something can be learnt of the functions of the parts
of animals from direct observation, scarcely anything in the case of
plants; and it is only necessary to read Cesalpino and the herbals
of the 16th century to see how helpless the botanists were in every
case in presence of questions concerning the possible physiological
meaning of vegetable organs, when they ventured beyond the conceptions
of the root as the organ of nourishment, and of the fruit and seeds
as the supposed ultimate object of vegetable life. The physiological
arrangements in vegetable organs are not obvious to the eye; they
must be concluded from certain incidental circumstances, or logically
deduced from the result of experiments. But experiment presupposes the
proposing a definite question resting on a hypothesis; and questions
and hypotheses can only arise from previous knowledge. An early attempt
to connect the subject with existing knowledge was made in the use
of the comparison of vegetable with animal life, a comparison which
Aristotle had employed with small success. Cesalpino, provided with
more botanical and zoological knowledge, endeavoured to arrive at more
definite ideas of the movement of the nutrient juices in plants, and
when Harvey discovered the circulation of the blood in the beginning of
the 17th century, the idea at once arose that there might be a similar
circulation of the sap in plants. Thus a first hypothesis, a definite
question was framed, and attempts were made to decide it by more exact
observation of the ordinary phenomena of vegetation, and still better
by experiment; and though a discussion which lasted nearly a hundred
years led to the opinion that there is no circulation of sap in plants
corresponding to the circulation of blood in animals, the result was
obtained by the aid of this hypothesis derived from a comparison
between animals and plants. The important discovery that leaves play
a considerable part in the nourishment of plants, was to some extent
an incidental product of the investigation of the former question, and
it preceded that of the decomposition of carbon dioxide by the green
parts of plants by more than a hundred years. To give another example;
it was obviously a comparison of certain phenomena in vegetable life
with the propagation of animals which paved the way for the discovery
of sexuality in plants; long before Rudolf Jacob Camerarius made his
decisive experiments (1691-1694) on the necessary co-operation of the
pollen in the production of seeds capable of germination, the idea
had been entertained that there might be an arrangement in plants
corresponding to the sexual relation in animals, though that idea was
highly indistinct and distorted by various prepossessions. In like
manner the interest excited by the discovery of the irritability of
the Mimosae in the 17th century, and of similar phenomena of movement
in plants at a later time, was mainly due to the striking resemblance
suggested between animals and plants; and the first researches into
the subject were obviously intended to answer the question whether
the movements in plants are due to conditions of organisation similar
to those in animals. In all cases of this kind it was matter of
indifference whether the analogies presupposed were finally confirmed
after prolonged investigation, as in the question of sexuality, or
disproved as in that of the circulation of the sap. The result was
of less importance than the obtaining points of departure for the
investigation. It answered this purpose to adopt certain actual or only
apparent analogies between plants and animals, and to assume, to some
extent to invent, certain functions for the apparently inactive organs
of plants, and to interrogate them upon the point. Scientific activity
was set in motion, and it mattered not what the result might be. In
all questions connected with the phenomena of life, our own life is not
only the starting-point but also the standard of our conceptions; what
animate nature is as opposed to inanimate we discern first by comparing
our own being with that of other objects. From our own vital motions we
argue to those of the higher animals, which we comprehend immediately
and instinctively from their conduct; by aid of these the motions
of the lower animals also become intelligible to us, and further
conclusions from analogy lead us finally to plants, whose vitality
is only in this way made known to us. While plants were thus even in
ancient times regarded as living creatures and allied to animals,
further reflection naturally suggested the idea that the phenomena of
animal life would be reproduced in plants even in details. We learn
from the botanical fragments of Aristotle that this was in fact the
way in which the first questions in vegetable physiology arose; they
assumed a more definite form with Cesalpino, and later physiologists
repeatedly made use of similar conclusions from analogy. The historian
of this branch of botanical science must seek no other beginning of
it, for it had no other and could have no other from the nature of
the case. And if preconceived analogies between plants and animals
often proved deceptive and mischievous, yet continued investigation
gradually brought to light more important and more essential points of
agreement between the two kingdoms; it has become more and more evident
in our own days, that the material foundations of vegetable and animal
life are in the main identical,—that the processes connected with
nourishment, movement of juices, sexual and asexual propagation present
the most remarkable similarities in both kingdoms.
If the first founders of scientific vegetable physiology surrendered
themselves thoroughly to teleological views, this was owing to the
circumstances of the time, and it served indeed to promote the
first advances of the science. There was no need in the 17th and
18th centuries that a man should be an Aristotelian to presuppose
design and arrangements in conformity with design in all parts of
physiological investigation. This is everywhere and always the original
point of view which precedes all philosophy; but it is the part of
advanced science to abandon this position; and as early as the 17th
century philosophers recognised the fact that the teleological mode
of proceeding is unscientific. But the first vegetable physiologists
were not philosophers in the stricter sense of the word, and in their
investigations they accepted the teleological conception of organic
nature without question, because they regarded it as a self-evident
fact, that every organ must be purposely and exactly so made as to be
in a condition to perform the functions necessary for the permanence
of the whole organism. This conception was in accordance with views
then prevailing, and was even useful; it was no disadvantage in the
first beginnings of the science, that it should be supposed that
every, even the minutest, part of a plant was expressly contrived
and made for maintaining its life, for this was a strong motive for
carefully examining the organs of plants, which was the first thing
requisite. This is exemplified in Malpighi, Grew, and Hales, and
we shall see that even towards the end of the 17th century Konrad
Sprengel made splendid discoveries respecting the relations of the
structure of the flower to the insect world, while strictly carrying
out his teleological principles. The teleological view was injurious
to the progress of morphology from the first, though the history
of systematic botany shows how hard it was for botanists to free
themselves from such notions. The case was different with physiology;
so long as it was a question of discovering the functions of organs,
and learning the connection between the phenomena of life, teleology
proved highly useful if only as a principle of research. But it was
another matter when it became requisite to investigate causes, and to
grasp the phenomena of vegetation in their causal connection. To this
the teleological mode of view was inadequate, and it became necessary
indeed to discard it as a hindrance, in spite of the difficulty of
explaining adaptation in the arrangements of organisms from any other
than the teleological point of view. It is sufficient here to say that
this difficulty is satisfactorily removed by the theory of selection.
This theory is become as important in this respect to physiology, as
the theory of descent is to systematic botany and morphology. If the
theory of descent finally liberated the morphological treatment of
organisms from the influence of scholasticism, it is the theory of
selection which has made it possible for physiology to set herself free
from teleological explanations. Only an entire misunderstanding of the
Darwinian doctrine can allow anyone to reproach it with falling back
into teleology; its greatest merit is to have made teleology appear
superfluous, where it seemed to naturalists in former times, in spite
of all philosophical objections, to be indispensable.
If the comparison of plants with animals as well as the teleological
conception of organisms promoted the first attempts at the
physiological investigation of plants, other influences of decisive
importance came into play when the time came for endeavouring to
conceive and explain the causes and conditions of the functions, which
had then been ascertained at least in their most obvious features.
Phytotomy was here the chief resource. In proportion as the inner
structure of plants was better known and the different kinds of tissue
better distinguished, it became possible to bring the functions of
organs, as made known by experiment, into connection with their
microscopic structure; phytotomy dissected the living machine into its
component parts, and could then leave it to physiology to discover from
the structure and contents of the tissues, how far they were adapted to
perform definite functions. Obviously this only became possible when
the phenomena of vegetation had been previously studied in the living
plant. For example, the microscopic examination of the processes
which take place in fertilisation could first be made to yield further
conclusions, after sexuality itself, the necessity of the pollen to
the production of fruitful seeds, had been proved by experiment; in
the same way the anatomical investigation of wood could only supply
material for explaining the mode in which water rises in it, when it
had first been ascertained by experiment that this happens only in the
wood, and so in other cases.
The relation between physiology and physics and chemistry suggests
similar considerations; it is necessary to make some preliminary
remarks in explanation of this relation, because we often meet with
the view, especially in modern times, that vegetable physiology is
virtually only applied physics and chemistry, as though the phenomena
of life could be simply deduced from physical and chemical doctrines.
This might perhaps be possible, if physics and chemistry had no further
questions to solve in their own domains; but in fact both are still
as far distant from this goal, as physiology is from hers. It is true
indeed, that modern vegetable physiology would be impossible without
modern physics and chemistry, as the earlier science had to rely on
the aid of the physics and chemistry of the day, when she was engaged
in forming a conception of ascertained vital phenomena as operations
of known causes. But it is equally true, that no advance which physics
and chemistry have made up to the present time would have produced
any system of vegetable physiology, even with the aid of phytotomy;
history shows that a series of vital phenomena in plants had been
recognised in the 17th and 18th century, at a time when physics and
chemistry had little to offer, and were in no condition to supply
explanations of any kind to the physiologist. The true foundation of
all physiology is the direct observation of vital phenomena; these must
be evoked or altered by experiment, and studied in their connection,
before they can be referred to physical and chemical causes. It is
therefore quite possible for vegetable physiology to have reached a
certain stage of development without any explanation of the phenomena
of vegetation from physics or chemistry, and even in spite of erroneous
theories on those subjects. What Malpighi, Hales, and to some extent
Du Hamel produced, was really vegetable physiology, and of a better
kind than some moderns are inclined to believe; and their knowledge was
derived from observations on living plants, and not from the chemical
and physical theories of their time. The discovery even of important
facts, for example, that green leaves only can form the food suitable
to effect the growth and formation of new organs, was made a hundred
years before that of the decomposition of carbon dioxide by the green
parts of plants, at a time indeed when chemistry knew nothing of carbon
dioxide and oxygen. A whole series of physiological discoveries might
be mentioned, which were distinctly opposed to chemical and physical
theories, and even served to correct them. We may give as examples,
the establishment of the facts that roots absorb water and the
materials of food without giving up anything in return, which seemed
quite unintelligible on the earlier physical theory of the endosmotic
equivalent; and that the so-called chemical rays of the physicists are
of subordinate importance in vegetable assimilation, while contrary
to the prevailing notions of physicists and chemists the yellow
portions of the spectrum and those adjacent to it actively promote the
decomposition of carbon dioxide. From what doctrines of the physicists
could it have been concluded, that the downward growth of roots and the
upward growth of stems was due to gravitation, as Knight proved in 1806
by experiments on living plants; or could optics have foreseen that
the growth of plants is retarded by light, and that growing parts are
curved under its influence. Our best knowledge of the life of plants
has been obtained by direct observation, not deduced from chemical and
physical theories. After these preliminary remarks we may proceed to
give a rapid survey of the progress of vegetable physiology.
1. That the first beginnings of vegetable physiology were made about
the time that chemistry and physics began to take their place among
the true natural sciences, is no proof that they called vegetable
physiology into existence. She, like general physiology, mineralogy,
astronomy, geography, owed her origin to the outburst of the spirit of
enquiry in the 16th and 17th centuries, which feeling the emptiness
of the scholastic philosophy set itself to gather valuable knowledge
by observation in every direction. It was in the second half of the
17th century that societies or academies for the study of the natural
sciences were founded in Italy, England, Germany, and France under the
influence of this feeling; the first works on vegetable physiology
play a very prominent part in their transactions; not to speak of less
important cases, it was the Royal Society of London which published
between 1660 and 1690 the memorable works of Malpighi and Grew; the
first communications of Camerarius, which form an epoch in the history
of the doctrine of sexuality, appeared in the journals of the German
Academia Naturae Curiosorum, and the French Academy undertook about the
same time to organise methodical researches in vegetable physiology
under Dodart’s direction, though the results it is true did not answer
to the goodness of the intention. This period of movement in all
branches of science, when the greatest discoveries followed one another
with marvellous rapidity, witnessed also the first important advances
in vegetable physiology; such were the first investigations into
the ascending and descending sap, especially those made in England,
Malpighi’s theory which assigned to leaves the functions of organs of
nutriment, Ray’s first communications on the influence of light on
the colours of plants, and above all the experiments of Camerarius,
which proved the fertilising power of the pollen. It was the period of
first discoveries; the attempts at explanation were certainly weak;
but phytotomy which was just commencing its own work lent aid from
the first to physiology, while physics and chemistry could do but
little for her. On the other hand, the predilection for mechanics
and mechanical explanation of organic processes in Newton’s age bore
fair fruit in Hales’ enquiries into the movement of sap in plants;
his ‘Statical Essays’ of 1727 connect closely with the works before
mentioned which had laid the foundations of the science, and with
this important performance the first period of its history reaches a
distinctly marked conclusion.
This time of vigorous advance was followed by many years, in which
no notable work was done and no great discovery effected; there was
active disputation on what had been already ascertained, but it did not
lead to any deeper conception of the questions or to new experimental
determinations.
2. About the year 1760 new life was infused into the consideration of
various branches of vegetable physiology. Du Hamel’s ‘Physique des
arbres’ (1758) gave a summary of former knowledge and added a number of
new observations, and from that time till the beginning of the present
century a series of important discoveries was made. The doctrine of
sexual propagation, which had scarcely been advanced since the time of
Camerarius, and was disfigured by the theory of evolution, found an
observer of the first rank in Koelreuter (1760-1770), who threw new
light upon the nature of sexuality by his experiments on the artificial
production of hybrids; he was the first who carefully studied the
arrangements for pollination, and pointed out the remarkable connection
between them and insect-life. These relations were afterwards (1793)
examined in greater detail by Konrad Sprengel, who arrived at such
astonishing and far-reaching results, that they were not even
understood by his contemporaries, nor was their significance fully
appreciated till quite modern times and in connection with the theory
of descent.
No less important was the advance made in the doctrine of the
nourishment of plants. Between 1780 and 1790 Ingen Houss proved, that
the green parts of plants absorb carbon dioxide under the influence
of light and eliminate the oxygen, and thus obtain the carbon which
plants accumulate in organic combinations, but that all parts of
plants also absorb at all times smaller quantities of oxygen, and
exhale carbon dioxide, and so perform a process of respiration exactly
corresponding to that of animals. He was soon followed by Theodore
de Saussure with more thorough investigation of these processes,
and with proofs that the ash-constituents of a plant are no chance
or unimportant addition to its food, as had been hitherto commonly
supposed (1804). The influence also of general physical forces on
vegetation was established in some important points, though not yet
submitted to searching examination. Thus Senebier showed in the period
between 1780 and 1790 the great effect which light exercises on the
growth and green colour of plants, and De Candolle at a later date
discovered its operation in the case of leaves and flowers that show
periodic movements. Still more important was Knight’s discovery in 1806
that the upright growth of stems and the downward direction of the main
roots are determined by gravitation.
3. This second period of important discoveries was also followed by
a relapse, and again doubts were raised as to the correctness of the
very facts which had been best established; attempts were made under
the influence of preconceived opinions to invalidate or ignore these
facts, and to substitute for them theories that wore the guise of
philosophy. The so-called nature-philosophy, which had long been a
great hindrance to morphology, proved in like manner injurious to
vegetable physiology; the doctrine of the vital force especially stood
in the way of every attempt to resolve the phenomena of life into their
elementary processes, to discern them as a chain of causes and effects.
The ash-constituents of plants, and even their carbon, were traced to
this vital force, and misty notions connected with the word polarity
were used to explain the direction of growth and much beside. In like
manner the influence of the nature-philosophy was brought to bear on
the established results of the sexual theory to the destruction of all
sound logic, and the sexuality of plants was once more openly impugned
in the face of Koelreuter’s investigations. This state of things
continued till some time after 1820, but then it began to improve once
more. L. C. Treviranus examined and refuted the errors of Schelwer and
Henschel in 1822; in England Herbert conducted new and very valuable
investigations into the question of hybridisation; and it was in this
period that Carl Friedrich Gärtner studied and experimented on normal
fertilisation and the production of hybrids during more than twenty
years; his conclusions, published in exhaustive works in 1844 and in
1849, finally settled the more important questions connected with the
sexual theory about the same time that Hofmeister established the
microscopic embryology of Phanerogams on a firm foundation.
Other parts also of vegetable physiology had been considerably advanced
before 1840; Theodore de Saussure observed in 1822 the production of
heat in flowers and its dependence on respiration; ten years later
Goeppert proved the rise of temperature in germinating and vegetating
organs. Dutrochet stimulated enquiry by his researches in various
branches of the science between 1820 and 1840; he was the first to
apply the phenomena of diosmosis to the explanation of the movement
of sap in plants with a lasting influence on the further progress of
physiology. Chemical investigations were less fruitful in results,
though they served to collect a considerable material of single facts,
which could afterwards be turned to theoretical account.
The close of this period, which began with unprofitable doubts, but
in which much was set in a train for further development after 1840,
is marked by the publication of some important compilations, in which
all that had as yet been done in vegetable physiology was presented in
a connected form. In addition to Dutrochet’s collected works (1837)
three comprehensive compendia of vegetable physiology made their
appearance, one by De Candolle, which was translated into German by
Roeper and published with many improvements and additions in 1833
and 1835; this was followed by a work on vegetable physiology by L.
C. Treviranus, 1835-1838, and lastly by Meyen’s ‘Neues System der
Pflanzenphysiologie,’ 1837-1839. These works exhibit the characteristic
features of the period chiefly in this, that physiology finds as yet
no strong support in phytotomy, while the old views of vital force are
brought face to face with more exact physico-chemical explanations of
processes of vegetation.
4. We have already pointed out the wonderful impulse given to the
study of morphology and phytotomy, of embryology and cells about the
year 1840; it was shown also that this was due in a great measure
to discarding the errors of the nature-philosophy and the idea of
vital force, and requiring in the place of such speculations exact
observation and systematic induction, and how Schleiden’s ‘Grundzüge’
soon after 1840 vigorously met the demands of the newer time in
these respects, but without satisfying them by the positive results
obtained. The rapid progress made by phytotomy and the doctrine of
cells in the hands of von Mohl and Nägeli proved specially favourable
to vegetable physiology, by making it possible to follow the processes
of fertilisation in the interior of the ovule. The formation of the
pollen-tube from the pollen-grain had been observed long before
1840, and Schleiden in 1837 had proposed the view that the embryo
of Phanerogams was formed at the end of the pollen-tube by free
cell-formation after it had entered the embryo-sac. But Amici in 1846
and Hofmeister in 1849 showed that this notion was erroneous, and
that the germ-primordium is in existence in the embryo-sac before the
arrival of the pollen-tube and is excited by it to further development,
to the forming the embryo. Similarly Hofmeister’s further observations
on the embryology of Vascular Cryptogams and Mosses left no doubt,
that the spermatozoids of these groups of plants discovered by Unger
and Nägeli serve to fertilise the germ-cell or egg-cell previously
formed in the female organ and to excite it to further development
(1849, 1851). Soon after the sexual act was observed in various
Algae, and these afforded the best opportunity for solving by the
aid of the microscope the questions which experiment had still left
open. Thuret showed in 1854, how the large egg-cells in species of
Fucus are surrounded and fertilised by spermatozoids, and he even
succeeded in producing hybrids by fertilising the egg-cells of one
species with the spermatozoids of another; but it was still uncertain
whether simple contact of the male and female organs was sufficient, or
whether fertilisation is due to the mingling of the substance of the
spermatozoid and the germ-cell; the question was settled by Pringsheim
in 1855; he saw the male organ of fertilisation of a fresh-water alga
penetrate into the substance of the egg-cell and be dissolved in it,
and this proceeding was afterwards observed in higher Cryptogams and is
represented in its simplest form in the sexual act of the Conjugatae,
which De Bary described at length in 1858 and like Vaucher regarded as
a sexual process.
When we consider to what an extent the time and power of work of the
most eminent botanists was devoted after 1840 to long and difficult
observations on the minute anatomy of plants, on cell-formation,
embryology and the history of the development of organs, we cannot
wonder if other parts of vegetable physiology, which require
experiments on vegetation in plants, were cultivated but little and
by the way only; but these studies also gained firmer footing in the
advance of phytotomy, which supplied the physiologist with a more
definite idea of the organism in which the phenonema of vegetative life
are produced.
The chemistry of the food of plants was one of the strictly
physiological subjects, which like the sexual theory was studied
without intermission and with considerable success in the period from
1840 to 1860, but chiefly or entirely by chemists, who connected
their investigations into the processes of nutrition in plants with
Saussure’s results. Agricultural chemists were chiefly engaged till
nearly 1860 with the questions, whether all or certain constituents
of the ash of a plant are indispensable parts of its food, and whence
these constituents are derived, and with cognate considerations on
the exhaustion of the soil by cultivation and its remedy by suitable
manuring. In France Boussingault had undertaken experimental and
analytical investigations on these subjects before 1840, and it was
he who in the course of the next twenty years made the most valuable
physiological discoveries; of these the most important was the fact
that plants do not make use of free atmospheric nitrogen as food, but
take up compounds of nitrogen for the purpose. In Germany the interest
in such questions was increased by the instrumentality of Justus
Liebig, who gathered from the knowledge that had been accumulated up
to 1840 all that was fundamental and of real importance, and drew
attention to the great practical value of the theory of the nutrition
of plants in agriculture and in the management of woods and forests;
considerable state provision was soon made for investigations of the
kind, but these often wandered from the right path for the reason,
that being designed to promote practical interests they lost sight
of the inner connection between all vital phenomena. Still a great
mass of facts was accumulated, which careful sifting might afterwards
render serviceable to pure science. Some of the best agricultural
chemists deserve the credit of vindicating purely scientific as well
as practical points of view, and explained in comprehensive works the
general subject of the nutrition of plants, so far as it was possible
to do so without going deeply into their organisation; among these
were Boussingault and the Germans Emil Wolff and Franz Schulze. But
the questions of the nutrition of plants, which are connected with the
chemical processes of assimilation and metabolism within them, remained
still undecided, though some valuable preliminary work on these points
dates from this time.
In comparison with this important advance in the sexual theory and the
doctrine of the nutrition of plants little was done in the branches
of vegetable physiology which remain to be mentioned, and that little
appeared in an unconnected and fragmentary state; different observers
established the connection between the temperature of plants and
oxygen-respiration; some new single facts were discovered in connection
with the downward curvature of roots, Brücke published in 1848 an
excellent enquiry into the movements of Mimosa-leaves, and Hofmeister
showed in 1857 that the phenomenon, then known as bleeding in the
vine and some other trees, takes place in all woody plants, and not
in spring only but in every period of the year, if the requisite
conditions are present. These and many other isolated observations were
very valuable for the future, but were not used at the time to frame
comprehensive theories, because no one devoted himself exclusively to
questions of the kind with the perseverance, which in these difficult
subjects can alone lead to certain results and to a deeper insight
into the inner connection of the phenomena. Surprisingly small was the
addition to the knowledge of the movement of sap in plants, and still
less was discovered respecting the external conditions of processes of
growth and the movements connected with them. The important question of
the dependence of the phenomena of vegetation on temperature, was it
is true not wholly neglected; but the mistake was made of attempting a
short cut by multiplying the total period of vegetation of a plant by
the mean daily temperature, in the hope of finding in this product an
expression for the total warmth required by a given plant; this mistake
was especially misleading in the geography of plants.
The more valuable knowledge which had been gathered up to 1851 was
brought together by von Mohl in his often-mentioned work on the
vegetable cell with equal perspicuity and conciseness, and current
views were critically examined; vegetable physiology generally was
expounded at greater length but with less critical sifting in
Unger’s text-book of 1855; these were the two books which did most to
disseminate a knowledge of the subject up to 1860, and they performed
their task with credit; that which appears in Schacht’s books after
1852 under the head of vegetable physiology rests on such imperfect
acquaintance with this branch of science, as to diminish rather than
increase its reputation.
* * * * *
Passing from this preliminary survey to a more detailed account of the
subject, it will be found necessary to keep the history of the sexual
theory distinct from other questions in vegetable physiology. This
mode of proceeding is required by the fact, that the establishment and
further elucidation of the decisive points in the sexual theory were
made independently of the rest of physiology, so that the historical
continuity would be interrupted and the account rendered obscure by
any attempt to connect the development of the theory chronologically
with other topics. In like manner the doctrine of the nutrition of
plants and of the movement of the sap was developed uninterruptedly and
in independence of other physiological matters; it will be advisable
therefore to devote a separate chapter to those subjects also. Earlier
discoveries respecting the movements of the parts of plants and the
mechanics of growth will be briefly recounted in a third chapter.
CHAPTER I.
HISTORY OF THE SEXUAL THEORY.
1. FROM ARISTOTLE TO R. J. CAMERARIUS.
It will contribute to a correct appreciation of the discoveries made
towards the end of the 17th century by Rudolph Jacob Camerarius and his
successors in regard to the sexual relations of plants, if we first
make ourselves acquainted with all that was known of the matter up to
that time from Aristotle downwards; we shall learn at the same time
how extremely unfruitful was the superficial observation of the older
philosophy in a question in which inductive research only could lead to
real results.
That Aristotle[90] like many others after him reckoned sexual
fertilisation among processes of nutrition, and thus failed to perceive
the specific and peculiar character of the latter, is shown distinctly
by his assertion, that the nutritive and propagative power of the
soul is one and the same. This hasty generalisation was associated
in Aristotle’s mind with another error arising from very defective
experience, which led him to bring sexuality in organisms into causal
connection with their movement in space. He tells us in his botanical
fragments, that in all animals which have the power of locomotion, the
female is distinct from the male, one creature being female, another
male, but both being of the same species, as in humankind. In plants
on the contrary these powers are combined and the male is not distinct
from the female; each plant therefore reproduces itself and emits
no fertilising material; and he adds, that in animals which do not
move, as those that have shells and those that live attached to some
other substance, male and female are not distinguished, for their
life resembles that of plants; at the same time they are called male
and female by resemblance and analogy, and there is a certain slight
distinction. In like manner some trees produce fruits while others do
not, though they aid fruit-bearing trees in the production of fruit, as
happens in the case of the fig-tree and the caprifig.
In comparison with these views of Aristotle those of his disciple
Theophrastus[91] appear to some extent enlightened, and to rest on a
wider experience, but even his observation supplies nothing of interest
on the subject; for he says that some blossoms of the ‘mali medicae’
produce fruit, and that some do not, and that it should be observed
whether the same thing occurs in other plants, which he might easily
have done for himself in his own garden. He is more concerned with
putting his knowledge into logical order, than with answering the
question whether there is any sexual relation in plants. It is certain,
he says, that among plants of the same species some produce flowers and
some do not; male palms, for instance, bear flowers, the female only
fruit[92]; and he concludes the sentence by the remark, that in this
lies the difference between these plants, and those which produce no
fruit, and that it is obvious that there must be a great difference
in the flowers. In his third book ‘De Causis’ (c. 15, 3) he says,
that terebinths are some male and some female, and that the former are
barren and are therefore called male. That Theophrastus in all these
matters trusted to the relations of others is shown by a passage in
the same book (c. 18, 1), where he says, ‘What men say, that the fruit
of the female date-palm does not perfect itself unless the blossom
of the male with its dust is shaken over it, is indeed wonderful,
but resembles the caprification of the fig, and it might almost be
concluded that the female plant is not by itself sufficient for the
perfecting of the fœtus; but this cannot be the case in one genus or
two, but either in all or in many.’ We observe the grand style in which
the Greek philosopher dismisses this important question, and how far he
is from condescending to make an observation for himself.
It appears that in Pliny’s time the hypothesis of a sexual difference
in plants had grown up and become confirmed in the minds if not of
writers, yet of those who occupied themselves with nature; Pliny in his
‘Historia Mundi,’ describing the relation between the male and female
date-palm, calls the pollen-dust the material of fertilisation, and
says that naturalists tell us that all trees and even herbs have the
two sexes[93].
If this theme supplied little material for reflection to philosophers,
it did not fail to excite the fancy of the poets. De Candolle cites
the verses of Ovid and Claudian on the subject, and passing over
the intervening centuries for a very sufficient reason notices the
lively poetic description of two date-palms in Brindisi and Otranto
by Jovianus Pontanus in 1505. But nothing was gained in this way for
natural science.
Treviranus in his ‘Physiologie der Gewächse[93]’ (1838), II. p. 371,
has well described the state of knowledge on this subject among the
botanists of Germany and the Netherlands in the 16th century. ‘The
idea of a male sex in such plants as Abrotanum, Asphodelus, Filix,
Polygonum mas et femina, was founded only on difference of habit, and
not on the parts which are essential to it. But it should be observed
that it is the less learned among the older botanists, Fuchs, Mattioli,
Tabernaemontan, who make most frequent use of this mode of designating
plants; the more learned, as Conrad Gesner, de l’Écluse, J. Bauhin
employ it only in the case of a plant already known. De l’Écluse it
is true in describing the plants which he found often notes the form,
colour, and even the number of the stamens; in Carica Papaya he calls
the individual with stamens the male, and the one with carpels the
female, since he holds them to belong to different sexes, though of the
same species; but he is satisfied with saying, that it is affirmed that
the two are so far connected, that the female produces no fruit if the
male is separated from it by any great distance (‘Curae posteriores,’
42).
The case of the botanists above-mentioned is simply one of ignorance;
in the botanical philosopher Cesalpino on the contrary we see a
consequence of the Aristotelian system, which leads him distinctly to
reject the hypothesis of separate sexual organs in plants as opposed to
their nature. It is difficult to understand how De Candolle, at page 48
of his ‘Physiologie végétale,’ can say that Cesalpino recognised the
presence of sexes in plants. His conception of vegetable seed-grains
as analogous to the male seed in animals must have made it impossible
for him to understand sexuality in plants. So too his notion that the
seed is derived from the pith as the principle of life in plants,
in connection with which he says at page 11 of the first of his
sixteen books; ‘Non fuit autem necesse in plantis genituram aliquam
distinctam a materia secerni, ut in animalibus, quae mari et femina
distinguuntur.’ He regarded the parts of the flower which surround the
ovary, or are separate from it, together with the stamens as simply
envelopes of the foetus; and though he knew, as has been already shown,
that in some plants, the hazel, chestnut, Ricinus, Taxus, Mercurialis,
Urtica, Cannabis, Mais, the flowers are separate from the fruit, and
even mentions that the barren individuals are called male, and the
fruit-bearing female, he understood this only as a popular expression,
without really admitting a sexual relation. Respecting the words male
and female he says at page 15: ‘Quod ideo fieri videtur quia feminae
materia temperatior sit, maris autem calidior; quod enim in fructum
transire debuisset, ob superfluam caliditatem evanuit in flores,
in eo tamen genere feminas melius provenire et fecundiores fieri
aiunt, si juxta mares serantur, ut in palma est animadversum, quasi
halitus quidam ex mari efflans debilem feminae calorem expleat ad
fructificandum.’
There is no mention of the pollen here, still less any attempt to
extend what had been observed in dioecious plants to the ordinary
cases, in which flowers and pistil, as Cesalpino would say, are united
in the same individual. His view of the relation between the seed and
the shoot, cited above on page 47, shows that he conceived of the
formation of seeds as only a nobler form of propagation than that by
buds, but not essentially distinct from it. The idea of sexuality in
plants was not in fact consonant with Cesalpino’s interpretation of
Aristotelian teaching.
Prosper Alpino’s account (1592) of the pollination of the date-palm
contains nothing new, except that he had seen it in Egypt himself[94].
The Bohemian botanist Adam Zaluziansky[95] made no observations of his
own, but attempted in 1592 to reduce the traditional knowledge on the
subject to some kind of theory. The foetus, he says, is a part of the
nature of plants, which they produce out of themselves, and is thus
distinguished from the shoot which grows from the plant, as a part
from the whole, but the other as a whole from a whole. He quotes Pliny
almost word for word where he says, that observers of nature maintain
that all plants are of both sexes, but in some the sexes are conjoined,
in others they are separate; in many plants the male and female are
united, and these have the power of propagation in themselves, like
many androgynous animals; and he explains this, more explicitly than
Aristotle, from defect of locomotion in plants. This is the case, he
says, with the majority of plants. In some, as the palm, the male
and female are separated, and the female without the male produces
no fruit, and where the dust from the male does not reach the female
plant by natural means, man can assist. Zaluziansky like other writers
is anxious that plants of different sexes should not be taken for
different species. He refers also to the popular distinction of many
plants into male and female according to certain external peculiarities.
Jung again must certainly have known the facts and views that were
current in his time; but there is nothing in his botanical writings to
show that he entertained the idea of a real sexuality in plants, of the
necessity of the co-operation of two sexes in the work of propagation.
It might almost be believed that the most learned and serious men, such
as Cesalpino and Jung, were just those, who regarded the hypothesis of
sexuality in plants as an absurdity, and shrunk from its consideration.
This impression is conveyed too by Malpighi’s ‘Anatomie des Plantes.’
It was Malpighi who gave the first careful account of the development
of the seed, and studied the earlier stages in the growth of the embryo
in the embryo-sac; and yet even he says nothing of the co-operation
of the dust contained in the anthers in the formation of the embryo,
and does not once mention the views of former writers. Malpighi,
like Cesalpino, regarded the formation of seeds as only another kind
of ordinary bud-formation, and propagation as only another kind of
nutrition. He mentions (p. 52) incidentally that plants with unfruitful
flowers are designated as male, but treats this as a popular expression
merely, and ultimately propounds the theory that the stamens and the
floral envelopes remove a portion of the sap from the flower, in order
to purify the sap for the production of the seeds (p. 56).
In all accounts of the theory of sexuality in plants, a botanist
otherwise unknown in history, Sir Thomas Millington, is named as the
person who first claimed for the stamens the character of male organs
of generation. The only record of the fact, however, is contained in
the following words of Grew in his ‘Anatomy of Plants’ (1682), ch. 5,
sect. 3, p. 171: ‘In conversation on this matter (namely the connection
of the stamens, called by Grew the attire[96], with the formation of
seeds) with our learned Savilian Professor Sir Thomas Millington, he
told me he was of opinion that the attire served as the male organ in
the production of the seed. I replied at once, that I was of the same
opinion, and gave him some reasons for it, answering at the same time
some objections that might be brought against it.’ Grew gives on p.
172 the following summary of his ideas on the subject[97]; it would
appear, he says, that the attire serves to remove some superfluous
parts of the sap, as a preparatory process to the production of seed.
As the floral envelopes (foliature) serve to remove the volatile and
saline sulphur-parts, so the attire serves to lessen and adjust the
gaseous, in order that the seed may become more oily and its principles
be better fixed. Here we find ourselves on the ground of the chemistry
of the day, in which sulphur, salt, and oil play the chief parts.
Consequently, continues Grew, the flower has usually a stronger smell
than the attire, because the saline sulphur is stronger than the
gaseous, which is too subtle to affect the sense. Closely adhering to
Malpighi’s view he goes on to compare these processes in the flower
with processes in the ovary of animals, inasmuch as they qualify
the sap in the ovary for the approaching formation of seed, and he
says that as the young and early attire before it opens contains the
superfluous part of the female organ, so after it is opened it probably
performs the office of the male. But how confused his ideas still were
on this point may be further seen by examination of the passage which
follows in his book (page 172, section 7), where, speaking of the
single flowers in the head of the Compositae, he regards the blade,
that is the style and stigma, of the floral attire as a portion of a
male organ, and the globulets (pollen-grains) and other small particles
upon the blade and in the thecae (anthers) of the seed-like attire as
a vegetable sperm, which subsequently when the parts are duly matured
falls down upon the seed-case and so touches it with a prolific virtue.
He meets the objection, that the same plant must consequently be both
male and female, with the fact, that snails and other animals are
similarly constituted. That the pollen-grains communicate a prolific
virtue to the ovary (uterus) or to its juices by simply falling upon
it, he thinks is rendered probable by comparing this with the process
of fertilisation in many animals, and here Grew has some curious
remarks. The section closes with the observation that to expect
complete similarity in this matter between plants and animals, is to
require that the plant should not only resemble an animal, but should
actually be one.
If now we ask ourselves, what it really was that was gained from
Millington and Grew, we find that it was simply the conjecture, that
the anthers produce the male element in fertilisation, and that this
view was closely connected in their minds with the strangest chemical
theories and analogies from animal life. It is remarkable by what
indirect ways science sometimes advances. If Grew had only been
prepared to assume some kind of sexuality in plants, he need only have
taken up Theophrastus’ statement, that the anther-dust of the male
palm is shaken over the female to produce fertilisation; and since
both Grew and Malpighi observed the pollen in the anthers, they might
at once and in reliance on this experiment of a thousand years before
have come to the conclusion that the stamens are the male organs. But
Grew never mentions the ancient views and experiences. Like other
writers before Camerarius, he made no attempt to answer the question
by experiment. It was a step in advance, when Ray in his ‘Historia
Plantarum’ (1693), I. cap. 10, p. 17; II. p. 1250, threw some light on
the very obscure train of thought in Grew’s mind, and did something to
put it on the right track, by referring to the case of dioecious plants
and to the old experience of the date-palm, but he too made no attempt
to settle the question by experiment. The true discoverer of sexuality
in plants, Camerarius, was however engaged in the experimental solution
of the problem two years before the appearance of Ray’s ‘Historia
Plantarum.’ Ray’s remarks on the subject in the preface to his ‘Sylloge
Stirpium’ (1694) are only assertion founded on no experiments. But
if any are prepared to attribute greater value to the utterances of
Grew and Ray, the comparison of them with the way in which Camerarius
addressed himself to the question will show at once, that it was he
who so far advanced the theory of the subject as to make it accessible
to experimental treatment, as he undoubtedly was the first who not
only undertook experiments on the subject but carried them out with
the skill which will appear in the following section. Linnaeus was
right when he says in his ‘Amoenitates’ (1749), I. p. 62, that it was
Camerarius who first clearly demonstrated (perspicue demonstravit) the
sexuality of plants and the mode of their propagation.
2. ESTABLISHMENT OF THE DOCTRINE OF SEXUALITY IN PLANTS BY RUDOLPH
JACOB CAMERARIUS. 1691-1694.
We have seen that all that was known with regard to sexuality in
plants up to 1691 was comprised in the facts related by Theophrastus
concerning the date-palm, the terebinth, and the ‘malus medica,’ and in
the conjectures of Millington, Grew, and Ray, while Malpighi’s views in
opposition to these later authors were considered to be equally well
founded. The sexuality of plants could only be raised to the rank of
a scientific fact in one way, that namely of experiment; it had to be
shown that no seed capable of germination could be formed without the
co-operation of the pollen. All historic records concur in proving,
that Camerarius was the first who attempted to solve the question
in this way, and that he followed up this attempt by many other
experiments. It is quite another question how the fertilising matter
reaches the germ which is capable of being fertilised, and this could
not be entertained till experiment had established the fact, that the
pollen is absolutely indispensable to fertilisation.
To Johann Christian Mikan, Professor of Botany in Prague, is due
the merit of having collected the scattered and therefore almost
forgotten writings of RUDOLPH JACOB CAMERARIUS[98], and published
them, together with some similar works of Koelreuter, at Prague in 1797
under the title, ‘R. J. Camerarii Opuscula Botanici Argumenti.’ This
book, apparently little known, will be my principal authority for the
following remarks. The short preliminary communications are printed
without alteration from the ninth and tenth year of the second, and
from the fifth and sixth year of the third decury of the Ephemerides
of the Leopoldina; the letter to Valentin, which will be noticed again
further on, together with an abstract of the same and an answer of
Valentin, are given according to Gmelin’s edition of 1749.
Camerarius had observed, that a female mulberry-tree once bore fruit,
though no male tree (amentaceis floribus) was in its neighbourhood,
but that the berries contained only abortive and empty seeds, which he
compared to the addled eggs of a bird. His attention was roused, and
he made his first experiment on another dioecious plant, Mercurialis
annua; he took in the end of May two female specimens of the wild plant
(they were usually called male, but he knew them to be the female) and
set them in pots apart from others. The plants throve, the fruit was
abundant and filled out, but when half ripe they began to dry up, and
not one produced perfect seeds; his communication on this subject is
dated December 28, 1691. In the third decury of the Ephemerides, year
5, he relates that in a sowing of spinach he had found monoecious as
well as dioecious plants, as Ray had observed in Urtica romana, and he
himself again in three other species. The disregard of this fact was
afterwards the cause of erroneous interpretation of the experiments and
of doubt about sexuality.
But Camerarius’ chief composition on the subject of sexuality in
plants is his letter ‘De sexu Plantarum,’ which is often mentioned but
apparently little read, and which he addressed to Valentin, Professor
in Giessen, on Aug. 25, 1694. It is the most elaborate treatise on the
subject which had as yet been written, or indeed which appeared before
the middle of the 18th century, and contains more profound observations
than were made by any other botanist before Koelreuter. The style
contrasts favourably with the style of the writers of the time, and
is thoroughly that of modern natural science; it combines perfect
knowledge with careful criticism of the literature of the subject;
the construction of the flower is explained more clearly than it had
ever been before, or was again for a long time after, and expressly
for the purpose of making the meaning of his experiments on sexuality
intelligible. The whole tone of the letter shows that Camerarius was
deeply impressed with the extraordinary importance of the question, and
that he was concerned to establish the existence of sexuality by every
possible means.
After detailed examination of the parts of the flower, the anthers and
pollen, the behaviour of the ovules before and after fertilisation,
the phenomena of double flowers and similar matters, from all which he
cautiously deduces the meaning of the anthers (apices), he proceeds
to bring forward direct proofs. He says, ‘In the second division of
plants, in which the male flowers are separated from the female on the
same plant, I have learnt by two examples the bad effect produced by
removing the anthers. When I removed the male flowers (globulos) of
Ricinus before the anthers had expanded, and prevented the growth of
the younger ones but preserved the ovaries that were already formed,
I never obtained perfect seeds, but observed empty vessels, which
fell finally to the ground exhausted and dried up. In like manner I
carefully cut off the stigmas of Mais that were already dependent, in
consequence of which the two ears remained entirely without seeds,
though the number of abortive husks (vesicularum) was very great.’
He then refers to his former communications to the Ephemerides on
dioecious plants, and says that the case of the spinach confirmed these
results. After alluding to similar relations in animals he continues,
‘In the vegetable kingdom no production of seeds, the most perfect
gift of nature, the general means for the maintenance of the species,
takes place, unless the anthers have prepared beforehand the young
plant contained in the seed (nisi praecedanei florum apices prius ipsam
plantam debite praeparaverint). It appears, therefore, justifiable to
give these apices a nobler name and to ascribe to them the significance
of male sexual organs, since they are the receptacles in which the
seed itself, that is that powder which is the most subtle part of the
plant, is secreted and collected, to be afterwards supplied from them.
It is equally evident, that the ovary with its style (seminale vasculum
cum sua plumula sive stilo) represents the female sexual organ in the
plant.’ Further on he assents to Aristotle’s theory of the mixture of
sexes in plants, and adduces Swammerdam’s discovery of hermaphroditism
in snails, which he says is the exception in animals but the rule in
plants. One erroneous notion which was only seen to be erroneous a
hundred years later by Konrad Sprengel, and not finally refuted till
within the last few years, was his belief that hermaphrodite flowers
fertilise themselves, and this by comparison with the snails he thinks
is strange, though most botanists till down to our own times, in spite
of Koelreuter and Sprengel, did not find it strange. That sexuality
in plants was admitted by botanists, Ray excepted, at the close of
the 17th century at most in a figurative sense, but that Camerarius
conceived of it as in the animal kingdom, and sought to make this
conception prevail, is apparent from the strong expressions, which he
uses to show that in dioecious plants the distinction between male and
female plants is not to be understood figuratively. He says that the
new foetus, the young plant contained in the seed, is formed inside
the coat of the seed after the plant has flowered, exactly as the new
foetus is formed in animals. The authority of the ancients was still
great at that time, for Camerarius thinks it necessary to insist that
the views of Aristotle, Empedocles, and Theophrastus are not opposed
to his sexual theory. Camerarius appears as the true investigator
of nature, endowed with the true discerning spirit in disregarding
the question which had already been raised with respect to animals,
whether the ovum or the spermatozoid (vermis) produces the foetus,
because the first thing to be done was to establish the fact of a
sexual difference, not the mode of generation; he thinks it certainly
desirable to examine and see what the pollen-grains contain, how far
they penetrate into the female parts, whether they advance uninjured
as far as the seed which receives them, or what they discharge if they
burst before reaching it. He does full justice to Grew’s services in
connection with the knowledge of the pollen and its function.
It does all honour to the scientific spirit in Camerarius, that he
raises a number of objections to his own theory; one was, that Lycopods
and Equisetaceae produce, as he thinks, no young plants from their
pollen; he suspected therefore that they have no seed. It should be
remembered that the germination of Equisetaceae and Lycopods was not
observed till the 19th century. An objection, more important at the
time, was that a third ear of a castrated maize plant contained eleven
fertile seeds, of whose origin he could give no account. He was even
more disturbed by finding that three plants of hemp taken from the
field and cultivated in the garden produced fertile seeds, and he tries
to explain it by supposing various ways in which pollination might have
taken place unobserved. This led him to make a fresh experiment; next
year he placed a pot containing seedlings of hemp in a closed room;
three male and three female plants grew up; the three male were cut off
(not by himself) before their flowers opened; the female produced a
great number of abortive seeds, but also a good many fruitful ones. His
opponents and those who sought to appropriate his honours fastened,
as is usual, on these failures, without being able to account for the
experiments which had been successful. The statement of his failures is
our best proof of the exactness of his observations, for we now know
the cause of failure, which Camerarius himself observed, but did not
apply in explanation. We may assume that he would have cleared up this
point in his splendid investigations in a quieter time, for at the end
of his letter he laments the unjust war then raging; it was the time
of the predatory campaign of Louis XIV. To his letter is appended a
Latin ode of twenty-six stanzas by an unknown poet, probably a pupil
of his own; it is an epitome of the ‘Epistola de sexu Plantarum,’ as
Goethe’s well-known poem contains the chief points of his doctrine
of metamorphosis, but it resembles Goethe’s composition in no other
respect; it begins
Novi canamus regna cupidinis,
Novos amores, gaudia non prius
Audita plantarum, latentes
Igniculos, veneremque miram.
3. DISSEMINATION OF THE NEW DOCTRINE; ITS ADHERENTS AND OPPONENTS.
1700-1760.
No part of botany has so often engaged the pen of the historian, as the
doctrine of sexuality in plants; but the majority of writers have not
gone to the original sources for their information, and the consequence
has been that the merits of the real founders and promoters of the
doctrine have often been thrown into the shade for the benefit of
others; even German botanists have ascribed the services of Camerarius
to Frenchmen and Englishmen, because they were unacquainted with his
writings, or were unable to judge of the question and its solution.
We shall here endeavour to show from the records of the 18th century
how far anyone before Koelreuter really contributed anything of value
to the establishment of the sexual theory. As is usually the case
in great revolutions in science, some simply denied the new theory,
many adopted it without understanding the question, others formed a
perverse and distorted conception of it under the influence of reigning
prejudices, while others again sought to appropriate to themselves
the merit of the real discoverer; there were but few who with a right
understanding of the question advanced it by new investigations.
