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Title: Disease in Plants
Author: Ward, H. Marshall
Language: English
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DISEASE IN PLANTS
DISEASE IN PLANTS
BY
H. MARSHALL WARD, Sc.D., F.R.S.
FELLOW OF SIDNEY SUSSEX COLLEGE, HONORARY FELLOW OF CHRIST'S COLLEGE
AND PROFESSOR OF BOTANY IN THE UNIVERSITY OF CAMBRIDGE
PRESIDENT OF THE BRITISH MYCOLOGICAL SOCIETY, AND FELLOW OF THE
LINNEAN AND ROYAL HORTICULTURAL SOCIETIES; HONORARY FELLOW
OF THE MANCHESTER LITERARY AND PHILOSOPHICAL SOCIETY
AND OF THE BOTANICAL SOCIETY OF EDINBURGH
London
MACMILLAN AND CO., LIMITED
NEW YORK: THE MACMILLAN COMPANY
1901
_All rights Reserved_
GLASGOW: PRINTED AT THE UNIVERSITY PRESS
BY ROBERT MACLEHOSE AND CO.
PREFACE.
It has often been represented to me that the cultivators of plants,
among whom are to be included planters and foresters, as well as
agriculturists and gardeners of every kind, are more particularly
concerned with, and interested in, the maladies themselves of the plants
they grow, than in the life-history of the fungi, insects or other
organisms to which they are due, or in the physiological processes which
are involved; and although it is impossible to really understand any
disease unless we also understand the processes by which it is brought
about, there is room for sympathy with the point of view of the
cultivator. He says, in effect, "I do not want to know all about the
biology of the fungus of wheat-rust, or of the _phylloxera_, nor do I
want to learn what experts can tell me about the action of bacteria in
soil, or the process of starch-formation in the leaves: I have neither
the time nor the means to master these details. What I want is guidance
as to what is wrong with my tomatoes, apple trees, chrysanthemums, fir
trees, turnips, etc., and what I am to do to set things right." Just
so. With the latter part of this cry one must sympathize, much as a
doctor does with the wail of the parent who calls him in to cure his
sick child--we need not stop to classify or compare the motives of the
parent and the cultivator, and perhaps I had done better to select a
breeder of sheep with his flock and a veterinary doctor in the
illustration, but we will let it pass; and as regards the former part of
the cry, I do not know that the plant-doctor can expect the cultivator
to be initiated in the aetiology of the disease any more than the
physician expects the parent to understand the biology of the typhoid
bacillus. That both the cultivator and the parent would be the better
for a real knowledge of the disease in either case must be admitted--nay
insisted on, provided the knowledge _is_ real--but we have to deal with
facts, and it is a fact that the clients of both doctors are impatient
of the details of the case.
Now, of course, I am aware that no short cut or "royal road" to science
exists, and if a man is going to train up trees or other plants, he
ought to know all about them in health and in sickness, in youth and in
old age, and he ought to learn everything about the soil they grow in,
the air that surrounds them, the enemies that beset them, and all the
multifarious relations of these one to another; but when I look at my
boy and reflect how much his nurse, his schoolmaster, his tutor, his
doctor, and his parents _ought_ to know successively and simultaneously
about him in sickness and in health, and about his surroundings, etc., I
begin to wonder whether there is not after all something to be said for
the cultivator's point of view.
Moreover, the cultivator knows a good deal about his plants which I do
not know, and although I should much like to know it, his plea of want
of time rings in my ears and the conviction strikes home that one ought
to try and meet his views, and tell him something about disease as
manifested in plants without insisting on his becoming a professional
mycologist, entomologist, agricultural chemist, and philosopher.
Of course, beyond a certain point, it is his lookout how much the
information is worth, and its educational value--a very different
matter--is sure to suffer from any restrictions imposed on the treatment
of the subject; but if the theme of disease in plants, treated from a
general point of view--I was about to write "treated in a popular
manner," but that is impossible until physiology and mycology are more
widely taught--enables him to understand better the questions he puts to
himself, and, still more, if it stimulates him to enquire further into
the inexhaustible field of science glimpsed at, something may come of
it.
The purpose of these essays is to treat the subject of disease in plants
with special reference to the patient itself, and to describe the
symptoms it exhibits and the course of the malady, with only such
references to the agents which induce or cause disease as are necessary
to an intelligent understanding of the subject, and of the kind of
treatment called for. Consequently I have avoided any unnecessary
classification or elaborate descriptions of parasitic fungi or insects,
histological details of the tissues of plants, chemical and physical
details regarding the soil, and even matters purely physiological as far
as possible. Several admirable works on these subjects are already
available, and must be referred to for further details.
It is, however, quite out of the question to avoid technicalities,
though I have chosen the simpler course wherever it was found feasible,
and have tried to so employ the examples selected that the student who
wishes to go further into the matters dealt with may turn to special
treatises for further information. For one eminently technical section I
ought perhaps to apologise, but the temptation to try and set forth, in
concrete form and suitable for the purposes of this book, some account
of what is known of the most essential and profound factors concerned in
the difficult question of the nature of life and death, health and
disease, was great. Probably my apology should go further, and apply to
what after all must be failure to explore this mystery to the bottom: my
only excuse must be that it may stimulate others to go further.
It was an afterthought to add, in Part I., the considerations on the
factors which influence the plant regarded as a living machine, so to
speak, in order that the student may the better apprehend the point of
view taken of the bearings of the matters discussed in Part II.
With regard to references, it seemed a better plan to give, in the form
of notes after each chapter, the titles of the principal books and
papers on which a student may base a further course of reading, than to
overweight the pages of what is, after all, merely an introductory
sketch to a huge subject, with detailed quotations from the numerous
sources of information made use of. I have freely expressed my own
opinions, but the sources for others are, I hope, as freely given. It
will, however, be understood that I have not aimed at a complete
bibliography, and, particularly, I have only given foreign references
where it seemed that adequate treatment of the subject could not be
found in English.
My sincere thanks are due to Mr. F. Darwin, F.R.S., who has kindly
looked through many of the proofs, and given me the benefit of several
suggestions: and to my wife for the very material aid she has afforded
me in the preparation of the index.
H. MARSHALL WARD.
CAMBRIDGE,
_November, 1900_.
CONTENTS.
_PART I.--SOME FACTORS._
CHAPTER I.
PAGE
THE PLANT AND ITS SURROUNDINGS, 1
CHAPTER II.
THE PLANT AND ITS FOOD, 7
CHAPTER III.
THE PLANT A LIVING MACHINE, 15
CHAPTER IV.
METABOLISM, 23
CHAPTER V.
ROOTS AND ROOT-HAIRS, 35
CHAPTER VI.
THE FUNCTIONS OF ROOT-HAIRS, 45
CHAPTER VII.
THE BIOLOGY OF SOIL, 56
CHAPTER VIII.
HYBRIDISATION AND SELECTION, 69
_PART II.--DISEASE IN PLANTS._
CHAPTER IX.
PHYTOPATHOLOGY. DERIVATION AND MEANING, 85
CHAPTER X.
HEALTH AND DISEASE, 91
CHAPTER XI.
CAUSES OF DISEASE, 99
CHAPTER XII.
CAUSES OF DISEASE. THE LIVING ENVIRONMENT, 108
CHAPTER XIII.
NATURE OF DISEASE, 119
CHAPTER XIV.
NATURE OF DISEASE (_Continued_), 130
CHAPTER XV.
SPREADING OF DISEASE AND EPIDEMICS, 142
CHAPTER XVI.
THE FACTORS OF AN EPIDEMIC, 149
CHAPTER XVII.
REMEDIAL MEASURES, 159
CHAPTER XVIII.
VARIATION AND DISEASE, 168
CHAPTER XIX.
SYMPTOMS OF DISEASE, 179
CHAPTER XX.
SYMPTOMS OF DISEASE (_Continued_), 186
CHAPTER XXI.
ARTIFICIAL WOUNDS, 194
CHAPTER XXII.
NATURAL WOUNDS, 204
CHAPTER XXIII.
EXCRESCENCES, 212
CHAPTER XXIV.
EXCRESCENCES (_Continued_), 222
CHAPTER XXV.
EXUDATIONS AND ROTTING, 227
CHAPTER XXVI.
NECROTIC DISEASES, 240
CHAPTER XXVII.
MONSTROSITIES AND MALFORMATIONS, 246
CHAPTER XXVIII.
PROLIFERATIONS, 257
CHAPTER XXIX.
GRAFTS, 262
CHAPTER XXX.
LIFE AND DEATH, 271
INDEX, 293
_PART I._
SOME FACTORS.
CHAPTER I.
THE PLANT AND ITS SURROUNDINGS.
_The plant the central object of study--soil, climate,
atmosphere, etc., are factors of its environment. Agricultural
chemistry. The plant a machine. Physiology._
If I were asked to sum up the most important result of the numerous
advances made during the past decade in agriculture and forestry, I
should reply--the clearer and wider recognition of the fact that the
plant itself is the centre of the subject, and not the soil, climate,
season, or other factors of its environment. Until comparatively recent
times it was the habit of farmers, foresters, planters, and gardeners,
all the world over, to look upon the plant as a mere item or as a
mysterious if important one in their calculations, and to regard the
soil as the chief factor in their studies.
Now all is changing, and the world is gradually awakening more and more
to the recognition of the truth that the soil and the clouds and the
atmosphere are merely reservoirs of more or less inert materials, from
which the living plant draws its supplies, and works them up, by means
of energy focussed from the sun, into new plant substance.
In other words, the more far-seeing pioneers of scientific agriculture
and forestry, etc., are recognising that agricultural chemistry is not
the be-all and end-all of agricultural science; but that, in place of
the study of the chemical analyses of dead soil, water, air, and
plant-remains, which has so long held sway, largely owing, I think, to
the influence of Liebig, the student should have his attention more
concentrated on the living plant itself and on the physiological actions
which make up its life. He must regard the living plant as a sort of
working machine--infinitely more complex than any machine made by man,
but a machine nevertheless--the purpose of which is to store up energy
from the sun, and so to add to our wealth on this planet, at the expense
of the extra-terrestrial universe.
It is not, be it noted, that the new study proposes to ignore or abandon
the old studies: modern physiology owes too much to the physics and
chemistry on which it is partly based, and to the labours of De
Saussure, Ingenhousz, Priestley, and others, for that. But it is that
the new study recognises that the central point, to which all views must
be focussed, is not the one that it was formerly supposed to be. The
student is still taught that the chemistry of soils yields valuable
information, and that lessons of importance are derived from comparisons
of the analyses of the ashes, etc., of plants; but he is no longer able
to cherish the hope, however remotely, that such studies solve his most
important problems.
The scene--or rather the point to which attention is now directed--is
the living, working, energy-accumulating plant itself, and not the dead
store of materials in the soil. The reason for the change is not far to
seek: it is due to the enormous strides made in the study of the
physiology of plants during the last quarter of a century, and the
subject abounds in examples illustrating the marvellous advances that
have been made, and at the same time showing how, in the progress of
researches, made for their own sake--_i.e._ in pursuit of satisfaction
for the intense curiosity of the scientific man--all kinds of side
issues turn up which prove to be of value in practice, and suggestive of
further thinking.
At the beginning of the nineteenth century--_i.e._ about 1820--the best
thinkers were giving up the old ideas that the environment supplied
food, as such, to plants, and had recognised that the plant takes up
substances from without and rearranges these in its own body.
The next twenty years or so form a very dark interval in plant
physiology, chiefly owing to the influence of the assumption of a
special "vital force," an assumption which was not allowed merely to
serve as a hypothesis put forward to stimulate research and suggest
better ideas, but which gained a hold over men's powers of reasoning to
an extent which now appears monstrous and phenomenal.
Many errors crept in during this reign of terror, one of the most fatal
of which was De Candolle's revival of the idea of "spongioles"; and
another, equally disastrous in many of its effects, was the conception
of a sort of vegetable food-extract, humus, existing in the soil in a
form peculiarly suitable for direct use by plants. It was during this
period that the confusion between the processes of respiration and
carbon-dioxide assimilation arose, and exerted its effects for evil into
our own day.
The now astounding statement that oxygen-respiration in plants did not
occur, laid the foundation of many subsequent difficulties, and so did
the positive and authoritative views on the uses of minerals to the
plant. Liebig, in fact, stood in the invidious position of being a high
authority on purely chemical questions, who was impelled to give
opinions on matters which can only be solved by physiological
experiments: his great service was to clear up mistakes as regards the
chemistry of soils and of plants--his great mistakes were due to his
pronouncing on physiological matters; and it may be doubted whether his
great services to the purely chemical side of subjects connected with
agricultural matters are the more to be admired, or the disastrous
influence of his statements on subjects which do not belong to the
domain of chemistry should be the more deplored. Be that as it may, he
handed on to succeeding generations some weighty errors as regards
plant-life, and taught the agriculturist to regard chemical analyses of
soils and plant ashes with a reverence which obstructed progress for
some time. As a set-off to this we must place his contributions to the
destruction of the bugbear vitalism, which was simply preventing
enquiry, and his services in bringing together and sifting with power
and originality all that had been then acquired as regards the chemistry
of the plant, the soil, and the atmosphere.
That Liebig was indispensable in 1840-1850 is one thing; but that his
influence should extend to the present day is quite another, and his
inevitable mistakes were almost as powerful for future evil, as his
clear exposition of the chemistry of his day was productive of immediate
good.
Boussingault, working at the same time, 1837-1855, but experimentally
with the living plant, taught us more about these matters than
any investigator of the time, though it is very probable that the
stimulus of Liebig's speculations, good and bad, had its effect in
impelling Boussingault to devote his splendid methods to problems of
plant-nutrition. Boussingault's contributions to our knowledge of the
composition of the dead plant cannot be over-estimated; but he did more
than this, because he so clearly apprehended the necessity for asking
his questions directly of the living plant, instead of deducing from
chemical principles what might be supposed to occur in it; and although
future researches showed that even so careful an investigator solved a
problem of first importance--viz. the question of the fixation of free
nitrogen--the wrong way, it will be found that so far as he did go his
conclusions were sound, and well calculated to inspire the confidence
with which the world received them. As we are here concerned more
especially with the botany of agriculture, however, it is unnecessary to
dwell longer on these matters, or on the similar and even more extensive
experiments, of world-wide reputation, carried on for so many years, and
still being carried on under the liberal auspices of Sir John Lawes, at
Rothamsted. Moreover it may be necessary to return to some of these
points later on.
NOTES TO CHAPTER I.
The reader will find a further general account of these
matters in Sachs' _Lectures on the Physiology of Plants_,
especially Lectures I. and XII., Engl. ed., Oxford, 1887. He
may then proceed to Pfeffer's _Physiology of Plants_, Engl.
ed., 1899, chapter I., and to the account of the history of
the subject in Sachs' _History of Botany_, Oxford, 1890,
especially pp. 359-375 and 445-524. References to more special
literature will be found in Pfeffer.
CHAPTER II.
THE PLANT AND ITS FOOD.
_The food of plants--"Vital force"--Other errors--Liebig and
Boussingault--The botany of agriculture. The synthesis of
carbohydrates--The physiology of plant-nutrition. The
persistence of misconceptions._
The year 1860 may be regarded as a landmark of importance in the history
of plant physiology, for it was in that year that Sachs discovered that
the bringing together of water and carbon-dioxide, in the green
chlorophyll-corpuscles of the plant exposed to sunlight, results in the
formation of the grains of starch found in these corpuscles.
Previous to this date Dutrochet (1826-37) had introduced the then crude
idea of osmosis into physiology; vegetable anatomy had improved, and the
modern conceptions of the living cell, protoplasm, nucleus, etc., were
slowly looming; sieve-tubes had been discovered, and the proteids and
starch in various parts of the plant examined; and the suggestion was
abroad, replacing Liebig's idea that plant acids were the first
products of carbon-assimilation, that some substance, of a slimy nature,
was manufactured in the cells of the leaves and thence distributed as
the formative material from which the plant constructed its parts. Davy
and Boussingault had even surmised that a carbohydrate might be the
first-formed product in assimilation.
There can be little doubt that Sachs' classical proof, by direct
physiological observation and experiment, first brought forward the
truth of organic synthesis in the plant in a concrete and convincing
form.
But it did more than that. It laid the foundation of the modern
physiology of plant-nutrition on ground already prepared by De Saussure
and the earlier workers; for, in addition to emphasising the truth of
organic synthesis--a truth which had been gradually impressing itself on
the world for some years--Sachs' discovery showed clearly the real
meaning of carbon-assimilation as a process for obtaining combustible
food, which the plant then proceeds to make use of.
Many points were rapidly cleared up at once, or if not explained were at
least put into a strong light for further enquiry, and plant-nutrition
soon ceased to be the mysterious subject for all kinds of wild
conjectures that it had hitherto been.
The meaning of thin leaves, with numerous stomata and finely ramified or
divided vascular bundles, became more apparent, as also did the
significance of the ascending transpiration current; the storage of
starch-grains in tubers, medullary rays, roots, seeds, etc., obtained
meanings not understood before; the spread of roots in the soil, and the
gradually discovered properties of the finer rootlets and of the
root-hairs, fitted naturally into their places; and, in short, a
thousand facts, otherwise isolated, became collated into an intelligible
system, full of suggestions for new work, such as has since gone on and
is now being pursued with an activity and success never before realised
in the history of science.
As time went on, while the general truth of Sachs' views was confirmed,
a number of detailed discoveries were made which seemed to contradict
them in certain points. It was found that not all leaves form starch,
for some contain sugar or oil; but Holle and Godlewski proved
experimentally that this oil may be replaced by starch if the conditions
of assimilation are slightly modified. More recently Hébert discovered
that the stalks and leaves of grasses contain a peculiar form of gum,
which was formerly confounded with starch, a substance not abundant in
them. Then came Schimper's discovery of starch-forming corpuscles,
which, if supplied with sugar, are able to form starch-grains in the
dark, as in tubers, etc., underground; and as subsequent researches have
proved that the chlorophyll-corpuscles--which are morphologically the
same as the starch-forming corpuscles and can be replaced by them--are
also able to form starch-grains from sugar, as proved by the experiments
of Boehm, Acton, Meyer, Laurent, Bokorny, Saposchnikoff, and others, it
soon became evident that nothing essential needed altering in Sachs'
view that starch is the first visible product of carbon-dioxide
assimilation, only it became clearer that the starch-grains are built up
by the protoplasm from glucose or some similar body, and represent so
many packets of reserve materials put by for the present because not
required for the immediate needs of the cell.
Boussingault showed, about thirty years ago, that assimilation soon
stops in green leaves if cut off from the plant, not because the leaves
die, but owing to some "maximum capacity" being attained. Sachs had
shown that the starch passes down to other parts of the plant in
solution as glucose.
Neither time nor space will permit me to go into the enormous field of
research and results opened up by these and similar observations made
between 1860-70. It must suffice to say that they led to the discovery
and study of the diastatic and other enzymes in the leaves and other
green parts of plants, and to a clearer understanding of what was
already known of them in seeds, and this knowledge reacted at once on
our insight into the processes of transport of reserve materials and
constructive materials from one part of the plant to another, matters
which will be referred to later on.
It remains to explain Boussingault's difficulty as regards the cessation
of assimilation. Recent researches confirm the view that at least three
causes are at work to bring about the inhibition of the
carbon-assimilation: first, the chlorophyll-corpuscles become filled to
excess with starch, which cannot get away because all the passages are
full and the products are inhibiting the further action of the enzymes
which should dissolve the solid granules; secondly, the leaf being
detached from the plant explains why the soluble products cannot get
away, for this makes a great difference in the rate of exhaustion of the
leaf; and, thirdly, the same fact involves that the leaf can obtain no
further supply of salts of potassium, etc., without which elements the
processes in question cannot go on.
These and numerous other deeper insights into the process of
assimilation, obviously strengthen the force of Sachs' discovery; though
it by no means necessarily follows that starch-grains are always the
resting form of the products of assimilation, and we now know that such
is often not the case: we now have much deeper glimpses into the initial
products of carbon-assimilation than Sachs had in 1860, but this
enhances rather than detracts from the importance of his splendidly
worked-out discovery. Put more generally, we may now say that the
process of carbon-dioxide assimilation in green leaves under the
influence of light is a process of synthesis--photo-synthesis--resulting
in the building up of a carbohydrate such as sugar, inulin or starch
from the elements carbon, hydrogen and oxygen.
But it must not be supposed that the importance of Sachs' discovery, and
the rapid consequent extensions of our knowledge, did their work
forthwith in disabusing men's minds of old and erroneous notions. To say
nothing of numerous smaller misconceptions which still held their ground
owing to the stupendous ignorance of plant-physiology which prevailed,
we find incompetent teachers and text-books were still propagating ideas
worthy of ancient times. The confusion between oxygen-respiration and
the gas interchanges in carbon-assimilation was by no means eliminated
even recently, though it can no longer withstand the deliberate
onslaughts now made on it. That the roots take up food as such from the
soil, and that that food is directly employed by the plant for its
nutrition is even yet implied in daily conversation around us; and
although matters have advanced so far that everyone now knows that the
substances at the roots must be in solution, ere they can be received
into the plant, it sometimes leads to astonishing replies, if we press
the question very far as to how the absorption takes place, in an
elementary examination of agricultural students. That manures are foods
to the plant, that sap circulates, that transpiration is of use to keep
the plant cool, and wood is a "porous body," etc., are only a few of the
misconceptions still current, in a decade that has found publishers for
a work advocating that roots are congealed sap, and that the leaves of
plants absorb the moisture and dust of the air, and so provide the plant
with food, and for a paper explaining the action of root-hairs as tubes
with open pores at their tips. But the gravest misapprehensions current
among us are due to the crude ideas as to what a plant really is: this,
I take it, is owing to the difficulty of grasping what physiologists
mean by organised structure, and leads to regarding the living being
either as a mere aggregation of chemical compounds, built up by the
ordinary play of chemical forces, as we know them, acting on dead
matter, or, as in the days before organic chemistry, as a mysterious
entity endowed with "vital force," and with properties not amenable to
scientific investigation. The mistaken notions as to the powers of roots
to "select" those substances which the plant requires, and to reject
useless ones was merely an expression of this belief.
The rock on which all are liable to come to grief--the chemist or
physicist who requires all his facts in terms of analyses and
proportions by weight, and therefore takes too mechanical a view of the
subject, or the man who is not scientifically trained at all, and
therefore is more liable to go to the other extreme and regard the plant
as a mysterious something which grows and has poetical associations and
traditions--is the great fact of organised structure, and it is the
recognition of this fact and some of its consequences which has altered
the whole position of the subject, and brought the study of the plant
into the domain of physiology. The living plant, its structure and
organisation, the functions of its mechanism, and its relations to the
environment, thus forms a subject apart from that which concerns the
chemical composition of the plant and its environment, and this
distinction designates, in a word, as it were, the change which has been
brought about by modern biology.
A point to be emphasised to the utmost where agricultural students are
concerned is that the essential process of feeding is the same in a
green plant, a fungus, and an animal; the greatest confusion still
exists with regard to this matter, owing to misconceptions as to the
real meaning of the functions of the chlorophyll-corpuscles when
supplied with carbon-dioxide and water and the energy of the sun's rays.
The plant does not feed on carbon-dioxide, any more than it feeds on
oxygen--it feeds on the organic material after it has been constructed,
and the chlorophyll-function is merely one mode of obtaining supplies of
such organic substance.
NOTES TO CHAPTER II.
In addition to the references in the last chapter, the student
should consult Sachs' _Lectures_, XVII.-XIX., and Pfeffer's
_Physiology_, pp. 287-329, for the further development of this
subject. An excellent résumé, with new facts and points of
view, will be found in Dr. Horace Brown's "Address to the
Chemical Section," _British Association Reports_, Dover, 1899;
and "Chemistry and Physiology of Foliage Leaves" in _Trans.
Chem. Soc._, 1893, p. 604. See also Blackman, "Experimental
Researches on Vegetable Assimilation and Respiration," _Phil.
Trans._, 1895; and Parkin, "Formation, etc., of Carbohydrates
in Monocotyledons," _Phil. Trans._, 1899.
CHAPTER III.
THE PLANT A LIVING MACHINE.
_The plant a machine into which energy and material are
taken--Carbon assimilation--Feeding--Accumulation and
transformations in the plant. The action of light--The
chlorophyll-function._
The relations of the plant to the environment can only be understood by
taking into account the results of modern physiological discoveries.
These teach us that the living plant is a highly complex machine, the
details of its organisation and structure being much more numerous and
much more closely correlated at numerous points, than the parts of any
other machine known to us.
They also teach us that it is supplied with energy from without, as any
other machine; and that when so supplied, and properly working, the
living structure or machinery does work, also as other machines. But
modern physiology goes further, in that it renders some account of the
ways by which the external energy is taken into the plant, and there
applied to do work, or stored up for a time in order that it may be used
to do work at some future time.
The accumulation of energy thus ensured is associated with corresponding
changes of material substance, and the principal means for bringing this
about is recognised in the assimilation of carbon-dioxide--photo-synthesis.
In this process energy enters the chlorophyll-corpuscle in the form of
the radiant energy of the sun, it is there directed in the mechanism of
the protoplasm, so as to do work on the molecules of water and
carbon-dioxide which have also been brought into the machinery; this it
does, breaking asunder their stable structure into unstable bodies,
which then re-combine in different ways to form a carbohydrate, such as
starch, and this starch is temporarily stored as grains, while oxygen
escapes.
Each starch-grain, therefore, is to be regarded as a packet of matter
and of potential energy, as it were, capable of yielding up the latter
at any future time, when put under such circumstances that it must do
so. Such stores of energy-yielding substance, if I may use the
much-abused phrase, form the principal food of the plant--or of an
animal, if it steps in and takes them--and we now see that the process
of carbon-dioxide assimilation, as it has perhaps unfortunately been
called, is not the same thing as the process of feeding, for the
_feeding_--_i.e._ the nutrition proper--of the plant does not begin
until the _food_ has been thus obtained.
We now see what the real position of the plant is, to its environment,
whether the latter be living or dead. From our point of view, the plant
serves as a centre for bringing together the substances obtainable from
the soil, and those derived from the atmosphere, and so focussing and
directing the radiant energy of the sun upon these substances, that they
are broken up, and some of their constituents synthesised, with
absorption of energy, into a body, such as starch, containing more
energy than did the original substances taken together or separate. It
matters little whether the actual carbohydrate thus synthesised is
starch, or sugar or inulin: the point is that energy has been gained
from outside and bound up with the acquired material for further use.
But modern physiology has carried matters much further than this, and
especially in the three following directions.
In the first place, it has shown that much of the energy thus stored
from without in the plant is again liberated in the process of oxygen
respiration, and expended partly as appreciable heat and partly as
driving force for stimulating the machinery of the living plant to
further activities.
In the second place, part of it is rearranged with the rearrangement of
the molecules with which the energy is bound up, as it were, so that
work of various kinds is done _in_ the machinery of the plant: I refer
to various metabolic and surface-actions resulting from the peculiar
mode of presentment of the resulting substances, for instance the
production of osmotic pressures in the cell.
And, thirdly, part of the synthesised substance is worked up into higher
bodies, by processes which obviously entail the further doing of work on
the constituents.
The further pursuit of this theme would evidently carry us beyond the
more immediate subject of this book; but I want to make clear that
recent researches render it more and more certain that the living plant
is a complex piece of co-ordinated machinery which brings together
matter and energy from the external universe, and then gets work out of
these.
This proposition is the more important because the whole question of the
enrichment of our planet with new food, new building materials, and new
fuel, to compensate the daily losses, depends on it, and is of course to
be referred fundamentally to the acquirement of new supplies of energy
from the sun. Enormous activity has been displayed by physiologists,
since 1860, in attempting to solve the question, which of the many
different rays known to proceed from the sun are absorbed by the
chlorophyll-corpuscle, and directed to the performance of the work above
referred to.
The names of Draper, Sachs and Pfeffer stand forth prominently as
pioneers in this; while those of Lommel, Engelmann, Timiriazeff and
Langley have been among the most active in making important
contributions to the subject, and in attempting to answer the further
questions connected with the mode in which the chlorophyll is concerned
in utilising the energy of the solar radiations. The point is one of
supreme importance, because it goes on all fours with modern questions
as to the rays of light absorbed or dispersed in our atmosphere at
different seasons of the year, or in special climatic conditions, to say
nothing of its other scientific aspects. Unfortunately, however, we have
no satisfactory explanation of the actual rôle played by the chlorophyll
substance itself, in spite of much industrious work which has been done
in the subject in this country and elsewhere. As regards the rays
employed, it was first proved that the most effective belong to the red
end of the visible spectrum, and that the effect as measured by the
amounts of oxygen given off, and of starch formed in given periods of
time, is more or less proportionable to the intensity of the solar
light. Then it was established that no monochromatic light is so
powerful as the white light from which it was obtained, though the
relative numbers expressing the activity in the red and yellow regions
may stand to those in the blue as something like 12:1. The latest
results place the maximum assimilation in the red-orange, and this
coincides with the maximum absorption in the chlorophyll. If we may
accept the current views as to the distribution of energy in the
spectrum of solar light, which depends on the complete absorption of all
the rays by a black body, where they are estimated as heat, we have the
interesting result that the agricultural or forest plant is adapted to
catch and retain, broadly speaking, just those particular rays which
possess most energy.
The probability is increasing that the protoplasmic machinery is the
really effective mechanism in the process, and we may figure this
machinery as so holding or presenting the molecules of carbon-dioxide
and water to the impact of the light-vibrations, that the latter are
enabled to undo the molecular structure; the atomic combinations thereby
liberated may then be supposed to form a body like formic-aldehyde,
which by polymerisation becomes a carbohydrate of the nature of a sugar
such as glucose, which the protoplasm then builds up into its substance
and subsequently deposits as starch, and stores temporarily in the form
of grains or as amorphous material.
This is partly hypothetical, and is largely due to the careful
deductions of the chemists, but there are very many facts now to hand
which bear out its probability, especially the recent advances in our
knowledge of the sugars, and the experimental feeding of leaves and
plants deprived of starch with such substances as dextrose, levulose,
maltose, and other sugars, as well as glycerine and other bodies which
should be convertible into, or yield them, if the theory is true. In
this last connection, the careful and extensive experiments of Acton, A.
Meyer, Boehm, and Laurent should be mentioned. It would be interesting
to enlarge upon Engelmann's beautiful physiological experiments in
connection with this subject of absorption of solar energy, where the
maximum accumulation of oxygen-loving bacteria at those parts of a green
alga which lie in the red-orange of the spectrum, are used as indicators
of the maximum oxygen evolution (and therefore of the maximum
carbon-dioxide assimilation), but space will not admit of this. For a
similar reason I must also pass over the same observer's experiments
with plants which assimilate in protoplasm behind a red instead of a
green substance, and which absorb chiefly other rays between the yellow
and blue, with the remark that they also seem to imply that it is the
protoplasmic machinery which turns the energy on to the carbon-dioxide
molecule, the coloured screen being secondary in the matter. Recent
experiments which show that green plants will not assimilate
carbon-dioxide in a light which has passed through a solution of
chlorophyll--and therefore left its red rays behind; nor behind a screen
of iodine dissolved in carbon-dioxide--which lets no visible rays
between the red and blue pass--should be noticed, as showing the
importance of the chlorophyll and the special rays referred to, however;
and I ought at least to mention Timiriazeff's beautiful proof, published
in 1890, that if, on the leaf of a plant left in the dark long enough to
render it free of starch, a bright solar spectrum is steadily projected
for 3-6 hours, the chlorophyll then removed by alcohol and the
decolorised leaf placed in iodine, the image of the spectrum is
reproduced by the different intensities of the starch bands, blue with
iodine, in the different parts. Here, again, the maximum coloration
coincides with the maximum absorption in and near the red.
Microscopic observations and photo-chemical experiments alike convince
us that the chlorophyll-corpuscle is itself a complex piece of
protoplasmic machinery, working for and with the rest of the plant, and
there can be little question as to the greater accuracy of our reasoning
on the whole question I am discussing, since Meyer, Schimper,
Pringsheim, and others have established the importance of its structural
peculiarities.
I must now pass on to consider another aspect of the question of
carbon-assimilation.
NOTES TO CHAPTER III.
In addition to the references in the last chapter, the reader
may be referred to Sachs' _Lectures_, XXV., and Pfeffer's
_Physiology_, pp. 329-356, where the voluminous literature is
given.
CHAPTER IV.
METABOLISM.
_Quantities of starch formed, and their significance for the
plant. The absorption of energy--the conversion of energy in
the plant. The plant is a complex machine for concentrating
and storing energy and material from without._
Sachs measured the increase in dry weight (due to the carbohydrates
formed in the chlorophyll-corpuscles) per square meter of leaf-surface,
exposed for a definite period, by drying rapidly at 100° C. equal areas
of the leaves concerned, and comparing the weights.
Of course the results are not to be pushed too far, in view of the fact
that some of the starch is continually passing away to be utilised, and
of the difficulties of comparing the weather, the intensity of light,
currents of air, hygroscopic conditions of atmosphere, and other
variable factors which influence the matter. For instance, the stomata
open and close to different extents according to the conditions of
light and moisture, and this affects the whole mechanism of
transpiration especially, and therefore the supplies of water and
mineral salts. Nevertheless, some interesting and valuable results have
been obtained in connection with this important subject.
It was found, for instance, that the foliage of a sun-flower or of a
vegetable-marrow may be forming starch at a rate of considerably over a
gram per hour in every square meter of leaf-surface exposed on a fine
day; while in particularly clear and warm sunny weather Sachs obtained
as much as 24 to 25 grams per square meter per diem.
When one reflects that 200 square meters is not an extravagant estimate
for the area of leaf-surface exposed on a tree, for a period which even
in our latitudes may be considerably over 100 days of, say, ten hours'
light, we need no longer wonder at the rapidity with which wood is
produced in the stems, and similar estimates (which I have purposely
kept lower than the estimates for continental and tropical climates) may
suffice to show how quickly potatoes or the ears of corn, etc., may fill
up with the starch or other carbohydrates which render them valuable as
crops. We want more measurements in these connections, moreover, for
there are several ways in which they are of scientific value and
practical importance.
It is evident from what has been said that every grain of starch formed
represents so much energy, packed away for the moment in the
storehouses of the plant; and we know that--quite apart, however, from
intermediate transformations of the energy thus stored--this energy
reappears in the kinetic state eventually, when the starch is burned
off, in presence of oxygen, and transformed into carbon-dioxide and
water. It matters not how quickly or how gradually this combustion
occurs, or whether it is accomplished by burning in a fire, or by slow
and complex stages in respiration or metabolism: the point is that the
unit of weight of starch yields so many units of heat when its structure
tumbles down to the original components, carbon-dioxide and water.
Clearly, if we know how many units of heat are yielded by the combustion
of one gram of starch, we can obtain an estimate of the amount of
energy, measured in terms of heat, which the foliage gains and stores
up--an estimate which will approach the truth in proportion as our
estimate of the total assimilative activity is correct.
A word of warning is necessary here, however, for those best acquainted
with physiology recognise that however useful such calculations as the
above may be, and undoubtedly are, to give a general idea of the fact
that the energy represented is large, it would be a mistake to suppose
that such estimates give even an approximate measure of the energy of
potential which may be got from the carbohydrate, and still less of the
amount of work that may be got from its employment, according to the way
it is employed or presented in the plant. To take a single instance
only. If the carbohydrate is rapidly burned off to carbon-dioxide and
water, very little is got out of it in the way of work--most, if not
all, of the energy set free escapes as heat: whereas if the carbohydrate
is slowly and gradually oxydised, passing through various stages and
giving rise to powerfully osmotic bodies in the process, or if it is
built up into protoplasm, or into the structure of a cell-wall,
relatively enormous quantities of work may be got out of its
surface-energy, and heat may be absorbed. Whence it follows that we
cannot measure the power for physiological work of a body by merely
obtaining its heat of combustion, any more than we can infer its
significance in metabolism from its chemical properties.
The general conclusion that the plant stores large quantities of energy
may of course be arrived at by simply estimating the enormous quantities
of food-material which we obtain annually from agricultural plants.
Modern physiologists have attempted to proceed further than this,
however, in their essays to form an estimate of the relations between
the available energy in the solar rays and that used and stored in the
plant.
If we reflect on such phenomena as the cool shade of a tree, and the
deep gloom of a forest, and on experiments which show that an ordinary
leaf certainly lets very little of the radiant energy of the spectrum
pass through it, it becomes evident that many of the rays which fall on
the leaf are absorbed in some form, and it becomes very probable that
much of the solar energy, other than that we term light, is retained in
the leaf for other purposes than assimilation--or, at least, no other
conclusion seems possible in view of all the facts. Engelmann's
researches with purple bacteria are almost conclusive on this point, and
we may regard it as extremely probable that the plant makes other uses
of rays, perceived by us as heat-rays, as sources of energy. Researches
on the influences of temperature on assimilation and other functions
point to the same conclusion; and Pfeffer and Rodemann definitely state
that heat is converted into work in the osmotic cells. And the study of
the absorption bands in the spectrum of the living leaf becomes more
intelligible in the light of these conclusions. Moreover, the fact that
a plant still carries on processes of metabolism when active
transpiration has lowered its temperature below that of the surrounding
air--and the plant therefore receives heat from the environment--points
to similar conclusions.
The importance of the conclusion is immense, for even if the plant had
no other sources of energy than the darker heat rays of the solar
spectrum, it is clear that it ought to be able to do work.
The above may suffice for the general establishment of the conclusion
that the plant absorbs more radiant energy than it employs solely for
assimilation, and emphasises our deduction that it is a machine for
storing energy.
The question now arises, how is this relatively enormous gain in energy
employed by the plant? Our answer to the question is not complete, but
modern discoveries in various directions have supplied clues here and
there which enable us to sketch in some degree the kinds of changes that
must go on.
Not the least startling result is that, important as carbon-assimilation
is as the chief mode of supplying energy, it is not the only means that
the plant has of obtaining such from the environment, and it is even
possible--not to say probable--that energy from the external universe
may be conveyed into the body of the plant in forms quite different from
those perceptible to our eyes as light.
In the most recent survey of this domain, it is pointed out that we may
distinguish between radiant energy, as not necessarily or obviously
connected with ponderable matter, and mechanical energy, which is always
connected in some way with material substance. All mechanical
performances in the plants depend on transformation of some form of
these, evident either as actual energy doing mechanical work, or as
energy of potential ready to do work.
In so far as molecular movements are concerned, we have the special form
of chemical energy. The evolution of heat, light and electricity by
plants are instances of radiant energy, and so on.
Many transformations of energy in the plants are due to non-vital
processes--_e.g._ transpiration, warping actions, etc., but we cannot
always draw sharp lines between the various cases. Nor can we directly
measure the work done in the living machinery; but from the effects of
pressures and strains, the lifting of heavy weights, driving of
root-tips into soil, osmotic phenomena, etc., it is certain that the
values may be very high.
The following classes of processes in living protoplasm and cells may be
taken as indicators. First we have transformation of chemical energy,
without which continued life is impossible: in many cases--_e.g._ the
processes connected with oxygen respiration--these result in the
development of heat. Secondly, we have those remarkable manifestations
of energy known as osmotic processes, which depend on surface actions,
and with which may be associated other surface effects, such as
imbibition, secretion, etc., and in connection with which heat may be
evolved or absorbed. It is true the substances which exhibit the
properties here referred to may be produced, or placed in position, by
chemical energy, or they may be absorbed by roots, etc.; but the
proximate energy exhibited by them is not derived from chemical energy,
and may be out of all proportion to the chemical energy of the substance
or substances concerned. Moreover it is significant to note that a
highly oxydised body may develop much osmotic energy, as well as a
highly combustible one.
It is of the greatest importance to realise the truth that much work can
be, and is done in the living plant, by conversions of energy of
potential independent of and out of proportion to the chemical energy
available by decomposing the substances concerned; even the heat of
respiration may be superfluous here, for the plant may absorb heat from
without, and convert it into work.
Tensions often arise in the plant, and do work expressed as
movements--_e.g._ the springing of elastic Balsam fruits, stamens of
_Parietaria_, etc.
Osmotic energy not only results in enormous pressures and tensions, but
causes movements by diffusion and diosmosis, and any given osmotic
substance which carries this energy with it is not necessarily formed
always in the same way in the cell--_e.g._ glucose may arise from
starch, or from carbon-dioxide, or from oil.
Surface-energy is also expressed in the powerful attractions for water
exhibited in imbibition, swelling, capillarity, absorption, surface
tensions, etc.
Transpiration induces relatively enormous disturbances of equilibrium,
and does work in moving water quite independent of chemical energy.
Again, what may be termed excretion-energy, as expressed in the
separation of a solid body--_e.g._ a crystal--from a solution, may be
for our purposes regarded separately. Any change in the condition of
aggregation of a substance in the plant may result in movements and the
overcoming of resistances.
It will be evident from this short digression--and this is the point I
wish to emphasise--that in the interval between the securing of a grain
of starch, representing so much energy won from the external universe,
and the reconversion of this grain into its equivalent carbon-dioxide
and water, by respiration, resulting in the loss of the above energy as
heat, the starch referred to may have undergone numerous transformations
in the living machinery of the plant, and have played at various times a
rôle in connection with the most various evolutions of energy.
If we try to picture a possible case, we may take the following. A given
starch-granule, after being built up in the chlorophyll-corpuscle, is
decomposed, and yields part of itself as glucose, which passes down into
other parts of the plant in solution. Part of it is merely re-converted
into starch, and temporarily stored: another part passes into the arena
of oxydation-processes, the sum of which constitute respiration, and may
serve for a time in the molecules of an organic acid: yet another part
may be converted into a constituent of the cellulose cell-walls; while
part may be brought into play in the reconstruction of protoplasm.
In this last connection a discovery made by Schulze about 1878, and
followed up later by Pfeffer, Palladin, and others is of importance.
Seedlings growing in the dark, or in an atmosphere devoid of
carbon-dioxide in the light, become surcharged with nitrogenous bodies
known as amides, formed during the breaking down of the proteids in the
destructive process preceding and accompanying respiration: if the
seedlings are allowed free access to light and carbon-dioxide, however,
the amides disappear. The explanation is that they are combined with
some of the materials of the carbohydrates, and again built up into the
material of the living protoplasm.
Returning to our hypothetical starch-grain--or, rather, its parts--we
have some of it retained as starch, in excess, simply because it is not
needed at the moment: another portion gives up its energy in
respiration, and this does work on the spot, or is lost as heat; or in
the body of an organic acid, or its salt, the part in question may do
lifting or pressing work by osmosis, or cause diffusion-currents from
one cell to another. In the constitution of the cell-wall we may have
part of our starch-grain aiding in imbibition or in the establishment of
elastic tensions in turgidity: and, finally, parts may be built up into
the living protoplasmic machinery of the plant.
What is true for the starch-grain is also true for any particle of salt,
or water, or gas which enters into the metabolism of the living plant,
regard being paid to the particular case, and circumstances in each
case.
Enough has been said to show that the plant cannot be properly studied
merely as the subject of chemical analysis or of physical investigation;
you might as well expect to understand a watch by assays of the gold,
silver, steel and diamonds of which its parts are made up, or to learn
what can be got out of the proper working of a lace machine by
analysing the silk put into it, and the fabric which comes out, and by
taking the specific gravity of its parts and testing the physical
properties of its wheels and levers.
This is not the same thing as denying the value of such knowledge, in
the case of either the dead machine or the living plant: it is merely
emphasising the supreme importance of the study of the structure and
working of the active machinery in both cases.
Nor is it pertinent to remark on the apparent hopelessness of physiology
being at present able to explain the seemingly infinite complexity of
the living machinery of protoplasm and its activities. The modern
locomotive is also a complex affair in its way, but it is profitable to
investigate it and to know all one can of its working and possibilities,
for obvious reasons: a little reflection will convince us that it is
also worth while to investigate that complex machine, the plant--the
working organism which alone can really enrich a country. Moreover, we
ought to be encouraged by the satisfactory progress now being made, and
the splendid practical results which are accruing, rather than dismayed
by the prospect of unflagging labour which will be required in the
future.
Enough has perhaps been said to establish the general truth that the
plant is a complex machine for storing energy and material from outside,
and we have seen that modern research has at least gone a long way
towards determining how the living machine works.
It is hardly necessary to point out that important practical
consequences may result from these phenomena of the accumulation of
surplus starch or other carbohydrates in the leaves during the day, and
of their disappearance during the night into the lower parts of the
plant. For instance, foliage cut for fodder in the morning is far poorer
in starch than if cut in the evening, and it would be very instructive
to have experiments made on a large scale to test the result of feeding
caterpillars or rabbits, for instance, with mulberry, vine, or other
leaves in the two conditions.
Again, we now see what complications may arise if a parasitic organism
gains access to the stores of carbohydrates in process of accumulation,
or attacks and injures the machinery which is building up such
materials, etc.
NOTES TO CHAPTER IV.
The student who desires to pursue this subject further should
read Sachs' _Lectures_, XX. and XXV., and Pfeffer's
_Physiology_, pp. 442-566, but he will hardly arrive at the
best that has been done without consulting Pfeffer's "Studien
zur Energetik der Pflanzen" in the _Abhandl. der Math.-Phys.
Classe der Kgl. Sachss. Gesellsch. der Wiss._ (Leipzig, 1892),
p. 151; and Kassowitz, _Allgemeine Biologie_ (Vienna, 1899),
Bk. I., pp. 1-127.
CHAPTER V.
ROOTS AND ROOT-HAIRS.
_Older views as to root-hairs--Root-hairs and their
development--Surface--Variations--Conditions for maximum
formation--Minute structure--Adhesion to particles of
soil--Functions._
On the roots of most plants are to be found delicate, silky-looking,
tubular prolongations of some of the superficial cells, known as
root-hairs. Malpighi (1687) seems to have been the first to observe
them, and he took them for capillary tubes. Grew (1682) seems to have
been responsible for the view that the roots act like sponges in taking
up water.
Simon (1768) was probably the originator of the idea that these
root-hairs were excretory tubules, a view that became very popular at
the beginning of this century.
Meyer (1838) was perhaps the first to give a comparative account of
them, and he supposed them to be delicate prolongations of the
root-surface to facilitate the absorption of water.
The real importance of these organs, however, has only become apparent
since Sachs, in 1859, recognised their relations to the particles of
soil between which they extend and to which they cling.
In 1883 Schwarz made a very thorough study of their biological
character, and in 1887 Molisch gave us new facts as to their physiology.
Our knowledge of them has been rendered very much more intimate by the
researches of Pfeffer and De Vries on osmotic and plasmolytic phenomena,
and they serve as an excellent study of some of the best results of
modern physiology.
In the normal case, such as is exemplified by a seedling wheat or bean,
the root-hairs arise some distance behind the growing tip of the root,
an obvious adaptation which prevents their being rubbed off by the soil,
as they would be if developed on parts still actively lengthening. As
those behind die off, new ones replace them in front, and so we find a
wave of succession of functionally active root-hairs some little
distance behind the tip of the root: the same order of events holds for
each new rootlet as it emerges from the parent root, and so successive
borings in the soil, made by the diverging root-tips, are thoroughly
explored by these root-hairs.
Measurements have shown that in various plants the surface of root on 1
mm. of length is increased by the root-hairs in proportions given in
the following table:
------------+---------------------+-----------------+--------------
PLANT. | Area of surface | Area of | No. of times
| without root-hairs. | root and hairs. | greater.
------------+---------------------+-----------------+--------------
Maize, | 4.52 sq. mm. | 25.13 sq. mm. | 5.5
Pea, | 4.71 sq. mm. | 58.33 sq. mm. | 12.4
Scindapsus, | 14.02 sq. mm. | 261.9 sq. mm. | 18.7
------------+---------------------+-----------------+--------------
--which sufficiently establishes the general proposition that the area
of the root-surface is enormously increased by these hairs.
But this does not give us any definite idea of the length of the
cylinders of soil explored by these surfaces, until we find that plants
such as an ordinary sunflower, hemp, or vegetable-marrow may have roots
penetrating into a cubic meter of soil, in all directions, and so
closely that probably no volume so large as a cubic centimeter is left
unexplored. Clark found by actual measurement that the roots of a large
gourd, if put end to end, extended over 25 kilometers, and Nobbe gives
520 meters for the roots of a wheat. Vetches may go nine feet deep, and
oats more than three feet. The Sal, a tree of the forests of India, has
roots which penetrate to a depth of 50 to 60 feet.
Some rough notion of the lengths, superficies and penetrating capacities
of the roots of a large tree may be gathered from the above, but it is
doubtful whether we can form any adequate ideas as to the millions of
root-hairs which must be developed along the course of these
subterranean boring organs.
One of the most striking results of modern enquiry into these matters,
is the discovery that the number and superficial area of these
root-hairs, on one and the same plant, may vary to a large extent
according to the structure, as it were, of the soil, and the degree of
moisture it is capable of retaining; or, with the same soil, according
to the amount of water which it receives and holds. Correlations have
also been observed between the development in length and surface of the
rootlets themselves.
The following illustrations will suffice to show this:
Six young wheat-plants in soil kept constantly wet, developed roots the
total length of which measured 365 mm. each, on the average, and almost
devoid of root-hairs.
Six similar plants in soil only moderately moist, averaged 668 mm., and
were well furnished (though not densely covered) with root-hairs.
Six similar plants in soil which would be termed dry, averaged 371 mm.,
but were densely covered with rich crops of root-hairs.
Further researches have shown that the conditions which rule the
development of the root-system and root-hairs in the soil are very
complex, and not always easy to trace. The most general statements we
can make are the following:
There is an optimum degree of moisture in the soil which promotes the
maximum development of root-hairs. If the soil is too wet they are not
developed.
These facts are of importance as correlated with the ease or difficulty
experienced by the roots in obtaining water, and plants such as our
ordinary agricultural plants show this very distinctly.
Although, as shown in the experiments with wheat, the short roots in dry
soil were more densely covered with root-hairs than the much longer
roots in moderately moist soil, subsequent closer investigation shows
that the total quantity and area of root-hairs is less in the former
case than in the latter.
The greatest number of root-hairs are developed on roots which are
growing at their best: too much moisture may prevent the formation of
root-hairs: too little may induce dense growths of root-hairs locally,
but the total number is reduced.
Another set of events which exerts influence on the development of
root-hairs is the composition of the dilute solution--water containing
dissolved salts--which surrounds them in the soil.
Thus, Schwarz found that when similar oat and wheat plants were grown
with their roots in solutions of various salts, the results differed as
follows:
Oats in a 15 per cent. solution of calcium chloride developed no
root-hairs, though they formed in a 5 per cent. solution, and were very
numerous in a 0.5 per cent. solution, or in water alone. In a 10 per
cent. nutritive solution the plants developed no root-hairs, though
they were abundant in a 1 per cent. solution.
Wheat plants with their roots in a 15 per cent. solution of potassium
nitrate bore no root-hairs, but they were numerous in a 2 per cent.
solution of the same salt.
These are extreme cases, for, although the roots were not killed, they
were strongly inhibited in their growth by the more concentrated
solutions. However, experiments of this kind at least bring vividly
before us what variations are possible, and suggest that similar events
on a smaller scale may occur in a soil which yields large quantities of
soluble substances, _e.g._ when freshly manured. Obviously these facts
have a practical significance as regards kind of soil, drainage, season
(_e.g._ drought or wet), etc.
But there are other factors which rule the development of root-hairs,
and some experiments by Lesage show that the correlations between the
development of root-hairs and roots are probably much more complex than
had been suspected; for he finds that if the lateral rootlets of a Bean,
in a water culture, are suppressed, the main rootlet develops numerous
and very long hairs to compensate the loss in surface, a matter of
obvious importance in the discussion of cases where roots have been
injured in the soil.
Before proceeding further it is necessary to look a little more closely
into the structure of a single hair.
It is a tubular prolongation of a single cell of the external covering
of the young root, usually about 1 to 3 mm. in length, and 0.01 to 0.10
mm. in diameter. In special cases the root-hairs of some water plants
may reach 5 to 18 mm. in length, but of course I am referring to the
ordinary land plants of agriculture and forestry. This tubular
prolongation is closed and rounded off at the distal free end, and opens
at the proximal end into the cell of which it is a protrusion.
The whole structure is bounded by an extremely delicate and elastic wall
of cellulose, which Frank says is of special composition, almost too
thin to measure in many cases, but often somewhere near 0.005 to 0.001
mm. in thickness. This thin membrane is remarkably permeable by water,
or dilute solutions, as is shown by the rapidity with which a root-hair
collapses if exposed to evaporation, or with which dense solutions
abstract water from it, or with which solutions may be seen to penetrate
it under the microscope.
Overlying the thin cell-wall proper, on the outside, is a thin
gelatinous layer, a product of alteration of the outermost lamellæ of
the former.
Closely lining the proper cell-wall on the inside, is an extremely thin
layer of living protoplasm, and somewhere in this protoplasm is a
distinct cell-nucleus.
The interior of the tube is filled with cell-sap, and it is the osmotic
pressure of this cell-sap which keeps the whole living instrument tense
and rigid, and the thin protoplasmic film close pressed against the
cellulose cell-wall.
Nothing whatever can pass into the cell-sap, or out from it, without
traversing both the lining of living protoplasm and the cell-wall.
If we gently pull a living root, of wheat, pea, mustard, etc., from a
normal soil, we find particles of soil so closely adherent to the
root-hairs that they cannot all be washed off without tearing the hairs:
the root-hairs establish relations of contact with these particles, so
close that they are cemented to the solid surfaces by means of the
gelatinous layer already referred to. This peculiarity has the following
consequences. In the first place, the enormous holdfast, ensured by the
millions of points of adherence, enables the plant to withstand even
powerful lever actions from above, and provides fixed points against
which the root-tips can work as they drive deeper into the soil. In the
second place, the intimate contact of the root-hairs and particles of
soil, ensures that the films of water held by surface-action on the
soil-particles and root-hairs shall be in continuity with the water
saturating the cell-walls of the latter, and therefore with the
protoplasm and cell-sap in their interior. The importance of this at
periods when the soil is "dry" will be obvious, when we reflect that no
soil is ever naturally so dry that surface-films of water are absent
from the particles.
The fact that the root-hair contains living protoplasm, enables us to
understand to a certain extent the results of the following
experiments.
If we have a leafy and healthy plant, with roots, bearing numerous
root-hairs, properly established in suitably moist soil in the pot, the
roots cease to absorb water if the temperature of the soil falls below a
certain minimum, though they recommence to do so if the temperature is
raised again: this has nothing to do with the temperature of the upper
parts of the plant, or of the air, and the latter may be so high that
the plant rapidly droops from loss of water at the leaves, which is not
being compensated owing to the inactivity of the roots.
Similarly we may have the air so cold, at a time when the soil is warm
enough to keep the root-hairs actively at work, that the plant becomes
surcharged with water, which escapes from the leaves like drops of dew.
The temperatures necessary to cause these disturbances in the action of
the living root-hairs vary for different plants, and even for different
varieties of the same species.
Similar arrestation of the functions of the roots may be brought about
by removing the oxygen from the soil around the root-hairs, and
replacing it by carbon-dioxide, or the vapour of chloroform. If not kept
too long in such a condition, the plant recovers rapidly on admitting
atmospheric oxygen, which is always present in a normal well-drained
soil both as gas in the capillary interspaces, and dissolved in the
water on the surfaces of the particles. If the access of oxygen is
delayed, however, as often happens in rainy seasons and in wet soils,
the root-hairs are killed, and rot sets in. A good instance of this has
lately been given by Heinricher in the case of potatoes.
NOTES ON CHAPTER V.
For the further pursuit of this subject the reader should
consult Sachs' _Lectures_, II. and XV.; Sorauer, _A Popular
Treatise on the Physiology of Plants_, 1895, chapters II. and
IV., and Pfeffer's _Physiology_, pp. 149-163. The principal
paper on root-hairs referred to in the text is Schwarz, "Die
Wurzelhaare der Pflanzen," in _Unters. aus dem bot. Inst. zu
Würzburg_, I. Heft 2, 1883, p. 140, where a very exhaustive
account of these organs will be found.
CHAPTER VI.
THE FUNCTIONS OF ROOT-HAIRS.
_Excretions from root-hairs--Osmotic phenomena--Turgescence--
Plasmolysis--Control of the protoplasm in absorption, etc.
Selective absorption._
We see then that the root-hairs are the active living instruments in
absorbing the water (containing small quantities of dissolved
substances) of the soil.
If the living root-hairs are so numerous and so active, however, a
natural inference is that they must exert some influence on the
composition or arrangement of their environment. All the teachings of
modern physiology go to show that such a living cell as I have sketched
cannot carry on its life, brief though it be--the root-hairs are active
for about four or five days--without forming substances of the nature of
excreta, and we should expect some of these to pass out to the soil.
Sachs showed, in 1860, that roots growing in contact with polished
marble corrode the surface of the mineral, and Nobbe, in 1876, showed
that the roots of seedlings reduce potassium permanganate, a fact which
Molisch confirmed in 1887. The latter observer also proved that living
root-hairs secrete substances which colour a solution of guaiacum blue,
oxidise pyrogallic acid and other organic substances, and rendered it
probable that they excrete some substance which inverts cane-sugar, and
in some cases even small quantities of a diastatic enzyme.
Molisch also confirmed an old observation, that roots excrete
carbon-dioxide; and he and Czapek showed that the root-hairs excrete
acids more permanent in their nature than carbonic acid, and published a
method for showing this by means of a dilute solution, slightly
alkaline, of phenolphthalein.
Molisch declared that the substances secreted by root-hairs may even be
observed, dissolved in drops which ooze from the surfaces of the
root-hairs.
That these root-excretions, and particularly the acids, may be of
service in dissolving and rendering more available various constituents
of the soil is an obvious suggestion, and it is borne out by Sachs'
discovery of the corrosion of marble, and by Molisch's observation that
living roots slowly corrode ivory if continuously kept in contact with
it.
But a deeper insight into the physiology of these organs was only
possible when the meaning of the phenomena of osmosis had been rendered
clearer by the researches of Pfeffer and De Vries in 1877.
De Vries showed that the turgescence of the living cell can be
diminished, and even reduced to nothing, by placing the cell in contact
with solutions of substances which attract water from the cell-sap: as
the turgescence diminishes, the cell contracts, owing to the elasticity
of the cell-wall, which was previously distended; if the abstraction of
water continues, the living protoplasmic membrane lining the cell-wall
contracts away from the latter. He then proved that no injury need
accrue to the cell by this process of plasmolysis, since the turgescence
can be restored by washing out the salt with a more dilute solution, or
with pure water; and the cell may go on living and even growing as
before. These phenomena can only be produced in cells where the
protoplasmic lining is intact and alive.
Pfeffer showed that the whole matter depends on the properties of the
living protoplasmic membrane, which, so long as it is alive, has the
power of governing the entrance or exit of dissolved substances, but is
as a rule freely permeable for water. If, then, substances with a
powerful attraction for water are formed in the cell cavity, and of such
a nature that the protoplasm does not permit their free diffusion to the
exterior, they attract water, and hold it fast, and so set up the
condition of hydrostatic pressure known as turgescence, the limit of
which depends on the attainment of a state of equilibrium between the
elastic reaction of the cell-wall and the distending power of the
absorbed water. When this limit is reached, water begins to filter back
again through the cell-wall. Numerous researches during the last fifteen
years have shown that the sap of such a living cell as the root-hair is
charged with substances of various degrees of osmotic power; bodies like
sugars, amides, vegetable acids and their salts, being formed by the
metabolic activity of the protoplasm and accumulated there. Moreover, we
now know that the salts of the vegetable acids in particular are
effective, and the researches of Warburg and Palladin in 1886 have
placed it beyond reasonable doubt that these acids are continually being
developed and destroyed in the living cell during normal growth and
respiration, and that great variations as to quantity may be brought
about by alterations in the conditions of the environment--_e.g._
temperature, oxygen, etc.
If, now, we bring a solution of some salt, such as potassium nitrate,
which has a powerful attraction for water, on the outside of the living
root-hair, the question whether the water remains in the cell, or passes
out of it, merely depends on whether the substances inside or that
outside have the most powerful attraction on the water in the sap, since
the protoplasm allows water to pass freely.
But the protoplasmic lining may affect the whole matter in another way;
for it may allow the dissolved salt, or other substance, in the solution
outside or inside the cell to pass through it also, or it may take it
up and fix it, or break it up or otherwise alter it.
More recent researches, and especially those of Pfeffer, have shown that
these diosmotic properties of the living protoplasm are of the utmost
importance in the whole matter of absorption of substances from the
soil.
Let us suppose the following case. A root-hair, in full vigour, is
allowed to bathe freely in a dilute solution of various substances, such
as sugar, potassium nitrate, phosphates, sulphates and carbonates of
iron, soda, lime, magnesium and others known by experiment to be
harmless to its life.
Now it turns out to be by no means a foregone conclusion that all or any
of the substances, even though freely soluble in the water, can pass
through the protoplasm into the interior of the cell. Some may be
allowed easy access, others may only be permitted to pass in small
quantities, and others, again, may be absolutely refused access by the
delicate living filter, so long as it is vigorously alive. Nor, as
proved by numerous experimental cultures since De Saussure's time, is
the entrance of a salt, etc., ruled by its indispensability or otherwise
in the economy of the plant. And it is important to notice that only
experiment can prove the point and determine which substances are
absorbed and which refused by the root-hair.
If we now suppose the protoplasm to give rise to powerfully osmotic
substances which accumulate in the sap-vacuole, but which are not
permitted free egress through the protoplasm (and the formation of such
bodies will occur if the protoplasm is actively respiring), the
conditions for absorption of water, with or without any dissolved salts,
which the protoplasm allows to traverse it, are set up.
But the above supposed case is realised, as Pfeffer showed in 1886, when
he found by a series of beautiful experiments that certain aniline dyes
can accumulate in living root-hairs, and other living cells, whereas
others cannot pass the living protoplasm. After accumulating for some
time, the dye may either remain stored there, or may eventually diffuse
out.
Pfeffer made another discovery, of equal importance, namely, that under
the influence of dilute organic acids, such as citric acid, the
permeability of the living protoplasm may be altered, so that it allows
substances to pass which could not otherwise have traversed it. De Vries
had also shown that the condition of the protoplasm affects its power of
retaining the colouring matter in the sap of the Beet: so long as the
protoplasm is alive, the crimson sap is retained, even when the cell is
plasmolysed, but immediately it begins to die the colour escapes through
it. A similar case exists when the chlorophyll-corpuscles retain their
colour in living cells known to be charged with acids: so long as the
protoplasm is alive and normally active the green bodies are protected.
These, and numerous other experiments of the same kind, prove that the
healthy root-hair is a living instrument for taking up dilute solutions
out of the soil, and holding them in the sap-cavity for a time. If
killed, by frost for instance, it loses these powers.
The researches of the last ten years have also shown that a time comes
when the turgid cell, if an isolated one, and if sufficient supplies of
water are present, is so tightly distended that the surplus water begins
to diffuse out again under the pressure proper to the hydrostatic
conditions set up.
Now we arrive at a very critical point.
When the water, or dilute solution of various substances, begins to
exude under pressure from the living root-hair, what is to prevent its
escape into the soil? And if it thus diffuses out, where is the object
of absorption?
The questions are obviously pertinent, and they may seem the more so in
that the cells adjoining the root-hair on its inner side are also
turgid, and possess similar properties to those of the root-hairs. To
establish a condition of things which shall bring about the inward flow
of the absorbed water, one of the three following cases is conceivable.
(1) The cells, as we pass radially into the root, have different
properties on the wall of the two sides; or (2) they are more and more
greedy of water owing to some process of extraction of their water by
tissues in the centre of the root; or (3) these successive series of
cells possess osmotically more powerful contents at periods coincident
with the escape of the water from the now osmotically weaker root-hairs.
A little reflection will show that where we have a group of such cells
as the above, all capable of absorbing water and dilute solutions and of
becoming turgid, movements of the absorbed water must go on until all
the cells are in equilibrium, as regards their osmotic pressures.
Now the living rootlet is just such a system, the various cells of which
are in different conditions of osmotic pressure at any given time: some
of these cells are old, and their protoplasm is allowing sap to filter
out under pressure: others are in the height of their vigour, and their
protoplasm extremely impervious to the highly osmotic sap-constituents
which it itself is forming actively: others are too young to have
attained their full turgescence: while others again are in stages
intermediate between the above.
There is another point of importance, however, to explain some
peculiarities in the absorption of these dilute solutions of salts,
etc., by the root-hairs from the soil, and by cells lying deeper in the
plant from these root-hairs.
It is easy to understand that if a root-hair absorbs a given
substance--say calcium sulphate, for illustration--and hands it over to
other cells unchanged, a time must be supposed to arrive when, the sap
of all the cells being equally charged with calcium sulphate, no more
could be absorbed: the rate of absorption of this particular substance,
and the quantity absorbed, up to the hypothetical point of equilibrium
chosen, would then depend simply on the ease with which its molecules
traversed the living protoplasmic membrane, and the degree of their
solubility in the sap.
But now suppose the following new factor to come in. Suppose that
calcium sulphate undergoes decomposition in some one of the internal
cells of the system of absorbing cells, or that it is even merely
crystallised out in such a cell, or in any other way removed from
solution (_e.g._ by deposition in cell-walls). This alters the state of
affairs considerably. The separation of the molecules from the
sap-solution is itself a cause for the flow of more of the solution to
the cell concerned, and such causes of diffusion are very common in the
plant.
The importance of this principle consists in that it lies at the base of
the whole question of selective absorption, application of manures, and
the rotation of crops; and those who are acquainted with the excellent
analytical results of De Saussure, Boussingault, Wolff, Trinchinetti,
Gödechen, etc., and the water-culture experiments of Sachs, Nobbe, and
others, will understand what an illuminating effect on these points was
produced by the above generalisation, which we owe especially to
Pfeffer's splendid researches into the nature of osmotic phenomena.
It will now be clear, I hope, why we regard the living root-hairs as
instruments--as pieces of living machinery--for the active absorption of
water, with substances dissolved in it, from the soil; and it will also
be evident, I think, that no one can form a proper conception of this
matter of absorption, so important in all agricultural questions, unless
he pays attention to these biological phenomena. It was hopeless to
expect to understand these matters merely in the light of chemical
analyses of plants and soils, and one expression of this hopelessness
was the belief in the power of roots to select only the substances
useful to it. We now know that the expression "selective power of roots"
has a totally different meaning from that implied in the minds of the
last generation of agriculturalists, and it would be easy to devise
experiments, with solutions of different strength, where the plant
should be made to take up relatively large quantities of harmless, but
useless minerals, etc., and to starve in the midst of plenty of the
elements proper to its structure, simply because the former are offered
in a form in which they easily traverse the protoplasm of the
root-hairs, while the latter are presented in a form unsuitable for
absorption. That all these matters are of importance in regard to
manuring and choice of soils, etc., needs no emphasising.
These remarks, of course, do not detract from the value of good
comparative chemical analyses, when viewed in the light of physiological
knowledge, as I need hardly say; but they do, and emphatically so,
attack the position that such analyses alone can explain the problems of
agriculture.
On the other hand, we must not rest satisfied with the suggestions so
far put forward to account for the processes referred to, since it is
impossible to overlook the fact that in their present form they merely
afford proximate explanations, and are too crudely mechanical for
finality.
NOTES ON CHAPTER VI.
In addition to the works referred to in the last chapter, the
student should consult Pfeffer's _Physiology_, pp. 86-149, and
pp. 410-441. With reference to water cultures, Sachs'
_Lectures_, XVII., may also be consulted. The standard work on
ash constituents of plants is Wolff, _Aschen-analysen_, 1871
and 1880, an indispensable book of reference in this
connection, though there are others, quoted in Pfeffer, where
further literature may also be found.
CHAPTER VII.
THE BIOLOGY OF SOIL.
_Soil not a dead matrix--Organic materials--The living
organisms of the soil--Their activities--Their numbers and
importance. Abandonment of the notion that chemical analysis
can explain the problem._
It is customary to regard the soil, between the particles of which the
root-hairs of plants are distributed, as if it were merely a dead matrix
of smaller or larger pieces of rock, such as sand, gravel, stones, etc.,
and organic remains, such as bits of wood, leaves, bones, etc., with
water and air in their interstices. As matter of fact, however, soil is
a much more complex body than was suspected until comparatively recent
times.
It is, of course, beyond the scope of this book to go into the different
varieties of soils, their structure or arrangement, and the chemical
nature of their constituent rocks and the débris mingled with the
latter. For the same reason I must pass over the curious properties of
soils in relation to the solutions they yield to water in contact, the
manner in which they retain some of these solutions and allow others to
pass easily, and the remarkable double decompositions which go on in
them. Moreover, I must assume as known the chief physical properties of
ordinary soils with respect to the phenomena of capillarity, absorption
of heat, action of frost, and so forth.
But all ideas as to the nature of soil based merely on the study of its
chemistry and physics are misleading, and it is in just the
establishment of this truth that modern discoveries in Agricultural and
Forest Botany have played so important a part.
From the facts that organic débris is found chiefly at the surface of
the earth, and that the smallest particles are held in suspension by the
water near the surface, it is comprehensible why such organic remains
abound in the upper parts of the soil, where the rootlets with their
absorbing root-hairs are also found, because they must have oxygen. The
rule is, therefore, that an ordinary soil consists of upper strata, rich
in organic materials and in oxygen, and a subsoil, poorer in these
substances.
Among these organic materials are countless myriads of living beings,
especially fungi and bacteria, which require oxygen and organic
materials for their subsistence, and it depends on the open or close,
moderately moist or damp, warm or cold nature of the soil, and on some
obviously connected factors, how far down these aërobic organisms can
thrive. As we go deeper down they become fewer and fewer, and gradually
disappear, and (neglecting certain anaërobic bacteria of putrefaction)
they are rarely found in marked abundance more than a few inches below
the surface soil.
These aërobic fungi and bacteria are the great agents of continued
fertility of a soil, and it is they which, living and multiplying in the
moist and well-aerated warm interstices of a rich open soil, carry out
the useful destruction of organic matter, breaking it up into mineral
and gaseous bodies, which are then dissolved in the water bathing the
root-hairs or escape into the atmosphere. In this work of destruction
they are aided by the oxygen of the air and the solar heat: their own
fermentative action is also accompanied by a marked rise of temperature,
and the carbon-dioxide and other products of their activity all go to
complicate the chemical changes going on in the soil around the roots.
Duclaux has calculated that _Aspergillus niger_, a common mould fungus,
can break down organic substances, such as carbohydrates, at such a rate
that a metre cube of the fungus would decompose more than 3000 kilogr.
of starch in a year, and this may serve as an example giving some idea
of the possibilities in soil.
Analyses of waters containing large quantities of organic matter, as
they enter such open soils as those referred to, compared with the
drainage water after passing through the upper strata, show that the
carbonaceous and nitrogenous materials are broken down to more or less
completely oxidised simpler compounds, and that the following chief
changes result. The ammonia and some other nitrogenous bodies remain
behind in the soil, as also do the phosphoric acid and much of the
potash; whereas large quantities of nitric and nitrous acids, together
with much sulphuric acid, chlorides, and calcium salts pass away in the
drainage. These facts are obviously highly important in agriculture.
Experiments on sewage farms have shown also that the upper soil retains
most of the bacteria of the sewage. Koch found at Osmont, near Berlin,
that whereas the different sewage waters contained numbers so enormous
that each cubic centimeter probably held 38,000,000 germs, the different
drainage waters held only 87,000 per c.cm.; and the whole process of
water-filtration through sandy soils depends on these well-known facts.
Recent experiments in connection with soil-filtration, however, bring
out the further facts that the oxidations which organic matters undergo
in the soil--and without which they are useless to the higher
plants--are enormously enfeebled if the upper layers of soil are
sterilised, so as to deprive them of the myriads of aërobic bacteria,
fungi and yeasts which they normally contain, and there can no longer be
any doubt as to the importance of the biology of the soil in connection
with the preparation of materials suitable for absorption in solution by
the root-hairs of agricultural and other plants.
The researches of the last ten years have brought to light a long list
of forms, comprising yeasts, such as Hansen's _Saccharomyces
apiculatus_, fungi and bacteria which live and grow in the soil, finding
their water and food supplies in the interstices, and under conditions
which we now know to be very diverse. They are usually more numerous, in
species and individuals, in cultivated farm and garden soils than in
woods, prairies, and untilled lands; but the geological nature of the
strata, the closeness and otherwise of the soil, its damp or dry
character and its average temperature (which depends on many things
besides latitude or altitude) and other factors co-operate to rule their
distribution and numbers. The fact that cultivated land is so well
supplied with manures, air, etc., is of great importance in relation to
their relative abundance there, and it is extremely probable that the
use of artificial manures lessens their numbers considerably as compared
with land on which stable and other animal manures are employed.
A list of the soil-bacteria which have been isolated and more or less
carefully cultivated and examined would comprise about fifty species;
but it is certain that, as at present classified and named, many more
species are to be discovered in any ordinary soil.
The fungi are apparently even more numerous than the bacteria, and we
may rest satisfied for the present with the general statement that the
life-actions of the myriads of individuals of these organisms in the
soil completely alter the question of soil-water as understood by the
last generation of agriculturalists.
But there is another aspect of this question of soil-organisms which has
grown in importance of late to such an extent that we are more than ever
justified in regarding the biology of soil as far more vital to the
interests of the plant than its physical or chemical properties. With
many of the fungi in the soil the roots of plants have to compete--just
as plant competes with plant--for water, salts, and other
food-materials. The toadstools which are so conspicuous in fields and
forests spring from mycelia which ramify in the ground, and are busily
breaking down the remains of other organisms, and just such fungi are
known to store up relatively large quantities of salts of potassium and
phosphorus--the very salts which are so valuable to crops and occur so
sparingly in most soils, but which the extensively spread fungus mycelia
can gradually accumulate. Some of these fungi, moreover, are more active
in their antagonism, and actually attack and pierce the roots as
destructive parasites, but I pass these by for the present, as they form
the subject for further consideration when we come to the diseases of
plants.
It is obvious that the competition of fungi with root-hairs for mineral
salts, oxygen, etc., may be at times acute, and it is extremely probable
that cases of so-called sterility of soil, where a particular soil is
found unsuitable for a crop, may sometimes be due to this
over-competition.
The researches of recent years, however, and especially those of Frank,
Winogradsky, Hellriegel, and Stahl, have brought to light a series of
relationships between certain of these soil-organisms and the higher
plants which place the matter of soil-biology in quite new lights.
On the one hand it has been discovered that groups of bacteria are the
active agents in bringing about the destruction of organic nitrogenous
matter with the formation of ammonia, in oxidising this ammonia to
nitrous and to nitric acids, which combine with bases in the soil to
form the corresponding salts; while, on the other hand, other forms can
decompose the nitrates and reduce them to nitrites, or set free ammonia
or even nitrogen from them. Moreover, there are certain species which
can fix the free nitrogen of the atmosphere, and start the cycle of
up-building of this inert element into the complex higher compounds we
term organic. It is impossible to over-estimate the importance of these
processes of nitrification and denitrification going on in the soil
about the root-hairs of the higher plants.
But, in addition to this circulation of nitrogen in the soil, it
turns out that the life-actions of bacteria, and not mere chemical
decompositions, are largely responsible for the circulation of
carbon, of iron, of sulphur and other elements formed from the
decomposition--also by bacterial and fungal agency--of animal and
vegetable remains in the soil.
Even more startling are the biological relations in the soil between
the absorbing roots of the higher plants and some of these bacteria and
fungi, for it has now been established beyond all doubt that certain
fungi enter the living roots and there flourish not as mere destructive
parasites, but as messmates not only tolerated by the plant, but even
indispensable to its welfare. It is probable that nearly half the plants
of our fields, moors, and forests entertain such fungi in their
root-tissues. The curious, and long-known nodules on the roots of
leguminous plants--peas, beans, clover, etc.--are filled with bacteria
which enable these plants to avail themselves of the free nitrogen of
the air, and so enrich the soil with nitrogenous substances.
The roots of most forest trees, orchids, and plants of the moorlands,
meadows and marshes are similarly occupied by fungi, which in some way
convey salts--probably especially phosphates and potassium compounds--to
the plant in return for the small tax of organic carbon-compounds it
exacts from the latter. In some cases at any rate, as Bernard has lately
shown, the very existence of the plant depends on its seedling roots
obtaining this advantageous attachment and co-operation (symbiosis) of
the fungus immediately on germination.
These remarks must suffice to illustrate this part of my subject, and to
emphasise the statement that the question whether a given plant can be
grown in a given soil, is by no means one of simply the physical and
chemical constitution of the latter. The plant will have to run the
gauntlet of a long series of vicissitudes brought about by the presence
or absence, relative proportions and vigour, and specific nature of the
organisms in the soil at its roots, and it is easy to see that many
cases of disease may be due to the absence of advantageous bacteria or
fungi, or to circumstances which disfavour their life, as well as to the
predominance of competing organisms.
It will now be evident that the old points of view must be abandoned,
and with them, especially, the widely prevalent notion that chemical
analyses of the plant and soil can explain the real problems of
agriculture.
It was of course an enormous advance in the science when, thanks to the
splendid labours of the chemists, at the end of the last century and the
beginning of this, we obtained that preliminary knowledge of the
constitution of the air, and of the composition of the water, acids and
salts, etc., which plants require for their food-materials and
life-processes. Much was gained by De Saussure's establishment of the
fact of oxygen respiration, though we now understand by the term
something very different from, and much more complex than, what he
understood by it, as, also, much had been gained by the previously
acquired knowledge of the gas-exchanges in carbon-assimilation: nor must
we forget the services of those who proved, by laborious analyses,
continued for long periods, what chemical compounds are found in the
tissues of plants, and in the soils at their roots and the atmosphere
which surrounded them. We must also remember many other contributions
which have been furnished, and are still being furnished by the chemist;
and I for one hope that his labours will continue to go hand in hand
with those of the physiologist.
But, when all due honour is paid to the scientific chemist, it must
still be allowed that his problems are different from the real problems
of agriculture. To take one set of instances alone. The chemist can
analyse a given soil or a given manure, and can even go a long way
towards making them, but his analyses do not tell us what conditions are
necessary in order that their ingredients may be presented to the roots
so as to be absorbed and become built up into the plant. Chemistry told
us that carbon was fixed from the air, but physiological experiments
determined how this meant the synthesis of certain definite
carbohydrates--this, too, in the face of the powerful authority of the
chemist Liebig, who supposed that the vegetable acids were the results
of the assimilation of carbon. Wolff, De Saussure, and other chemists
have done yeoman service in showing that different plants, growing in
the same soil, contain different proportions of mineral substances; but
it was by means of water-cultures, and other physiological researches,
such as those of Pfeffer on osmotic phenomena and of Schwarz and Molisch
on root-hairs, that the puzzling question of selective absorption, by
means of the living root-hairs, came into the arena of our knowledge.
In every case--and, as already said, I am not undervaluing the work
done--the chemist has left us only on the threshold of the real
problem. He has stood outside the factory in which the real work we want
to know about is being carried on, and has told us of so many tons of
this material being carried in at the gates, and of so many tons of that
coming out; he has even burnt down the factory, and all its contents and
machinery, and has then told us how many tons of the various materials
were there at the time; but this is not what we want, valuable as the
information is, and still more will be. What we want, and what we expect
to obtain, is more information regarding what is done with the materials
in the factory: what machinery they are put into, and how they are put
in: what stages they go through, and how the stages follow one another:
what wear and tear has to be endured, and how we can step in and stop
the working of the machine for our own benefit at the best possible
time.