The botanists who endeavoured to aid in determining the matter by their
own observations may be distinguished into those, to whom the important
point was the enquiry whether the pollen is absolutely necessary to
the formation of seed, such as Bradley, Logan, Miller, and Gleditsch,
and those who like Geoffroy and Morland assumed that sexuality was no
longer an open question, and who were bent on observing in what way
the pollen effects fertilisation in the ovule. But there was another
class of writers altogether, who, believing that they could deal with
the subject without making observations and experiments of their own,
either like Leibnitz, Burckhard, and Vaillant, simply accepted the
results of the observations of others on general grounds, or like
Linnaeus and his disciples, endeavoured to draw fresh proofs from
philosophical principles, or like Tournefort and Pontedera, simply
rejected the idea of sexuality in plants. Lastly, we might mention
Patrick Blair who did nothing himself, but merely appropriated the
general results of Camerarius’ observations, and has had his reward
in being quoted even by German writers as one of the founders of the
sexual theory[99].
We have now to see what was really brought to light by further
experiment and observation. BRADLEY appears to have been the first who
experimented on hermaphrodite flowers with a view to establish the
sexuality of plants (‘New improvements in Gardening’ (1717), I. p.
20). He planted twelve tulips by themselves in a secluded part of his
garden, and as soon as they began to flower removed the anthers; the
result was, that not one of them produced seeds, while four hundred
tulips in another part of the same garden produced seeds in abundance.
Twenty years pass by before another experiment is made. JAMES
LOGAN[100], Governor of Pennsylvania, an Irishman by birth, set some
plants of maize in each corner of a plot of ground, which was forty
feet broad, and about eighty long, and experimented on them in various
ways. In October he noted the following results:—the cobs of the
plants, from which he had removed the male panicles when the stigmas
were already dependent, presented a good appearance; but closer
examination showed that they were unfertilised, with the exception of
one which was turned in the direction from which the wind might have
conveyed pollen from other plants. On the cobs, from which he had
removed some of the stigmas, he found exactly as many grains as he had
left stigmas. One cob, which had been wrapped in muslin before the
appearance of the stigmas, produced only empty husks.
Miller’s experiments in 1751, which Koelreuter has extracted from the
‘Gardener’s Dictionary,’ part II[101], are specially interesting,
because the aid of insects in pollination was then observed for the
first time. Miller planted twelve tulips, six or seven ells apart, and
carefully removed the stamens as soon as the flowers began to open;
he imagined that he should thus entirely prevent fertilisation; some
days after he saw some bees load themselves with pollen in an ordinary
tulip-bed and fly over to his imperfect flowers. After they were gone,
he observed that they had left on the stigmas a quantity of pollen
sufficient for fertilisation, and these tulips did in fact produce
seed. Miller also kept some female plants of spinach apart from the
male, and found that they bore large seeds without embryos.
Professor GLEDITSCH, Director of the Botanic Garden in Berlin,
described in the same year (‘Histoire de l’Académie royale des sciences
et des lettres’ for the year 1749, published in 1751 at Berlin), an
experiment on the artificial fertilisation of Palma dactylifera folio
flabelliformi, which was no doubt our Chamaerops humilis, since he says
himself in page 105 that it was Linnaeus’ Chamaerops, and Koelreuter
speaks of the plant in his report by that name. This treatise, in point
of scientific tone and learned handling of the question, is the best
that appeared between the time of Camerarius and that of Koelreuter.
We learn from the introduction, that in the year 1749 there were few
who doubted the existence of sexuality in plants. The author says
that he has endeavoured to attain to perfect conviction on the point
by many years’ experiments with plants of the most various kinds. Of
late years he had chiefly selected dioecious plants for investigation,
Ceratonia, Terebinthus, Lentiscus, and the species of date-palm
which is commonly called Chamaerops. After relating the formation of
fertile seeds in Terebinth and the mastic-tree produced by artificial
pollination, he turns to Chamaerops, of which species Prince Eugene
had repeatedly caused specimens of considerable size to be brought
over from Africa; a specimen cost as much as a hundred pistoles; but
they died without flowering. ‘Our palm in Berlin,’ he continues, ‘is
a female, and may be eighty years old; the gardener asserts that it
has never borne fruit, and I have myself never seen fertile seeds on
it during fifteen years.’ As there was no male tree of the kind in
Berlin, Gleditsch procured some pollen from the garden of Caspar Bose
in Leipsic. In the course of the nine-days’ journey the greater part
of the pollen escaped from the anthers, and Gleditsch feared that it
was spoilt; but he was reassured by the Leipsic botanist Ludwig, who
had had experience in Algiers and Tunis, and who informed him that the
Africans usually employ dry pollen that has been kept for some time for
the purpose of fertilisation. Though the flowering of the female tree
was nearly over, he strewed the loose pollen on its flowers, and tied
the withered inflorescence of the male plant to a late-blowing shoot of
the female. The result was that fruit ripened in the following winter,
and germinated in the spring of 1750. A second attempt conducted in a
similar manner produced an equally favourable result[102].
Koelreuter, who repeats this account in his ‘Historie der Versuche,’
a record of the experiments made between the years 1691 and 1752 on
the sexes of plants, ends his narrative with these words: ‘These
are, as far as I know, all the attempts which have been made and
described since the year 1691 to prove the existence of sexes in
plants.’ Koelreuter’s book was written to show that experiment only can
determine the question of sexuality in the vegetable kingdom, and that
no one beside Camerarius, Bradley, Logan, Miller, and Gleditsch had
pursued this method up to 1752.
While these botanists occupied themselves with the question whether
there was a distinction of sexes in the vegetable kingdom, we meet with
two writers at the beginning of the 18th century who regard sexuality
as proved, and who take up the question of the mode in which the
pollen brings about the formation of the embryo. Both were adherents
of the theory of evolution, bad observers, and not familiar with
the literature of the subject. The first is SAMUEL MORLAND. In the
‘Philosophical Transactions’ of 1702 and 1703, p. 1474, he names Grew
as the man who had observed that the pollen answers to the male semen,
but he makes no allusion to Camerarius’ experiments, the only ones
which had as yet been made. He himself suggests that the young seeds
may be compared to unfertilised ova, while the pollen-dust (farina)
contains embryo plants, one of which must find its way into every
ovule (ovum) in order to fertilise it. If so, the style must be a tube
through which the embryos pass into the ova. He supposes the pollen in
Fritillaria imperialis to be washed by wind and rain from the stigma
through the style into the ovary, without reflecting that the movement
must be an upward one in the hanging flower. If I could prove, he says,
that embryos are never found in unfertilised seeds, this would be a
demonstration; but I have never been so fortunate as to settle this
point. He does not mention that Camerarius had shown this ten years
before; he can only give as the main argument for his conjecture,
that in beans the embryo lies near the orifice of the seed-coat (the
micropyle), which shows that he was not aware that the two large
bodies in the seed of the bean (the cotyledons) belong to the embryo,
a fact which his countrymen Grew and Ray had already pointed out. It
appears therefore, that Morland supplied no answer to the question how
fertilisation takes place; his treatise contains nothing more than the
assertion that the embryo is already contained in the pollen-grain, and
that it reaches the seed through a hollow style and is there developed,
an entirely erroneous and not even an original idea, for it was the
offspring of the theory of evolution which was at that time in vogue.
GEOFFROY’S communications (‘Histoire de l’Académie royale des
sciences,’ Paris, 1714, p. 210) contain a few more facts. He mentions
neither Grew, Camerarius, nor even Morland, but connects his own
observations of 1711 on the structure and purpose of the more important
parts of the flower with those of Tournefort, who was a decided
opponent of the doctrine of sexuality in plants. The parts of the
flower are hastily described, figures are given of some forms of
pollen-grains, and the notion that the style is a tube receives some
apparent confirmation from the experiment of drawing water through
the style of a lily. The view that the pollen is not an excrement, as
Tournefort and Malpighi had maintained, is defended partly by arguments
which prove nothing, for instance, by the erroneous assertion that
the anthers are always so disposed that the extremity of the pistil
must necessarily receive their dust. The only proof offered for the
fact that seeds are infertile if deprived of the co-operation of the
pollen, is a very hasty account of some experiments with maize and
Mercurialis. The result of these experiments, as well as some other
remarks of Geoffroy, remind us of the text of Camerarius’ letter to an
extent which mere accident will hardly account for. If Geoffroy really
made these experiments, which is open to some doubt, yet they were
made fifteen years later than those of Camerarius, who did make the
same experiments among others and has described them better. Geoffroy
next endeavours to show how the pollen effects the fertilisation,
and offers two views on the subject; first, that the dust contains
much sulphur and is decomposed on the pistil, the more subtle parts
forcing their way into the ovary, where they set up a fermentation
and cause the formation of the embryo; the second view is, that the
pollen-grains already contain the embryos, which find their way into
the seeds and are there hatched. This is Morland’s notion, who however
is not mentioned. Geoffroy considers the latter to be the more probable
hypothesis, chiefly because no embryo is found in the ovule before
fertilisation, and also because the seed of the bean has an orifice
(the micropyle); it does not occur to him that these facts speak as
much for the first as for the second view.
Enough has been produced to show that Morland and Geoffroy contributed
nothing either to the establishment of the fact of sexuality in
plants, or to the decision of the question how the pollen effects
fertilisation in the ovule. Nevertheless I have mentioned these two
men immediately after those who really developed the sexual theory,
because they at least took their stand on experience, and endeavoured,
though unsuccessfully, to demonstrate conditions of organisation
which should explain the process of fertilisation. We come now to the
names of men—Leibnitz, Burckhard, Vaillant, Linnaeus—who are usually
supposed to have aided in establishing the sexual theory, but who
may be proved to have contributed nothing whatever to the scientific
demonstration of that doctrine. First as regards the philosopher
Leibnitz; he says in a letter of 1701, from which Jessen has quoted
the most important parts in his ‘Botanik der Gegenwart und Vorzeit,’
1864, p. 287: ‘Flowers are closely connected with the propagation
of plants, and to discover distinctions in the mode of propagation
(principiis generationis) is very useful,’ etc.; again, ‘A new and
extremely important point of comparison will be hereafter supplied
by the new investigations into the double sex in plants,’ alluding,
according to Jessen, to those of Camerarius and Burckhard. We shall
not expect to find that Leibnitz made experiments himself, and the
words quoted merely indicate that he wished to see the parts of the
flower employed for purposes of classification, because according to
the observations of others they are the instruments of propagation.
The remark applies in a still higher degree to Burckhard, who in his
letter to Leibnitz of 1702, quoted above on p. 83, further developed
the idea intimated by Leibnitz, for he too accepted the sexuality of
plants as an established and self-evident truth. The address with which
Sebastian Vaillant opened his lectures at the Royal Gardens in Paris in
1717 has often been noticed by the historians of botany. De Candolle,
who assigns to him an important share in developing the sexual theory,
says[103], that in this address he propounded the sexuality of plants
most expressly and as an acknowledged fact, and that he described very
graphically the way in which the anthers fertilise the pistil, into
which description little that was correct probably found its way, since
it required Koelreuter, Sprengel, and the botanists of quite modern
times to clear up this point. Vaillant therefore can only have the
credit of an eloquent description of what was then accepted. However,
De Candolle goes on to say what Vaillant’s discoveries were, and on
the following page we read that Linnaeus confirmed these discoveries
in the year 1736 in his ‘Fundamenta Botanica,’ and made skilful use of
them in the year 1735 in laying the foundations of his sexual system.
We have already in the second chapter of the first book explained the
confusion of ideas which lies at the bottom of these and many similar
statements, and in the same chapter have sufficiently indicated
our opinion respecting Linnaeus’ share in the establishment of the
doctrine of sexuality. It was the character of Linnaeus’ mind to
attach slight value to the experimental proof of a fact, even when,
like that of sexuality, it could only be proved by experiment; from
the point of view of his scholastic philosophy it was more important
with him to deduce the existence of this fact, in what seemed to
him the philosophic way, from the idea of the plant or from reason,
and in doing so to drag in a variety of analogies from the animal
kingdom; hence he acknowledged the services rendered by Camerarius,
but troubled himself little about his experiments which alone could
decide the question, while he undertakes himself to prove the existence
of sexes in plants on grounds of reason and the like in his peculiar
fashion. How he did this in the ‘Fundamenta’ and in the ‘Philosophia
Botanica’ has been already shown. Here we will briefly notice the
often-quoted dissertation, ‘Sponsalia Plantarum,’ in the first volume
of the ‘Amoenitates Academicae’ (1749). He first gives the views of
Millington, Grew, Camerarius and others; then on p. 63 he accepts the
statement of Gustav Wahlboom, that he, Linnaeus, had devoted infinite
labour to this question in 1735 in the ‘Fundamenta Botanica,’ and had
there (§§ 132-150) proved the sexes of plants with so great certainty
that no one would hesitate to found on it a detailed classification
of plants. Here then we have once more the construction of Linnaeus’
so-called sexual system introduced into the question of sexuality,
as if it had anything whatever to do with the establishing the
existence of sexes in plants, and as to the infinite labour (infinito
labore) which Linnaeus is supposed to have given to the question,
the paragraphs cited from the ‘Fundamenta’ contain the scholastic
subtleties quoted in Book I. chap. 2, but not one single really new
proof. The arguments in the dissertation we are considering are of
exactly the same kind, and it is itself only a lengthy paraphrase of
Linnaeus’ propositions in the ‘Fundamenta Botanica,’ illustrated by
experiments made by others, and with the addition of a few unimportant
observations, some of which are misinterpreted. We read, for instance,
p. 101, ‘Nectar is found in almost all flowers, and Pontedera thinks
that it is absorbed by the seeds that they may be the longer preserved;
it might seem that bees must be hurtful to flowers, since they
carry away the nectar and the pollen;’ but Linnaeus, differing from
Pontedera, remarks that ‘bees do more good than harm, because they
scatter the pollen on the pistil, though it is not yet ascertained what
is the importance of the nectar in the physiology of the flower.’ This
fact of the assistance rendered by insects, which was soon afterwards
better described by Miller, is not further examined in this place, for
Linnaeus goes on to speak of gourds, that they do not perfect their
fruit under glass, because the wind is prevented from effecting the
pollination.
One experiment only is mentioned, but not the person by whom it
was made. We read at p. 99 that in the year 1723 in the garden of
Stenbrohuld, the male flowers of a gourd in bloom were daily removed,
and that no fruit was formed. Soon after allusion is made to the
artifices used by gardeners to obtain hybrid varieties of tulips and
cabbage, but the matter is treated rather as agreeable trifling. In the
third volume of the Amoenitates of the year 1764, in which Koelreuter’s
first enquiries into hybridisation had been already published, we find
a dissertation on hybrids by Haartman, which was certainly written
as early as 1751. In this treatise the necessary existence of hybrid
forms is concluded from philosophic principles, as Linnaeus had deduced
sexuality from similar principles; no experiments are made, but
certain forms are arbitrarily assumed to be hybrids; a Veronica spuria
gathered in the garden at Upsala in 1750 is asserted to be the product
of Veronica maritima as the mother and of Veronica officinalis as the
father, but the only reason for assigning the paternity to the latter
plant is that it grew close by. We find also a Delphinium hybridum
stated on similar grounds to be the offspring of Delphinium elatum
fertilised by Aconitum napellus, and a Saponaria hybrida to have arisen
from the pollination of Saponaria officinalis by a Gentiana; and we are
told among other things that Actaea spicata alba is the offspring of
Actaea spicata nigra fertilised by Rhus toxicodendron. It is obvious
that in all this there was no observation of decisive facts, but simple
conclusions from arbitrary premises.
We conclude therefore that neither Linnaeus nor his disciples in the
interval that elapsed between the labours of Camerarius and Koelreuter
contributed a single new or valid proof to the establishment of the
fact, that there is a sexual difference in plants and that hybrids are
formed between different species; and if many later botanists talked
of the great services rendered by Linnaeus to the sexual theory, and
even regarded him as its most eminent founder, this arose partly from
the fact that they were unable to distinguish between his scholastic
deductions and scientific proof, and partly from that confusion of the
idea of sexuality with a classification of plants founded on the sexual
organs, to which we have before called attention. Such a confusion
of ideas gave rise to the claims which Renzi asserted on behalf of
Patrizi, but which Ernst Meyer, in his ‘Geschichte der Botanik,’ iv.
p. 420, has refuted on this very ground. Even in our own century De
Candolle has been blamed by Johann Jacob Roemer for not giving Linnaeus
the credit of being the actual founder of the sexual theory.
A few words in conclusion on those writers, who after Camerarius’
investigations still denied sexuality in plants, because they knew
nothing of what had been written on the subject or were incapable of
appreciating scientific proof. Tournefort must first be mentioned
on account of the great authority which he enjoyed with botanists
during the first half of the 18th century. In his ‘Institutiones rei
herbariae’ of the year 1700 (Book I. p. 69), with which we have already
made acquaintance, he treats of the physiological significance of the
parts of the flower, apparently in entire ignorance of Camerarius’
researches, and at any rate with a leaning to Malpighi’s views. He
makes the petals take up nourishment from the flower-stalks, which
they further digest and supply to the growing fruit, while the
unappropriated parts of the sap pass through the filaments into the
anthers and collect in the loculaments, to be afterwards discharged as
excreta. Tournefort even doubted the necessity of the pollination of
the female date-palm. The truth is that he was not well acquainted with
the facts, and was led astray by his preconceptions. The same was the
case with the Italian botanist Pontedera; in his ‘Anthologia’ of 1720
he reproduces Malpighi’s unlucky notion, and at the same time makes the
ovary absorb the nectar for the perfecting of the seed; he regarded
the male flower in dioecious plants as a useless appendage.
Valentin, to whom Camerarius addressed his famous letter ‘De sexu
plantarum’ in 1694, did his correspondent a disservice in publishing a
short abstract of it, which contained some gross misapprehensions of
the facts[104]. Alston in 1756 relying on these incorrect statements
disputed the conclusions of Camerarius, and doubted the sexual
importance of the stamens on very insufficient grounds. More reasonable
doubts were suggested by a German botanist, Möller, who observed that
female plants of spinach and hemp produced seeds even after the removal
of the male plants, and appealed to the apparently asexual propagation
of Cryptogams; these objections were answered by Kästner of Göttingen,
who pointed to the fact that dioecious plants, the willow for instance,
sometimes bear hermaphrodite flowers. The botanists in question would
never have entertained these doubts, if they had read and understood
the writings of Camerarius, or had been acquainted with the literature
of the subject.
4. THE THEORY OF EVOLUTION AND EPIGENESIS.
We have already observed the influence of the theory of evolution on
the doctrine of the fertilisation of plants in the case of Morland
and Geoffroy. We learn more about it in the work, already quoted,
of the philosopher Christian Wolff, ‘Vernünftige Gedanken von den
Wirkungen der Natur,’ Magdeburg, 1723; it will be well to give his
own words, for they will serve to show at the same time the amount of
knowledge possessed by a cultivated and well-read man in the country
of Camerarius and thirty years after the appearance of his treatise
on the sexuality of plants. In the second chapter of the fourth part,
which treats of the life, death, and generation of plants, Wolff
says: ‘Ordinarily plants are produced from seeds, for the seed not
only contains the plant in embryo but also its first food.’ He says
that propagation by means of buds is equally natural, for each bud
contains a branch in little. ‘We find inside in the flower a number of
stalks disposed in a circle, and something at the top of each, which
is full of dust and lets the dust fall on the upper part of that which
holds the seed; this organ is compared by some to the genitals of the
animal, and the dust to the male seed; they think also that the seed
is made fruitful by the dust, and that therefore the embryo must be
conveyed by the dust into the seed-case and there be formed into seeds.
I have proposed to examine into the matter, but I have always let it
escape me.’ ... ‘Since all that has been hitherto adduced is found
also in flowers which spring from bulbs, and it is also certain that
the leaves of bulbs have consequently embryos in them ... it is easy
to see that the embryos must come from the leaves of the bulbs. And
since they could as easily be conveyed from there into the seed-grains
with the sap, as into the dust which is produced in the upper part of
the flower, I am inclined to think that this is the true account of
the matter and that it will be confirmed by experience. But now comes
the main question, whence come the embryos into the sap; since they
have not an external figure only but an internal structure also, it is
not plain how they can be formed either by the mere inner movement of
the sap, or by separation of certain parts.... And this is certainly
more credible, that the embryos already exist in little in the sap and
the plant, before they are brought by some change into the condition
in which they are met with in the seed and in buds. But there is the
further question where they were previously. They must either lie one
in another in a minute form, as Malebranche especially maintains, or
they are brought from the air and the earth with the nourishing sap
into the plant, an idea which Honoratus Fabri advanced and Perrault and
Sturm developed after him. According to the first opinion the first
seed-grain must have contained everything in itself, which has grown
from it to this hour.’ But this demand goes beyond even Wolff’s powers
of belief; for, says he, it is too great a tax on the imagination to
conceive of this inclosing of germs one in another like box within box.
It is well known that such notions as these were very prevalent in
the 18th century, and that the spermatozoids of animals were thought
to lend considerable support to them; even Albert Haller after 1760
was an adherent of the theory of evolution. However confused Wolff’s
general train of thought may be, we should notice his perception of
the fact, that the theory of evolution does away with the sexual
significance of the anthers. We shall see by-and-bye, that Koelreuter
was able to form a very different idea of sexual propagation. His great
importance in the history of the sexual theory will be best learnt
from a consideration of the speculative views of his predecessors
and contemporaries. It will not be amiss therefore to disregard
chronology for a while, and to notice here the views of the Baron von
Gleichen-Russworm, and the feeble arguments of Kaspar Friedrich Wolff
against the theory of evolution. The first-named writer in his work
‘Das Neueste aus dem Reich der Pflanzen,’ 1764, relying principally on
microscopic observation of the contents of pollen-grains, supported the
view that the granules in them answer to spermatozoids in animals, and
that they find their way into the ovule and are there developed into
embryos. Yet Gleichen was at the same time a zealous supporter of the
sexual theory, and endeavoured to meet well-known objections to it by
pointing to the occurrence of female flowers on male plants of spinach;
he also made some experiments on maize and hemp in the interests of
the theory. He did not perceive that hybrids supply convincing proof
against the theory of evolution, but he rightly appealed to them as
affording strong arguments in favour of sexuality. His real knowledge
of hybrids is partly drawn from the statements of Linnaeus, with
which we have already made acquaintance; he even describes a hybrid
between a goat and a cow, and other similar ones, and he is angry
with Koelreuter for fixing such narrow limits to the occurrence of
hybrids; thus the first person who produced hybrids systematically in
the vegetable kingdom must submit to be scolded for refusing to accept
the imaginary hybrids of his contemporaries. Gleichen’s book and the
selection from his microscopic discoveries, which appeared in 1777,
abound in good detached observations; he was the first who saw and
figured the pollen-tubes of Asclepias, without of course suspecting
their real nature and importance.
Kaspar Friedrich Wolff is usually said to be the writer who refuted the
theory of evolution. It is certainly true that in his dissertation for
his doctor’s degree in 1759, the well-known ‘Theoria generationis,’
he appeared as the decided opponent of evolution; but the weight of
his arguments was not great, and the hybridisation in plants which was
discovered at about the same time by Koelreuter supplied much more
convincing proof against every form of evolution. Wolff conceived of
the act of fertilisation as simply another form of nutrition. Relying
on the observation, which is only partly true, that starved plants are
the first to bloom, he regarded the formation of flowers generally as
the expression of feeble nutrition (vegetatio languescens). On the
other hand the formation of fruit in the flower was due to the fact,
that the pistil found more perfect nourishment in the pollen. In this
Wolff was going back to an idea which had received some support from
Aristotle, and is the most barren that can be imagined, for it appears
to be utterly incapable of giving any explanation of the phenomena
connected with sexuality, and especially of accounting for the results
of hybridisation. Wolff may have rejected the theory of evolution on
such grounds as these, but he failed to perceive what it is which is
essential and peculiar in the sexual act.
5. FURTHER DEVELOPMENT OF THE SEXUAL THEORY BY JOSEPH GOTTLIEB
KOELREUTER, AND KONRAD SPRENGEL. 1761-1793.
Camerarius had shown by experiment that the co-operation of the pollen
is indispensable to the production in plants of seeds containing an
embryo, and later observers had confirmed the fact of sexuality by
further and varied experiments. The next step in the strict scientific
investigation of the matter was to determine by the same method of
experiment the share of each principle, the male and the female, in the
formation of the new plant which resulted from the sexual act. When
pollen and ovule belong to the same individual plant, the offspring
assumes the same form and the question remains undecided. It was
necessary to bring together the pollen and ovule of different plants;
this must show whether some characters are derived to the offspring
from the pollen, and others from the ovule, and what the characters are
which are thus distinguished, supposing of course that such a union of
different forms is possible. The answer to these questions could only
be obtained by experiment, that is by artificial hybridisation; for
until hybrid forms had actually been produced in this manner, it must
be quite unsafe to assume that certain wild plants owed their origin to
cross-fertilisation.
Camerarius had already raised the question in his letter, whether
cross-fertilisation in plants is possible, and had added another,
whether the progeny varies from its parents (an et quam mutatus
inde prodeat foetus). Bradley is our authority for the statement
that a gardener in London had obtained a hybrid between Dianthus
caryophyllus and Dianthus barbatus by artificial means as early as
1719; but KOELREUTER[105] was the first who investigated the question
scientifically and thoroughly. He was the first moreover who recognised
all its importance, and he applied himself to it with such admirable
and unexampled perseverance and judgment, that the results which he
obtained are still the best and most instructive, though a thousand
similar experiments have been made since his time. He also made the
first careful study of the different arrangements inside the flower in
their connection with the sexual relation, discovered the purpose of
the nectar and the co-operation of insects in pollination, and proposed
that view of the sexual act, which with some considerable modification
we must still in the main consider to be the true one, namely, that it
is a mingling together of two different substances.
If we compare Koelreuter’s writings, which are full of matter in a
small compass, with all that was produced after Camerarius, we are
astonished not only at the abundance of new thoughts, but still more
at their wonderful clearness and perspicuity, and the sureness of the
foundation laid for them in observation and experiment. In reading the
observations of Linnaeus, Gleichen, and Wolff on the sexual theory
we step into a world of thought which has long been strange and is
scarcely intelligible to us, and which in the present day possesses
only a historical interest. Koelreuter’s works on the contrary seem
to belong to our own time; they contain the best knowledge which we
possess on the question of sexuality, and have not become antiquated
after the lapse of more than a hundred years. We see by his example
that one really gifted thinker with the requisite perseverance will
effect more in a few years, than many less gifted observers in the
course of many years. But the same thing happened now, which happens
often in similar cases and which happened to Camerarius; a much longer
time elapsed before others learnt to understand the meaning and
importance of Koelreuter’s labours, than he had found necessary for
making his discoveries.
Koelreuter’s most important and best-known work appeared in four
portions in 1761, 1763, 1764 and 1766 under the title, ‘Vorläufige
Nachricht von einigen das Geschlecht der Pflanzen betreffenden
Versuchen und Beobachtungen’; we shall endeavour to give a brief
summary of the more important results.
At different places in this work occur remarks and experiments on
arrangements for pollination, which up to that time had been seldom and
only hastily observed. As the pollen-tube had not yet been discovered,
and Koelreuter himself set out with the view, that a fluid finds its
way from the pollen-grains as they lie on the stigma to the ovules, it
was important first of all to determine the quantity of pollen which is
required for the complete fertilisation of an ovary; with this object
in view Koelreuter counted the pollen-grains formed in a particular
flower and compared them with the number required to be applied to
the stigma in order to effect complete fertilisation, and he found
that the latter number was much the smaller. For instance, he counted
four thousand eight hundred and sixty-three pollen-grains in a flower
of Hibiscus venetianus, while from fifty to sixty were sufficient to
produce more than thirty fertile seeds in the ovary; in Mirabilis
jalapa and Mirabilis longiflora he counted about three hundred grains
of pollen in the anthers, while from two to three or even one sufficed
for fertilisation in the one-ovuled ovary. He also tried, whether in
flowers with divided and even deeply-cleft styles fertilisation could
be effected in all compartments of the ovary through one of them only,
and he found that it could.
Koelreuter directed special attention to the arrangements, by which in
the natural course of things the pollen finds its way from the anthers
to the stigmas. He ascribed perhaps too much to the agency of the wind
and the oscillations of the flower from any cause; at the same time he
was the first who recognised the great importance of the insect-world
to pollination in flowers. ‘In flowers,’ he says, ‘in which pollination
is not produced by immediate contact in the ordinary way, insects are
as a rule the agents employed to effect it,’ (later observation has
shown that they are generally so employed even in cases where actual
contact is possible), ‘and consequently to bring about fertilisation
also; and it is probable that they render this important service if
not to the majority of plants at least to a very large part of them,
for all the flowers of which we are speaking have something in them
which is agreeable to insects, and it is not easy to find one such
flower, which has not a number of these creatures busy about it.’ He
noticed the dichogamous construction in Epilobium, but did not further
pursue his observation. He next examined the substance in flowers which
is agreeable to insects; he collected the nectar of many flowers in
considerable quantities, and found that it gave after evaporation of
the water a kind of sweet-tasted honey; this honey was unpalatable only
in Fritillaria imperialis, which is avoided by the humble-bees. He had
no doubt therefore, that bees procure their honey from the nectar of
flowers. How greatly he was interested in the relations between the
existence of certain plants and that of certain animals, relations
which were neglected till Darwin once more brought them into notice in
quite recent times, is shown by his investigation into the propagation
of the mistletoe (1763); he calls special attention to the fact, that
not only must the pollination of this plant be effected by insects, but
that the dissemination of its seeds is also exclusively the work of
birds, and that the existence of the plant therefore is dependent on
two different classes of living creatures.
Again we find observations on the movements of anthers and stigmas,
especially those caused by sensitiveness. Count Giambattista dal Covolo
had made the first observations in 1764 on the sensitiveness of the
anthers of thistle-like plants, and had endeavoured to explain their
mechanism. Koelreuter did not trouble himself about this point, so much
as about the connection between the irritability of the anthers and the
pollination of the stigmas. He took into consideration the sensitive
stamens of Opuntia, Berberis and Cistus, which Du Hamel had already
noticed, and discovered for himself the irritability of the lobes of
the stigma in Martynia proboscidea and Bignonia radicans. He noticed
that the lobes when touched close, but soon open again; but if pollen
is placed upon them, they remain closed till fertilisation is secured.
How perfectly insects effect the pollination of flowers he showed by a
comparative trial, in which he applied pollen himself to three hundred
and ten flowers with a brush, while he left the same number to the
operation of insects; the number of seeds formed in the latter case was
very little less than in the former, though the insects had to contend
with unfavourable weather.
He endeavoured also to ascertain the time required for the quantity
of ‘seminal matter’ sufficient for fertilisation to reach the ovary
after pollination; he also showed that pollination is followed by
fertilisation without the aid of light; later botanists incorrectly
maintained the contrary.
Koelreuter was less successful in his observations on the structure of
pollen-grains; here the microscope was indispensable and microscopes
were still very imperfect. Nevertheless he discovered that the
outer covering of the pollen-grain consists of two distinct coats,
and noticed the spines and sculpturings on the outer coat and its
elasticity; he observed the lids of the orifices in the exine of
Passiflora coerulea, and went so far as to see the inner coat in
moistened pollen-grains protrude in the form of conical projections,
which then however burst and allowed the contents to escape. But he
explained the pollen-tube, which he had thus seen, incorrectly by
supposing that these projections were intended to prevent the bursting
of moistened grains. It was not till sixty or seventy years later that
the matter was fully understood. Koelreuter supposed the contents of
the pollen-grain to be a ‘cellular tissue,’ and the true fertilising
substance to be the oil which adheres to the outside of the grains,
but is formed inside them and finds its way out through fine passages
in the coat. The bursting of the pollen-grains, which his opponent
Gleichen thought must take place to allow of the escape of his supposed
spermatozoids, seemed to him an unnatural proceeding.
Starting from the hypothesis, that the oil which clings to the
pollen-grains is the fertilising substance, Koelreuter propounds his
view of the process of fertilisation in accordance with the chemical
notions of the day; he first rejects the idea that the pollen-grains
themselves can reach the ovary, and then says: ‘Both the male seed and
the female moisture on the stigmas are of an oily nature, and therefore
when they come together enter into a most intimate union with one
another, and form a substance which, if fertilisation is to ensue,
must be absorbed by the stigma and conveyed through the style to the
so-called ovules or unfertilised germs.’ Koelreuter therefore made the
fertilisation really take place on the stigma, the mingled male and
female substance making its way into the ovary and there producing the
embryos in the seed. He had expressed this view before in 1761; he
repeated it in 1763 with the idea that the male and female moistures
unite together, as an acid and an alkali unite to form a neutral
salt; a new living organism is the result at once or later of this
union. In an investigation which he made in 1775 into the conditions
of pollination in Asclepiadeae he reverted to this idea, and insisted
that the act of fertilisation in the whole vegetable and animal kingdom
is a mingling of two fluids. But at a later period he seems to have
no longer considered the moisture of the stigma to be the female
principle, for experiment had taught him, that if a stigma exchanges
the moisture from another stigma for its own, and is then dusted with
its own pollen, no hybrid form is produced[106]. In any case Koelreuter
had a more correct idea of the nature of sexual fertilisation than
any of his predecessors, and it was one specially adapted to enable
his contemporaries to understand the results of experiments in
hybridisation, while the hybrids themselves supplied most convincing
arguments against the prevailing theory of evolution.
We have arrived at Koelreuter’s most important performance, the
production of hybrids. Here was a case for skilful experimentation,
not for microscopic observation, and here he obtained results in which
nothing afterwards required to be changed, but which when combined
with later observations have been used for the discovery of general
laws in hybridisation. The first hybrid which he obtained by placing
the pollen of Nicotiana paniculata on the stigmas of N. rustica,
produced pollen that was impotent; but he soon after obtained hybrids
from the two species which produced seeds capable of germination, and
in 1763 he described a considerable number of hybrids in the genera
Nicotiana, Kedmia, Dianthus, Matthiola, Hyoscyamus, and others. In the
last portion of his great work (1766) he speaks of eighteen attempts
to obtain hybrids with five native species of Verbascum, and submits
Linnaeus’ views on hybrid plants, which we have already described,
to a withering criticism. He shows at the same time from experiment,
that if the stigma of a plant receives its own pollen and pollen from
another plant at the same time, the former only is effectual, and that
this is one reason why hybrids which can be raised artificially are
not found in nature. We must not attempt to give a detailed account of
his famous hybrids of the third, fourth, and fifth degrees, nor of his
experiments on other points, such as the reverting of hybrids to the
original form by the repeated employment of its pollen; the value of
these experiments for theoretical purposes was afterwards fully brought
out by Nägeli.
It is impossible to rate too highly the general speculative value of
Koelreuter’s artificial hybridisation. The mingling of the characters
of the two parents was the best refutation of the theory of evolution,
and supplied at the same time profound views of the true nature of the
sexual union. It was shown by his numerous experiments that only nearly
allied plants and not always these are capable of sexual union, which
at once disposed of Linnaeus’ vague ideas in the judgment of every
capable person, though it was long before science candidly accepted
Koelreuter’s results. The plant-collectors of the Linnaean school as
well as the true systematists at the end of the 18th century had little
understanding for such labours as Koelreuter’s, and incorrect ideas on
hybrids and their power of maintaining themselves prevailed in spite
of them in botanical literature. Hybrids were necessarily inconvenient
to the believers in the constancy of species; they, disturbed the
compactness of their system and would not fit in with the notion that
every species represented an ‘idea.’
Koelreuter’s doctrines however did not always fall on unfruitful soil;
two botanists at least were found in Germany who adopted them, Joseph
Gärtner the author of the famous Carpology and father of Carl Friedrich
Gärtner who at a later time spent twenty-five years in experimenting
on fertilisation and hybridisation, and Konrad Sprengel who took up
Koelreuter’s discovery of the services rendered by insects and arrived
at some new and very remarkable results.
JOSEPH GÄRTNER made no fresh observations on sexuality himself, but
in the Introduction to his ‘De fructibus et seminibus plantarum’
(1788) he made use of Koelreuter’s results for the purpose of
distinguishing more clearly between different kinds of propagation,
and strengthening his own attack on the theory of evolution. The
germ-grains or spores of cryptogamic plants were at that time often
regarded on insufficient grounds as true seeds; Gärtner distinguished
them from seeds, because they are formed without fertilisation and
yet are capable of germination, whereas ovules become seeds capable
of germination only under the influence of the pollen. He distinctly
denied the sexuality of the Cryptogams; it was not till fifty years
later that strict scientific proof was substituted in this department
of botany for vague conjecture, and it was more in the interest of
true science in Gärtner’s day to deny sexuality in the Cryptogams
altogether, than to take the stomata in Ferns with Gleichen, or the
indusium with Koelreuter, or the volva in Mushrooms for the male organs
of fertilisation. Gärtner rightly appealed to Koelreuter’s hybrids
against the defenders of the theory of evolution; and to those who saw
in the seed only another form of vegetative bud, he said, that the bud
can produce a new plant without fertilisation but that the seed cannot.
We have already given an account in the chapters on Systematic Botany
of the services rendered by Gärtner to the knowledge of the seed in
its immature and in its mature condition; as regards the process of
fertilisation he adopted in the main Koelreuter’s view, that it is
the result of the union of a male and female fluid, from which the
germ-corpuscle in the ovule is developed by a kind of crystallisation.
Konrad Sprengel also fully committed himself to this view, and was
thereby prevented from understanding the process of fertilisation in
Asclepiadeae.
In KONRAD SPRENGEL[107] we encounter once more an observer of genius,
like Camerarius and Koelreuter, who however surpassed them both in
boldness of conception and was therefore even less understood by his
contemporaries and successors, than they had been by theirs. The
conclusions, to which his investigations led him, were so surprising,
they suited so little with the dry systematism of the Linnaean school
and with later views on the nature of plants, that they had become
quite forgotten when Darwin brought them again before the world and
showed their important bearing on the theory of descent. As Camerarius
first proved that plants possess sexuality, and Koelreuter showed that
plants of different species can unite sexually and produce fruitful
hybrids, so now Sprengel showed that a certain form of hybridisation
is common in the vegetable kingdom, namely the crossing of different
flowers or different individuals of the same species. In his work,
‘Das neu entdeckte Geheimniss der Natur in Bau und Befruchtung der
Blumen,’ Berlin, 1793, he says at page 43: ‘Since very many flowers
are dioecious, and probably at least as many hermaphrodite flowers
are dichogamous, nature appears not to have intended that any flower
should be fertilised by its own pollen.’ This was however only one of
his surprising conclusions; still more important perhaps was the view,
that the construction and all the peculiar characters of a flower can
only be understood from their relation to the insects that visit them
and effect their pollination; here was the first attempt to explain the
origin of organic forms from definite relations to their environment.
Since Darwin breathed new life into these ideas by the theory of
selection, Sprengel has been recognised as one of its chief supports.
It is highly interesting to read, how this speculative mind by the
study of structural relations in flowers, which were apparently trivial
and open to the eyes of all men, first arrived at ideas which in the
course of a few years were to lead to such far-reaching results. He
says: ‘In the summer of 1787 I was attentively examining the flowers
of Geranium sylvaticum, and observed that the lower part of the petals
was provided with slender rough hairs on the inside and on both edges.
Convinced that the wise framer of nature has not produced a single hair
without a definite purpose, I considered what end these hairs might be
intended to serve. And it soon occurred to me, that on the supposition
that the five drops of juice which are secreted by the same number of
glands are intended for the food of certain insects, it is not unlikely
that there is some provision for protecting this juice from being
spoiled by rain, and that the hairs might have been placed where they
are for this purpose. Since the flower is upright, and tolerably large,
drops of rain must fall into it when it rains. But no drop of rain can
reach one of the drops of juice and mix with it, because it is stopped
by the hairs, which are over the juice-drops, just as a drop of sweat
falling down a man’s brow is stopped by the eye-brow and eye-lash, and
hindered from running into the eye. An insect is not hindered by these
hairs from getting at the drops of juice. I examined other flowers
and found that several of them had something in their structure,
which seemed exactly to serve this end. The longer I continued this
investigation, the more I saw that flowers which contain this kind of
juice are so contrived, that insects can easily reach it, but that
the rain cannot spoil it; but I gathered from this that it is for the
sake of the insects that these flowers secrete the juice, and that it
is secured against rain that they may be able to enjoy it pure and
unspoilt.’ Next year, following out an idea suggested by the flowers of
Myosotis palustris, he found that the position of spots of different
colours on the corolla have some connection with the place where the
juice is secreted, and with the same ready reasoning as before he came
to the further conclusion: ‘If the corolla has a particular colour in
particular spots on account of the insects, it is for the sake of the
insects that it is so coloured; and if the particular colour of a part
of the corolla serves to show an insect which has lighted on the flower
the direct path to the juice, the general colour of the corolla has
been given to it, in order that insects flying about in search of their
food may see the flowers that are provided with such a corolla from a
long distance, and know them for receptacles of juice.’
He afterwards discovered that the stigmas of a species of Iris were
absolutely unable to be fertilised in any other way than by insects,
and further observation convinced him more and more, ‘that many,
perhaps all flowers, which have this juice, are fertilised by the
insects which feed on it, and that consequently this feeding of insects
is in respect of themselves an end, but in respect of the flowers only
a means, but at the same time the sole means to a definite end, namely,
their fertilisation; and that the whole structure of such flowers can
be explained, if in examining them we keep in sight the following
points, first, that flowers were intended to be fertilised by the
agency of one or another kind of insects, or by several; secondly, that
insects in seeking the juice of flowers, and for this purpose either
alighting upon them in an indefinite manner, or in a definite manner
either creeping into them or moving round upon them, were intended to
sweep off the dust from the anthers with their usually hairy bodies or
with some part of them, and convey it to the stigma, which is provided
either with short and delicate hairs, or with a viscid moisture, that
it may retain the pollen.’
In the summer of 1790 he detected dichogamy, which he first observed
in Epilobium angustifolium. He found, ‘that these hermaphrodite
flowers are fertilised by the humble-bee and by other bees, and that
the individual flower is not fertilised by its own pollen, but the
older flowers by the pollen which the insects convey to them from
the younger.’ Having observed the same thing in Nigella arvensis,
he afterwards found exactly the opposite arrangement in a species of
Euphorbia, in which the stigmas can receive the pollen by the aid of
insects only from older flowers.
He goes on to say that he grounds his theory of flowers on these
his six chief discoveries made in the course of five years; he then
gives his theory at length, first of all explaining the nature of
juice-secreting glands (nectaries), and organs for receiving or
covering the nectar, and the contrivances for enabling insects to find
their way readily to the juice. He calls attention to Koelreuter’s
excellent observations on the fertilisation of nectar-bearing flowers
by insects, and notices that no one has hitherto shown that the whole
structure of such flowers has this object in view, and can be fully
explained by it. He finds the chief proof of this important proposition
in dichogamy.
‘After the flower,’ he says, ‘has opened in dichogamous plants, the
filaments have or assume either all at once or one after another a
definite position, in which the anthers open and offer their pollen for
fertilisation. But at this time the stigma is at some distance from
the anthers and is still small and closed. Hence the pollen cannot be
conveyed to the stigma either by mechanical means or by insects, for
there is as yet properly no stigma. This condition of things continues
a certain time. When that time is elapsed, the anthers have no longer
any pollen, and changes take place in the filaments the result of which
is that the anthers no longer occupy their former position. Meanwhile
the pistil has so changed that the stigma is now exactly in the place
where the anthers were before, and as it now opens, or expands the
parts of which it is composed, it often fills about the same space as
the anthers filled before. Now the spot, which was at first occupied by
the ripe anthers and is now occupied by the ripe stigma, is so chosen
in each flower, that the insect for which the flower is intended cannot
reach the juice without touching with a portion of its body the anthers
in a young flower, and the stigma in an older; it thus brushes the
pollen from the anthers and conveys it to the stigma, and so the pollen
of the younger flower fertilises the older.’ It has been already said,
that Sprengel was also acquainted with the opposite form of dichogamy;
and the result of his explanation of both kinds is the conclusion,
that some flowers can only be fertilised by the aid of insects, and
he adds that some cases are to be found, in which the arrangements
in the flower are of such a nature as to involve the injury and even
the death of the insect that gives its services. Further on he tells
us, that all flowers, ‘which are without a proper corolla and have no
calyx of any importance in its place, are destitute of nectar, and
are not fertilised by insects but by some mechanical means, as by the
wind, which either blows the pollen from the anthers on to the stigmas,
or shakes the plant or the flower and makes the pollen fall from the
anthers on to the stigmas.’ He observes, that such flowers always
produce a light pollen and in large quantities, whereas the pollen
of nectar-bearing plants is heavy. Then he shows how his principles
explain all the physiological characters of flowers, position, size,
colour, smell, form, time of flowering and the like.
Sprengel set out with the idea, that the nectar and certain
arrangements in flowers are expressly intended for the service of
insects; but his investigations led him ultimately to the conclusion,
that insects themselves serve not only to effect the fertilisation of
plants generally, but also in all ordinary cases to bring about the
crossing of different flowers of the same plant or of different plants
of the same species. There remained a question, which from Sprengel’s
strictly teleological point of view especially required an answer,
what was the object of this crossing of flowers or individual plants?
Sprengel was content, as we have seen, with simply stating the fact,
and with saying, that nature apparently did not choose that any flower
should be fertilised by its own pollen. Who would make it a reproach
to the discoverer of such remarkable and widely-prevalent phenomena
in nature, that he did not answer this question and give the final
touches to the body of doctrine which he created, and which could
only be developed by many experiments and the labour of long years?
Neither his worldly circumstances nor the reception accorded to his
work with all its genius were such as to encourage him to undertake
the solution of this last and most difficult problem, even if he had
been inclined to do so. Botanists were just at that time and for some
time after preoccupied with views, which allowed such biological and
physiological facts in vegetable life to lie neglected, nor were
Sprengel’s results at all favourable to the doctrine of the constancy
of species; from that point of view the wonderful relations between the
organisation of flowers and that of insects must have seemed absurd and
repulsive. In such cases it is the character of less-gifted natures,
rather to deny the facts or to disregard them, than to sacrifice their
own favourite views to them; this is one explanation of the neglect
which Sprengel’s book met with everywhere. Then notwithstanding the
labours of a Camerarius and a Koelreuter there were many even at the
beginning of our own century who still doubted the sexuality of plants.