The physiologist proceeds empirically, by experimenting with the living
machinery. He recognises the parts and their structure, and tries to
find out what they are doing: he knows that the laws of physics and
chemistry cannot be traversed, but he sees these laws at work under
special and very complex and peculiar conditions. He therefore, as the
results of his experiments, sets new questions--or old questions under
new conditions, if you like--and undoubtedly wants the help of both
chemist and physicist; or, if it is preferred, the chemist and physicist
may attack the problems, but they must familiarise themselves with the
peculiar mechanism of the organism concerned, and cannot hope to attain
success without experimenting with it. I confess it seems to me as
reasonable to look upon scientific agriculture as a branch chiefly of
chemistry as it would be to look upon horse-breeding or pigeon-rearing
from the same point of view; and why the professed chemist's advice is
regarded as so comforting and final in the one case and not in the other
is one of those mysteries which seem inherent in human nature.
The central point in agriculture is the plant: get the most out of
it--the energy-winning machine which alone can keep the animals and
everything else connected with the farm going--and all the rest follows.
The old agriculture has taken a gloomy view of things, and especially on
account of a large variable which it blames for many ills, namely, the
season or climate. Perhaps the old agriculture has not sufficiently
recognised that Nature grows plants in accordance with the fact that
variation is not peculiar to the weather: if the seasons vary, so do
fruit and other produce and the plants which yield them; and since man
cannot hope to control the one variable, possibly relief will be found
in doing more, within his limits, towards controlling others.
In any case he cannot hope to succeed without study of the physiology of
the plant.
NOTES TO CHAPTER VII.
An admirable short account of soil in its relation to
root-hairs is given in Sachs' _Lectures_, XV.; but for a more
exhaustive treatment of the subject of soil the reader is
referred to King, _The Soil_ (Wisconsin, 1895), or Warrington,
_Lectures on the Physical Properties of Soil_ (Oxford, 1900);
Larbalétrier, _L'Agriculture_ (Paris, 1888), chapters II. and
III. There is also a very good account in Bailey, _The
Principles of Agriculture_ (London, 1898), chapters I.-III.
With reference to the organisms in soils and the
decompositions they bring about, the student should consult
Kramer, _Die Bakteriologie in ihren Beziehungen zur
Landwirthschaft_ (Wien, 1890), and Lafar, _Technical Mycology_
(Engl. edition, 1898), sections V., VIII., and IX.
CHAPTER VIII.
HYBRIDISATION AND SELECTION.
_The crossing of varieties of wheat, etc.--The essentials of
fertilisation--Rimpau's experiments--Hybrids and selected
varieties._
In the more hopeful view of the case which the new agriculture will have
to take, it will recognise the physiological truth that since the living
plant is the important and variable machine which constructs the produce
looked for, and since that machine will work best in proportion as its
needs are properly satisfied; therefore in cases where the needs of a
given type of the machine cannot be efficiently provided for, it will be
well to select some other type which will take what supplies and
conditions can be offered. Of course, this is already recognised to a
certain extent, as is implied in the practices of "rotation of crops,"
selection of "pedigree wheats" and mixtures of "pasture grasses," and in
decisions as to the quality of land according to the kinds of weeds
found on it, and so forth; but I am convinced that the agriculturist of
the future--and the same applies to the horticulturist, planter and
forester--will have to concern himself more systematically with the
working and the variability of the plant, and particularly with what
Darwin termed Variation under Domestication, than has always been the
custom in the past. The subject of the plasticity of cultivated plants,
and especially of hybrids, is in one sense an old one; but much work is
being done which proves, as such work is apt to do, that very much more
may be done by well-planned experiments on the selection of new
varieties raised by hybridising and cultivation.
In illustration of this point, a short summary of some of the results of
crossing different species of wheat, barley, oats, peas, beet, etc., may
serve to show what has been gained and what may be hoped for in these
directions. It should be stated that much has been done and is being
done in this country as well as abroad, as witness English varieties of
corn, peas, and potatoes, and the recent experiments on crossing various
kinds of maize in America.
The hybridiser grows his cereals, etc., in pots until ready for
crossing, and then takes them into the laboratory, removes the weaker
spikelets, and takes out the young stamens from the flowers left on the
plant. The female plant is then ready, and the flowers covered with
paper caps. The pollen, obtained by a clean wet brush from the plant
chosen as the father, is then carefully placed in position on the
stigmas, and the caps replaced. The pollination is repeated
occasionally, and care taken that no uncrossed flowers develop later. In
this way a few seeds or grains are got to start with.
This would be the place to introduce an account of the enormous advances
made by the botanists of the last decade or two in the study of the
microscopic phenomena of fertilisation. Without going into
details--which would more than occupy all the space at command--I may
recall the discoveries of Strasburger and his pupils, and of Guignard,
which have supplemented the earlier discoveries of De Bary, Cohn, and
Hofmeister, by establishing the facts that the essential point in
fertilisation is the fusion of two nuclei, and the bringing together in
the fused mass of two extremely minute thread-like coiled bodies, the
so-called chromatosomes or filaments, one of which is derived from the
male and the other from the female parent. The particulars as to the
marvellous adaptations to secure the union of these two infinitesimally
minute threads, their behaviour immediately before and after union, and
many other points must be passed over, as I have only space to emphasise
the one crowning discovery that these tiny filaments of nuclear
substance are the material carriers of all the hereditary properties of
the parents to the young plant which their union initiates.
It must not be supposed that the above statements are based on any
meagre foundation of facts. The attraction of the fusing nucleated
masses had been demonstrated over and over again by Tulasne, De Bary,
Strasburger and others; but Pfeffer brought the matter to a crisis by
discovering the attractive (chemotactic) substance emitted in given
cases, and by collecting the fertilising bodies by its means into
artificial tubes.
The fusion of the nucleated bodies in the sexual act was observed by
Strasburger in the living plant a few years ago, and numerous later
observers have confirmed it. Meanwhile all the stages of approach and
contact of the essential filaments of the nuclear substance have been
traced, as also all the stages of the transference of half of each
filament, male and female, into each of the first two cells of the very
young embryo-plant.
Moreover, the essentials are found to be the same in the animal kingdom
also, and the bearing of all these discoveries on the phenomena of
reproduction, variation, and heredity in living organisms has been and
is of the highest importance, for they support, control, explain and
correct so many of the splendid results of Knight, Kölreuter, Sprengel,
Hildebrand and Hermann Müller, and in every direction throw side-lights
into the crevices of that magnificent structure, the theory of Natural
Selection, erected for all time by our countryman, Charles Darwin.
To return now to experiments on crossing. It is found that the first
products of the crossing appear exactly alike; they may have characters
intermediate between those of the father and mother, or they may
resemble one more than the other, but all the seeds of the same cross
do it in the same way.
On then sowing the seeds of the plants produced from this first cross,
variations begin to appear. Most of the progeny revert to one or other
of the parent forms, others show all conceivable combinations of their
characters, and a few may give rise to entirely new characters. In
succeeding generations the reversions are preponderant, and, supposing
no care is taken to prevent it, the whole of the offspring gradually go
back to the ancestral type.
Some important consequences result, however, if systematic care is
brought to bear on the matter. This tendency to variation in the second
generation of crossed plants has often been noted, and it bears out very
distinctly the conclusions to which Darwin came.
The hybridiser takes advantage of this variation, as others have done,
to select some forms and rigidly suppress others, in order to obtain
well-marked varieties of the plants he experiments with. In
illustration, I may take the following from Rimpau's account of his
experiments on crossing wheat: By crossing a white English long-eared,
dense wheat, and celebrated as a heavy cropper, with a red, looser
German wheat, remarkable for its resistance to winter cold, Rimpau hoped
to obtain a variety uniting both the above qualities. As regards the
property of resistance, he failed, and he eventually gave up the
attempts in face of the advantages offered by the so-called
_Square-heads_, which then came into the market. His experiments, even
with the above varieties, are worth noting, however, for they show how
promising the results of carefully conducted crossing and selection may
be.
The crossing was done in 1875, in both directions. In 1876 the few
grains obtained were found to yield plants almost all alike, with the
long loose ear of the German parent, but the paler colour of the English
wheat.
In 1877 the plants, obtained by sowing the finest grains, were found to
consist of pure white, pure red, and of forms which appeared to vary and
revert in all possible degrees as regards colour, density, and other
characters intermediate between these.
By carefully separating the closest and densest white wheats from the
closest and densest red ones, he got in 1878 a large number of each
coming nearer to the type sown than did the mongrel forms intermingled
with them: these reversions and intermediate forms were then rigidly
eliminated, and only the deepest coloured and densest red and white
forms again sown.
In 1879 these two chosen varieties were constant, so far as concerned
those selected from the crossing of female English white with male
German red wheat, and the following year proved the constancy of the red
variety in the reciprocal cross. In 1886 all four varieties--_i.e._ the
two reds and the two whites of both the crossings--had become constant.
Still more instructive are the results of the cross between the same
white English non-bearded wheat and a red German bearded wheat.
The first results of the crossing in 1875 showed the loose ear of the
German mother, but was paler in colour; while the influence of the
English father was shown by the absence of beard.
From the reversions and mixtures of the mongrels showing reminiscences
of the parents in all degrees in 1877, rigid selections and re-sowings
were made as before, and Rimpau eventually got four very distinct
varieties, two red and two white, a bearded and a beardless form of
each, and these were declared fixed and constant in 1879-1882.
Passing over many similar results, and merely noting a very successful
variety got from a cross between a very early ripening loose red
American wheat and the dense heavy cropping English Square-head--the
crossed variety which has proved very suitable for certain light soils
and dry climates on the Continent, which demand very rapid ripening, and
are therefore of great physiological and technical interest--I must pass
on to note the curious result of the successful hybridisation of wheat
and rye. This cross has been effected several times, and first in this
country according to reports from Edinburgh (1875), New York (1886), and
elsewhere, and Rimpau's careful experiments seem to leave no doubt on
the matter.
First I must remind you that wheat (_Triticum_) differs from rye
(_Secale_) in several marked characters, such as the breadth and shape
of the glumes, the number of flowers in the spikelet, etc.; and that the
cultivated rye differs from cultivated wheats in the characters of the
straw, in having long ears, and in its flowering glumes remaining widely
divaricated for some days when in flower.
In 1888 Rimpau removed the young stamens from the German wheat referred
to, and pollinated the stigmas with pollen from a long-eared rye. Four
sound grains were obtained, looking like wheat-grains.
The history of one of these grains was as follows: In 1889 it yielded
ears which were peculiarly narrow and long, and its stalks were also
much longer than the wheat: the flowers remained exposed, with widely
open paleae, for several days, and the grains were very peculiar, though
wheat-like.
Fifteen of the best grains were selected, and in 1890 three of the
resulting plants proved to be a wheat of the Square-head type and one
quite sterile. The others retained the elongated, narrow, brownish-red
ears, the flowering glumes again opening wide for some days. This last
is a characteristic of rye, but not of wheat.
A long series of natural hybrids of wheat, barley, and oats are also
described and discussed by Rimpau, as well as artificial crosses--some
very remarkable--of barleys, but they must be passed over here.
Peas rarely become hybridised naturally. According to Darwin, H. Müller,
and Focke, the flowers are little visited by insects in our countries,
though the mechanism points to their adaptation for pollination by large
bees.
Rimpau confirms Darwin, H. Müller, and Ogle as to the self-fertilisation
of our cultivated peas. Nevertheless, as is well known, marked varieties
have been obtained by artificial crossing by Gärtner, Knight, Laxton,
and others, especially in this country.
At the same time experiments show that while it is very easy to obtain
artificial hybrids of such plants, and there is no fear of natural
inter-crossing, the forms are remarkably unstable as yet. Similarly
unsatisfactory results were obtained with beet. As experiments are still
going on, however, we may expect to hear more about these and other
results.
It is probable, from recent experiments by De Vries, Correns, and
others, that a remarkable regularity, expressed by Mendel in the form of
a law, obtains in the variations which result from hybridising.
In considering these illustrative cases, it is necessary to thoroughly
apprehend that two procedures are involved. In the first place we have
the cross-pollination leading to the formation of the hybrid plant by
cross-fertilisation. But experience shows that this would lead to very
uncertain results if the plant-breeder did not supplement them by the
second and extremely important process of rigid selection--_i.e._ by
choosing the best of the progeny and breeding from them apart from the
parent-forms, and gradually intensifying, as it were, the variations in
certain directions which have been started by the crossing.
It is by selection, careful culture, and repeated selection that so much
has been done in obtaining the innumerable new varieties of roses,
sweet-peas, orchids, orchard fruits, cereals, grapes, strawberries,
melons, tomatoes, early potatoes, etc., brought forward by numerous
breeders of plants in all countries, as will readily be understood if
reference be made to the work of Hays and Webber in America; Saunders in
Canada; Garton, Sutton, Veitch, Bateson, and others in this country.
Nor is it necessary that the new materials for selection to work upon
should be started by hybridisation. Grafting, change of conditions, and
even variations so vaguely understood that we term them "spontaneous,"
may supply the starting-points for changes in the characters of plants,
so remarkable after intensification by breeding that people find it
difficult to believe they can have come from one stock.
Here, however, I must conclude, merely remarking that the above sketch
is a mere outline of the subjects modern agriculture and horticulture
concern themselves with. There are hundreds of problems connected with
the germination of seeds, on which valuable recent work has been done by
Klebs, Green, Horace Brown, and others; with the resistance of seeds
and seedlings to high and low temperatures, a subject opened out by
Sachs, Kny, De Vries, Krasan, Just, Höhnel, Dewar, Dyer, and others;
with the conditions of vegetation which affect the various functions of
growth, respiration, assimilation, transpiration, and so forth, on which
I cannot even touch in these pages.
Meanwhile I hope I have succeeded in impressing upon you the grand fact
that the plant is a living and very complex engine, driven by the
radiant energy of the sun, and capable of doing work thereby, and this
just as truly as any heat-engine is driven by chemical energy gained by
means of the sun's rays, or as a water-mill is driven by power which
must be referred to the energy of potential in the head of water placed
in position by the sun's work in evaporation. Fundamentally the whole of
life and work on our planet is to be referred to the one great source of
energy which renders possible the establishment of differences of
potential.
This machine, then, doing work in various ways, adapts itself--or goes
to the wall--to the conditions of its work among competing organisms or
opposing circumstances. Curiously enough, while in some cases it suffers
from the competition, in others it is benefited by its life-actions
fitting in between those of other organisms, which in their turn
supplement it. In other words new types of this engine, capable of doing
the work in various ways, are obtainable; some are good types for the
conditions afforded, others are bad ones.
Examples of both will occur in the further exposition of the subject.
Man's position in regard to the struggle is that of an intelligent being
who steps in at certain stages and protects, fosters, and in every way
favours the agricultural plant--the living machine--and sees that every
opportunity is given it to do its best work in the best way--from his
points of view!
NOTES TO CHAPTER VIII.
The foundation of any course of reading on hybridisation
and selection should be Darwin's _Effects of Cross and
Self-Fertilisation in the Vegetable Kingdom_, which, with his
books _On the Origin of Species by means of Natural Selection_
and _The Variation of Animals and Plants under Domestication_,
will prepare the student for the long course of reading
necessary for a full appreciation of what has been done in
this department of science.
From the numerous works which followed these I should select
Bailey's _Survival of the Unlike_, London, 1896, and
_Evolution of our Native Fruits_, New York, 1898, as
especially useful for the reader of this book, to which may
also be added _Plant Breeding_, New York, 1896, by the same
author, as giving numerous facts and practical directions of
value. Further, the "Hybrid Conference Report," _Journ. Roy.
Hort. Soc._, 1900, abounds in facts and information. Rimpau,
_Landw. Jahrb._, vol. xx., 1891, p. 239. The student who
wishes to get towards the root of the matter will hardly be
able to dispense with Strasburger's _Neue Untersuchungen über
die Befruchtungsvorgang bei den Phanerogamen_, Jena, 1884. An
interesting summary of recent work on _Xenia_ and "double
fertilisation" will be found in _Bull. No. 22, U.S. Dept. of
Agric._, 1900. See also _Nature_, Mar. 15, 1900, p. 470.
If he wishes to explore the vast region of controversial
literature that opens up from these points, and which is far
beyond the purpose of this book, he may consult the literature
collected in Kassowitz' _Allgemeine Biologie_, Wien, 1899, B.
II., and the references in the works quoted; also,
Strasburger, "The Periodic Reduction of Chromosomes in Living
Organisms," _Ann. Bot._, viii., 1894, p. 281. For "Mendel's
Law," see Correns in _Ber. d. deutsch. bot. Gesellsch._, vol.
xviii., 1900, p. 158.
_PART II._
DISEASE IN PLANTS.
CHAPTER IX.
PHYTOPATHOLOGY. DERIVATION AND MEANING.
_History. References in the Bible--Greeks and Romans--
Shakespeare--Rouen law--Superstitions--Malpighi and Grew--
Hales--Unger--Berkeley--De Bary, etc. Physiology and Biology
--Diagnosis--Etiology--Therapeutics. Study of causes._
Phytopathology, from Greek words which signify to treat of diseases of
plants, comprises what is known of the symptoms, course, and causes of
the diseases which threaten the lives of plants, or bring about injuries
and abnormalities of structure. As a distinct and systematised branch of
botany it is a modern study, the history of which only dates from about
1850, though the subject had been treated more or less disjointedly by
several authors during the preceding century, and isolated records of
diseased crops, fruit-trees, etc., exist far back in the history of
Europe. The existence of mildews and blights on cereals indeed was
observed and recorded by the writers of the older books of the Bible,
half a dozen references to such blights being found in the Old
Testament, as well as others to blasted fig trees, etc., in the New
Testament. Aristotle, about 350 B.C., noticed the epidemic nature of
wheat-rust. The Greeks and Romans were so well acquainted with such
diseases that their philosophers speculated very shrewdly as to causes,
while the people dedicated such pests to special gods. As regards the
Middle Ages, we know little beyond the fact that blights and mildews
existed, but Shakespeare's reference in _King Lear_ (Act III., Sc. 4)
leaves no doubt as to his acquaintance with mildew in the 17th century,
and other authorities bear out the same. Even the law took cognisance of
the danger of wheat-rust in 1660 in Rouen (Loverdo). Prior to the 18th
century, however, only meagre notes on the subject occur scattered here
and there among other matters, and much superstition existed then and
later regarding these as other diseases.
Malpighi, in 1679, gave excellent figures of leaves rolled by insects
and of numerous galls, the true nature of which he practically
discovered by observing the insect piercing the tissues; previous
observers--Pliny knew that flies emerge from galls, but thought the
latter grew spontaneously--having nothing but superstitions and
conjectures to offer. Grew, in 1682, also gave a capital figure and
description of a leaf mined by "a small flat insect . . . which neither
ranging in breadth nor striking deep into the leaf, eats so much only
as lies just before it, and so runs scudding along betwixt the skin and
the pulp of the leaf, leaving a whitish streak behind it, where the skin
is now loose, as the measure of its voyage"--a by no means inadequate
description of the injury and its cause.
During the eighteenth century several academic treatises or
dissertations dealing with diseases of plants appeared.
But as a rule we only find disjointed notes. Hales (1727-33) discusses
the rotting of wounds, canker, and a few other matters, but much had to
be done with the microscope ere any substantial progress could be made.
With the nineteenth century, and the founding of the modern theories of
nutrition by Ingenhousz, Priestley, and De Saussure, we find a new era
started. As the discoveries of the microscopists continued to build up
our knowledge of the anatomy of plants and began to elucidate the
biology of the fungi and other cryptogams, while the chemists and
physiologists laid the foundations of our modern science of plant life,
it gradually became possible to tabulate and classify plant diseases,
and discuss their symptoms and causes in a more scientific manner. Even
in 1833, however, Turpin, and a far better observer, Unger, regarded
parasitic fungi as due to diseased outgrowths of chlorophyll-corpuscles
and parenchyma cells, views shared by Meyen (1837) and Schleiden (1846).
We may pass over the various treatises of Wiegmann (1839), Meyen (1841),
Raspail (1846), Kühn (1859), and a number of other works of the period,
merely referring with emphasis to Berkeley's admirable papers in the
_Gardener's Chronicle_ (1854) for a summary of what was then known. All
these works antedate De Bary's _Morphologie und Physiologie der Pilze,
etc._ (1866), in which he brought together the results of his researches
during the decade, proving the real nature of parasitic diseases and
infection as worked out by experiments between 1853 and 1863.
This work put the whole subject of parasitic diseases of plants and
animals on a new footing, and paved the way for the modern treatment of
plant pathology as elaborated in the treatises of Frank (1880 and 1895),
Sorauer (1886), Kirchner (1890), and others, to which the reader is
referred for further details. I will merely quote the following passage
from Raspail's _Histoire Naturelle de la Santé et de la Maladie_, 1846
(vol. ii., p. 176), in illustration of the views entertained by high
authorities just prior to De Bary's work: "L'insecte qui produit les
_erineum_, _uredo_, _æcidium_, _xyloma_, _puccinia_, n'est donc plus
pour nous un insecte inconnu, mais un _acarus_ (grise), un _aphis_
(puceron) ou un _thrips_, qui produit au printemps une déviation, etc."
And this view, that fungi already well known to mycologists were called
forth by the punctures of insects, was regarded as not out of harmony
with the idea that the fungus itself was an abnormal outgrowth of the
tissues of the host.
The proper study of plant pathology presupposes and involves a knowledge
of the physiology of plants, of the normal relations of the latter to
their environment, and of the biology of those animals and plants
(principally insects and fungi) which are parasitic on them. It is of
the first importance to understand that a disease is a condition of
abnormal physiology, and that the boundary lines between health and
ill-health are vague and difficult to define. As with the study of the
diseases of man and other animals, so with those of plants, the practice
resolves itself into the accurate observation and interpretation of
symptoms (_Diagnosis_) on the one hand, and of causes (_Aetiology_) on
the other, before any conclusions of value can be drawn as to preventive
or remedial measures (_Therapeutics_). In plants, however, symptoms of
disease are apt to exhibit themselves in a very general manner, or at
any rate it may be that our perceptions of them differentiate symptoms
due to very different reactions imperfectly, probably because the
organisation of the plant is less specialised than that of animals. The
turning yellow and premature falling of leaves, for instance, is a
frequent symptom of disease; but it may be due to a long series of
different causes of ill-health--_e.g._ drought, too high or too low a
temperature, light of insufficient or of excessive intensity, a
superfluity of water at the roots, the presence in the tissues of
parasitic fungi, or that of worms or insects at the roots or elsewhere,
poisonous gases in the air, soil, etc., and so forth. Consequently the
science of plant pathology is much concerned with the direct action of
external causes, which are probably less obscure than in the case of
animals, though by no means always obvious. Such considerations at any
rate seem to account for the fact that most authorities on plant
pathology base their classification on the causes of disease, there
being few noteworthy exceptions.
NOTES TO CHAPTER IX.
The bibliography here quoted will be found in Berkeley,
"Vegetable Pathology," _Gardener's Chronicle_, 1854, p. 4;
Plowright, _British Uredineæ and Ustilagineæ_, 1889; Eriksson
and Henning, _Die Getreideroste_, Stockholm, 1896; De Bary,
_Comparative Morphology and Biology of the Fungi_, etc., 1887;
Frank, _Die Krankheiten der Pflanzen_, 1895-96, and scattered
in the works referred to in them and in the text.
CHAPTER X.
HEALTH AND DISEASE.
_Variation--Disease--Comparison to a top. Health--Extinction
of species--Natural demise. Examples of complex interactions
in health--Interference, and tendencies to ill-health._
When we come to enquire into the causes of disease, it appears at first
an obvious and easy plan to subdivide them into groups of factors which
interfere with the normal physiology of the plant. Scientific experience
shows, however, that the easy and the obvious are here, as elsewhere in
nature, only apparent, for disease, like health, is an extremely complex
phenomenon, involving many reactions and interactions between the plant
and its environment. If we agree that a living plant in a state of
health is not a fixed and unaltering thing, but is ever varying and
undergoing adaptive changes as its life works out its labyrinthine
course through the vicissitudes of the also ever-varying environment,
then we cannot escape the conviction that a diseased plant, so long as
it lives, is also varying in response to the environment. The principal
difference between the two cases is, that whereas the normal healthy
plant varies more or less regularly and rhythmically about a mean, the
diseased one is tending to vary too suddenly or too far in some
particular directions from the mean; the healthy plant may, for our
present purposes, be roughly likened to a properly balanced top spinning
regularly and well, whereas the diseased one is lurching here, or
wobbling there, to the great danger of its stability. For we must
recognise at the outset that disease is but variation in directions
dangerous to the life of the plant. Health consists in variation also,
but not in such dangerous grooves. That the passage from health to
disease is gradual and ill-defined in many cases will readily be seen.
In fact we cannot completely define disease. Mere abnormality of form,
colour, size, etc., is not necessarily a sign of disease, in the usual
sense of the word, otherwise the striking variations of our cultivated
plants would suggest gloomy thoughts indeed, whereas we have reason to
believe that many cultivated varieties are more healthy--in the sense of
resisting dangerous exigencies of the environment--than the stocks they
came from. Strictly speaking, no two buds on a fruit-tree are alike, and
the shoots they produce vary in position, exposure, number, and vigour
of leaves, and so forth. The minute variations here referred to are not
seen by the ordinary observer, but those who bud, graft and multiply by
cuttings on a large scale know that such bud-variations are important,
quite apart from more extensive "sports" which occasionally occur.
On the other hand, we have reason to believe that many species have died
out gradually as the environment altered. These plants died because they
did not vary sufficiently, or did not vary in the right directions; they
became diseased with respect to the then prevailing conditions of normal
physiology or health.
Disease, therefore, may be said to be variation of functions in
directions, or to extents, which threaten the life of the plant, the
normal in all cases being the state of the plant characteristic of the
species.
Even now, however, we have not obtained a complete definition, because,
since all plants die sooner or later, we have not excluded the natural
demise of the individual or its parts, and no one would call the
autumnal fall of leaves, or the withering of an annual after flowering,
death from disease. Clearly then the idea of disease implies danger of
premature death, and probably this is as near as we shall get to a
satisfactory definition. Since this matter is of primary importance for
our present theme, I will add the following instances for consideration.
A plant in perfect health and in the fullest exercise of all its
functions, has its roots in a soil which is suitably warmed and aerated,
contains the right quantities of water which dissolve just the proper
proportions of all the essential mineral salts, but nothing poisonous,
while the soil itself has a texture such that the roots and root-hairs
can extend and do their utmost in absorbing.
The leaves above are exposed to just the right intensity of light, in
air which is not too dry, and is of suitable temperature and
composition, containing no poisonous exhalations, etc.; and as the
foliage is gently moved by the breeze, it manufactures carbohydrates at
the optimum rate in the chlorophyll, and the so-called "elaborated sap"
containing the dissolved organic food-supplies is prepared in the
tissues in maximum quantities and of just the right degrees of
concentration and quality for use in the buds, stem, roots, etc., for
which it is destined as they draw on the supplies.
Between these assimilating organs, the leaves, and the absorbing roots,
we have in the stem the wood, with its vessels adapted in quantity and
calibre to convey the water containing dissolved salts from the
absorbing roots to the leaves (to say nothing of other parts) and,
separated from this wood by the cambium, we find the sieve-tubes and
cortical tissues in suitable quantity conveying the "elaborated
sap"--the solutions of organic food-materials from the leaves down to
the roots, up to the buds, and elsewhere. Joining these cortical and
wood tissues are adapted series of medullary rays which, apart from
other connections, bring about the necessary interchanges of water and
"elaborated sap" with the cambium, the formative tissue which has to be
fed and served by them, and which by its growth supplies new vessels and
sieve-tubes, etc., to carry the continually increasing quantities of
water and food substances as the roots and leaves increase in number and
area, and thus enables this ideally correlated system to go on working
at maximum energy.
Now suppose the same plant with its roots in an unsuitable soil--too dry
or too poor in mineral supplies, for instance--the transpiring leaves
above cannot obtain sufficient water and salts to supply their needs,
but we will suppose hypothetically that they still assimilate under the
same ideal conditions as before. The supplies now coming to the cambium
are diminished, since the want of water and minerals compels the leaves
to put aside any excess of carbohydrates (_e.g._ as stored
starch-grains), and the plastic materials which do pass to the cambium
so deficient in water cannot be directly utilised, and a starvation
period sets in. Consequently the cambium forms less wood, and this will
contain fewer and smaller vessels, and so reduce the conducting
passages: fewer sieve-tubes also are constructed, and the paths of the
water current and food supplies narrowed, which of course reacts on the
tissues everywhere. The reserve substances may slowly be dissolved and
distributed, however, and considerable quantities be passed in course of
time into the roots, which, as opportunity offers, gradually employ them
in making new roots, and if the disturbance has not gone too far and
the conditions do not become unfavourable, an increased root-supply may
by its larger absorbing area gradually establish the former state of
equilibrium of functions. But this at the expense of the plant, which is
smaller, has fewer leaves and narrower water channels, etc., than a
plant not thus checked, and it may take a long time to make up for the
loss of time and stature thus incurred. Indeed if the plant is an annual
no recovery at all may occur, the reserves passing into fruit and seeds
instead of slowly supplying the roots as described.
If it be asked, can such a condition of affairs as that described really
occur, we have only to think of a transplanted specimen with its roots
maimed and put into unsuitable soil, or of plants in the open with
feeding roots gnawed by an insect, etc., or of a tree hitherto in
equilibrium with its fellows in a plantation suddenly set free by
thinning and so forth.
Now take the case where the roots are maintaining their maximum
functional activity, but the leaves--owing to want of light, too much
moisture or too low a temperature of the air--are functionally
depressed. Here we get a state of over-saturation with water set up, the
tissues are turgid to bursting point, what supplies do traverse the
sieve-tubes, cortex, etc., do so slowly and are excessively diluted, and
the cambium again forms less wood, but the lumina of the vessels are
larger and the lignification less complete. Growth in length is
excessive, but more leaves are formed, though they are apt to be
abnormally thin and may be small. Little or no reserves are stored
anywhere, and the watery tissues contain dangerously diffusible
substances which may render them an easy prey to parasitic fungi. Here
again, however, if the disturbance of equilibrium has not gone too far,
and if the season permits, the new leaves may come into full activity
and the situation be saved by transpiration and assimilation gradually
increasing and restoring the equilibrium. But, as before, the plant has
suffered, and shows the effect in its weak shoots, retarded flowering,
and other ways.
Such plight as is here described may actually be attained in greenhouses
where over-watering is the fault, and even in the open it is not
uncommon in rainy summers, or in plantations where dominant trees get
the upper hand and partially shade more slowly growing species, or in
fields where rank grass is allowed to overwhelm crops of lower stature.
Now it will be evident that either of these typical cases of temporary
disturbance of functional equilibrium may be carried too far: in the
first case the plant may wilt and wither, in the second it may rupture
and rot, to take these eventualities only. And yet it is difficult to
call these indispositions diseases: they are rather examples of extreme
departures from the normal standard of health, just on the borderland
between health and disease. A step further, as it were, and disease
supervenes: certain tissues die from want of water, and a necrotic area
is formed, or the cortex bursts and a wound is formed in another way, or
some fungus gets a hold, and so on. These abnormal states are
particularly apt to predispose the plant to disease--insects revel in
such semi-wilted leaves and shoots crammed with reserves, and fungi in
the water-logged leaves of the second case, while a cold dry wind is
peculiarly fatal to such tissues.
NOTES TO CHAPTER X.
The reader may consult Hartig, _Diseases of Trees_, Eng. ed.,
1894, Introduction; Sorauer, _Pflanzen Krankheiten_, pp. 1-12,
and Frank, _Die Krankheiten der Pflanzen_, B. 1, p. 5, for
definitions of disease.
CHAPTER XI.
CAUSES OF DISEASE.
_A. External causes--I. Non-living environment: soil,
atmosphere, temperature--II. Living environment: plants,
animals--Complex interactions--Predisposing causes--No one
factor works alone--Tangled problems of natural selection
involved. B. So-called internal causes._
It is customary to classify the causes of disease in plants into two
principal groups--(1) those due to the action of the non-living
environment--soil, atmosphere, physical conditions such as temperature,
light, etc.; and (2) those brought about by the activities of living
organisms--plants and animals of various species. Before passing to
further subdivisions under these two heads, however, it is necessary to
observe that no disease can be efficiently caused by an organism alone,
since its powers for injury as a parasite, or otherwise, are affected by
its non-living environment as well as by the host-plant. For instance,
the spores of a parasitic fungus which would infect and rapidly destroy
a potato plant in moist warm weather may be showered on to such a plant
with impunity if the air remains dry and cool--or on to a cabbage under
any circumstances as far as we know.
Again, probably no one factor of the non-living environment ever
suffices to induce a disease, possibly because no such thing as only one
change at a time ever occurs. For instance, it is difficult to say, when
a soil becomes sodden with water, whether the excess of water and
dissolved matters, the want of air displaced by the water, the lowering
of the temperature, or the accumulation of foul products, etc., is the
principal factor in causing the damage which results, and we have to
determine by the balance of experimental evidence which is the dominant
factor in all such cases.
The study of aetiology of disease is in fact only a particular case of
that of aetiology in general. Plants at high altitudes in the Alps
acquire very different characteristics from the same species in the
plains. Is this due to the low temperature, the rarer atmosphere, the
more intense illumination, the changes in moisture, etc., etc.? The
question is more difficult than it appears at first sight, and we must
remember that, complex as are the factors working on the host, they are
equally complex in their actions on a parasite attacking the host,
whence the resulting disease becomes indeed a tangled problem of natural
selection.
Finally it remains to say a few words about a numerous class of cases
where no external cause of disease can be discovered. It was formerly
the custom to group such cases of "Internal Causes" by themselves, but
apart from the fact that many of these mysterious diseases have
subsequently been shown to be due to the action of external agencies,
the whole question of internal causes resolves itself into one of
relations between the plant and its surroundings, and it becomes evident
that no inherited or internal disease can be regarded as explained until
we know the external causes which have so modified the structure and
working of the living cells as to make them abnormal in their reactions
to other parts of the plant. "Internal causes" of disease, therefore, is
a phrase expressing our ignorance, but somewhat more emphatically than
usual. If this is clearly understood there seems no reason against its
employment for the time being in the artificial scheme of classification
we require. With regard to external causes due to the non-living
environment, excess or deficiency of materials in the soil, water, or
atmosphere plays an important part, and--since we may neglect purely
aquatic plants--it is customary to speak of diseases due to unsuitable
soils or to injurious atmospheric influences. For instance, any
deficiency in the supplies of the necessary mineral salts (compounds of
calcium, magnesium, potassium with sulphuric, nitric and phosphoric
acids, etc.) leads to pathological changes, as also does the lack of the
necessary traces of iron. But it is equally true that the presence of
such ingredients in excess or in combinations unsuited to the plants
also leads to disaster, as also does the presence of minerals or other
compounds which poison the root-hairs--_e.g._ products of decomposition,
soluble salts of copper and other poisons. That these matters are bound
up with the whole question of manuring and of proper soil-analyses will
be evident.
Another essential factor is the nature and quantity of organic materials
in the soil, whether leaf-mould and decomposing vegetable remains,
stable manures, or other animal matters, all of which affect different
species very differently, and produce very different results in
different soils. It is necessary to apprehend in this connection what
has been stated above: that soil is not a mere dead structureless
medium, and that the root-hairs of ordinary plants cannot deal with
large quantities of putrefying organic matter: that a good soil must
abound in useful bacteria and fungi to render such substances
available--and in very various ways--and that it must be open and
aerated, of proper temperature and suitably supplied with water, and so
forth, or disaster will result. Here, again, then we are brought into
close contact with all that is known of fermentation, nitrification, and
the various biological changes going on in soil, and the application of
such knowledge to the practice of manuring and tillage in all its forms.
In view of the above remarks, the danger of "over-feeding," in this
sense, has a real meaning for horticulturists, though it must not be
forgotten that no substance is really a food until it is assimilable
into the protoplasm: manures, etc., are food-materials, not food. The
futility of mere chemical analyses to prove what a plant requires is now
well known, and it is only on the basis of long and carefully conducted
experiments that we can ever discover what a particular plant in a
particular soil, situation, and climate requires for healthy
development. Again, the quantity of water in soil may be too great or
too small for given species, and this either on the average for the
year, or during critical periods only; and it is obviously important
whether the excess or deficiency is due to improper supplies of water,
the depth or shallowness of the soil, its retentive powers, or the
nature of the sub-soil and so on, again bringing the whole matter into
connection with our understanding of the physical constitution and
structure of soils, and the nature of soil-drainage.
For instance, a common way of killing ferns is to keep the roots and
soil wet and the air and fronds dry, whereas the natural habitats
provide for wet and shaded fronds and well-drained soil.
It may be noted here that in most cases where gardeners speak of plants
being killed under the "drip" of trees--_e.g._ Beech, the injury is due,
not to the effects of water but to the shade: the loss of light is so
great that the shaded plants die of inanition because their leaves are
not able to provide sufficient carbohydrates.
Closely bound up with this is the question of the gases in soils. Apart
from the disastrous effects of poisons--_e.g._ coal gas escaping from
pipes under pavements in towns, etc., diseased conditions often result
from deficiency of oxygen at the root-hairs, due to imperfect aeration
of soils, brought about by stagnant water, excess of animal matter, and
so forth.
Unsuitable constitution of the atmosphere is also a fruitful source of
disease, though its effects are commoner in closed stoves and
greenhouses than in the open. Nevertheless the continual exhalation of
sulphurous fumes, chlorine, and other poisonous gases in the
neighbourhood of manufacturing centres or of large smoky towns,
volcanoes, etc., play their part in injuring plants; and excessive
moisture in the form of mist, rain, etc., is also important. All these
matters bring us at once into the region of physiology, and only an
intelligent appreciation of what is known about the action of the
atmosphere on the soil and the plant will save the peasantry of a
country from a hopeless mysticism but little removed from that of the
Middle Ages, when blights and other evils were vaguely referred to the
river-mists, thunder clouds, and easterly winds.
If we summarise the above as the material factors of the environment, we
may classify another set of external non-living causes of disease as the
non-material factors. Such are principally the following:
The space at the disposal of plants greatly affects their welfare. The
crowding of roots in the soil and of foliage in the air, resulting in
the loss of light to the leaves, involves deficiency of all the
materials referred to above--minerals, organic materials, gases, and
water--and no better illustration of the intense struggle for existence
among these apparently passive and motionless beings, plants, can be
given than an over-crowded seedbed or plantation. If left to themselves
such over-stocked areas exhibit to the keen eye of the trained observer
all the phases of starvation, weakness, wounding, rot, and, so to speak,
brutal dominance of the stronger over the weaker which it is the object
of cultivation to prevent. Here, then, we are brought face to face with
the true significance of thinning and weeding out, pruning, and similar
processes.
Unsuitable temperature is one of the commonest of all sources of
disease, for every plant is adapted to certain ranges of temperature,
and best adapted to a given optimum somewhere between the maximum and
minimum temperature for each function. Consequently any serious
departure from the mean may bring about physiological disturbances of
the nature of disease, and this in very various ways, as exemplified by
the results of frost, sun-scorching, drought, hail-storms, forest fires,
and so forth.
As a predisposing factor to disease abnormal temperature effects play a
great part. Many wound-fungi gain their entrance through frost-cracks,
bruises due to hailstones, or into tissues chilled below the normal.
No less remarkable are the diseases primarily due to insufficient or
improper exposure to light, which affects the chlorophyll-apparatus and
the process of carbon-assimilation and through these the whole
well-being of the plant. Every plant is adapted to certain ranges of
light intensity, and most cultivators know how impossible it is to grow
shade plants in fully exposed situations, and how easily plants which
live in open sunny situations are "drawn" and killed by shade. It is
equally important to have the right kind of light, as disastrous
experiences with greenhouses glazed with glass which cut off certain
rays of light have taught. Here, again, it is important to notice that
the optimum intensity or quality of light may differ for different
functions and organs of the plant, as is shown by many adaptations on
the part of species growing in natural situations--_e.g._ bud
protection, orientation of leaves, etc.--and it may be taken as a rule
that etiolated plants are peculiarly susceptible to other diseases.
As regards other factors of the inorganic environment, disasters which
come within the scope of our subject may be brought about by many
agencies, the mechanical effects of snow and hail, wind, avalanches,
etc., the effects of lightning, and so forth, being a few of them.
NOTES TO CHAPTER XI.
For other detailed classifications of the causes of disease
the reader is referred to the works of Sorauer and of Frank
referred to in the last chapter. Also Kirchner, _Pflanzen
Krankheiten_, Stuttgart, 1890.
Of more historical importance are the older classifications of
Berkeley, _Gardeners' Chronicle_, 1854, and Re, _Gardeners'
Chronicle_, 1849-50. These latter are interesting as showing
the very different views held by the earlier workers, and
comparison of these with the modern views helps to mark the
progress of physiology during the half century which has
intervened.
CHAPTER XII.
CAUSES OF DISEASE. THE LIVING ENVIRONMENT.
_Causes due to animals--Vertebrata--Wounds, etc.--Invertebrata
--Insects, etc.--Plants as causes of disease--Phanerogams,
weeds, etc.--Cryptogams, fungi--Epidemics, etc._
Passing now to those causes of disease which are connected with the
living environment, we may obviously divide them into two groups of
agents, animals and plants.
Among animals, the various vertebrata, including man, are especially
responsible for the larger kinds of wounds and wholesale destructive
processes due to breakage, stripping of leaves and bark, cutting and
biting, and so forth. Cattle, rabbits, rats and mice, squirrels and
birds of various kinds stand out prominently as enemies to trees and
other plants, to which they do immense injury in various ways by their
horns, teeth, claws, and beaks; and the damage which an ignorant
gardener or forester can do with his ill-guided footsteps, axe, spade,
and knife can only be appreciated by one who knows the habits of plants.
It is among the invertebrata, however, especially insects and worms,
that the most striking agents of disease in plants are to be found, for,
with the exception of certain rodents--and we may logically include also
human invasions--vertebrate animals do not often appear in such numbers
as to bring about the epidemics and scourges only too commonly caused by
insect pests.
Insects injure plants in very various ways. Some, such as locusts,
simply devour all before them; others, _e.g._ caterpillars, destroy the
leaves and bring about all the phenomena of defoliation. Others, again,
eat the buds--_e.g._ _Grapholitha_; or the roots--_e.g._ wire-worms, and
so maim the plant that its foliage and assimilation suffer, or its roots
become too scanty to supply the transpiration current. Many aphides,
etc., puncture the leaves, suck out the sap, and produce deformations
and arrest of leaf-surface, as well as actual loss of substance, and
when numerous such insects induce all the evils of defoliation. Others,
such as the leaf-miners, tunnel into the leaves, with similar results on
a smaller scale.
It must be remembered that a single complete defoliation of a herbaceous
annual, or even of a tuberous plant like the potato, so incapacitates
the assimilatory machinery of the plant, that no stores can be put aside
for the seeds, tubers, etc., of another year, or at most so little that
only feeble plants come up.
In the case of a tree the case is different, and since most large trees
in full foliage have far more assimilatory surface than is actually
necessary for immediate needs, a considerable tax can be paid to
parasites or predatory insects before the stores suffer perceptibly.
Still, it should be recognised that the injury tells in time, especially
in seed years.
Many larvae of beetles, moths, etc., bore into the bark and as far as
the cambium or even into the wood or pith of trees, the local damage
inducing general injuries in proportion to the number of insects at
work: moreover, the wounds afford points of entrance for fungi and other
pests.
Galls and similar excrescences result from the hypertrophy of young
living tissues pierced by the ovipositors of various insects, and
irritated by the injected fluid and the presence of the eggs and larvae
left behind. They may occur on the buds, leaves, stems, or roots, as
shown by various species of _Cynips_ on oak, _Phylloxera_ on vines,
etc., in all cases the local damage being relatively small, but the
general injury to assimilatory, absorptive, and other functions is great
in proportion to the number of points attacked.
Many grubs--larvae of flies, beetles, etc.--bore into the sheaths or
internodes of grasses, or the pith of twigs, or into buds, fruits, and
other organs of plants, and do harm corresponding to the kind and amount
of tissues injured.
Various species of so-called eelworms--Nematodes--also cause gall-like
swellings on young roots, or they invade the grains of cereals.
Finally, various slugs and snails cause much injury by devouring young
leaves and buds and diminishing the assimilatory area.
Plants as agents of disease or injury fall naturally into the two main
categories of flowering plants (Phanerogams) and Cryptogams, among which
the fungi are the especially important pests.
Beginning with weeds, we find a large class of injurious agents. Weeds
damage the plants we value by crowding them out in the struggle for
existence, as already stated, and when the weed-action is simply due to
superfluous plants of the same species, we speak of overcrowding. But it
must not be overlooked that the competition between crowded plants of
the same species--where every individual is acting as a weed to the
others--may be more dangerous than between plants and weeds belonging to
other species and genera, because in the former case they are struggling
for the same minerals and other necessary food-materials: a matter of
importance in connection with the rotation of crops.
The question of allowing grass to grow at the foot of fruit trees, as in
orchards, is a good case in point. Such grass may increase the damp and
shade, thus favouring fungi at one season, and dry up the moisture of
the soil to the injury of the fine superficial roots at another, as well
as exhaust the soil, owing to the competition of the roots for salts
and other materials. On the other hand, the checking of surface roots by
competition with the grass has been claimed as advantageous. In this
connection probably the whole question of the composition of the turf
arises, as well as that of possible cropping for hay, and manuring.
As regards any particular weed, the cultivator should learn all he can
respecting its duration, seeding capacity, method of dissemination, the
depth and spread of its root-system, and any other particulars which
enable him to judge when and how to attack it. It is only necessary to
see the victory of such drought-resisting weeds as _Hieracium
pilosella_, Plantains, _Hypochaeris_, on lawns to realise how weeds may
win in the struggle for existence with the finer grasses.
Many so-called weeds are, however, partially parasitic, with their roots
on the roots of others--_e.g._ _Rhinanthus_, _Thesium_, etc., and much
damage is done to meadow grasses and herbage by the exhaustive tax which
these semi-parasites impose.
This is carried still further in the case of such root-parasites as
_Orobanche_, where the host-plant is burdened with the whole support of
the pest, because the latter, having no chlorophyll, is entirely
dependent on the former for all its food.
Even ordinary climbing plants may injure others by shading them, either
by scrambling over their branches--_e.g._ Bramble, or twisting their
tendrils round the twigs--_e.g._ Bryony, or twining round them--_e.g._
Woodbine, _Convolvulus_, etc. The principal direct injury is in these
cases owing to the loss of light suffered by the shaded foliage, but
the weed-action is often increased by the competition of their
roots--_e.g._ briars; and in the case of woody climbers the gradually
increased pressure of the woody-coils round the thickening stems
compresses the cambium and cortex of the support and induces strictures
and abnormalities which may be fatal in course of time.
Epiphytes, or plants which support themselves wholly on the trunks,
branches, or leaves of other plants, also injure the latter more
especially by shading their foliage--_e.g._ tropical Figs, Orchids,
Aroids, etc.; and similar damage is done by our own Ivy, the main roots
of which are in the soil, but the numerous adventitious roots of which
cling to the bark.
When the climber or epiphyte is also parasitic, as in the case of the
Dodder, _Loranthus_, Mistletoe, etc., the direct loss of substance
stolen from the host by the parasite comes in to supplement any effect
of shading that the latter may bring about if it is a leafy plant.
Of Cryptogams, apart from a few epiphytic ferns, and the intense
weed-action of certain Equisetums, the rhizomes and roots of which are
as troublesome as those of twitch and other phanerogamic weeds, it is
especially the fungi which act as agents of disease, and which, as we
now know, are _par excellence_ the causes of epidemics.
The action of fungi may be local or general; and restricted, slow and
insidious, or virulent and rapidly destructive.
Examples of local action are furnished by _Schinzia_, which forms
gall-like swellings on the roots of rushes; _Gymnosporangium_, which
induces excrescences on the stems of junipers, and numerous leaf-fungi
(_Puccinia_, _Æcidium_, _Septoria_, etc.), which cause yellow, brown, or
black spots on leaves, as well as by _Ustilago_, which attacks the
anthers or the ovary of various plants, and so forth. In such cases the
injury done by a few centres of infection is very slight, but prolonged
action may bring into play secondary effects such as the gradual
destruction of the cambium round a branch, when, of course, the effect
of ringing results; or if the fungus becomes epidemic and myriads of
leaf-spots are formed, the destruction of foliar tissue, gradual taxing
of the assimilatory cells, etc., may end in rapid defoliation, and
renewed attacks soon exhaust the plants and lead to sterility and death,
as often occurs with Uredineae--_e.g._ the coffee leaf-disease.
It is highly probable that such fungi are particularly exacting owing to
their exhausting demands for compounds of potassium, phosphoric acid,
and other bodies.
Examples of virulent and rampant general action are afforded by finger
and toe in turnips, etc., where the roots are invaded by
_Plasmodiophora_, which induces hypertrophy and rotting of the roots;
and by the damping off of seedlings, where the fungus _Pythium_ rapidly
invades all parts of the seedlings and reduces them to a water-logged,
putrefying mass; or the potato-disease, which is due to the rapid
spread of _Phytophthora_ in the leaves and throughout the plant, which
it blackens and rots in a few days.
Many fungi not in themselves very virulent or aggressive do enormous
harm owing to the secondary effects they induce. Some of the
tree-killing hymenomycetes, such as _Agaricus melleus_, for instance,
penetrate the wood of a pine at the collar, and the result of the large
flow of resin which results is to so block up the water passages that
the tree dies off above with all the symptoms of drought. Similarly, the
_Peziza_ causing the larch disease, having obtained access to the stem
about a foot or so above the ground, will gradually kill the cambium
further and further round the stem, and so girdle the tree as
effectually as if we had cut out the new wood all round. In all such
cases--and the same applies to the leaf-diseases referred to above--the
fungus may be compared to an army which is not strong enough to invade
the whole territory, but which, by striking at the lines of
communication, cuts off the supplies of water, food, etc., and so brings
the struggle to an end. Indeed we might compare the cases of fungi which
attack the root and collar, and so strike at and cut off the water
supply, to a compact army which at once cuts off the enemy from his
narrow base; whereas the innumerable units which bring about an epidemic
attack on the leaves, and so surround the enemy and cut off his food
supplies all round, is rather like a much larger army which cannot get
in beyond the natural barriers of the tissues, and so puts a _cordon_
all round the territory and seizes the multitudes of food-stuffs at the
frontiers. The end result is similar in both cases, but the methods of
warfare differ.
Many fungi, however, though they make their presence noticeable by
conspicuous signs, cannot be said to do much damage to the individual
plant attacked. The extraordinary malformations induced by parasites
like _Exoascus_, which live in the ends of twigs of trees and stimulate
the buds to put out dense tufts of shoots, again densely
branched--Witches' brooms--are a case in point. Also the curious
distortions of nettle stems swollen and curved by _Æcidium_, of maize
stems and leaves attacked by _Ustilago_, and of the inflorescences of
_Capsella_ by _Cystopus_, etc., are not individually very destructive;
it is the cumulative effects of numerous attacks, or of large epidemics,
which tell in the end.
Some very curious effects are due to fungi such as _Æcidium elatinum_,
which, living in the cortex of firs, stimulate buds to put out shoots
with erect habit, and with leaves which are radially disposed, annually
cast, and differently shaped from the normal--characters quite foreign
to the species of fir in its natural condition.
Equally strange are the shoots of _Euphorbia_ infested with the æcidia
of _Uromyces_, those of bilberries affected with _Calyptospora_, etc. In
all these cases we must assume a condition of toleration, so to speak,
on the part of the host, which adapts itself to the altered
circumstances by marked adaptations in its tissue developments, mode of
growth and so forth.
This toleration is perhaps most marked in the case of those cereals
which, though infected by the minute mycelium of _Ustilago_ while still
a seedling, nevertheless go on growing as apparently healthy green
plants indistinguishable from the rest, although the fine hyphae of the
parasite are in the tissues and keeping pace with the growth of the
shoots just behind the growing points. As the grains of the cereal begin
to form and swell, however, the hyphae suddenly assume the part of a
dominant aggressor, consume the endosperm of the enlarging seed, and
replace the contents of the grain with the well-known black spores known
as Smut.
NOTES TO CHAPTER XII.
The reader will find a summary of such fungi as are here
concerned in Massee, _A Text-Book of Plant Diseases_, 1899, or
Prillieux, _Maladies des Plantes Agricoles_.
For further details the student should consult the works of
Frank and Sorauer referred to in the notes to Chapter IX., and
Tubeuf, _The Diseases of Plants_, Engl. ed. 1897, pp. 104-539.
For experiments on the effects of grass on orchard trees, see
_Report of the Woburn Experimental Fruit Farm_, 1900, p. 160.
For the further study of weeds, the interesting bulletins of
the Kansas State Agricultural College, 1895-1898, will show
the reader what may be done in the matter of classifying them
according to their biological peculiarities.
In regard to insects, the reader will find the following list
embraces the subject: Somerville, _Farm and Garden Insects_,
1897; Theobald, _Insect Life_, 1896; Ormerod, _Manual of
Injurious Insects_, 1890, and _Handbook of Insects Injurious
to Orchards, etc._, 1898.
The admirable series of publications of the U.S. Department of
Agriculture under the editorship of Riley and Howard, and
entitled _Insect Life_, 1888-1895, also abounds in
information.
Further, Taschenberg's _Praktische Insektenkunde_, 1879-1880,
and Judeich and Nietsche, _Lehrbuch der Mitteleurop. Forst.
Insektenkunde_, 1889.
For an elementary introduction to the study of fungus
diseases, see Marshall Ward, _Diseases of Plants_, Soc. for
Promoting Christian Knowledge, London.
CHAPTER XIII.
NATURE OF DISEASE.
_General and local disease--General death owing to cutting-off
supplies, etc.--Disease of organs--Tissue-diseases, e.g.
timber--Root-diseases--Leaf-diseases, etc.--Diseases of
Respiratory, Assimilatory, and other organs--Physiological and
Parasitic diseases--Pathology of the cell--Cuts--Cork--Callus
--Irritation--Stimulation by protoplasm--Hypertrophy._
On going more deeply into the nature of those changes in plants which we
term pathological or diseased, it seems evident that we must at the
outset distinguish between various cases. A plant may be diseased as a
whole because all or practically all its tissues are in a morbid or
pathological condition, such as occurs when some fungus invades all the
parts or organs--_e.g._ seedlings when completely infested by _Pythium_,
or a unicellular Alga when invaded by a minute parasite; or it may die
throughout, because some organ with functions essential to its life is
seriously affected--_e.g._ the roots are rotten and cannot absorb water
with dissolved minerals and pass it up to the shoot, or all the leaves
are infested with a parasite and cannot supply the rest of the plant
with organic food materials, in consequence of which parts not directly
affected by any malady become starved, dried-up, or poisoned or
otherwise injured by the results or products of disease elsewhere.
In a large number of cases, however, the disease is purely local, and
never extends into the rest of the organs or tissues--_e.g._ when an
insect pierces a leaf at some minute point with its proboscis or its
ovipositor, killing a few cells and irritating those around so that they
grow and divide more rapidly than the rest of the leaf tissues and
produce a swollen hump of tissue, or gall; or when a knife-cut wounds
the cambium, which forthwith begins to cover up the dead cells with a
similarly rapid growth of cells, the callus. Numerous minute spots due
to fungi on leaves, cortex, etc., are further cases in point, the
mycelium never extending far from the centre of infection.
Many attempts have been made to classify diseases on a basis which
assumes the essential distinction of the above cases, and we read of
diseases of the various organs--root-diseases, stem-diseases,
leaf-diseases, and so forth; or of the various tissues--timber-diseases,
diseases of the cambium, of the bark, of the parenchyma, and so on.
Furthermore, attempts have been made to speak of general functional
disease, of diseases of the respiratory organs, of the absorptive
organs, and so forth, as opposed to local lesions.
Critical examination, however, shows that no such distinctions can be
consistently maintained, partly because the organs and functions of
plants are not so sharply marked off as they are in animals, the
diseases of which have suggested the above classification, and partly
because all disease originates in the cells and tissues, and it is a
matter of detail only that in some cases--_e.g._ severe freezing or
drought of seedlings, or when some ingredient is wanting in the
soil--the diseased condition affects practically every cell alike from
the first, while in others it spreads more or less rapidly from some one
spot.
Even the distinction into physiological diseases _versus_ parasitic
diseases cannot be maintained from the standpoint of the nature of the
disease itself. All disease is physiological in so far as it consists in
disturbance of normal physiological function, for pathology is merely
abnormal physiology, no matter how it is brought about. This is not
saying that no importance is to be attached to the mode in which disease
is incurred or induced: it is merely insisting on the truth that the
disease itself consists in the living cell-substance--the
protoplasm--not working normally as it does in health, and this, whether
want of water, minerals, or organic food be the cause, or whether the
presence of some poison or mechanical irritant be the disturbing agent,
as also whether such want or irritation be due to some defect in soil
or air, or to the ravages of a fungus or an insect.
This being understood I need not dwell on the common fallacy of
confounding the fungus, insect, soil or other agent with the disease
itself, or of making the same blunder in confusing symptoms with
maladies. In this sense, wheat rust is not a disease: it is a symptom
which betrays the presence of a disease-inducing fungus, the Rust
fungus. Similarly, chlorosis is not a disease: it is a symptom of
imperfect chlorophyll action, and the best proof of the truth of both
statements is that in both cases the fundamental disease-action is the
starvation of the cell-protoplasm of carbohydrates and other essential
food matters--in the one case because the fungus steals the
carbohydrates as fast as the leaves can make them, in the second because
the leaf is unable to make them.
The foundation of a knowledge of disease in plants therefore centres in
the understanding of the pathology of living cells.
If a suitable mass of living cells is neatly cut with a sharp razor the
first perceptible change is one of colour: the white "flesh" of a potato
or an apple, for instance, turns brown as the air enters the cut cells,
and the microscope shows that this browning affects cell-walls and
contents alike. The cut cells also die forthwith; and the oxygen of the
air combining with some of their constituents forms the brown colouring
matter which soaks into the cell-walls. The uninjured cells below them
grow longer, pushing up the dead débris, and divide across by walls
parallel to the plane of the wound, and so form series of tabular cells
with thin walls, which also soon turn brown and die, the cell-walls
meanwhile undergoing changes which convert them into cork. The living
cells deeper down are now shut off from the outer world by a skin, of
several layers, of cork-cells, which prevent the further free access of
air or moisture. During the period of active cell-division which
initiates the cork, the temperature of the growing cells rises: a sort
of fever (wound-fever) is induced, evidently owing to the active
respiration of the growing cells.
This healing by cork occurs in any tissue of living cells exposed by a
cut--leaf-tissue, young stem or root, fruit, cambium, etc.; and the same
applies to any other kind of cutting or tearing injury--such as a prick
with a needle or the proboscis of an insect, a stripping, or even a
bruise.
Such healing is prepared for and carried out very thoroughly in the case
of falling leaves and cast branches, the plane of separation being
covered by a cicatrix of cork.
If the cell-tissue under the wound is actually growing at the time,
however, a further process is observed when the wound-cork has been
formed. The uninjured cells below go on growing outwards more vigorously
than ever, the pressure of the overlying tissues taken off by the cut
having been removed, and, lifting up the cork-layer as they do so, they
rapidly divide into a juicy mass of thin-walled cells which is of a
cushion-like nature and is termed a _Callus_. This callus is at first a
homogeneous tissue of cells which are all alike capable of growing and
dividing, but in course of time it undergoes changes in different parts
which result in the formation of tracheids, vessels, fibres and other
tissue-elements, and even organs, just as the embryonic tissues of the
growing points, cambium, etc., of the healthy plant give origin to new
growths. Such wound-wood, however, is apt to differ considerably in the
arrangement, constitution and hardness of its parts as compared with
normal wood, and its peculiar density and cross-graining are often
conspicuous.
If instead of a simple tissue, the cut or other wound lays bare a
complex mass such as wood, the resultant changes are essentially the
same to start with. The living cells bordering the wound form cork, and
then those deeper down grow out and form a callus. The exposure of the
wood however, entails alterations in its non-living elements also. The
lignified walls of tracheids, fibres, etc., turn brown to a considerable
depth, and this browning seems to be--like all such discolorations in
wounds--due to oxidation changes in the tannins and other bodies
present: the process is probably similar to what occurs in humification
and in the conversion of sap-wood into heart-wood in trees. Such wood is
not merely dead, but it is also incapable of conveying water in the
lumina of its elements, which slowly fill with similarly dark-coloured,
impervious masses of materials termed "wound-gum," the nature of which
is obscure, but which slowly undergoes further changes into resin-like
substances.
The exposure of wood by a wound results also in another mode of stopping
up the vessels and so hindering the access of air, loss of water, etc.,
for the living cells of the medullary rays and wood-parenchyma grow into
the lumina of the larger vessels through the pits, forming _thyloses_,
again a phenomenon met with in heart-wood. In Conifers the stoppage of
the lumina is increased by deposition of resin, which also soaks into
the cell-walls and the wounded wood becomes semi-translucent owing to
the infiltration.
Every living cell in an active condition is irritable, and one of the
commonest physiological reactions of growing tissues is that of
responding to the touch of a resistant body, as is vividly shown by the
movements of the Sensitive plant, _Dionaea_, etc., and by those of
tendrils, growing root tips, etc., on careful observation. We have
reason for stating that if a minute insect, too feeble to pierce the
cuticle, cling on to one side of the dome-shaped growing point of any
shoot, the irritation of contact of its claws, hairs, etc., would at
once cause the protoplasm of the delicate cells to respond by some
abnormal behaviour; and, as matter of experiment, Darwin showed long ago
that if a minute piece of glass or other hard body is kept in contact
with one side of the tip of a root, the growth on the side in contact is
interfered with. Moreover we know from experiments on heliotropism,
thermotropism, etc., that even intangible stimuli such as rays of light,
etc., impinging unsymmetrically on these delicate cells cause
alterations in their behaviour--_e.g._ arrest or acceleration of growth.
Perhaps the most remarkable class of stimulations, however, is that due
to the presence of the entire protoplasmic body of one organism in the
cell of another, each living its own life for the time being, but the
protoplasm of the host cell showing clearly, by its abnormal behaviour,
that the presence of the foreign protoplasm is affecting its physiology.
A simple example is afforded by Zopfs' _Pleotrachelus_, the amoeboid
protoplasmic body of which lives in the hypha of _Pilobolus_, causing it
to swell up like an inflated bladder, in which the parasite then forms
its sporangia. The _Pleotrachelus_ does not kill the _Pilobolus_, but
that its protoplasm alters the metabolic physiology of the latter is
shown by the hypertrophy of the cells, and by the curious fact that it
stimulates the _Pilobolus_ to form its sexual conjugating cells,
otherwise rare, an indication of very far-reaching interference with the
life-actions of the host.
An equally remarkable example is that of _Plasmodiophora_, the amoeboid
naked protoplasm of which lives and creeps about in the protoplasm of a
cell of the root of a turnip, to which it gains access through the
root-hairs. It does not kill the cell, but stimulates its protoplasm to
increased activity and growth and division, itself dividing also and
passing new amoebae into each new daughter-cell of the host. Here the
processes of stimulation, hypertrophy and further division are repeated,
until hundreds or thousands of the turnip root-cells are infected. The
externally visible result is the formation of distorted swellings on the
root (Finger and Toe), most of the cells of which are abnormally large
and filled with amoeboid _Plasmodiophora_ protoplasm, which finally
devours the turnip-protoplasm and itself passes over into spores. Here
we have most convincing proof of the stimulation of protoplasm by other
protoplasm in direct contact with it; and that the metabolism of the
host-cells is profoundly altered is shown not only by the abnormal
growth of the cells, but also by the starvation of the rest of the
turnip plant as the _Plasmodiophora_ gets the upper hand. We have here,
in fact, a local intracellular parasitic disease, gradually invading
large tracts of tissue and eventually inducing general disease resulting
in death--a state of affairs reminding us of cancer in animals.
Irritation and hypertrophy of cells, however, may be induced by
parasites which never bring their protoplasm into direct contact with
that of the host. Many Chytridiaceae penetrate the cells of plants, and
grow inside them as short tubes, vesicles, etc., the protoplasm of which
is separated by their own cell-walls from that of the host-cell;
nevertheless hypertrophy and abnormal cell-divisions and secretions are
induced, and the effect even extends to neighbouring cells--_e.g._
_Synchytrium_--showing that some influence is exerted through cells
themselves not directly affected. This latter point need not surprise us
now we know that the cells of plant-tissues are connected by fine
protoplasmic strands passing through the separating cell-walls.
But the invading plant need not actually enter the cells, and may still
stimulate them through both its own and their own cell-walls to abnormal
growth. This is well shown by the intercellular mycelium of _Exoacus_
and _Exobasidium_, and the latter affords an excellent illustration of
the far-reaching effects of hyphae on the cells (of _Vaccinium_) into
which they do not penetrate. Not only are the cells stimulated to grow
larger and divide oftener than normally, thus producing large gall-like
swellings, but the chlorophyll disappears, the cell sap changes colour
to red, the numerous compound crystals normally found in the tissues
diminish in number and are different in shape, large quantities of
starch are stored up, and even the vascular bundles are altered in
character. All these changes indicate very profound alterations in the
physiological working of the protoplasm of the cells of the host, and
yet the fungus has done its work through both its own cell-walls and
those of the host.
Even harmless endophytic algae in the intercellular spaces of plants may
stimulate the cells in their immediate neighbourhood to increased
growth, _e.g._ _Anabaena_ in the roots of Cycads.
NOTES TO CHAPTER XIII.
With reference to cork-healing and wound-fever the student may
consult Shattock "On the Reparative processes which occur in
Vegetable Tissues," _Journal of the Linnean Society_, 1882,
Vol. XIX., p. 1; and Shattock "On the Fall of Branchlets in
the Aspen," _Journal of Botany_, 1883, Vol. XXI., p. 306. Also
Richards, "The Respiration of Wounded Plants," _Annals of
Botany_, Vol. X., 1896, p. 531; and "The Evolution of Heat by
Wounded Plants," _Ann. of Bot._, Vol. XI., 1897, p. 29.
For details and figures respecting callus, see Sorauer,
_Physiol. of Plants_, p. 175.
In respect to the irritable movements referred to see Darwin,
_The Power of Movements in Plants_, 1880, chapter III. The
recent work of Nawaschin, _Beobachtungen ueber den feineren
Bau u. Umwandlungen von Plasmodiophora_, Flora, Vol. LXXXVI.,
1899, p. 404, should be read for details and literature
concerning "Finger and Toe."
CHAPTER XIV.
NATURE OF DISEASE (_Continued_).
_Actions of poisons in small doses--Results of killing a few
cells--Malformation--Enzymes--Secretions and excretions--
Acids, poisons, etc.--Chemotactic phenomena--Parasitism--
Epiphytes and endophytes--Symbiosis--Galls._
Physiological research has shown that the respiratory activity of cells
may be increased by small doses of poisons, and even that growth may be
accelerated by them--_e.g._ chloroform, ether--and, still more
remarkable, that fermentative activity may be enhanced by minute doses
of such powerful mineral poisons as mercuric chloride, iodine salts,
etc., and that the cells may be gradually accustomed to larger doses
without injury. Unfertilised eggs of insects have been started into
growth by treatment with acids and those of frogs with mercury salts,
and the germination of beans quickened by various poisonous alkaloids.
In other words, graduated doses of poison may alter the physiological
activity of living cells, inducing pathological phenomena, while larger
doses kill them.
Now we know at least one parasitic fungus which poisons the cells of its
host, and kills them, with similar symptoms to those resulting from
excessive doses of the above-named toxic agents. _Botrytis_ hyphæ,
living in the cell-walls of plants, but not entering the cells, excretes
a poison which kills the protoplasm, and the fungus then feeds on the
debris. Numerous other fungi form powerful poisons, but we do not know
whether or how they employ them--_e.g._ Ergot.
It is obvious that if all the young cells of a root-tip or of the apex
of a shoot, or those of a young leaf, are growing and dividing
regularly, the killing of one or a few cells at one point on the side of
the organ must result in irregularities--in malformation--of the adult
organ. This has been proved experimentally by destroying a few cells
with a needle. It can also be done by planting a minute mycelium of
_Botrytis_ laterally on a young organ--_e.g._ a very young lily-bud. The
fungus adheres to the surface, kills a few epidermis cells, and forms a
foxy-red spot, which becomes concave as the dead cells lose water and
dry. Since the rest of the bud goes on growing, however, while this dead
point remains stationary, the latter gradually becomes the centre of a
concavity, the growing tissues having grown round it: the bud is
deformed. Numerous cases of malformed organs are explained in this way;
a minute insect has bitten or pierced the young tissue, or a fungus has
killed a minute area, or a drop of acid condensed from fumes in the air
is the lethal agent, and so forth. And even on a much larger scale we
see the same kinds of agents at work. Wherever a patch of cells is
killed whilst those around go on growing, there must result some
deformation of the resulting organ, since had the injury been withheld
the number and sizes of the cells now fixed in death would have
increased and covered a larger area: they now serve to pull over to
their side the still living and growing cells. The same results follow
on any lateral wound: the killed spot of tissue serves as a point round
which the continued growth of other parts of the organ turns. Hence the
malformation is in these cases a secondary effect, and not, as in simple
hypertrophy, a direct effect of the action of the cells involved in the
injury.
There is another class of bodies secreted by fungi, however, which act
directly on cells, viz. enzymes--that is, soluble bodies which are able
to dissolve cellulose (_cytases_), starch (_diastases_), proteids
(proteolytic enzymes), and other substances, by peculiar alterations in
their constitution. It is by means of its _cytase_ that _Botrytis_
hyphae pierce the cellulose walls of plants, and no doubt in all cases
where fungi pierce cell-walls it is by the solvent action of such a
cytase, and similarly when haustoria penetrate into the cells. It is
also by means of these starch-dissolving enzymes (diastases) and
proteolytic enzymes, etc., that the hyphae inside the cells are enabled
to make use of the starch, proteids, etc., they find there.
All living cells form materials, resulting from the activity of the
protoplasm, which we may compare with the refuse or by-products formed
in any great manufacturing industry: these by-products have to be got
rid of if they are injurious or noisome (_excretions_), and if
not--_i.e._ if they are capable of further use (_secretions_)--they have
to be stored away till required. Some of the most prominent of these
bodies excreted by fungi are, as we have seen, poisonous acids, such as
oxalic acid, enzymes, and organic poisons, such as those in ergot. But
similar enzymes, acids, poisons, etc., to those found in fungi are also
found in the cells of other plants and animals; for only by means of
their solvent actions can processes like digestion and assimilation of
the starchy and other materials into the body-substance be accomplished,
and we have seen that it is a general property of living cells to form
acids, and other excretions and secretions.
Now we know very little about what may happen when an organism--say a
fungus--secreting especially one kind of enzyme or poison or other
active substance, comes into intimate contact with another--say a
leaf-cell--which secretes predominantly others, but what we do know
points to the certainty that various complications will occur.
For instance, if certain bacteria which prefer an alkaline medium, and
yeasts which prefer an acid environment are mixed in a saccharine
solution, it depends on the reaction of the liquid which organism gains
the upper hand: if the liquid is acid the yeast may dominate the
bacteria; if alkaline it may be suppressed by them.
That a parasite may be prevented from successfully attacking a
particular plant is shown by the failure of _Cuscuta_ to establish its
haustoria in poisonous plants such as _Euphorbia_, _Aloe_, etc., and it
has been pointed out that poisonous secretions in the cells of the plant
protect them against the penetration of fungi. This cannot be taken as
meaning that any poison protects against any parasite, however, for
_Euphorbia_ is itself subject to attacks of Uredineae, and _Pangium
edule_, which contains prussic acid and is extremely poisonous to most
animals, is eaten with avidity by several insects, while nematode worms
can live in its tissues. This is no more remarkable, however, than the
fact that _Fontaria_, a myriapod, secretes prussic acid in its own
tissues, or than that certain glands of the stomach secrete free
hydrochloric acid, and _Dolium_ forms sulphuric acid in its glands.
There is yet a further point to notice here. It has been proved that
certain substances formed in plant-cells, not necessarily nutritive,
attract the hyphae of parasitic fungi or repel them, according to the
kind and degree of concentration. So clear has this proof been made that
it was possible in experiments conducted apart from a host plant, to
make the hyphae on one side of an artificial membrane--_e.g._
collodion--penetrate it by placing one of these attractive
(_chemotropic_) substances in suitable proportions on the other side.
The hyphae dissolved holes in the membrane by means of enzymes and
plunged into the attractive substance on the other side.
The foregoing sketch gives us a glimpse into the causes at work in
parasitism.
Suppose a fungus on the outside of the epidermis of a young organ--say a
leaf. It may be unable to penetrate into the plant, and finding no
suitable food outside it dies: or it may be satisfied with the traces of
organic matter on the epidermis and then lives the life of a saprophyte.
Or it may be able to establish a hold-fast on the tender epidermal
surface, but without entering the cells, and irritate the developing
organ by contact stimulation, inducing slight abnormalities; if in its
further, purely superficial growth such an epiphyte covers large areas
of the leaf, and especially if the hyphae are dark coloured--_e.g._
_Dematium_ and other "Sooty Moulds"--injury may be done to the leaf
owing to the shading action which deprives the chlorophyll below of its
full supply of solar energy. Some epiphytes, however, are able to fix
their hyphae to the epidermis by sending minute peg-like projections
into the cuticle--_Trichosphaeria_, _Herpotrichia_--while others send
haustoria right through the outer epidermal walls--_e.g._
_Erysiphe_--and thus supplement mere contact-irritation and shading by
actual absorption from the external cells. Here the fungus is a
parasitic epiphyte.
A stage further is attained in those fungi which enter the stomata and
live in the intercellular spaces--_e.g._ many Uredineae and
_Phytophthora_--and many such intercellular endophytes increase their
attack on the cells by piercing their walls with minute (_Cystopus_) or
large and branched (_Peronospora_) haustoria, or even eventually pierce
the cells and traverse them bodily (_Pythium_). In all these cases it is
clear that conflicts must occur between poison and antidote, acid and
alkali, attractive and repellent substances, enzyme and enzyme, etc., as
was hinted at above; and the same must take place when the parasite is
endophytic and intracellular from the first, as in Chytridiaceae, etc.,
the zoospores of which pierce the outer cell-walls and forthwith grow
into the cells. There are also fungi which, while able to pierce the
outer cell-walls, and grow forward in the thickness of the wall itself,
cannot enter the living cells themselves--_e.g._ _Botrytis_. In the
example mentioned, the fungus excretes a poison, oxalic acid, which
soaks into and kills the cells next its point of attack: into these dead
cells it then extends, and, invigorated by feeding on them, extends into
other cell-walls and excretes more poison, and so on.
On the basis of the foregoing it seems possible to sketch a general view
of the nature of parasitism. In order that a fungus may enter the cells
it must be able to overcome not only the resistance of the cell-walls,
but that of the living protoplasm also: if it cannot do the latter it
must remain outside, as a mere epiphyte, or at most an intercellular
endophyte. If it can do neither it must either content itself with a
saprophytic existence or fail, so far as that particular host-plant is
concerned. Its inability to enter may be due to there being no
chemotropic attraction, or to its incapacity to dissolve the cell-walls,
or to the existence in the cell of some antagonistic substance which
neutralises its acid secretions, destroys its enzymes or poisons, or is
even directly poisonous to it.
Moreover when once inside it does not follow that it can kill the cell.
The protoplasm of the latter may have been unable to prevent the fungus
enemy from breaking through its first line of defence--the cell-wall,
but it may be quite capable of maintaining the fight at close quarters,
and we see signs of the progress of the struggle in hypertrophy,
accumulation of stores, and other changes in the invaded cells and their
contents.
Finally, the invested or invaded cell may so adapt itself to the demands
of the invader that a sort of arrangement is arrived at by which life in
common--_Symbiosis_--is established, each organism doing something for
the other and each taking something from the other. In this latter case,
which is often realised--_e.g._ lichens, leguminous plants and the
organisms in their root-nodules, mycorrhiza, etc.--we leave the domain
of disease, which supervenes indeed if the other symbiont is lacking.
Some interesting facts bearing on the matters here under discussion,
have been obtained from the study of _Galls_, the curious outgrowths
found on many plants and due to the action of insects.
A typical gall exhibits three distinct and characteristic layers of
tissue surrounding the hollow chamber in which the larva of the insect
lies, viz., an outer layer of soft cells forming a parenchyma covered
with an epidermis, and frequently also with a layer of cork; an inner
stratum consisting of very thin-walled delicate cells filled with
protoplasmic and reserve food-materials on which the larva feeds; and
between the two a more or less definite layer of thick-walled
sclerenchyma cells which serve as a protection against accidents to the
larva as the outer layer shrivels or rots, or if it is exposed to the
attack of marauders. This layer may be absent from galls which have a
short life only. Vascular bundles run into the outer layer from the
leaf-veins or the stele of the shoot, etc. Such galls abound in tannin,
and are frequently of use in the arts on this account: they also contain
starch, and proteid substances and crystals of calcium oxalate. When the
larva has consumed the stores of food material and reached the adult
stage it eats its way out and escapes.
The growth of such a gall is preceded by the laying of an egg on or in
the embryonic tissue of a leaf, stem, or other young part, and it is
interesting to note that only organs in the meristematic stage can form
galls, and that it is by no means necessary that the tissues should be
wounded. Moreover, the egg as such is incapable of stimulating the plant
tissues, but when it hatches, the resulting larva, beginning to feed on
the cells, irritates the tissues and rapid growth and cell-division
occur, as in the case of other wounds or of fungus attacks. The actual
wound made by the ovipositor heals up at once. It is evident from
numerous recent researches that these true galls are not due to any
poisonous or irritating liquid injected by the parent, but that the
stimulus to the tissue formation is similar to that exerted by a wound.
The young gall is in fact a callus enclosing the living larva, and it is
the continued irritation of the latter which keeps up the stimulation.
The final shape and constitution of the gall depend on mutual
reactions--not as yet explained in detail--between the species of plant
and the species of gall-insect concerned, as may readily be seen from
the extraordinary variations in size, shape, colouring, hairiness and
other structural peculiarities of the galls on one species of, for
instance, the common oak. From what we have learnt about fungus
parasites, however, there can be little doubt that reactions between the
cells and the larva of the insect occur, resembling those which take
place between the cells and the hyphae of the fungus, and this is borne
out by the study of other hypertrophies due to animals; _e.g._ Nematode
worms in roots, and the remarkable galls--the simplest known--on
_Vaucheria_, caused by the entrance into this alga of a species of
_Notommata_, which induces a different gall on each of the various
species of its host plants.
It must be concluded that the formation of the _Vaucheria_ gall is
induced by the mechanical irritation which the Rotifer causes in the
protoplasm. These galls are comparable to the hypertrophies in
_Pilobolus_ caused by the presence of _Pleotrachelus_.
Attempts to induce the development of galls artificially by injecting
formic, acetic and other vegetable acids, poisons and other substances
into the tissues have, however, failed, and even the substances
contained in the insect or gall itself only produced negative results.
Nothing further was obtained than slight callus formations in some
cases. Nor have experimenters succeeded in obtaining more than slight
distortions by fixing insects on the growing leaves in such positions
that they could scratch the epidermis.
We must therefore conclude that very complex interactions between the
plant and insect are here concerned, among which may be the infiltration
of some liquid from larva to plant--many of these gall larvae are
strongly scented, and Kustenmacher says that fluids excreted by the
larva are absorbed by the gall-tissue apparently as nutriment. This
would point to the symbiotic character of galls and their guests.
NOTES TO CHAPTER XIV.
With regard to the action of poisons in small doses see
further Johannsen, _Das Aether-Verfahren beim Fruhtreiben_,
Jena, 1900, and, for _Botrytis_, see Marshall Ward, "A Lily
Disease," _Annals of Botany_, Vol. II., 1889, p. 388.