Even after Knight and William Herbert, with a right understanding of
the question left open by Sprengel, had obtained experimental results
which helped to answer it, the new doctrine did not make its way. The
earlier simple-minded but consistent teleology had been succeeded
by a rejection of all teleological explanations in the treatment of
physiological questions, and this spirit conduced to make Sprengel’s
results seem inconvenient in proportion as they appeared to admit only
of such explanation. With regard to phenomena of this kind botanists
before 1860 were in a position, in which they were without the means
of forming a judgment; they shrank from the teleological point of
view and from believing with Konrad Sprengel, that every, even the
least-obvious, arrangement in an organism was the direct work of a
Creator; but they had nothing better to put in the place of this idea,
and hence Sprengel’s discoveries not being understood were neglected
till Darwin recognised all their importance, and by opposing the theory
of descent and selection to the principle of design was in a position
not only to show that they had a scientific meaning, but also to employ
them as powerful supports of the theory of selection. Then, too, it
became possible rightly to appreciate the contributions of Knight,
Herbert, and K. F. Gärtner to the further completion of Sprengel’s
doctrine, for their discoveries also were for a while neglected. A
few years after the appearance of Sprengel’s book, Andrew Knight[108]
relying on the results of experiments made for the purpose of comparing
self-fertilisation and crossing in the genus Pisum, laid down the
principle, that no plant fertilises itself through an unlimited number
of generations; in 1837 Herbert summed up the results of his numerous
experiments in fertilisation in the statement, that he was inclined
to believe that he attained a better result, when he fertilised the
flowers from which he wished to obtain seeds with pollen from another
individual of the same variety or at least from another flower, than
when he fertilised it with its own pollen; K. F. Gärtner came to the
same conclusion after experiments in fertilising Passiflora, Lobelia,
and species of Fuchsia in 1844. In these observations lay the first
germ of the answer to the question left undecided by Sprengel, why
most flowers are so constructed that fertilisation can only be fully
effected by the crossing of different flowers or of different plants of
the same species; the artificial crossings of this kind, which Knight,
Herbert, and Gärtner compared with the self-fertilisation of single
flowers, showed that crossing procures a more complete and vigorous
impregnation than self-fertilisation. It was but a short step from this
fact to the idea, that the arrangements in the flower discovered by
Sprengel together with the aid of insects serve to secure the strongest
and most numerous progeny possible. Darwin was the first who fixed his
eye distinctly on this idea also, in order to employ it in his theory
of selection, and sought support for it in a number of experiments made
after 1857.
6. NEW OPPONENTS OF SEXUALITY AND THEIR REFUTATION BY EXPERIMENTS.
1785-1849.
Those who have read the writings of Camerarius and Koelreuter
carefully find it difficult to believe, that after their time doubts
were still entertained not about the manner in which the processes
of fertilisation are accomplished but about the actual existence
of difference of sex in plants. And yet such doubts were expressed
repeatedly during the succeeding sixty years in various quarters and
with the greatest confidence, and this not in consequence of increased
accuracy in experimental research or of contradictions that could
be proved in the views of the founders of the sexual theory, but
because a number of observers made unskilful experiments and obtained
contradictory results, or made inaccurate observations on the plants on
which they experimented, and generally had not the requisite experience
and circumspection. Such were Spallanzani and later Bernhardi, Giron
de Bouzareingue and Ramisch. Schelver, his pupil Henschel, and their
adherents erred still more grossly and from a different cause; they
thought themselves justified by preconceived opinions and conclusions
from the nature-philosophy in denying facts established by experiment.
The destructive effects of the nature-philosophy on the powers of
the understanding at the beginning of the 19th century was shown
in the case of many botanists, who were no longer able to estimate
the result of simple experiments, and to trace back the phenomena
of nature to the scheme of causes and effects. As Linnaeus once
imagined that he could prove sexuality in plants on philosophical
grounds and paid comparatively slight attention to their behaviour
as shown by experiment, so we have in Schelver a nature-philosopher
who conversely endeavoured to prove the impossibility of sexuality in
plants on philosophical grounds. As Linnaeus deduced sexuality from
the nature or idea of the plant, Schelver denied it from the same
nature or idea; as a matter of logic one was as much in the right as
the other, but the question could not be decided in this way but only
by experiment. However our nature-philosophers thought it advisable
to get some empirical support for their theories, and they found it
in SPALLANZANI[109]. He published his enquiries into fertilisation
in animals and plants under the title ‘Expériences pour servir à
l’histoire de la génération des animaux et des plantes,’ Geneva,
1786; his account of those relating to plants, with which only we
are concerned, betrays a very defective acquaintance with botanical
literature, for he reckons Cesalpino among those who had admitted
sexuality in plants. His experiments themselves testify to very slight
knowledge of the biological considerations by which the cultivation of
plants for experiment must be guided, and generally little botanical
acumen, as is often the case with amateurs who without sufficient
preparation suddenly turn their attention to questions of vegetable
physiology; his treatment of his topics is superficial, his criticism
of others is dogmatic and bitter without exciting confidence in the
author’s own skill and judgment. His experiments were often undertaken
in haste and with little consideration, and some of them were made on
plants the least suitable for such investigations, as for instance
on Genista, beans, peas, radishes, Basilicum, Delphinium. It is no
matter of surprise therefore that in the case of some plants, as
Mercurialis and Basilicum, he arrived at the conclusion that the pollen
is necessary to the production of fertile seeds, while he makes others,
as the gourd, the water-melon, hemp, and spinach produce such seeds
without fertilisation. His countryman Volta, a greater man, repeated
his experiments and impugned the results which he had obtained from
them.
Such was the character of the experiments to which Franz Joseph
Schelver, Professor of Medicine in Heidelberg appealed in his ‘Kritik
der Lehre von dem Geschlecht der Pflanzen,’ 1812. It is unnecessary
to give a detailed account of this strange production of a mind
misled, even though a considerable number of German botanists as late
as 1820 took its nonsense for profound wisdom. Schelver dismissed
the experiments of Camerarius in four lines, and while he treated
Koelreuter with contempt, he praised Spallanzani as the weightiest
author on the subject. The statements of Camerarius and Koelreuter
are true, he said, but they do not prove the fertilisation. He is
more concerned to decide the question from the nature of vegetative
life, and from this nature constructed by himself he concludes that
the organs of plants are of no use at all, that they cannot even tend
to be of use to one another and to propagate life together, because
this one end of their action can be a living one only where all the
parts are present at the same time, which of course disposes of the
fertilising effect of the pollen; accordingly he does not refer the
effect of a male plant on a neighbouring female plant, which results
in the formation of seeds, to pollination by the former, but it is the
proximity itself which has the fertilising effect. But these are very
insufficient specimens of his reasoning.
The writings of his pupil Henschel[110] are even worse than those
of his master, and the worst of these is his large work ‘Von der
Sexualität der Pflanzen’ of 1820. He thought himself obliged to prove
the doctrines of the nature-philosophy by countless experiments; but
the way in which these are devised, managed and described displays
the extreme of dulness and incapacity to form a sound judgment. The
doubt which must occasionally rise in the mind of the reader as to the
accuracy of his reports, and the remarks which have been made on this
point by Treviranus and Gärtner, are not needed to disgust him with the
scientific efforts of this writer.
It would be superfluous to give an account of the contents of
Henschel’s book, which is interesting from the pathological rather
than from the historical point of view. To what an extent better men
than Henschel even later than 1820 lost under the influence of the
nature-philosophy their capacity for judging such questions as we are
discussing, how even investigators of merit thought it worth while
to treat the productions of Schelver and Henschel with a certain
respect, is shown among other works, by a collection of letters, which
were published by Nees von Esenbeck as a second supplement to the
‘Regensberg Flora’ of 1821, and by the later remarks of Goethe on the
metamorphosis of plants, to be found in Cotta’s edition of his works
in forty volumes (vol. xxxvi. p. 134) under the title ‘Verstäubung,
Verdunstung, Vertropfung.’ But there were some who set themselves
distinctly against these pernicious ideas, such as Paula Schrank
(‘Flora,’ 1822, p. 49) and C. L. Treviranus, who published in 1822
a full refutation of Henschel in his ‘Lehre von dem Geschlecht der
Pflanzen in Bezug auf die neuesten Angriffe erwogen.’ A few stray
supporters of the dying nature-philosophy were still to be found at a
later time; among them Wilbrand, Professor in Giessen, who (‘Flora,’
1830, p. 585) adopted the very subtle distinction that there is in
plants something analogous to sexuality in animals, but no real
sexuality. We see in the whole literature of the nature-philosophy
an incapability of judging of experiments simply with the sound human
understanding; an imaginary something was constantly introduced into
the results of experiments which had not the remotest connection with
their conditions and results.
The doubts expressed by Bernhardi in 1811, by Girou in 1828-30, and by
Ramisch in 1837 were of a different kind: these men made experiments
and judged of them in a scientific manner; but they were insufficiently
acquainted with what had been done before them, and their experiments
were not devised with the requisite knowledge of the conditions of the
problem, or carried out with sufficient precautions. Camerarius and
Ray had noticed in the previous century the occasional occurrence of
male flowers on female plants of spinach, hemp and mercury; and yet
the observers above mentioned chose these plants for their experiments
without being on their guard against the possible appearance of these
exceptional circumstances, or of other means of pollination.
We see then that doubts were entertained till as late as after 1830
with regard either to sexuality in plants altogether, or to its general
prevalence in Phanerogams; the Cryptogams were not mentioned, for they
were assumed to be devoid of sex in spite of many valuable observations
of earlier times. The great majority of botanists however admitted the
sexual significance of the organs of the flower; most of them rested in
entire faith on Linnaeus’ authority, while some were able to appreciate
the experimental proofs of Camerarius, Bradley, Logan, Gleditsch and
Koelreuter. But all who took up the subject in earnest between 1820
and 1840 were naturally led to desire that the question should once
more be thoroughly examined. The Berlin Academy of science had offered
in 1819 at Link’s suggestion a prize for an essay on the question,
whether there is such a thing as hybrid fertilisation in the vegetable
kingdom, in the hope of stimulating botanists to new investigations
into the decisive points in the sexual theory. The only reply to
this offer, an essay by Wiegmann which was not sent in till 1828, did
not come up to the requirements of the Academy, and was rewarded with
only half the prize. The Dutch Academy at Haarlem was more successful
when induced by Reinwardt in 1830 to propose the question in a
somewhat altered form and in connection with practical horticulture.
This prize was contended for by KARL FRIEDRICH GÄRTNER[111], whose
essay delayed by circumstances till 1837 received the prize of honour
and an extraordinary reward. But the whole body of his results,
derived from the experimental researches of five-and-twenty years,
were not published till 1849 and then in a large volume, ‘Versuche
und Beobachtungen über die Bastardzeugung,’ Stuttgart, 1849, having
been preceded by an introductory work of equal extent, ‘Versuche und
Beobachtungen über die Befruchtungsorgane der vollkommeneren Gewächse
und über die natürliche und künstliche Befruchtung durch den eigenen
Pollen.’ The two works together are the most thorough and complete
account of experimental investigation into sexual relations in plants
which had yet been written. They are a brilliant termination of the
period of doubt with respect to sexuality in plants which succeeded
to the age of Koelreuter—a termination which coincides in time with
the lively discussion which was being maintained on the strength of
microscopical investigations by Schleiden and Schacht on the one
side and by Hofmeister on the other respecting the processes in the
formation of the embryo.
Gärtner’s writings derive their importance not so much from new and
surprising discoveries or brilliant ideas and unexpected combinations,
as from their very searching examination into all the circumstances and
relations which can come under consideration in the sexual propagation
of Phanerogams. His experiments in hybridisation, of which he kept
most exact accounts, exceeded the number of nine thousand; in these
and in normal cases of pollination he studied all the sources of error
which could in any way affect his experiments, and took into careful
consideration all the conditions of fertilisation connected with the
development of the plant itself and with its external circumstances; at
the same time he examined critically all that had been written on the
subject, and submitted every experiment reported by former observers to
the test of his own wide experience. The volume on self-fertilisation
is a complete account of the biology and physiology of flowers. The
phenomena connected with the unfolding and fertilisation of the flower
are described from the writer’s own observations, some of which are
quite new; it specially investigates the relations between the calyx,
the corolla, the secretion of nectar and the opening of the anthers,
also the temperature of flowers and the physiological processes in
the ovary, the style and the stigma; all that was then known of
irritability and the phenomena of movement in the flower and in the
organs of fructification was collected together and elucidated by fresh
observations, and thus a picture was drawn complete to the smallest
detail of the life of a flower, such as we do not yet possess of any
other organ. It would be idle to think of giving in a small compass
a clear idea of the wealth of these observations. But all this was
only preliminary to the main point, the proof that Camerarius was
right, that notwithstanding the objections of a hundred years the
co-operation of the pollen is indispensable to the formation of the
embryo in the growing seed, and that plants therefore have sexuality
exactly as animals have it. Gärtner did not content himself with simply
making new experiments in fertilisation; he refuted the objections of
Spallanzani, Schelver, Henschel, Girou and others in detail from fresh
experiments and from other sources of information, paying particular
regard to all the circumstances which could come under consideration
in each case; he exposed the inaccuracy of the observations of the
opponents of sexuality point by point, and finally called attention to
a number of remarkable phenomena observable in the ovary even before
fertilisation, and to the circumstances under which the pollen may find
its way to it in cases where ordinary pollination has been apparently
prevented. These observations once more confirmed the existence of
sexuality in plants, and in such a manner that it could never be
again disputed. When facts were observed in 1860, which led to the
presumption that under certain circumstances in certain individuals of
some species of plants the female organs might produce embryos capable
of development without the help of the male, there was no thought of
using these cases of parthenogenesis to disprove the existence of
sexuality as the general rule; men were concerned only to verify first
of all the occurrence of the phenomena, and then to see how they were
to be reasonably understood side by side with the existing sexuality,
as had to be done also in the corresponding cases in the animal kingdom.
Gärtner’s work on hybridisation had been preceded by other enquiries
into the same subject, those namely of Knight mentioned above at the
beginning of the century, and Herbert’s more ample investigations
published in his work on Amaryllideae in 1837. Gärtner did not neglect
to compare his observations at all points with the results of his
predecessors, especially those of Koelreuter, and he deduced from
the astonishing mass of material a number of general propositions
respecting the conditions under which the production of hybrids is
possible, the results of crossing, and the causes of failure. A
special interest attaches to his mixed and compound hybrids, to his
experiments on the various degrees of influence which foreign pollen
exercises on the behaviour of the female organ, and the connection of
this point with the formation of varieties. It is impossible to give
a more distinct account of Gärtner’s results without entering into
discussions which would exceed the limits of a historical survey. It is
the less necessary to do so, since Nägeli undertook in 1865 to give a
summary view of all the important results to be found in the wealth of
material supplied by Koelreuter, Herbert and Gärtner[112]. Gärtner’s
experiments in hybridisation were conducted at Calw in Würtemberg,
the place where Koelreuter had made his in 1762 and 1763. And thus
it was in two small cities of Würtemberg that the foundations of the
sexual theory were laid and the theory itself perfected, as far as
it could be by experiment only, by three of the most eminent among
observers. Camerarius in Tübingen, Koelreuter and K. F. Gärtner in Calw
contributed so largely to the empirical establishment of the theory,
that all that was done by others would seem of small importance, if
artificial pollination only were in question. But Koelreuter was
imperfectly acquainted with the methods by which pollination is
usually effected in nature; Sprengel was the first who saw into all
their more important relations, and the fact must not be concealed,
that Gärtner in regarding Konrad Sprengel’s observations as unworthy
of serious consideration, neglected the most fruitful source of new
and magnificent results. His careful study of the secreting of nectar
and of the sensitiveness of the organs of fertilisation, and his many
observations on other biological relations in flowers, would have
found their natural termination, if he had connected them at all
points with Sprengel’s general conclusions respecting the relation
of the structure of the flower to the insect world. This Gärtner
entirely failed to do, and hence in this case also it was reserved
for Darwin’s wonderful talent for combination to sum up the product
of the investigations of a hundred years, and to blend Koelreuter’s,
Knight’s, Herbert’s, and Gärtner’s results with Sprengel’s theory
of flowers into a living whole in such a manner, that now all the
physiological arrangements in the flower have become intelligible both
in their relations to fertilisation, and in their dependence on the
natural conditions under which pollination takes place without the aid
of man. Here, as in morphology and systematic botany, Darwin found
the premisses given and drew the conclusion from them; here too the
certainty of his theory rests on the results of the best observers, on
investigations which find in that theory their necessary logical and
historical consummation.
7. MICROSCOPIC INVESTIGATION INTO THE PROCESSES OF FERTILISATION IN
THE PHANEROGAMS; POLLEN-TUBE AND EGG-CELLS[113]. 1830-1850.
Those who were convinced of the sexuality of plants had endeavoured as
early as the previous century to form some idea with the help of the
microscope of the way in which the pollen effects the formation of the
embryo in the ovule. We may pass over Morland’s and Geoffroy’s very
rude attempts in this direction: Needham (1750), Jussieu, Linnaeus,
Gleichen, and Hedwig imagined that the pollen-grain bursts upon the
stigma, and that the granules it contains make their way downwards
through the style to the ovules, and are there either hatched into
embryos or assist in their production. This way of conceiving the
matter was closely connected with the theory of evolution which then
prevailed, and seemed to find some countenance in the seed-corpuscles
of animals; it was also supported by the observation that pollen-grains
placed under the microscope in water often burst and discharge their
contents in the form of a granular mucilage. It has been already
mentioned that Koelreuter rejected this view; he declared the bursting
of the pollen-grains to be contrary to nature, and considered the oil
which exudes from the grains to be the fertilising substance. This
view was adopted by Joseph Gärtner and Sprengel, but it fell into
disesteem, while that of Needham and Gleichen commanded some assent
some years longer. The next question was, how the granular contents of
the pollen-grain reach the ovules. Accident supplied a starting-point
for further consideration. Amici, who was examining the hairs on the
stigma of Portulaca for another purpose, saw on that occasion (1823)
the pollen-tube emerge from the pollen-grain, and the granular contents
of the latter, commonly known as the fovilla, execute streaming
movements like the well-known movement in Chara. The desire to verify
this remarkable fact, and to discover how the fertilising substance
is absorbed by the stigma, led Brongniart in 1826 to examine a great
number of pollinated stigmas. He succeeded in establishing the fact
that the formation of pollen-tubes is a very frequent occurrence.
The want of perseverance in following out his observation and a
prepossession in favour of Needham’s old theory prevented him from
discovering the course of the pollen-tubes all the way to the ovules;
he supposed, indeed, that after penetrating into the stigma they open
and discharge their granular contents, and he maintained distinctly
that these are analogous to the spermatozoids in animals, and are
the active part of the pollen. But now Amici addressed himself more
earnestly to the question, and in 1830 he not only followed the
pollen-tubes into the ovary, but also observed that one finds its way
into the micropyle of each ovule.
Thus the question was suddenly brought near to its solution, when
observers began to wander from the right path in different directions.
Robert Brown showed in 1831 and 1833 that the grains in the
pollen-masses of Orchids and Asclepiads put forth pollen-tubes as in
other plants, and that fine tubes are found in the ovary of Orchids
in which pollination has taken place; but he was in doubt about the
connection of these tubes with the pollen-grains, and rather inclined
to think that they were formed in the ovary, though possibly in
consequence of the pollination of the stigma. Schleiden went wrong in a
very different way, and by so doing made the question as prominent in
botanical research, as was that of the origin of cells at this time.
He published in 1837 some excellent investigations into the origin and
development of the ovule before fertilisation, certainly the best and
most thorough of the day. He at the same time showed that Brongniart’s
and Brown’s doubts were unfounded, and confirmed the statement of
Amici, that the pollen-tubes make their way from the stigma to the
ovule, which they enter through the micropyle. But he made them
push forward a little too far, for he asserted positively that ‘the
pollen-tube pushes the membrane of the embryo-sac before it, making an
indentation, and its extremity then appears to lie in the embryo-sac.
The extremity of the tube now swells out into a round or oval shape,
and cell-tissue forms from its contents; the lateral organs, one or
two cotyledons, are then produced, the original apical point remaining
more or less free and forming the plumule. The portion of the tube
underneath the embryo and the fold of the embryo-sac which envelopes
it are divided off sooner or later and disappear, so that the embryo
now really lies in the embryo-sac.’ This view, which appears to rest
on direct observation and is illustrated by figures which answer to
the description, corresponds with the old theory of evolution and has
a striking approximation to the ideas of Morland and Geoffroy; and if
it were correct, it would like these imply the necessity of pollination
to the formation of seeds that should contain embryos, but at the same
time it would do away with that which is the essential point in the
sexuality of plants, for the ovule would merely be the spot adapted to
the hatching of the embryo formed from the pollen. Schleiden’s idea
was at once adopted by Wydler, Gelesnow and various other botanists,
and especially by Schacht, but the most eminent microscopists withheld
their assent. Amici was the first who openly opposed the new doctrine;
before the Italian congress of savants at Padua in 1842 he endeavoured
to prove that the embryo is not formed at the end of the pollen-tube,
but from a portion of the ovule which was already in existence before
fertilisation, and that this part is fertilised by the fluid contained
in the pollen-tube. But the choice of a gourd, a plant eminently
unsuitable for his purpose, prevented his discovering the exact
details of the process, and Schleiden did not hesitate to denounce his
assertions in 1845 in the plainest terms. But in the next year (1846)
Amici produced decisive proof for the views which he had maintained;
he showed from the Orchidaceae, which were peculiarly well adapted
for such investigations, not only that Robert Brown’s doubts above
mentioned were without foundation, but, which is the main point, that
a body, the egg-cell, is present in the embryo-sac of the ovule before
the arrival of the pollen-tube, and that this body is excited by the
presence of the pollen-tube to further development, the formation of
the embryo. He gave a connected account on this occasion for the first
time of the whole course of these processes from the pollination of the
stigma to the perfecting of the embryo.
The correctness of the account given by Amici was confirmed in the
following year by von Mohl and Hofmeister, the latter of whom described
in detail the points which were decisive of the question from a
variety of plants, and illustrated them by very beautiful figures in
a more copious work, ‘Die Enstehung des Embryo der Phanerogamen,’
Leipzig, 1849. Tulasne also came forward in opposition to Schleiden’s
theory, being thoroughly convinced that there was no actual contact
of the pollen-tube with the egg-cell, denying indeed the existence of
the egg-cell before fertilisation. Thus a vehement controversy arose
on the subject; a prize offered by the Institute of the Netherlands
at Amsterdam was awarded to an essay of Schacht’s in 1850, which
defended Schleiden’s theory, and illustrated it by a great number of
drawings giving incorrect and indeed inconceivable representations of
the decisive points. Von Mohl says very admirably on this occasion
(‘Botanische Zeitung,’ 1863, Beilage, p. 7): ‘Now that we know that
Schleiden’s doctrine was an illusion, it is instructive, but at the
same time sad, to see how ready men were to accept the false for the
true; some renouncing all observation of their own dressed up the
phantom in theoretical principles; others with the microscope in hand,
but led astray by their preconceptions, believed that they saw what
they could not have seen, and endeavoured to exhibit the correctness of
Schleiden’s notions as raised above all doubt by the aid of hundreds
of figures, which had every thing but truth to recommend them; and
how an academy by rewarding such a work gave fresh confirmation to
an experience which has been repeatedly made good especially in our
own subject during many years past, namely that prize-essays are
little adapted to contribute to the solution of a doubtful question
in science.’ In this case the prize-essay had been refuted before it
appeared by von Mohl, Hofmeister and Tulasne. Schacht adhered all the
more firmly to Schleiden’s theory; after further controversy, in which
other writers of less authority took part, Radlkofer published in 1856
a complete review of the question, which fully confirmed Hofmeister’s
observations, and gave incidentally an account of Schleiden’s views
in the altered form which they had by that time assumed; this account
showed in fact that Schleiden had completely retracted his former
opinions, and in this retractation Schacht was soon after compelled to
follow him, having become acquainted with facts observed in the ovule
of Gladiolus, which were obviously irreconcilable with Schleiden’s
theory.
Hofmeister had from the first directed special attention to the
questions, whether any bodies are found in the pollen-tube which answer
in any way to spermatozoids, and whether any opening can be perceived
at the end of the tube. He found indeed forms in Coniferae in 1851,
which reminded him of the male organs of fertilisation in the higher
Cryptogams; but the pollen-tube was closed both in them and in the
rest of the Phanerogams, in which moreover its outer coat attains to a
considerable thickness. There remained therefore only the hypothesis,
that a fluid substance passes through the walls of the pollen-tube and
of the embryo-sac and effects the fertilisation of the egg-cell; thus
it was not the theory of preformation of the last century, to which
Brongniart still adhered, but the view represented by Koelreuter, which
ultimately proved to be nearer the truth, though it may be said that
all that remained of that view was, that the fertilising substance in
the Phanerogams is a fluid. The granular contents of the pollen-grains,
which were supposed to be spermatozoids, have since been partly found
to be only innocent starch-grains and drops of oil.
8. DISCOVERY OF SEXUALITY IN THE CRYPTOGAMS. 1837-1860.
By the year 1845 no one capable of forming a judgment on the question
any longer doubted the existence of different sexes in Phanerogams.
But it was not so with the Cryptogams, though a number of facts were
acknowledged at this time which seemed to point to the conclusion, that
a moment arrives sooner or later in the course of their development
also, when a sexual act is accomplished. But the question had not
as yet been systematically studied; no experimental investigations
had been made, or observations of such a kind as to demonstrate the
necessity of sexual union.
The great majority of botanists in the second half of the 18th century
had no longer any doubt that the stamens were organs of reproduction,
and they were anxious to prove the existence of similar organs in
the Cryptogams; they rested in this matter on external resemblances
and analogies, which they interpreted in a more or less arbitrary
manner. The obvious external resemblance between the antheridia and
archegonia in Mosses and the sexual organs in the Phanerogams led
Schmidel and Hedwig to consider them to be stamens and ovaries, and the
conjecture was correct, though the true nature of the moss-fruit had
to be learnt in another way. Micheli, Linnaeus and Dillen, trusting
still more to external appearance and with slight knowledge of these
plants, had before this taken the fruit for a male flower, and in
the case of the rest of the Cryptogams the best botanists were only
feeling their way in the dark with no certain experience to guide
them. It is not necessary to give a particular account of the views
which originated in this way; one or two may be mentioned by way of
example. Koelreuter regarded the volva of Mushrooms, Gleditsch and
Hedwig certain tube-like cells in their lamellae, as the male organs
of fertilisation. Gleichen took the stomata, Koelreuter the indusium,
Hedwig even the glandular hairs of Ferns for anthers. It was not yet
suspected that the course of development and the whole morphology of
the Cryptogams could not be so compared with that of the Phanerogams;
correct and incorrect assumptions with regard to the sexual organs
of the Cryptogams were alike devoid of scientific value, being mere
guesses and vague conjectures. Nor was the state of things much better
even in the first years of the 19th century; and if by that time a
number of occasional observations had been made which could afterwards
be turned to scientific account, these were as yet only isolated facts
without scientific connection, and every one was at liberty to concede
or to refuse sexual organs to the Cryptogams generally at his own
discretion. Meanwhile observations gradually accumulated, and towards
1845 it began to be possible by critical examination of them to arrive
at something like a clearer understanding of this part of botany.
The majority of botanists readily accepted Schmidel’s and Hedwig’s
opinion with respect to the Mosses; Vaucher had as early as 1803
maintained that the long-known conjugation of Spirogyra was a sexual
act; Ehrenberg observed in 1820 the conjugation of a Mould, Syzygites;
Bischoff and Mirbel explained the organisation of the antheridia of
the Liverworts in 1845, while Nees von Esenbeck saw the spermatozoids
of Sphagnum in 1822 and Bischoff those of Chara in 1828, though they
were at first taken for Infusoria, an opinion maintained by Unger as
late as 1834. But it was Unger[114], who in 1837, after careful study
of the spermatozoids of the Mosses in 1837, declared them to be the
male organs of fertilisation; in 1844 Nägeli discovered corresponding
forms on the prothallium of Ferns, which had till then been called a
cotyledon, and in 1846 the spermatozoids of Pilularia, the products of
the small spores which Schleiden had explained to be the pollen-grains
of that plant.
These facts were of the highest importance, but little was to be made
of them as long as the female organ in the plants in question, the
Mosses excepted, was unknown, and meanwhile it was only the resemblance
between vegetable and animal spermatozoids which led to the conjecture,
that the one had the same sexual significance as the other.
Light was suddenly thrown upon the subject, when Count Lesczyc-Suminsky
discovered in 1848 on the supposed cotyledon (prothallium) of Ferns
both the antheridia and the peculiar organs, inside which the embryo or
young fern is formed. Though the statements respecting the structure
and development of these female organs and of the embryo were
inaccurate in some important points, yet the place was now indicated
where it might be presumed that the fertilisation by the spermatozoids
takes place; and as the history of the germination of the rest of
the vascular Cryptogams was to some extent known through the earlier
labours of Vaucher and Bischoff, the organs of fructification of these
plants might now be sought, where they are really to be found. But
an erroneous idea respecting the meaning of the small spores of the
Rhizocarps propounded by Schleiden had first to be put out of the way,
and this was done by an appeal to the discovery of Nägeli mentioned
above and by the investigations of Mettenius. Then in 1849 Hofmeister
supplied a connected description of the germination of Pilularia
and Salvinia, in which the decisive points as regards the sexual
act were clearly set forth, and the connection of the spermatozoids
with the fertilisation of the egg-cells in the archegonium was
established. He did the same for Selaginella, which is very unlike the
Rhizocarps and Ferns, and in which the spermatozoids are developed
from smaller spores, and fertilise the egg-cells in archegonia formed
in the prothallium of the large spores. By comparing the processes
of germination in these plants with those of Ferns and Mosses, he
succeeded in throwing entirely new light on the whole of the morphology
of these classes of plants, and thus made it possible for the first
time to compare them with one another and with the Phanerogams, and to
form a right estimate of the sexual act in the Muscineae and Vascular
Cryptogams in its relation to the history of the development of these
plants. Hofmeister arrived at the following conclusion from his
observations in 1849: ‘The prothallium in the vascular Cryptogams is
the morphological equivalent of the leaf-bearing Moss-plant, while the
leafy plant of a Fern, of a Lycopodium and a Rhizocarp answers to the
capsule of the Moss. In Mosses as in Ferns there is an interruption
of the vegetative development by sexual procreation, an alternation
of generations; this takes place in the Vascular Cryptogams very soon
after germination, in the Mosses much later.’ The vast importance
of this discovery to systematic botany has been already noticed.
The conception of these relations developed by Hofmeister was not
less important to the doctrine of the sexuality of plants; it swept
away at one stroke all the old false analogies between Phanerogams
and Cryptogams and brought to light the real agreement; Hofmeister
had detected in the archegonium of the Cryptogams the body which is
developed there, as in the ovule of the Phanerogams, into an embryo
after fertilisation, namely the germinal vesicle or egg-cell. Here
was the point of departure for all further systematic comparison in
the sexual propagation of Cryptogams and Phanerogams. All beside was
of secondary importance, even the fact, that the fertilisation of the
egg-cell in the Cryptogams is not effected by a pollen-tube, but by
spermatozoids. It was now easy to show the corresponding relations of
generation in the other cases which Hofmeister had not yet observed.
Hofmeister’s statements and conclusions respecting Selaginella and
Isoetes were confirmed and some additions made to them by Mettenius in
1850, and in 1851 appeared Hofmeister’s exhaustive work ‘Vergleichende
Untersuchungen,’ in which the mode of production of the embryo in
Coniferae was represented as an intermediate form between those of
Phanerogams and Cryptogams. Further contributions were made to the
knowledge of the subject; Henfrey confirmed Hofmeister’s results in
the case of Ferns; Hofmeister himself and Milde observed in 1852 the
history of fertilisation in Equisetaceae, and the former supplied at
the same time a more complete account of the development of Isoetes; in
1855 he described the decisive points in Botrychium and Mettenius in
1856 those in Ophioglossum.
The processes of development before and after fertilisation were
now cleared up by all these discoveries, but the direct observation
of the act of fertilisation was still wanting. Hofmeister (‘Flora,’
1857, p. 122) describes the state of affairs in the following terms:
‘While numerous investigations had thrown a clear light on the
character of the male and female organs, and on the way in which the
embryo is formed by repeated division of the egg-cell present before
fertilisation, we continued quite in the dark respecting the particular
nature of the fertilisation. Observation and experiment had established
the fact, that the influence of the spermatozoids on the archegonia was
required to produce an embryo in the latter. Female moss-plants[115]
separated from the male, macrospores in the Vascular Cryptogams
separated from the microspores, had in all cases proved unproductive;
but it was not even certainly known to what point in the female organ
the spermatozoids force their way. It is true that Lesczyc and after
him Mercklin had seen the entry of moving spermatozoids into the mouth
of archegonia in Ferns; but Lesczyc’s account of the part which he
supposed them to play there afterwards, was proved to be an illusion.
I had myself observed motionless spermatozoids halfway down the neck
of archegonia of an Equisetum; but nothing was to be learnt of the
manner in which the spermatozoid affects the egg-cell. Then it happened
that in the spring of 1851, being engaged in observing the development
of the organs of vegetation of Ferns, I repeatedly saw spermatozoids
moving about in the basilar cells which enclose the egg-cell in the
archegonia of Ferns, and the majority of them even playing about the
egg-cell. Their movements were put an end to during the observation
by the commencement of changes, which the contents of young vegetable
cells which have been cut open usually experience under the prolonged
influence of water.’ Later observations leave no doubt now that in the
Muscineae and Ferns single spermatozoids force their way into the naked
egg-cell of the archegonium.
The question was first set at rest in the Algae, where the process of
fertilisation could be seen directly and without exposing the objects
to destructive influences. That sexual propagation occurs in the Algae
also had seemed probable, since Decaisne and Thuret in 1845 discovered
organs in species of Fucus, and Nägeli in 1846 in Florideae, which
scarcely admitted of any other explanation. Alexander Braun also had
called attention to the formation of two kinds of spores in a large
number of fresh-water Algae. But as yet there was only conjecture. Then
Thuret proved by experiment in 1854, that in the genus Fucus the large
egg-cells must be fertilised by very small swarming spermatozoids, in
order to set up germination; both organs can be collected separately
and in numbers in this genus, and be brought together at pleasure;
Thuret even succeeded in obtaining hybrids. Pringsheim first observed
in 1855 the formation of spermatozoids in the little horns of Vaucheria
and established the fact that spores capable of germination are not
formed unless the spermatozoids approach the egg-cell. To Thuret’s
statements he added the very important one, that the remains of
spermatozoids may be recognised on the surface of the contents of
the fertilised egg-cell of Fucus, which is already surrounded by a
membrane. About the same time Cohn published his observations on
Sphaeroplea annulina, which confirmed the fact of the approach of
the spermatozoids to the egg-cells, which consequently, as in Fucus
and Vaucheria, form a cell-wall and are rendered capable of further
development.
Still the decisive observation had not yet been made; no one had
yet seen how the two fertilising elements behaved at the moment of
fertilisation. Pringsheim had the good fortune to make this observation
in one of the commonest of fresh water Algae, Oedogonium. There he saw
the moving spermatozoid first come into contact with the protoplasmatic
substance of the egg-cell, and then force its way into it, blend with
it and dissolve. And thus the first observation was made, which proved
decisively that a real intermixture takes place of the male and female
elements of fertilisation; this important fact was confirmed by De Bary
in the same year.
Now that it was once established, that fertilisation in Cryptogams
consists in the blending together of two naked bodies of protoplasm,
the spermatozoid and the egg-cell, it was reasonable to conclude that
conjugation in Spirogyra and generally in Conjugatae, was an act of
fertilisation, only in this case the two fertilisation-elements are
not of different size and shape, but similar in appearance. To this
conclusion De Bary arrived in 1858 in his monograph of the Conjugatae.
This extension of the idea of fertilisation to cases in which the
uniting cells are to outward appearance alike, was of special value
to the theory of sexuality, as was seen in the sequel, when other
forms of fertilisation were observed which made it necessary still
further to extend the idea of sexuality. In 1858 Pringsheim discovered
arrangements for fertilisation in another group of Algae, the
Saprolegnieae, which to outward appearance at least departed widely
from those hitherto known in the lower plants.
Thus between the years 1850 and 1860 a number of fundamental facts
were discovered, and were afterwards confirmed and extended by fresh
observations in the course of the following years. It does not fall
within the limits of this work to notice the many discoveries that
were made in this part of botanical science after 1860; we will only
remark, that between 1860 and 1870 the processes of fructification were
observed by Thuret and Bornet in Florideae, and especially by De Bary
and his pupils in Fungi, in some of which very peculiar forms were
brought to light. No doubt any longer exists that difference of sex
prevails generally in the Thallophytes also, though it is still an open
question, whether it may not be wanting in some of the very simplest
and smallest kinds.
One of the most important results of these investigations is
obviously the striking resemblance between many of the processes of
fertilisation in Cryptogams and in the lower animals; here is another
confirmation of the fact, often brought out in other ways by modern
zoological and botanical research, that the points of resemblance in
the vegetable and animal kingdoms appear most plainly, if we compare
together the simplest forms to be found in both; we have in this
fact a plain proof also, that both kingdoms have been developed from
like common elements, as the theory of descent implies. With respect
to the true nature of fertilisation itself, which is evidently a
similar process in the main in animals and plants, we can only say at
present, that it amounts in all cases to a material blending together
of the contents of two cells, neither of which is capable of further
development by itself, while the product of the combination is not only
capable of such development, but unites in itself the characteristics
of the two parent forms and transmits them to its descendants. That
fertilisation is not the intimate union of two bodies possessing a
definite form, but that the male fertilising substance at least may
be a simple fluid, appears to be distinctly shown by the process in
Phanerogams; and we may assume, that in Cryptogams also, the sexual act
is not affected by the form of the fertilisation-elements, though a
certain shape and power of movement is necessary for the conveyance of
the fertilising substance to that which is to be fertilised.
CHAPTER II.
HISTORY OF THE THEORY OF THE NUTRITION OF PLANTS.
1583-1860.
That plants take up certain substances from their environment for the
purpose of building up their own structures could not be a matter of
doubt even in the earliest times; it was also obvious, that movements
of the nutrient material must be connected with this proceeding. But it
was not so easy to say, what was the nature of this food of plants, in
what manner it finds its way into and is distributed in them, and what
are the forces employed; it was even for a long time undecided, whether
the food taken up from without suffers any change inside the plant,
before it is applied to purposes of growth. Such were the questions
which had engaged the attention of Aristotle, and which formed the
chief subject of Cesalpino’s physiological meditations.
But the questions respecting the nutrition of plants acquired a much
more definite shape in the latter half of the 17th century, when the
various phenomena of vegetation began to be more closely observed, and
some attempt was made to understand their relations to the outer world.
Malpighi, the founder of phytotomy, was the first who undertook to
explain the share which belongs to the different organs of the plant
in the whole work of nutrition; guided by analogy, he perceived that
the green leaves are the organs which prepare the food, and that the
material so prepared by them passes into all parts of the plant, there
to be stored up or employed for purposes of growth. But this gave no
insight into the nature of the substances from which plants prepare
their food. On this point Mariotte endeavoured to give such information
as could be obtained from the chemistry of his day; and he has the
merit of having shown, in opposition to the old Aristotelian notion,
that plants convert the food-material which they derive from the ground
into new chemical combinations, while the earth and the water supply
the same elements of nutrition to the most different kinds of plants.
It could not escape the notice of physiologists even of that time,
that the water which plants take up from the ground introduces into
them but very small quantities of matter in solution. Van Helmont in
the first half of the 17th century had shown this by an experiment,
the results of which, however, led him to think that plants were able
to produce both the combustible and incombustible parts of their
substance from water. Hales at the beginning of the 18th century formed
a different opinion, being led by the evolution of the gases in the dry
distillation of plants to conclude, that a considerable part of their
substance was absorbed in a gaseous form from the atmosphere.
The views propounded by Malpighi, Mariotte, and Hales contained the
most important elements of a theory of the nutrition of plants; fully
understood they would have taught that one part of the food of plants
comes from the earth and the water, and another part from the air;
that the leaves change the materials thus obtained in such a manner as
to produce from them the substance of plants and to apply this to the
purposes of growth; but the ideas were not combined in this way, for
during some years after their time botanists were chiefly engaged in
observations on the movement of the sap in plants, and they arrived
even on this point at very obscure and even contradictory results,
because they overlooked the function of the leaves which had already
been recognised by Malpighi. All insight not only into the chemical
processes in the nutrition of plants, but also into the mechanical laws
of the movement of the sap, and generally into the whole internal
economy of plants, depends on a knowledge of the fact, that it is only
the cells which contain chlorophyll, and therefore in the higher plants
the leaves chiefly as consisting largely of such cells, which have the
power of converting the gaseous food supplied by the atmosphere into
the substance of the plant with the aid of the materials taken up from
the soil. This fact is of fundamental importance to the whole theory of
the nutrition of plants; it is only by a knowledge of it that we can
explain the movement of material connected with nutrition and growth,
the dependence of vegetation on light, and to a great extent also the
function of the roots.
But this principle could not be discovered till the new chemical
system founded by Lavoisier took the place of the old phlogistic
chemistry, and it is remarkable that the discoveries, which laid the
foundation of modern chemistry in the period between 1760 and 1780,
contributed essentially to the establishment at the same time of the
modern doctrine of the nutrition of plants. Ingen-Houss, in reliance
on Lavoisier’s antiphlogistic views on the composition of air, water,
and the mineral acids, succeeded in proving that all parts of plants
are continually absorbing oxygen and forming carbon dioxide, but that
the green organs at the same time under the influence of light absorb
carbon dioxide and exhale oxygen; and as early as 1796 he considered
it probable that plants obtain the whole mass of their carbon from
the carbon dioxide of the atmosphere. Soon after (1804) de Saussure
proved, that plants, while they decompose carbon dioxide, increase in
weight by a greater amount than that of the carbon which they retain,
and that this is to be explained by the fact that they at the same
time fix the elements of water. He likewise showed that the small
quantities of saline compounds, which plants take up from the soil, are
a necessary part of their food, and that it was at least probable, that
the nitrogen of the atmosphere does not contribute to the formation of
nitrogenous substances in plants. Senebier had before insisted on the
fact, that the decomposition of carbon dioxide under the influence of
light only takes place in green organs.
Thus the most important points in the nutrition of plants were
discovered by Ingen-Houss, Senebier and de Saussure. But, as often
happens in the case of discoveries of such magnitude, their ideas
were for a long time exposed to great misunderstanding. They were
better appreciated in France than in any other country; Dutrochet and
De Candolle were able to see the importance of the interchange of
gases in the green organs to the general nutrition and respiration;
but others, and especially German botanists, were not content with
these simple chemical processes as the foundation of the whole system
of nutrition and consequently of the whole life of the plant; the
theory of the vital force, which was elaborated in connection with the
nature-philosophy during the first years of the 19th century, and was
generally accepted by philosophers and physiologists, chemists and
physicists, preferred to supply the plant with a mysterious substance
for its food, which had its source in the life itself and which it
called humus. The most obvious considerations, which must at once have
shown that this humus-theory was absurd, were entirely overlooked;
and thus in the face of de Saussure’s results the food of plants was
once more referred entirely to the soil and the roots, as it was in
the earliest times; one of the consequences of this humus-theory in
combination with the vital force was that the ash-constituents of
plants were supposed to be merely accidental admixtures or stimulants,
or to be directly produced in the plant by the vital force.
In the period between 1820 and 1840 the reaction set in from different
quarters against the theory of vital force; chemists succeeded in
producing by artificial means certain organic compounds, which had
hitherto been regarded as products of that force; Dutrochet discovered
in endosmose a process, which served to refer various vital phenomena
in plants to physico-mechanical principles; de Saussure and others
showed that the heat of plants is a product of respiration, and by
1840 the earlier theory of a vital force might be looked upon as
antiquated and obsolete. It remained to restore to their rights the
observations of Ingen-Houss and de Saussure, which under the influence
of that theory and of the notions respecting the humus had been so
utterly misconstrued. Liebig set aside the humus-theory in 1840,
and referred the carbon of plants entirely to the carbon dioxide
of the atmosphere, and their nitrogenous contents to ammonia and
its derivatives; he claimed the components of the ash as essential
factors in the nutrition, and taking his stand on the general laws of
chemistry endeavoured to obtain chiefly by the method of deduction an
insight into the chemical processes of assimilation and metabolism.
The whole theoretical value of the facts discovered by Ingen-Houss,
Senebier and de Saussure was first made apparent by the connection
which Liebig succeeded in establishing between the phenomena of
nutrition. The doctrine of nutrition burst suddenly into new life;
firm ground was gained, and the botanist, no longer distracted by
the difficulties raised by the vital force but resting on physical
and chemical principles, might now resume the task of investigation.
Oxygen-respiration denied by Liebig was first of all re-established
by von Mohl and others. Liebig’s views on the source of nitrogen in
plants and on the importance of the ash-constituents rested chiefly on
general considerations and observations and on calculation, and had now
to be tested by systematic investigation and especially by experiments
on vegetation in individual plants. And here the place of honour must
be assigned to Boussingault, who pursued the path of pure induction
as contrasted with Liebig’s deductive mode of proceeding, gradually
improved the methods for experimenting on vegetation, and soon
succeeded in so producing plants in a purely mineral soil free from all
humus, that he finally settled the question of the derivation of the
carbon from the atmosphere and of the source of the nitrogen also.
He showed from the plants thus artificially nourished, and with due
consideration of the many sources of error which beset the question,
that the uncombined nitrogen of the atmosphere does not contribute to
the nutrition of plants, but that a normal increase in the nitrogenous
substances in a plant takes place when the roots take up nitrates as
well as the necessary constituents of the ash.
With the exception of some doubts which still remained respecting the
necessity of certain constituents of the ash, such as sodium, chlorine
and silicic acid, the source of the materials which take a part in
the chemistry of the nutrition of plants was known before 1860; but
the knowledge obtained with regard to processes in the interior of
the plant, the origination of organic substances in the processes of
assimilation, and the further changes which they undergo was still
fragmentary and uncertain, and led to no general and conclusive results.
1. CESALPINO.
Aristotle had sought to determine the nature of the materials which
plants take up as food, and had laid down the proposition, that the
food of all organisms is not simple but composed of various substances.
This view was correct, but he united with it the erroneous notion,
that the food of plants is elaborated beforehand in the earth, as in a
stomach, and is made applicable to purposes of growth, so as to exclude
the necessity of any separation of excrements in the plant; this error
was refuted by Jung, as we shall see, but nevertheless it continued to
live as late as into the 18th century, and ultimately quite spoilt Du
Hamel’s theory of nutrition.
Cesalpino, whom we have learnt to regard as a faithful and gifted
disciple of Aristotle, directed his speculations to the mechanical
rather than to the chemical side of the question, and chiefly tried to
explain the movement of the nutrient sap in plants. He had a larger
stock of material drawn from experience at his disposition than his
master, and it is instructive therefore to make a nearer acquaintance
with his views, because they show how far the old philosophy was in a
condition to turn better empirical knowledge than Aristotle possessed
to a satisfactory use; they will also show that Cesalpino’s first
essays led him to views which can no longer be said to be strictly
Aristotelian.
In the second chapter of the first book of the work from which we have
already quoted, ‘De plantis libri XVI,’ 1583, he raises the question,
in what way the food of plants is taken in and their nutrition
accomplished. In animals we see the food conveyed from the veins to the
heart, which is the laboratory of the warmth of the body, and after it
has been finally perfected there, spread abroad through the arteries
into all parts of the body; and this is effected by the operation of
the force (spiritus) which is generated in the heart from the food.