The subject of enzymes has been exhaustively treated by Green,
_The Soluble Ferments and Fermentations_, Cambridge, 1899, to
which the reader is referred for literature. I have taken the
statements regarding _Fontaria_ and _Dolium_ from Kassowitz,
_Allgemeine Biologie_, p. 182. The two most important works on
chemotactic phenomena are Pfeffer, "Uber Chemotaktische
Bewegungen," etc., _Unters. aus dem Bot. Inst. zu Tubingen_,
B. II., p. 582, and Miyoshi, "Die Durchbohrung von Membranen
durch Pilzfaden," _Pringsh. Jahrb. f. Wiss. Bot._, B. XXVIII.,
1895, p. 269, and from these the further literature can be
traced. As regards the nature of parasitism see Marshall Ward,
"On Some Relations between Host and Parasite," etc., being the
Croonian Lecture delivered before the Royal Society, _Proc.
Roy. Soc._, Vol. 47, p. 393. On Symbiosis, see Marshall Ward,
"Symbiosis," _Annals of Botany_, 1899, Vol. XIII., p. 549,
where the literature is collected. For a general account of
galls the reader may consult Kerner, _The Natural History of
Plants_, Eng. ed., 1895, Vol. II., pp. 527-554, and Adler,
_Alternating Generations, A Biological Study of Oak Galls_,
etc., 1894.
CHAPTER XV.
SPREADING OF DISEASE AND EPIDEMICS.
_Dissemination of fungi by the aid of snails, rabbits, bees,
and insects--Man--Distribution in soil, on clothes, through
the post, etc.--Worms, wind--Puffing of spores--Creeping of
mycelia--Lurking parasites--Spread of insects and other
animals--Losses due to epidemics._
The dissemination of plant diseases is a subject which has been far too
much neglected, but our knowledge of it is slowly increasing. The spores
of fungi such as Rusts and Erysipheae are often carried from plant to
plant by snails; those of root-destroying and tree-killing Polyporei by
rabbits, rats, and other mammals which rub their fur against the
hymenophores. Bees have been shown to carry the spores of _Sclerotinia_
and infect the stigmas of Bilberries, etc., with them; and flies convey
the conidia of Ergot from grain to grain. Insects, indeed, of all kinds
are great disseminators of disease--as witness also the part played by
mosquitoes in transferring the malaria parasite to man--and beetles,
bees, flies, etc., of all sorts probably play more active parts in this
work than has yet been proved, since they not only carry spores attached
like pollen to their hairy bodies, but in many cases in their alimentary
canal, to be spread later in the dung.
The part played by man in conveying fungi from plant to plant counts for
much. Not only do gardeners and farm labourers carry spores on their
boots and clothes as they pass from infected to non-infected areas, but
carted soil and manure are frequently infested with spores of Smuts,
_Fusarium_, _Polyporus_, and the sclerotia or rhizomorphs of
_Sclerotinia_, _Agaricus melleus_, _Dematophora_, etc. Man also sends
diseases through the post, and by rail and ship, by spores or mycelia
attached to seedlings, bulbs, fruits, flowers, etc., as shown in several
cases of potato, vine, hollyhock, lily, and hyacinth diseases. Every
time a carpenter saws a piece of fresh timber with the saw which has
been used previously for cutting wood attacked with dry rot, he risks
infecting it with the fungus. Similarly in pruning: every cut with a
knife which the gardener has used on infected branches may infect the
tree.
Cuttings made with a soil-contaminated knife and stuck into ordinary
soil in dirty boxes covered with equally dirty glass, present every
chance for infection by soil organisms; bacteria and fungi obtain access
to the vessels, and derive plenty of food from the juices, and the
wonder is not that so many cuttings "damp off," but that any are raised
at all under ordinary conditions.
That worms bring buried spores to the surface can hardly be doubted
after Pasteur's experiments with Anthrax, and the principle of Darwin's
discoveries of the important bearing of the habits of earthworms on this
subject, and that the soil attached to the feet of ducks and other birds
teems with small seeds, applies to fungi also. Wind is also responsible
for distributing fungus-spores over wide areas, as may be easily proved
by fixing a glass slide smeared with glycerine in the course of a breeze
passing over an infected area.
But although the fungi are, generally speaking, passive in regard to
their distribution, such is by no means always the case. Apart from the
fact that some forms attract insects by means of honey dew (Ergot), or
by sweet odours (Spermogonia, _Sclerotinia_), the zoospores of
_Pythium_, _Phytophthora_, etc., are motile, and although they cannot
move far in the films of water in which they travel, nevertheless in a
wet potato field, with the wind flapping the leaves one against the
other, some dissemination of importance must be actively brought about,
and similarly with the amoebae of _Plasmodiophora_ in the soil.
The shooting of ascospores into the air by certain species of _Peziza_,
from the discs of which the spores may be seen to puff out in clouds,
affords further evidence that fungi cannot be regarded as entirely
passive in respect to distribution of their spores. But when we come to
certain of the soil fungi--_e.g._ _Agaricus melleus_, _Dematophora_,
etc.--the active creeping forward by growth in the soil of their
rhizomorphs and mycelial strands afford examples of active spreading of
considerable importance in the vineyard and forest, since they pass from
root to root and from tree to tree and may infect the entire area in
course of time.
Not the least significant mode of dissemination is that by which what I
have termed "lurking parasites" are spread: such are fungi which attach
themselves to the seeds, fruits, tubers, etc., of other plants and so
obtain all the advantages of being carried and sown with the
latter--_e.g._ Ustilagineae and Uredineae which adhere to grain,
_Verticillium_, _Nectria_, etc., in potatoes and other plants.
The spread of diseases due to animals, especially insects, is of course
more active, in consequence of the motility of the distributing agents.
This is most marked in the winged species, of which locusts, beetles,
moths and butterflies, flies and wasps furnish well-known examples; and
is not inconsiderable in the case of wingless and merely creeping
species. It is noteworthy that many forms wingless in the parasitic
stage are winged at certain periods, _e.g._ the females of _Phylloxera_.
That man also spreads insect pests is well known and acted upon, as
witness the phylloxera laws--which, however, it is to be feared too
often only illustrate once more the adage concerning the shutting of the
stable door after the horse has gone.
It would be tedious to attempt anything like a complete account of the
estimates of loss in different countries, due to the ravages of insects
and fungi, but the following examples should surely serve to convince
anyone of the magnitude of these losses and of the economic importance
of the whole question, and the reader may be referred to the special
literature for further details.
The coffee leaf-disease of Ceylon, due to the fungus _Hemileia_, is
estimated to have cost that Colony considerably over £1,000,000 per
annum for several years. One estimate puts the loss in ten years at from
£12,000,000 to £15,000,000. The hop-aphis is estimated to have cost Kent
£2,700,000 in the year 1882. In 1874 the Agricultural Commissioner of
the United States estimated the annual loss, due to the ravages of
insects on cotton alone, to amount to £5,000,000; and in 1882 the annual
loss to the United States due to insects, calculated for all kinds of
agricultural produce, was put at the appalling figure of from
£40,000,000 to £60,000,000 sterling. In India, the annual loss due to
wheat-rust alone has recently been estimated at 4,000,000 to 20,000,000
rupees, and one insect alone is said to have cost the cotton planters a
quarter of the crop--valued at seven crores of rupees--in bad years.
Similarly, in Australia the annual loss from wheat-rust has been put at
from £2,000,000 to £3,000,000. In 1891 the loss in Prussia alone from
grain-rusts was officially estimated at over £20,000,000 sterling. Need
more be said? Even allowing for considerable exaggerations in such
estimates it is clear that the damage to crops in any country soon
amounts to sums which even at low rates of interest would easily yield
incomes capable of supporting the best equipped laboratories and staffs
for investigations directed to the explanation of the phenomena in
detail, the sole basis on which intelligent preventive and therapeutic
measures can be based. But it is far from likely that the estimates are
exaggerated. The planting and agricultural communities are as a rule
opposed to the publication of statistics--or at least have been so in
various countries and at different times--and if we knew the damage done
to all crops even in our own Empire, the results would probably astonish
us far more than the above figures have done.
NOTES TO CHAPTER XV.
On the dissemination of fungi, the reader will find Fulton,
"Dispersal of the Spores of Fungi by the Agency of Insects,"
_Ann. Bot._, Vol. III., 1889, p. 207, and Sturgis, "On Some
Aspects of Vegetable Pathology and the Conditions which
Influence the Dissemination of Plant Diseases," _Botanical
Gazette_, Vol. XXV., 1898, p. 187, both useful papers. Further
information will be found in Zopf, _Die Pilze_, Breslau, 1890,
pp. 79-95 and 228, and Wagner, "Ueber die Verbreitung der
Pilze durch Schnecken," in _Zeitschr. f. Pflanzen Krankh._,
1896, p. 144. The estimates as to losses due to epidemics are
taken from Watt, _Agricultural Ledger_, Calcutta, 1895, p. 71;
Balfour, _The Agricultural Pests of India_, London, 1887,
pp. 13-15; Eriksson and Henning, _Die Getreideroste_; the
publications of the U.S. Department of Agriculture, _The
Kew Bulletin_, and elsewhere. The reader will find further
examples in Massee, _Text-Book of Plant Diseases_, 1899, pp.
47-51. Both these subjects are well worth further attention,
and I know of no complete account of them.
CHAPTER XVI.
THE FACTORS OF AN EPIDEMIC.
_Illustrations afforded by the potato disease--The larch
disease--The phylloxera of the vine._
When we come to enquire into what circumstances bring about those severe
and apparently sudden attacks on our crops, orchards, gardens, and
forests by hosts of some particular parasite, bringing about all the
dreaded features of an epidemic disease, we soon discover the existence
of a series of complex problems of intertwined relationships between one
organism and another, and between both and the non-living environment,
which fully justify the caution already given against concluding that
any cause of disease can be a single agent working alone.
The statement of prophecy that a particular insect or fungus need not be
feared, because it is found to do so little harm in particular cases or
districts examined, will thus be seen to be a dangerous one: any pest
may become epidemic if the conditions favour it!
In 1844 and 1845 the potato disease assumed an epidemic character so
appalling in its effects that it is no exaggeration to say that it
constituted a national disaster in several countries. It was stated at
the time that this disease had been known for some time in Belgium, in
Canada and the United States, in Ireland, in the Isle of Thanet, and in
other parts of the world. Similar, but less devastating epidemics have
occurred in various years since. It was generally noticed during such
epidemics that the plants themselves were full of foliage, surcharged
with moisture, and of a luxuriant green colour promising abundant
crops. The now well-known spots, at first pale and then brown and
fringed with a whitish mould-like growth--the conidiophores of the
_Phytophthora_--were observed during the dull cloudy and wet weather,
cooler than usual, when the atmosphere was saturated for days together,
in July and August. The actual amount of rain does not appear to have
been excessive, but most observers seem to agree that dull weather with
moist air had succeeded a warm forcing period of growth. So rapidly did
the disease run its course that in a few days nearly all the plants were
a rotting blackened mass in the fields, and the potatoes dug up
afterwards were either already rotten or soon became so in the stores.
Further experience has confirmed this, and we now know that the epidemic
is very apt to appear in any region where potatoes are grown on a large
scale, in dull moist weather, especially in fields exposed to mists,
heavy dews, etc., about July and August, when the foliage is full and
turgid. Similarly on heavy wet soils, unless the season is remarkably
open and dry; but also on dry light soils in rainy seasons. So evident
was this that many believed that the mists and dew brought the
disease--harking back to the superstitions of earlier days. We must
remember that prior to 1860 the life-history of _Phytophthora_ was not
known. Since De Bary's proof of the germination of the zoospores and of
the infection of the leaves, the course of the hyphae in them and in the
haulms, the origin of the conidia, etc., and the confirmation by
numerous competent observers of the true fungus nature of this disease,
we are now in a position to understand the principal factors of the
various epidemics of potato disease.
It is not merely that the potato-fields afford plenty of food for the
fungus, and that the dull weather causes the tissues to be surcharged
with moisture, owing to diminished transpiration, but the mists and
dew--to say nothing of actual rain and the flapping of wet
leaves--favour the germination and spread of the zoospores throughout
the field. Whether the dull light also favours the accumulation of
sugars in the tissues, and the partial etiolation of the latter implies
less resistance to the entering hyphae, may be passed over here, but in
any case it is clear that we have several factors of the non-living
environment here favouring the parasite and not improving the chances
of the host, even if they do not directly disfavour it.
As another instance I will take the Larch-disease, which is due to the
ravages of a Peziza (_Dasyscypha Willkommii_) the hyphae of which obtain
access by wounds to the sieve-tubes and cambium of the stem, and
gradually kill them over a larger and larger area and so ring the tree,
with the symptoms of canker described below.
Now the Larch fungus is also to be found on trees in their Alpine home,
but there it does very little damage and never becomes epidemic except
in certain sheltered regions near lakes and in other damp situations.
How then are we to explain the extensive ravages of the Larch disease
over the whole of Europe during the latter half of this century? The
extensive planting, providing large supplies for the fungus, does not
suffice to explain it, because there are large areas of pure Larch in
the Alps which do not suffer.
In its mountain home the Larch loses its leaves in September and remains
quiescent through the intensely cold winter, until May. Then come the
short spring and rapid passage to summer, and the Larch buds open with
remarkable celerity when they do begin--_i.e._ when the roots are
thoroughly awakened to activity. Hence the tender period of young
foliage is reduced to a minimum, and any agencies which can only injure
the young leaves and shoots in the tender stage must do their work in a
few days, or the opportunity is gone, and the tree passes forthwith
into its summer state.
In the plains, on the contrary, the Larch begins to open at varying
dates from March to May, and during the tardy spring encounters all
kinds of vicissitudes in the way of frosts and cold winds following on
warm days which have started the root-action--for we must bear in mind
that the roots are more easily awakened after our warmer winters than is
safe for the tree.
It amounts to this, therefore, that in the plains the long continued
period of foliation allows insects, frost, winds, etc., some six weeks
or two months in which to injure the slowly sprouting tender shoots,
whereas in the mountain heights they have only a fortnight or so in
which to do such damage. That the lower altitude and longer summer are
not in themselves inimical to Larch is proved by the splendid growths
made by the trees first planted a century ago. Then came the epidemic of
Larch-disease: the fungus, which is merely endemic--_i.e._ obtains a
livelihood here and there on odd trees, or groups of trees in warmer or
damper nooks--in the Alps, was favoured by the more numerous points of
attack afforded to its spores by injuries due to insects--_Coleophora_,
_Chermes_, etc.--and frost wounds, as well as by the longer periods of
moist dull weather, and the longer season of foliation. Moreover, as
time went on almost every consignment of young Larch-trees sent abroad
was already infected. Here again, then, we find the factors of an
epidemic consisting in events which favour the reproduction and spread
of a fungus more than they do the well-being of the host.
As a third illustration I will take the case of an insect epidemic. In
1863 a disease was observed on vines in the South of France which
frightened the growers as they realised its destructive effects: the
roots decayed and the leaves turned yellow and died before the grapes
ripened, and such vines threw out fewer and feebler shoots the following
year, and often none at all afterwards. In 1865 the disease was
evidently becoming epidemic near Bordeaux, and in 1868 it was shown to
be due to an insect, _Phylloxera_, the female of which lays its eggs on
the roots, where they hatch. The louse-like offspring sticks its
proboscis into the tissues as far as the central cylinder. The irritated
pericycle and cortex then grow and form nodules of soft juicy
root-tissue at which the insect continues to suck. Rapid reproduction
results in the majority of the young rootlets being thus attacked, and
since they cannot form their normal periderm and harden off properly
they rot, and admit fungi and other evils, in consequence of which the
vine suffers also in the parts above ground.
Evidence that the general damage is due to the diminished root-action is
found in the peculiarly dry poor wood formed in the "canes" of diseased
plants.
By 1877 the epidemic had spread to the northern limits of the French
vineyards, and by 1888 half the vines in the country were attacked, and
the yield of wine reduced from half a million hectolitres to 50,000
only. Meanwhile the disease had spread to Italy, Germany, Madeira,
Portugal, and even to the Cape, though not in epidemic form as in the
Bordeaux centre whence it spread.
Now it appears that _Phylloxera_ has long been in the habit of doing
damage to vines in America, where, however, it attacks the leaves, on
which it makes pocket-like galls, rather than the roots. Moreover, there
are species and varieties of American vines which, even when planted in
Europe, do not suffer at all from this insect at the roots, either
because the rootlets do not push out at the same season as those of the
European form, or because they form wood more rapidly and completely, or
secrete resinous and other matters distasteful to the insect in greater
quantity and are thus capable of healing the wounds, or in some other
way they do not respond to the attack or suit the insect. In any case
the attack on the leaf rather than the root seems to be the exception in
European vineyards and the rule in American species, and we appear to be
face to face with a problem of specific predisposition to this
particular malady. That the resistant properties of the vines of
America--not all, only particular species and varieties are thus
"immune"--can be utilised has been proved by European growers; and not
only so, for Millardet and others have shown that the European vine
grafted on to these resistant stocks suffer less than when on their own
roots. It has also been shown that hybrids can be obtained which are
resistant.
But the most curious point of all is that _Phylloxera_ was itself a
native of America, and came thence to Europe. It had played its part
with certain fungi in ruining all the attempts to introduce the European
vine into America many years ago. A recent authority on the evolution of
American fruits writes as follows:
"All the most amenable types of grapes had long since perished in the
struggle for existence, and the types which now persist are necessarily
those which are, from their very make-up or constitution, almost immune
from injury, or are least liable to attack . . . the _Phylloxera_ finds
tough rations on the hard, cord-like roots of any of our eastern species
of grapes. But an unnaturalised and unsophisticated foreigner, being
unused to the enemy and undefended, falls a ready victim; or if the
enemy is transported to a foreign country the same thing occurs."
Further proof that it is in the "constitution" of the European vine that
the want of resistance to _Phylloxera_ resides, is furnished by the fact
that in California and the Pacific states the European vine was
introduced with more success, but is now suffering badly because
_Phylloxera_ has spread there also. It must not be overlooked, however,
that we are as yet very ignorant of all that is implied in the word
"constitution" as used above.
If we enquire further why the _Phylloxera_ epidemic was so much worse
in the Southern vineyards than in the more Northern ones of Germany, the
opinion seems to prevail that the warmer climates favour the insect.
Further, it appears that, in Italy, the vines in loose open soil,
provided it is equally rich in mineral food-materials and offers no
disadvantages as regards drainage, suffer less than those in closer
soils, the reasons alleged being that the young roots can push out more
rapidly and widely, and so obtain holdfasts with greater distances
between them.
NOTES TO CHAPTER XVI.
The student may obtain further information on the history of
the Potato disease by consulting the following: Berkeley,
"Observations, Botanical and Physiological, on the Potato
Murrain," _Journal of the Horticultural Society_, Vol. I.,
1846, p. 9; De Bary, _Die Gegenwärtig herrschende Kartoffel
Krankheit_, etc., Leipzic, 1861; and the pages of the
_Gardeners' Chronicle_ from 1860-1900.
For the Larch disease he should consult Hartig, _Unters. aus
der Foist. Botanischen Inst. München_, B. I., 1880; and
Willkomm, _Microscop. Feinde des Waldes_, B. II., 1868.
For _Phylloxera_ the literature is chiefly in the _Comptes
Rendus_ and other French publications since 1875, and in the
Reports of the U.S. Dept. of Agriculture.
For a summary of the facts concerning the life-histories of
the parasites referred to above, see Frank, _Krankheiten der
Pflanzen_, and Marshall Ward, _Diseases of Plants_, p. 59, and
_Timber and Some of its Diseases_, London, 1889, chapter X.
Also Marshall Ward, "On some Relations between Host and
Parasite in certain epidemic Diseases of Plants," _Proc. Roy.
Soc._, Vol. XLVII., 1890, pp. 393-443; and "Illustrations of
the Structure and Life-history of Phytophthora infestans,"
_Quart. Journ. Microsc. Soc._, Vol. XXVII., 1887, p. 413; also
Marshall Ward, "Researches on the Life-history of Hemileia
vastratrix," _Journ. Linn. Soc._, Vol. XIX., 1882, p. 299; and
"On the Morphology of Hemileia vastatrix," _Quart. Journ.
Microsc. Soc._, 1881, Vol. XXI., p. 1.
CHAPTER XVII.
REMEDIAL MEASURES.
_Preventible diseases--The principles of therapeutics--Powders
and their application--Spraying with liquids--Nature of
chemicals employed--Employment of epidemics and natural
checks--The struggle for existence._
It may be said that in no connection is the proverb "Prevention is
better than cure" more applicable than with this subject, and
undoubtedly the best utilitarian argument that can be used in favour of
a thorough study of the causes of disease is that only by understanding
these causes is there any hope of avoiding the exposure of crops, garden
plants, forest trees, etc., to the attacks of preventible diseases.
Moreover, only an intelligent appreciation of the causes of a disease
will enable the cultivator to take steps to mitigate their effects when
once the damage has begun its course. Every cultivator learns by
experience or by precept that there are some things he must avoid in
dealing with certain plants, or otherwise they will not succeed; in
other words they will succumb to diseased conditions and die. It is
partly owing to the want of systematisation of this knowledge, and its
extension in other directions, that such extraordinary blunders are made
in ignorant practice, and trees for instance are planted in low-lying
frost beds which would succeed in slightly higher situations, or seeds
subject to damping-off are sown in beds rife with the spores of
_Peronospora_ or _Pythium_, and so forth.
Many diseases, however, are not preventible in the present state of our
knowledge, or prevailing conditions are such that the risk must be run
of endemic diseases gradually becoming epidemic, and thus the natural
desire for some means of checking the ravages of some pest or another
has led to innumerable trials to minimise the effects by prophylactic
measures. The procedure almost invariably followed where parasites are
concerned, consists in either dusting the plants with some chemical in
the form of a powder, or spraying it with a liquid, or occasionally in
enveloping the plant in some gas, in each case poisonous to the insect-
or fungus-pest. The principal rules to be observed are: (1) the poison
employed must be sufficiently strong or concentrated to kill the
parasite, but not sufficiently powerful to injure the host; (2) it must
be applied at the right period, as suggested by a knowledge of the
life-history of the fungus or insect in question.
Obviously it is of no use to apply such topical remedies to a parasite
while it is spending the greater part of its life inside the tissues of
the host. Further, questions of expense of the materials employed and of
the labour of applying them help to limit the adoption of such measures.
Among the various kinds of powders employed, finely divided sulphur, or
a mixture of sulphur and lime, have been used with success in some
cases--_e.g._ against Hop mildew and other epiphytic Erysipheae, and
against red spider, aphides, etc., the gaseous sulphur dioxide evolved
being the efficacious agent. In other cases pyrethrum or tobacco powder,
wood ashes, etc., have been employed against insects. Such powders are
applied by hand or by means of bellows, and are very easily manipulated
in most cases, though, like all such applications, the dangers of
concentration at particular spots owing to uneven distribution, or of
dilution and washing off by rain, have to be incurred.
Far more numerous are the various liquids which have been employed for
washing, spraying, or steeping the affected parts of diseased plants.
Water alone, or aqueous decoctions or emulsions of various
kinds--_e.g._, quassia, tobacco, soap, or aloes, have been widely
employed against insects such as green fly, red spider, etc. In
greenhouses, where the leaves can be washed by hand or thoroughly
syringed, and the concentration and time of action thoroughly
controlled, such liquids are often serviceable, but great practical
difficulties are apt to interfere with their use in the open field.
The principal liquids employed against fungi have been copper sulphate
and other metallic compounds (Bordeaux mixture, Eau Céleste, etc.),
various compounds of arsenic (_e.g._ "Paris green"), potassium sulphite,
permanganate, etc., and emulsions of carbolic acid, petroleum, and such
like antiseptics, for the exact composition of which the special
treatises must be consulted. Some of these, especially Bordeaux mixture,
have been experimented with on a very large scale, especially in
America, and various forms of spraying machines have been introduced for
dealing with large areas.
It is clear that these spraying operations are more particularly adapted
to field crops such as Turnips, Hops, Vines, Potatoes, and to garden and
greenhouse plants than to woods and plantations; as a rule they cannot
be applied to forest trees--though they have been used in orchards--or
to roots, seeds, and other parts in the soil, and many special forms of
treatment have been devised for particular cases of these kinds.
One of the oldest of these is the steeping of grain in solutions of
copper, or in hot water, just before sowing, and the practical
eradication of Bunt and, partially, of Smut is due to this practice,
which has lately been adapted to potatoes, the principle being that the
parasitic germs shall be killed while still adhering to the outside of
the seeds, tubers, etc., before germination. "Finger and Toe" due to
_Plasmodiophora_ has been successfully dealt with by the application of
lime, but we do not know whether the effect is owing to indirect actions
in the soil, to direct actions on the plasmodia, or to the increased
production of root-hairs induced by liming.
_Phylloxera_ has been treated by plunging into the soil near the roots
small blocks of some slowly-soluble medium, such as gelatine,
impregnated with carbon-bisulphide, the volatile fumes of which kill the
insect, and even more drastic remedies have been tried along similar
lines. In America orchard trees infested with insects or fungi have been
covered one by one with light tents, and the vapours of prussic acid,
burning sulphur, and other poisons allowed to act inside the tent. In
all such cases it must be remembered that uncontrolled ignorance of the
properties of poisons on the part of the operator may lead to disaster,
and the same applies to the much easier treatment of greenhouses, and
cases where poisoned food is laid about for insects or vermin.
Attempts, not altogether unsuccessful on the small scale, have also been
made to introduce epidemic diseases among rats, mice, and locusts and
other insects, by inoculating some of them with parasitic bacteria or
fungi (_Empusa_, _Isaria_, etc.), and then allowing them to run loose in
the hope that they will communicate the disease to their fellows. The
introduction of lady-birds into districts infested with Coccideae and
similar pests which they devour, is also recorded as successful, as also
the importation of birds into forests plagued with caterpillars. It must
not be over-looked, however, that man's interference with the existing
balance of events in the natural struggle for existence is occasionally
disastrous, as witness the results of importing rabbits into Australia,
goats into the Canary Islands, and sparrows in various countries.
Darwin's well-known illustration of the inter-relations between clover,
bees, field-mice, and cats (_Orig. of Species_, 6th ed., 1876, p. 57),
which shows the astounding probability of the dependence of such a plant
on the number of cats in the neighbourhood, well illustrates the
situation.
Mere mention must be made of other special treatments.
Caterpillars and larger animals are often picked by hand or their
natural enemies--_e.g._ birds, are encouraged in forests. Locusts are
caught in nets, trenches, etc., and buried. Woodlice, slugs, etc., are
often trapped by laying attractive food such as carrots and overhauling
the traps daily: similarly with earwigs. Rings of tar round tree stems
have been employed to prevent caterpillars creeping up them.
American Blight has been treated by rapidly flaming the stems. Syringing
with hot water has also been employed for vines affected with mildew,
mealy bug, etc.
With regard to the alleged immunity from devouring insects of certain
poisonous plants, it has been pointed out that _Pangium edule_, which
abounds in prussic acid, is infested with a grub, and ivy is
occasionally eaten by caterpillars.
Another point as regards insect pests is the well-known destructive
effect of a cold, wet spring on the young larvae. The use of cyanide of
potassium requires especial care, but has been described as easily
carried out with success in greenhouses.
It seems probable that lady-birds, the larvae of wasp-flies and
lace-wings, and ichneumon-flies as well as wrens can keep down aphides.
For an example of the treatment of a complex case of "chlorosis" with
mineral manures, the reader may consult the _Gardeners' Chronicle_, 1899
(July), p. 405. Many similar cases have been recorded, but it should not
be overlooked that very complex inter-relations are here involved.
Charlock has been successfully dealt with by applying 5 lbs. of copper
sulphate in 25 gallons of water to each acre of land while the weeds are
young.
In all these cases the guiding idea is derived from accurate knowledge
of the habits of the insect, fungus, or pest concerned, and obviously
the procedure must be timed accordingly. It is a particular case of the
struggle for existence, where man steps in as a third and (so to speak)
unexpected living agent.
It is clear from our study of the factors of an epidemic that one of the
primary conditions which favour the spread of any disease is provided
by growing any crop continuously in "pure culture" over large areas.
This is sufficiently exemplified by the disastrous spread of such
diseases as Wheat-rust, Larch-disease, Potato-disease, Phylloxera,
Hop-disease, Sugar-cane disease, Coffee-leaf disease, and numerous other
maladies which have now become historic in agricultural, planting, and
forest annals. Providing the favourite food-supply in large quantities
is not the only factor of an epidemic, but it is a most important one in
that it not only facilitates the growth and reproduction of a pest, but
affords it every opportunity of spreading rapidly and widely.
Moreover, Nature herself shows us that such pests are kept in check in
her domain by the struggle for existence entailed by innumerable
barriers and competitors. As matter of experience also it is found that
rotation of crops, planting forests of mixed species, and breaking up
large areas of cultivation into plantations, fields, etc., of different
species afford natural and often efficient checks to the ravages of
fungus and insect pests. Over and over again it has been found that a
fungus or an insect which is merely endemic so long as it is isolated in
the forest, where its host is separated from other plants of the same
species by other plants which it cannot attack, becomes epidemic when
let loose on the continuous acres so beloved of the planter. And the
same reasoning applies to the success of such pests on open areas from
which the birds or other enemies of the pest have been driven. True, we
cannot always trace the tangled skein of inter-relationships between one
organism and another in Nature: the recognition of the principle of
natural selection and the struggle for existence is too recent, and our
studies of natural history as yet too imperfect to lay all the factors
clear, but no observant and thoughtful man can avoid the truth of the
general principle here laid down. The history of all great planting
enterprises teaches us that he who undertakes to cultivate any plant
continuously in open culture over large areas must run the risk of
epidemics.
NOTES TO CHAPTER XVII.
The principal literature, now very voluminous, on this subject
is contained in the publications of the U.S. Department of
Agriculture from 1890 onwards. See especially _Bulletins_,
Nos. 3, 6, and 9; _Farmers' Bulletin_, No. 91, 1899; and _The
Journal of Mycology_ during the same period. See also Lodeman,
_The Spraying of Plants_, London, 1896. A summary of the
principal processes will be found in Massee, _Text-Book of
Plant Diseases_, pp. 31-47.
With regard to the history of the subject, which still needs
writing, the reader should not overlook Roberts, "On the
Therapeutical Action of Sulphur," _St. George's Hospital
Reports_, date unknown, but subsequent to the following:
Berkeley, _Introduction to Cryptogamic Botany_, 1857, p. 277,
with references. These are, I believe, with the references to
steeping of wheat in De Bary, _Unters. über d. Brandpilze_,
Berlin, 1853, among the first attempts to utilise such
remedies.
Further facts will be found in the pages of the _Gardeners'
Chronicle_, especially since 1890, and in _Zeitsch. f.
Pflanzen-krankheiten_ since 1891.
CHAPTER XVIII.
VARIATION AND DISEASE.
_Predisposition and immunity--Pathological conditions
vary--Hardy varieties--"Disease-proof" varieties--Disease
dodging--Thick skins--Indian wheats, etc. Cell-contents
vary--Citrus, Cinchona, Almonds, etc. Double ideals in
selection--Cultivation of pest and host-plant--Variations of
fungi--Bacteria--Specialised races--Difficulties--Experiment
only will solve the problems._
The numerous and often expensive failures in the application of any
prophylactic treatment, have proved an acute stimulus to the research
for other ways of combating the ravages of plant diseases. It is a
matter of every-day experience that particular varieties of cultivated
plants may suffer less from a given disease than others in the same
district; also that one and the same species may suffer badly in one
country and not in another--_e.g._ the Larch in the lowlands of Europe
as contrasted with the same tree in its Alpine home, and the various
species of American Vines in Europe.
These matters, in the hands of astute observers, are turning the
attention of cultivators and experts to new aspects of the question of
plant diseases, namely, the possible existence of immunity, and the
breeding of disease-proof varieties; and the existence on the part of
the host plant of predispositions to disease which may depend on some
factors in the plant or in the environment over which it is possible to
exercise control, or which, if known, can be avoided.
The matter is complicated by the recent demonstration of the fact that
parasites also vary and can adapt themselves to altered conditions, as
is shown by the history of the coffee-leaf disease (_Hemileia_) in
Ceylon, and by Eriksson's results with Wheat-rusts (_Puccinia_) and
various experiments with _Coleosporium_ and other Uredineae; but there
are good grounds for concluding that hybridisation, grafting, and
selection of varieties may do much towards the establishment of races
which will resist particular diseases, as shown by Millardet's
experiments with Vines, and the results obtained by Cobb and others with
Wheat.
The great difficulty with so-called "disease-proof varieties" is to test
them under similar conditions in different countries, and for a
sufficient period of time. A particular race of Wheat may behave very
differently in Norfolk, Devonshire, and Northumberland, and the recent
introduction of the purely experimental method in this connection is a
marked advance. However rough the experiments may of necessity have to
be, it is only by such means that data can be gradually accumulated.
Having now obtained some insight into the factors concerned in disease,
let us enquire further into the bearing of variation on these. It is
evident that pathological conditions may vary; indeed they are
themselves symptoms of variation, as we have seen. The history of all
our cultivated plants shows abundantly that many of the variations
obtained by breeding in our gardens, orchards, fields, etc., involve
differences of response on the part of the plant to the very agencies
which induce disease. Every year the florists' catalogues offer new
"hardy" varieties; but a hardy variety is simply, for our present
purpose, one which succumbs less readily to frost, cutting winds, cold
damp weather, and so forth. If anyone doubts that hardy varieties have
been gradually bred by selection, I refer him to the evidence collected
by De Candolle, Darwin, Wallace, Bailey and others. When we come to
enquire into the causes of "hardiness," however, difficulties at once
beset us. The adaptation may express itself in a difference in the time
of flowering or leafing, the exigencies of the season being "dodged," as
it were, in a manner which was impossible with the original stock, as
appears to have occurred with Peaches in America; or it may be expressed
in deeper rooting, as is said to be the case in some Apples, or in the
acquirement of a more deciduous habit, or in actually increased
resistance to low temperatures. In such cases we cannot trace what
alterations have occurred in the cells and tissues concerned, though we
may be sure that some changes do occur.
No experienced cultivator doubts that some varieties of Potato, Wheat,
Vine, Chrysanthemum, etc., suffer more from epidemic diseases than
others, and our yearly catalogues furnish us with plenty of promises of
"disease-proof" varieties. Here also we may imagine several ways in
which a particular variety may resist or escape the epidemic attacks of
fungi which in the same neighbourhood decimate other varieties. If we
could breed a variety of the Larch which opened its buds later than the
ordinary form in our northern plains, the probability of its escaping
the Larch-disease would be increased in proportion to the shortness of
the period of tender foliation described on p. 153. It has been claimed
for certain varieties of Wheat that increased thickness of the cuticle
and fewer stomata per square unit of surface have diminished the risk of
infection by Rust fungi, and for certain varieties of Potato, that the
thicker periderm of the tuber protects them against fungi in the soil.
That certain thick-skinned Apples, Tomatoes, and Plums pack and store
better than those with a more tender epidermis seems proved--that is to
say, they suffer less from fungi which gain access through bruises and
other wounds; but it cannot be said that any convincing proof is yet to
hand explaining in detail why some races of wheat resist Rust, or why
the roots of American Vines suffer less from _Phylloxera_ than others.
One of the most extraordinary cases known to me in this connection is
the unconscious selection on the part of native Indian cultivators,
perfectly ignorant of the principles involved, of spring and autumn
forms of Rice, Wheat, Castor Oil, Sugar Cane, Cotton, and other crops.
"It has been estimated that Bengal alone possesses as many as 10,000
recognisable forms of rice." Now there is not the slightest ground for
doubt that these have been unconsciously bred from the semi-aquatic
native species during the many centuries of Indian agriculture, and
nevertheless they have, among other peculiar races, some hill-breeds
which they cultivate on dry soils and without direct inundation. That is
to say, they possess tropical and temperate races differing far more
than our spring and summer wheats.
Something has been gained, then, if we can show that there is nothing
absurd or hopeless in the search for disease-proof or resistant races,
and I think this can be done. We must not forget that the ideal usually
set before himself by a breeder of plants has hitherto been almost
exclusively some standard of size, form, colouring, and so forth, of the
flower, or of taste and texture of the fruit, tuber, etc., though
experiments with _Cinchona_, with brewery yeasts, and other plants
remind us that variations in other directions have been attended to
also.
Now it is obvious that in breeding sour limes and sweet oranges the
cultivator is selecting, and intensifying by selection, very different
metabolic processes in the cell: he can test the results of these, and
so the selection proceeds.
The question is, Could he select at the same time those variations in
cell activity which express themselves in properties of the flower,
fruit, foliage, etc., he desires, as well as such variations as aid the
cells in repelling fungi, insects, or exigencies of the non-living
environment?
That more or less disease-proof varieties could be selected if that
object alone were kept in view can hardly be doubted; plenty of examples
exist already which show that the necessary variations to work upon
exist in just those secretions of protoplasm, etc., which we have seen
are concerned in repelling or attracting parasites.
The Sweet Almond has lost the power of producing amygdalin and prussic
acid in its cells; Cinchona plants vary immensely in the quantity of
quinine formed, and in European hot-houses may even form none at all;
some varieties of Maize have sugar and dextrine instead of starch in
their endosperms, or coloured instead of clear sap in the aleurone
layer, and recent researches prove that they can transmit these
peculiarities to hybrid offspring; non-poisonous bacteria have
frequently been got from poisonous species simply by cultivation under
special conditions, and pigmented forms can be bred into non-pigmented
races.
But we see that the difficulty of selection is increased in the case
postulated above, because two ideals are to be worked up to, and they
may conceivably be incompatible. Not necessarily so, however, for
breeders have solved such problems before in obtaining early _and_ heavy
cropping races of potatoes, wheat, etc., sweet _and_ large grapes,
strawberries, etc., hardy _and_ brilliant flowers, and so forth.
There is, however, another aspect of this question of variability in
organisms in this connection to be considered. Ever since cultivation
began man has probably been cultivating not only the crops he desires,
but also the pests which infest them, and if variation of his chosen
plants occurs--and no one will deny that--surely variation of the fungi
and insects which live on them also takes place. That this is so can be
demonstrated, though, since it is not part of my theme to go into the
question of peculiarities of species and races of parasites, the subject
must here be passed over with a few remarks only.
Recent researches have shown not only that fungi vary immensely in form
and morphological characters according to the amount and kind of
food-materials put at their disposal, thus bringing the whole question
of polymorphism into the domain of experimental physiology, but that
their capacities for infection, spore formation, etc., are also capable
of variation and are dependent on the quality and quantity of food
supplies, water, as well as on the temperature, illumination, and other
factors of the environment. This is true of parasites as well as of
saprophytes. _Botrytis_ forms conidia only in darkness and in moist air.
Klebahn found that a _Puccinia_ growing on _Digraphis_ infected
_Polygonatum_ readily and completely, _Convallaria_ imperfectly, whereas
if sown on _Majanthemum_ it only just infected the plant and then
remained sterile, while it refused to infect _Paris_ at all. Magnus has
shown that _Peronospora parasitica_ can only infect meristematic
tissues, and that when it co-exists with _Cystopus_ on _Capsella_, as is
usually the case, it enters the latter plant by infecting the gall-like
pustules of hypertrophied tissue induced by that parasite. Numerous
parasitic fungi can only penetrate particular parts of plants. For
instance, the _Ustilago_ of wheat can only infect the young seedling,
and grows for weeks as a barren mycelium, only becoming a dominant
fungus in the endosperm. Numerous other examples could be given, but
these suffice to show some of the ways in which the nature of the food
substratum supplied by the host affects the fungus. It is obvious that
if the nature of this food changes, the fungus is also affected, and no
doubt this is the principal reason why Rust-fungi, for instance, vary so
much in their vigour and reproductive power on different wheats and
grasses, though the other factors of the environment must also be of
influence on them as well as on the hosts.
But--and this is the second point--modern research is also showing that
the various species of Rust-fungi have split up into different varieties
or specialised races, according to the particular host plants they
inhabit. For instance there are special varieties or races of the
particular species known as _Puccinia graminis_, the wheat rust, each of
which grows well on various kinds of grain and grasses but refuses to
infect others. Thus, the variety which infects Wheat refuses to infect
Barley or Oats, while that variety which grows on Rye will not take on
Wheat and so forth. Now it is important to notice that these specialised
races are indistinguishable one from another by their visible
microscopic characters: they are all botanically of the species
_Puccinia graminis_ which forms its æcida on the Barberry. We must
therefore conclude that we have here the same phenomenon as that met
with in culture-races of bacteria which, having been fed for several
generations on media rich in proteids, refuse to grow on media rich in
carbohydrates, or when attenuated races are developed by culture under
special conditions.