In plants on the contrary we see no veins, or other channels, nor do
we feel any warmth in them, so that it is difficult to understand
how trees grow to so great a size, since they seem to have much less
natural heat than animals. Cesalpino explains this enigma by saying,
that animals require much food for maintaining the activity of the
senses and the movements of their organs. The larger quantity of animal
food also requires larger receptacles, namely the veins. Plants on the
other hand need less food, because this is only used for purposes of
nutrition, or to a very small extent for the production of internal
heat as well, and therefore they grow more vigorously and bear more
fruit than animals. At the same time plants are not without internal
heat, though it cannot be perceived by the touch because all objects
seem cold to us, which are less warm than our organ of feeling. That
plants moreover have veins, though only narrow ones in accordance with
the small mass of their food, is shown by those which yield a milky
juice, such as Euphorbia and Ficus, which when cut bleed like the
flesh of animals; Cesalpino adds ‘and this is very frequent also in
the vine,’ which shows that he made no distinction between milky juice
and the exuding water of the weeping vine-stock. These narrow veins
cannot be seen on account of their fineness; but in every stem and in
every root things may be discerned which like nerves in animals can be
split longitudinally and are called the nerves of the plant, or also
certain thicker things, such as those which branch in most leaves and
are there called veins. These should be considered as food-passages
and as answering to the veins in animals; but plants have no main vein
like the vena cava in animals, but many fine veins pass from the root
to the heart of the plant (cor, root-neck, see above, Book I. chap. 2),
and ascend from it into the stem; for it was not necessary that the
food should be collected in a common receptacle in plants, as it is in
the heart in animals, where this is necessary for the production of
the spiritus, but it was sufficient that the fluid in plants should be
changed by contact with the medulla cordis (in the root-neck), as it is
changed in animals in the marrow of the brain or in the liver; and in
these organs the veins are very narrow, as they are in plants.
Since plants have no sense-perception, they cannot seek their food
like animals, but they draw up the moisture from the ground into
themselves in a way of their own; but it is not easy to see how this
takes place. Cesalpino, in trying to explain this, gives us a glimpse
into the physics of the day, and we observe also to our surprise an
attempt made to explain phenomena in living creatures by physical
laws, a step beyond the limits of Aristotelian modes of thought and in
the right direction. It is not the ratio similitudinis, which draws
iron to the magnet, that can cause the attraction of the juice by the
roots, for then the smaller would be drawn to the larger; and if the
attraction of the fluid of the earth by the roots were the same thing
as the attraction of the iron by the magnet, the moisture of the earth
would draw out the juice from the plant, which is just what does not
happen. Nor can it be the ratio vacui; for since not moisture only
but air also is contained in the earth, the plant would be filled not
with juice but with air. But Cesalpino hits upon a third kind of cause
by which juices may be drawn into the plant. Do not many dry things,
he says, in accordance with their nature attract moisture, as linen,
sponge and powder, while others repel it, as the feathers of many birds
and the herb Adiantum, which are not wetted even when dipped in water;
but the former absorb much water, because they have more in common with
it than with air; of this kind Cesalpino thinks those parts of plants
must be, which the nourishing soul employs to take in food. Therefore
these organs are not traversed by a continuous canal such as the veins
in animals, but formed like the nerves of a fibrous substance; and thus
the power of suction (bibula natura) conveys the moisture continually
to the place, where the principle of internal heat is placed, just as
may be seen in the flame of a lantern, to which the wick continually
conducts the oil. The absorption of the moisture is also increased
by the outer warmth, for which reason plants grow more vigorously in
spring and summer.
That Cesalpino had no suspicion of the use of the leaves in the
nutrition of plants appears incontestably from his repeating the
Aristotelian idea, that the leaves are only for the protection of young
shoots and fruits from air and sun-light; this idea is no result of
speculation, but came simply from observing a vineyard in a hot country.
2. FIRST INDUCTIVE EXPERIMENTS AND OPENING OF NEW POINTS OF VIEW IN THE
HISTORY OF THE THEORY OF THE NUTRITION OF PLANTS.
All that Aristotle and his school, Cesalpino not excepted, are able to
tell us about the phenomena of vegetable life, was the result of the
most every-day observations, none of which were critically and exactly
tested to ascertain their actual correctness, while the larger part
of their physiological axioms were not derived from observations on
plants at all, but from philosophical principles, and especially from
analogies taken from the animal world.
The first step towards a scientific treatment of the doctrine of
nutrition was an enlargement and critical examination of the materials
to be gained from experience; nor were any difficult observations or
experiments needed to discover contradictions between the truths of
nature and the old philosophy; all that was necessary was to look into
things more closely and to judge of them with less prejudice.
In this way Jung was led to oppose one important point of the
Aristotelian account of nutrition. In the second fragment of his work
‘De plantis doxoscopiae physicae minores’ is to be found a remark,
which is evidently directed against the notion that plants receive
their food already elaborated from the earth, and therefore give
off no excrements[116]. Plants, says Jung in accord with Aristotle,
appear not to need a thinking soul (anima intelligente), which would
be able to distinguish wholesome from unwholesome food, and Aristotle
therefore provided them with food which had already been perfectly
prepared in the earth. But Jung takes another view founded on actual
observation. It is very possible, he says, that the openings in the
roots which take in liquid matter are so organised, that they do not
allow every kind of juice to enter, and who can say that plants have
the peculiarity of only absorbing what is useful to them, for like
all other living creatures they have their excreta, which are exhaled
through the leaves, flowers, and fruits. But among these he reckons the
resins and other exuding liquids, and says that it is possible after
all that a large part of the juices of plants escapes by imperceptible
evaporation, as happens in animals.
According to Aristotle’s view the plant itself was quite passive in
the work of nutrition; since food was offered to it which had been
already prepared for it in the earth, growth was to some extent merely
a process of crystallisation unaccompanied by chemical change. In
pointing to the formation of excreta Jung on the contrary ascribed a
chemical activity to the plant, and by supposing that the organisation
of the root was such as to prevent the entrance of certain matters
and to favour that of others, he made the plant co-operate in its own
nourishment, though he did not assume that it needed a thinking soul
for this purpose.
Johann Baptist van Helmont[117], physician and chemist, and a
contemporary of Jung, took up a position still more decidedly opposed
to Aristotelian doctrines. He rejected the four elements of that
philosophy, and regarding water as a chief constituent of all things
he considered that the whole substance of plants, the mineral parts
(the ash) as well as the combustible, was formed from water. Thus while
Aristotle made the component parts of plants be introduced into them by
water in a state ready for use, Van Helmont, on the contrary, ascribed
to the plant the power of producing all kinds of material from water.
It would scarcely have been necessary to mention this resistance to old
dogmas, originating as it did in the notions of the alchemists, if Van
Helmont had not made an attempt to establish his views by experiment;
this was the first experiment in vegetation undertaken for a scientific
purpose of which we have any information, and it was repeatedly
quoted by many later physiologists, and employed in support of their
theories. He placed in a pot a certain quantity of earth, which when
highly dried weighed two hundred pounds; a willow-branch weighing five
pounds was set in this pot, which was protected by a cover from dust,
and daily watered with rain-water. In five years’ time the willow had
grown to be large and strong, and had increased in weight by a hundred
and sixty-four pounds, though the earth in the pot, when once more
dried, only showed a loss of two ounces. Van Helmont concluded from
this experiment that the considerable increase of weight in the plant
had been gained entirely at the cost of the water, and consequently
that all the materials in the plant, though distinct from water,
nevertheless come from it.
These objections to Aristotelian teaching on the part of Jung and Van
Helmont remained isolated and unproductive, but an incentive to new
investigations in vegetable physiology was supplied from a different
quarter, and its influence lasted till far into the 18th century. This
was the suggestion, that not only does a nutrient sap taken up by
the roots ascend to the leaves and fruits of plants, but that there
is also a movement of the same sap in the opposite direction in the
rind. But this idea assumed from the first two different forms. Some
botanists, evidently resting on the analogy of the circulation of the
blood in animals, supposed that there was also an actual circulation of
the sap in plants; others on the contrary were content with supposing
that while the watery sap absorbed by the roots rises in the wood, an
elaborated sap capable of ministering to growth moves in the rind, the
laticiferous vessels, and the resin-ducts. The two views were at a
later time repeatedly confounded together, and those who refuted the
first believed that they had refuted the other also. It appears that a
physician from Breslau, Johann Daniel Major[118], Professor in Kiel,
first gave expression to the opinion, that there is a circulation of
the nourishing substance in plants as in animals; and from this time
to the end of the 18th century the circulation of the juices of plants
was a favourite subject of discussion, but more often chosen by the
impugners of the doctrine than by its defenders.
The better form of the idea, namely, that there is a return-movement
of material towards the root, combined with the view, that the leaves
are the organs which produce the substances required for growth from
the crude material supplied to them, was expressed by Malpighi as early
as 1771 in the shape of a well-considered theory. In his ‘Anatomes
plantarum idea’ of that year he devotes the last pages to a short
account of the theory of nutrition, as he understood it. He regarded
the fibrous constituents of the wood as the organs for conducting the
sap taken up by the roots, and the vessels as air-passages, which he
named tracheae on account of their resemblance to the tracheae of
insects. He was in doubt whether the air came from the earth through
the roots, or from the atmosphere through the leaves, for he had
never succeeded in finding openings for the entrance of air in the
roots or the leaves; but he thought it more probable that the air is
absorbed by the roots, because they are well supplied with tracheae,
and air has besides a tendency to ascend. Beside these fluid-conducting
fibres and air-conducting tracheae in the wood he called attention
to the existence of special vessels, which conduct peculiar juices
in many plants, as the laticiferous vessels, gum-passages, and
turpentine-canals.
Respecting the movement of the juices, he notices that the direction
may be reversed, because shoots planted upside down send out roots
into the earth from what is organically their upper end, and grow into
trees; and though they do not grow vigorously, yet the experiment
proves that the movement of the sap in them is in the reverse
direction.
After these preliminary remarks he proceeds to prove, that it is in
the leaves that the crude juices of nutrition undergo the change which
fits them for the maintenance of growth. The way in which Malpighi
arrives at this view is as simple as it is original. He considers
the cotyledons of young plants to be genuine leaves (in leguminibus
seminalis caro, quae folium est conglobatum), as is shown in the
gourd, where the cotyledons grow into large green leaves. Liquid is
conveyed to them through the radicle, and a portion of the substances
which they contain passes from them into the plumule to make it grow,
which it will not do if the cotyledons are removed; hence he concludes
that all other leaves also are intended to elaborate (excoquere) the
nutritive juice contained in their cells, which the woody fibres have
conveyed to them. The liquids mingled together in their long passage
through the network of fibres are changed in the leaves by the power
of the sun’s rays, and blended with the sap before contained in their
cells, and thus a new combination of the constituent parts is effected,
transpiration proceeding at the same time; he compares the whole
process with that which goes on in the blood of animals.
We see that Malpighi’s view of the function of the leaves in nutrition
approaches very closely to the truth, as closely indeed as was at
all possible in the existing condition of chemical knowledge. He was
induced by the results of anatomical investigation to carry this view
farther and indeed correctly; he supposed that the parenchymatous
tissue of the rind acts in the same way as the leaves; but he went a
step too far in assigning the function of the leaves to the colourless
parenchyma also, which only serves for the storing up of assimilated
matter. He says we must ascribe a character similar to that of the
leaf-cells to the corresponding cells in the rind and to those also
which lie transversely in the wood (the medullary and cortical rays),
and that it is not unreasonable to conclude that the food of the plant
is elaborated and stored up in these cells. As he makes no sharp
distinction between elaboration and mere storing up, he ascribes the
function of the leaves to the parenchyma of fleshy fruits also and to
the scales of bulbs; he concludes from the exudations from stumps of
trees and from the cut surfaces of other parts of plants, that they are
filled with reserve-matter (asservato humore turgent).
Thus the essential points in Malpighi’s theory of nutrition in the year
1671 were, that the vessels of the wood are primarily air-conducting
organs, that the leaves elaborate the crude sap for purposes of
growth, that the sap so elaborated is stored up in different parts of
the plant, and that the fibrous elements of the wood convey upwards
to the leaves the crude materials of nutrition which are absorbed by
the roots. No mention is made of a circulation of juices, comparable
to the circulation of the blood, though this idea was in later times
often imputed to him; and we find by his later remarks, that while he
was in no doubt as to the elementary organs which convey the ascending
sap, he confined himself to conjecture with respect to the way by
which the sap elaborated in the cell-tissue of the leaves, rind and
parenchyma generally is carried on its further course. But he was in
no doubt about the direction of that course; he believed that this sap
forces itself downwards through the stem into the roots, and upwards
in the branches above the leaves and so into the fruit. Thus Malpighi
had formed a more correct idea of the movement of assimilated matter
than the majority of his successors who introduced the very unsuitable
expression, ‘descending sap.’ He further thought it probable that the
elaborated sap passes through the bast-bundles[119], but without a
continuous flux and reflux (absque perenni et considerabili fluxu et
refluxu); that it rests to some extent in the laticiferous vessels,
but that it is also driven sometimes, when occasion requires, by
transpiration and external causes into the higher parts of the plant,
where it is the means of maintaining growth and nutrition. These later
remarks also are better than much that was said about the movement of
the sap in the 18th and even in the 19th century, and at all events
they prove that to speak of Malpighi as a defender of the circulation
of the sap in Major’s sense, as was often done in later times, was an
entire misunderstanding of his views.
Malpighi published his theory in a brief and connected form in 1671; it
appeared again further worked out in detail in the fuller edition of
the Phytotomy in 1674; he attributed a special value to his discovery,
that plants require air to breathe as much as animals, and that the
vessels of the wood answer in function to the tracheae in insects and
to the lungs in other animals; he recurs also several times to the
importance of leaves as organs for the elaboration of the food.
If we compare Malpighi’s theory of the nutrition of plants with the
views of his predecessors, we cannot help seeing, that it was an
entirely new creation, in which Aristotelian doctrines had no share.
If his successors had apprehended the important and essential points
in his doctrine and had striven by experimenting on living plants
to support and illustrate them by new facts, we should have been
spared many erroneous notions which established themselves in the
theory, and made it a perfect chaos of misconceptions. That particular
misconception, which we have already mentioned more than once,
namely, that Malpighi, like Major and Perrault after him, assumed a
continuous circulation of the juices of the plant, necessarily involved
an incorrect idea of the function of the leaves; that function was
by many later writers either quite neglected, or sought for chiefly
in transpiration, the chemical activity of the leaves being quite
overlooked.
Malpighi’s theory can hardly be said to take into consideration the
chemical nature of the food of plants; it is chiefly occupied with the
relation of the organs to the main points in the nutritive process;
its foundations are for the most part laid in the anatomy of the
plant. Grew, who in all essential points adopted Malpighi’s views,
but without doing much to advance them by his lengthy discussions on
particular questions, made some attempt to extend the knowledge of
the chemistry of the subject; but his notions were entirely borrowed
from the corpuscular theory of Descartes, and he may be said to have
constructed his own chemical processes; the consequence was that he
usually overlooked the points that were of fundamental importance, and
brought nothing to light that could assist the further development of
the theory of nutrition. But there is another writer, whose name is in
the present day known to few in the history of vegetable physiology,
but whose ideas on the chemistry of plants are of great interest. This
writer is MARIOTTE[120], the discoverer of the well-known law of gases,
one of the greatest physicists of the latter half of the 17th century,
who also enriched the physiology of the human body with some valuable
discoveries. We have a tolerably copious treatise of Mariotte’s in the
form of a letter to a M. Lantin in the year 1679, to be found in the
‘Œuvres de Mariotte,’ Leyden, 1717, under the title, ‘Sur le sujet
des plantes.’ It is highly instructive to gather from this letter the
ideas of one of the most famous and ablest of the natural philosophers
of that day on chemical processes and conditions in the nutrition of
plants, a few years after the appearance of Malpighi’s great work and
about the time that Grew’s Phytotomy was being published. It is to be
expected that Mariotte should give but an incidental and superficial
attention to the more delicate structure of plants; but we are
compensated for this by his making us acquainted with everything
fundamentally important and new which could at that time be said on
the chemistry of the food of plants. Speaking of the ‘elements’ or
‘principles’ of plants, Mariotte propounds three hypotheses. The first
is, that there are many immediate principles (principes grossiers et
visibles, evidently what we should call proximate constituents) in
plants, such as water, sulphur or oil, common salt, nitre, volatile
salt or ammonia, certain earths, etc.; and that each of these immediate
constituents is a compound of three or four more simple principles,
which have united together into one body; nitre for instance has its
‘phlegma’ or tasteless water, its ‘spiritus,’ its fixed salt, and other
things; common salt in the same way has the like constituents, and it
may be assumed with much probability, that these more simple principles
also are compounds of parts that differ among themselves, but are too
small to be distinguished by any artificial means as to figure or any
other characters. Having shown how certain principles unite together,
he goes on to say, that he is unwilling to ascribe to them any sort
of consciousness (connaissance) by which they seek to unite together;
but he thinks that they are endowed with a natural disposition to
move towards one another, and to unite closely as soon as they touch
one another; though it is very difficult to define the nature of this
disposition, it is enough to know that there are many instances of such
movements to be found in nature; thus heavy bodies move towards the
centre of the earth, and iron to the magnet; nor are these movements
more difficult to conceive, than that of the planets in their courses
or of the sun round its axis, or that of the heart in a living animal.
With this first hypothesis Mariotte places himself, in opposition to
the Aristotelian doctrine with its entelechies and final causes which
prevailed at that time among botanists and physiologists, upon the
firm ground of modern science with its atoms, and its assumption of
necessarily active forces of attraction.
Mariotte’s second hypothesis more specially concerns the chemical
nature of plants; he supposes that several of his principes grossiers
are contained in every plant, and he endeavours first to explain their
source; the motes in the air, he says, which when burnt by lightning
smell of sulphur, are carried by rain into the earth, and parts of
them are taken up into the plant. Moreover distillation in all plants
produces a water, which the chemists call phlegma, and also acids and
ammonia, and if the residuum is burnt there remains an ash, from which
we obtain an earth which is without taste and insoluble in water,
and fixed salts; these salts differ from one another according as
they are mixed with more or less acid and ammoniacal spirit or other
unknown principles, which the fire could not volatilise. It is not to
be wondered at that these principles are found in plants, since they
derive their food from the earth which contains them. We see how great
has been the advance since the time when Van Helmont believed that he
had proved by his experiment, that all the materials in plants come
from pure water.
It remained to confront one view of the source of the substances in
plants, which was also drawn from the treasure-house of Aristotelian
conceptions, and was still in vogue. It was supposed that the very
materials of which the plant is composed were contained in their own
form in the earth, and had only to be taken up by the roots. Aristotle
had himself said: ‘Everything feeds on that of which it consists, and
everything feeds on more than one thing; whatever appears to feed only
on one thing, as the plant on water, feeds on more than one thing, for
earth in the case of the plant is mixed with the water; therefore the
country-people water plants with mixtures of things.’ This passage
might leave some doubt about Aristotle’s view, if we did not find the
following: ‘As many savours as there are in the rinds of fruits, so
many it is plain prevail also in the earth. Therefore also many of
the old philosophers said, that the water is of as many kinds as the
ground through which it runs[121].’ These passages taken with those
quoted above show that Aristotle made the substances required for the
growth of plants reach them from the earth ready elaborated, as has
been before observed; and this view, still maintained in Mariotte’s
time, may yet be met with among those who are ignorant of physiology.
It is interesting then to see, how vigorously Mariotte exposes the
incorrectness and absurdity of this idea, though he has no new
discovery to help him. In his third hypothesis he maintains, that
the salts, earths, oils, and other things, which different species
of plants yield by distillation, are always the same, and that the
differences are due entirely to the way in which these principes
grossiers and their simplest parts are united together or separated,
and he proves it thus: If a bonchretien pear is grafted on a wild one,
the same sap, which in the wild plant produces indifferent pears,
produces good and well-flavoured pears on the graft; and if this
graft has a scion from the wild pear again grafted on it, the latter
will bear indifferent fruit. This shows that the same sap in the stem
assumes different qualities in each graft. But still more forcible is
his proof of the fact, that plants do not take their substance direct
from the earth, but produce it themselves by chemical processes. Take
a pot, he says, with seven to eight pounds of earth and grow in it any
plant you like; the plant will find in this earth and in the rain-water
which has fallen on it all the principles of which it is composed in
its mature state. You may put three or four thousand different kinds of
plants in this earth; if the salts, oils, earths were different in each
species of plant, all these principles must be contained in the small
quantity of earth and rain-water which falls upon it in the course of
three or four months, which is impossible; for each of these plants
would yield in the mature state a dram of fixed salt at least and two
drams of earth, and all these principles together with those which are
mixed with the water would weigh at least from two to three ounces, and
this multiplied by four thousand, the number of the species of plants,
would give a weight of five hundred pounds.
These arguments like those of Jung, and in the main also those of
Malpighi, rested on facts which were on the whole as well known in
ancient times as in the 17th century; but no one had before given heed
to considerations, which were in themselves quite sufficient to do
away with the Aristotelian teaching on the subject of the nutrition of
plants.
In the second part of his letter Mariotte discusses the phenomena of
vegetation which depend on nutrition; he compares the endosperm in the
seed with the yolk of the egg in animals, and the entrance of the water
into the roots with its rising in capillary tubes; he takes the milky
juice to be the nutrient sap and compares it with arterial blood, the
other watery juices answering to venous blood. He says something quite
new about the pressure of the sap; he notices the high pressure at
which the sap stands in plants, and concludes from it that there must
be contrivances in them, which allow of the ingress of the water but
not of its egress. The existence of the pressure is well demonstrated
by the outflow from plants which contain milky juice when they are
wounded, and is compared with the pressure on the blood in the veins.
Equally striking is his further conclusion, that the pressure of the
sap expands the roots, branches, and leaves, and so contributes to
their growth. The sap, he adds, would not be able to remain at this
pressure, if it did not enter by pores, which forbid its return.
In these remarks lay the first germs of speculation on the growth
of plants, such as we shall meet with in Hales also in a somewhat
different form, but in the backward state in which phytotomy then was
they could not at present be further developed; we shall recur to them
further on, though in a different connection.
Mariotte concluded that the primary sap finds its way into the plant
through the leaves as well as through the roots from the fact, that if
a branch is taken from a tree, and one of its smaller branches kept in
water, another will remain fresh for some days; the conclusion was not
quite justified, as the future showed. His remarks on the necessity of
sun-light to nutrition, on the ripening of fruit, and other matters,
rests on very imperfect experience and need not be noticed.
The characteristic and the important point in Mariotte’s theory of
nutrition is the marked contrast between his point of view in natural
science and the Aristotelian and scholastic doctrines still widely
diffused, and thus he is led to declare war also against Aristotle’s
vegetable soul. He connects his remarks on this point with a fact which
excites his astonishment, namely that every species of plant reproduces
its properties so exactly; no explanation of this fact, he says, is
gained by the assumption of a vegetable soul, of which no one knows
what it is. He declares as decidedly against the theory of evolution,
also much in vogue in his day. In opposition to the notion that all
future generations are shut up one inside another in the seeds of a
plant, he thinks it much more probable that the seeds only contain the
essential substances, and that their influence on the crude sap brings
about the successive formation of the rest of the constituents of the
plant, a view which we may still allow to be correct. He regards the
whole process of nutrition and life in plants as a play of physical
forces, as the combination and separation of simple substances, but
he believes at the same time that he can prove the commonly received
doctrine of spontaneous generation to be a necessary conclusion from
this view. On this point he went wrong from want of sufficient and
well-sifted experience, for he regarded it as a proof of generatio
spontanea that numerous plants spring up from the soil thrown out from
ditches and swamps that have been laid dry. ‘We may therefore suppose,’
he says, ‘that there are in the air, in the water, and in the earth
an infinite number of minute bodies so fashioned that two or three
uniting together may make the beginning of a plant, and represent the
seed of such a plant, if they find a soil favourable to their growth.
But it is not probable that this little complex body contains already
all the branches, leaves, fruits, and seeds of this plant, and still
less that this seed contains all the branches, leaves, flowers, etc.,
which proceed ad infinitum from the first germination.’ The contrary
he thinks is proved by the fact, that a rose-bush which has lost its
leaves in the winter may produce in the next year nothing but leafy
shoots from its flower buds, which shows that the blossoms were not
previously formed in those buds, and that a similar conclusion is to be
drawn from another fact, that the seeds of one and the same fruit-tree
or of a melon produce descendants that differ from one another by
variation; here we have an argument against the theory of evolution
much more to the purpose than the greater part of those which were
alleged against it before Koelreuter obtained his hybrids.
Other prejudices also of his day were opposed by Mariotte, and on
good grounds; the medicinal effects, commonly known as the ‘virtutes’
of plants, played an important part in the botany, and still more in
the medicine and chemistry of that time. He rejects the old theory of
heat and cold, moisture and dryness, things supposed to be essentially
immanent qualities of the substance of plants and used to explain
their medicinal effects, and pointing to the fact, that poisonous
plants grow in the same soil as harmless ones and side by side with
them, he concludes, as he had before concluded, that different plants
do not derive their peculiar constituents immediately from the soil,
but that they form them themselves by separation and combination of
the common principles. Finally he declared against one of the grossest
errors which had come down from the previous century, the ‘signatura
plantarum,’ which supposed that the medicinal properties of plants
could be deduced from their external features, and especially from
resemblances between their organs and the organs of the human body.
Mariotte insists that the medicinal properties of plants are to be
ascertained by trying them on sick people.
Mariotte’s letter, the most important parts of which have here been
given, presents us with a lively picture of the views which prevailed
in the second half of the 17th century respecting the life of plants;
it shows at the same time how an eminent investigator of nature,
adopting the principles of a more modern philosophy and knowing how
to make a skilful use of the facts that were known to him, was led
to oppose antiquated error, the result of prepossessions and want of
reflection. If we combine the views of Malpighi on the internal economy
of the plant, derived chiefly from its anatomy, with the chemical and
physical disquisitions of Mariotte, we have an entirely new theory of
the nutrition of plants, not only antagonistic to the Aristotelian
doctrine, but distinguished from it by a much greater wealth of ideas
and by more sagacious combinations.
These two men had in truth discovered all the principles of vegetable
life and nutrition, which could have been discovered in the existing
condition of phytotomy and chemistry; Mariotte especially had succeeded
in applying the very best that was to be obtained from the uncertain
chemical knowledge of his day to the explanation of the phenomena of
vegetation. Chemistry was at that time beginning to set herself free
from the notions of the medical science, the iatro-chemistry of a
former age, only to throw herself into the arms of the theory of the
phlogiston; and how little she could contribute to the explanation of
the processes of nutrition in plants, how little the methods then in
use were adapted to the examination of organised bodies, may be learnt
from a little book published in 1676 and again in 1679, ‘Mémoires pour
servir à l’histoire des plantes,’ which appeared indeed in Dodart’s
name, but which was compiled and approved by the body of members of
the Academy of Paris. It contains no results of investigation, but
a detailed scheme for researches into botanical science, and more
particularly into the chemical part of it. There we read, that plants
must be burnt slowly, in order that the destroying and transmuting
power of the fire may have less effect; the ‘virtutes plantarum’ play
an important part in the chemical examination of plants, and blood
was mixed with their juices, in order to discover their properties.
A writer named Dedu in a treatise, ‘De l’âme des plantes’ (1685)
derived the generation and growth of plants from the fermentation and
effervescence of the acids in combination with the alkalies, as Kurt
Sprengel informs us. It is by comparison with these and similar notions
that we recognise the full superiority of the utterances of Malpighi
and Mariotte respecting the nutrition of plants, and their sagacity
is still further shown by the fact, that there are some things which
they forebore to say, evidently because they thought that they were not
clearly proved.
The views of Malpighi and Mariotte on the nutrition of plants were
respected and often quoted by their contemporaries and immediate
successors; but as has happened in other cases unfortunately up to
recent times, much that was fundamentally important and significant
in them was neglected from the first for comparatively unimportant
matters, and the views of these clear thinkers were so mixed up with
indistinct ideas and actual misconceptions, that no real advance was
made, though a variety of new facts were from time to time brought to
light. It has been already noticed that Malpighi’s correct idea of
the connection of the leaves with the nutrition of the plant was at
a later time commonly supposed to be equivalent to Major’s theory of
circulation, and since the latter was for various reasons considered to
be incorrect, it was thought that Malpighi’s view was dismissed with
it. Yet even Major’s theory deserved the preference over the views of
those who assumed only an ascent of the sap in the wood, because it
at least attempted to account for certain phenomena of growth. It
found a new supporter in 1680 in the person of Claude Perrault, who
does not however appear[122] to have added anything essentially new
to Malpighi’s conclusive arguments for a returning sap. Nor did his
opponent Magnol in his very weak treatise published in 1709 succeed
in saying anything that will bear examination against the theory of
circulation, which he too ascribed to Malpighi.
Among the phenomena of vegetation in woody plants, there is scarcely
one so striking as the outflow of watery sap from wounded vines and
from some tree-stems in the spring. This phenomenon, like the outflow
of milky juice, gum, resin and the like, could not fail to be regarded
with lively interest by those who occupied themselves with vegetable
physiology in the 17th century. Even supposing the movements of water
in the wood and of the milky and other juices in their passages not
to be necessary accompaniments of the nutrition of plants, yet it was
natural that the physiologists of the 17th century should see in them
striking proofs of that movement of the sap which is connected with
nutrition, and should therefore make them a subject of study. It might
also seem to them that the problem in question was easy to solve,
for it was not till long after that it came to be understood that
these movements are in reality one of the most difficult questions of
vegetable physiology. We discover the interest taken in these matters
from a series of communications in the form of letters from Dr. Tonge,
Francis Willoughby, and especially from Dr. Martin Lister, to be found
in the Philosophical Transactions for 1670[123]. The phenomenon to
which these men chiefly directed their attention was just the one best
calculated to lead to misconceptions respecting the movements of water
in woody plants, namely that which is known as the bleeding of the
wood in winter, and which depends on entirely different causes from
those which produce the weeping of the vine and other woody plants in
spring; but the two things were supposed to be identical, and hence
arose an unfortunate confusion of ideas. Lister indeed showed that it
is possible to force water out of the wood of a portion of a branch
cut from a tree in winter time by warming it artificially, and then
to cause the water to be sucked in again by cooling it; but it was
reserved for a modern physiologist to prove that this phenomenon has
nothing to do with the bleeding of cut stems from root-pressure, and
cannot be used to explain it.
John Ray, who gave a clear and intelligent summary of all that was
known respecting the nutrition of plants in the first volume of his
‘Historia plantarum’ (1693), also communicated some experiments made
by himself on the movements of water in the wood. He follows Grew’s
nomenclature, who called the ascending sap in the wood lymph and the
woody fibres therefore lymph-vessels, and notices particularly that
the lymph especially in spring cannot be distinguished in taste or in
consistence from common water. He agrees with Grew that in spring the
lymph fills the true vascular tubes of the wood and oozes from them
in cross sections, while in summer these are filled with air, and the
lymph at that time, when there is strong transpiration in woody plants,
ascends only in the lymph-vessels, that is in the fibrous elements of
the wood and the bast. By suitable incisions Ray proved that the lymph
can also move laterally in the wood; and by causing water to filter in
opposite directions through pieces of a branch cut off at both ends, he
refuted those who thought that the cavities of the wood and especially
the vessels were furnished with valves to hinder the return of the
lymph. But his knowledge of the mechanical causes of the movement of
water in the wood was not very great.
Some years elapsed before Hales’ labours added materially to the
progress which had been already made in the study of these processes
in vegetation. His important services to vegetable physiology close
our present period, but before we pass on to them, we must first
notice a few less important writers. The pages of Woodward and Beale
on transpiration and the absorption of water are not very valuable
contributions to the theory of nutrition. The fact stated by Woodward,
that a Mentha growing in water took up and discharged by evaporation
through the leaves forty-six times as much water as it retained in
itself, was perhaps the most important of all that he discovered, but
his own conclusions from it were of no value.
None of Malpighi’s doctrines had from the first excited so much
attention as the one which makes the air which is necessary for the
respiration of the plant circulate in the spiral vessels of the wood,
as it does in the tracheae in insects; while Grew and Ray after him
agreed with Malpighi in the main, his countryman Sbaraglia in 1704
ventured even to deny the existence of such vessels, and before long
phytotomy was fallen into such a state of decadence that the question,
whether there were any vessels, or as they were then called spiral
vessels, at all, was repeatedly affirmed and as often denied again,
and ultimately it was thought better in the interest of physiological
questions to take counsel of experiment rather than of the microscope.
Thus in 1715 Nieuwentyt endeavoured with the help of the air-pump to
make the air contained in the vessels issue in a visible form under
a fluid. Here we again encounter the philosopher Christian Wolff as
a zealous representative of vegetable physiology in Germany; in the
third part of his work, ‘Allerhand nützliche Versuche,’ 1721, among
other experiments he mentions some which confirmed the presence of air
in plants; the question was more interesting, in the state in which
physics and chemistry then were, than that of the anatomical character
of the air-conducting organs. Wolff submitted leaves lying in water
containing no air to the vacuum of the air-pump, and saw air-bubbles
issue, especially on the under side; but when he allowed the
atmospheric pressure to come into play again the leaves became filled
with water, and a piece of fir-wood treated in a similar manner sank
after the infiltration. In similar experiments with apricots air issued
from the rind and especially from the stalk. Wolff’s pupil Thümmig
described similar experiments in his ‘Gründliche Erläuterung der
merkwürdigsten Begebenheiten in der Natur,’ 1723, and both continued in
this question, as in all their physiological and phytotomical views,
faithful adherents of Malpighi, as it was wisest then to be. We must
linger a moment longer over Christian Wolff, because he published a
few years later a general view of the nutrition of plants in a popular
form. Wolff’s services in the dissemination of natural science in
Germany seem not to have been as highly appreciated up to the present
time as they deserve to be; his various works on natural science,
some of which took a wide range and were partly founded on his own
observations, were full of matter and for his time very instructive;
they contributed moreover to introduce more liberal habits of thought
at a time when gross superstitions, such as that of palingenesia,
reigned even among men who published scientific treatises in the German
Academy of Sciences (the ‘Acta of the Leopoldina).’ If Wolff’s own
scientific researches show more good will than skill, yet he had an
advantage over many others in a really philosophical training, a habit
of abstract thought which enabled him to fix with certainty on what
was fundamentally important in the observations of others, and thus
to expound the scientific knowledge of his day from higher points of
view. For this reason his work which appeared in 1723, ‘Vernünftige
Gedanken von den Wirkungen der Natur,’ deserves recognition. It is a
work of the kind which would now be called a ‘Kosmos,’ and treats of
the physical qualities of bodies generally, of the heavenly bodies
and specially of our own planet, of meteorology, physical geography,
and lastly of minerals, plants, animals and men. In accordance with
his chief object, general instruction, it is written in German and
in a good homely style, and contains the best information that was
at that time to be obtained on scientific subjects; among these he
gives an account of the processes of nutrition in plants, in which he
made careful and intelligent use of all that had been written on the
subject, bringing together all the serviceable material which he could
gather from Malpighi, Grew, Leeuwenhoek, Van Helmont, Mariotte and
others into a connected system, and occasionally introducing pertinent
critical remarks. If we consider the state of scientific literature in
Germany in the first years of the 18th century, we shall be inclined
to assign as great merit to comprehensive text-books of this popular
character as to new investigations and minor discoveries. Wolff’s
chapter on nutrition has however a special interest for us, because it
contains several observations of value which were lost sight of after
his time. These refer chiefly to the chemistry of nutrition and touch
many problems which were not solved before our time; for instance,
the statement that it is a well-known fact that the earth loses its
fruitfulness, if much is grown on it; that it requires much to feed
it, and must be manured with dung or ashes; in these few words we have
the questions of the exhaustion of the soil, and the restitution of
the substances taken from it by the crop, brought into notice by Wolff
at this early period. ‘It should be particularly noted,’ continues
Wolff, ‘how fruitful nitre makes the soil; Vallemont has praised
the usefulness of nitre, and has mentioned other things which have
a like operation by reason of their saline and oily particles, such
as horn from the horns and hoofs of animals; dung likewise contains
saline and oily particles, which are present in the ash also, and we
see therefore that such particles should not be wanting, if a plant
is to be fed from water. The seed also, which supplies the first food
of the plant, shows the same thing, for there are none which do not
contain oil and salt, and there are many from which the oil may be
squeezed out; and oil and salt are found in all plants if they are
examined chemically.’ He insists on the correctness of the view taken
by Malpighi and Mariotte, that the constituents of the food must be
chemically altered in the plant. Since every plant, he says, has its
own particular salt and its own particular oil, we must readily allow
that these are produced in the plant and not introduced into it. But at
the same time since plants cannot grow where the soil does not supply
them with saline and especially with nitrous particles, it is from
these that the salts and oils in the plant must be produced, and the
water also changed into a nutritious juice. Further on he alludes to
the saline, nitrous and oily particles which float in the air, and says
that daily experience shows that most of the substance of putrefying
bodies passes into the air, and that if we admit light through a
narrow opening into a dark place, we can see a great number of little
particles of dust floating about; water also readily takes up salt and
earth, and mineral springs show that metallic particles are mixed with
it. Therefore there is no reason to doubt that rain-water also contains
a variety of matters which it conveys to the plant. Alluding once more
to the chemical changes in the constituents of the food which must be
supposed to take place in the plant, he connects the subject with some
remarks on the organs of plants, in which he closely follows Malpighi;
he says that these changes cannot take place in tubes, because the
sap merely rises or falls in them; we can only therefore suppose that
it is in the spongy substance (the cellular tissue) that the nutrient
sap is elaborated, and accordingly the vesicles or utriculi are a kind
of stomach; but the change in the water can only be this, that the
particles of various substances which are in rain-water are separated
from it and united together in some special manner, and this cannot be
effected without special movements. But his ideas on these movements
in the sap are somewhat obscure. He employs the expansion of the air
and the capillarity of the woody tubes as his moving forces. He agrees
decidedly with those who postulated a returning sap as well as an
ascending crude sap, but he appeals in this matter to Major, Perrault,
and Mariotte, and not to Malpighi; yet like Malpighi he notices the
growth of trees set upside down as a proof that the juices can move
in opposite directions in the conducting organs, and with Mariotte he
ascribes the enlargement of growing organs to the expanding power of
the juices which force their way into them.
But these well-meant efforts on the part of Christian Wolff, and indeed
all that was done from Malpighi and Mariotte to Ingen-Houss to advance
the knowledge of the nutrition of plants, was thrown into the shade by
the brilliant investigations of STEPHEN HALES[124] in whom we see once
more the genius of discovery and the sound original reasoning powers of
the great explorers of nature in Newton’s age. His ‘Statical Essays,’
first published in 1727, reappeared in two new editions in English, and
afterwards in French, Italian and German translations; in the last with
a preface by Christian Wolff. This was the first work devoted to a more
complete account of the nutrition of plants and of the movements of the
sap in them, and while it noticed what had been already written on the
subject, it was chiefly composed of the author’s own investigations.
An abundance of new experiments and observations, measurements and
calculations combine to form a living picture of the whole subject.
Malpighi endeavoured to discover the physiological functions of organs
by the aid of analogies and a reference to their structure; Mariotte
discerned the main features of the connection between plants and
their environment by combining together physical and chemical facts;
Hales may be said to have made his plants themselves speak; by means
of cleverly contrived and skilfully managed experiments he compelled
them to disclose the forces that were at work in them by effects made
apparent to the eye, and thus to show that forces of a very peculiar
kind are in constant activity in the quiet and apparently passive
organs of vegetation. Penetrated with the spirit of Newton’s age,
which notwithstanding its strictly teleological and even theological
conception of nature did endeavour to explain all the phenomena of life
mechanically by the attraction and repulsion of material particles,
Hales was not content with giving a clear idea of the phenomena of
vegetation, but sought to trace them back to mechanico-physical laws as
then understood. He infused life into the empirical materials which he
collected by means of ingenious reflections, which brought individual
facts into connection with more general considerations. Such a book
necessarily attracted great attention, and for us it is a source of
much valuable instruction on matters of detail, though we now gather up
the phenomena of vegetation into a somewhat differently connected whole.
His investigations into transpiration and the movement of water in
the wood were greeted with the warmest approbation. He measured the
quantity of water sucked in by the roots and given off by the leaves,
compared this with the supply of moisture contained in the earth,
and endeavoured to calculate the rapidity with which the water rises
in the stem, and to compare it with the rapidity of its entrance
into the roots and its exit by the leaves. The experiments, by which
he showed the force of suction in wood and roots, and that of the
root-pressure in the case of the bleeding vine, were particularly
striking and instructive. His measurements and the figures, on which
he founded his calculations, were not so exact as they were often at a
later time supposed to be, but he was himself satisfied with obtaining
round, approximative numbers; these under given circumstances supplied
a sufficient basis for propositions which were new and afforded a
certain amount of insight into the economy of the plant. This mode of
proceeding showed his understanding; for the case of living bodies
is different from that of metals and gases; in these we seek for
constants which can then be inserted in general formulae, and to which
therefore the nicest accuracy is applied; but in plants we have to
deal with individual cases, and it is from a right interpretation of
the measurements taken from them that we can arrive at general laws of
vegetation.
To show that the forces of suction and pressure which operate in
plants are not something sui generis, but prevail also in dead matter,
in other words that they are an example of the general attraction of
matter, a subject of particular interest at that time, Hales observed
the absorption of water by substances with fine pores; and measured
the force employed. These processes he compared with the force which
swelling peas exert on the obstacles which they encounter, and thus
obtained a more correct idea of the forces concerned in the movement of
water in the plant than that given by the capillarity of glass-tubes,
which Mariotte and Ray had employed to illustrate them.
Hales failed to appreciate the value of Malpighi’s observations on
the function of leaves, and was induced by the copiousness of the
evaporation of water from their surfaces to overrate the physiological
importance of that process; hence he saw in leaves chiefly organs of
transpiration, which raise the sap by suction from the roots through
the stem. In accordance with this view he denied the existence of a
descending sap in the bark, and only admitted that the ascending sap
in the wood might possibly sink in the night in consequence of the
lowering of the temperature, like the quicksilver in a thermometer, and
that so far there might be a return-movement. This was the weak point
in Hales’ system.
One of his most important discoveries has generally been overlooked
even in modern times, probably because it was entirely neglected by
his successors in the 18th century; he was the first who proved,
that air co-operates in the building up the body of the plant, in
the formation of its solid substance, and that gaseous constituents
contribute largely to the nourishment of the plant; consequently
that neither water, nor the substances which it carries with it from
the earth, alone supply the material of which plants are composed,
as had been generally imagined. He showed also with the aid of the
air-pump, and better than Nieuwentyt and Wolff, that air enters the
plant not only through the leaves but also through apertures in the
rind, and circulates in the cavities of the wood. He then connected
this with the fact which he had confirmed by numerous experiments,
that large quantities of ‘air’ are obtained from vegetable substance
by fermentation and dry distillation; the air thus set free by
fermentation and heat must in his opinion be condensed and changed to
a solid condition during the period of vegetation. He says in chap. 7,
that we find by chemical analysis (dry distillation) of vegetables,
that their substance is composed of sulphur, volatile salt, water
and earth; these principles are all endowed with mutual power of
attraction (of their parts). But air also enters into the composition
of the plant, and this in its solid state is powerfully attractive,
but in an elastic condition has the highest powers of repulsion. It is
on infinitely various combinations, actions, and reactions of these
principles that all activity in animal and vegetable bodies depends. In
nutrition the sum of the forces of attraction is greater than that of
the forces of repulsion, and thus the viscid ductile parts are first
produced, and then by evaporation of the water the harder parts.
But if the latter again absorb water, and the forces of repulsion
consequently gain the preponderance, then the consistence of the
vegetable parts is dissolved, and this decomposition restores to them
the power of forming new vegetable products; therefore the stock of
nutritive substance in nature can never be exhausted; this stock is the
same in animals and plants, and is fitted by a small change of texture
to feed the one or the other.
He goes on to say, that it results from his experiments, that leaves
are very useful for the nourishing of the plant, inasmuch as they
draw up the food from the earth; but they seem also to be adapted to
other noble and important services; they remove the superfluous water
by evaporation, retaining the parts of it that are nutritious, while
they also absorb salt, nitre, and the like substances, and dew, and
rain; and since, like Newton, he regarded light as a substance, he
concludes by asking: ‘may not light, which makes its way into the outer
surfaces of leaves and flowers, contribute much to the refining of the
substances in the plant?’
It might be gathered from these expressions that Hales attributed
importance for purposes of nutrition only to the substances suspended
in the air; but this was not the case; for we read in the 6th
chapter, that he had proved by experiment that a quantity of true
permanently elastic air is obtained from vegetable and animal bodies by
fermentation and dissolution (dry distillation); the air is to a great
extent immediately and firmly incorporated with the substance of these
bodies, and it follows therefore that a large quantity of elastic air
must be constantly used in forming them.
But Hales not only regards the air as a nourishing substance, but
he sees also in its elasticity, which counteracts the attraction of
other substances, the origin of the force which maintains the internal
movements in the plant. He says that if all matter were endowed only
with forces of attraction, all nature would at once contract into an
inactive mass; it was therefore absolutely necessary in order to set
in movement and animate this huge mass of attracting matter, that a
sufficient quantity of strongly repellent and elastic matter should be
mixed with it; and since a large portion of these elastic particles
are constantly changing to a solid condition through the attraction of
the other parts, they must be endowed with the power of again assuming
their elastic condition, when they are set free from the attracting
mass. Thus the formation and dissolution of animal and vegetable bodies
go on in constant succession. Air is therefore very important to the
production and growth of animals and plants in two ways; it invigorates
their juices while it is in the elastic state, and contributes much to
the firm union of the constituent parts, when it has become fixed.
We see what good use Hales could make of the small stock of ideas in
physics and chemistry at his disposal, and that he succeeded with their
help in rising to a point of view, from which he was able to form some
idea of the phenomena of vegetation in their most important relations
to the rest of nature, and in their inner course and connection. But
his successors did not comprehend the fundamental importance of these
considerations, and made no use of the pregnant idea, that a much
larger part of the substance of plants comes from the air and not from
the water or the soil; they were for ever wondering that so little is
furnished by the soil to the plant, as Van Helmont had shown, though
they did not confess to supposing that the water was changed into the
substance of the plant, as he had imagined. Thus physiologists lost
sight of the principle, which might long before the time of Ingen-Houss
have sufficiently explained the most important of all the relations of
the plant to the outer world, namely that it derives its food from the
constituents of the atmosphere, and so neglected further experimental
enquiry into the matter; they quoted and repeated Hales’ experiments
and observations again and again, but forgot that which in his mind
bound all the separate facts together.
Hales is the last of the great naturalists who laid the foundations of
vegetable physiology. Strange as some of their ideas may seem to us,
yet these observers were the first who gained any deep insight into the
hidden machinery of vegetable life, and handed down to us a knowledge
both of individual facts and of their most important relations. If we
compare what was known before Malpighi’s time with the contents of
Hales’ book, we shall be astonished at the rapid advance made in less
than sixty years, while scarcely anything had been contributed to the
subject in the period between Aristotle and Malpighi.