Now since such physiological races as I have described are by no means
confined to _Puccinia_ but are also known in _Melampsora_,
_Gymnosporangium_ and other fungi, we must conclude from this and from
what we know of variation in plants and animals generally, that
variation and adaptation are common among parasites, insects as well as
fungi.
These considerations will serve to show moreover that the question of
breeding disease-proof varieties of our cultivated plants is complicated
by the danger of our breeding at the same time adapted races of their
pests. It appears at first sight extremely improbable that we should
escape the danger by breeding from those specimens of our plants which
have best survived a fungus epidemic. Still, it must not be forgotten
that "hardy varieties," and races adapted to other exigencies of the
non-living environment, have been bred by selection--and nevertheless
this variable non-living environment is always with us. The matter is
therefore simply and solely one of experiment, and the retort that a
disease-resisting variety of any particular plant has not yet been
raised is no more valid than the objection that a true blue primrose has
not yet been obtained: whether the same remark can be made with regard
to any hope of a _disease-proof_ plant may be another matter, but in any
case it must be made more cautiously in the light of our present
experience.
NOTES TO CHAPTER XVIII.
The reader will find more on this subject in Bailey's
_Survival of the Unlike_ and the literature quoted in the
notes to Chapter VIII.
For varieties of Indian Wheats, etc., see Watt, _Agricultural
Ledger_, Calcutta, 1895.
For a discussion on so-called "Disease-proof Wheats" consult
Eriksson & Henning, _Die Getreideroste_.
Magnus' paper is in the _Berichte der Deutschen bot.
Gesellsch._, 1894, p. 39.
Concerning physiological races and adapted varieties of
_Puccinia_, etc., see Eriksson, "A General View of the
Principal Results of Swedish Research into Grain Rust,"
_Botanical Gazette_, vol. 25, 1898, p. 26.
For an account of Wheat-rust see Marshall Ward, "Illustrations
of the Structure and Life-history of _Puccinia graminis_,
etc.," _Ann. of Bot._, 1888, Vol. II., p. 215.
CHAPTER XIX.
SYMPTOMS OF DISEASE.
_Discolorations--Pallor--Etiolation--Laying of Wheat--
Chlorosis--Yellowing--Albinism--Variegation--Uprooting,
Exposure and Wilting of seedlings._
Everybody knows in a general way when the geraniums in the window pots
are drooping from want of water, or when the young Wheat is sickly, or
the Pear-trees "blighted," and we have now to see how far we can
systematise the knowledge that has been gained in course of time
regarding the signs which sick plants exhibit.
_Pallor._--Under this heading, which includes all cases where the normal
healthy green colour is replaced by a general sickly yellow or pale hue,
ultimately resulting in death of the parts if not arrested, we have
several totally distinct diseases of the chlorophyll apparatus, each
recognised by the co-existence of other subordinate symptoms. The
principal varieties of pallor usually met with are the following:
_Etiolation_ is due to insufficient intensity of light, the pale sickly
yellow organs being unusually watery and deficient in vascular tissue,
the internodes abnormally long and thin, and the leaves generally
reduced in size, or, in some plants also "drawn."
Forced Endive, Rhubarb, Asparagus, and earthed Celery afford examples of
etiolation purposely induced. The want of light causes the true
chlorophyll colouring matter to remain in abeyance, and consequently the
plant as a whole suffers from carbohydrate starvation.
_Laying_ of Wheat and other cereals is a particular case of etiolation.
The seeds having been sown too thickly, the bases of the haulms, owing
to the etiolation and consequent lack of carbohydrates, suffer from want
of stiffening tissues, and the top-heavy plants fall over.
_False etiolation_ depends on a similar abeyance of the chlorophyll, but
in this case due to too low a temperature. It is often seen in Wheat and
other monocotyledons when the young leaves unfold in cold weather in
spring. The symptoms of "drawing" and tenderness are however absent.
Pallor due to too intense illumination must be kept sharply distinct
from etiolation, the pale green or yellow hue being here due to the
destruction of the chlorophyll by insolation, and the accessory symptoms
of "drawing" are wanting.
_Chlorosis_ is a form of pallor where the chlorophyll grains themselves
are fully developed, but their green pigment remains in abeyance owing
to a deficiency of iron in the soil, and can often be cured by adding
traces of a ferrous salt. The distinction between _Icterus_, where the
organs are only yellow, and _Chlorosis_ proper, where they are nearly
white cannot always be maintained. In the typical case only those organs
whose cells are still young can become green on adding iron.
_Yellowing_ or _False Chlorosis_ may be experimentally induced by too
much carbon-dioxide in the atmosphere. It also often ensues when the
roots of plants in the open are waterlogged, owing to the stagnant water
not only driving air from the root-hairs but accumulating dissolved
substances which poison the plant. Trees frequently thus suffer from
"wet feet" when their roots have penetrated down to a sodden impervious
subsoil.
_Yellowing_ accompanied by _Wilting_ is a predominant symptom in most
cases where transpiration is more active than root-absorption beyond a
certain limit, as is well known in cases of prolonged drought. It may
also be caused in evergreens by the foliage transpiring actively in
bright January weather, for instance, while the ground is frozen and the
chilled root-hairs cannot absorb.
In other cases similar appearances are traceable to insects devouring
the roots, _e.g._ wireworms, and the malady is sometimes enhanced by
their accumulations so fouling the wet soil that the roots die off,
owing to want of oxygen and to the excess of carbon-dioxide and
poisonous matters.
Yellowing may also result from the presence of poisonous or acid gases
in the atmosphere or soil, such as chlorine, hydrochloric acid,
sulphurous acid, etc., in the neighbourhood of chemical works, or from
the escape of coal-gas in streets, etc., points of importance in
connection with the use of fungicides and insecticides.
Yellowness is the prevailing symptom in many cases of fungus attack of
the roots or collar of the plant, the resulting stoppage of
transpiration being also sometimes supplemented by rotting of the roots,
and the consequent deprival of oxygen and accumulation of foul gases. In
other cases Fungi, and even Bacteria, have been found to have made their
way into the principal vessels, the lumina of which they stop up, thus
reducing the transpiration current.
Certain insects may also induce a general yellowing and wilting of
plants by entering or destroying the tissues concerned in the
transpiration--_e.g._ _Oniscus_, the Frit Fly, and _Cecidomya_, the
Hessian Fly, which attack young winter wheat within the sheaths and
cause the plants to turn yellow and wilt.
_Albinism_ and _Variegation_ are apparently due to causes totally
different from any yet mentioned. Church's analyses have shown that
albino leaves contain more water and less organic matter than green ones
of the same plants, but not necessarily less ash constituents. The
composition of the ash points to there being more potash and less lime
in the white organs than in the green ones, and, speaking generally, the
former are related to the latter much as young leaves are related to
mature ones.
The whole matter is complicated by the behaviour of certain _variegated_
plants--_e.g._ Ribbon grass, _Calla_, _Abutilon_, which are usually
regarded as partial albinos.
Meyen showed long ago that such variegated plants, if grafted on green
ones, may induce the development of variegated leaves on both scion and
stock, and Morren and others have not only confirmed this but have also
shown that variegation may be inherited through the seed. Nevertheless
some care has to be taken with many of these variegations lest rich
soil, bright light, and other favourable treatment favour the
restitution of the green colour. These facts may be interpreted in
various ways. Some disturbance of physiological functions of the roots,
due to unfavourable conditions of soil, may be the cause; but Beijerinck
has lately published some results which show that some of these albino
diseases can be induced by inoculating normal plants with the juice of
spotted ones even though such juice has been filtered through porcelain,
and concludes that a "_contagium fluidum vivum_" of the nature of a
transmissible enzyme is the agent which disturbs the physiology of the
infected cells.
Koning, while confirming these results in the main, refers them to a
micro-organism so small that it traverses the porcelain filter.
_Upheaval of seedlings._--This is a common form of injury, resulting in
death by drought and exposure, especially in seedling pines, wheat,
etc., in soils exposed to alternate freezing and thawing during spring
when there is no snow to protect the plants. The soil freezes during the
night, and during the thaw next day water accumulates just below the
surface. The freezing is then repeated, and, partly owing to the
expansion of the forming ice and partly to the mechanical effect of the
ice-crystals in the interstices, the surface of the soil is lifted and
draws the roots with it. During the succeeding thaw the soil particles
fall away from the lifted root-fibres, and frequent repetition of these
processes results in such complete exposure of the roots to the full sun
that the plantlet falls over and wilts.
_Exposure of roots_ is also sometimes effected by winds displacing sandy
soils liable to shifting in dry weather, and the resulting wilting of
the plants thus exposed at their roots may be supplemented by damage due
to the repeated impact of the wind-driven sharp grains of sand, which
act like a sand-blast and erode the tissues.
In many of the cases given above the principal result is the weakening
or destruction of the chlorophyll action. This means a loss of
carbohydrates--sugars, starches, etc.--and in so far a starvation of the
plant. The injurious effects are quantitative and cumulative: if large
areas of foliage are concerned, or if the effect lasts a long time, the
plant suffers from loss of food, and may die. In those cases where the
effect is due to the cutting off of supplies at the roots, and where the
yellowing is a secondary symptom, the disease is more general in
character, and recovery is often impossible, because the loss of water
cannot be compensated, and the results may be further complicated by the
gradual penetration of poisonous matter into the cells. It is frequently
necessary, though sometimes very difficult, to decide which is the
primary and which secondary (or tertiary, etc.) symptoms in the order of
their importance, and the diagnosis may be complicated by a number of
accessory factors which it is impossible to treat generally.
NOTES TO CHAPTER XIX.
The principal cases here described are dealt with in works on
plant physiology, and in the works of Sorauer and Frank
already referred to.
As regards damage due to uprooting of seedlings by frost, see
Fisher, "Forest Protection" (Engl. ed. of Hess' _Forstchutz_),
in Schlich's _Manual of Forestry_, Vol. IV., 1895, pp.
439-442.
On Albinism, see Church, "A Chemical Study of Vegetable
Albinism," _Journ. Chem. Soc._, 1879, 1880, 1886.
Beijerinck's results are contained in his paper, "Ueber ein
Contagium vivum fluidum," etc. (with English abstract), in
_Verhandl. d. Kon. Akad. v. Wetensch, te Amsterdam_, 1898.
Koning's paper is in _Zeitschr. f. Pflanzenkrank._, Vol. IX.,
1899, p. 65. See also _Nature_, Oct. 11, 1900, p. 576.
CHAPTER XX.
SYMPTOMS OF DISEASE (_Continued_).
_Spotted leaves--The colours of spots--White, yellow, brown,
and black spots on leaves--Parti-coloured spots--The browning,
etc., of leaves._
_Discoloured spots_ or patches on the herbaceous parts of plants,
especially leaves, furnish the prominent symptoms in a large class of
diseases, due to many different causes, and although we cannot maintain
this group of symptoms sharply apart from the last, as seen from the
considerations on _albinism_, it is often well marked and of great
diagnostic value. By far the greater number of spot-diseases are due to
fungi, but this is by no means always the case. The most generally
useful method of subdividing the classes, though artificial like all
such classifications, will be according to the colour of the spots or
flecks, which, moreover, are usually found on the leaves. It is
necessary to note, however, that various conditions may modify the
colour of spots on leaves. Many fungi, for instance, induce different
coloured spots according to the age of the leaf or other organ attacked,
or according to the species of host, the weather, etc. Moreover the
spots due to these parasites are frequently yellow when young and some
other colour, especially brown or black, when older.
_Scale_ is the name given to the characteristic shield-like insects
(_Mytilaspis_, _Aspidiotus_, etc.) which attach themselves to branches
of Apples, Pears, Oranges, Camellias, and numerous other plants, and
suck the juices. It is the female insect which has the body broadened
out into the "scale," under which the young are brought up. Enormous
damage has been done by some forms--_e.g._ the San José scale in the
United States.
The superficial resemblances of the patches of eggs of some Lepidoptera
to Aecidia and other fungi may be noted in passing--_e.g._ _Bombyx
neustria_ on Apple twigs, _Aporia Crataegi_.
_White_ or _greyish spots_ are the common symptom marking the presence
of many Peronosporeae and Erysipheae in or on leaves, _e.g._
_Peronospora Trifoliorum_, _P. parasitica_ on Crucifers, etc., and
_Sphaerotheca_ on Hops; also _Septoria piricola_, _Cystopus_, _Entyloma
Ranunculi_, etc.
White spots are also caused by insects such as _Tetranychus_ (red
spider) on Clover and other plants.
_Yellow_, or _Orange-coloured Spots_. In cases where these occur on
leaves, and in the case of grasses, etc., on the leaf sheaths as well,
they commonly indicate the presence of Uredineae, and sections under
the microscope will show the mycelium in the tissues beneath. Species of
_Uromyces_, _Puccinia_, etc., in the Uredo state have the spots powdery
with spores; _Aecidia_ show the characteristic "cluster cups," and so
forth. These spots are often slightly pustular, and in some cases
markedly so.
Other fungi also induce yellow spots on leaves--_e.g._ _Phyllosticta_ on
Beans, _Exoascus_ on Poplars, _Clasterosporium_ on Apricot leaves,
_Synchytrium Succisae_ on _Centaurea_, etc.
Yellow spots are also a frequent symptom of the presence of Aphides, of
Red Spider, etc. Thus the minute golden yellow spots sometimes crowded
on Oak leaves are due to _Phylloxera_ punctures.
Yellow patches are formed on the large leaves of _Arisarum_ by a species
of parasitic Alga, _Phyllosiphon_, which lives in the mesophyll. Many
tropical leaves are spotted yellow by epiphytic Algae--_e.g._
_Cephaleuros_.
It must be noticed that many fungi produce yellow spots or flecks in the
earlier stages, which turn brown or black as the fructifications appear,
_e.g._ _Dilophia graminis_, _Rhytisma acerinum_.
The yellow-spotted leaves of _Farfugium grande_ (_Senecio Kaempferi_)
are so like those of _Petasites_ attacked with _Aecidium_ in its early
stages, that an expert might be deceived until the microscopic analysis
was completed.
_Red spots_, varying from rusty or foxy red to bright crimson, are the
symptomatic accompaniment of several fungi, the former often
characterising the teleutospore or aecidium stage of Uredineae--_e.g._
_Aecidium Grossulariae_--the latter sometimes indicating the presence of
Chytridiaceae.
Red spots are also caused by _Gloeosporium Fragariae_ on Strawberry
leaves, _Polystigma rubrum_ on Plums.
Crimson spots on Apple and Pear leaves are also due to _Phytoptus_: they
turn brown later.
_Brown spots_ or flecks, varying in hue from dull slaty brown to deep
red browns, are a common symptom of Fungus and Insect diseases, the
colour often indicating the death of the tissues, rather than any
special peculiarity of the action of the parasite. Good examples are
furnished by the Potato-disease, and by _Peronospora viticola_,
_Sphaerella vitis_ and other disease-fungi of the Grape Vine. The
teleutospore stage of many Uredineae also occurs in deep brown spots.
Black spots and flecks are exceedingly common symptoms of the presence
of fungi, _e.g._ _Fusicladium_ on Apples and Pears, and the pycnidial
and ascus stages of many Ascomycetes--_e.g._ _Phyllachora graminis_. The
teleutospore stages of species of _Puccinia_, _Phragmidium_, etc., are
also so deep in colour as to appear almost black.
_Scab_ on Pears is due to the presence of _Fusicladium_, which indurates
the outer skin of the fruit causing it to crack under pressure from
within, and to dry up, the deep brown to black patches of fungus
persisting on the dead surface.
Black spots on grasses and sedges are caused by Ustilagineae, and are
commonest in the grain, the soot-like powdery spores (Smut) being very
characteristic. _Ustilago longissima_ induces black streaks on the
leaves. Many of these fungi cause distortions or pustules on leaves and
other organs.
Brown and black leaf spots are frequently furnished with concentric
contours arranged round a paler or other coloured central point--_e.g._
_Cercospora_ on Beans, _Ascochyta_ on Peas.
Brown spots with bright red margins are formed in young Beans by
_Gloeosporium_.
Species of _Fumago_, _Herpotrichia_, etc., may cover the entire surface
of the leaf with sooty patches, or even weave the leaves together as if
with black spider-webs.
_Mal nero_ of the Vine is a particular case of black spotting and
streaking of the leaves for which no satisfactory explanation is as yet
to hand. As with Chestnuts, Walnuts, and other plants containing much
tannin, the dark spots appear to be due to this substance, but whether
the predisposing cause is a lack of some ingredients in the soil, or
some temperature reaction, or fungi at the roots, is as yet unknown. The
most recent explanation puts the disease down to the action of bacteria,
but the results obtained by different workers lead to uncertainty.
The "dying back" of leaves, especially of grasses, from the tip, is
usually accompanied by a succession of colours--yellow, red, brown, to
black--and is a common symptom of parching from summer drought; and
spots of similar colours, frequently commencing at the margins of
leaves, are characteristic symptoms of the injurious action of acid
gases in the air.
Brown and blackish spots on Pears are caused by a species of _Thrips_.
In many cases the minute spots of Rust-fungi on one and the same leaf
are bright orange yellow (_uredo_), deep brown, or almost purple-black
(_teleutospores_), foxy-red brown (older uredospores), or dead slaty
black where the old teleutospores have died off--_e.g._ _Uromyces Fabae_
on Beans, _U. Pisi_ on Peas, etc.
_Parti-coloured leaves._--The leaves sometimes start shrivelling with
red edges, while yellow, red, and finally brown and black blotches
appear on the lamina, from no known cause--_e.g._ Vines. In other cases
similar mimicry of the autumnal colouring of leaves results from the
action of acid gases.
_Burning_ is a common name for all cases where the leaves turn red or
red-brown in hot, dry weather, and many varieties are distinguished in
different countries and on different plants, because species react
dissimilarly. The primary cause is usually want of water--drought.
_Foxy leaves_ are a common sign of drought on hot soils, and the disease
may usually be recognised by the gradual extension of the drying and
fox-red colour proceeding from the older to the younger leaves, and from
base to apex--_e.g._ Hops.
_Coppery leaves._--The leaves of the Hop, etc., may show yellow spots
and gradually turn red-brown--copper-coloured--as they dry; the damage
is due to _Tetranychus_, the so-called Red Spider. These cases must of
course be carefully distinguished from the normal copper-brown of
certain varieties of Beech, Beet, _Coleus_, etc.
_Silver-leaf._--The leaves of Plum, Apple, and other fruit trees often
obtain a peculiar silvery appearance in hot summers, the cause of which
is unknown.
Discolorations in the form of confluent yellow and orange patches, etc.,
resembling variegations, are not infrequently due to the ravages of Red
Spider and mites--_e.g._ on Kidney Beans.
_Sun-spots._--Yellow spots, which may turn brown or black according to
the species of plant affected and the intensity of the action, are often
caused by the focussing of the solar rays by lens-like thickenings due
to inequalities in the glass of greenhouses, or by drops of water on
them or on other leaves, _e.g._ Palms, _Dracaena_, etc. The action is
that of a burning glass, and extends throughout the leaf-tissues. Young
grapes, etc., may also be injured in this way. Water-drops on the glass
can only act long enough to produce such injuries if the atmosphere is
saturated. The old idea that a drop on a leaf can thus focus the sun's
rays into the tissues beneath is not tenable.
Here again we see that the disease-agencies concerned in producing the
symptoms described in this chapter, agree for the most part in so far
that the principal effect is generally the disturbance of chlorophyll
action in the spots or flecks on the leaves, and the rendering useless
of these areas so far as providing further food-supplies is concerned.
The effects may be due merely to the shading action of a
parasite--_e.g._ epiphytic fungi--or to actual destruction of the
tissues invaded--_e.g._ by endophytic fungi--or the tissues may be
burnt, poisoned, etc. In so far the results are again quantitative and
cumulative, and the amount of damage depends on the number and size of
the spots or other areas affected, and the proportion of foliage
involved, as well as the length of time the injurious action is at work.
But, again, it must be remembered that several symptoms may co-exist,
and matters may be complicated by the spread of the destructive agent,
or its consequences, to other parts, and in some cases we are quite
uninformed as to the true nature of the disease.
NOTES TO CHAPTER XX.
Further information regarding these "leaf-diseases" will be
found in special works dealing with the fungi and insects
which cause them. In addition to works already quoted, the
reader may also be referred for Fungi to Massee, _A Textbook
of Plant-diseases caused by Cryptogamic Parasites_, London,
1899; or Prillieux, _Les Maladies des Plantes Agricoles_,
1895. See also Marshall Ward, Coffee-leaf Disease, _Sessional
Papers_, XVII., Ceylon, 1881, and _Journ. Linn. Soc._, Vol.
XIX., 1882, p. 299.
The question of "Sun-spots" has been dealt with by Jönnson in
_Zeitschr. f. Pflanzenkrankh._, 1892, p. 358.
CHAPTER XXI.
ARTIFICIAL WOUNDS.
_The nature of wounds and of healing processes--Knife wounds--
Simple cuts--Stripping--Cuttings--Branch-stumps and pruning--
Stool-stumps--Ringing--Bruises._
_Wounds._--All the parts of plants are exposed to the danger of wounds,
from mechanical causes such as wind, falling stones or trees, hail,
etc., or from the bites of animals such as rabbits, worms, and insects,
and although such injuries are rarely in themselves dangerous, they open
the way to other agencies--water, fungi, etc., which may work great
havoc; or the loss of the destroyed or removed tissues is felt in
diminished nutrition, restriction of the assimilative area, or in some
other way.
We have seen that living cells die when cut, bruised, or torn; and that
the cells next below in a layer of active tissue are stimulated by the
exposure to increased growth and division, and at once produce a layer
of cork, the impervious walls of which again protect the living cells
beneath. This is found to occur in all cell-tissues provided the cells
are still living, and it matters not whether the wound occurs in the
mesophyll of a leaf, the storage parenchyma of a Potato-tuber, the
cortex of a root or stem, or in the fleshy parts of a young fruit, the
normal effect of the wound is in all cases to call forth an elongation
of the uninjured cells beneath, in a direction at right angles to the
plane of the injured surface, which cells then divide by successive
walls across their axis of growth: the layers of cells thus cut off are
then converted into cork, by the suberisation of their walls. Further
changes may then go on beneath the protective layer of wound-cork thus
produced, and these changes vary according to the nature of the cells
beneath: the cambium forms new wood, the medullary rays similar rays,
cortex new cortex, and so on.
_Knife-wounds._--Artificial cuts in stems are easily recognised and soon
heal up unless disturbed. Several cases, differing in complexity, are to
be distinguished. The simplest is that of a longitudinal, oblique, or
horizontal short cut in which the point of the knife severs all the
tissues of the stem down to the wood. The first effect usually observed
is that the wound gapes, especially if longitudinal, because the cortex,
tightly stretched on the wood cylinder, contracts elastically. This
exposes the living cortex, phloem and cambium to the air, and such
tissues at once behave as already described above: the cells actually
cut die, those next below grow out under the released pressure, and
these give rise to cells which become cork. As the growth and
cell-division continue in the cells below this thin elastic cork-layer,
they form a soft herbaceous cushion or _callus_ looking like a thickened
lip to each margin of the cut. Each lip soon meets its opposite
neighbour, and the wound is closed over, a slight projection with a
median axial depression alone appearing on the surface. The depression
contains the trapped-in callus-cork squeezed more and more in the plane
of the cut as the two lips of callus press one against the other, and
sections across the stem and perpendicular to the axis of the cut show
that this thin cork, like a bit of brown paper, alone intervenes between
the cambium, phloem and cortex respectively of each lip, as each layer
attempts to bridge over the interval. If the healing proceeds normally,
these layers, each pressing against the trapped cork-film, and growing
more and more in thickness, shear the cork-layer and tear its cells
asunder, and very soon we find odd cells of the cambium of one lip
meeting cambium cells of the other, phloem meeting phloem, and cortex
cortex, and the normal thickening of the now fused layers previously
separated by the knife goes on as if nothing had happened, the only
external sign of the wound being a slight ridge-like elevation, and,
internally, traces of the dead cells and cork trapped here and there
beneath the ridge. When the conjoined cambium resumes the development
of a continuous layer of xylem and phloem, no further trace of the
injury is observable, unless a speck of dead cells remains buried
beneath the new wood, and indicates the line where the knife point
killed the former cambium and scored the surface of the wood in making
the wound.
_Stripping._--Now suppose that, instead of a mere slit with the
knife-point, a strip of bark is removed down to the wood. Exactly the
same processes of corking and lip-like callus formation at the edges of
the wound occur, but of course the occlusion of the bared wood-surface
by the meeting of the lips occupies a longer time. Moreover, the living
cells of the medullary rays exposed by the wound on the wood-surface
also grow out under the released pressure, and form protruding callus
pads on their own account. In course of time the wood is again
completely covered by the coming together over its face of these various
strips of callus, but two important points of difference are found, as
contrasted with the simpler healing of the slit-wound. In the first
place the exposed wood dries and turns brown, or it may even begin to
decay if moisture and putrefactive organisms act on it while exposed to
the air; and, in the second place, the normal annual layer of wood--or
layers, as the case may be--formed by the cambium only extends over that
part of the stem where the cambium is still intact, and is entirely
wanting over the exposed area. Thus, if it takes two years for the
cambium to extend across the wound, a layer of wood will be formed all
round the intact part of the stem, from lip to lip of the cut tissues
during the first year; then a second annual layer outside this will be
formed during the second year, but extending further over the edges of
the wound, and nearly complete, because the cambium has now crept
further across the wounded surface to meet the opposite lip of cambium;
and during the third year, when the cambium has once more become
continuous over the face of the wound, the annual wood layer will be
complete. But, of course, this last layer covers in the edges of the two
previously developed incomplete wood-layers as well as the exposed and
brown, dry, or rotten dead face of the wood. It also covers up the
trapped-in brown cork and any débris that accumulated in the wound, and
this "blemish," though buried deeper and deeper in the wood during
succeeding annual deposits of wood-layers, always remains to remind us
of the existence of the wound, the date of which can be fixed at any
future time by counting the annual rings developed subsequently to its
formation. Obviously, also, the deficiency of wood at this place makes
itself visible on the outside by a depression.
_Cuttings._--When a cutting of _Pelargonium_, Willow, or other plant is
made, we have a typical knife-wound, the behaviour of which is very
instructive in illustration of plant-surgery, and may be most easily
seen by keeping it in damp air instead of plunging it into sand or
soil.
All the living cells actually cut or bruised turn brown and die as
before; those beneath--_e.g._ the living pith, medullary rays, cambium,
phloem, and cortex, grow out under the released pressure and form a
callus, the outermost layer of which becomes cork, while those below,
abundantly supplied with food-materials, proceed to spread, as if
flowing over the surface of the cut wood, and rapidly occlude the wound.
Meanwhile new roots are formed adventitiously from the cambium just
above the plane of section, and push out through the cortex into the
damp air, and if the cutting had been in soil it would now be capable of
independent existence. It is important to keep cuttings upright, as the
roots only spring from the lower end. Such cuttings can be obtained not
only from stems, but also from roots and even leaves.
Callus-formation is not confined to the basal end of a cutting; it has
nothing to do with position, but is a reaction to the wound stimuli,
independent of light, gravitation, etc. As time goes on, however, the
internal organisation of the erect cutting usually reacts on the callus
at either end, and roots only rise from the lower one, while shoot-buds
may form in the upper one, though it is possible to bring about the
formation of buds from the lower end also.
_Branch stumps._--A more complex example is furnished by a branch cut
off short some distance--say a foot--from the base, where it springs
from the trunk. As before, the immediate effect of the section is the
formation of a callus from the cambium, phloem and cortex, which begins
to rise as a circular occluding rim round the wood. The transpiration
current in the trunk, however, is not deflected into the 12 inches or so
of amputated branch, because there are no leaves to draw the water up
it, and so the stump dries up and the cortex and cambium die back to the
base, leaving the dead wood covered with shrivelled cortical tissues
only. This dead stump gradually rots under the action of wet, fungi, and
bacteria, and since the pith and heart-wood afford a ready passage of
the rot-organisms and their products into the heart of the trunk, we
find in a few years a mere stump of touch-wood and decayed bark, which
falls out at the insertion like a decayed tooth, leaving a rotten hole
in the side of the trunk.
If, however, instead of allowing the basal part of the amputated branch
to protrude as a stump, we cut it off close to the stem, and shave the
section flush with the normal surface of the latter, the callus formed
by the cambium, etc., rapidly grows over the surface, and soon forms a
layer of cambium continuous with that of the rest of the stem. The wound
heals, in fact, much as if it were a strip-wound, and beyond a slight
prominence for a year or two no signs are visible from the outside after
the occlusion. Of course these matters depend on the relative thickness
of branch and stem, and if much wood is exposed the dangers of rot and a
resulting hollow in the stem are increased. It is interesting to note
how much thicker the callus lips are at the sides of the wound than
above and below, owing to differences in the distribution of the
nutrient materials.
_Stool-stumps._--When a tree is felled, the stump may, if the section is
close to the ground and kept moist, begin to form a thick rim-like
callus round the wood, in which adventitious buds soon make their
appearance, and grow out into so-called _Stool-shoots_. The products of
assimilation of these, and the stores accumulated in the stump, often
suffice to feed the callus sufficiently to enable it to grow over and
completely occlude the wound, if the wood surface is not too large, or
so long exposed that rotting processes have meanwhile set in.
_Ringing._--If the strip of cortical tissues and cambium is removed all
round the stem, exposing the wood in a form of a ring, complications may
ensue owing to the following circumstances. A well-marked callus appears
at the upper edge of the wound, because, the transpiration current up
the young wood not being stopped, plenty of water and salts from the
soil can reach the leaves; but the nutritive materials supplied by the
latter are accumulated at the upper lip of the wound owing to the
stoppage there of their descent in the phloem, cortex, etc. No such
callus-lip appears at the lower margin of the wound owing to want of
these supplies. Consequently the occlusion and healing of the ring-wound
only takes place from above downwards, and if the ring of cortical
tissues removed is a broad one, the healing may be a long process, or
may even be indefinitely delayed, a thicker and thicker callus
projecting over from above. For similar reasons no annual wood layers
are formed below, but only above the wound, and thus the branch or tree
may die. The latter contingency is the more likely the further up the
tree the ringing takes place, owing to the risk of drying up which
threatens the exposed wood, and to the consequent interruption of the
transpiration current, and the likelihood that lateral shoots below the
wound may divert the water to their own leaves. If the ringing occurs
low down on a stem, and the environment remains damp, the upper thick
callus may put out new roots; the part above the wound then behaves like
a cutting. If the ringing is done on a young and vigorous branch of an
old tree, the lower lip may receive supplies from the leaves of branches
below the wound, or from shoots which spring from adventitious buds
close to it, and the wound may heal over normally. Such healing may be
rendered more certain by keeping the wounded surface moist--_e.g._ by
means of damp moss, and so encouraging the formation of callus-bridges
from the medullary rays.
If on ringing a tree or a branch the young wood is removed as well as
the cambium and cortical layers, the death of the parts above the wound
is almost certain, owing to the stoppage of the transpiration current:
the exceptions to this rule depend simply on the existence of other
channels of communication, such as internal phloems, very thick
sap-wood, and so forth.
_Bruises._--If a branch or woody stem is struck sharply, with a hammer,
for instance, the bruised cortex, phloem and cambium are killed by the
blow, and the general effect is as if these tissues had been removed at
that spot by the knife, but with the following complications. The
bruised cortical tissues rapidly dry as they perish, and may adhere to
the wood below. Consequently the still sound parts bordering on the
wound are not released from pressure, but, on the contrary, have to
advance towards each other over the surface of the wood under still
greater pressures, in part due to the tightening of the whole cortex as
the dead parts dry and contract, and in part due to the above-mentioned
adherence of the latter to the wood. It results from this that such
wounds heal very slowly and badly, and when the killed patch at last
ruptures, wound-fungi, insects, and other injurious agencies may get in
and do irreparable damage, as has been found to occur in cases where
such wounds have been made in striking trees to shake down insects,
fruit, etc.
NOTES TO CHAPTER XXI.
The essential facts regarding wounds and healing by occlusion
are given in Marshall Ward, _Timber and some of its Diseases_,
1889, chapters viii. and ix., and in Laslett, _Timber and
Timber Trees_, 1894, chapters iv. and v. More detailed
treatment will be found in Frank, _Krankh. d. Pflanzen_, B. 1.
cap. 2, where the special literature is collected. The reader
may also consult Hartig, _Diseases of Trees_, Engl. ed. 1894,
pp. 225-269.
CHAPTER XXII.
NATURAL WOUNDS.
_Burrows and excavations. Bark-boring--Wood-boring--Wood
fungi--Leaf-miners--Pith flecks--Erosions. Skeleton leaves--
Irregular erosions--Shot holes. Frost cracks--Strangulations--
Spiral grooving._
Natural wounds are produced in a variety of ways during the life of the
plant, and, generally speaking, are easily healed over by the normal
process if the area destroyed is not too large, and the parts remaining
uninjured are sufficiently provided with foliage, or with supplies of
food-materials stored up in the roots, rhizomes, medullary rays, etc.,
to feed a vigorous callus.
The nature of such wounds and the mode of healing are explained by what
we know of artificial wounds, and it only remains to point out that the
principal danger of ordinary wounds is not so much the direct traumatic
action, because the simpler organisation of the plant does not involve
matters connected with shock, loss of blood, etc., as in animals; the
danger consists, rather, in their affording access to other injurious
agents, especially fungi, and the treatment of wounds frequently
resolves itself into cutting or pruning in order to get clean surfaces
which can heal readily.
Wounds on leaves imply loss of foliar surface--_i.e._ of chlorophyll
action--and the remarks on page 193 apply.
_Burrows_ may be taken as comprising all kinds of tunnel-like
excavations in the various organs of plants, including those cases where
insects burrow into hollow stems of grasses, etc., as indicated by the
perforations they make in the outer tissues.
_Bark-boring_ is done by many species of beetles, especially
_Scolytidae_, which excavate characteristically formed branching
passages tangentially in the inner bark of Conifers and other trees.
Some of them also bore down to the surface of the sap wood (_e.g._
_Tomicus bidentatus_) or even burrow right into the latter (_e.g._ _T.
lineatum_). It commonly happens that the external apertures show up
clearly, owing to the brown dust and excrement, sometimes accompanied by
turpentine, which exude from them. Many of these Bark beetles only
attack trees which are already injured by fire, lightning, etc.;
possibly they cannot bore through a cortex which swamps them with sap,
as a vigorous one might do.
_Wood-boring_ is also done by many of the bark-beetles as well as by
Longicorns, _e.g._ _Saperda_ in Poplars and Willows, the young shoots
of which often show characteristic swellings with lateral holes
indicating the points of exit. From the external apertures comminuted
wood, like saw-dust, is frequently ejected in quantity and betrays the
presence of the insects. Certain wood-wasps (_Sirex_) and the larvae of
moths (_Cossus_) also make large perforations in the wood of Willows and
other trees, often destroying it completely. In the case of these larger
borers, whose tunnels may be as broad as the little finger, the foul
smell as well as abundant "saw-dust" betray the evil.
Excavations in wood are by no means caused only by insects: several of
the larger Hymenomycetes--_Stereum_, _Thelephora_, _Polyporus_,
etc.--tunnel the timber in characteristic ways and often after a fashion
very suggestive of insects. They usually obtain access through
fractures.
_Tunnels_ in leaves are invariably due to the activity of miners
belonging to the smaller moths and beetles--_e.g._ _Tinea_, _Orchestes_,
etc.--the larvae of which eat out the mesophyll but leave the covering
epidermis or cuticle untouched, and since the insect bores forwards
only, in an irregular track, and leaves its excrement in the winding
passage, the effect is very characteristic.
Whitish leaf tunnels in Peas are excavated by _Phytomyza_.
Characteristic foxy-red tunnels are mined in the leaves of Apples by
_Lyonetia_, _Coleophora_, etc.
_Falling of fruit_, of Apples, Plums, Apricots, etc., before they are
ripe, is frequently due to insects, of which the various species of
_Grapholitha_ or _Carpocapsa_ are conspicuous: the fallen fruits show a
small hole leading by a labyrinth of passages to the "core" or "stone,"
and in which the grub and its excrement are visible. The cutting off of
the vascular bundles and disturbance of the water supply only partly
explain the premature fall.
_Pith-flecks_ are minute brown specks or patches found in the
wood-layers of many trees, and consist of dead parenchymatous
thick-walled cells, reminding one of the structure of pith. They are
explained as due to the borings of minute insects, _Diptera_ or Beetles,
the larvae of which pierce the cortex and phloem and bore their way into
the cambium. The latter then occludes the tunnels by filling them up
with cells, and continuing its wood-forming activity gradually buries
them deeper and deeper in the wood. Such pith-flecks are common in
Willow, Birch, Alder, _Sorbus_, etc. It is possible that they may be due
to other causes also in other trees.
_Erosions_ or _irregular wounds_ on leaves are caused by large numbers
of grubs and caterpillars and other insects, such as earwigs, as well as
slugs, snails, and other animals; but it must by no means be assumed
that all marginal leaf wounds, for instance, are caused by animals,
since many fungi which rot the tissues, as explained below (p. 208),
also cause such erosions, the putrescent parts falling out--_e.g._ the
Potato disease.
_Skeleton leaves_ frequently result from the ravages of caterpillars,
which leave the coarser ribs and veins untouched, but much finer
skeletons with the minute veins almost intact may be found on plants
infested with certain insects--_e.g._ _Selandria_ on Cherries.
Skeletonised patches on Cherry leaves, often pink or brown-pink, are
eaten out by this grub.
_Shot-holes_ are perforations in leaves presenting the appearance, from
their more or less rounded shape, of gunshot wounds. They may be due to
insects which bore through the young leaves while still folded in the
bud--_e.g._ Willow Beetle--or which gnaw out the tissue--_e.g._ the
Beech Miner. Similar but usually more torn and irregular holes are eaten
out by many caterpillars--_e.g._ the Cabbage Moth.
Shot-holes on Peas may be the work of Thrips.
Leaf perforations are commonly caused by severe hail-storms, the
hail-stones beating right through the thin mesophyll. Certain chemicals
used for spraying have also been known to cause shot-holes by killing
the tissue beneath the standing drops.
There is, however, a class of shot-holes in thin leaves which are due to
the action of minute fungi, the mycelium of which so rots the tissues in
a more or less circular area round the point of infection, that, in wet
weather, the decomposing mass falls out and leaves a round hole--_e.g._
certain Chytridiaceae, Peronosporeae, _Gloeosporium_, _Exoascus_, etc.
If dry weather supervenes these holes frequently dry at the edges, and
the leaves appear as if eaten out.
Shot-holes in Cherry, Walnut, Tobacco, and Plum leaves are due to
_Phyllosticta_, in Cherry leaves also to _Clasterosporium_, and in
Potato leaves to _Haltica_.
_Frost-cracks._--The trunks of trees exposed to the north-east, and
occasionally with other aspects, are apt to show longitudinal ridges
which realise on a larger scale the features of healed wounds scored
with a knife. These wounds are due to the outer layers of wood losing
water from their cell-walls as it congeals to ice in their lumina, more
rapidly than do the warmer internal parts of the trunk; as this drying
of the wood causes its shrinkage, especially in the tangential
direction, the effect of a sudden frost and north-east wind is to rend
the wood, which splits longitudinally with a loud report, as may often
be heard in severe winters. Since the cortex and bark are ruptured at
the same time the total effect resembles that of a deep knife-cut, and
the same healing processes result on a larger scale when the wood swells
and closes up the wound again in spring. But this recently-closed lesion
is evidently a plane of weakness, and if a similarly severe winter
follows the wound reopens and again heals, and so on, until after a
succession of years a prominent _Frost-ridge_ results, which may finally
heal completely if milder winters ensue or the tree be eventually
protected.