3. FRUITLESS ATTEMPTS TO EXPLAIN THE MOVEMENT OF THE SAP IN PLANTS.
1730-1780.
If those, who studied the nutrition of plants and especially the
movement of their sap in the period between Hales and Ingen-Houss, had
kept a firm hold on Malpighi’s view, that the nutritive substances are
elaborated in the leaves, and had combined it with Hales’ idea, that
plants derive a large portion of their substance from the air, they
would have had a principle to guide them in their investigations into
the movement of the sap; and by experimenting on living plants they
might have succeeded in giving a more definite expression to these
ideas, even though chemistry and physics supplied during that time no
new aids. We have said already that such was not the course of events;
physiologists confined their attention to the obvious phenomena of
vegetation, and trusted in so doing to gain a firmer footing, but in
this they never got beyond a commonplace and unreflecting empiricism,
because their observation was without an object, and their conclusions
without a principle. They wandered from the right direction, as always
happens when observation is not guided by a well-considered hypothesis;
and their conceptions were rendered more obscure by their imperfect
acquaintance with one of the most important aids to understanding
the movement of the sap, namely the structure of the more delicate
parts of the plant, the knowledge of which had not advanced since the
days of Malpighi and Grew. Since most of them made no phytotomical
investigations of their own, and only partially understood the
descriptions of those writers, they had to be content with misty and
often quite inaccurate ideas of the inner structure of wood and bark,
and yet expected to obtain an insight into the movement of the sap
in them. In reading the writings of Malpighi, Grew, Mariotte, Hales
and even Wolff, notwithstanding many mistakes in details we find a
pleasure in the connected reasoning, and in the sagacity which knew how
to distinguish between what was important and what was not; whereas
the observers, whom we have now to mention, give us only isolated
statements, nor have we the satisfaction of feeling that we are
conversing with men of superior understanding.
We may pass over the unimportant writings of Friedrich Walther (1740),
Anton Wilhelm Platz (1751) and Rudolph Böhmer (1753), as merely barren
exercises; but some notice should be taken of those of De la Baisse
and Reichel, since these authors at least endeavoured to bring to
light something new. But the method which they employed of making
living plants suck up coloured fluids was calculated to give rise to
serious errors both at the time and afterwards. Magnol had mentioned
experiments of the kind in 1709, and the Jesuit father Sarrabat, known
by the name of DE LA BAISSE, occupied himself with them and described
them in a treatise, ‘Sur la circulation de la sêve des plantes,’ 1733,
which received a prize from the academy of Bordeaux[125]. He set the
roots of different plants in the red juice of the fruit of Phytolacca,
and found that in two or three days the whole of the bark of the roots
and especially the tips of the root-fibres were coloured red inside.
It was a natural conclusion at that time, that it was these parts which
chiefly absorbed the red colouring matter, and in fact this opinion was
maintained till quite recent times, and it was on such results that
Pyrame de Candolle founded his theory of the spongioles of the root,
which is still accepted in France. At present it is known, that the
bark and especially the youngest tips of the fibres of the root are not
coloured under these circumstances, until they have been first poisoned
and killed by the colouring matter; these experiments therefore,
which have been frequently repeated since De la Baisse’s time, prove
nothing respecting the action of living roots, but they were from the
first the cause of a pernicious error in vegetable physiology, which
as we shall see gave rise to others also. One result however of De la
Baisse’s experiments was less misleading; he placed the cut ends of
branches of woody plants in the coloured fluid, and found that not only
the general body of the wood, but the woody bundles which pass from
it into the leaves and parts of the flowers, were coloured red, while
the succulent tissue of the bark and leaves remained uncoloured. It
appeared therefore that the red juice passed only through the wood,
and a somewhat bold analogy might lead to the further conclusion that
this is true also of the nutrient substances dissolved in the watery
sap; but the view so stated is not at present considered to be correct,
and that the sap which ascends from the roots to the leaves, the
water especially, is conveyed through the wood only, and not through
the rind, had been already sufficiently proved by the experiments of
Hales and others. The uncritical treatment of experiments of this kind
by GEORG CHRISTIAN REICHEL[126] afterwards led to new errors, though
his dissertation, ‘De vasis plantarum spiralibus,’ shows to advantage
by the side of similar productions of the day owing to its careful
notices of the literature, and the author’s original researches in
phytotomy. Reichel was not satisfied with the arguments of Malpighi,
Nieuwentyt, Wolff, Thümmig and Hales for the view that the vessels of
the wood contain air. He observed quite correctly, that if branches
are cut off from woody and herbaceous plants and the cut surfaces
are placed in red decoction of brazil-wood, the red colouring matter
spreads through all the vascular bundles, even those of the flowers
and fruit; but on examination with the microscope he found the red
fluid to some extent in the cavities of the vessels, and hastily
concluded that they too in the natural condition convey sap and not
air. His description and his drawing show however, that only some
vessels had received any of the red fluid and that none of these were
filled with it. Reichel and the many who repeated his statements
forgot to ask whether the vessels had contained air or fluid before
the experiment, or whether the result would have been the same, if
plants with uninjured and living roots had absorbed the coloured fluid,
and no divided vessels had therefore come in contact with it. There
was no reason why observers of that day should not have been alive to
the simple consideration, that the vessels of a branch parted from
the stem and placed in a fluid must necessarily show the capillary
action of narrow glass tubes if they are filled with air in their
natural condition, and that in the experiment the transpiration of the
leaves must favour the ascent of the red juice in the cavities of the
vessels, as was to be gathered from other and better experiments made
by Hales. But these obvious reflections were not made; the supposed
results of the experiment were heedlessly accepted, and the unfounded
notion, that vessels are natural sap-conducting organs, was set up
in opposition to the trustworthy decision of Malpighi and Grew, that
they convey air. Thus on the strength of badly interpreted experiments
one of the most important of physiological discoveries was called in
question, and a hundred years later there were persons, who, relying
on the same experiments as Reichel, supposed that the vessels of
the wood convey the ascending sap, a view which made it impossible
from the first to arrive at any real understanding of the movement
of the sap in plants provided with organs of transpiration. But even
the other great discovery which we owe to Malpighi, that leaves are
organs for elaborating the food, was denied by Bonnet, who substituted
for it the utterly false view, that they chiefly serve to absorb
rain-water and dew. BONNET[127], who had previously done good service
to insect-biology, and had discovered the asexual propagation of
aphides, having injured his eyes in these studies, found an agreeable
pastime in a variety of experiments on plants. Much that he did was
unimportant, yet he obtained some results, which could afterwards be
turned to account by more competent persons, for the weakness of his
own judgment is shown even in his more serviceable observations, such
as those on the curvature of growing plants. We notice the same defect
in his observations on the part played by leaves in the nutrition of
the plant. It shows the character of the time that a book like Bonnet’s
‘Recherches sur l’usage des feuilles des plantes,’ a mere accumulation
of undigested facts, should have been generally considered an important
production. He tells us, that his attention was called by Calandrini
to the fact, that the structure of the under side of leaves seems to
show that they were intended to absorb ‘the dew that rises from the
ground’ and introduce it into the plant. Starting from this sensible
suggestion, as he calls it, he proceeded to make a variety of senseless
experiments with leaves, which were cut off from their plants, and
having been smeared over with oil or other hurtful substances were laid
on water, some on their upper some on their under side, the object
being to note the time which they took to perish. It is impossible to
imagine worse-devised experiments on vegetation; for if Bonnet wished
to test Calandrini’s ‘sensible’ conjecture, he ought certainly to have
left the leaves on the living plants and have observed the effect of
the supposed absorption of dew on the vegetation. It is to be observed,
that by rising dew he evidently meant aqueous vapour, for the real dew
descends chiefly on the upper side of the leaf; and what could he have
expected to learn by laying cut leaves on water? how could this prove
that leaves absorb dew? Nevertheless Bonnet came to the conclusion that
the most important function of leaves was to absorb dew, and in order
to make this result agree with Hales’ investigations on transpiration,
he propounded the theory[128], that the sap which rises by day from
the roots into the stem is carried by the woody fibres assisted by
the air-tubes into the under side of the leaves, where there are many
stomata to facilitate its exit (evaporation). At the approach of night,
when the leaves and the air in the air-tubes are no longer under the
influence of heat, the sap returns to the roots; then the under side
of the leaves commences its other function; the dew slowly rising
from the earth strikes against it, condenses upon it, and is detained
there by the fine hairs and by other contrivances (this really takes
place to a much greater extent on the upper side). The fine tubes of
the leaves absorb it at once, (this is evidently not so, since the dew
increases in quantity till sunrise), and conduct it to the branches,
whence it passes into the stem. He thought so highly of this strange
theory, that he believed he found in it a teleological explanation of
the heliotropic and geotropic curvature of leaves and stems, two things
which he did not distinguish, and of the position of leaves on the
stem. Bonnet’s view of the functions of leaves, foolish as it is, is
historically important and therefore required to be noticed, because it
was really accepted during many years in preference to the older and
better ideas, and because it shows how the power of judging of such
matters had fallen off since Malpighi’s time. It appears to have been
the praise lavished on Bonnet by his contemporaries that made later
physiologists, who might have known better, take him for an authority
on the nutrition of plants. His experiments on the growth of plants
in another material than earth are if possible more worthless than
those with cut leaves. Here too the idea was not his own; for hearing
that land-plants had been grown in Berlin in moss instead of earth,
he made numerous experiments of the kind, and found that many plants
grow vigorously in this way, and bloom and bear seed. But the theory
of nutrition gained nothing by these experiments, which were only a
childish amusement. The few pages which Malpighi wrote on the nutrition
of plants are worth more than all Bonnet’s book on the use of leaves;
the former by the help of some simple considerations and conclusions
from analogy really discovered the use of leaves; Bonnet on the faith
of many unmeaning experiments ascribed to them another function than
the true one.
We are unable to pass a much more favourable judgment on the views
respecting the nutrition of plants of another writer, who otherwise
did good service to vegetable physiology, and to whom we shall return
in our last chapter. It is true that DU HAMEL[129], of whom we speak,
was not an investigator of nature, as were Malpighi, Mariotte or
Hales; compared with those great thinkers he was only a compiler, and
a somewhat uncritical one. But he was not a dilettante in science,
like Bonnet; he made the vegetable world the subject of serious and
diligent study, and he endeavoured to turn the results of that study
to practical account. Long familiarity with plants gave him a kind
of instinct for the truth in dealing with them, as is shown in his
observations and experiments, many of which are still instructive;
but he had neither that faculty of combination which can alone bring
a meaning out of experiments and observations in physiological
investigations, nor the power to distinguish between matters of
fundamental and secondary importance. So thinks also his biographer Du
Petit-Thonars.
The merits and the faults here mentioned are combined in an especial
degree in Du Hamel’s most famous work, ‘Physique des arbres,’ which
appeared in two volumes in 1758 and is a text-book of vegetable anatomy
and physiology with numerous plates. His remarks on the nutrition of
plants and the movement of the sap are a lengthy compilation chiefly
from Malpighi, Mariotte and Hales, though he has not succeeded in
appropriating exactly that which is theoretically important or adopting
the most commanding points of view. He introduces the results of his
own experiments into his account, and these are often instructive in
themselves, but are never made use of to establish a definite view with
respect to the connection between the processes of nutrition. He hits
upon the right view only when he is dealing with plain and obvious
matters; for instance, he restores the vessels of the wood to their old
rights, and concludes from experiments, as had been already done in the
17th century, that an elaborated sap moves in the reverse direction in
the rind; so too he perceives that if bulbs, tubers, and roots, with
or without the help of water which they have absorbed, produce shoots
and even flowers, this must be done at the expense of material laid
up in reserve, but he does not turn this fact to any further account.
But he utterly spoilt the best part of his subject; he made the leaves
nothing but pumps that suck up the sap from the roots; he quotes
Malpighi’s better view as a curiosity, and never mentions it again; but
he accepts Bonnet’s unfortunate theory, though he himself adduces many
facts, which make for Malpighi’s interpretation of the leaves. He is
almost more unsuccessful with chemical points in nutrition; he repeats
Mariotte’s statements with regard to the necessity of a chemical change
in the nutrient substances in the plant, and even supplies further
proof of it; but he cannot shake off the Aristotelian dogma, that
the earth like an animal stomach elaborates the food of plants, and
that the roots absorb the elaborated matter like chyle-vessels (II.
pp. 189, 230). He concludes from his own attempts to grow land-plants
without earth and in ordinary water that the latter supplies the plant
with very little matter in solution, but he makes no use of Hales’
statements with regard to the co-operation of the air in the building
up of the plant, and ends by saying (II. p. 204) that he only wished to
prove that the purest and simplest water can supply plants with their
food, which his experiments do not prove. Thus almost all that Du Hamel
says on the nutrition of plants is a mixture of right observations
in detail with wrong conclusions, and reflections which never rise
above the individual facts and give no account of the connection of
the whole. These faults appear in a still higher degree in a later
and almost more comprehensive work, the ‘Traité théorique et pratique
de la végétation’ of Mustel (1781). The further the distance from the
founders of vegetable physiology, the larger were the books that were
written on the subject; but the thread that held the single facts
together became thinner and thinner, till at last it broke. The theory
of nutrition, like a forced plant, needed light that it might recover
strength. This light came with the discoveries of Ingen-Houss, and
with the mighty strides made by chemistry after 1760 in the hands of
Lavoisier.
4. THE MODERN THEORY OF NUTRITION FOUNDED BY INGEN-HOUSS AND THEODORE
DE SAUSSURE. 1779-1804.
The two cardinal points in the doctrine of the nutrition of plants,
namely that the leaves are the organs which elaborate the food, and
that a large part of the substance of the plant is derived from the
atmosphere, were established, as we have seen, by Malpighi and Hales,
and employed by them in framing their theory; it remained to supply a
direct and tangible proof of the fact that the green leaves take up a
constituent of the atmosphere and apply it to purposes of nutrition. It
was evidently the want of such direct proof which caused the successors
of the first great physiologists to overlook the importance of the
propositions thus obtained by deduction, and so to grope their way in
the dark with no principle to guide them.
The discoveries of Priestley, Ingen-Houss and Senebier, and the
quantitative determinations of de Saussure in the years between 1774
and 1804, supplied the proof that the green parts of plants, and the
leaves therefore especially, take up and decompose a constituent of
the air, while they at the same time assimilate the constituents of
water and increase in weight in a corresponding degree; but that this
process only goes on copiously and in the normal way, when small
quantities of mineral matter are introduced at the same time into
the plant through the roots. The discoveries and facts, from which
this doctrine proceeded, were those which overthrew the theory of
the phlogiston, and from which Lavoisier deduced the principles of
modern chemistry; the new theory of the nutrition of plants was indeed
directly due to Lavoisier’s doctrines, and it is necessary therefore
to take at least a hasty glance at the revolution which was effected
in chemistry between 1770 and 1790. It is a well-known fact[130] that
this revolution dates from the discovery of oxygen-gas by Priestley in
1774. Priestley himself was and continued to be a stubborn adherent of
the phlogiston; but his discovery was made by Lavoisier the basis of an
entirely new view of chemical processes. By the combustion of charcoal
and the diamond, Lavoisier proved as early as 1776 that ‘fixed air’
was a compound of carbon and ‘vital air.’ In like manner phosphoric
acid, sulphuric acid and, after a preliminary discovery by Cavendish,
nitric acid also were found to be compounds of phosphorus, sulphur
and nitrogen with vital air; in 1777 Lavoisier showed that fixed air
and water are produced by the combustion of organic substances, and
after establishing within certain limits the quantitative composition
of fixed air, he named it carbonic acid, and the gas which had up
to that time been known as vital air he called oxygen. Cavendish
in 1783 obtained water by the combustion of hydrogen-gas, and then
Lavoisier proved that water is a compound of hydrogen and oxygen. These
discoveries not only did away step by step with the old theory of the
phlogiston, and supplied the principles of modern chemistry, but they
also affected exactly those substances which play the most important
part in the nutrition of plants; every one of these discoveries in
chemistry could at once be turned to account in physiology. In 1779
PRIESTLEY discovered that the green parts of plants occasionally
exhale oxygen, and in the same year Ingen-Houss described some fuller
investigations, which showed that this only takes place under the
influence of light, and that the green parts of plants give off carbon
dioxide in the dark, as those parts which are not green do both in
the light and the dark. A correct interpretation of these facts was
not however possible in 1779; it was not till 1785 that Lavoisier
succeeded in setting himself quite free from the old notions, and
developed his antiphlogistic system into a connected whole. It should
be mentioned that he had discovered in 1777 that the respiration of
animals is a process of oxidation which produces their internal heat,
heat being the product of every form of combustion. This fact was
equally important for vegetable physiology, but it was some time before
it was used to explain the life of plants.
The establishment of the fact, that parts of plants give off oxygen
under certain circumstances, did little or nothing to further the
theory of their nutrition[131]; and that was all that vegetable
physiology owes to Priestley. Ingen-Houss on the other hand determined
the conditions under which oxygen is given off, and further showed that
all parts of plants are constantly giving rise to carbon dioxide; on
these facts rests the modern theory of the nutrition and respiration of
plants, and we must therefore consider that Ingen-Houss was the founder
of that theory. But since we are dealing here with a discovery of more
than ordinary importance, it seems necessary to go more closely into
the details.
A work of Priestley’s appeared in 1779, which was translated
into German in the following year under the title, ‘Versuche und
Beobachtungen über verschiedene Theile der Naturlehre,’ and contained
among other things the writer’s experiments on plants. His way of
managing them was eminently unsuitable, nor did he arrive at any
definite and important result, though he expressed the idea which
had led him to make them clearly enough, where he says, ‘If the air
exhaled by the plant is of better character (richer in oxygen) than
atmospheric air, it follows that the phlogiston of the air is retained
in the plant and used there for its nourishment, while the part which
escapes, being deprived of its phlogiston, necessarily attains a higher
degree of purity.’ After he had ceased his experiments with plants in
1778, he observed that there was a deposit of matter in the water in
some vessels which he had used for them, and that it gave off a very
‘pure air’; a number of further observations taught him that this
air was given off only under the influence of sun-light; Priestley
himself did not suspect that the deposit in question, afterwards known
as Priestley’s matter and found to consist of Algae, was a vegetable
substance.
In the same year (1779) appeared the first book by INGEN-HOUSS[132], in
which the subject was treated at length; it was called, ‘Experiments
on Vegetables, discovering their great power of purifying the common
air in the sunshine and of injuring it in the shade and at night,’
and was at once translated into German, Dutch and French. The title
itself shows that the author had observed more and more correctly
than Priestley. But he did not come to an understanding of the
inner connection of the facts, till Lavoisier completed his new
antiphlogistic theory. He says himself in his essay, ‘On the nutrition
of plants and the fruitfulness of the earth,’ which appeared in 1796,
and was translated into German with an introduction by A. v. Humboldt
in 1798, that when he published his discoveries in 1779, the new system
of chemistry was not yet fully declared, and that without its aid he
had been unable to deduce the true theory from the facts; but that
since the composition of water and air had been discovered, it had
become much easier to explain the phenomena of vegetation. But in order
to establish his priority he says on p. 56, that he had been fortunate
enough to find out the real cause why plants at certain times vitiate
the surrounding air, a cause which neither Priestley nor Scheele had
suspected. He had discovered, he says, in the summer of 1779, that
all vegetables incessantly give out carbonic acid gas, but that the
green leaves and shoots only exhale oxygen in sun-light or clear
daylight. It appears therefore that Ingen-Houss not only discovered the
assimilation of carbon and the true respiration of plants, but also
kept the conditions and the meaning of the two phenomena distinct from
one another. Accordingly he had a clear idea of the great distinction
between the nutrition of germinating plants and of older green ones,
the independence of the one, the dependence of the other, on light;
and that he considered the carbon dioxide of the atmosphere to be the
main if not the only source of the carbon in the plant, is shown by
his remark on a foolish assertion of Hassenfratz that the carbon is
taken from the earth by the roots; he replied that it was scarcely
conceivable that a large tree should in that case find its food for
hundreds of years in the same spot. There was a certain boldness in
these utterances of Ingen-Houss, and a considerable confidence in
his own convictions, for at that time the absolute amount of carbon
dioxide in the air had not been ascertained, and the small quantity of
it in proportion to the other constituents of air would certainly have
deterred some persons from seeing in it the supply of the huge masses
of carbon which plants accumulate in their structures.
Before Ingen-Houss in the work last mentioned explained the results
of his observations of 1779 in accordance with the new chemical
views, and laid the foundations of the doctrine of nutrition in
plants, JEAN SENEBIER[133], of Geneva, made protracted researches
into the influence of light on vegetation (1782-1788), and founded
on their results a theory of nutrition, which he published in 1800
in a tediously prolix work in five volumes entitled, ‘Physiologie
végétale.’ In this work some valuable matter was concealed in a host of
unimportant details and tiresome displays of rhetoric, which for the
most part are beside the question. But it must be acknowledged that
Senebier was better provided with chemical knowledge than Ingen-Houss,
and that he brought together all the scattered facts that the chemical
literature of the day offered, in order to obtain a more complete
representation of the processes of nutrition. It was of especial
importance at that time to insist on the principle that the processes
of nutrition within the plant must be judged by the general laws of
chemistry; organised beings, said Senebier, are the stage, on which
the affinities of the constituents of earth, water, and air mutually
influence each other; the decompositions however are generally the
result of the influence of light, which separates the oxygen of the
carbon dioxide in the green parts of plants. He insists (II. p. 304)
upon this among other facts, that the simple constituents of all plants
are the same, and the differences are only quantitative. He then brings
before us the simple and compound constituents of plants one after the
other, and among them light and heat figure as material substances,
in accordance with the view of the time. He treats at great length
the old question of the meaning of the salts in the plant, and it is
instructive to observe how he tries to decide whether the nitrates,
sulphates and ammonia, which are found in the sap of plants, are
introduced from without, or are formed in them from their constituent
elements; he concludes finally that the former is the more probable
opinion. That the greater part at least of the carbon of plants comes
from the atmosphere could scarcely be a matter of doubt with those
who knew the writings of Ingen-Houss; but Senebier devotes special
attention to this question; he endeavours to take all the co-operating
factors into the calculation, and especially to prove once more that
the oxygen given off from the plant in light comes from the carbon
dioxide which has been absorbed, that the green parts only and no
others are able to effect this decomposition, and that there is a
sufficiency of carbon dioxide in nature to supply the food of plants.
But although he convinced himself that green leaves decompose the
carbon dioxide which surrounds them in a gaseous form, he supposed that
it is chiefly through the roots that this substance finds its way with
the ascending sap into the leaves, and this view often gave occasion to
further error in later writers.
The tedious prolixity of Senebier’s book was one reason why it never
enjoyed the measure of appreciation and influence which it deserved;
but it was also thrown into the shade by the appearance of a work of
superior excellence, distinguished at once by the importance of its
contents, by condensation of style, and by perspicuity of thought.
This work was the ‘Recherches chimiques sur la végétation’ of THÉODORE
DE SAUSSURE[134] (1804), which contained new observations and new
results, and what was still more important, a new method. Saussure
adopted for the most part the quantitative mode of dealing with
questions of nutrition; and as the questions which he put were thus
rendered more definite, and his experiments were conducted in a most
masterly manner, he succeeded in obtaining definite answers. He knew
how to manage his experiments in such a manner that the results were
sure to speak plainly for themselves; they had not to be brought out
by laborious calculation from those small and, as they are called,
exact data, which less skilful experimenters use to hide their own
uncertainty. The directness and brevity with which precise quantitative
results are expressed, the close reasoning and transparent clearness
of thought, impart to the reader of de Saussure’s works a feeling
of confidence and security such as he receives from scarcely any
other writer on these subjects from the time of Hales to our own.
The ‘Recherches chimiques’ have this in common with Hales’ ‘Statical
Essays,’ that the statements of facts which they contain have been
made use of again and again by later writers for theoretical purposes,
while the theoretical connexion between them was constantly overlooked,
as we shall have reason to learn in the following section. It is not
every one who can follow a work like this, which is no connected
didactic exposition of the theory of nutrition, but a series of
experimental results which group themselves round the great questions
of the subject, while the theoretical connection is indicated in short
introductions and recapitulations, and it is left to the reader to form
his own convictions by careful study of all the details. It was not de
Saussure’s intention to teach the science, but to lay its foundations;
not to communicate facts, but to establish them; the style therefore,
as might be expected, is dry and unattractive; the writer seems to
confine himself too anxiously within the limits of what is given in
experience, and there is no doubt that many errors in later times might
have been avoided if the inductive proof of de Saussure’s doctrines had
been accompanied with a deductive exposition of them of a more didactic
character.
The processes of vegetation examined by de Saussure were, for the most
part, the same as those which Ingen-Houss and Senebier had studied
at length and correctly described in their general outlines. But de
Saussure went beyond this, and by means of quantitative determinations
struck a balance between the amount of matter taken up and given off
by the plant, thereby showing what it retains. In this way he made two
great discoveries: that the elements of water are fixed in the plant
at the same time as the carbon, and that there is no normal nutrition
of the plant without the introduction of nitrates and mineral matter.
But we cannot form a due idea of de Saussure’s services to physiology
without going further into the detail of his work.
We will first consider his investigations respecting the assimilation
of carbon in plants. Here we have the important result, that larger
quantities of carbon dioxide in the atmosphere surrounding the plants
are only favourable to vegetation if the latter are in a condition to
decompose them, that is, if they are in sufficiently strong light; that
every increase in the amount of carbon dioxide in the air in shade or
in darkness is unfavourable to vegetation, and that if that increase is
greater than eight times in the hundred it is absolutely injurious. On
the other hand he found, that the decomposition of carbon dioxide by
the green parts in light is an occupation that is necessary to them,
that plants die when they are deprived of it. The first clear insight
into the chemical processes which accompany the decomposition of carbon
dioxide in the interior of the plant was obtained by perceiving, that
plants by appropriating a definite quantity of carbon make a much
more than proportionate addition to their dry substance, and that this
is due to the simultaneous fixation of the component parts of water.
The full significance of this fact could only be apprehended at a
later time, when the theory of the combinations of carbon, organic
chemistry, had been further developed. As regards the importance of the
decomposition of carbon dioxide by the green organs under the influence
of light to the whole nourishment of the plant, de Saussure arrived
by more definite proofs than Ingen-Houss had given at the result,
that only a small portion of the substance of plants is derived from
the constituents of the soil in solution in water, but that the great
mass of the vegetable body is built up from the carbon dioxide of the
atmosphere and the constituents of water; he convinced himself of this
partly by considering the small quantities of matter which the water is
able to dissolve from a soil capable of sustaining vegetation, partly
by experiments in vegetation and considerations of a more general
character.
Not less important were de Saussure’s investigations into
oxygen-respiration by plants, which taken simply as a fact, had been
previously discovered by Ingen-Houss. But de Saussure showed that
growth is impossible without this process of respiration, even in
germinating plants, though these are rich in assimilated matter. He
further showed that green leaves and opening flowers, and generally
the parts of plants which are distinguished by greater activity of
vital processes, require more oxygen for respiration than those in a
less active and resting state. He determined the loss of weight which
the organic substance of germinating plants suffers from respiration,
and found it to be greater than was proportionate to the weight of
carbon exhaled; but the chemical science of his day did not supply
him with a certain explanation of this fact. Lastly, de Saussure at a
later time (1822) discovered the chief relations between the internal
heat of flowers and their consumption of oxygen, and thus we see that
he supplied the most important elements in the modern theory of the
respiration of plants, though he did not fully explain their mutual
connection.
It evidently was the received opinion before the time of Ingen-Houss,
and in spite of Hales’ views, that plants derive the larger part of
their food from the constituents of earth and water. But when it became
known that the carbon, which is the chief constituent of vegetable
substance, comes from the atmosphere, and it was considered that much
the larger part of that substance is combustible, it naturally became
a question whether the incombustible ingredients which form the ash
take any part in the nutrition of plants. This question was by many
physiologists answered in the negative; but de Saussure maintained
the contrary view. He insisted that certain ingredients, which are
found in the ash of all plants, must not be regarded as accidental
admixtures, and that the small quantities in which they occur are no
proof that they are not indispensable; and he showed from a large
number of analyses of vegetable ash, which for a long time were
unsurpassed in excellence, that there are certain relations between
the presence of certain substances in the ash and the condition of
development of the organs of the plant; for instance, he found that
young parts of plants capable of development were rich in alkalies and
phosphoric acid, while older and inactive portions were richest in
lime and silicic acid. Still more important were the experiments in
vegetation, by which he showed that plants, whose roots grow not in
earth but in distilled water, only take up as much ash-constituents as
corresponds with the particles of dust which fall into the water; and
further, that the increase in the organic combustible substance of a
plant so grown is very insignificant, and consequently that there is
no normal vegetation where the plant does not take up ash-constituents
in sufficient quantity,—a result of the highest importance to the main
question. Unfortunately de Saussure neglected to state these results
with due emphasis and to point out their fundamental importance, and
consequently doubts were entertained even till after 1830 respecting
the necessity of the constituents of the ash to vegetation.
It was known in de Saussure’s time that nitrogen entered into the
substance of living plants; the question was, whence it was obtained.
As it was known that four-fifths of the atmosphere consists of
nitrogen, it was natural to suppose that it is this which the plant
makes use of for forming its nitrogenous substance. De Saussure
endeavoured to settle the question by the volumetric method, which,
as was afterwards discovered, was not in this case to be trusted.
Nevertheless he arrived at the right conclusion, that plants do not
assimilate the nitrogen of the atmosphere; this gas must therefore be
taken up by the roots in some form of chemical combination. He made
no experiments on growing plants to decide what that form was, but
contented himself with the conjecture that vegetable and animal matter
in the soil and ammoniacal exhalations from it supply the nitrogen in
plants. This question, first ventilated certainly by de Saussure, and
afterwards the subject of protracted discussion, was finally settled
fifty years later by the experiments of Boussingault.
In connection with his researches into the importance of the
constituents of the ash, de Saussure proposed the question whether
roots take up the solutions of salts and other substances exactly in
the form in which they offer themselves. He found first of all that
very various and even poisonous matters are absorbed by them, and
that there is therefore no such power of choice, as Jung had once
supposed; on the other hand, it appeared that the solutions do not
enter unchanged into the roots, for in his experiments in every case
the proportion of water to the salt absorbed was greater than the
proportion between them in the solution, and that some salts enter the
plant in larger, some in smaller quantities, under circumstances in
other respects the same. But at this time, and for a long time after,
it was not possible to understand and rightly explain these facts; the
theory of diffusions was not yet known, and fifty or sixty years were
to elapse before light was thrown on the questions thus raised by de
Saussure.
Such were the most important contents of de Saussure’s publication
in 1804. His later contributions to the knowledge of some important
questions in vegetable physiology will be mentioned further on. A
comparison of the contents of the ‘Recherches chimiques’ with what
was known of the chemistry of the food of plants before 1780 excites
the liveliest astonishment at the enormous advance made in these
twenty-four years. The latter years of the 18th century had proved
still more fruitful, if possible, as regards the theory of nutrition
than the latter years of the 17th; both periods have this in common,
that they developed an extraordinary abundance of new points of view
in every branch of botanical science. They resemble each other also in
the circumstance that they were both followed by a longer period of
inactivity; the time from Hales to Ingen-Houss was highly unproductive,
and so also were the thirty years that followed the appearance of de
Saussure’s great work, though it must be admitted that some good work
was done during that period in France, while in Germany the new theory
was grossly misunderstood by the chief representatives of botany, as we
shall see in the following section. It should be mentioned however that
one of these misconceptions, which was not removed till after 1860, was
caused by de Saussure himself. He had observed that the red leaves of a
variety of the garden Orache disengage oxygen from carbon dioxide, as
much as the green leaves of the common kind. In this case he was hasty,
and concluded from this single observation that the green colour is not
an essential character of the parts which decompose carbonic acid; if
he had only removed the epidermis of the red leaves he would have found
that the inner tissue is coloured as dark green as the ordinary green
leaves. He who was usually so extremely careful as an observer was for
once negligent, and later writers, as is apt to happen, fixed exactly
on this one weak point, and repeatedly called in question one of the
most weighty facts of vegetable physiology, namely, that only cells
which contain chlorophyll eliminate oxygen.
5. VITAL FORCE. RESPIRATION AND HEAT OF PLANTS. ENDOSMOSE. 1804-1840.
During the twenty years that followed the appearance of de Saussure’s
chemical researches the theory of the nutrition of plants can scarcely
be said to have been advanced in any one direction, while much that
had already been accomplished was not even understood. Various
circumstances worked together to introduce misconceptions in this
province of botany; above all others the inclination, more strongly
pronounced than ever at this period, to attribute to organisms a
special vital principle or force, which was supposed to possess a
variety of wonderful powers, so that it could even produce elementary
substances, heat, and other things out of nothing. Whenever any
process in such organisms was difficult to explain by physical or
chemical laws, the vital force was simply called in to bring about the
phenomena in question in some inexplicable manner. It was not that the
question was now raised, which at a later time engaged the attention
of profounder thinkers, whether there was a special agent operating in
organic bodies beside the general forces which govern inorganic nature;
for a careful examination of this question would certainly have led
to the most earnest efforts to explain all the phenomena of life by
physical or chemical laws. On the contrary, it was found convenient to
assume this vital force as proved, and to assign it as the cause of a
variety of phenomena, thus escaping the necessity of explaining the
way in which the effects were produced; in a word, the assumption of
a vital force was not a hypothesis to stimulate investigation, but a
phantom that made all intellectual efforts superfluous.
Another hindrance to the progress of physiology, especially where
questions of nutrition turned on the movement of the sap, was the
backward condition of the study of the inner structure of plants,
as described in the second book. For instance, the question of
the descending sap was complicated in the strangest way by Du
Petit-Thouars’s theory of bud-roots that descend between the bark
and the wood; Reichel’s unfounded idea of the rising of the sap in
the tubes of the wood was generally accepted, and a still worse
error was maintained by some, that the intercellular spaces of the
parenchyma are true sap-conveying organs. In 1812 Moldenhawer had
to insist, but without producing any general conviction, that the
vessels of the wood contain air, and Treviranus in 1821 that the
stomata serve for the entrance and exit of air. We need not notice
here what nature-philosophers like Kieser said about nutrition and
the movement of the sap; but even those who were far from adopting
the extravagancies of this school were incapable of either making
use of or carrying on the labours of Ingen-Houss, Senebier, and de
Saussure. We may adduce in proof of this statement the remarks of
Link on the function of leaves in his ‘Grundlehren der Anatomie und
Physiologie,’ 1807. He says at p. 202 that their function is according
to Hales transpiration, according to Bonnet absorption, according
to Bjerkander the exudation and secretion of a variety of fluids,
according to Hedwig the storing up of juices, and inasmuch as leaves
increase the green surfaces of plants, bear stomata and hairs, and
hold a quantity of juices in their abundant parenchyma, we may ascribe
all these functions, but none of them exclusively, to leaves; the only
thing peculiar to them is that they convey elaborated juices to the
young parts. Their great work, the decomposition of carbon dioxide, he
does not mention. But this neglect of the doctrines of Ingen-Houss,
Senebier, and de Saussure was common, especially in Germany; it is seen
in the efforts made to prove once more the existence of a descending
sap in the rind, just as it had been proved in the two previous
centuries, by the result of removing a ring of bark from the stem,
and by similar experiments; whereas the simple consideration that it
is only in the green leaves that carbonaceous vegetable substance is
formed, would have made the existence of what was known as a descending
sap appear to be a matter of course, and must have led to a much
clearer conception of the matter. But this consideration was either
quite overlooked or only mentioned incidentally by those who occupied
themselves with experiments on the movement of the descending sap. This
is the case in Heinrich Cotta’s ‘Naturbeobachtungen über die Bewegung
und Function des Saftes in den Gewächsen,’ 1806, in many respects an
instructive work, and in Knight’s otherwise serviceable experiments
on the growth in thickness of trees. It was not till after 1830 that
De Candolle and Dutrochet perceived that the fact that the green
leaves are assimilating organs must be decisive of the question of the
movement of the sap in the stem.
No progress was made with the general doctrine of nutrition between
1820 and 1840 except in one point, the absorption of oxygen by all
parts of plants; here something was done to consolidate the theory
and to enrich it with new facts; it was indeed a subject more adapted
to the views of the day, because it at once suggested a variety of
analogies with the respiration of animals. Grischow showed in 1819
that Fungi never decompose carbon dioxide, but absorb oxygen and
give off carbon dioxide. Marcet carried the subject further in 1834,
after de Saussure had published in 1822 an excellent investigation
into the absorption of oxygen by flowers; in this work we have the
basis laid for the theory of vegetable heat, to which we shall
return. But Dutrochet was the first who made an elaborate comparison
of the respiration of plants and animals (1837), and showed that
not only growth, as de Saussure had already perceived, but also the
sensitiveness of plants depends on the presence of oxygen, that is on
their respiration. The recognition of the fact, that the inhalation of
oxygen plays the same part in plants that it does in animals, prepared
the way for the view that heat in plants is simply a result of their
respiration, as it is in animals. It is not necessary to describe at
length the experiments which were made on heat in plants before 1822;
they were one and all vitiated by a want of clearness in the statement
of the question, which made success impossible; it was assumed that
this heat by raising the temperature of the plant would make itself
felt by surrounding objects, and it was sought for exactly where it is
least to be found, in the wood, in fruits and tubers, and generally in
resting, inactive parts. Moreover the previous experiments, collected
in Goeppert’s book ‘Ueber die Wärmeentwicklung der Pflanzen,’ 1830,
were so unskilfully managed that they could not possibly lead to any
result. Nor could the question whether plants really develope internal
heat, as animals do, be determined by a few cases of active development
of heat in flowers, because an idea was prevalent at the time in
connection with the theory of a vital force, that flowers as the organs
of reproduction alone possessed the power of generating heat.
Lavoisier had clearly perceived in 1777 that the combustion of
substances containing carbon by inhaled oxygen was the source of
animal heat, and had proved it by experiments. Senebier, who first
observed the rise of temperature in the inflorescence of Arum by
the thermometer, had at least suggested in his work on physiology
of 1800 (iii. p. 315) that a vigorous absorption of oxygen might be
the cause of the phenomenon. Bory de St. Vincent reported in 1804
that Hubert, the owner of a plantation in Madagascar, had observed
among other things that the air in which the flowering spike of one
of the Aroideae had developed its heat could support neither animal
respiration nor combustion. These indications were however disregarded,
until de Saussure in 1822 proved directly the connection between
the absorption of oxygen and the rise of temperature in flowers. It
was however a long time before heat in plants was conceived of as
a general fact necessarily connected with their respiration. This
conception would have swept away the whole mass of facts accumulated
by Goeppert in his book of 1830, from which he tried to prove (p. 228)
that plants at no period of their life possess the power of generating
heat—a view which he retracted however in 1832, when he had observed
a rise of temperature in germinating plants, bulbs, tubers, and in
green plants, when collected into heaps. How difficult it was for
physiologists under the dominion of the ‘vital force’ to hold firmly
to the simple principle of natural heat, and not to be led away by
isolated observations, is shown by the expressions of De Candolle in
1835, and still more by those of Treviranus in 1838. It is therefore
refreshing to see Meyen in his ‘Neues System’ (1838), vol. ii, warmly
asserting this principle, and making the development of heat in plants
a necessary consequence of their respiration and of other chemical
processes. Meyen himself produced no new observations; but Vrolik
and De Vriese showed by laborious experiments in 1836 and 1839 the
dependence of the generation of heat in the flowers of Aroideae on
the absorption of oxygen. A higher importance as regards the general
principle attaches to the attempt of Dutrochet in 1840 to prove that
even growing shoots generate small quantities of heat, as shown by a
thermo-electric apparatus. Some of the details in these observations
are open to objection; but it cannot be denied that they are based on
a clear recognition of the general principle, though they ignore the
consideration that the generation of heat in plants is not necessarily
accompanied with a rise in temperature, since cooling causes may be
acting at the same time with greater effect. However the doctrine
of the natural heat of plants was in the main established by the
observations of de Saussure, Vrolik, De Vriese, and Dutrochet, and
by Meyen’s and Dutrochet’s assertion of the principle laid down by
Lavoisier, though thirty years elapsed before it became an accepted
truth in vegetable physiology.
The crude idea of a vital force was deprived of one of its chief
supports when it was recognised that the natural heat of organisms
was a product of chemical processes induced by respiration, for this
had been regarded since the time of Aristotle as more peculiarly an
effect of the principle of life. And now another discovery was made,
equally calculated to promote the reference to mechanical principles of
those general and important phenomena of life which had hitherto been
indolently ascribed to the operation of the vital force. It appears
to be a matter of indifference whether Professor Fischer of Breslau
is or is not to be considered as the true discoverer of endosmose in
1822, for it is certain that it was DUTROCHET[135] who first studied
the subject with exactness, and above all perceived its extraordinary
value for the explanation of certain phenomena in living organisms.
He repeatedly called attention to this value in the years between
1826 and 1837, and endeavoured to refer a variety of phenomena in
vegetation to this agency. He had first observed the operation of
endosmose in its mechanical effects in living bodies; the escape of
the zoospores of an aquatic Fungus and the ejection of the sperm from
the spermathecae of snails first led him to the hypothesis, that the
more concentrated solutions inclosed in organic membranes exercise
an attraction on surrounding water, which, forcing its way into the
inclosed space, is there able to exert considerable powers of pressure.
To Dutrochet must always belong the merit of having brought into
notice this mechanical effect of endosmose and of employing it to
explain a number of vital phenomena. Many things in which a mechanical
explanation had not been hitherto thought of could now be traced to a
mechanical principle, the effects of which could be exhibited and more
accurately studied by means of artificial apparatus. Dutrochet rightly
attached a special value to the fact, that all states of tension in
vegetable tissue could be at once explained by endosmose and exosmose,
though, as so often happens in such matters, he may have extended
his new principle to cases where it was not applicable, as we shall
see below. His account of the nature of endosmose itself must now be
considered to be obsolete, nor did the mathematician Poisson or the
physicist Magnus about 1830 succeed in framing a satisfactory theory
on the subject. It was discovered in the course of the succeeding
twenty or thirty years, that the phenomena observed by Dutrochet, and
which he called endosmose and exosmose, were only complicated cases of
hydro-diffusion, which with the diffusion of gas forms an important
part of molecular physics. Dutrochet, like his immediate successors,
conducted his investigations into osmose with animal and vegetable
membranes, the latter being of a complex structure; with these he
always observed in addition to the endosmotic flow of water into the
more concentrated solution, an escape of the solution itself, and from
this he concluded that there must always be two currents in opposite
directions through the membrane which separates the two fluids, that,
as he expresses it, the endosmose is always accompanied with exosmose.
This error, which was even developed later into a theory of the
endosmotic equivalent, has had much to do till recently with making it
impossible or difficult to refer certain phenomena of vegetation to
the processes of hydro-diffusion. To mention only one case, Schleiden
rightly observed that if endosmose, as Dutrochet understood it, is
the sole cause why water is absorbed by the roots, there must also
be a corresponding exosmose at the roots; and this, which was called
root-discharge, Macaire Prinsep thought he had actually discovered,
and even Liebig firmly believed in its existence till a recent period,
although the researches of Wiegman and Polstorff (1842) and later
more careful investigations showed, that there was no noticeable
discharge by exosmose to answer to the great quantity of water with
substances in solution in it which is taken up by the roots. Again,
Dutrochet’s theory of endosmose did not fully explain the way in which
the several substances which feed the plant find their way into and
are disseminated in it. But notwithstanding these and other defects it
deserved the greatest consideration, because it gave the first impulse
to the further development of the theory of diffusion, and contained
a mechanical principle which might serve to explain very various
phenomena in vegetation as yet unexplained. Dutrochet hastened to apply
it to this purpose, where it was at all possible to do so, and chiefly
in his treatise on the ascending and descending sap (‘Mémoires,’ 1837,
i. p. 365), which was superior to anything which had been written
on the movement of the sap in plants in its clear conception of the
question and in perspicuity of treatment. It should be especially
mentioned that Dutrochet formed a true estimate of the functions of the
leaves as regards both the ascending and descending sap, and to some
extent pointed out the fault which lies at the bottom of the earlier
experiments with coloured fluids. After communicating a number of good
observations on the paths of the ascending and descending sap, and
noticing particularly that in the vine the vessels of the wood serve
for the movement of the sap only in spring, when vines bleed, but
that they are air-passages in summer, when transpiration causes the
most copious flow of water in the wood, he proceeds to consider the
forces which effect the movement of the ascending sap in the wood both
in spring and summer. He first of all judiciously distinguishes two
things which had been before always mixed up together, the weeping of
severed root-stocks and the rise of the sap in the wood in transpiring
plants. The first is caused, he thinks, by impulsion, the other by
attraction; we should now say, that in weeping root-stocks the water
is pressed upwards, in transpiring plants drawn up. He then refers the
phenomenon of impulsion to endosmose in the roots, and without going
much into detail as regards the anatomical conditions, he compares
a weeping root-stock to his own endosmometer, in the tube of which
the fluid that has been sucked in rises by endosmose and even flows
over; it is true that no very thorough understanding of the matter
was gained in this way, but at any rate the principle which was to
explain it was indicated. He then endeavours to explain the movement
of the water which ascends in the wood of transpiring plants by the
action of endosmose from cell to cell. In this he failed entirely,
as was afterwards perceived; but he succeeded in showing that all
the mechanical explanations that had been previously attempted were
incorrect, and the whole treatise, though unsatisfactory in its main
result, contains a great number of ingenious experiments and acute
remarks.
With the exception of Théodore de Saussure, who occupied himself
exclusively with chemical questions in physiology, Dutrochet was the
only vegetable physiologist in the period between 1820 and 1840 who
studied all its more important questions thoroughly and experimentally;
his treatise on the respiration of plants, which has been already
mentioned, is excellent in itself, and was of the greatest importance
at the time it appeared, because it brought the chemical processes in
respiration, the entrance and exit of the gases, for the first time
into correct connection with the air-passages in the plant, with the
stomata, the vessels, and the intercellular spaces, and submitted the
composition of the air contained in the cavities of plants to careful
examination. It was the best work on the respiration of plants in
the year 1837 and for a long time after; and if Dutrochet made the
mistake of regarding the oxygen which is disengaged from the plant
itself in the light as the chief agent in respiration, and the oxygen
directly absorbed from the atmosphere as only subsidiary to this, he
compensated for it by recognising the importance of the fact, that only
cells which contain chlorophyll decompose carbon dioxide, and still
more by correctly distinguishing between respiration by the absorption
of oxygen and the decomposition of carbonic dioxide in light; these
two processes were at that time and afterwards very inappropriately
distinguished as the diurnal and nocturnal respiration of plants, and
this misleading expression maintained itself in spite of Garreau’s
protest in 1851 till after 1860, when a modern German physiologist
succeeded in establishing the true distinction between respiration and
assimilation in plants. Another mischievous complication arose about
1830 connected with the expression, circulation of the sap; it was
thought that an argument for such a circulation even in the higher
plants was to be found in the ‘circulation of the sap’ (protoplasm) in
the cells of the Characeae, which had been detected by Corti and more
exactly described by Amici; Dutrochet (Mémoires, I. p. 431) exposed
this confusion of ideas, and has the merit of refuting at the same time
the absurd theory of the ‘circulation of the vital sap,’ for which
Schultz-Schultzenstein had received a prize from the Academy of Paris.