_Strangulations._--We are now in a position to understand the so-called
strangulations which result when woody climbers, telegraph wires, etc.,
kill or injure trees by tightly winding round them. If strong wire is
twisted horizontally round a stem, the growth in thickness of the latter
causes the trapping of the cortex and cambium, etc., between the wire
and the wood, and a ringing process is set up in consequence of the
death of the compressed tissues. A callus then forms above the wound, as
in the case of true ringing by means of a cut, and eventually bulges
over the upper side of the wire: in the course of years this overgrowth
may completely cover in the wire, and, pressing on to the lower lip of
the wound, may at length fuse with the cambium below. Hereafter the
thickening rings of wood are continuous over the buried wire. The
process is obstructed by all the impediments referred to in dealing with
ringing, and of course the stem thickens more above than below the wire.
If the sapwood is thin, and the bark is so thick as to put great
obstacles in the way of the junction of the upper and lower cambiums,
death may result--the tree is permanently ringed. (See p. 201.)
_Spiral grooves_ are frequently met with where Wood-bine or other woody
climbers have twined round a young stem or branch, the upper lip of the
groove always protruding more than the lower. If a kink or a crossing of
two plants or branches of the twiner results in a complete horizontal
ring, the results are as in the above cases of ringing and
strangulation. Naturally grooved walking sticks are often seen.
_Buried letters, etc._--These processes of healing by occlusion enable
us to understand how letters of the alphabet, cut into the wood of
trees, come to be buried deep in the timber as successive annual rings
cover them in more and more. Chains, nails, rope, etc., have frequently
been found thus buried in wood.
NOTES TO CHAPTER XXII.
In addition to the notes to the last chapter, the reader may
be referred to Fisher in Vol. IV. of Schlich's _Manual of
Forestry_, Chap. VI., for an account of Hess' excellent work
on Boring Beetles, etc.
The authority on Wood-fungi is Hartig, see especially his
_Zersetzungs-erscheinungen des Holzes_, the principal results
of which are condensed in his _Diseases of Trees_ already
referred to. As regards "Pith-flecks," the reader should
consult Frank, _Krankh. der Pflanzen_, B. I., p. 212: the
subject needs further investigation.
CHAPTER XXIII.
EXCRESCENCES.
_Herbaceous excrescences, or galls--Erineum--Intumescences--
Corky warts, etc.--Pustules--Frost-blisters--Galls and Cecidia
--Root nodules._
_Excrescences_, or out-growths of more or less abnormal character from
the general surface of diseased organs, are very common symptoms, and
widely recognised. They are due to hypertrophy of the tissues while the
cells are young and capable of growth, and may be induced by a variety
of causes, among which the stimulus of insect-punctures and of the
presence of insect eggs are best known; but that of fungi, though less
widely recognised, plays an equally important part, and, as we shall
see, galls and other excrescences may be due to widely different agents.
_Galls_ or _Cecidia_ are protuberances of the most varied shapes,
colours, and sizes found on herbaceous parts attacked by insects, fungi,
etc. In the simplest cases the insects only pierce and suck the young
cellular tissue--_e.g._ _Phytoptus_, Aphides, etc.--but in others the
stimulus to hypertrophy starts by the puncture of the embryonic tissue
of a leaf, root, etc., by the ovipositor of the female insect, which
then lays an egg--_e.g._ _Cynips_, _Cecidomyia_, etc.--the presence of
which appears to intensify the irritating action, or such only occurs
when the young larva escapes.
Our knowledge of the primary cause of gall-formation amounts to very
little. Generally speaking, only embryonic or very young cellular tissue
reacts, and galls on adult leaves and branches have usually been
initiated long before. The same gall-insect may induce totally different
galls on different plants, or even on different parts of the same plant,
and different insects call forth different galls on any one plant. These
facts point clearly to the co-operation of both plant and insect in the
gall-formation, and the best hypothesis yet to hand is to the effect
that a gall is a hypertrophy of cells, the normal nutrition, growth, and
division of which have been disturbed owing to the action of some poison
or other irritant derived from the insect, or fungus, or other organism.
Attempts have been made to reproduce galls by injecting the juices of
similar galls into the tissue, but as yet without success, and this may
point to the co-operation of mechanical irritation during the
hypertrophy in normal gall-formation.
Galls, in the broad sense, are not always preceded by a wound, however.
Insects on the outside of young tissues may cause such irritations that
the parts in contact with the animal are arrested in their growth, while
those further away grow more rapidly--_e.g._ where Mites, etc., cause
puckers and leaf-rolling. In true galls the hypertrophy may consist
merely in the enlargement of cells already present, and no new
cell-divisions and, still less, changes in the nature of the tissues
result--_e.g._ some pocket galls on _Viburnum_, _Pyrus_, etc., and the
hairy outgrowths of the epidermis known as _Erineum_. In other cases
there is not only hypertrophy of existing cells, but new cell-divisions
are instituted: these cell-divisions may be confined to the direction
perpendicular to the epidermis, and the tissues grow only in the
direction of the surface, producing puckerings--_e.g._ the Aphis galls
on _Ribes_, Phytoptus galls of _Salvia_, leaf galls on _Tilia_, _Acer_,
_Alnus_, etc., and the curious galls on Plums due to _Cecidomyia Pruni_,
and which must not be confounded with the "pocket plums" and similar
galls due to Exoasci.
In a third series of cases, cell-divisions occur parallel to the surface
of the leaf, and galls are formed which grow in thickness, and develop
the most extraordinary and complicated new tissues--proteid-cells
surrounding the egg or larva deposited inside, followed by a protective
layer of sclerenchyma encasing this food layer, and around this again
softer tissues which may assume the structures and functions of
respiratory tissues, water-storing tissues, starch reservoirs,
assimilatory, or protective tissues of various kinds, and over all may
be a well-marked epidermis, with stomata, or cork with lenticels.
The chief seat of these hypertrophies and--what is more
remarkable--development of new tissue elements not found elsewhere in
the leaves, or even in the species, is the mesophyll, and various
speculations and hypothesis have been founded on these curious
phenomena.
_Erineum._--The simplest excrescences on plants are certain hair-like
developments of epidermal cells due to the irritation of species of
_Phytoptus_, and similar insects which rise in clusters on the surfaces
of leaves and by their colours, consistence, arrangement in patches,
spots, etc., so simulate fungi that Persoon was deceived by them and
gave them the genus name _Erineum_. They occur on most of our trees,
_e.g._ Poplar, Lime, Oak, and are very common in the Tropics. Usually
pale or even white at first, they turn brown as the hair-like outgrowths
die and lose their sap, but since the latter may be bright
coloured--yellow, red, purple,--the patches are sometimes very
conspicuous objects on smooth leaves.
In many cases these hairs exactly resemble in shape and other characters
the abnormal root-hairs found on roots exposed to the effects of
poisonous reagents, or of unsuitable food-materials, or the rhizoids
developed from wounded Algae, etc.
_Intumescences_ are similar trichomatous outgrowths not associated with
insects or fungi, and due to some disturbance of the balance between
transpiratory and assimilatory functions of their leaves, as indicated
by the less localised occurrence and by their non-appearance when the
plant is under favourable cultural conditions. Structures not unlike
these have been artificially induced by exposure to particular lights,
and also by painting spots with dilute corrosive sublimate, indicating
that poisons may impel the epidermis cells to grow out abnormally.
_Corky warts._--Several forms of disease are known in which the
pathological condition is expressed by the formation of cork in unwonted
places and quantities. The _Scab_ or _Scurf_ of Potatoes is a case in
point. The tissue of the lenticels absorbs water and the outermost cells
are cut off by cork and die: the cells below them burst the dead
bark-like masses thus formed, and again cork is formed and cuts off the
outer masses, and the rough cork warts--_Scab_ or _Scurf_--are the
result.
The causes predisposing to scab have been variously assigned to
dampness, want of lime, action of bacteria and fungi--_e.g._
_Sorosporium_, _Oospora_, _Spongospora_,--the latter making their way
into the ruptured tissue of the lenticels and irritating the cells to
further growth.
It seems probable that several different kinds of scab exist in
Potatoes, as well as in roots--_e.g._ Beets, and the whole subject needs
further investigation. The scab-like rough scaly bark of Pear trees in
dry districts may also be mentioned here.
_Cork-wings_ are well known on the young branches of Elms, Maples, etc.,
some varieties of which have received specific names on this account.
_Corky excrescences_ on leaves occur occasionally in the Gooseberry,
Holly and other plants, for which no cause has been discovered.
Lenticels are also formed on some leaf-galls, and are remarkable as
being structures not normal on leaves.
_Pustules._--This term may be employed generally for all slight
upheavals of the surfaces of herbaceous organs, which subsequently burst
and give egress to the spores, etc., of the organism causing them, or
merely fray away at the top if no organism is discoverable. They are
often due to fungi--_e.g._ _Synchytrium_, _Protomyces_, _Cystopus_, and
Ustilagineae,--and we may extend the use of the general term also to
those cases where the _stroma_ of the fungus itself bursts through the
cortex of older parts and forms the principal part of the
pustule--_e.g._ _Monilia_, forming white or grey pustules on Apples,
_Roestelia_ and other Æcidia, forming yellow or orange pustules on
leaves, etc.; _Cucurbitaria_ and _Nectria_ (red) breaking through the
cortex of trees, and _Phoma_ and numerous other Ascomycetes which form
black cushions. _Pustules_ on the leaves of _Lysimachia_, _Ajuga_, etc.,
are due to the parasitic Alga _Phyllobium_.
Cylindrical stem swellings are caused by _Calyptospora_: they are due to
the hypertrophy of the cortex of Bilberry stems permeated by the
hyphae. _Epichloë_, which clothes the sheaths and halms of grasses with
its stroma, at first snowy white and later ochre-yellow as the
perithecia form, is another example.
The cylindrical layer of eggs of a moth such as _Bombyx_ on a twig must
not be confounded with these cases.
_Frost-blisters_ are pustule-like uprisings of the cortex, where the
living tissues below have formed a callus-like cushion into the cavity
beneath the dead outer parts of the cortex which were killed by the
frost; they occur on the stems of young Apples, Pears, etc.
_Galls_ in the narrower sense are tissue outgrowths usually involving
deeper cell-layers. They are so varied and numerous that classification
is difficult. For symptomatic purposes we may divide them as follows:
_Leaf-galls._--A well-marked type is that of the _pocket-galls_ or
_bladders_ in which the whole thickness of the leaf is as it were pushed
up like a glove-finger at one spot, so that if the upper surface of the
leaf forms the outside of the gall the lower surface is its lining. Such
galls are common on Limes (_Phytoptus_), _Glechoma_ (_Cecidomyia_), Elms
(_Tetraneura_), etc. Similar localised extension of the leaf surface,
compelling it to rise up like a pocket, are caused by fungi--_e.g._
_Taphrina_ on Poplars, _Exoascus_ on Birches, etc., _Exobasidium_ on
Bilberries, Rhododendrons, etc.
Another type is that of the _Gall-apple_, so well known on Oaks, where
the spherical swelling is solid--except for the inner cavity containing
the eggs--_Neurotus_, _Cynips_, _Hormomyia_, etc. These are comparable
in general characters to the nodules on roots.
Fungus galls with similar external features when young are found on
Maize (_Ustilago Maydis_), and betray their nature by the black powdery
spores as they mature.
Bud galls on Willows are due to _Cecidomyia_, which causes several
internodes to swell out into a greenish barrel-shaped mass, from which
leaves may spring.
Small irregular excrescences on Willow stems are referred to
_Phytoptus_, and another species of the same insect induces similar
swellings on Pines which are not surcharged with resin.
_American Blight_, or Woolly Aphis, on Apples especially, causes the
tumour-like swellings covered with sticky white fluff, which is a waxy
excretion of the insect. Galls on _Pilea_, in Java, are due to an
Alga--_Phytophysa_.
_Root-nodules_ or _nodosities_ are frequently caused by insects--_e.g._
_Centhorhynchus_, a beetle which attacks Crucifers, _Cynips_ and allied
"gallflies" of Oaks, and the notorious _Phylloxera_. But similar
root-galls are produced by Nematode worms, _Heterodora_, on Beets,
Tomatoes, Cucumbers and numerous other plants, and by the Slime fungus
_Plasmodiophora_, and it is not always easy to distinguish such cases
from the fungus-galls (_Mycocecidia_) on the roots of Alders, _Juncus_,
and Leguminoseae where the symbiosis of bacteria or fungi with the
roots are of benefit to the plant. _Urocystis Leimbachii_ forms similar
nodules at the collar of young plants of _Adonis_.
_Heterodora javanica_ passes into the cortex of sugar-cane roots through
fissures, and makes its way to the place where a young rootlet is about
to emerge; here it sticks its beak into the growing-point and remains
fixed.
Molliard has shown that in the roots of Melons, _Coleus_, etc.,
_Heterodora_ causes the cells in immediate contact with its head, and
which would normally become vessels of the xylem, to swell up into huge
giant-cells, with their walls curiously folded, and containing large
supplies of proteids and numerous nuclei, reminding us of the food-layer
of insect galls and of the tapetal layer of pollen-sacs. While the
stimulus exerted by the Nematode thus induces hypertrophy and storage
with food-substances of these cells, those of the next layers undergo
reticulate thickenings of their walls. Again instances of the evolution
of new tissue elements by the action of the foreign organism.
So far as galls on leaves are concerned the amount and kind of damage
done are in proportion to the area of chlorophyll action put out of play
for the benefit of the plant, and the remarks already made on p. 193
apply here also. Where buds are destroyed the effects may of course
extend further, but it rarely happens that leaf-galls are so abundant as
to maim a tree permanently. Nevertheless we must remember that cases
like _Phylloxera_ are notorious.
Far more dangerous, however, are the root-galls due to such insects,
because here the damage is not so local: the water-supplies are cut off,
and injurious consequences result from the absorption of the products of
decomposition in the soil.
NOTES TO CHAPTER XXIII.
In addition to the literature on galls quoted in the Notes to
Chapter XIV., the reader should consult Dale "On certain
Outgrowths (Intumescences) on the green parts of Hibiscus,"
_Proc. Cambr. Phil. Soc._, Vol. X., 1899, p. 192, and _Brit.
Ass. Rep._, Bradford, 1900.
The detailed study of the anatomy and histology of Galls has
been recently undertaken by Küster, "_Beitrage zur Kenntniss
der Gallenanatomie_," Flora, B. 87, 1900, p. 117, where the
principal references will be found.
On the root-galls due to Nematodes see Atkinson in _Science
Contributions from the Agric. Expt. Station, Alabama_, Vol.
I., p. 1, 1889; Percival, "An Eel-worm disease of Hops" in
_Natural Science_, Vol. VI., 1895, p. 187; and Molliard in
_Revue générale de Botanique_, Apl., 1900, p. 157, where the
histology is dealt with.
The nodules of the roots of Leguminoseae are not part of the
subject of this work: the literature is collected in _Science
Progress_, 1895, Vol. III., p. 252, and Dawson, _Phil.
Trans._, 1900.
CHAPTER XXIV.
EXCRESCENCES (_continued_).
_Cankers--Burrs--Sphaeroblasts, and other excrescences of
woody tissues--Witches' Brooms._
_Cankers_ are irregular excrescences due to the perennial struggle
between tissues attempting to heal up a wound, and some organism or
other agent which keeps the lesion open. A canker always originates in a
wound affecting the cambium, and usually in a small wound such as an
insect puncture or frost nip; if undisturbed the dead parts would heal
over by cork and callus, but if recurring frost-cracks break open the
coverings, or if insects or fungi penetrate the callus and invade the
cambium, irregularities of growth due to the occluding tissue on the one
hand, and continued growth of the still unimpaired cambium on the
opposite side of the injured shoot on the other, result in the canker.
Frost cankers occur on fruit-trees, Vines, Beeches, etc.
Cankers due to insects are found on Apples, the cortex of which is
punctured by the woolly Aphis (_Schizoneura_) while the twigs are young,
and the wound is kept open by the insects nestling in crevices in the
occlusion tissues. Species of _Coccus_, _Lachnus_, and _Chermes_ also
produce cankers on forest trees.
Cankers due to fungi usually originate in a wound primarily due to an
insect puncture or bite, or to frost, the invading fungus hyphae making
their way into the wounded tissues and gradually extending more and more
into the cambium and the occluding callus. Among the best known of these
wound fungi which cause cankers are _Dasyscypha Willkommii_ the peziza
of Larch disease, _Nectria ditissima_ and _N. cucurbitula_ on Beech and
Conifers; less common are _Scleroderris_ on Willows, _Aglaospora_ on
Oaks and some others.
_Peridermium Pini_ and _Aecidium elatinum_ also cause cankers under
certain conditions, as also does _Gymnosporangium_, but in these cases
the fungi are more truly parasitic.
In some cases--_e.g._ Ash, Pine, Olives--bacteria are concerned as
associated organisms in the cankering of trees.
_Burrs_ or _Knauers_ are irregular excrescences, principally woody, with
gnarled and warted surfaces. They are frequently due to some previous
injury, such as the crushing or grazing of cortical tissues by
cart-wheels. The excitation of the tissues thus wounded results in the
development of shoots from adventitious or dormant buds at the base of
old tree trunks, or in the starting of the same process where a branch
has been broken off. The new bud begins to develop a shoot, but soon
dies at its tip owing to paucity of food-supplies to the weak shoot,
while new buds at its base repeat the process next year with the same
result, and each of these again in turn, and so on. The consequence is
an extremely complex nest of buds, all capable of growing in thickness
and putting on wood to some extent, but not of growing out in length. In
course of time this mass may attain dimensions measurable by feet,
forming huge rounded and extremely hard-knotted burrs, the cross-section
of which shows the vascular tissues running irregularly in all
directions, and, owing to the very slow growth, extremely dense and
hard. The dark spots in such sections--_e.g._ Bird's-eye Maple--are the
cut bud-axes all fused together, as it were. On old Elms such burrs are
common at heights on the stem which preclude the assumption of any
coarse mechanical injury, and similar structures occur on the boles of
other forest trees suddenly exposed to light by the felling of their
companions, which suggests that these epicormic shoots result from some
disturbance due to the action of light.
_Witches' Brooms_ are irregular tufts of twigs often found among the
branches of trees such as Birches, Hornbeam, etc., where they look like
crows' nests, and similar structures are to be found on Silver Firs and
other conifers. In the former case they are due to _Exoascus_, in the
latter to _Aecidium_, fungi which are perennially parasitic in the
shoots, and stimulate the twiggy development of a number of buds which
would normally have remained in abeyance, or not have been formed at
all, and only do so now in a fashion different from that of normal
branches.
Rosette-like formations, depending on similar disturbing causes on the
part of insects, occur in conifers--_e.g._ _Gastropacha Pini_.
Dense tufts of twiggy shoots may be developed on many trees by pruning
in such a way as to stimulate the shooting out of basal buds which would
otherwise remain dormant, _e.g._ Elm, Ash, and thus it occurs that
injuries such as frost, insect bites, etc., may induce the production of
such tufts in a tree crown. The dense nests of stool-shoots thrown up
from felled tree-stumps are of essentially the same nature--partly
adventitious and partly dormant buds being enabled to grow out because
they can now be supplied with materials previously carried beyond them
while the trunk was still there. Suckers, if repeatedly cut down, may
also behave similarly.
_Wood-nodules_ or _Sphaeroblasts_ are curious marble-like masses of wood
which protrude with a covering of bark from old trunks of Beeches, etc.,
and can be readily dug out with a knife. The nodule has arisen by the
slow growth of the cambium of a dormant bud, the base of which separated
at an early date from the wood beneath; the cambium then closed in over
the base and laid on thickening rings all round the axis of the bud
except at the extreme apex. When the separation occurred the cambium of
the wood beneath covered over the previous point of junction, and thus
the woody bud was pushed out with the bark, and now protrudes covered
with a thin layer of the latter. Similar nodules are occasionally found
on Apple trees.
NOTES TO CHAPTER XXIV.
For further information on Cankers the student should read
Marshall Ward, _Timber and some of its Diseases_, Chapter X.
Further, the discussion as to the causes of canker in Frank,
_Krankheiten der Pflanzen_, B. I., p. 207, and B. III., pp.
167 and 172, and various papers in _Zeitschrift für
Pflanzen-krankheiten_.
CHAPTER XXV.
EXUDATIONS AND ROTTING.
_Tumescence--Rankness--Bursting of fruits, etc.--Root rot--Rot
of fruits--Bulb diseases--Flux--Honey-dew--Slime flux--
Resinosis--Gummosis--Manna._
I put together in one artificial class a varied group of diseases, the
principal symptom of which is the escape of fluids from the tissues,
under circumstances which betray an abnormal state of affairs, often
obvious, but sometimes only to be inferred. In many of these cases
bacteria abound in the putrefying mass, and some evidence exists for
connecting these microbes causally with the disease in a few of the more
thoroughly investigated cases, but in no case has this been sufficiently
demonstrated; and considering the ease with which bacteria gain access
_via_ wounds caused by insects and fungi, as well as by other agents,
the necessity for rigid proof must be insisted upon before we can accept
such alleged examples of _Bacteriosis_.
_Tumescence._--It occasionally happens that herbaceous parts of plants
pass into a condition of over-turgescence from excess of water in the
tissues, an abnormal state which indicates pathological changes
resulting from various causes, often not evident and therefore regarded
as internal. Such disease was formerly termed _Oedema_ or _Dropsy_.
This disease is frequently due to the excessive watering of pot plants
with large root systems and deficient foliage, in hot-houses with a
saturated atmosphere: it is, therefore, primarily referable to
diminished transpiration. It can sometimes be brought about by covering
potato plants, for instance, with a bell-jar in moist, hot weather; and
this, and the prevalence of the disease in hot-houses as compared with
plants grown out of doors, point to the above explanation. Similar
phenomena do occasionally occur out of doors in hot, moist situations or
during wet seasons, however, and the watery shoots of rank vegetation
are merely particular cases of the same class. Moreover, the well-known
tendency to succulence of sea-side varieties of plants which have thin
herbaceous leaves when growing inland, points to the action of the
environment in these matters, excess of salts being no doubt one factor
in such cases.
_Rankness_ affords another example where superfluity of water is
concerned, though it does not involve simply this, because the plant may
also contain excessive quantities of nitrogenous and mineral matters
taken up by the roots.
Rankness is, in fact, in many respects analogous to etiolation in so far
as the tissues are soft and surcharged with water, but it differs
fundamentally in the deep green of the chlorophyll: this may lead to
abundant assimilation if free access of air and drier conditions can be
gradually brought about. Any sudden drying, however, may be fatal to the
tender tissues.
Rankness commonly depends on excess of food materials, especially
nitrogenous manures, as may be seen in meadows and cornfields where the
manure heaps have remained on the ground and saturated it to excess as
compared with the rest of the soil; this may often be observed with
weeds, etc., in the neighbourhood of farm-buildings. If the period of
rank growth is accompanied and followed by days of suitably bright
sunshine and dry air, the increase of vegetative structures usually
results in increased flowering, heavy crops, or strong wood; but if the
rankness continues too long, or is accompanied by wet and dull weather,
the watery tissues are peculiarly susceptible to attacks of fungi and
insects, and to damage by sudden frosts or chilly winds. Rankness
affords, in fact, a typical illustration of predisposition to disease.
_Damping off._--When seedlings are too closely crowded in beds kept too
damp, or in moist weather, they are very apt to rot away, with all the
symptoms--spreading from a centre, contagious infection, mycelia on and
in the tissues, etc.--of a fungus attack. The commonest agent concerned
is one of the species of _Pythium_, the propagation of which is favoured
by the rank, over-turgid, and etiolated conditions of the plants.
Species of _Mucor_, _Botrytis_, and other fungi, may also be met with.
_Bursting_ of fleshy fruits, such as Tomatoes, Grapes, etc., is due to
over-turgescence in rainy weather or excessively moist air. But the
phenomenon is by no means confined to such organs. Hot-house plants when
oedematous not infrequently put out watery blisters from the cortex or
leaves, which rupture; and the stems of fleshy fasciated (_e.g._
Asparagus) or blanched and forced plants (_e.g._ Celery, Rhubarb) are
particularly apt to crack here and there from the pressure of the
turgescent tissues on the strained epidermis. Beets, Turnips, and other
fleshy roots show the same phenomena in wet seasons. That these ruptures
and exposures of watery tissues afford dangerous points of entry for
parasites and moulds will be obvious--_e.g._ _Edelfäule_, a rotten
condition of the grapes in the Moselle district.
_Root-rot_ is a common disease in damp, sour clay soils after a
continuance of wet weather--_e.g._ Wheat, especially if root-drawn and
exposed to thaw water.
In the disease known as Beet-rot, the roots turn black at the tip, where
the tissues shrivel and become grooved and wrinkled extensively. Inside
the flesh also blackens and finally rots. In earlier stages, only the
vascular bundles are brown and blocked with gum-like substances. In
advanced stages there is much gummy material in the lumina, and even
large cavities filled with this gum may be found.
The rot of Cherries, Pears, Apples, Plums, etc., in store may be due to
several fungi, of which _Botrytis_, _Monilia_, _Mucor_, _Penicillium_,
and _Aspergillus_ are the chief. The fruit may be attacked while still
on the tree, but very often fungi and bacteria gain access to the
tissues, through bruises, cracks, etc., formed in the fruit lying in the
storage baskets or on the shelves.
Rot in Onions, Hyacinth bulbs, etc., is frequently due to the access of
_Botrytis_ or _Sclerotinia_, followed by moulds, yeasts, and bacteria in
the stores.
_Sour-rot_ in Grapes, and other fleshy fruits which need much sun to
ripen them, is probably a usual result of continued cold, wet weather at
the cropping season, setting in when the fruits are beginning to swell.
_Flux._--It is a common event to see fluids of various kinds issuing
from wounds in trees, or congealing in more or less solid masses about
them; and owing to the prevailing tendency to compare plant diseases
with those of animals, we find such expressions as _Gangrene_, _Ulcer_,
and so forth, applied to these "open sores." In so far as such
outflowings frequently indicate diseased states of injured tissues which
are incapable of healing up, the analogy is perhaps a true one; but it
must be remembered that very different structures and processes in
detail are concerned. Moreover, liquid excretions more or less
indicative of diseased states are by no means confined to wounds or
definitely injured tissues, in which case such terms are wholly
misapplied.
_Honey-dew._--The leaves, or other organs, of many plants are sticky in
hot weather, owing to the excretion of a sweet liquid containing sugar,
the consistency and colour of which vary according to circumstances.
This honey-dew must not be confounded with the normal viscidity of
certain insectivorous plants--_e.g._ Sundew--or with the sticky
secretion on the internodes of species of _Lychnis_, etc., where it
plays the part of a protection against minute creeping things.
Honey-dew is often met with on Lime trees, Roses, Hops, etc. In many of
these cases the honey-dew is excreted by Aphides, which suck the juices
of the leaves and pour out the saccharine liquid from their bodies. The
sweet fluid is in its turn sought after by ants, and also serves as
nutritive material for various epiphytic fungi--_e.g._ sooty mould,
_Capnodium_, _Fumago_, and _Antennaria_--which give the leaves and
honey-dew a brown or black colour. Certain _Coccideae_ also excrete
honey-dew, especially in the tropics.
At least one case is known where honey-dew is formed as the result of
the parasitic action of a fungus, namely _Claviceps purpurea_ in its
conidial stage on the stigmas of cereals, and this may be compared with
the sweet odorous fluid excreted by the spermogonia of certain
_Aecidia_. In both cases the sweet fluid attracts insects which
disperse the spores.
Honey-dew may also be formed without the agency of fungi or insects,
when hot and dry days are followed by cool nights, with a saturated
atmosphere, _e.g._ _Caesalpinia_, _Calliandra_ and other trees in the
tropics, which are called rain trees owing to the numerous drops of
fluid which drip from the leaves under the abnormally turgescent
conditions referred to.
_Cuckoo-spit._--The leaves of Willows, Meadow grasses and herbs, etc.,
are often seen with froth on them, in which is a green insect,
_Aphrophora_, which sucks the juices from the tissues and excretes the
frothy watery cuckoo-spit from its body.
_Slime-flux._--The trunks of trees may sometimes be observed to pour out
a slimy fluid from cracks in the bark, or from old wounds, or branch
scars. In some cases, _e.g._ in Oaks, the slime has a beery odour and
white colour, and abounds in yeasts and other fungi to the fermentative
activity of which the odour and frothiness are due. In other cases the
slime is red _e.g._--Hornbeam; or brown--_e.g._ Apple and Elm; or
black--_e.g._ Beech, the colour in such cases being due to the mixture
of yeasts, bacteria, and fungi with which these slimes abound. The
phenomenon appears to be due to the exudation of large quantities of sap
under pressure--root pressure--and is primarily a normal phenomenon
comparable to the bleeding of cut trees in spring: the fungi, etc., are
doubtless saprophytes, but their activity is concerned with the
putrefactive processes going on in the diseased wood, and which may lead
to rotting of the timber.
The origin of the wounds in the bark and cortex, and which extend into
the wood and other tissues as the putrefactive and fermentative
processes increase, appears to be in some cases at least due to
lightning.
_Resin-flux_ or _Resinosis_.--The stems of Pines and other conifers are
apt to exude resin from any cut or wound made by insects, or by the
gnawing of other animals; but in many cases the flow is due to fungi,
_e.g._ _Peridermium_, the hyphae of which invade the medullary rays and
resin canals and thus open the way to an outflow through cracks in the
bark. _Agaricus melleus_ not only invades the resin passages, but
stimulates the tree to produce abnormal quantities of resin, which flows
down to the collar and roots, and exudes in great abundance at the
surface of the soil. Various other plants also exude resin from wounds,
and in some cases the flux seems to be increased by degeneration of the
tissues, _e.g._ _Copaifera_.
_Gummosis._--Cherries, Apricots, Acacias, and many other trees are apt
to produce abnormal quantities of gum, which flows from any wound or
exudes through cracks in the bark. Degeneration of the wood-cells, and
especially of the cell-walls of a soft wood formed by abnormal activity
of the cambium, points to its origin being due, in some cases at any
rate, to a conversion of the cellulose, and fungi are sometimes found in
the masses of gum; but beyond the fact that _gummosis_ is a pathological
phenomenon we know very little of the disease.
With regard to such gumming, it is significant how frequently pruned
trees--Cherries, Oranges, Lemons, Plums, etc.--suffer.
_Manna flux._--Certain trees, such as the Manna Ash, species of
Tamarisk, etc., yield manna from wounds, and in some cases the latter
are due to insects, _e.g._ _Cicada_.
The Potato-disease is best known by the pale whitish fringe, giving an
almost mealy appearance to the margins of the brown to black patches in
damp weather. In dry weather the brown patches shrivel and dry, and as
they are apt to be at the edges and tips of the leaflets, these curl up.
The young disease spots are yellowish, and the leaves of badly affected
plants are apt to be sickly yellow throughout.
This Potato-disease due to _Phytophthora_ must be distinguished from the
curling and puckering, with wilting and browning of the leaves and
yellow glassy look of the stems, due to the invasion of the vessels by a
fungus which lurks in the tubers, and gains access thence to the shoots.
In the disease traceable to _Phytophthora_ the stock remains green and
the leaves plump and plane, and only the brown patches slough out in wet
or shrivel in dry weather, and are bordered by the pale whitish zone of
conidiophores.
In the leaf-curl the yellow and flaccid appearance of all the leaves of
a stalk, or even of the plant, is the striking symptom, and the stem
soon droops and blackens just above the soil, a white mould appearing
also at the black spots. Subsequently black spots appear higher up, and
bacteria gain an entrance. The stolons rot, and eventually the roots and
the leaves wither. The tubers appear sound, but are small; they are apt
to rot in the store, the vascular zones turning brown.
This leaf-curl has been ascribed to _Pleospora_, _Polydesmus_,
_Verticillium_, and other parasites, as well as to excessive manuring
and other agencies, but it still needs explanation.
Rot of Potato tubers in the soil, or in store, may be brought about by
very different agents.
If _Phytophthora_ has obtained access, the fungus hyphae spread between
the cells, starting from the haulm, and cause the flesh to turn
yellowish and then brown in patches. On the exterior are discoloured
patches, depressed, with the flesh beneath brown and soft. The mycelium
spreads mostly in the outer layers, which though they turn deep brown
remain firm.
Wet rot of potatoes may be due to various fungi, and, in excess of
water, to putrefactive bacteria (_e.g._ _Clostridium_), which destroy
the cell-walls. The flesh becomes soft, then soup-like, and finally
putrefies to a liquid mass with a vile smell of butyric acid, etc., in
which the starch grains may be seen floating.
Tubers are often found with the cork burst and peeling in shreds, the
flesh more or less converted into a putrid and stinking pulp, with a
spotted brown boundary of partly destroyed but firmer tissue between the
dark utterly rotten and the white and still firm healthy flesh. The
principal agent in the destruction of the tissues is _Clostridium_, an
anaerobic bacillus which consumes the cell-walls but leaves the starch
intact. Hence a thoroughly decomposed tuber consists of a cork bag full
of starch and foetid liquid. In the dried condition the flesh shows a
brown marbling; this passes into a soft soupy starchy part, and here and
there may be violet grey cavities lined with _Spicaria_, _Hypomyces_,
etc., the white stromata of the latter often appearing externally. The
excavations are filled with loose starch grains, and bounded by cork and
cambium formed in the peripheral cells. The cell-walls eventually
undergo slimy decomposition.
_Spicaria_, _Fusisporium_, various moulds, and bacteria may all be
associated with wet-rot.
Dry-rot of Potatoes is also due to various fungi and bacteria, but the
destructive action goes on slowly, owing to there being no more moisture
than the tissues afford. The flesh becomes excavated here and there,
owing to the slow destruction of the cell-walls by _Clostridium_: the
destroyed tissues are brown, and the uninjured starch grains powder them
all over. Finally the whole shrunken mass has a crumbly consistency.
When the flesh remains white, but assumes a powdery consistency and
dry-rot, with the cork destroyed here and there, Frank refers the
damage to _Phellomyces_. Where the dry-rot is due to _Fusarium_ the
chalk-white stromata may often be detected breaking through the
periderm; but it must be remembered that the soil-contaminated, broken
skin of a potato-tuber is a favourable lurking spot for many fungi, and
_Periola_, _Acrostalagmus_, and others have been detected therein.
Brown spots, depressed into the flesh, sometimes result from the ravages
of _Tylenchus_, the minute worms being found in the diseased tissues.
In some cases the flesh turns watery and soft, grey, almost glass-like,
starting at the haulm end, and this may be owing to the invasion of
_Rhizoctonia_.
NOTES TO CHAPTER XXV.
The rotting of bulbs, roots, etc., has been much discussed
during the last few years in the pages of the _Gardeners'
Chronicle_, _Zeitschrift für Pflanzenkh._, and elsewhere. The
principal references to Bacteriosis--the rot in which bacteria
are stated to be the primary agent causing these and similar
diseases--may be found in Massee, _Diseases of Plants_, pp.
338-342, and more fully in Russell, _Bacteria in their
Relation to Vegetable Tissue_, Baltimore, 1892; and in Migula,
_Kritische Uebersicht derjenigen Pflanzen-krankheiten, welche
Angeblich durch Bakterien verursacht werden_, Semarang, 1892.
The most convincing accounts, however, are since that date;
see Smith, "Pseudomonas Campestris," _Cent. f. Bakt._, B.
III., 1897, p. 284, and Arthur and Bolley, _Bacteriosis of
Carnations_, Perdue University Agr. Expt. Station, 1896, Vol.
VII., p. 17. Woods has lately shown that this disease is due
to Aphides only, the bacteria having nothing to do with the
disease primarily, _Stigmonose_, _Bull. 19_, U.S. Dept. Agr.,
1900; but it is necessary to bear in mind that actual
penetration of the cell-walls from without must be proved, as
De Bary proved it for germ-tubes of fungi, before the evidence
that Bacteria are truly parasitic in living plants can be
called decisive. This is a difficult matter, but until it is
settled we do not know whether these organisms are really
parasitic in the sense that _Phytophthora_ is, or merely gain
access by other means--I have traced them through dead
fungus-hyphae--to the vessels, dead cell-walls, etc. The proof
of infection _via_ water pores and vessels is given for one
species by Harding, "Die Schwarze Faulnis der Kohls," etc.,
_Cent. f. Bakt._, Abh. II., B. VI., 1900, p. 305, with
literature.
Concerning the "Damping off" of seedlings, see Marshall Ward,
"Observations on the Genus Pythium," _Quart. Journ. Microsc.
Soc._, Vol. XXIII., 1883, p. 485, and Atkinson, _Bull. 94 of
Cornell University Agric. Expt. Station_, 1895, p. 233.
On Bacteriosis in Turnips, see Potter, _Proc. R. S._ 1901,
Vol. LXVII., p. 442.
CHAPTER XXVI.
NECROTIC DISEASES.
_Patches--Frost-patches--Bruising due to hail, shot, etc.--
Fire--Sun-burn or scorching--Sun-cracks. Dying-back--Frost--
Fungi--Wound fungi--Defoliation by insects--Defoliation by
hand--Staghead._
_Necrosis._--This is a general term for cases where the tissues
gradually turn brown or black in patches which die and dry up, the dead
area sometimes spreading slowly and invading the usually sharply
demarcated healthy tissues around. It is a common phenomenon on the more
slender stems or branches of trees, especially those with a thin cortex,
and the terms _Brand_ or _Scorching_ sometimes applied signify the
recognised resemblance between burnt patches and these dead areas of
necrotic tissue.
Necrosis is often due to frost, which kills the cortex of Pears, Beech,
etc., in patches of this kind. The dead cortex and cambium stick to the
wood beneath and contract as they dry. The living cambium and cortex
around them then begin to push in callus towards the centre of the
necrotic area; but since this callus is formed under the pressure of the
cortical tissues it does not form a thick lip or margin to the healing
wound, as it does in a Canker, but insinuates itself with thinned-off
edges between the wood and the dead tissue, or at most traps a little of
the latter in the final closing up of the wound. It is easy to see how
such an area of Necrosis may become a Canker if the dead tissues split
or slough off, and fungi or insects obtain access to the callus at the
margins of the area, setting up the disturbances described on p. 222. As
matter of fact many Cankers--_e.g._ those of the Larch disease, and
those due to _Nectria_, or Aphides, etc.--often begin as flattened or
depressed areas of Necrosis started by frost, and many small necrotic
patches would eventually become Cankers if not healed up by the callus.
Necrosis may also be due to the bruising of the tissues by large
hailstones, to gun-shot wounds, or to any form of contusion which kills
the living cells of cortex and cambium.
Necrosis is a natural and common result of fire, and it frequently
happens after forest-fires which have run rapidly through the dry
underwood, fanned by steady winds, that the lower parts of the boles are
scorched on one side only. The killed cambium and cortex then dry up in
black necrotic patches, which may eventually heal up by intrusion of
callus from the uninjured parts.
_Sun-burn_ or _Scorching_.--If thin-barked trees, such as Hornbeam,
Beech, Firs, etc., which have been growing in partial shade owing to
dense planting, are suddenly isolated by thinning, the impingement of
the sun's rays on the south-west side during the hottest part of summer
days may kill the cambium, and produce necrosis of the cortical tissues,
and such necrotic patches heal very slowly or not at all, because the
dead tissues have contracted so tightly on to the wood below that the
callus cannot readily creep between.
_Sun-cracks_ are due to intense insolation on the south side of trees in
clear weather in early spring, causing the drying and contraction of the
wood and its coverings down that side of the tree: the contracted
tissues consequently split, as in the case of frost-cracks, the healing
up of which is very similar.
_Dying-back._--All that is true of the necrosis of cortical tissues in
small patches also applies to cases where the whole of the outer tissues
of thin twigs and branches die of inanition owing to a premature fall of
leaves--_e.g._ after a severe attack of some insect or fungus pest. The
consequent arrest of the transpiration current and the proper supply of
nutriment to the cambium and cortex explain the phenomena. The younger
branches of Coffee trees suffering from severe attacks of leaf-disease
are often denuded of leaves and die back from the causes mentioned, the
whole of the outer tissues becoming necrotic, and drying up tight on to
the wood, because other branches with functionally active leaves on them
divert the transpiration current, and drought and inanition supervene.