We shall recur in the next chapter to Dutrochet’s minute investigations
into the movements connected with irritability in plants, which he
also endeavoured to refer to endosmotic changes in the turgidity of
the tissues, but he did not do justice to the anatomical conditions of
the problem. And here we may take occasion to remark, that Dutrochet’s
works were often undervalued, especially in Germany, greatly to the
detriment of vegetable physiology. His younger German contemporaries,
von Mohl and Schleiden, and at a later time Hofmeister, were right
in pointing out what was erroneous and sometimes arbitrary in his
mechanical explanations of various movements in plants, and it cannot
be denied that he was sometimes led into obscure and doubtful views,
as for instance when without any apparent connection he regarded the
inhalation of oxygen as a mechanical condition of the rising of the
sap and also of heliotropic curvatures, and that his attempts at
explanation were not seldom forced and improbable; but all this does
not prevent it from being true, that an attentive reader will still
gain much instruction from his physiological writings and be excited
by them to examine for himself. Dutrochet was a decidedly able man and
an independent thinker, who it is true was often led astray by his
prejudices, but at the same time manfully protested against the old
traditional way of dealing with physiological ideas, and substituted
careful examination both of his own and others’ investigations for
the accumulation and comfortable retailing of isolated observations
which was then the fashion. After de Saussure’s ‘Recherches chimiques’
Dutrochet’s ‘Mémoires pour servir a l’histoire anatomique et
physiologique des végétaux et des animaux,’ 1837, are without doubt
the best production, which physiological literature has to show in the
long period from 1804 to 1840. If later botanists, instead of dwelling
on his faults, had developed with care and judgment all that was
really good in his general view of vegetable physiology, this branch
of botanical science would not have declined as it did in the interval
between 1840 and 1860. We shall discover the greatness of Dutrochet as
a vegetable physiologist by comparing his work above-mentioned with the
best text-books of the subject of the same time, those of De Candolle,
Treviranus, and Meyen; not one of them comes up to Dutrochet’s Mémoires
in acuteness or depth.
The three text-books just mentioned contained little or nothing new
either in facts or ideas on the subject of the nutrition of plants;
all three were rather compilations of what was already known, and
differed from each other only in their selection of material and in
the form which each sought to give to the general theory; but this is
a reason why we should take a nearer look at them, that we may learn
how the spirit and tendencies of the time were reflected in vegetable
physiology, and made themselves felt particularly in the theory of
nutrition.
De Candolle’s work appeared in French in 1832 in two volumes, the
first only being devoted to the subject of the nutrition of plants,
and in German in 1833 with many valuable annotations by the translator
Roeper, under the title, ‘Pflanzen-physiologie oder Darstellung der
Lebenskräfte und Lebensverrichtungen der Gewächsc.’ It suffers, in
common with the other two books we have mentioned on the same subject,
and with the earlier works of Du Hamel, Mustel, and other writers, from
a too discursive mode of treatment, which has the effect of burying
the points of fundamental importance under a huge mass of facts and
statements from other writers. It contains much that might have been
omitted as obsolete, and much empirical material of a purely chemical
nature, which could not at that time be applied to the purposes of
physiology. Nevertheless, it deserved the great consideration which
it enjoyed for a long time, especially in Germany, for its author had
undertaken to treat vegetable physiology as a separate and peculiar
branch of knowledge, not ignoring at the same time its connection with
and dependence on physics, chemistry, phytotomy, and biology proper,
and thus to give a full and complete delineation of vegetable life;
whereas the best works that had been written since Du Hamel’s time,
especially on the nutrition of plants, had proceeded from chemists and
physicists or from plant-growers like Knight and Cotta, who treated
the subject in a one-sided manner, each from his own point of view,
and made no attempt to give a connected account of all the phenomena
of vegetation. For this reason De Candolle’s ‘Physiologie végétale’ is
the most important performance that appeared after Du Hamel’s ‘Physique
des Arbres’; and if we wish to know what progress was made in vegetable
physiology generrally, and in the doctrine of nutrition particularly,
in the period from 1758 to 1832, we have only to compare the contents
of these two books. That this progress was a considerable one, appears
plainly from a short summary at the end of the first volume of the
general theory of nutrition, as De Candolle himself conceived it; this
summary will show us at the same time that he aimed rather at giving
a clear account of the whole of the internal economy of the plant,
than at searching into the moving forces, the causes and effects. From
this he was necessarily withheld by his assumption of a vital force.
He distinguished four kinds of forces; the force of attraction which
produces the physical, and that of elective affinity which causes
the chemical phenomena; then the vital force, the original source of
all physiological, and the soul-force, the cause of all psychical
phenomena. Only the first three of these forces operate in the plant,
and though it is necessary to find out what phenomena in vegetation are
due to physical or chemical causes, yet the main task of the vegetable
physiologist is to discern those which proceed from the vital force,
and the chief mark of such phenomena is that they cease with the death
of the plant (p. 6). Of course therefore all the peculiar phenomena of
nutrition, which are manifested only in the living plant, come within
the domain of the vital force. It must be allowed, however, that De
Candolle has made a very moderate use of the vital force, and confines
himself wherever he can to physical and chemical explanations; and when
he has recourse to the vital force, it is owing less to the influence
of his philosophical point of view than to the fact that his account is
based rather on tradition and information at second hand than on actual
research. It is true that De Candolle was perhaps better acquainted
than any contemporary botanist with the physics and chemistry of his
day, and it is part of his great merit that he should have acquired
so much knowledge on these subjects while engrossed in his splendid
labours as a systematist and morphologist; but he betrays, at least
in his later years, a want of practice in the study of physics and
a want also of the habit of mind which this imparts, and which is
more important to the physiologist than a knowledge merely of many
facts. But this defect is still more apparent in Treviranus and Meyen,
whose works on physiology were published soon after that of the great
systematist.
De Candolle first brings together all the facts in physiology which
have been discovered from the beginning, not omitting the chemical
researches of more modern times into the substance of plants, and then
gives a general delineation of the processes of nutrition in the plant:
‘The spongioles (an unfortunate invention of his own which has not
yet disappeared from French books, and plays a great part in Liebig’s
latest work)—the spongioles of the roots, being actively contractile
and aided by the capillarity and hygroscopic qualities of their tissue,
suck in the water that surrounds them together with the saline organic
or gaseous substances with which it is laden. By the operation of
an activity which is manifested principally in the contractility of
the cells and perhaps also of the vessels, and is maintained by the
hygroscopic character and capillarity of the tissue of the plant and
also by the interspaces produced by exspiration of the air and by other
causes, the water sucked in by the roots is conducted through the wood
and especially in the intercellular passages to the leaf-like parts,
being attracted in a vertical direction by the leaves and in a lateral
direction by the cellular envelope (cortical parenchyma) at every
period of the year, but chiefly in the spring; a considerable part of
it is exhaled all day long through the stomata into the outer air in
the form of pure water, leaving in the organs in which the evaporation
takes place all the saline, and especially all the mineral particles
which it contained. The crude sap which reaches the leaf-like parts of
the plant there encounters the sun-light, and by it the carbonic acid
gas held in solution by the sap, whether derived from the water sucked
in by the roots or from the atmospheric air, or being part of that
which the oxygen of the air produced with the surplus carbon of the
plant, is decomposed in the day-time; the carbon is fixed in the plant
and the oxygen discharged as gas into the air. The immediate result of
this operation appears to be the formation of a substance which in its
simplest and most ordinary state is a kind of gum consisting of one
atom of water and one of carbon, and which may be changed with very
little alteration into starch, sugar, and lignine, the composition
of which is almost the same. The nutrient sap thus produced descends
during the night from the leaves to the roots, by way of the rind and
the alburnum in Exogens, by way of the wood in Endogens. On its way it
falls in with glands or glandular cells, especially in the rind and
near the place where it was first formed; these fill themselves with
the sap and generate special substances in their interior, most of
which are of no use in the nutrition of the plant, but are destined
either to be discharged into the outer air or to be conducted to other
parts of the tissue. The sap deposits in its course the food-material,
which becoming more or less mixed up with the ascending crude sap in
the wood, or sucked in with the water which the parenchyma of the
rind draws to itself through the medullary rays, is absorbed by the
cells and chiefly by the roundish or only slightly elongated cells,
and is there further elaborated. This storing up of food-material,
which consists chiefly of gum, starch, sugar, perhaps also lignine,
and sometimes fatty oil, takes place copiously in organs appointed for
the purpose, from which this material is again removed to serve for
the nourishment of other organs. The water, which rises from the roots
to the leaf-like parts of the plant, reaches them in an almost pure
state, if it passes quickly through the woody parts, the molecules of
which are but slightly soluble. If, on the other hand, the water flows
through parts in which there is much roundish cell-tissue filled with
food-material, it moves more slowly and mixes with this material and
dissolves it; when it is drawn away from these places by the vital
activity of the growing parts, it reaches them not as pure water but
charged with nutrient substances. The juices of plants appear to
be conveyed chiefly through the intercellular passages. The vessels
probably share in certain cases in these functions, but serve generally
as air-canals. The cells appear to be the really active organs in
nutrition, since decomposition and assimilation of the juices take
place in them. Cyclosis (of Schultze’s vital sap[136]) is a phenomenon
which appears to be closely connected only with the preparation of
the milky juices, and to be caused by the actively contractile nature
of the cell-walls or of the tubes. Woody and other substances are
deposited in every cell in different quantities according to their
kinds and the accompanying circumstances, and clothe their walls; the
unequal thickness of the layer so deposited appears according to Hugo
von Mohl to have given rise to the supposition of perforated cells;
that is, the parts of the cell-wall that remain transparent appear
under the microscope as pores. Every cell may be regarded as a body
which prepares juices in its interior; but in vascular plants their
activity stands in such a connection with a complex of organs, that a
single cell does not represent the whole organism, as may be said of
the cells of certain cellular plants, which are all like one another.
There is no circulation in plants like the circulation in animals, but
there is an alternating ascent and descent of the crude sap and of the
formative sap which is often mixed with it. Both these phenomena depend
perhaps on the contractile power in cells that are still young, and if
so, this power would be the true vital energy in plants.‘
What is strange to us in De Candolle’s theory of nutrition is due
chiefly to the predominance of the vital force; yet at the same time
it gives the facts in their general connection, and its best feature
is, that the true function of the leaves, the decomposition of carbon
dioxide in light and the production of organisable substance, is made
the central point of the whole circle of the processes of nutrition.
Very different in this respect were the views of the two most eminent
German vegetable physiologists at the close of the period before
us, Treviranus and Meyen, though they are not in accord with one
another in their general conception of the subject. It may be said
that all the prejudices and errors, built up on the foundation of the
hypothesis of a vital force during the first thirty years of the 19th
century, culminated in Treviranus; while others were already setting
up the mechanical explanation of the phenomena of vegetation as the
one object to be attained, Treviranus produced once more the whole
machinery of the obsolete doctrine of the vital force, and with such
effect, that his ‘Physiologie der Gewächse’ was already obsolete when
it appeared in 1835. The second volume of Meyen’s ‘Neues System der
Pflanzenphysiologie’ was a striking contrast to the work of Treviranus;
Meyen endeavours as far as possible to trace back the phenomena of
vegetation to mechanical and chemical causes, though he does not
often succeed in bringing anything to light that is new or of lasting
service. He, like Treviranus, was deficient in sound training in
chemistry and physics; they did not stand in this respect, as Hales and
Malpighi once did, at the highest point of knowledge of their time. At
the same time there was a great difference in the way in which each
dealt with the writings of his predecessors; Treviranus, who had done
good service in former years in phytotomy, was not equal to the task
which he had now undertaken; his physiological expositions are marked
by feebleness of thought and by an inability to survey as from a higher
ground the connection between the facts; he distrusts all that had been
done during the previous thirty years, and almost everywhere appeals
to the publications of the 18th century; he lives indeed in the ideas
of the past, without gaining vigour from the forcible reasoning and
freshness of thought of a Malpighi, a Mariotte, or a Hales. Meyen’s
treatment of his subject is on the contrary fresh and vigorous; he does
not disregard the old, but he holds chiefly to the modern conquests
of science; while Treviranus with singular ill-luck constantly
overlooks what is valuable in itself and important in its results,
Meyen generally picks out the best things from the books before him;
Treviranus timidly avoids expressing any view decidedly and maintaining
it; Meyen, amid the multiplicity of the labours which we have already
described, finds no time to arrange his thoughts, is hasty in judgment
and often contradicts himself. But with all these defects, he is still
the champion of the new tendencies that were being developed, while
Treviranus lives entirely in the past, and shows no trace of the
actively creative spirit which was soon to burst forth so mightily in
every branch of natural science.
If we examine what both these writers have said on the subject of
the nutrition of plants, we shall find that the difference in their
general views in physiology as described above appears at once in
their treatment of the work of suction in the roots, and of the
means by which the sap ascends; here in Treviranus the vital force
is everything; it makes the vessels of the wood conduct the juices
from the roots into the leaves, with other antiquated notions of the
kind; Meyen on the contrary adopts Dutrochet’s position, and even
rejects De Candolle’s spongioles. Treviranus knows not what to make of
respiration; Meyen explains it without hesitation as a function that
answers to respiration in animals, and finds in it the main cause of
the natural heat which Treviranus derives in the old mystical fashion
from the vital force. In one point however they agree, namely, in a
complete misconception of the connection between the decomposition of
carbon dioxide in the leaves and the general nutrition of the plant.
It is necessary to the understanding of the confusion of ideas which
had crept at this time into the doctrine of nutrition, and to a right
estimate of the services of Liebig and Boussingault on this point, that
we should look a little more closely into the chemical part of the
theory of nutrition in Treviranus and Meyen.
Treviranus in the introduction to his book repudiated the idea of a
vital force separable from matter, but he was in fact a prisoner
within that circle of ideas, and he made a much freer use of the vital
force than De Candolle; he went even farther than this, and in his want
of chemical experience he hit upon the grossly materialistic notion of
a vital matter [I. p. 6]. This vital matter is a half-fluid substance,
which may be obtained from all bodies that were once alive by boiling
and by decay; it is formed from other elements, but it is itself the
true elementary matter with which alone physiology has to do; it is
common to the animal and vegetable kingdom, and is purest when in the
form of mucilage, albumen, and gelatine; that animals and plants alike
consist of this vital matter explains the circumstance, that plants
serve as food for animals and animals as food for plants. He goes on to
show that a similar unctuous substance, called by chemists extract of
the soil, and considered by many of them to be an important ingredient
in the nutrition of plants, is their true and proper food. This extract
of the soil was therefore the vital matter which plants take up; it
was natural that Treviranus should no longer attribute any importance
to the decomposition of carbon dioxide in the leaves, especially
as he was unable to understand the chemical connection of all that
Ingen-Houss, Senebier, and de Saussure had written. He explained the
co-operation of light in the nutrition of plants to be a merely ‘formal
condition,’ and the salts in solution in the water of the soil were in
his opinion stimulants for the use of the extremities of the roots,
which were thus put into a condition of ‘vital turgescence’; and as the
functions of the leaves, such as Malpighi and Hales had conjectured,
and Ingen-Houss, Senebier, and de Saussure had proved it to be, had no
existence for Treviranus, he made the assimilation of the soil-sap take
place on its way, as it flowed upwards and downwards through the plant.
We see that nothing can be conceived more deplorable than this theory
of nutrition; it would have been bad at the end of the 17th century, it
is difficult to believe that it could have been published thirty years
after de Saussure’s work.
There is much in the details of Meyen’s views on the chemical
processes in the nutrition of plants that is better than what we find
in Treviranus; it is a great point that he concluded from earlier
experiments, that the salts which find their way with the water into
the roots are not merely ‘stimulants’ but food-material, and, as was
before said, he explained the respiration of oxygen by plants correctly
in accordance with de Saussure’s observation. But he too stumbled over
the assimilation of carbon; he, like so many before and after him, was
confused by the simple fact, that gaseous matter takes part both in the
nutrition and the respiration of the plant; and taking the processes in
both cases for processes of respiration, he considered the absorption
of oxygen to be the only important and intelligible function, and the
decomposition of carbon dioxide in light to be a matter of indifference
as regards the internal economy of the plant. Instead of ascertaining
by a simple calculation, whether the apparently small quantity of
carbon dioxide in the atmosphere was perhaps sufficient to supply
vegetation with carbon, he simply declared it to be insufficient,
and because plants will not flourish in barren soil merely by being
supplied with water containing carbon dioxide, he gave up the
importance of that gas altogether. He too found the humus-theory, which
had been constructed by the chemists, more convenient for his purpose,
and like Treviranus derived the whole of the carbon in plants from
‘extract’ of the soil, without any close attention to the facts of the
case; he refused to believe that the soil is rendered not poorer but
richer in humus by the plants that grow on it. It is obvious then that
the account given by Treviranus and Meyen of the chemical processes
that take place in the nutrition of plants, though correct in some of
the details, could afford no true general view of the processes of
nutrition, because it entirely misconceived the cardinal points in the
whole theory, namely the source of the carbon, and the co-operation
of light and of the atmosphere; and thus the best results of the
observations of Ingen-Houss, Senebier, and de Saussure were lost upon
the German vegetable physiologists.
6. SETTLEMENT OF THE QUESTION OF THE FOOD-MATERIAL OF PLANTS. 1840-1860.
We have noticed in the previous section the rise of views during
the period between 1830 and 1840 which were calculated to make the
hypothesis of a vital force appear superfluous, at least as an
explanation of certain important phenomena in vegetation; such were
the referring the natural heat of plants to chemical processes, and
the movement of the sap to osmose; in the domain of chemistry also,
in which Berzelius had in the year 1827 made the distinction between
organic and inorganic matter to consist in the fact, that the former
is produced under the influence of the vital force, the opinion was
openly expressed that such an intrusion of the vital principle could
not be allowed, since organic compounds had been repeatedly produced
from inorganic substances by artificial means, and therefore without
its aid. The general tendency of scientific thought was now in fact
unfavourable to the nature-philosophy of former days; it inclined to
free itself from the obscurity that attended the idea of a vital force,
and to assert the belief, that chemical and physical laws prevail alike
outside and inside all organisms; this idea became an axiom with the
more eminent representatives of natural science after 1840, and if not
always expressed in words, was made the basis of all their attempts to
explain physiological phenomena.
Thus a freer course began to open for the intellectual movement of the
time even before the year 1840, and strict inductive research, and
above all the establishment of facts and closer reasoning were now
demanded in the question of the nutrition of plants, as they were also
in the domain of morphology and phytotomy. But in dealing with the
theory of nutrition, the first thing required was not the discovery
of new facts so much as the forming a correct appreciation of the
discoveries of Ingen-Houss, Senebier, and de Saussure, and clearing
away the misconceptions that had gathered round them. The chief modern
representatives of vegetable physiology, De Candolle, Treviranus, and
Meyen, had increased the difficulty of the task by neglecting to keep
the several questions of their science, the chemical especially and
the mechanical, sufficiently distinct from one another. The question,
what are the materials which as a rule compose the food of plants,
though one of the first and most immediate importance, had been very
imperfectly investigated, while attention had been diverted to a
confused mass of comparatively unimportant matters, and the solution
of that question had been rendered impossible for the time by the
humus-theory, an invention of chemists and agriculturists, which
Treviranus and others had fitted so readily into the doctrine of a
vital force. To Liebig belongs the merit of removing these difficulties
and all the superfluous matter which had gradually gathered round the
subject, and of setting forth distinctly the points which had to be
considered; this was all that was required to ensure a satisfactory
solution of the problem, for former observations had supplied an
abundance of empirical material. But some points of minuter detail were
brought out in the course of his investigations which required new and
comprehensive experiments, and for these a most capable and successful
observer was found between 1840 and 1850 in the person of Boussingault.
But before we go on to give a fuller account of the work of Liebig and
Boussingault, we may mention a circumstance which serves to indicate
the character of the revolution in scientific opinion before and after
1840. An anonymous ‘Friend of science’ had put a prize at the disposal
of the Academy of Göttingen for an answer to the questions, ‘whether
the inorganic elements, which are found in the ashes of plants, are
found in the plants themselves, in cases where they are not supplied
to them from without; and whether these elements are such essential
constituents of the vegetable organism, as to be required for its full
development.’ The first question appears in the present day absurd,
since it implies the possibility of elementary matter coming into
being, and of certain special elements coming into being in the plants
themselves, an idea however not unfamiliar to the nature-philosophy
and the vital force school. It was easy for Wiegman and Polstorff,
the authors of the essay that gained the prize (1842), men of the
new school, to answer the first question in the negative, and indeed
their answer to the second question involved a negative answer to the
first. The investigations made by Wiegman and Polstorff in connection
with the subject of the second question were conducted in a thoroughly
intelligent manner, though they set out from the hypothesis that a
certain quantity of compounds of humic acid, as it was called, must
be present in the food-mixtures. Their experiments, better adapted to
the purpose than any previous ones, showed convincingly that it is
necessary to the normal nutrition of the plant that it should take up
the constituents of the ash; the observers also took into consideration
a number of other questions connected with nutrition, in which however
we may already sec the influence of Liebig’s book which had come out
during their investigations.
This work was the one entitled ‘Die organische chemie in ihrer
Anwendung auf Agricultur und Physiologie,’ which appeared first in
1840 and was afterwards repeatedly reprinted and enlarged. The name of
the author, the first chemist of Germany, raised an expectation that
the questions respecting nutrition would be dealt with otherwise than
they had hitherto been, and this expectation was more than fulfilled
by the novelty and boldness with which Liebig cleared up the most
important points of the theory, seized upon all that was essential
and fundamental, and disregarded the unimportant matter which had
before only served to confuse the question. Moreover, he was able
to rest on long-accepted facts in just those points which were the
most important, and on these he had only to throw the light of his
chemical knowledge to dispel the previous darkness. In accordance with
his main purpose, which was to apply organic chemistry and vegetable
physiology to the service of agriculture, Liebig directed the severity
of his criticism first of all against the humus-theory constructed
by chemists and agriculturists and thoughtlessly adopted by various
physiologists; this was the first thing that must be got rid of, if
the question was to be answered, of what substances does the food
of plants consist, for the humus-theory was at once incorrect, and
the product of a want of reflection which overlooked facts which lay
before men’s eyes. Liebig showed that what was known as humus is not
diminished but constantly increased by vegetation, that the quantity
in existence would not suffice for any length of time for the support
of a vigorous vegetation, and that it is not taken up by plants. This
once established, and Liebig’s calculations left no doubt on the
point, there remained one source only for the carbon of the plant,
namely, the carbon dioxide of the atmosphere, with regard to which it
was shown by a very simple calculation resting on eudiometric results
that its quantity is sufficient to supply the vegetation of the whole
earth for countless generations. It is true that Liebig in his zeal
went much too far, when he found something contradictory in the true
respiration of plants, because it is connected with the elimination of
carbon dioxide, and simply denied its reality. On the other hand the
theoretical significance of the fact established by de Saussure, that
the elements of water are assimilated at the same time as the carbon,
was now for the first time clearly explained. Liebig was better able to
realise the importance of this fact for the theory of nutrition than
de Saussure had been. But these weighty points were not the ones which
attracted most attention with the adherents and opponents of Liebig;
the practical tendency of his book made the discussion, to which it
gave rise especially among chemists and agriculturists, turn rather on
the question of the source of the nitrogen in the substance of plants.
The humus-theory had made the nitrogen like the carbon enter the plant
in the form of organic compounds. De Saussure in his great work of 1804
had named ammonia as a compound of nitrogen which might be taken into
consideration with others, but he arrived at no definite conclusion.
Liebig, from different points of view and in reliance on his own
investigations into the nature of nitrogen and its compounds, arrived
at the result, that ammonia must ultimately be the sole source of the
nitrogen in the plant, and that the ammonia in the atmosphere and in
the soil is quite sufficient to supply vegetation with the requisite
amount of nitrogen just as the carbon dioxide of the atmosphere is
the sole source of the carbon of the plant; and so he concluded that
‘carbon dioxide, ammonia, and water contain in their elements the
requisites for the production of all the substances that are in animals
and plants during their life-time. Carbon dioxide, ammonia, and water
are the ultimate products of the chemical process of their putrefaction
and decay.’
Liebig was less happy, at least as regards his mode of treating the
subject, in his remarks on the necessity and specific importance of
the constituents of the ash to the nutrition of plants. Instead of
insisting on an experimental answer to the question, what constituents
of the ash are absolutely indispensable to the health of one or all
plants, he lost himself in ingenious chemical theories, intended to
show the operation of inorganic bases in fixing vegetable acids, the
extent to which different bases can replace each other, and similar
matters.
It is not requisite for our purpose to follow Liebig in his
applications of his theoretical remarks to agriculture, still less
to occupy ourselves with the sensation and the discussions which his
work excited among practical and theoretical farmers and agricultural
chemists. The scientific value of Liebig’s considerations on the
nutrition of plants stood out in a purer and more definite form for
the vegetable physiologists, who turned their attention chiefly to
the points mentioned above. It is true that Liebig’s work encountered
lively opposition from these men also, and the two foremost
representatives of vegetable physiology at that time, Schleiden and von
Mohl, criticised it unsparingly; this was due partly to the deductive
method adopted by Liebig, to which botanists were unaccustomed in
physiological questions, and partly to the derogatory expressions in
which he indulged against the vegetable physiologists, whom he held
responsible with the botanists generally for all the absurdities
connected with the humus-theory. Von Mohl asked, and justly, whether
de Saussure, Davy, Carl Sprengel, Berzelius and Mulder, the real
founders of the theory, were botanists. But it was unnecessary for von
Mohl, Schleiden and others to feel touched by Liebig’s reproach, at
least so far as it was addressed to professed physiologists, for they
were no more physiologists than Davy, Berzelius or Mulder. Professed
vegetable physiologists, official public representatives of vegetable
physiology there were none, and then as now every one who occupied
himself occasionally with questions of the kind was called a vegetable
physiologist. In this way the contest became a dispute about words,
and Liebig, von Mohl and Schleiden lost an excellent opportunity for
influencing public opinion in favour of the idea, that it was high
time to establish public official representatives of so important a
branch of science, who should devote themselves entirely to it; how
could it be expected that Professors of botany, who were required
by the government and the public to work for the advancement of
systematic botany, phytotomy, and medical botany, to give instruction
in these subjects, and to devote a large portion of their time to the
management of botanic gardens, should do much to promote the study of
vegetable physiology, which demands very considerable acquaintance also
with physics and chemistry? and where were the laboratories and the
instruments for the serious prosecution of this branch of science? But
these questions were not raised, and the old state of things remained
for the time unchanged.
As regards the scientific questions at issue between Liebig and von
Mohl, Schleiden, and various agricultural chemists, the contest was
chiefly about matters of secondary importance, and among these might
be included the objection that Liebig knew scarcely anything of the
anatomy of the plant. The main point was, that he had corrected
mistaken views as to the way in which plants are fed, had refuted
gross errors, had shown what was fundamental and essential and what
was unimportant. Everything that was written on the subject after 1840
shows that he did all this completely; the publications called forth
by the controversy on his book occupied in the main the ground which
Liebig had cleared. Now every body knew all at once what was meant by
the decompositon of carbon dioxide in the green parts of plants, that
the constituents of the ash are not mere seasoning to the vegetation,
and the like; firm ground had been won for all, a number of scientific
truths had become common property for ever; this did not of course make
it less meritorious in others, to test the rest of Liebig’s theories,
or even to correct his great mistake about the respiration of plants,
as was done emphatically by von Mohl.
It would not be consistent with the design of this work to go into all
the details of the discussion excited by the appearance of Liebig’s
book, into questions for instance respecting the first products
of assimilation in plants, and their further transformations by
metabolism. Whether the primary use of the basic mineral constituents
is merely to fix the vegetable acids, whether these acids are the
first products of assimilation, or whether carbo-hydrates are the
immediate result of that process, and similar questions, were for some
time only matter of conjecture, deduction and combination, unsupported
by certain observation obtained by suitable methods; it was not
till after 1860 that new paths were struck out on these subjects,
and important results achieved. More important at the time for the
advance of the science was the further examination of the question
respecting the source of the nitrogen which plants assimilate; it was
the more necessary that this point should be finally settled, because
Liebig’s deductions still gave room for many doubts, and the first
of vegetable physiologists, de Saussure, in his later days made the
mistake of coming forward in opposition to Liebig as a defender of the
humus-theory, maintaining (1842) that ammonia or the nitrates are not
themselves the food-material of plants, but only serve to dissolve
the humus. Others also found it difficult to give up entirely the
old and favourite doctrine of the humus; though von Mohl and others
acknowledged that the carbon of plants is mainly derived from the
atmosphere, yet they thought themselves obliged to assign to the humus,
on account of the nitrogen which it contains, a very important share
in promoting vegetation. Under these circumstances it was extremely
fortunate for physiology that BOUSSINGAULT took up the question. He
had occupied himself before the appearance of Liebig’s book with
experimental and analytical investigations into germination and
vegetation, and specially into the source of nitrogen in plants. His
experiments in vegetation in 1837 and 1838 produced no very decisive
results; but he continued them for some time longer, improving his
methods of observation from year to year; and between the years 1851
and 1855 he succeeded in establishing with all certainty as the result
of many repeated trials, that plants are not capable of assimilating
the free nitrogen of the atmosphere, but that a normal and vigorous
vegetation is produced, when they are supplied with nitrogen from the
nitrates in the soil. It appeared also that plants will flourish in
a soil from which all trace of organic substance has been removed by
heat, if a nitrate is added to the constituents of the ash; this proves
at the same time that the whole of the carbon in such plants is derived
from the carbon dioxide of the atmosphere without the co-operation
of the humus, and that consequently the favourable effect of a soil
rich in humus on vegetation must be due to other causes than those
which were assumed by the humus-theory. We cannot describe the further
services rendered by Boussingault to the theory of nutrition, for this
would take us too much into technical details, and the best and most
important of his results were first given to the world after 1860,
and do not fall therefore within the limits of this history. But it
should be mentioned that Boussingault must be considered the founder
of modern methods of conducting experiments in vegetation. Liebig had
before spoken in terms of sufficient severity of the miserable way
in which experiments on the subject of the nutrition of plants were
managed after de Saussure’s time till later than 1830, but he did not
himself introduce better methods; this was reserved for Boussingault.
One instance may be given; those who desired to decide the question of
the humus by experiment, such as Hartig in conjunction with Liebig and
others, generally adopted the plan of supplying plants with compounds
of humus-acid, and seeing what would be the result. Boussingault did
as Columbus with the egg; he simply made plants supply themselves
with food in a soil artificially deprived of all trace of humus and
containing a mixture of food-material, in order to prove beyond
question that they do not need humus.
In Germany also Prince Salm-Horstmar made similar experiments to
those of Boussingault; he occupied himself chiefly in determining the
relative importance of the acids and bases of the ash in the nutrition
of plants, whether any and which of them are indispensable; these are
questions which approached their solution only after 1860, and some are
not yet decided.
The establishment of the facts, that plants containing chlorophyll
derive the whole of their carbon from the carbon dioxide of the
atmosphere, and that the latter is also the original source of the
carbon in plants and animals which do not contain chlorophyll; further
that the nitrogen which plants assimilate is derived from ammoniacal
salts or nitrates, and that the alkalies, alkaline earths in the form
of sulphates and phosphates, are indispensable ingredients in the food
of plants, must be considered to be the great results of the labour
bestowed on the theory of nutrition in the period from 1840 to 1860,
while the way was also prepared for many points, which were afterwards
of the first importance in the enquiry.
On the other hand the advance made in the theory of the movement of
the sap from the time of Dutrochet till nearly 1860 was so small as
to be scarcely worth mentioning; yet it was an advance, that the
physiological value of the doctrine of endosmose was more and more
highly estimated, and that more solid proofs of the theory itself
and a more exact acquaintance with osmotic processes were making it
possible to explain more of the details of the movement of material
in the plant, though the whole question was far from being finally
settled. One discovery must be specially mentioned, the establishment
by Hofmeister in 1857 of the fact, that the phenomenon observed for
centuries in the grape-vine and other trees, and more recently in Agave
and in many tropical climbing plants, known by the name of bleeding or
weeping and supposed to be confined to certain periods of vegetation,
not only occurs in all plants with true woody cells, but may be
produced in them at all times by suitable means. The knowledge of this
fact was an aid to the investigation of the cause of the weeping.
The theory of the descending sap was in the least advanced condition
during this period; appeal was still made to experiments of the kind
which Malpighi, Du Hamel, and Cotta had made, and which in reality show
nothing more than that in dicotyledonous woody plants a food elaborated
in the leaves is carried downwards through the cortex. As soon as it
was understood, that all organic substance originates in the leaves, a
fact which no one could doubt after 1840, no experiment was required
to prove that the formative matter necessary for the growth of the
roots, buds, and fruit, must be conducted to those parts from the
leaves. It could no longer be a question whether such a movement of
assimilated material takes place; it remained only to consider what are
the conducting tissues, and what is the nature of the substances which
are produced in the leaves and conducted to the rest of the organs.
Both questions in accordance with the organisation of the plant could
be properly answered only by microchemical methods, and these were not
adopted and further developed till after 1857. We have already said
that nothing certain was known even as late as 1860 about the chemical
combinations formed by assimilation in the leaves; De Candolle supposed
that the primary formative sap was a gum-like substance, from which
the rest of the various vegetable substances were secreted in the
different tissues. Theodor Hartig, who had done good service between
1850 and 1860 by his investigations into the starch in the wood of
trees and into proteid in seeds, by the discovery of sieve-tubes, by
observations on the amount of water in woods at different times of the
year, and by other contributions to botanical science, also occupied
himself with the subject of the descending sap, which he conceived of
as a formless primary mucilage, from which, as from De Candolle’s gum,
the various substances in the plant were deposited as it travelled
through the plant. He says (‘Botanische Zeitung’ for 1858, p. 341),
‘The crude sap is changed in the leaves into primitive formative sap,’
and ‘the formation of solid reserve-material (from this) involves
the elimination of large quantities of watery fluid.’ The occasional
remarks of vegetable physiologists of all sorts between 1840 and
1860 prove, that similar ideas respecting the formation of a primary
mucilage of this kind in the leaves were generally entertained.
CHAPTER III.
HISTORY OF THE DOCTRINE OF THE MOVEMENTS OF PLANTS (PHYTODYNAMICS).
It will scarcely be doubted at the present day, that the mechanical
laws of growth, of geotropic and heliotropic curvatures, of the
various kinds of periodic movements, of the twining of tendrils
and climbing plants, and of movements dependent on irritation, may
be referred to a common principle, and that in all these movements
besides the elasticity of the cell-walls the still unknown qualities
of the protoplasm play the most important part, and that consequently
the ‘streamings’ of the protoplasm, the movements of swarmspores and
similar occurrences must be ranked with these phytodynamical phenomena.
From this point of view phytodynamics would appear to be one of the
most important foundations of vegetable physiology. The recognition of
this fact is however of very recent date, and to imagine that such a
conception of the movements of plants was present to the minds of the
early physiologists would be to attribute to the past ideas to which
it was entirely a stranger. These movements were scarcely noticed even
as curiosities in former ages, and it was not till the end of the 17th
century that some attention began to be paid to them; and very slow
progress was made at a later time in disentangling the relations which
come under consideration and which are some of them very complicated,
in determining the dependence of the phenomena on external influences,
and explaining to some extent their mechanical conditions.
Single movements of parts of plants are noticed in a cursory manner by
some early writers. Varro was the first who mentioned the heliotropic
movements of the stalks of many flowers, which he says were for that
reason called heliotropic flowers; in the following century Pliny
says that the leaves of clover close when bad weather is approaching;
Albertus Magnus in the 13th century, Valerius Cordus and Garcias del
Huerto in the 16th, thought the daily periodical movements of the
pinnate leaves of some Leguminosae worth recording; Cesalpino noticed
the movements of tendrils and climbing plants, and was surprised to
see that the latter to some extent seek for their supports. These were
every-day phenomena, but the striking sensitiveness of the leaves
of Mimosa pudica introduced from America could not fail to attract
attention, and so we find an essay on the causes of it in Robert
Hooke’s ‘Micrographia’ of 1667. The irritability of the stamens of
Centaurea had been already mentioned by Borelli in 1653.
1. We meet with the first speculations on the subject at the end of
the 17th century. Ray in his ‘Historia Plantarum’ (1693) commences
his general considerations on the nature of the plant with a succinct
account of phytodynamical phenomena, and introduces the whole by a
sentence of Jung: ‘Planta est corpus vivens non sentiens,’ etc. Though
Ray, like Cesalpino, seems to believe in the Aristotelian soul of
plants, yet he does on the whole endeavour to explain the movements
which he describes by physical and mechanical laws; he thinks that
the irritability of Mimosa in particular is not due to sensation,
but to known physical causes; the movement of the leaf when it is
touched is caused by a contraction, which again is due to a withering
or relaxation of its parts. He endeavours to apply the knowledge
of his time to the explanation of the mechanical process: leaves,
he says, remain tense only because the loss by evaporation is kept
constantly supplied by the water that flows to them from the stem;
if then in consequence of a touch the sap-passages of the leaves are
pressed together, the supply of water is not sufficient to prevent
their becoming relaxed. Ray mixes up together the movements from
irritability and the daily periodical movements, as was done till
recent times; the latter, he says, occur not only in the leaves of
Leguminosae, but in almost all similar pinnate leaves, and with these
periodical movements of leaves he places also the periodical opening
and closing of the flowers of Calendula, Cichorium, Convolvulus, and
others. That these last movements are due to changes of temperature
appeared to him to be proved by an experiment of Jacob Cornutus on
flowers of Anemone, which, when cut off and placed in a well-closed
box in a warm place, opened at an unusual time if the flower stalk
only was dipped in warm water. This fact, afterwards forgotten and
discovered again a few years ago, of the dependence of the movements of
flowers on changes of temperature, was applied by Ray to explain the
periodical movements of leaves, which, to use his own expression, fold
themselves together as the cold of night draws on, and open again with
the day, and as he thought that these movements are of the same kind
as the movements of irritability in Mimoseae, he tries to explain how
cooling has the same effect as a touch. It was natural in the existing
state of science to assume that changes of temperature were the first
causes of various movements, for a thrust was at that time almost the
only recognised cause of motion. Hence Ray explained the movements
of growing stems which are now called heliotropic by a difference of
temperature on the opposite sides. A certain Dr. Sharroc had observed
the stem of a plant on which he was experimenting grow towards that
part of a window, where the air found free entrance through an
opening; from this circumstance, and from the rapid elongation of the
stems of plants growing under cover, which he ascribed to the higher
temperature, Ray concluded that cold air hinders the growth of the side
of a stem on which it falls, and that this side must become concave.
Thus Ray used the etiolation of plants grown under cover to explain
their heliotropic curvatures, as De Candolle did one hundred and
forty years later, only with this difference, that he described the
rapidity with which forced plants shoot up to the higher temperature,
De Candolle to want of light. On the other hand Ray knew perfectly well
that the green colour of leaves is not produced by the access of air
but by the light, for, as he says, plants become green under glass, and
not under an opaque cover; and if they become less green under glass
than in the open air, this is because the glass absorbs certain rays of
light and reflects others. Ray however, like almost all later observers
till quite recent times, did not keep the elongation and bleaching of
etiolated plants sufficiently distinct; his account of this phenomenon
is spoilt by the presence of much that is obscure.
It has been justly observed by other writers on botanical subjects
that no notice is usually taken of one of the most remarkable of the
phenomena of which we are here speaking, because, being a matter of
every-day occurrence, it is simply accepted as something obviously in
accordance with the nature of things; this is the fact, that the main
stems of plants grow vertically upwards and their main roots downwards.
To the French academician Dodart, whom we have already encountered
in the history of the theory of nutrition, is due the great merit of
being the first to find this apparently simple phenomenon really very
remarkable; he convinced himself by experiments on germinating plants,
that these vertical positions are caused by curvatures, and endeavoured
to discover the physical reason why the main roots if placed in an
abnormal position escape from it by curving in the downward direction,
and the main stems in the upward direction, till they both reach the
vertical line. It was a matter of minor importance that his mechanical
explanation, which supposed that the fibres of the roots contract on
the moister side and those of the stem on the same side lengthen, was
quite unsatisfactory; it was much more important that these remarkable
phenomena were made the subject of scientific enquiry, and we find that
various observers soon after directed their attention to them, and
exercised their acuteness in attempts at explaining them; to these
attempts we shall return in a future page.
A still more universal phenomenon than the vertical growth of stems
and roots is the growth of plants generally, and it required as
much or even more of the spirit of enquiry to propose the question,
whether this growth can be explained by mechanical laws, and what that
explanation is. Mariotte touched on this question in 1679, but only
incidentally, and supposed that the stretching of the pith, which meant
at that time the whole of the parenchymatous tissue, was the cause of
the growth of the parts of plants. This idea might have had its origin
in the Aristotelian notion that the pith is the seat of the vegetable
soul, but Mariotte endeavoured to give physical reasons for it. Hales
in his ‘Statical Essays’ of 1727 went much more minutely into the
question of the growth of plants. Following the train of thought in his
doctrine of the nutrition of plants, he introduces his observations on
their growth with the remark, that plants consist of sulphur, volatile
salts, earth, water, and air, the first four of which attract one
another, and therefore form the solid, inert part of the substance of
plants; the air behaves in a similar manner as long as it is kept by
the other substances in a solid condition; but as soon as it is set at
liberty it is capable of expansion. On this power of expansion in the
air, by which the juices of plants are quickened and strengthened, he
builds his mechanical theory of growth, according to which the plastic
parts of the plant assume a state of tension, and as the air enters
into combination with other substances and so becomes fixed, warmth and
movement are excited, and these make the particles of sap assume by
degrees a form and shape. These principles supplied his starting-point.
To get a clearer idea of the way in which the growth of the parts
of plants proceeds, he made equi-distant punctures in young stalks
and leaves, and found that the intervals between them increased by
growth more in the younger intervening parts than in the older. In the
course of these observations he is particularly struck by the great
longitudinal extension which accompanies growth, because, as he says,
the vessels still continue hollow, as a glass tube when drawn out to
its utmost extent retains its canal. He finds Borelli’s idea confirmed,
that the young shoot grows by the extension in length of the moisture
in the spongy pith; and he endeavours to explain the fact that the
growing shoot does not extend equally in the transverse direction, and
so become spherically rounded off like an apple, from the nature of the
structure of the cell-tissue. That the air enclosed in the tissue and
the sap with it presses into the shoot with sufficient force to produce
so great an extension, he thinks is proved by his experiments, which
show him the great force with which the water rises in the bleeding
vine, and forces itself into swelling peas; it is known, he says, that
water acts with great force when it is heated in a vessel, for water
can be driven into the air by heat; the sap in plants is composed of
water, air, and other active ingredients, and makes its way with great
force into the tubes and cells, when it is heated by the sun.
2. The course of the 18th century gradually increased the number
of the phytodynamical phenomena, to which physiologists paid more
or less attention, and repeated attempts were made to explain them
on mechanical principles. These attempts were for the most part
unsatisfactory, because movements distinct in kind from one another
were mixed up together, their dependence on external influences was not
distinctly perceived, and the knowledge of the anatomical structure of
the parts which exhibited the movements was, owing to the decline of
phytotomy, extremely imperfect. Moisture and warmth played the chief
part in these explanations, but their mode of operation was expressed
in the most general terms; the mechanical processes in plants were
described much in the way in which a person with very indefinite ideas
as to the nature of steam and the construction of the inside of a
steam-engine might speak of its movements. The majority of writers,
in accordance with the tendencies of the age, professed their desire
to refer the phenomena of life in plants not to an unknown principle
called the soul, but to mechanical and physical causes; but they did
not apply their minds to the examination of these phenomena with that
strenuous effort, which in this subject especially could alone lead to
a complete and satisfactory explanation of them.
Linnaeus studied the periodical movements of flowers in 1751 and those
of leaves in 1755, but a mechanical explanation of them was not to be
expected from him; he contented himself with pointing out the external
conditions of these phenomena in many species, with classifying them,
and giving the periodical movements a new name by calling the positions
assumed by night the sleep of plants; nor did he use the word at all in
a metaphorical sense, for he saw in this sleep of plants a phenomenon
entirely analogous to sleep in animals. That the sleep-movements were
not capricious but due to external influences was with him a necessary
consequence from the nature and idea of the plant, which was that of
a living and growing being, only without sensation. But it should be
mentioned that he stated correctly that the movements connected with
the sleep of plants are not caused by changes of temperature, or not
by these only, but by change of light, since they take place in the
uniform temperature of a conservatory.
Linnaeus’ account of these kinds of movement was only formal, it is
true, but still it was well-arranged and clear; the treatment of the
same and other movements by his contemporary Bonnet was quite the
reverse. It is scarcely possible to imagine anything more shapeless,
such an utter confusion of things entirely different from one another,
as is to be found in Bonnet’s experiments and reflections on the
various movements of leaves and stems in his work on the function
of leaves, published in 1754; geotropic and heliotropic curvatures,
nutations and periodic movements, are all run one into another; a
person who understands something of the subject may find here and
there single things in his experiments that may be turned to account,
but he was himself unable to make any use of them. He set out with a
preconception which prevented him from the first from understanding
what his experiments showed him; it was his object to prove from a
multitude of instances, that stalks and leaves so curve, twist and turn
in all cases, that the under sides of the leaves are directed towards
the ground, in order that they may be able to suck up the dew, which
according to his theory is the chief nutriment of plants and rises from
the ground. It is no great merit in him, that amid all this confusion
a correct observation here and there forced itself upon him, as for
instance that organs, chiefly such as are young and ductile, if they
are put out of their natural position, endeavour to recover it by
bending and twisting. On the other hand his conclusions with regard to
the mechanical causes of these movements are utterly inane; the least
skill in judging of the results of his experiments must have led him to
very different ideas; warmth and moisture, he says, appear to be the
natural causes of movement, but warmth is more effective than moisture,
and the warmth of the sun more effective than that of the air. This
explanation is unsuitable to just those cases which he chiefly studied,
the geotropic and heliotropic curvatures. In one point only he arrived
ultimately at a right judgment, namely that the great lengthening of
the stem, the small size attained by the leaves and the want of colour
in plants grown under cover, are caused by partial or entire absence of
light; Ray however had shown this before as regards the colour.
Though Du Hamel, like many later writers, treated Bonnet’s
investigations, uncritical as they were and without plan, with great
respect, he gave himself a much better account of the various movements
of plants. In the sixth chapter of the fourth book of his ‘Physique
des arbres,’ 1758, he discussed all the phenomena of the kind that
were known to him under the title: ‘On the direction of stem and
roots, and on the nutation of the parts of plants.’ Under the head
of upright or oblique direction of the stem and roots, he speaks of
geotropic, heliotropic, and some other curvatures; then follows a
chapter on etiolation, and under the title, ‘Movements of plants, which
approximate to some extent to the voluntary movements of animals,’ he
enquires into the periodical and sensitive movements of the leaves of
Mimosa; he winds up with a short account of Linnaeus’ flower-clock, and
of the hygroscopic movements of the valves of fruits. The movements
of tendrils and climbing stems, of which Du Hamel seems to have known
little, are not mentioned in this connection; but they are noticed in
a former chapter with hairs, thorns and similar things,—a plan which
Cesalpino also adopted. If this way of dealing with the different
movements of plants is to be taken as a classification of them, it
was a very unsatisfactory one; for it separates like things, and
brings together things unlike; still it is an improvement on Bonnet’s
arrangement, while the author gives us also some new and valuable
observations. He may claim to be the first who made heliotropic
curvature depend on light, and it is a significant fact that he got
this conclusion from Bonnet’s experiments. After examining, like Hales,
into the distribution of growth in shoots, and discovering that this
ceases with the commencement of lignification, he proposed to himself
the question: at what spots does the lengthening of the roots take
place, and he found from suitable experiments that every root-fibre
grows only at its terminal portion, which is a few lines in length,
and that no other part of it increases in length. In the chapter on
the direction of the parts of plants he examines the explanations
which had been given of heliotropic curvatures. Astruc and de la Hire
had supposed the weight of the descending sap to be the cause of the
downward curvature of the roots, and the lighter vapours which ascend
in the tissue to be the cause of the upward curvature of the stem;
Bazin on the contrary attributed the geotropism of the roots to the
moisture in the earth. Du Hamel undertook to determine whether the
moisture, the low temperature, or the absence of light in the earth
made the roots curve downwards, and he was obliged by the result of
his experiments to deny that they do. But he was unfortunate in his
own explanation of the movements which we should now call geotropic,
heliotropic and periodic, for he came to the conclusion that the
‘direction of the vapours’ inside the vessels of the plant and round
about the plant has more to do with producing these movements than
any other causes, and that if warmth and light appear to influence
them, it is perhaps only because they produce vapours or communicate a
definite movement to them. As regards the movements of the leaves of
Mimosa, Du Hamel repeated the experiment made by Mairan in 1729, in
which the periodic movement continued even in constant darkness; he
found that this was so, and concluded that the periodic movements of
Mimosa do not essentially depend on temperature and changes of light;
Hill had determined in 1757 that the alternation of day and night was
the cause of the movements connected with the sleep of plants, because
he found that darkness artificially produced in the day-time made the
plants assume the nocturnal position; but Zinn in 1759 came to the same
conclusion as Mairan and Du Hamel. It was not till long after that the
question was to some extent cleared up by Dutrochet. Du Hamel thought
it necessary to give a formal refutation of the opinion expressed by
Tournefort, that the movements of plants are produced by muscles, and
to show that Tournefort’s vegetable muscles are hygroscopic fibres.
We have to mention in conclusion, that Du Hamel was the first who
observed that the two branches of a vine-tendril twine in opposite
directions round a support that happens to be between them; he also
appears to have been the first who compared the irritability of the
stamens of Opuntia and Berberis with that of Mimosa-leaves; the
stamens of Berberis were afterwards examined by Covolo in 1764, by
Koelreuter 1788, by Smith in 1790, and by others, but without leading
to any discoveries respecting the nature of the irritability. Dal
Covolo’s famous essay on the stamens of the Cynareae (1764) produced
no absolutely final result, but it contained some particulars which
threw light on the mechanical laws of these movements of irritability.
Koelreuter, who studied these objects in 1766, thought less of
discovering a mechanical explanation of them, than of finding arguments
in the irritability of the stamens for the necessity of insects to
pollination. An entirely new kind of movement was discovered by Corti
in 1772 in the cells of Chara, which is now known as the circulation
of the protoplasm; this form of movement in plants appeared at first
to bear no resemblance whatever to the phytodynamic processes then
known, and it was not brought into connection with them till a long
time after; on the contrary an erroneous idea soon began to prevail,
that it was a real rotation of the sap, as understood by the early
physiologists; this idea held its ground till far into the 19th
century, and being combined with mistaken notions respecting the
movements of latex, was developed by Schultz-Schultzenstein into the
doctrine of the circulation of the vital sap. For a time indeed Corti’s
discovery was forgotten, and had to be reproduced by Treviranus in
1811. A somewhat similar fortune attended the discovery of the movement
of the Oscillatorieae by Adanson in 1767, which misled Vaucher into
pronouncing them to be animals.
3. Imperfect as were the theoretical efforts of the 18th century in
this branch of botanical study, yet they aimed at tracing the various
movements back to the play of physical forces. But in the closing
years of the century another order of ideas, injurious to the healthy
progress of science, made its appearance in this, as in other parts of
botany and zoology. Even the majority of those who had no sympathy with
the nature-philosophy and its phraseology, believed that there was in
organised bodies something of a special and peculiar nature; because
the attempts made to explain the phenomena of life by mechanical laws
were on the whole unsatisfactory, all such explanations were looked
upon as impossible and even absurd, while it was forgotten that the
vital force, which was to explain everything, was a mere word for
everything that could not be explained in the life of organisms. This
vital force was personified, and seemed to assume a really tangible
form in the movements of plants. But the moment that a phenomenon was
handed over to this force, all further investigation was abandoned;
the idea with regard to phytodynamical phenomena especially was that
of the peasant, who could only explain the movement of the locomotive
by supposing that there was a horse shut up in it. Moreover the
knowledge of the inner structure of plants was at its lowest point
at the end of the 18th century; the spiral threads which could be
unwound were the only structural element whose form was to some extent
understood, and their hygroscopic movements were supposed to be due
to a combination of the pulsations of the vital force with the spiral
tendency of the plant. At the same time whole bundles of vessels were
taken for spiral fibres, or were supposed to consist of them, and
these were thought to be vegetable muscles, which contract under the
influence of various kinds of irritation, and so cause the movements
in the organs of plants; but it was forgotten that in the organs which
exhibit the most striking movements, such as sensitive leaves and
leaves that suffer periodical changes of position, these ‘muscles’
occupy a central position which unfits them for the function ascribed
to them. It would be unprofitable and wearisome to give many examples
of what is here stated, though many might easily be collected; it will
suffice to quote some sentences only from Link’s ‘Grundlehren der
Anatomie und Physiologie’ of 1807; they are particularly instructive,
because Link declared against the nature-philosophy and professed to
be on the side of inductive science. Under the head of movements of
plants, he discussed geotropic curvatures and other movements in the
superficial manner of the time and only to come to the conclusion, that
the direction of growth of stems and roots is caused by a polarity of
a definite kind in every plant, from which we may argue, he says, ‘to
higher connections of our planet in the world of space.’ He says again,
‘that it is natural to conjecture that light is the cause of the sleep
of plants,’ and then gives the contradictory statements of Hill, Zinn,
and De Candolle, all jumbled together into an inextricable tangle in a
fashion which sets all maxims of reasonable discussion at defiance. He
then puts aside all attempts at mechanical explanation with the remark,
that plants observe their regular times of sleep even when kept in the
dark and at a low temperature, for this evident habituation is one of
the most important marks of vitality. He is led to similar results by
Desfontaine’s observation, that a Mimosa, exposed to the shaking of
a wheeled vehicle, closes at first but then opens again. Speaking of
the rapid oscillations of the leaves of Hedysarum gyrans and similar
movements, he rejects Percival’s idea of a will in plants, but says
that the attempts to derive them from mechanical or chemical causes has
only led to solemn trifling.
It is plain that men who could print such remarks as these and still
worse than these, could not possibly effect anything in the province
of botany which we are considering. The broad and shallow stream of
such opinions as these flowed on till later even than 1830, but it
ran dry at last when its supplies were cut off by the effect of new
discoveries, and scientific investigation again gained the upper hand.
Some calmer thinkers, who could not rest content with empty words, had
meanwhile been pursuing the path trodden by Ray, Dodart, Hales, and
Du Hamel, and by experiment and earnest reflection had brought new
facts to light, which were at least calculated to pave the way for the
mechanical explanation of phytodynamical phenomena. Senebier in his
‘Physiologic végétale’ (1700) had described some minute researches
which he had made into the subject of etiolation; and though he made
the great mistake of attributing the want of colour in the leaves
and the excessive elongation of the stems to the decomposition of
carbon dioxide which does not take place in the dark, yet he gave an
example of genuine scientific investigation and again expressed its
true spirit, when he said that the Linnaean phrase, ‘the sleep of
plants,’ was unsuitable, because the sleeping leaves are not relaxed,
but continue as stiff as in the day-time. De Candolle also, like
Senebier, experimented in 1806 on the influence of light on vegetation,
and succeeded in proving that the daily period of leaves may be
reversed by artificial illumination; he was, as we have said above,
an adherent of the theory of a vital force, but only made use of it
when physical explanations failed him. The same year, (1806) is the
date of a brilliant discovery, which was extremely inconvenient to the
thorough-going adherents of the nature-philosophy and the vital force,
and did much to bring the scientific study of the movements of plants
back to the right path. ANDREW KNIGHT[137] showed by experiment that
the vertical growth of stems and primary roots is due to gravitation;
he attached germinating plants to a rapidly revolving wheel, and thus
exposed them to the centrifugal force, either alone or combined with
gravitation; the radicles, which normally follow gravitation, here
took the direction of the centrifugal force, while the stems assumed
the opposite direction. The next question was, why gravitation makes
the roots and stems take exactly opposite directions, why, that is,
in a plant placed in a horizontal direction, the root-end curves
downwards and the stem upwards. Knight supposed that the root, being
of a semi-fluid consistence, is bent downwards by its own weight,
while the nutrient sap in the stem moves to the underside and causes
stronger growth there, until by means of the curvature so produced the
stem assumes the upright position. Here too, as in Dodart’s case, it
was no great misfortune that the explanation proved afterwards to be
insufficient; it served at the time to explain as much as was then
known of the matter. The spirit of true scientific research displayed
in Knight’s explanation of geotropism was expressed in many other
contributions which he made to vegetable physiology; two only must be
mentioned here. He showed in 1811 that under suitable conditions roots
are diverted from the vertical direction by moist earth, an observation
which was confirmed by Johnson in 1828 and afterwards forgotten. More
attention was excited by his discovery in 1812, that the tendrils
of Vitis and Ampelopsis are negatively heliotropical, that is, that
they turn away from the source of light. A few other cases of this
kind of heliotropism have since been discovered, and they are highly
interesting, because they teach that there is the same opposition in
the relations of plants to light as in their relations to gravitation.
Knight possessed some of the direct and bold reasoning power of his
countryman Hales; he defied the vital force, and was always ready with
a mechanical explanation, if it was at all possible to find one. Thus
he explained the twining of tendrils by supposing that the pressure of
the support drives the juices to the opposite side, which consequently
grows more vigorously and causes the curvature, which makes the tendril
wind round the support. This theory was at all events better than the
one which von Mohl sought to put in its place in 1827, and no better
one was suggested till very recently. Much the same may be said of
Knight’s explanation of geotropic curvatures; it is true that Johnson
showed in 1828 that the ends of roots as they curve downwards set in
motion a heavier weight than themselves, and therefore do not simply
sink down, and Pinot in 1829, that they force their way even into
quicksilver, and that consequently Knight’s theory, at least as regards
the roots, is unsatisfactory; but no better theory has yet been found,
and his view also of the progress in the upward curvature of the stem
has not given place to any one that can be said to be more generally
accepted.
It was the commonly received opinion till after 1820 that the
movements of the parts of plants are produced by the spiral vessels,
or, which meant the same thing in those days, by the vascular bundles.
It was an important event therefore when Dutrochet proved in 1822,
that the movements of the leaves of Mimosa were due to the alternate
expansion of the antagonistic masses of parenchyma in the pulvinus or
cushion of succulent tissue found at the articulation, and that the
central vascular bundle follows passively their curvatures. Lindsay had
indeed arrived at the same conclusion from similar experiments as early
as 1790, but his unprinted essay on the subject was first produced by
Burnett and Mayo in 1827. Meanwhile Dutrochet had also found that light
influences the movements of the leaves in different ways; alternation
of light and darkness excites them to motion, while leaves which have
become rigid in continued darkness are restored by light to their
normal condition of sensitiveness.
Much attention was bestowed in the period between 1820 and 1830 on
various questions connected with the movements of the organs of plants.
In 1826 the faculty of medicine in Tübingen offered a prize for an
essay on the peculiar nature of tendrils and climbing plants, which
was intended to bring into discussion all the points which required
to be cleared up before a more thorough understanding of the whole
subject could be obtained. The two essays which gained the prize were
published in 1827. One was by Palm, the other by von Mohl, both of very
different value. Palm’s essay is a good and careful college-exercise;
but there is nothing of this character in von Mohl’s. The skill
of the composition, the exact knowledge of the literature of the
subject, the wealth of personal experience, the searching criticism,
the prominence given to all that is fundamental and important, the
feeling of certainty and superiority which the book inspires, all
unite to make the reader forget that it is not the work of a mature
and professed naturalist, but of a student of two-and-twenty years
of age. This academical prize-essay on the structure and twining of
tendrils and climbing-plants was one of von Mohl’s best works, and
altogether the best that appeared on the subject before Darwin wrote
upon it in 1865; at the same time it must be said that von Mohl did
not explain the exact mechanical processes in the tissues, for he
assumed a sensitiveness in both cases which causes the winding round
the support, and thought that this sensitiveness must be conceived of
‘dynamically’ and not ‘mechanically.’ Nevertheless von Mohl conducted
his investigation up to this point according to strict rules of
inductive science, and studied the facts which were capable of being
established by observation and experiment with an exactness such as
had not yet been applied to any case of movement in plants. It was
a genuine production of its author, strictly inductive up to the
point at which deduction became necessary. Von Mohl pointed out in it
essential differences in the behaviour of tendrils and climbing plants,
and the corresponding distinction between the organs which have to
be considered in each case, and he made the important discovery that
contact with the support acts as a stimulus on the tendril, though
he was wrong in supposing that the climbing stem also is similarly
affected. He at once assented to Dutrochet’s new view, that it is not
the vascular bundles but the layers of parenchyma which produce the
movements. He distinctly rejected the notion constantly repeated,
though with some hesitation, since the time of Cesalpino, that tendrils
and climbing-plants ‘seem to seek for’ their supports, as also the idea
which many had adopted without reflection from Grew, that the varying
direction of a climbing-stem is due to the varying influence of the
course of the sun and moon, and showed that the movements of nutation
in the stem are sufficient to explain the apparent seeking for the
support; it is true that he did not fully explain the corresponding
phenomena in tendrils, but he saw enough to set aside the old ideas.
We must not here go further into his many, and for the most part
excellent, observations; some of course had afterwards to be corrected,
but the important point was, that his full investigation of the
subject showed how such phenomena must be studied, if we are to arrive
at a strictly mechanical explanation of them.
If von Mohl had attempted to give a mechanical explanation of the
processes in the tissue of twining organs he must necessarily
have failed from ignorance of the agency of diffusion, which must
certainly be taken into consideration. This agency was not discovered
by Dutrochet till the year (1826) in which von Mohl undertook his
investigation, and some time elapsed before it was sufficiently
understood to be successfully applied to the explanation of phenomena
in vegetation. Dutrochet did indeed attempt so to apply his theory in
1828, and showed that changes in the turgidity of tissue are produced
by endosmose and exosmose, and consequently that a new mechanical
method of explanation had been discovered for processes which had been
usually referred to a supposed vital principle; but in his later and
more detailed researches into geotropism, heliotropism, periodical
movements and movements of irritability, which he collected together
in his ‘Mémoires’ of 1837, he fell into two different mistakes: he
assumed conditions of size and stratification in cells which do not
actually exist, for the purpose of explaining very various kinds of
curvature by endosmose, and he was not satisfied with endosmose in
the parenchyma; he postulated changes in the vascular bundles also,
which were supposed to be produced by the influence of the oxygen
in a way which he did not explain. Thus there were blots in his
explanation of separate processes, and his mechanical theories remained
unsatisfactory; but it is worthy of recognition and was most important
for the development of phytodynamics, that he was thoroughly in earnest
in his purpose of explaining every movement in plants by mechanical
laws. Even the opponents of such explanations were obliged to go deeply
into mechanical relations in order to refute him, and no one could any
longer be imposed upon by the simple assertion that all depends on the
vital force; so devoted a partisan of vital force as Treviranus had to
deal with endosmose as an established principle. Moreover Dutrochet’s
copious investigations presented such an abundance of interesting
observations, delicate combinations, and suggestive considerations,
that the study of them is still instructive and indeed indispensable to
any one who is occupied with such researches. Comparison of his papers
in the ‘Mémoires’ of 1837 with what was before known on the mechanical
laws of the movements of plants leaves us in no doubt that energetic
mental effort had taken the place of the old complacent absence of
thought.
Still no single movement had as yet been fully explained on mechanical
principles; but by the year 1840 clearer views had been attained
on the whole subject; the co-operation of external agencies was in
substance recognised, and the different forms of movement were better
distinguished, though much still remained to be done in this direction;
and as regards the mechanical changes in the tissue of the parts
capable of movement, a factor had been given in endosmose which must be
taken into account, though it might be necessary to seek a different
mode of applying it.
4. Before proceeding to give some account of the theoretical efforts
that were made in this subject between 1840 and 1860, it should be
mentioned that new cases of movement in plants had been discovered.
Dutrochet observed that the stem in the embryo of Viscum is negatively
heliotropic, and had carefully studied its behaviour; he opposed the
old notion that the geotropic downward curvature is peculiar to main
roots, and that that is the reason why they are in ‘polar’ opposition
to the stem, by pointing to the shoots of the rhizomes of Sagittaria,
Sparganium, Typha, and other plants, which at least when young curve
downwards with some force; and on extending Knight’s experiment with
a rotating wheel he found that the leaves also exhibit a peculiar
geotropism. These observations and some new examples of periodical
movement and movements of irritability were connected without
difficulty with the forms of movement that had been long known in the
vegetable kingdom, and contributed to correct the views that had been
entertained respecting them. But this was not the case for a time with
two phenomena which also fall within the province of phytodynamics,
namely normal growth and the movements of the protoplasm, which exhibit
the two opposite extremes, so to speak, of the facts connected with
movement. Various measurements had been made of the growth of plants
since the beginning of the century, and attempts had been made to
establish its dependence on light and heat, but without any great
success. Treviranus had rediscovered the movements of the protoplasm
in 1811 in Nitella. Similar movements were repeatedly pointed out by
Amici, Meyen, and Schleiden in the cells of higher plants, but they
were taken for streamings of the cell-sap; it was still unknown that
all these were movements of the same organised substance, which moves
independently in water in the form of swarmspores. These phenomena,
especially the movements of swarmspores, were noticed and studied
separately between 1830 and 1840, but no one thought of bringing
both these movements and the mechanical laws of normal growth into
connection with the phenomena which had usually been treated together
under the head of movements in the vegetable kingdom. De Candolle and
Meyen did not mention them in this connection in their ‘Compendia’ of
1835 and 1839; Meyen on the contrary discussed the ‘circulation of the
cell-juice’ with nutrition, and the movement of swarmspores with the
propagation of Algae. The two writers just named, like Du Hamel before
them, divided into two main groups the movements in the vegetable
kingdom which had been long known and were usually put together, and
treated of geotropic and heliotropic curvatures and the movements of
tendrils and climbing plants under the head of direction of plants,
and the periodical movements and movements connected with irritability
under that of movements, though they gave no reasons for this
classification; it rested evidently on an indistinct feeling outrunning
clear perception—that in the one they were dealing with growing parts
of plants, in the other with parts which had ceased to grow. Dutrochet
made no such distinction, but he was the only one among the chief
representatives of vegetable physiology between 1830 and 1840 who had
thoroughly adopted the mechanical view of phytodynamical phenomena.
We have said that Treviranus was a warm adherent of the theory of
vital force. De Candolle and Meyen, it is true, endeavoured to explain
each separate movement if possible by mechanical laws, but in their
more general speculations they readily lapsed into antiquated views;
thus De Candolle speaks of the sensitiveness of Mimosa as a case of
extreme ‘excitability,’ and Roeper, in accordance with his other views,
translated De Candolle’s expression, autonomous movements, by the term
‘voluntary’ movements. The movements he is speaking of are those of
Hedysarum gyrans, and Meyen also terms them ‘voluntary’ movements,
and ranks them with those of Oscillatoria. That he was influenced
in this by a dim reminiscence of the old vegetable soul is shown by
the heading, ‘Of movements and sensation in plants,’ placed over the
section of his work in which the expression occurs; and in the last
chapter of this section, he attributes some kind of sensation to plants
on account of the evident marks of design in their movements, though he
veils his meaning in obscure and tortuous phrases.
5. The mists of the nature-philosophy and the vital force disappeared
from the phytodynamical province of botanical science after the year
1840. The methodical research of inductive science, which had still
to contend with them up to that time, was once more acknowledged as
the supreme guide and ruler. A few stray dissentients were still
to be found, but the general voice was against them. There was an
eager desire for exact investigation of the facts, in order to lay
a firmer foundation for future theory. But no conclusive results,
no such entirely new points of view were gained before 1860, as
were established during the same time in phytotomy, morphology, and
systematic botany. To these subjects the most eminent enquirers applied
their best powers almost exclusively, while phytodynamics vanished from
the field of view of the generality of botanists, and no one made them
the object of the comprehensive, intense, and effectual study, which
Dutrochet had previously devoted to them. At the same time his example
was not without a powerful effect. The working of endosmose was further
investigated and treated as a part of molecular physics. Greater
freedom was thus gained in the mechanical treatment of phytodynamical
questions, and a firmer basis secured by aid of the advances which were
being at the same time made in phytotomy. But with the exception of
Brucke’s essay on Mimosa (1848), the works produced during this period
were chiefly devoted to the critical examination of the writings of
previous observers, and whatever appeared that was new and positive
remained incomplete till after the date at which this history ends.
Under these circumstances we must be content to indicate briefly the
more important of the new discoveries and of the efforts made at this
time to advance the theory of the subject.
Several observers occupied themselves soon after 1840 with the
influence of light on the growing parts of plants. Payer maintained in
1843 that the radicles of various Phanerogams turn from the light, and
a controversy arose between him and Dutrochet on the point, in which
Durand took part in 1845, but no definite conclusion was arrived at
even as regards the certainty of the fact. The beautiful discovery of
Schmitz in 1843, that the Rhizomorphs grow more slowly in the light
than in the dark, and arc at the same time negatively heliotropic,
might have proved much more important; but the theoretical value of
this fact has till quite recently been entirely misconstrued. Sebastian
Poggioli had discovered in 1817 that highly refringent rays of light
were more heliotropically active, and the fact was confirmed by Payer
in 1842; but Dutrochet in 1843 maintained, and incorrectly, that it is
the brightness of the light, and not its refrangibility, which is the
determining factor. Zantedeschi found in 1843 that red, orange, and
yellow light are heliotropically inactive. Gardner on the contrary in
1844, and Guillemain in 1857, came with the help of the spectrum to
the conclusion that all its rays are heliotropically active, and the
question long remained hampered by these contradictory statements, till
it was taken up again in 1864. This was a similar case to that of the
question of the effect of variegated light on the elimination of oxygen
and the formation of chlorophyll. Daubeny had given attention to the
subject in 1836 and inclined to the view, that it was the brightness
of the light rather than its refrangibility which was the important
point; and Draper’s observation, made with the spectrum in 1844, that
the elimination of oxygen reaches its maximum in yellow light and
decreases on each side of it, was generally understood as though it
was a question only of the brightness of the light. It is only within
recent times that this view has been abandoned, and in the same way all
the investigations which have just been mentioned were not settled till
after 1860, and were scarcely turned to any theoretical account.
The bright point in the history of phytodynamics at this time is
Brücke’s treatise in 1848 on the movements of the leaves in Mimosa,
not only on account of the very important results which it records,
but still more for the exactness of its method which has made it a
model of research in these subjects. He first established the essential
difference between the periodical nocturnal position of the leaves of
Mimosa and the position which they assume when irritated, and showed
that the former is connected with an increase in turgidity, the latter
with relaxation; he showed further that if the upper half of the
organ is removed, the periodical movements and the irritability both
continue. Of great importance to the theory was the clear account
given of the tension which is produced between the vascular bundle and
the turgescent layer of parenchyma, and the reference of the periodic
movements and of those of irritation to the movements of water in the
antagonistic masses of parenchyma. The details were still imperfect,
but one great advantage was secured, namely, the doing away with the
mysticism associated with the idea of irritability, from which even von
Mohl was not entirely free.
A full enquiry into the downward curvature of roots, published by
Wigand in 1854, deserves mention, because it threw some light on the
theory of the strictly mechanical questions connected with a subject
which had been for some time neglected, and because, while containing
other instructive matter, it refuted the theory, founded on endosmose
and on the structure of tissue, which had been suggested by Dutrochet
and adopted by von Mohl, since it showed that one-celled organs also
exhibit geotropic curvatures. The great theoretical importance of the
fact that all the various phytodynamical phenomena, with the exception
of movements of irritability, are manifested in one-celled organs, was
not fully understood till after 1860.
It has been already observed, that no theoretical result was obtained
from the discovery of circulation in cells made by Corti in 1772, and
repeated by Treviranus in 1811. The same may also be really said of the
later observations of Amici, Meyen, and Schleiden, which went to show
that such movements occur very generally in vegetable cells. In like
manner the movements of swarmspores, of which a considerable number
of instances had been observed before 1840, were rather the subject
of astonishment than of scientific consideration. They could not in
fact find their place in the general system until Nägeli and von Mohl
discovered in 1846, that it is in the protoplasm that the so-called
movement of the cell-sap takes place, and Alexander Braun made it known
in 1848 that the swarmspores are naked masses of protoplasm, and indeed
true vegetable cells. A new substratum for the movements in plants,
and one of the simplest kind, was thus obtained; and Nägeli attempted
in 1849 a mechanical explanation of the movements of swarmspores, while
in 1859 De Bary exhibited in the Myxomycetes most instructive examples
of such movements. If Nägeli failed in his attempt, yet it seemed
possible that the protoplasm had an important share in the production
of all phytodynamic phenomena, and the idea appeared capable of a very
wide application when Unger pointed out in 1855 the resemblance between
vegetable and animal protoplasm. It is true that not one of these
later observations led to any conclusive results till after 1860; but
that the whole subject of phytodynamics had made considerable advance
as early as 1850 is apparent from the account given of it by von Mohl
in his ‘Vegetabilische Zelle’ of 1851, and by Unger in his ‘Lehrbuch
der Anatomie und Physiologie der Pflanzen’ of 1855. Von Mohl chiefly
exposes the unsatisfactory nature of the attempts that had been made to
explain the phenomena; Unger, on the other hand, shows how much that is
fundamentally important had been already established.
The mechanics of growth had not been included by former writers among
the phenomena of phytodynamics, nor was it so included by either Unger
or von Mohl. It seemed to be supposed that there was a fundamental
difference between growth and other movements in the vegetable
kingdom, and this idea was entertained even in the most recent times.
From the time of Mariotte and Hales no one had made the mechanical
laws of growth the subject of special investigation or theoretical
consideration; yet some observations had been made on the formal
relations of growth and its dependence on external influences. Ohlert
(1837) was the first after Du Hamel who studied the distribution of
growth in the root; Cotta in 1806, Chr. F. Meyer in 1808, Cassini in
1821, Steinheil and others made measurements in connection with the
same question in the stem, but only with the result of showing that the
distribution of growth at the internodes may vary very greatly, and
even Münter’s measurements in growing internodes in 1841 and 1843, and
Grisebach’s in 1843 led to no appreciable result, because the observers
neglected to apply the figures obtained to the theory of the subject.
It seemed to be generally supposed that it was enough simply to write
down the measurements in figures, and that a theoretical result would
spring into being of itself; on the contrary the real scientific work
begins after the figures are obtained. The same cause prevented the
observations which have yet to be mentioned from producing real fruit.
The influence of the variability of the temperature of the air[138],
and of the alternation of daylight and darkness on the longitudinal
growth of internodes and leaves after they have emerged from the
bud-condition, had often been investigated; Christian Jacob Trew
published in 1727 long-continued daily measurements on the flowering
stem of Agave Americana in conjunction with observations on temperature
and weather; a hundred years later similar observations were made by
Ernst Meyer in 1827, by Mulder in 1829, and by Van der Hopp and De
Vriese in 1847 and 1848; but Harting in 1842 and Caspary in 1856 were
the first who went at all deeply into the questions involved. These
observations, some of which were carefully made, led to no further
result than the discovery of the fact, which Münter indicated and
Harting applied to theoretical purposes but which no one else thought
worthy of attention, namely that the rate of growth increases at first
and independently of external causes, till it reaches a maximum, and
then decreases till at length it comes to an end; they did not even
establish a really practical method of observation. Scarcely two
observers arrived at the same result, because the questions respecting
the relations of growth in length to temperature and light had not
been clearly and distinctly put. Communications were published in the
periodicals, which simply tabled long-continued measurements of the
longitudinal growth of parts of plants, and gave an idea of constant
irregularity of growth, without suggesting any explanation of the
causes which produced it; so indistinct were the ideas of observers
on these subjects even after 1850, that the majority of them proposed
to themselves the question, what difference there is between growth
by day and by night; it did not occur to them that day and night
are not simple forces of nature, but different and very variable
complications of external conditions of growth, such as temperature,
light and moisture, and that such a mode of putting the question could
not possibly lead to the discovery of the relations established by
law, so long as the several factors were unknown which are included in
the conceptions of day and night. Harting’s essay of 1842 is superior
to those above mentioned, inasmuch as he distinctly endeavoured to
obtain from his measurements some definite propositions that might
be applied to the theory of the subject, and especially to give a
mathematical expression to the dependence of growth on temperature,
but his success in this particular point was not great. The idea, that
there must be a simple arithmetical relation to be discovered between
growth and temperature, had been suggested by Adanson in the previous
century, and it found many supporters in the period between 1840 and
1860: but it should be observed that the term growth was used in a
loose and popular sense to sum up all the phenomena of vegetation
in one expression. Adanson had supposed that the time occupied in
the unfolding of the bud was determined by the sum of the degrees
of the mean daily temperature, reckoned from the beginning of the
year; Senebier, and at a later time De Candolle, declared against the
existence of any such relation, but a similar idea was not only very
generally entertained after 1840, but it even came to be treated as a
probable natural law. Boussingault had pointed out that in the case
of cultivated plants in Europe and America, if the whole period of
vegetation expressed in days is multiplied by the mean temperature of
the same period, the products do not deviate widely from one another
in the same species. It was thereupon assumed that these deviations
are due to incorrect observation, and that such a constant product of
the period of vegetation and the mean temperature will be found in
every species. This product then received the unmeaning appellation
of the sum of the temperature. If such a relation between vegetation
and temperature really exists, it would necessarily follow that other
things, such as light, moisture, the soil, &c., have no influence at
all on the period of vegetation, not to speak of those internal causes
which help to complicate the simplest processes of growth. It is
unnecessary to expose in this place the absurdities involved in this
idea of the sum of the temperature; the needful remarks will be found
in the ‘Jahrbücher für wissenschaftliche Botanik’ of 1860, i. p. 370.
It is a remarkable fact however that such monstrous reasoning should
have been able to prejudice science in various ways even later than the
year 1860. A new science was actually invented and called Phaenology,
which accumulated thousands and thousands of figures, in order to
discover the sum of the temperature for every plant, and as this
crude empiricism showed that the simple multiplication of the period
of vegetation by the temperature gave no constant result, the square
of the temperature was tried and other tricks of arithmetic adopted.
Though Alphonse de Candolle as early as 1850 expressed well-founded
objections to the whole of this method of treating the subject, in
which the mean temperature played much too important a part, yet he
was so far unable to keep clear of the prevailing ideas, that he
thought he could express the effect of light by an equivalent number
of degrees of temperature, and so save the supposed law of temperature
in vegetation. To this idea may be traced his work on the geography of
plants, published in two volumes in 1855, which however contains a rich
treasure of personal experience and knowledge of the works of other
writers.
It appears then that scarcely any point of fundamental importance in
phytodynamics was cleared up before the period at which this history
closes; it was not till after that date that these questions began to
be studied from new points of view, and they are at the present time
still under discussion.
INDEX.
Adanson, 66, 116, 545, 561.
Aepinus, 257.
Agardh, 143, 160, 205, 352.
Albertus Magnus, 14.
Aldrovandi, 18.
Alpino, 380.
Alston, 402.
Amici, 223, 284, 371, 432, 434, 558.
Ammann, 39.
Aristotle, 4, 6, 13, 16, 43, 51, 219, 376, 450.
Astruc, 543.
Bachmann, 7, 39, 63, 74-76, 83, 101.
Baisse (de la Baisse), 483.
Banks, 139.
Bartling, 144, 145.
Batsch, 125, 137, 143.
Bauhin, Kaspar, 5, 6, 8, 12, 13, 17, 19, 24-26, 33, 39, 64, 80, 100,
115.
Bazin, 543.
Beale, 472.
Berkeley, 205.
Bernhardi, 109, 225, 256, 263-266, 347.
Bischoff, 161, 198, 207, 438, 439.
Blair, Patrick, 391.
Bock, Hieronymus, 3, 13, 14, 19, 24, 27, 28.
Boehmer, 248, 483.
Boerhaave, 78.
Bonnet, 163, 247, 486-488, 541.
Borelli, 536.
Bornet, 210, 443.
Boussingault, 373, 449, 531, 561.
Bradley, 391, 406.
Braun, A., 162, 165, 169, 170-181, 184, 208, 312, 314, 334, 336, 442,
558.
Bravais, 169.
Brisseau-Mirbel, 198, 224, 226, 250, 256, 259, 261, 262, 272-275, 284,
307, 311, 321.
Brongniart, Adolph, 147, 321, 432, 436.
Brown, Robert, 110, 112, 122, 139-144, 155, 151, 227, 323, 433.
Brücke, 339, 536, 557.
Brunfels, 3, 5, 13, 14.
Brunn, 255.
Buffon, 89.
Burckhard, 83, 391, 397.
Burnett, 550.
Calandrini, 486.
Camerarius, Rud. Jak., 60, 77, 81, 87, 361, 376, 385-390, 406.
Candolle, (_see_ De Candolle).
Caspary, 560.
Cassini, 559.
Cesalpino, Andrea, 5, 7, 9, 12, 17, 18, 23, 37, 40, 42-57, 61, 63, 80,
81, 103, 125, 163, 219, 220, 450.
Cessati, 213.
Choulant, 19.
Clusius (_see_ de l’Écluse).
Cohn, 209, 213, 442.
Comparetti, 249, 263, 282.
Corda, 184, 205.
Cordus, Valerius, 29, 536.
Cornulus, Jakob, 537.
Corti, 314, 513, 545, 558.
Cotta, 506, 559.
Covolo, dal, 410, 545.
Cramer, Karl, 203.
Dalechamps, 29, 30.
Darwin, Chas., 11, 12, 49, 53, 152, 169, 180, 183, 351, 431.
Daubeny, 557.
De Bary, 210, 213-215, 292, 314, 318, 339, 372, 443, 559.
Decaisne, 442.
De Candolle, Alphonse, 562.
De Candolle, Pyrame, 9, 71, 92, 110, 112, 122, 126-139, 307, 484, 515,
537, 554, 555, 561.
De la Baisse, 483.
De Lamarck, 127.
De l’Écluse, 13, 18, 19, 29-31, 55.
De la Hire, 543.
De l’Obel, 3, 6, 13, 17, 23, 26, 32, 35, 58, 64, 67.
Desfontaines, 136, 293, 307.
De Vriese, 508, 560.
Dillenius (Dillen), 76, 211, 437.
Dioscorides, 3, 4, 13, 15, 28, 34.
Dippel, 343.
Dodart, 538, 547.
Dodoens (Dodonaeus), 13, 18, 22, 29, 30.
Draper, 557.
Du Hamel du Monceau, 89, 247, 368, 488-491, 542-545, 559.
Du Petit-Thouars, 137, 489.
Durand, 556.
Dutrochet, 212, 370, 509-514, 550, 552, 553.
Ehrenberg, 208, 211, 322, 354, 438.
Eichler, 350.
Endlicher, 9, 110, 146, 333.
Erlach, 354.
Fabri, 403.
Fischer, 509.
Fogel, 59.
Frank, 39, 343.
Fries, Elias, 10, 111, 153, 205.
Fuchs, 3, 13, 14, 15, 18, 19, 20, 24.
Fürnrohr, 192.
Gärtner, Karl Friedrich, 370, 421, 427-430.
Gärtner, Joseph, 23, 110, 122-125, 207, 413.
Galen, 3, 15.
Garcias del Huerto, 536.
Gardner, 557.
Gaudichand, 293.
Geoffrey, 391, 395.
Gesner, Konrad, 18, 20, 29, 379.
Ghini, Luca, 18.
Girou de Bouzareingue, 422, 426.
Giseke, 137.
Gleditsch, 211, 212, 391, 393.
Gleichen-Russworm, 247, 249, 263 404, 431.
Goeppert, 184, 370, 507.
Goethe, 62, 144, 156-160, 263, 390.
Grew, Nehemiah, 69, 89, 93, 97, 221, 222, 223, 225, 231, 232, 234,
239-244, 263, 382-385, 551.
Grischow, 506.
Grisebach, 560.
Guillemain, 557.
Haartman, 400.
Hales, 89, 224, 363, 476-482, 539.
Haller, 66, 89, 404.
Hanstein, Johannes, 203, 343, 348, 350.
Hartig, Theodor, 301, 314, 342, 354, 532, 534.
Harting, 303, 560, 561.
Harvey, 205.
Hassenfratz, 495.
Hebenstreit, 76.
Hedwig, 123, 198, 207, 224, 253-255, 263, 283, 431, 437, 438.
Henfrey, 312, 335, 440.
Henschel, August, 422, 424, 425.
Herbert, William, 370, 420, 421, 431.
Hermann, 68.
Heucher, 76.
Hill, 76, 544.
Hofmeister, Wilhelm, 11, 118, 167, 170, 184, 199-203, 208, 209, 210,
228, 312, 318, 335, 336, 371, 439, 440.
Hooke, Robert, 221, 223, 229-232, 536.
Hornschuch, 206.
Ingen-Houss, 224, 368, 491, 493, 494-497.
Irmisch, 165.
Jessen, 397.
Johnson, 549.
Jungermann, 39.
Jung (Jungius), 40, 43, 58-63, 64, 73, 80, 115, 155, 221, 381,
454-456.
Jussieu, Antoine Laurent de, 9, 23, 77, 92, 109, 110, 116-122, 125,
155, 431
Jussieu, Bernard de, 9, 41, 109, 115.
Karsten, 313, 320.
Kessler, 19.
Kieser, 160, 283, 320.
Knaut, Christopher, 74, 76.
Knight, Andrew, 421, 431, 506, 548.
Kölliker, 313.
Koelreuter, 89, 122, 123, 247, 406-414, 431, 437, 544, 545.
Kützing, 205, 206.
Lantzius-Beninga, 198.
Lavoisier, 491, 492, 507.
Leeuwenhoek, 223, 244, 245, 259.
Leibnitz, 83, 391, 397.
Leitgeb, 203.
Lesczyc-Suminsky, 438, 441.
L’Heritier, 137.
Léveillé, 205.
Liebig, 373, 449, 525-531.
Lindley, 9, 147, 148.
Lindsay, 550.
Link, 161, 211, 225, 233-259, 261, 267-270, 310, 505, 546.
Linnaeus, 8-10, 37, 40, 41, 49, 56, 65, 71, 79-108, 113, 118, 397-402,
431.
Lister, 470.
Lobelius (_see_ de l’Obel).
Logan, James, 391, 392.
Ludwig, 76, 248.
Macaire, Prinsep, 511.
Magnol, 8, 470.
Mairan, 544.
Major, Johann Daniel, 456, 460, 469.
Malpighi, 44, 48, 63, 69, 89, 155, 221, 223, 231-239, 241, 262, 363,
366, 367, 381, 457-461.
Man, James, 258.
Marcet, 506.
Mariotte, 461-470, 539.
Mattioli, 3, 18, 29.
Mayo, 550.
Medicus, 255, 267.
Menzel, 39.
Mercklin, 441.
Mettenius, 198, 202, 439.
Meyen, 208, 225, 226, 259, 260, 284-292, 305, 310, 322, 333, 351, 508,
514, 523, 554, 555, 558.
Meyer, Chr., 559.
Meyer, Ernst, 18, 160, 161, 401, 560.
Micheli, 211, 437.
Mikan, 385.
Milde, 202, 440.
Millardet, 350.
Miller, 391, 392.
Millington, 382, 384, 385, 399.
Mirbel (_see_ Brisseau-Mirbel).
Mohl, Hugo von, 105, 161, 183, 192, 223, 226, 227, 259, 260, 284,
291-311, 318, 321, 325, 329, 336, 340, 349, 350, 351, 354, 355,
374, 529-532, 550, 551, 558.
Moldenhawer, J. J. P., 225, 257-261 276-284.
Morison, 7, 8, 63, 66-68, 101.
Morland, Samuel, 391, 394.
Morren, 208, 322.
Mulder, 303, 529, 560.
Müller, 343.
Münter, 560.
Mustel, 266, 267, 490.
Nägeli, 11, 63, 118, 161, 166, 183, 185, 193-197, 208, 226, 227, 297,
302, 312-316, 318, 326-334, 336, 340, 346-356, 438, 558, 559.
Naumann, 169.
Needham, 431.
Nees von Essenbeck, 160, 205, 212, 438.
Nieuwentyt, 472.
Oelhafen, 39.
Ohlert, 559.
Oken, 161.
Palm, 550.
Payen, 303.
Payer, 191, 556, 557.
Percival, 547.
Perrault, 403, 460, 470.
Persoon, 211.
Plato, 11.
Platz, Wilhelm, 483.
Pliny, 3, 13. 15, 34, 378.
Ploessl, 258.
Poggioli, 556.
Polstorff, 526.
Pontedera, 391, 399, 401.
Priestley, 491-494.
Pringsheim, 203, 209, 210, 213, 318, 372, 442, 443.
Radlkofer, 314, 350, 354, 435.
Ramisch, 422, 426.
Raspail, 320.
Ratzenberger, 19.
Ray, 7, 8, 39, 40, 59, 60, 63, 67, 68-74, 101, 115, 384, 471, 536-538.
Reichel, Christian, 484.
Rivinus (_see_ Bachmann).
Roemer, 401.
Roeper, 144, 371, 555.
Rudbeck, 76, 79.
Rudolphi, 211, 256, 258, 267-270.
Ruppius, 76.
Saint-Hilaire, Auguste de, 149.
Saint-Pierre, 137.
Salm-Horstmar, 532.
Sanio, Karl, 309, 316, 318, 341, 349, 350.
Sarrabat (_see_ de la Baisse).
Saussure, Theodore de, 126, 369, 370, 497-504, 506, 531.
Sbaraglia, 472.
Schacht, Hermann, 280, 283, 302, 305, 318, 337, 338, 341, 343, 345,
348, 434, 435.
Schaeffer, J. C., 211.
Scheffer, 39.
Schellhammer, 74.
Schelver, F. J., 422, 424.
Schimper, C. Friedr., 162-170.
Schimper, W. B., 198.
Schlechtendal, 192.
Schleiden, 63, 161, 179, 183, 188-193, 226, 297, 302, 311, 322, 323,
326, 341, 345, 433-436, 529, 558.
Schmidel, 20, 123, 197, 438.
Schmitz, 212, 556.
Schrank, Paula, 255, 425.
Schulz-Schulzenstein, 293, 300, 320, 545.
Schulze, Franz, 284, 318, 373.
Schulze, Max, 314, 339.
Schwann, 313.
Schwendener, 215.
Selligue, 258.
Senebier, 126, 224, 249, 369, 495-497, 547, 561.
Sharroc, 537.
Smith, 545.
Spallanzani, Lazaro, 422-424.
Sprengel, Konrad, 363, 368, 414-422.
Sprengel, Kurt, 66, 125, 224, 256, 259, 262, 263, 268, 320, 469.
Steinheil, 559.
Sternberg, 184.
Suminsky (_see_ Lesczye-Suminsky).
Thal (Thalius), 18.
Theophrastus, 3, 4, 13, 15-17, 34, 219, 377.
Thümmig, 248, 473.
Thuret, 209, 210, 314, 372, 442, 443.
Tonge, 470.
Tournefort, Pitton de, 7, 8, 39, 63, 76 79, 83, 101, 115, 391, 401,
544.
Tragus (_see_ Bock).
Trentepohl, 206, 207.
Treviranus, 19, 161, 256, 261, 267, 270-272, 275, 290, 310, 320, 425,
520-524, 545.
Trew, 560.
Trog, 212.
Tulasne, 213, 435.
Turpin, 320.
Unger, 161, 184, 198, 206, 227, 300, 305, 312, 314, 318, 325-329, 333,
336-340, 346, 375, 438, 559.
Vagetius, 59.
Vaillant, Sebastian, 391, 397, 398.
Valentin, 355, 386, 387, 402.
Van der Hopp, 560.
Van Deyl, 257.
Van Helmont, 455.
Varro, 535.
Vaucher, 126, 207, 372, 438, 545.
Voight, 160.
Volkamer, 39.
Vrolik, 508.
Wallroth, 215.
Walther, Friedrich, 483.
Weickert, 257.
Wiegman, 526.
Wigand, 105, 341, 558.
Wilbrand, 425.
Willoughby, 470.
Wolff, Christian, 221, 247, 402, 403, 472-476.
Wolff, Kaspar Friedr., 44, 155, 190, 249-253, 273, 275, 276, 319, 405.
Woodward, 472.
Wright, 257.
Wydler, 165.
Zaluziansky, 380, 381.
Zantedeschi, 557.
Zinn, 544.
THE END.
FOOTNOTES:
[1] It will be shown in a later chapter that Linnaeus’ sexual system
was intended to be artificial.
[2] Kurt Sprengel in his ‘Geschichte der Botanik,’ i. 1817, and
Ernst Meyer in his ‘Geschichte der Botanik,’ iv. 1857 have described
the connection between the first beginnings of modern botany and
the general state of learning in the 15th and 16th centuries; a
particularly interesting notice of Valerius Cordus from the pen
of Thilo Irmisch will be found in the ‘Prüfungsprogramm’ of the
Schwarzburg gymnasium of Sondershausen for 1862. Here, as throughout,
the present work will be confined to the investigation and description
of the development of strictly botanical ideas.
[3] Otto Brunfels, born at Mainz before the year 1500, was at first a
student of theology and a monk; becoming a convert to Protestantism he
was actively engaged at Strassburg first as a teacher and afterwards as
a physician; he died in 1534.
[4] Beside the herbals mentioned in the text, which may be regarded
as scientific works on botany, a considerable number of books on the
signature of plants were written in the 16th and 17th centuries in the
interests of medicine or medical superstition. It was believed that
certain external marks and resemblances between parts of plants and the
organs of the human body indicated the plants and the parts of them
which possessed healing virtues. Pritzel mentions by name twenty-four
works of the kind, which appeared between 1550 and 1697. The herbals
also noticed the signatures, and even Ray has an enquiry into the
subject.
[5] The fragments of Aristotelian botany which have come down to us
are to be found translated from Wimmer’s edition in Ernst Meyer’s
‘Geschichte der Botanik,’ i. p. 94.
[6] Ernst Meyer (Geschichte der Botanik) gives a full account of
Theophrastus, who was born at Lesbos A.C. 371 and died A.C. 286. An
edition of his work ‘De historia et de causis plantarum’ was published
by Theodor Gaza in 1483. See also Pritzel’s ‘Thesaurus literarum
botanicarum.’
[7] See L. C. Treviranus in his work, ‘Die Anwendung des Holzschnitts
zur bildlichen Darstellung der Pflanzen,’ Leipzig, 1855, and Choulant
‘Graphische Incunabeln,’ Leipzig, 1858.
[8] Konrad Gesner, born in Zürich in 1516, became after many
vicissitudes of fortune Professor of Natural History in his native
town, and died there of the plague in 1565. See Ernst Meyer,
‘Geschichte der Botanik,’ iv.
[9] Leonhard Fuchs, born at Membdingen in Bavaria in 1501, was
a student of the classics under Reuchlin in Ingolstadt in 1519,
and became Doctor of Medicine in 1524. Owing to his conversion to
Protestantism he led an unsettled life for some years, but was finally
made Professor of Medicine in Tübingen in 1535, and died there in 1566.
See Meyer, ‘Geschichte der Botanik,’ iv.
[10] Rembert Dodoens (Dodonaeus), born at Malines in 1517, was a
physician, and a man of varied culture; he published a number of
botanical works, some of them in Flemish, after 1552, and finally in
1583 his ‘Stirpium Historiae Pemptades vi’ (Antwerp). From 1574 to
1579 he was physician to the Emperor Maximilian II. In 1582 he became
Professor in Leyden and died in 1585. See Ernst Meyer, ‘Geschichte der
Botanik,’ iv. p. 340.
[11] Hieronymus Bock (Tragus) was born at Heiderbach in the Zweibrücken
in 1498; he was destined to the cloister, but embraced Protestantism
and became a schoolmaster in Zweibrücken and superintendent of the
Prince’s garden; he was afterwards preacher in Hornbach, where he
practised also as a physician and pursued his botanical studies; he
died in 1554. See Ernst Meyer, ‘Geschichte der Botanik,’ iv. p. 303.
[12] Pierandrea Mattioli, who was born at Siena in 1501 and died there
in 1577, was for many years physician at the court of Ferdinand I. He
wrote rather in the interests of medicine than of botany; his herbal,
originally a commentary on Dioscorides, was gradually enlarged and went
through more than sixty editions and issues in different languages. See
Meyer, ‘Geschichte der Botanik,’ vi.
[13] Charles de l’Écluse (Carolus Clusius) was born in Arras in 1526.
His family suffered from religious persecution in France, and he spent
the greater part of his life in Germany and the Netherlands; in 1573 he
removed to Vienna by the invitation of Maximilian II; in 1593 he became
professor in Leyden and died there in 1609. See Meyer, ‘Geschichte der
Botanik,’ iv, who gives full information respecting the eventful life
of this distinguished man.
[14] Jacques Dalechamps, a native of Caen, who died in 1588, was a
philologist rather than an original investigator of nature, as is
remarked by Meyer in his ‘Geschichte der Botanik,’ vi. p. 395.
[15] Mathias de l’Obel (Lobelius), the friend and fellow-countryman of
Dodoens and de l’Écluse, was born at Lille in 1538 and died in England
in 1616. A full account of this botanist will be found in Meyer.
[16] Kaspar Bauhin was born at Basle in 1550, and like his elder
brother John studied under Fuchs; he collected plants in Switzerland,
Italy, and France, and became professor in Basle; he died in 1624. Some
account is given of him and of his brother by Haller in the preface
to his ‘Historia Stirpium Helvetiae’ (1768), and by Sprengel in his
‘Geschichte der Botanik,’ i. p. 364 (1818).
[17] Andrea Cesalpino (Caesalpinus) of Arezzo was born in 1519. He was
a pupil of Ghini and professor at Pisa, and afterwards physician to
Pope Clement VIII. He died in 1603.
[18] We find it stated in Theophrastus that if the pith of the vine
is destroyed the grapes contain no stones; this evidently points to a
still higher antiquity for the view that the seeds are formed from the
pith; see the De causis plantarum, v. ch. 5, in the ‘Theophrasti quae
supersunt opera’ of Schneider, Leipzig, 1818.
[19] These words are quoted by Linnaeus in the ‘Philosophia Botanica,’
par. 159.
[20] See his biography by Guhrauer, ‘Joachim Jungius und sein
Zeitalter,’ Tübingen, 1850; on his place in philosophy consult Ueberweg
(‘Geschichte der Philosophie,’ iii. p. 119), who regards him as a
forerunner of Leibnitz.
[21] Morison served in the royal army against Cromwell, and after the
defeat of his party retired to Paris, where he studied botany under
Robin. He was made physician to Charles II and Professor of Botany in
1660, and Professor of the same faculty in Oxford ten years later. See
Sprengel, ‘Geschichte der Botanik,’ ii. p. 30.
[22] The wood-engraving of the 16th century had fallen into decay,
and engraving on copper-plate had taken its place. A thick volume of
figures of plants in the largest folio size engraved on copper, the
‘Hortus Eistädtensis,’ appeared in the beginning of the 17th century.
[23] John Ray, born at Black Notley in Essex, was also a zoologist
of eminence. He studied theology and travelled in England and on the
continent, and afterwards devoted himself entirely to science, being
supported by a pension from Willoughby. See Carus, ‘Geschichte der
Zoologie,’ p. 428.
[24] A. Q. Bachmann (Rivinus) was the third son of Andreas Bachmann,
a physician and philologist of Halle. He is said to have spent 80,000
florins on the publication of his works and the providing them with the
500 copper-plates with which they were illustrated. A life of him and
just estimate of his work, by Du Petit-Thouars, is to be found in the
‘Biographie universelle ancienne et moderne.’
[25] Tournefort was born at Aix in Provence, and received his early
education in a Jesuit college. He was intended for the Church, but
after his father’s death, in 1677, he was able to devote himself
entirely to botany. After travelling in France and Spain, he became
Professor at the Jardin des Plantes in 1683; but while thus engaged he
made various journeys in Europe, and in 1700 visited Greece, Asia, and
Africa—everywhere diligently collecting the plants which he afterwards
described.
[26] In addition to the Autobiography of Linnaeus, various accounts of
his life have been written, some of which are mentioned in Pritzel’s
‘Thesaurus Lit. Bot.’ A strange revelation of his character and
sentiments is to be found in his treatise on the ‘Nemesis divina,’
which he bequeathed to his son. Of this work Professor Fries has
unfortunately published an epitome only, which is noticed in the
Regensburg Flora, No. 44 (1851). On Linnaeus’ services to zoology, see
Carus, ‘Geschichte der Zoologie,’ München, 1872.
[27] Printed in Jessen’s ‘Botanik der Gegenwart and Vorzeit,’ p. 287.
[28] ‘Epistola ad Godofredum Gulielmum Leibnitzium etc. cum Laurentii
Heisteri praefatione,’ Helmstadii, 1750.
[29] See the excellent account of the Platonic and Aristotelian
philosophies and of scholasticism in Albert Lange’s ‘Geschichte des
Materialismus,’ second edition, 1874.
[30] The comparison of the vegetable seed with the egg in animals,
which is in itself incorrect, comes, as Aristotle tells us, from
Empedocles, and was a favourite one with the systematists.
[31] Linnaeus uses the word ‘herba’ for the older word ‘germen,’ which
with him means the ovary.
[32] It would not be difficult to prove that the doctrine of the
constancy of species is properly a conclusion from scholasticism, and
ultimately from the Platonic doctrine of ideas, and was therefore
assumed as self-evident before the time of Linnaeus, who only gave it a
more distinct and conscious expression; his arguments from experience
are without force. The strength of the dogma lies in its relation to
the platonico-scholastic philosophy, which the systematists followed,
more or less consciously, up to quite recent times.
[33] The authority for the contents of these dissertations is Wigand’s
‘Kritik und Geschichte der Metamorphose’ (1846).
[34] Bernard de Jussieu, born at Lyons in 1699, and at first a
practising physician there, was by Vaillant’s intervention called to
Paris, and after Vaillant’s death became Professor and Demonstrator at
the Royal Garden. He and Peissonel were among the first who declared
against the vegetable nature of the Corals. It is expressly stated in
his Éloge (‘Histoire de l’Académie Royale des Sciences,’ Paris, 1777)
that he founded his natural families on the Linnaean fragment. He died
in 1777.
[35] A. L. de Jussieu, born at Lyons, came to Paris to his uncle
Bernard in 1765. In 1790 he was a member of the Municipality, and till
1792 Superintendent of Hospitals. When the Annales du Museum were
founded in 1802, he resumed his botanical pursuits. In 1826 his son
Adrien took his place at the Museum. See his life by Brougniart in the
‘Annales des Sciences Naturelles,’ vii (1837).
[36] Joseph Gärtner was born at Calw in Würtemberg in 1732, and died
in 1791. He commenced his studies in Göttingen in 1751, where he was a
pupil of Haller. He travelled into Italy, France, Holland, and England
in order to make the acquaintance of famous naturalists, and worked
also at physics and zoology. In 1760 he was Professor of Anatomy in
Tübingen, and in 1768 became Professor of Botany at St. Petersburg;
but finding himself unable to bear the climate, he returned to Calw
in 1770, and gave himself up entirely to his book, ‘De fructibus
et seminibus plantarum,’ which he had already commenced. Banks and
Thunberg, one of whom had returned from a voyage round the world, the
other from Japan, handed over to him the collections of fruits which
they had made. His persistent study, partly with the microscope,
brought him near to blindness. There is an interesting life of Gärtner
by Chaumeton in the ‘Biographie Universelle.’
[37] Augustin Pyrame de Candolle sprang from a Provençal family, which
had fled from religious persecution to Geneva, where it was and is
still held in great estimation. He associated as a boy with Vaucher,
and on his first visit to Paris in 1796 with Desfontaines and Dolomieu,
and after his return to Geneva was a friend of Senebier. The elder
Saussure, and afterwards Biot, whom he assisted in an investigation in
physics, endeavoured to attach him to that study. He spent the years
from 1798 to 1808 in Paris, where he lived in close intercourse with
the naturalists of that city. Numerous smaller monographs, and the
publication of his work on succulent plants and of a new edition of
De Lamarck’s ‘Flore Française,’ occupied this earlier period of his
life. From 1808 to 1816 he was Professor of Botany at Montpellier.
During this time he made many botanical journeys in all parts of
France and the neighbouring countries, and wrote many monographs, his
essays on the geography of plants, and his most important work, the
‘Théorie élémentaire.’ From 1816 till his death in 1841 he resided
once more in Geneva, which had freed itself in 1813 from the enforced
connection with France established in 1798. Here De Candolle found
time to take part in political and social questions, in addition to an
almost incredible amount of botanical labour. (Notice sur la vie et les
ouvrages de A. P. De Candolle par De la Rive, Genève, 1845.)
[38] Robert Brown was the son of a Protestant minister of Montrose, and
studied medicine first at Aberdeen and afterwards in Edinburgh; he then
became a surgeon in the army, and was at first stationed in the north
of Ireland. When the Admiralty despatched a scientific expedition to
Australia under Captain Flinders in 1801, he was appointed naturalist
to the expedition on the recommendation of Sir Joseph Banks, F. Bauer
being associated with him as botanical draughtsman, Good as gardener,
Westall as landscape-painter; one of the midshipmen of the vessel
was John Franklin. In consequence of the unseaworthiness of the ship
Flinders left Australia, intending to return with a better one, but was
shipwrecked on the voyage and detained by the French at Port Louis as a
prisoner of war till 1810. The naturalists of the expedition remained
in Australia till 1805, when Brown returned to England with 4000 for
the most part new species of plants. Sir J. Banks appointed him his
librarian and keeper of his collections in 1810; he was also Librarian
to the Linnaean Society of London. In 1823 he received the bequest of
Banks’ library and collections, which were to be transferred after his
death to the British Museum; but by his own wish they were deposited
there at once, and he himself received the appointment of Custodian of
the Museum and remained in that position till his death. At Humboldt’s
suggestion Sir Robert Peel’s Ministry granted him a yearly pension of
£200. His merits were universally acknowledged, and Humboldt even named
him ‘botanicorum facile princeps.’
[39] Stephen Ladislaus Endlicher was born at Pressburg in 1805, and
abandoning the study of theology became Scriptor in the Imperial
Library at Vienna in 1828, and in 1836 Custos of the botanical
department of the Imperial Collection of Natural History. Having
graduated at the University in 1840, he became Professor of Botany and
Director of the Botanic Garden. His library and herbarium, valued at
24,000 thalers, he presented to the State, and with his private means
founded the Annalen des Wiener-Museums, purchased botanical collections
and expensive botanical books, and published his own works and works of
other writers. His official salary was small, and having exhausted his
resources in these various expenses, he put an end to his own life in
March 1849. Endlicher was not only one of the most eminent systematists
of his day, but a philologist also, and a good linguist. He wrote among
other things a Chinese grammar. See ‘Linnaea,’ vol. xxxiii (1864 and
1865), p. 583.
[40] John Lindley, Professor of Botany in the University of London, was
born at Chatton near Norwich in 1799, and died in London in 1865.
[41] Auguste de Saint Hilaire was born at Orleans in 1779, and died
there in 1853; he was Professor at Paris, and in 1840 published
his ‘Leçons de Botanique comprenant principalement la Morphologie
Végétale,’ etc. This work contains a somewhat diffuse account of P.
de Candolle’s doctrine of symmetry, together with Goethe’s theory of
metamorphosis and Schimper’s doctrine of phyllotaxis, and his own
views also on classification founded on the comparative morphology of
the day. It is marked by fewer errors than will be found in Lindley’s
theoretical writings, but it is less profound, and touches only
incidentally on fundamental questions; at the same time it possesses
historical interest as giving a lucid description of the state of
morphology before 1840.
[42] See Wigand, ‘Geschichte und Kritik der Metamorphose,’ Leipzig,
1846, p. 38.
[43] See Goethe’s collected works in forty volumes, Cotta, 1858, vol.
xxxvi.
[44] See Haeckel, ‘Natürliche Schöpfungsgeschichte,’ ed. 4, 1873, p. 80.
[45] Louis Marie Aubert du Petit-Thouars was born in Anjou in 1758 and
collected plants during many years in the Mauritius, Madagascar, and
Bourbon. He was afterwards Director of the Botanic Garden at Roule, and
became Member of the Academy in 1820. He died in 1831. His articles
in the ‘Biographie Universelle’ prove him to have been a writer of
ability. Preconceived opinions interfered with the success of his own
investigations, especially into the increase in thickness of woody
stems, and obstinate adherence to such notions prevented an unbiassed
interpretation of what he saw. See Flora, 1845, p. 439.
[46] K. F. Schimper, born in Mannheim in 1803, was at first a student
of theology in Heidelberg, but having afterwards travelled as a paid
collector of plants in the south of France, he applied himself to the
study of medicine. From 1828 to 1842 he was employed as a teacher in
the University of Munich, though occasionally engaged in exploring the
Alps, Pyrenees, and other districts, in the service of the King of
Bavaria. It was during this period of his life that he composed his
most important works on phyllotaxis, and essays on the former extension
of glaciers, and on the glacial period. He returned to the Palatinate
in 1842, and died at Schwetzingen in 1867 in the enjoyment of a pension
from the Grand duke of Baden.
[47] See Hofmeister, ‘Allgemeine Morphologie’ (1868), pp. 471, 479, and
Sachs, ‘Lehrbuch der Botanik,’ ed. 4 (1874), p. 195.
[48] See Nägeli, ‘Beiträge zur Wissenschaftlichen Botanik’ (1858), I,
pp. 40, 49.
[49] A comparison of the two theories and a refutation of Schleiden’s
assertion, that that of the brothers Bravais expresses better ‘the
simplicity of the law,’ will be found in ‘Flora,’ 1847, No. 13, from
the pen of Sendtner, and in Braun’s ‘Verjüngung,’ p. 126.
[50] This is not at all true of modern inductive science, which merely
forms a different idea of the connection, and has regard to the
relation between the percipient subject and the phenomena.
[51] See A. Bayer, ‘Leben und Wirken F. Unger’s,’ Gratz (1872), p. 52.
[52] See Darwin’s repudiation of this statement on p. 421 of Ed. 6 of
the ‘Origin of Species.’
[53] Casimir Christoph Schmidel was born in 1718 and died in 1792; he
was Professor of Medicine in Erlangen, and was the first who described
the sexual organs in various Liverworts.
[54] Lantzius Beninga, born in East Friesland in 1815, was a professor
in Göttingen, and died in 1871.
[55] Gottlieb Wilhelm Bischoff was born at Dürkheim on the Hardt in
1797, and died as Professor of Botany at Heidelberg in 1854. He wrote
various manuals and text-books which are careful and industrious
compilations, but being entirely conceived in the spirit of the times
preceding Schleiden they are now obsolete; his investigations however
into the Hepaticae, Characeae, and Vascular Cryptogams, illustrated
by very beautiful drawings from his own hand, are still of value; and
the same may be said of his ‘Handbuch der botanischen Terminologie und
Systemkunde’ on account of its numerous figures.
[56] Karl Adolf Agardh (1785-1859) was until 1835 Professor in Lund,
afterwards Bishop of Wermland and Dalsland. Jacob Georg Agardh, born
in 1813, was Professor in Lund. William Henry Harvey (1811-1866) was
Professor of Botany in Dublin. Friedrich Traugott Kützing, born in
1807, was Professor in the Polytechnic School of Nordhausen.
[57] C. G. Nees von Esenbeck published his ‘System der Pilze und
Schwämme’ in 1816; Th. F. L. Nees von Esenbeck, in conjunction with A.
Henty, a ‘System der Pilze’ in 1837. The first (1776-1858) was for a
long time President of the Leopoldina, Professor of Botany in Breslau,
and one of the chief representatives of the nature-philosophy. Elias
Fries, born in 1794, became Professor of Botany in Upsala in 1835; he
died in 1878. Léveillé (1796-1870) was a physician in Paris. August
Joseph Corda was born at Reichenberg in Bohemia in 1809, and became
custodian of the National Museum in Prague in 1835; he undertook a
journey to Texas in 1848, from which he never returned, having probably
perished by shipwreck in 1849. Weitenweber, in the ‘Abhandlungen der
Böhmischen Gesellschaft der Wissenschaft,’ Bd. 7, Prag, 1852, gives
a full account of this eminent mycologist. Corda was the first who
thoroughly applied the microscope to copying and describing every form
of Fungus that was known to him, and especially the minuter ones.
His ‘Icones Fungorum hucusque cognitorum’ (1837-1854) are still an
indispensable manual in the study of the subject.
[58] Jean Pierre Étienne Vaucher, the instructor and friend of P. de
Candolle, was a minister and professor in Geneva.
[59] Trentepohl’s communication is to be found in the ‘Botanische
Bermerkungen und Berichtigungen’ of A. W. Roth, Leipsic, 1807.
[60] Pier’ Antonio Micheli, born at Florence in 1679, was Director
of the Botanic Garden there, and died in 1737. Johann Jacob Dillen
(Dillenius), born in Darmstadt in 1687, was Professor of Botany
in Oxford, and died in 1747. These two botanists were the first
who submitted the Mosses and the lower Cryptogams to scientific
examination, and endeavoured to prove the presence of sexual organs in
these plants.
[61] Jacob Christian Schaeffer, born in 1718, was Superintendent in
Regensburg; he died in 1790.
[62] See Sachs, ‘Lehrbuch der Botanik,’ ed. 4 (1874), p. 245.
[63] Fr. Wilh. Wallroth, born in the Harz in 1792, was district
physician at Nordhausen. He died in 1857. See ‘Flora’ for 1857, p. 336.
[64] Robert Hooke, born in 1635 at Freshwater in the Isle of Wight,
was a man of marvellous industry and varied acquirement in spite of
a delicate constitution. He became a Fellow of the Royal Society in
1662, and was afterwards its Secretary and Professor of Geometry in
Gresham College. He died in 1703. There is a good account of him by de
l’Aulnaye in the ‘Biographie Universelle.’
[65] Marcello Malpighi, born at Crevalcuore near Bologna in 1628,
became Doctor of Medicine in 1653, and after 1656 was Professor in
Bologna, Pisa, Messina, and a second time in Bologna; in 1691 he was
named Physician to Innocent XII. He died in 1694. On his services
to comparative anatomy, and the anatomy of the human body, see the
‘Biographie Universelle’ and Carus, ‘Geschichte der Zoologie,’ p. 395.
[66] We read at p. 3: ‘Componuntur expositae fistulae (spirales)
zona tenui et pellucida, velut argentei coloris, lamina parum lata,
quae spiraliter locata et extremis lateribus unita tubum interius et
exterius aliquantulum asperum efficit; quin et avulsa zona capites seu
extremo trachearum tum plantarum tum insectorum non in tot disparatos
annulos resolvitur, ut in perfectorum trachea accidit; sed unica zona
in longum soluta et extensa extrahitur.’
[67] Nehemiah Grew, the son of a clergyman in Coventry, appears to
have been born in 1628. Having taken a Doctor’s degree in a foreign
University, he practised as a physician in his native town, and pursued
at the same time his phytotomical researches. He became Secretary to
the Royal Society in 1677, and published his ‘Cosmographia Sacra’ in
1701. He died in 1711. See the ‘Biographie Universelle.’
[68] Leeuwenhoek’s observations in animal anatomy were perhaps more
important than those which he made in botany. Carus (‘Geschichte der
Zoologie,’ p. 399) says of him: ‘While Malpighi used the microscope
with system and in accordance with the requirements of a series of
investigations, the instrument in the hands of the other famous
microscopist of the 17th century was more or less a means of gratifying
the curiosity excited in susceptible minds by the wonders of a world
which had hitherto been invisible. Still the discoveries, which were
the fruit of an assiduous use of the microscope continued during fifty
years, embraced many subjects and were important and influential. Anton
von Leeuwenhoek was born in Delft in 1632. Being intended for trade,
he had not the advantage of a learned education and is said even to
have been ignorant of Latin; his favourite occupation was the preparing
superior lenses, with which he incessantly examined new objects without
being guided at any time by a scientific plan. The Royal Society of
London, to whom he communicated his observations, made him a member of
their body. He died in his native town in 1723, being ninety years of
age.
[69] This subject will be noticed again in the history of the sexual
theory.
[70] C. F. Wolff was born at Berlin in 1733. He studied anatomy under
Meckel and botany under Gleditsch, in the Collegium Medico-chirurgicum
in that city. He afterwards resorted to the University of Halle, and
there made acquaintance with the philosophy of Leibnitz and Wolff,
which predominates too much in his dissertation, ‘Theoria Generationis’
(1759). Haller, the representative of the theory of evolution against
which this work was directed, replied to it in a kindly spirit and
entered into a correspondence with its youthful author. After lecturing
on medicine in Breslau, he was admitted to teach physiology and other
subjects in the Collegium Medico-chirurgicum in Berlin, but was twice
passed over in the appointment to professorships in that institution.
He received an appointment in the Academy of St. Petersburg from the
Empress Catherine II in 1766, and died in that city in 1794. See Alf.
Kirchhoff, ‘Idee der Pflanzenmetamorphose,’ Berlin, 1867.
[71] Johannes Hedwig, the founder of the scientific knowledge of the
Mosses, was born at Kronstadt in Siebenbürgen in 1730. Having completed
his studies at Leipsic, he returned to his native town, but was not
permitted to practice there as a physician because he had not taken
a degree in Austria. He consequently went back to Saxony and settled
first at Chemnitz, and in 1781 in Leipsic. Here he was appointed in
1784 to the Military Hospital, and became Professor extraordinary
of Medicine in 1786 and ordinary Professor of Botany in 1789. He
died 1799. He commenced his botanical studies as a student at the
University, and continued them in Chemnitz under trying circumstances,
till as Professor he was free to devote himself entirely to them.
[72] See P. Harting, ‘Das Mikroskop,’ §§ 433 and 434.
[73] Johann Jakob Bernhardi, born in 1774, was Professor of Botany in
Erfurt, and died there in 1850.
[74] Karl Asmus Rudolphi, born at Stockholm in 1771, was Professor of
Anatomy and Physiology in Berlin, and died there in 1832.
[75] Heinrich Friedrich Link was born at Hildesheim in 1767, and became
Doctor of Medicine of Göttingen in 1788. In 1792 he became Professor
of Zoology, Botany, and Chemistry in Rostock, Professor of Botany in
1811 in Breslau, and in 1815 in Berlin, where he died in 1851. He was
a clever man of very varied accomplishment, but not a very accurate
observer of details, and was held in estimation by many chiefly as
a good teacher and philosophic author of popular works on natural
science. He was one of the few German botanists in the early part of
the present century who aimed at a general knowledge of plants, and
combined anatomical and physiological enquiries with solid researches
in systematic botany. Of his many treatises on all branches of
botanical science, zoology, physics, chemistry, and other subjects,
his Göttingen prize essay must be considered to have contributed most
to the advancement of science. Von Martius somewhat overrates his
scientific importance in his ‘Denkrede auf H. F. Link’ in the ‘Gelehrte
Anzeigen,’ München (1851), 58-69.
[76] Ludolf Christian Treviranus, born at Bremen in 1779, became Doctor
of Medicine of Jena in 1801, and practised at first in his native
town, where he became a teacher at the Lyceum in 1807. In 1812 he
accepted the professorship in Rostock vacated by Link, and was again
his successor in Breslau. In 1830 he exchanged posts with C. G. Nees
von Esenbeck, who was a professor in Bonn; he died in that town in
1864. In the first part of his life he occupied himself chiefly with
vegetable anatomy and physiology, afterwards with the determination
and correction of species. His first works, which are noticed in
the text, and the treatises on sexuality and the embryology of the
Phanerogams, published between 1815 and 1828, are the most important
in a historical point of view. His ‘Physiologie der Gewächse’ in two
volumes (1835-1838) is still of value for its accurate information on
the literature of the subject; but it can scarcely be said to have
contributed to the advance of physiology, for its author adhered in it
to the old views, and especially to the notion of the vital force, at a
time when new ideas were already asserting themselves. The ‘Botanische
Zeitung’ for 1864, p. 176, contains a notice of his life.
[77] Charles François Mirbel (Brisseau-Mirbel) was born at Paris in
1776, and died in 1854. He began life as a painter, but having been
introduced by Desfontaines to the study of botany, he became Member
of the Institute in 1808, and soon after Professor in the University
of Paris. From 1816 to 1825 the cares of administration withdrew him
from his botanical studies, but he resumed them and became in 1829
Professeur des cultures in the Museum of Natural History. Mirbel was
the founder of microscopic vegetable anatomy in France. All that had
been accomplished there before his time was still more unimportant
than the work done in Germany. His writings involved him in many
controversies, and he made enemies by refusing in his capacity of
teacher to allow the importance at that time attributed to systematic
botany, but directed his pupils to the study of structure and the
phenomena of life in plants. We are told by Milne-Edwards that he
suffered much from the fierce attacks which were made upon him; he
sank at last into a weak and apathetic state, and was for some time
before his death unable to continue his studies or official duties
(‘Botanische Zeitung’ for 1855, p. 343).
[78] Johann Jakob Paul Moldenhawer was Professor of Botany in Kiel; he
was born at Hamburg in 1766, and died in 1827.
[79] On the doubts which were entertained till after 1812 on the
subject of stomata, see Mohl’s ‘Ranken und Schlingpflanzen’ (1827), p.
9.
[80] Franz Julius Ferdinand Meyen was born at Tilsit in 1804, and died
as Professor in Berlin in 1840. He applied himself at first to pharmacy
and afterwards to medicine, and having taken a degree in 1826 he
practised for some years as a physician. In 1830 he set out on a voyage
round the world under instructions from A. von Humboldt, and returned
in 1832 with large collections. He was made Professor in Berlin in
1834. There is a notice of his life in ‘Flora’ of 1845, p. 618.
[81] Hugo Mohl (afterwards von Mohl) was born at Stuttgart in 1805,
and died as Professor of Botany in Tübingen in 1872. His father held
an important civil office under the Government of Würtemberg. Robert
Mohl, also in the service of the Government, Julius Mohl, the Oriental
scholar, and Moritz Mohl, the political economist, were his brothers.
The instruction at the Gymnasium at Stuttgart, which he attended for
twelve years, was confined to the study of the ancient languages; but
Mohl early evinced a preference for natural history, physics, and
mechanics, and devoted himself in private to these subjects. He became
a student of medicine in Tübingen in 1823, and took his degree in 1828.
He then spent several years in Munich in intercourse with Schrank,
Martius, Zuccharini and Steinheil and obtained abundant material for
his researches into Palms, Ferns, and Cycads. He became Professor
of Physiology in Berne in 1832, and Professor of Botany in Tübingen
after Schübler’s death in 1835, and there he remained till his death,
refusing various invitations to other spheres of work. He was never
married, and his somewhat solitary life of devotion to his science was
of the simplest and most uneventful kind. He was intimately acquainted
with all parts of botanical science, and possessed a thorough knowledge
of many other subjects; he was in fact a true and accomplished
investigator of nature. A very pleasing sketch of his life from the pen
of De Bary is to be found in the ‘Botanische Zeitung’ of 1872, No. 31.
[82] But von Mohl expressed some doubts on this point in 1844
(‘Botanische Zeitung,’ p. 340).
[83] This tertiary layer was at first supposed by Theodor Hartig to be
of general occurrence; von Mohl in 1844 considered it to be present
only in certain cases.
[84] Anselm Payen (1795-1871) was born at Paris and was Professor of
Industrial Chemistry in the École des Arts et Métiers in that city. His
most important botanical works were his ‘Mémoire sur l’amidon,’ etc.,
Paris (1839), and his ‘Mémoire sur le développement des Végétaux,’
published in the Memoirs of the Academy of Paris.
[85] On this point, see von Mohl’s citation in ‘Flora’ of 1827, p. 13.
I have not myself been able to consult the originals.
[86] See Meyen, ‘Neues System,’ ii. 344.
[87] Franz Unger was born in 1800 on the estate of Amthof, near
Leutschach in South Steiermark, and was educated up to the age of
sixteen in the Benedictine Monastery of Gratz. Having gone through the
three years’ course of ‘philosophy,’ he turned his attention, by his
father’s wish, to jurisprudence; but he abandoned this study in 1820,
and became a student of medicine, first in Vienna, and afterwards in
Prague. From the latter place he made a vacation tour in Germany, and
formed the acquaintance of Oken, Carus, Rudolphi, and other men of
science, and in 1825 of Jacquin and Endlicher, with the latter of whom
he maintained an active correspondence on scientific subjects. Having
taken his degree in 1827, he practised as a physician in Vienna till
the year 1830, and after that date was medical official at Kitzbühl
in the Tyrol. During these years he continued the botanical studies
which he had commenced as a youth, and at Kitzbühl directed special
attention to the diseases of plants, to palaeontological researches,
and to enquiries into the influence of soil on the distribution of
plants. At the end of 1835 he became Professor of Botany at the
Johanneum in Gratz, and devoting himself there especially to the study
of palaeontology, he soon became the most eminent authority on that
subject. Having been made Professor of Vegetable Physiology in Vienna
in 1849, he applied himself more to physiology and phytotomy. He
retired from this position in 1866, and from that time forward lived as
a private individual in Gratz, promoting scientific knowledge by the
publication of popular treatises and the delivery of lectures. He died
in 1870. Information respecting his personal character and his varied
and copious labours in many departments of botanical science is given
by Leitgeb in the ‘Botanische Zeitung’ of 1870, No. 16, and by Reyer,
‘Leben und Wirken des Naturhistoriker Unger,’ Gratz, 1871.
[88] Hermann Schacht was born at Ochsenwerder in 1824, and died in 1864
in Bonn, where he had been Professor of Botany since 1859.
[89] See Sachs, ‘Lehrbuch der Botanik,’ ed. 4 (1874), p. 129 (p. 128 of
2nd English edition).
[90] See Ernst Meyer, ‘Geschichte der Botanik,’ I. p. 98, &c.
[91] The edition here used is that of Gottlob Schneider, ‘Theophrasti
Eresii quæ supersunt opera,’ Leipzig, 1818. See in addition to the
passages noticed in the text the ‘De Causis,’ l. I. c. 13. 4, and l.
IV. c. 4, and the ‘Historia Plantarum,’ l. II. c. 8.
[92] It should be understood that neither Theophrastus nor the
botanists of the 16th and 17th centuries considered the rudiments of
the fruit to be part of the flower; this, which was pointed out in the
history of systematic botany, seems to have been overlooked by Meyer,
‘Geschichte,’ I. p. 164.
[93] The passage is quoted in full in De Candolle’s ‘Physiologie
végétale,’ 1835, ii. p. 44. It is said there of the pollen, ‘Ipso et
pulvere etiam feminas maritare.’
[94] See De Candolle, ‘Physiologie végétale,’ p. 47.
[95] His ‘Methodus Herbaria’ is said to have been published in 1592.
The remarks in the text are made in reliance on a long quotation from
it in Roeper’s translation of De Candolle’s ‘Physiologie,’ ii. p. 49,
who had before him an edition of 1604.
[96] In the ‘Compositae,’ however, Grew called the single flowers the
florid attire, see p. 37.
[97] We may compare with this, pp. 38 and 39 of the first part of the
work which appeared in 1671, where Grew ascribed no sexual significance
to the stamens.
[98] Rudolph Jacob Camerarius was born at Tübingen in 1665 and died
there in 1721. Having completed the course of study in philosophy
and medicine, he travelled from 1685 to 1687 in Germany, Holland,
England, France, and Italy. In 1688 he became Professor Extraordinary
and Director of the Botanic Garden in Tübingen; in 1689 Professor
of Natural Philosophy; and finally, in 1695, First Professor of the
University, in succession to his father, Elias Rudolph Camerarius. He
was afterwards succeeded by his son Alexander, one of ten children.
There is an article on Camerarius in the ‘Biographie Universelle,’
from the pen of Du Petit-Thouars. His works on other subjects, as well
as those on the question of sexuality in plants, are distinguished by
ingenious conception and lucid exposition.
[99] See Patrick Blair’s ‘Botanic Essays,’ in two parts (1720), pp.
242-276. Even the Latin ode is borrowed without acknowledgment.
[100] The account in the text is taken from Koelreuter’s report in his
‘Historie der Versuche über das Geschlechte der Pflanzen,’ as given
at p. 188 of Mikan’s ‘Opuscula Botanici Argumenti.’ Logan’s work,
‘Experimenta et Meletamata de Plantarum Generatione,’ unknown to me, is
said by Pritzel to have been published at the Hague in 1739. Koelreuter
cites from a London edition of 1747.
[101] Koelreuter’s report in Mikan’s collection is again the authority
which is here relied on.
[102] Koelreuter says that he sent pollen of Chamaerops in 1766 to
St. Petersburg and to Berlin, where it was successfully employed by
Eckleben and Gleditsch. He wished to try how long the pollen retains
its efficacy.
[103] See Vol. II. p. 502, of the ‘Physiologie végétale.’
[104] See Mikan, ‘Opuscula Botanici Argumenti,’ p. 180.
[105] Joseph Gottlieb Koelreuter was born at Sulz on the Neckar in
1733, and died at Carlsruhe in 1806, where he was Professor of Natural
History, and from 1768 to 1786 Director also of the Botanic and
Grand-ducal Gardens. On giving up the latter position he continued his
experiments in his own small garden till the year 1790. Karl Friedrich
Gärtner in his work ‘Ueber Bastardzeugung’ of 1849, at p. 5 says that
after the latter date Koelreuter occupied himself with experiments
in alchemy; but this must be a mistake. Gärtner, loco cit., and the
‘Flora’ of 1839, p. 245, supply all that seems to be known of the life
of this distinguished man. The ‘Biographic Universelle’ contains no
account of him. It would appear that he was in St. Petersburg before
1766.
[106] See Gärtner, ‘Ueber Bastardzeugung’ (1849), p. 62. I have
unfortunately been unable to meet with the second continuation of
Koelreuter’s work.
[107] Christian Konrad Sprengel, born in 1750, was for some time Rector
at Spandau. There he began to occupy himself with botany, and devoted
so much time to it that he neglected the duties of his office, and even
the Sunday’s sermon, and was removed from his post. He afterward lived
a solitary life in straitened circumstances in Berlin, being shunned by
men of science as a strange, eccentric person. He supported himself by
giving instruction in languages and in botany, using his Sundays for
excursions, which any one who chose could join on payment of two or
three groschen. He met with so little support and encouragement that
he never brought out the second part of his famous work; his publisher
did not even give him a copy of the first part. Natural disgust at the
neglect with which his work was treated made him forsake botany and
devote himself to languages. He died in 1816. One of his pupils wrote a
very hearty eulogium on him in the ‘Flora’ of 1819, p. 541, which has
supplied the above facts.
[108] See Hermann Müller, ‘Befruchtung der Blumen durch Insecten,’
Leipzig (1873). p. 5.
[109] Lazaro Spallanzani was born at Scandiano in Modena, and died
at Pavia in 1799, where he was for a long time Professor of Natural
History. He made researches in very various questions of natural
science, and especially in animal physiology; but they seem to have
been conducted with the same want of care and deliberation which
appears in his experiments on sexuality in plants. A long article in
the ‘Biographie Universelle’ gives a detailed account of his scientific
labours.
[110] August Henschel was a practising physician and a University
teacher in Breslau.
[111] Karl Friedrich Gärtner, son of Joseph Gärtner, was born at Calw
in 1772, and died there in 1850. He attended lectures on natural
science at the Carlsacademie at Stuttgart, and then went first to Jena
for medical instruction, and in 1795 to Göttingen, where he was a pupil
of Lichtenberg. He took a degree in 1796 and settled as a physician
in his native town. Here he occupied himself at first with questions
of human physiology, and afterwards worked at the supplement to his
father’s ‘Carpologia.’ He collected notices and extracts for a complete
work on vegetable physiology. This design was never fulfilled, but it
led to his taking up the question of sexuality in plants, to which
he devoted twenty-five years (‘Jahresheft des Vereins für vaterl.
Naturkunde in Würtemberg,’ 1852, vol. viii, p. 16).
[112] See also Sachs, ‘Lehrbuch der Botanik,’ Leipzig, 1874.
[113] The more important works referred to in this section are Robert
Brown’s ‘Miscellaneous Writings,’ edited by Bennett, 1866-67; von
Mohl on G. Amici, in the ‘Botanische Zeitung,’ 1863, Beilage, p. 7;
Schleiden, ‘Ueber die Bildung des Lichens und Entsichung des Embryos,’
in ‘Nova Acta Academiae Leopoldinensis,’ 1839, vol. xi, Abtheilung,
1; Hofmeister, ‘Zur Uebersicht der Geschichte von der Lehre der
Pflanzenbefruchtung,’ in ‘Flora’ of 1867, p. 119.
[114] The authorities for these statements are collected by Hofmeister
in ‘Flora,’ 1857, p. 120, etc.
[115] W. P. Schimper, in his ‘Recherches anatomiques et morphologiques
sur les Mousses’ of 1850, had made some important statements respecting
the sterility of female moss-plants growing at a distance from male
specimens, and proved that the presence of male plants among females
that are otherwise barren renders them fruitful.
[116] See the Fragments of Aristotelian phytology in Meyer’s
‘Geschichte der Botanik,’ i. p. 120.
[117] J. B. van Helmont was born at Brussels in 1577, and died at
Villvorde near Brussels in 1644. He was a leading representative of the
chemistry of his day. Kopp, in his ‘Geschichte der Chemie,’ 1843, i. p.
117, has given a full account of his life and labours.
[118] J. D. Major, who was born at Breslau in 1639, and died at
Stockholm in 1693, is quoted by Christian Wolff, as well as by
Reichel (‘De vasis plantarum.’ 1758, p. 4) and others, as the founder
of the theory of circulation, which he propounded in 1665 in his
‘Dissertatio Botanica de planta monstrosa Gottorpiensi,’ etc. Kurt
Sprengel (‘Geschichte der Botanik, ii. p. 7) classes him also among the
defenders of the doctrine of palingenesia, a superstitious belief in
the reproduction of plants and animals from their ashes, which was used
to prove the resurrection of the dead.
[119] He says, ‘in mediis vasculis reticularibus,’ which when taken in
connection with his general histology, must be understood to mean the
bast-bundles.
[120] The date of the birth of Edme Mariotte is not known. He was a
native of Burgundy, and lived in Dijon at the time of his earliest
scientific labours. He was an ecclesiastic and became Prior of St.
Martin sous Beaune near Dijon; he was a Member of the Academy of
Sciences in Paris from its foundation in 1666, and was one of the first
Frenchmen who experimented in physics and applied mathematics to them.
He died in Paris in 1684 (‘Biographie Universelle’).
[121] See the Fragments of Aristotelian phytology in Meyer’s
‘Geschichte der Botanik,’ i. pp. 119, 125.
[122] His views are known to me only from Magnol’s paper in the
‘Histoire de l’Académie Royale des Sciences,’ 1709, and Sprengel’s
‘Geschichte der Botanik,’ ii. 20. Perrault’s treatise is according to
Pritzel’s ‘Thesaurus’ of the date of 1680, but is published in the
‘Œuvres divers de Perrault’ of 1721.
[123] Especially in pages 1165, 1201, 2067, 2119.
[124] Stephen Hales was born in the county of Kent in 1677 and was
educated at home without showing any special ability. At the age of
nineteen he became a member of Christ’s College in Cambridge, and there
showed his taste for physics, mathematics, chemistry, and natural
history. Nevertheless he took orders and held Church preferment in
different counties. He became a Member of the Royal Society in 1718,
and read before it his ‘Statical Essays.’ His ‘Hæmostatics’ appeared
in 1733. He made and published other investigations and discoveries
of very various kinds before his death in 1761. He was buried in his
church at Riddington, which he had rebuilt at his own cost, and the
Princess of Wales caused an inscription to his memory to be placed in
Westminster Abbey. See his Éloge in ‘Histoire de l’Académie Royale des
Sciences,’ 1762.
[125] See Sprengel, ‘Geschichte der Botanik,’ i. 229, and Reichel’s and
Bonnet’s works mentioned below.
[126] Georg Christian Reichel was born in 1727 and died in 1771. He was
Professor in the University of Leipsic.
[127] Charles Bonnet, born at Geneva in 1720, sprang from a wealthy
family, and was intended for the profession of the law, but gave
himself up from an early age to scientific pursuits, and especially to
zoology. He was afterwards a member of the great council of Geneva,
and wrote various treatises on scientific subjects, psychology, and
theology. He died on his properly at Genthod near Geneva in 1793. See
the ‘Biographie Universelle’ and Carus, ‘Geschichte der Zoologie,’ p.
526.
[128] See p. 35 of the German translation by Arnold, 1762.
[129] Henri Louis du Hamel du Monceau was born at Paris in 1700
and died in 1781. He had an estate in the Gatinais, and turned his
studies in physics, chemistry, zoology, and botany to account in the
composition of a number of treatises on agriculture, the management of
woods and forests, naval affairs, and fisheries. He was made Member
of the Academy in 1728 on presenting to it an essay on a disease then
raging in the saffron-plantations, and caused by the growth of a fungus
(‘Biographie Universelle’).
[130] See Kopp, ‘Geschichte der Chemie’ (1843), i. p. 306, and
‘Entwicklung der Chemie in der neucrenzeit’ (1873), p. 138.
[131] Still less was gained from an observation made by Bonnet, that
leaves exposed to sun-light in water containing air show bubbles
of gas on their upper surface. Bonnet expressly denied the active
participation of the leaves in the phenomenon, since the same thing
happens with dead leaves in water containing air.
[132] Jan Ingen-Houss, physician to the Emperor of Austria, practised
first in Breda, and afterwards in London. He was born at Breda in
Holland in 1730, and died near London in 1799.
[133] Jean Senebier, born at Geneva in 1742, was the son of a
tradesman, and after 1765 pastor of the Evangelical Church. On his
return from a visit to Paris he published his ‘Moral Tales,’ and at
the suggestion of his friend Bonnet competed for a prize offered at
Haarlem for an essay on the Art of Observation. He was awarded the
second place in this competition. In 1769 he became pastor at Chancy,
and in 1773 librarian of Geneva. At this time, among other literary
labours, he translated Spallanzani’s more important writings; he also
studied chemistry under Tingry, and carried out his researches into the
influence of light. In 1791 he wrote an article for the ‘Encyclopaedie
méthodique’ on vegetable physiology. The revolution in Geneva drove him
into the Canton Vaud, and there he composed his ‘Physiologie végétale,’
in five volumes. He returned to Geneva in 1799 and took part in a new
translation of the Bible. He died in that city in 1809 (‘Biographie
Universelle’).
[134] Nicolas Théodore de Saussure was born at Geneva in 1767, and died
there in 1845. He was the son of the famous explorer of the Alps, and
assisted his father in his observations on Mont Blanc and the Col du
Géant. In 1797 he wrote his treatise on carbonic acid in its relation
to vegetation, a prelude to his ‘Recherches chimiques’; the latter
work received great attention from the scientific world, and he was
made a corresponding member of the French Institute. He was a man of
literary tastes, and took part also in public affairs, being repeatedly
elected to the Council of Geneva. His preference for a secluded life
is said to have been the reason why he never undertook the duties of a
professorship. See the supplement to the ‘Biographie Universelle’ and
Poggendorf’s ‘Biographisch-litterarisches Handwörterbuch.’
[135] Henri Joachim Dutrochet, born in 1776, was a member of a noble
family which belonged to the department of the Indre and lost its
property during the revolution; he therefore adopted medicine as a
profession, and took his degree at the Faculty of Paris in 1806. He was
attached to the armies in Spain as military surgeon in 1808 and 1809;
but he retired as soon as possible from practice and devoted himself in
great seclusion to his physiological pursuits, living for some years in
Touraine. He was made corresponding member of the Academy in 1819, and
communicated his discoveries to that body. Becoming an ordinary member
in 1831, he spent the winter months from that time forward in Paris.
He died in 1847 after two years’ suffering from an injury to the head.
Dutrochet was one of the most successful champions, in animal as well
as vegetable physiology, of the modern ideas which displaced the old
vitalistic school of thought after 1820. See the ‘Allgemeine Zeitung’
for 1847, p. 780.
[136] See above on page 513.
[137] Thomas Andrew Knight, President of the Horticultural Society, was
born at Wormsley Grange, near Hereford, in 1758, and died in London in
1838.
[138] See ‘Arbeiten des botanischen Institutes in Würzburg,’ vol. i. p.
99.
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