Dying-back is frequently also a direct effect of early frosts, which
kill the thin twigs before the "wood is ripened," as gardeners say.
Dying-back is also a frequent result of direct frost action on thin
watery shoots or "unripe wood," and is apt to occur every year in
certain varieties of Roses, for instance, in particular situations, such
as "frost-beds," or aspects exposed to cutting winds, and so forth. The
necrosis which results may affect all the tissues, or only the cortex
and cambium, and the frequent accompaniment of all kinds of saprophytic
_Ascomycetes_ and moulds or other fungi is in no way causal to the
phenomenon.
Dying-back may also be caused by fungi, and not necessarily parasites,
for cases are often observed where saprophytes only are to be found in
the necrotic tissues of the cortex, having made their way in through
minute cracks, lenticels, etc.
A simple case is often seen in Chrysanthemums, Roses, etc., chilled and
wetted to danger point, but not frozen, during the nights of autumn. The
lowered resistance of the chilled tissues enables fungi like _Botrytis
cinerea_ to gain a hold, and the peduncles die-back with all the
symptoms of Necrosis, the fungus gaining power more and more as its
mycelium spreads in the dead tissues.
Many other cases are known where wound-fungi, such as _Nectria_,
_Cucurbitaria_, _Phoma_, etc., in themselves incapable of true
parasitism, gain a hold on the necrotic tissue of a wounded twig, and
having laboriously accumulated a vigorous mycelium saprophytically,
extend into other parts. In many of these cases the dying-back of the
twigs is expedited owing to the mycelium invading the medullary rays and
wood vessels, and so obstructing the transpiration current. The much
more rapid spread of the hyphae up into the parts thus killed
sufficiently indicates the fundamentally saprophytic character of such
fungi.
Dying-back in all its forms is a common result of defoliation by
insects, _e.g._ caterpillars, especially if it occurs when the wood is
depleted of reserve materials, and thus cannot supply the auxiliary buds
and enable the twigs to clothe themselves with a new flush of foliage, a
common danger in Conifers.
Any form of defoliation--_e.g._ excessive plucking of tea and mulberry
leaves, browsing of animals, etc.--exposes the twigs to the dangers of
dying-back, the accessory phenomena being similar to those already
described.
_Stag-head._--Old trees, though vigorous and in full foliage throughout
the crown generally, frequently lose the power of bearing leaves on
their topmost branches and twigs, which stand out bare and brown, and
fancifully resemble the antlers of a stag: hence the forester's name
"stag-head." This "top-dry" condition is frequently due to the removal
of litter, or to excessive draining, or to the roots having gradually
penetrated into unsuitable soil. The consequence is that some dry
summer the drought causes the breakage of the water columns above, and
the twigs die back.
Tropical trees may also become _stag-headed_ owing to the attacks of
_Loranthus_ and other parasites, the portions above the point of
attachment dying back from inanition.
Cases also occur in the tropics where the _stag-head_ condition is due
to the persistent roosting of frugiferous bats--"flying foxes"--which
tear the bark and foliage with their claws, and befoul the twigs
generally.
NOTES TO CHAPTER XXVI.
The principal literature as regards frost is given in the
works of Frank, Sorauer, and Hartig already referred to. An
excellent summary will be found in Hartig's _Diseases of
Trees_, p. 282, and in Fisher "Forest Protection," Vol. IV. or
Schlich's _Manual_, p. 423.
CHAPTER XXVII.
MONSTROSITIES AND MALFORMATIONS.
_Monstrosities--Teratology--Atrophy of organs--Shanking of
grapes--Barren fruit trees--Dwarfing--Distortions and
malformations--Fasciations--Flattened roots--Torsions--Curling
and puckering--Leaf rolling--So-called "spontaneous"
teratological changes._
_Monstrosities._--In a wide sense this term is applicable to many cases
here treated under other headings, and signifies any departure from the
normal standard of size, form, arrangement, or number of parts, and so
forth, due to arrest of growth, excessive growth of parts, or of the
whole organs, etc.
Such _teratological_ conditions are however by no means always
_pathological_: that is to say, they may be variations which do not
threaten the existence of the plant. In some cases they are clearly due
to exuberant nutrition, and although they may occasionally predispose to
disease, in others they show no evidence of doing so. The whole
practice of horticulture and agriculture abounds in examples of
teratological sports or varieties which are transmissible by seeds,
budding and grafting, and other means--_e.g._ double flowers,
hypertrophied floral organs (cauliflowers), seedless grapes and oranges,
crested ferns, etc.; and even when such varieties could not live as such
in a state of nature, there is evidence to show that many of them
readily revert to the original seed-bearing or single condition, and
adapt themselves to the altered environment.
Every part of the plant may exhibit teratological changes, and I shall
for the most part select cases in illustration which indicate approach
to pathological states, and group with them cases known to be
pathological in origin.
_Atrophy_ is a common phenomenon denoting dwindling or reductions in
size of organs due to insufficient nutrition, or arrest of growth from
various causes.
Atrophy of leaves is a common result of the attacks of parasitic fungi,
even when the latter induce local hypertrophy--_i.e._ excessive growth
of particular parts, _e.g._ _Synchytrium_ on Dandelions and Anemones.
_Puccinia suaveolens_ causes partial atrophy of the leaves of Thistles,
_Aecidium Euphorbiae_ of those of _Euphorbia_.
The carpels of Anemone are atrophied in plants attacked by _Aecidium_,
and the whole flower is suppressed in Cherries infested with _Exoascus
Cerasi_, while other fungi--_e.g._ _Cystopus_, _Exoasci_, etc.--cause
atrophy of the seeds, and numerous instances of atrophied grain occur in
plants infested with Ustilagineae.
Atrophy of the grains of cereals is sometimes due to the direct attack
of animals, _e.g._ eel-worms (_Tylenchus_) eat out the grains of Corn;
weevils and other beetles (_Curculio_, _Bruchus_, etc.) similarly devour
the contents of grain and nuts, the flowers of Peas and Apples, and so
forth, inducing atrophy of the parts left. Still more striking cases are
afforded by small insects which bore into the halms of cereals, and
cause atrophy of the whole ear--_e.g._ _Cephus_ in Wheat and Rye. Barley
occasionally withers after flowering, the grain atrophying from no known
cause, terms like _consumption_ given to the disease conveying no
information.
Atrophy of young fruits is commonly due to the flowers not
setting--_i.e._ some agent has interfered with the normal transference
of the pollen to the stigma. This may be due to excessive rain washing
out the pollen (_e.g._ Vine), to a lack of the necessary insects which
effect pollination, often seen in greenhouse plants; to the stamens
being barren--_e.g._ certain varieties of Vine--or to the premature
destruction of the stigmas by frost, as in Cherries, Pears, etc., or by
insects, as in Apples, or fungi, _e.g._ the infection of bilberries with
_Sclerotinia_; or even by poisonous gases, as is sometimes seen in
Wheat, etc., growing near alkali works. Drought is also a common cause
of atrophy of young Plums.
_Shanking of Grapes_ is a particular case of atrophy and drooping of the
immature fruits, due to the supplies being cut off by some agency. It
may arise from very various causes which bring about disease in the
leaves or roots, and should always be looked upon as a sign of weakness
in the Vine, the structure of which is affected, _e.g._ poor wood--or
the functions interfered with, _e.g._ water supplies deficient owing to
paucity of roots.
Barren Apple, Pear, Plum, and other flowers are often found to have been
bored through the petals while in bud, and the whole "heart" of the
flower eaten out by the grubs of _Anthonomus_, leaving the unopened buds
brown and dead, as if killed by frost or drought, and often erroneously
supposed to be so.
The wilting and shrivelling of Clover is sometimes due to _Sclerotinia_,
the mycelium of which pervades the roots and stock, on which the
sclerotia may be found. Lucerne is similarly killed in Europe by the
barren mycelium of _Leptosphaeria_, which may be found as a purple mat
on the roots.
_Dwarfing_ consists in partial atrophy of all the organs, and is a
common result of starvation in poor, dry, shallow soils, as may often be
seen in the case of weeds on walls or in stony places. Dwarfs which are
thus developed in consequence of perennial drought are not, however,
necessarily diseased, in the more specific sense of the word; their
organs are reduced in size proportionally throughout in adaptation to
the conditions, and simply carry out their functions on a smaller scale.
Dwarfing is frequently a consequence of the lack of food materials, or
of some particular ingredient in the soil, and in such cases is a
diseased condition of some danger; similar results may ensue in soils
containing the necessary chemical elements, but in unavailable forms.
Dwarfing may also be brought about by repeated maiming, nipping off the
buds, pruning, etc., as in the miniature trees of the Japanese; and the
case of trees continually browsed down by cattle, or of moor plants
perennially dwarfed by cutting winds, are further illustrations in the
same category, as are also those of certain alpine and moraine plants,
whose only chance of survival depends on their adapting themselves to
the repeated prunings suffered by every young shoot which rises into the
cutting winds, since there is no question of lack of food-materials in
these cases.
The practice of the Japanese is to pinch out the growing tips of the
shoots wherever they wish to prune back, and it is by the judicious use
of this heading in, and suitable pot-culture, that the dwarfs are made,
6-20 inches high at from 30-80 years old.
Dwarfing is often brought about by grafting on a slow-growing stock, and
this method is employed in practice, as are also heading in, pruning of
roots, and confinement in pots.
Dwarfing may also be due to poor or shrivelled--partially
atrophied--seeds or such as have had their endosperms or embryos injured
by insects or fungi, and although it is possible to nurse such dwarfs
into normal and vigorous plants with good culture, they do not usually
recover under natural conditions in competition with more vigorous
plants.
_Distortions_ or _Malformations_ may be defined as abnormalities in the
form of organs which concern all, or nearly all the parts, and do not
refer merely to swellings or excrescences on them or excavations, etc.,
in them.
_Fasciation._--Shoots of Asparagus, Pine, Ash, and many other plants are
occasionally expanded into broad ribbon-like structures often studded
with more than the normal number of buds or leaves, etc., such as would
be found on the usual cylindrical shoots. Such _fasciations_ are due to
several buds fusing laterally under compression when young and the whole
mass growing up in common, or, in a few cases, to the unilateral
overgrowth of one side of the terminal bud. Fasciations appear to depend
on excessive nutrition in rich soils. They may spread out above in a
fan-like manner, exaggerating the abnormality, or they may revert to the
original form. Some cases are more or less fixed by heredity--_e.g._
_Celosia_. Fasciated stems are frequently curved like a crozier, owing
to one edge growing more rapidly than the other.
Cauliflowers are really cultivated monstrosities. Fasciated Dandelions,
_Crepis_, monstrous Chrysanthemums, peloric _Linaria_, five-leaved
Clovers, spiral Teazels, etc., may all, if grown with care, be kept more
or less constant in the monstrous state. That is to say, the particular
kinds of variation here manifested can be maintained in proportion as
the external conditions controlling the variation are maintained. Such
conditions are chiefly rich supplies of food-stuffs, plenty of water and
air, suitable temperature and lighting, etc. Mutilations, favouring the
development of abnormal buds may also induce fasciations.
_Torsions_ or spiral twistings of stems also frequently arise among
plants grown in rich soils, and are often combined with
fasciations--_e.g._ Asparagus, _Dipsacus_; and De Vries has shown that
the peculiarity is not only transmissible by seed, but may be more or
less fixed by appropriate culture.
_Contortions_ of stems are often due to the unequal growth on different
sides of the stems owing to the presence of fungi--_e.g._ _Caeoma_ on
Pines, _Aecidium_ on Nettles, also _Puccinia_ on petioles of Mallow,
_Cystopus_ on inflorescences of _Capsella_, etc.
_Distortions_ of roots may be brought about in various ways by the
hindrances afforded by stones.
_Spiral roots_ occur occasionally in pot plants.
_Flattened roots_ usually result from compression between rocks, the
young root having penetrated into a crevice, and been compelled to adapt
itself later. The distortions of stems by constricting climbers, wire,
etc., have been described, and fruits--_e.g._ Gourds--are easily
distorted by means of string tied round them when young.
Distortions of leaves are very common, and are sometimes
teratological--_i.e._ due to no known cause--_e.g._ the pitcher-like or
hood-like _cucullate_ leaves of the Lime, Cabbage, _Pelargonium_, etc.,
and of fused pairs in _Crassula_. Also coherent, bifurcate, crested,
displaced and twisted leaves occasionally met with, and in some cases
fixed by cultivation, may be placed in this category.
_Puckers_ must be distinguished from pustules, since they consist in
local upraisings of the whole tissue, not swellings--_e.g._ the
yellowish green pockets on Walnut leaves, due to _Phyllereum_.
Puckered leaves in which the area of mesophyll between the venation is
increased by rising up in an arched or dome-like manner are sometimes
brought about by excessive moisture in a confined space.
_Leaf-curl_ is a similar deformation caused by fungi, such as _Exoascus_
on Peaches.
Wrinkling or puckering of leaves is also a common symptom of the work of
Aphides--_e.g._ Hops.
Characteristic curling and puckering, with yellow and orange tints, of
the terminal leaves of Apples, Pears, etc., are due to insects of the
genera _Aphis_, _Psylla_, etc.
Small red and yellow spots with puckerings and curlings of the young
leaves of Pears, the spots turning darker later on, are due to
_Phytoptus_.
_Leaf-rolling._--The leaves of Beeches, Poplars, Limes, and many other
plants, instead of opening out flat, are often rolled in from the
margins, or from the apex, by various species of _Phytoptus_,
_Cecidomyia_, or other insects, which puncture or irritate the
epidermis in the young stages and so arrest its expansion in proportion
to the other tissues. According as the lower or upper surface is
attacked the rolling is from the morphologically upper surface
downwards, or _vice versa_. Very often the mesophyll is somewhat
thickened where rolled and _Erineum_-like hairs may be developed--_e.g._
Lime. Many caterpillars also roll leaves, drawing the margins inward to
form shelters--_e.g._ _Tortrix viridana_, the Oak leaf-roller. Certain
beetles--_Rhynchitis_--also roll up several leaves to form a shelter in
which the eggs are laid.
Webs are formed among the mutilated leaves of Apples by the caterpillars
of _Hyponomeuta_.
It must be borne in mind that instances can be found of teratological
change of every organ in the plant--_e.g._ stamens transformed into
carpels or into petals; anthers partly polliniferous and partly
ovuliferous; ovules producing pollen in their interior, and so on, being
simply a few startling examples of what may happen. Such abnormalities
are frequently regarded as evidence of internal causes of disease, and
this may be true in given cases; in a number of cases investigated,
however, it has been shown that external agents of very definite nature
bring about just such deformations as those sometimes cited as examples
of teratology due to internal causes, and the question is at least an
open one whether many other cases will not also fall into this category.
The study of galls has shown that insects can induce the formation of
not only very extraordinary outgrowths of tissues and organs already in
existence, but even of new formations and of tissue elements not found
elsewhere in the plant or even in its allies; and Solms' investigations
on _Ustilago Treubii_ show that fungi can do the same, and even compel
new tissues, which the stimulating effects of the hyphae have driven the
plant to develop, to take part in raising and distributing the spores of
the fungus--_i.e._ to assume functions for the benefit of the parasite.
Molliard has given instances of mites whose irritating presence in
flowers causes them to undergo teratological deformations, and Peyritsch
has shown that the presence of mites in flowers induces transformations
of petals into sepals, stamens into petals. Similarly De Bary, Molliard,
Magnus, Mangin, and Giard have given numerous cases of the
transformation of floral organs one into another under the irritating
action of fungi, of which the transformation of normally unisexual
(female) flowers into hermaphrodite ones, by the production of stamens
not otherwise found there, are among the most remarkable.
These and similar examples suffice to awaken doubts as to whether any
teratological change really arises "spontaneously," especially when we
learn how slight a mechanical irritation of the growing point may induce
changes in the flower; _e.g._ Sachs showed that a sunflower head is
profoundly altered by pricking the centre of the torus, and Molliard got
double flowers by mechanical irritation.
NOTES TO CHAPTER XXVII.
For the details and classification of the multitude of facts,
the student is referred to Masters' _Vegetable Teratology_,
Ray Society, 1869, and the pages of the _Gardeners' Chronicle_
since that date.
Concerning torsions, etc., the student should read De Vries,
"On Biastrepsis in its Relation to Cultivation," _Ann. of
Bot._, Vol. XIII., 1899, p. 395, and "Hybridising of
Monstrosities," _Hybrid Conference Report_, _Roy. Hort. Soc._,
1900, Vol. XXIV., p. 69.
The reader will find an excellent account of the abnormalities
in flowers due to the action of parasitic insects and fungi in
Molliard, "Cécidies Florales," _Ann. des Sc. Nat._, Ser.
VIII., Bot., T. 1, 1895, p. 67.
CHAPTER XXVIII.
PROLIFERATIONS.
_Proliferations--Vivipary--Prolepsis--Lammas shoots--Dormant
buds--Epicormic shoots--Adventitious buds--Apospory and
apogamy._
_Proliferation_ consists in the unexpected and abnormal on-growing or
budding out of parts--stems, tubers, flowers, fruits, etc.--which in the
ordinary course of events would have ceased to grow further or to bear
buds or leaf-tufts directly. Thus we do not expect a Strawberry--the
swollen floral axis--to bear a tuft of leaves terminally above the
achenes, but it occasionally does so, and similarly Pears may be found
with a terminal tuft of leaves, Roses with the centre growing out as a
shoot, Plantains (_Plantago_) with panicles in place of simple spikes,
and so on.
We regard such cases as _teratological_, because they are exceptional
for the particular species, and as _pathological_ because they appear to
be connected with over-feeding in soils with excessive supplies of
available food-materials; but it should be noted that conditions quite
comparable to proliferation are normal in the inflorescences of
Pine-apples, some Myrtaceae, Conifers, etc., and that many instances of
proliferations come under the head of injurious actions of fungi,
insects, and other agents.
_Proliferation_ of tubers is sometimes seen in Potatoes still attached
to the parent plant in wet weather following a drought. The eyes grow
out into thin stolons, or forthwith into new tubers sessile on the old
tuber. Similarly in store we sometimes find the eyes transformed
directly into new tubers, and cases occur where the growth of the eye is
directed backwards into the softening tuber, and a small potato is
formed inside the parent one.
Threading is also occasionally met with in the "sets" when ripened too
rapidly in hot dry soils.
_Vivipary_ is a particular case of proliferation, in a certain sense,
where the seeds appear to germinate _in situ_, and we have small plants
springing from the flowers, reminding us of wheat which has sprouted in
the shocks in damp weather. In reality, however, the grains are here
replaced by bulbils which sprout before they separate from the
inflorescence. In varieties of _Poa_, _Polygonum_, _Allium_, _Gagea_,
etc., this phenomenon is constant in plants growing in damp situations.
_Prolepsis._--It frequently happens that branches or whole plants are
suddenly defoliated in summer,--_e.g._ by caterpillars or other
insects--at a time when considerable stores of reserves had already been
accumulated during the period of active assimilation. In such cases the
axillary buds, which would normally have passed into a dormant condition
over the winter had the leaves lived till the autumn-fall, suddenly
shoot out into _proleptic_ shoots (also termed Lammas shoots), and
reclothe the tree with foliage. The wood of the year in which this
occurs may exhibit a double annual ring, and the vigour of the tree is
likely to suffer in the following season and no fruit be matured.
Proleptic branches may also be due to the shooting out of accessory
buds--_i.e._ extra buds found in or near the leaf-axils of many plants,
such as Willow, Maples, _Cercis_, _Robinia_, _Syringa_, _Aristolochia_,
etc.--which do not normally come to anything, or do so only if a surplus
of food materials is provided.
_Dormant buds_, or _preventitious buds_, are such as receive no
sufficient supply of water and food materials to enable them to open
with the other buds in ordinary years, for in most trees only the upper
buds on the branches develop into new shoots. The lower buds do not die,
however, but merely keep pace with the growth in thickness of the parent
branch, and may be elongated sufficiently each year to raise the minute
tips level with the bark, their proper cambium only remaining alive but
not thickening the bud.
When, by the breaking of the branch above the insertion of the dormant
bud--or by pruning, defoliation by insects, etc.--the transpiration
current and supplies of food materials are in any way deflected to the
minute cambium and growing points of the dormant buds, they are
stimulated to normal growth, and may grow out as _epicormic shoots_ or
"shoots from the old wood." In many cases such epicormic shoots are
stimulated to grow out by suddenly exposing an old tree to more
favourable conditions of root-action and assimilatory activity, owing to
the felling of competing trees which previously hemmed it in from light
and air, and restricted the spread and action of its roots in the soil.
This is often seen in old Elms, Limes, etc.
It is by such means as the above that substitution branches are obtained
when a leader is broken or cut away.
_Adventitious buds_ are such as are newly formed from callus or other
tissues in places not normally provided with buds, as is often seen on
occluding wounds--_e.g._ stool shoots. They may also be developed on
roots, a fact utilised in propagating _Bouvardias_, Horse-radish, etc.,
by means of root-cuttings, and the _suckers_ of Plums and other fruit
trees are shoots springing from adventitious buds on roots.
Adventitious buds are also common on leaves (_e.g._ _Bryophyllum_,
Ferns, etc.), and are frequently induced on them by wounds--_e.g._
_Gesneria_, _Gloxinia_, etc. Even cut cotyledons may develop them, and
pieces of leafless inflorescence (Hyacinth), hypocotyl (_Anagallis_),
and in fact practically any wounded tissue with a store of reserve
materials may be made to develop them: thus they have been found arising
from the pith of Sea-kale, and are commonly developed from the cut bulb
scales of Hyacinths.
_Apospory_ and _Apogamy_ are particular cases of the production of
vegetative buds on the leaves in place of sporangia in Ferns (Apospory),
and on prothallia in place of Archegonia (Apogamy), in the latter case
induced by dry conditions and strong illumination.
NOTES TO CHAPTER XXVIII.
In addition to the literature quoted in the notes to Chapter
XXVII., the student should consult the works on Forest Botany
for the scattered information regarding adventitious buds. A
good account may be found in Büsgen, _Bau und Leben unserer
Waldbäume_, Jena, 1897.
For Apospory and Apogamy, see Lang "On Apogamy and the
Development of Sporangia upon Fern Prothalli," _Phil. Trans._,
vol. 190, 1898, p. 187, where the literature is collected.
CHAPTER XXIX.
GRAFTS.
_Grafting--Comparison with cuttings--Effects of environment--
Relations between scion and stock--Variation in grafts--
Grafting and parasitism--Infection--Pollination--Grafts-hybrids
--Predisposition of Natural grafts--Root-fusions._
Grafting is a process which consists in bringing the cambium of a shoot
of one plant into direct union with that of another, and is practised in
various ways, the commonest of which is as follows:
One plant--the _stock_--rooted in the ground, is cut off a short
distance above the surface of the soil, and a shoot from the second
plant--the _scion_--cut off obliquely with a sharp knife, is inserted
into a cleft in the stock, so that the two cambiums (and sometimes the
cortex and pith of each as well) are in close contact: the scion is then
tied in position, the wounds covered with grafting wax, and the whole
left until union of the tissues is completed. This union depends on the
formation of _callus_ at the cut surfaces, and the intimate union of the
ingrowing cells from each callus.
The development of the callus follows the course described for wounds,
cuttings, etc., and the union is exactly comparable to the union of the
two lips of a healing callus over a wound (see p. 197).
Grafting was known and practised far back in the ages. Virgil was well
acquainted with the process, and Theophrastus compared it with
propagation by cuttings.
The scion differs from a cutting, however, in having no roots of its
own: it is parasitic upon, or rather is in symbiosis with the stock, the
root and tissues of which intervene between it and the soil.
Consequently the selective absorption, size and number of vessels, and
innumerable other physiological and anatomical peculiarities of the
stock determine what and how much shall go up into the scion, while the
latter supplies the former with organic materials and rules what and how
much food, enzymes, and other secretions, etc., it shall receive to
build up its substance. Surely, then, if such factors as the nature of
the soil, the water and mineral supplies, the illumination, and the
various climatic factors of altitude can cause variations on a plant
direct, these and other factors are still more likely to be effective on
stock and scion, and each must affect the other.
Nevertheless opinions have differed much as to whether any important
effect is to be seen, and on no point more than on whether the scion
can affect the stock, in spite of such examples as _Cytisus Adami_,
_Garreya_ on _Aucuba_, Sunflower on Jerusalem Artichoke, etc. Recent
results, especially of experiments with herbaceous plants, show that not
only can the stock affect the scion (and _vice versa_) directly, but the
effect of the changes may be invisible on the grafted plant and only
show itself in the progeny raised from the seed of the grafted plant. In
other words, variation occurs in grafts either _directly_, as the
results of the effects of the environment on the graft, or owing to the
interaction of scion and stock, showing as changes in general nutrition
in the tissues concerned, etc., owing to special reactions of the
protoplasm of the uniting cells one on the other, and of the results of
the further protoplasmic secretions, sortings, and so forth, on the
cells developed as descendants of these in the further growth of the
graft: or _indirectly_, in that some of these changes so alter the
nature of the special protoplasm put aside for reproductive purposes,
that the resulting embryo in the seed transmits the effects, and they
show as variations in the seedling. If these results are confirmed they
should meet all objections that have been urged against the transmission
of acquired characters.
In fact there are analogies between grafting and parasitism which cannot
be overlooked, and should not be underestimated, their commonest
expression appearing in the alterations in stature, habit, period of
ripening, and so forth. These analogies are easily apprehended when we
compare parasites like the Mistletoe, _Loranthus_, or even such
root-parasites as the Broom-rapes and the Rhinanthoideae with grafts;
but they also exist in the case of many fungus-parasites, and we might
almost as accurately speak of _grafting_ some fungi on their hosts as of
_infecting_ the latter with them, especially when it is borne in mind
that the effect of the scion on the stock is by no means always to the
benefit of the latter, and that there are reasons for regarding the
action of some such unions as that of a sort of slow poisoning of the
stock by the scion. Why do we not here say that the stock has been
_infected_ by the scion?
The resemblances between pollination and the infection by fungus hyphae
may also be insisted upon. If we take into account Darwin's remarkable
experiments showing that in "illegitimate unions" the pollen exerts a
sort of poisonous action on the stigmas or ovules, it is possible to
arrange a series of cases starting with perfectly legitimate
pollinations where the pollen tube feeds as it descends the style on
materials provided by the cells, and proceeding to cases where the
pollen is more and more merely just able to penetrate the ovary and
reach the ovules, to the extreme cases where no union at all is
possible.
Side by side with such series could be arranged analogous cases where
fungus spores can enter and infect the cells of the host, and live
symbiotically with or even in them, or can penetrate only with
difficulty, or with poisonous effects, and finally cannot infect the
plant at all.
Less obviously, but nevertheless existing, are gradations in grafting to
be observed, where one and the same stock may be successfully combined
with a scion which improves it--or which is improved by it--or the scion
may unite but acts injuriously on it, or, finally, cannot be induced to
unite.
But we may go further than this in these comparisons. Just as the
results of pollination frequently induce far-reaching effects on distant
tissues--_e.g._ the swelling of Orchid ovaries, and rapid fading of the
floral organs--so also the effects of hyphae in the tissues may induce
hypertrophies, deflection of nutrient materials, and the atrophy of
distant parts--_e.g._ the curious phenomena observed in _Euphorbia_
attacked by _Uromyces_--and some of the distant actions in grafts may be
compared similarly.
Going still further, we may compare the effects of cross-breeding or of
hybridisation, where the _progeny_ show that changes have resulted from
the mutual interactions and reactions of the commingled protoplasm, with
Daniel's results, in which he obtains proof of such interactions of the
commingled protoplasmic cell-contents of grafts in the seedling progeny;
although there is no probability--we may even say possibility--in this
latter case that the effects are due to nuclear fusions, but only that
the germ-plasm of the seed-bearing plant has been affected by the
changes in the cell-protoplasm which nourishes it when the reproductive
cells are forming.
In the case of graft-hybrids the matter appears to be somewhat
different, and we may well suppose, with Strasburger, that the
commingling of characters observed in flowers, fruits, foliage, etc., on
shoots borne after grafting are due to the occurrence of nuclear fusions
during the union of the grafted tissues; though it is by no means
impossible that what has really happened is profound alterations in the
nuclear substance (germ-plasm) owing to its being nourished by
cell-protoplasm (somato-plasm) which has been itself affected by the
interchanges of substance between scion and stock, and therefore itself
furnishes a different nutrient medium from the unaltered cytoplasm of
either.
But even here we can find parallels among the ordinary phenomena of
plant reproduction. Maize plants with white endosperm containing starch,
if crossed by pollen from other plants with purple endosperm containing
sugar, bear seeds with purple endosperm containing sugar, and such
_Xenia_ may be compared to graft-hybrids in many respects.
I know of no case among fungus infections which could be compared
directly with these examples, and it is not at all likely that we shall
meet with any instance of a fungus-hypha handing over nuclear substance
to an egg-cell, and so affecting the latter that an embryo results. But
the case is not hypothetically impossible, although the distant
relationships of the two groups of organisms render it extremely
improbable among the higher plants. It is by no means so improbable,
however, that further research may show cases where the egg-cell of a
lower cryptogam--_e.g._ another fungus--may be affected either directly,
or indirectly, by the protoplasm of a parasitic or symbiotic hypha, as
suggested by the extraordinary phenomena of symbiosis.
Some of the variations in grafted plants are found to predispose the
plant to disease, or the reverse, and cases may be cited where the
resulting shoots, foliage, or fruits, or seedlings more readily fall a
prey to, or resist, parasitic fungi and insects than the ungrafted
plants. Daniel gives instances of such--_e.g._ among other examples,
Peas grafted on Beans yield seeds which suffer more from Erysipheae than
the normal seedlings. But the best known cases are those of Vines in
their relations to _Phylloxera_, already referred to (p. 155).
Several instances are also known where grafted plants show more or less
resistance to such factors of the environment as low temperatures;
grafted or budded Roses often suffer much from Erysipheae, and so forth.
Much research is still needed to determine how far these matters depend
on real alterations in the nature of the graft, or _are only true for
the localities in which the experiments have been made_, a point which
has, I think, been overlooked by all observers.
Grafted plants are apparently very much exposed to injury by slugs,
insects, and the invasions of parasites during the healing of the callus
and the fusion process. Here again it must not be overlooked that the
callus is, so to speak, a tit-bit of luscious, thin-walled, succulent
tissue; and, like all wounds, the graft affords entrance to parasites
such as _Nectria_ and Ascomycetes of various kinds, under circumstances
very favourable to their invasion.
_Natural Grafts._--It is by no means an uncommon event to find the
branches of Beeches, Limes, and other trees which have been accidentally
brought into contact during growth, joined where they cross. As they
press one against the other, they become naturally grafted, by that form
of the process known as _inarching_: except that in artificial inarching
the operator cuts off the cortical tissues of the two branches and
brings their cambial surfaces together, whereas in nature the cambiums
only come into contact after the destruction by pressure, or slight
abrasion, of the entrapped intervening tissues. The fusion occurs, in
fact, exactly as in the burying-in of a nail or wire, referred to on p.
211.
Natural grafts are very common among the roots of trees, and possibly
explain some queer cases of the apparent revivification of stumps of
trees not usually given to forming abundant stool shoots. It is regarded
as probable in some old forests that the majority of the roots of trees
of the same species are linked up together by such natural grafts, a
probability not diminished by the fact that such roots cross at many
points, and are easily grafted.
NOTES TO CHAPTER XXIX.
The student should read Bailey, _The Nursery Book_, 1896, for
details regarding the practice of grafting, and facts in
abundance can be obtained from the pages of the _Gardeners'
Chronicle_.
Concerning graft-hybrids and the variations of grafted plants
see Jouin, _Can Hybrids be obtained by Grafting?_ and
especially Daniel, "La Variation dans la Greffe," in _Ann. des
Sc. Naturelles_, S. VIII., Vol. 8, 1898, p. 1, and the
literature there collected. The whole subject is largely
controversial, and much work remains to be done.
CHAPTER XXX.
LIFE AND DEATH.
_Protoplasm--Hypothesis as to its structure and behaviour--
Assimilation--Growth--Respiration--Metabolism--Action of the
environment--Nuclear protoplasm--Pollination--Grafting--
Parasitism--Graft-hybrids--Life--Death--Variation--Disease._
We have seen that all the essential phenomena of disease concern only
the living substance--the protoplasm--of the plant, and that however
complex the symptoms of disease may be, the occurrence of
discolorations, lesions, hypertrophies, and so forth are all secondary
matters subsidiary to the fundamental alterations of structure and
function constituting the disease. It remains to see if we can adopt any
hypothesis as to the nature of this physical basis of life--the
protoplasm--which shall help us to understand still more clearly in what
must reside those processes which, so long as they proceed harmoniously
and uninterruptedly, constitute life and health, and which when
interfered with result in disease and death. The protoplasm of the
living plant-cell looks like a slimy translucent mass which has been
superficially compared in appearance to well-boiled sago or clear gum.
Fifty years of observations and experiments with it have convinced
physiologists that it is not a mere solution or emulsion, however, or
even a chemical compound in the ordinary sense of the term, although
chemical analysis gets little out of it beyond water, proteids,
carbohydrates and fats, and traces of certain mineral salts; for living
protoplasm does not respond to the laws of physics and mechanics in
obeying them, simply as do ordinary solutions and liquids. On the other
hand, the most delicate chemical manipulation fails us, because when
killed it is no longer protoplasm. Nor does the microscope advance
matters far, beyond convincing us that this marvellous material must
have a structure far more intimate than anything visible to the highest
magnifying powers at our disposal.
Nevertheless, some information is forthcoming from the comparative
examination of the protoplasm of numerous different kinds of organisms,
for we have learnt that certain ingredients and no others are necessary
for its composition--namely, carbon, hydrogen, oxygen, nitrogen,
phosphorus, sulphur, calcium[Note: See note at end of chapter.],
magnesium, potassium--and it is as a rule of no use trying to foist on
to it any substitute for any one of these. Moreover, these chemical
elements must be given in certain definite proportions and forms: for
instance it is of no use to offer the carbon and sulphur in such a form
as carbon disulphide, or the nitrogen and hydrogen in that of
hydrocyanic acid, but the carbon must be given to the protoplasm in the
form of a carbohydrate or in some similar form, the nitrogen as an
ammonium salt, nitrate or proteid, the sulphur as a sulphate, and so
forth, and thus water, air, carbohydrates, and the nitrates, sulphates,
and phosphates of potassium, calcium, and magnesium become the chief
natural sources of the essential ingredients. Again, we have learnt that
while there are different forms of protoplasm in the cell, and that
these react on each other, and go through cycles of arrangement and
rearrangements, the intimate structure must be of that kind termed
molecular--beyond the region of vision, just as is the microscopic
structure of a crystal; but, while like the latter affording evidence of
order and sequence when properly examined, the structural arrangements
and changes must be infinitely more complex.
All these, and numerous other results of enquiry, have led to the
conclusions that we must regard living protoplasm as a complex made up
of very large molecular units, each containing atom-groupings of the
elements named; and, partly on account of the large number of atoms they
contain, and partly due to the vibrations of absorbed heat, these units
must be extremely labile. Moreover, they are linked up into an
invisible and intricate meshwork, bathed in a watery liquid held in the
interstices somewhat as water is held in a sponge. In this imbibed
liquid are dissolved the substances, consisting of the same elements,
which are to serve as food, and which are to be taken up into the
molecular framework and built up into the structure of new molecular
units--or, as they may be shortly termed, molecules of protoplasm: in
the bathing liquid are also dispersed the fragments--again containing
the elements named--which have resulted from the breaking asunder of
some of the complex protoplasm molecules, and which are partly destined
to be used up again, partly to be burnt off in respiration, and partly
to be put aside as metabolic products such as reserves, secretions,
permanent structure, etc. Among the elements carried into this liquid
and dissolved in it the free oxygen of the air also plays an important
part.
As new molecules are formed, by mutual combinations of the
food-materials selected by molecular attractions, they are taken up into
the protoplasmic framework, and built in between those already in
existence, thus distending the whole, and we say that the protoplasm
_Assimilates_ food-materials and _Grows_. When distended beyond a given
degree, or disturbed in various other ways, the molecular framework
breaks, and some of the molecules are shattered, and as they fall to
pieces certain of their constituent parts containing carbon and hydrogen
forcibly combine at the moment of liberation with the oxygen in the
fluid around and are burnt off in the form of carbon-dioxide and water,
heat being of course evolved. This is the fundamental process of
_Respiration_.
It is probably the alternation of these processes of _Assimilation_--the
building up into the protoplasmic structure of new complex labile
molecules--and _Destruction_--the shattering of such molecules
with redistribution, oxidation, etc., of their fragments--which
constitute the fundamental process of life. Different authorities
attempt to explain the details of these processes in various ways,
but there is practical agreement on the one point, that life
consists in the alternate building up of new protoplasm from the
food-materials--_Assimilation_--and the breaking down of the molecular
complexes to simpler ones--_Disintegration_, or _Dis-assimilation_, as
we may call it. During the periods when assimilation prevails, and the
protoplasm increases in mass, we recognise _Growth_, and since this is
usually associated with the vigorous imbibition of water, owing to the
powerful osmotic attractions for that liquid exhibited by some of the
products, and with consequent further stretching of the invisible
molecular plexus, the growth may be so evident in increased size, that
we are accustomed to look upon the visible increase in volume alone as
growth; but it is essential to understand that growth of the protoplasm
is always proceeding during life, even when as many older molecules are
being shattered and dispersed as new ones are being formed by
assimilation, and when, therefore, no visible permanent enlargement
occurs. Similarly, during periods when disintegration of the molecules
prevails, we must not assume that the assimilation of new molecules is
not occurring and that growth is not proceeding. The two processes are
always going on during the active life of the protoplasm: in fact life
consists in the play of these processes, as already said.
That numerous chemical rearrangements of the atom-complexes take place
outside the protoplasmic molecules--both of those left unemployed in
assimilation and of those rejected during the destructive
processes--will be readily understood: many of the bye-products found in
plants, such as vegetable acids, alkaloids, colouring matters,
crystalline bodies, etc., etc., are due to these, so to speak,
fortuitous combinations and re-combinations.
The part played by respiration has often been misunderstood. It consists
in the burning off of some of the carbon and hydrogen of the shattered
protoplasm molecules, by means of the oxygen of the air, which finds its
way into the fluids around the protoplasm, and when it is active every
act of combustion--which is here an explosion--leads to the shattering
of more protoplasm molecules, and consequently to more respiratory
combustion of the products. If the supply of oxygen is limited the
breaking down of the molecules of protoplasm does not cease, but the
carbon and hydrogen which would otherwise have been oxidised are now in
part left to form other compounds in the surrounding liquid, and thus
incompletely oxidised bodies, such as vegetable acids, alcohols, etc.,
accumulate. Even in the complete absence of atmospheric oxygen the
protoplasm may go on breaking down and accumulating various compounds
containing relatively much carbon and hydrogen--so-called intramolecular
respiration; but in ordinary plants this process soon comes to an end,
because the blocking up of the molecular plexus leads to obstruction and
interferes with the normal assimilation and dis-assimilation, and, if
prolonged, leads to pathological conditions, and eventually death.
Here, then, we meet with a cause of disease, or of predisposition to
disease. The deprivation of oxygen interferes with the normal processes
of building up and breaking down of the protoplasmic molecules, and
bodies we term poisonous accumulate and may lower the vitality or even
bring life to an end.
During normal life other products of the disruption of the protoplasm
molecules are nitrogenous bodies, such as proteids, and these we have
reason to believe are used up again, acting as the nuclei, so to speak,
of the new molecules, and so being built up again with fresh
food-materials into the plexus, to be again set free, and again used up,
and so on. Others are the carbohydrates, such as cellulose, which pass
out of the molecule into an insoluble form, and are accumulated outside
the protoplasm in the form of cellulose membranes, and so forth. It is
these formed products of metabolism (Metabolites), especially cellulose
and bodies which result from its subsequent transformation, which
constitute the main permanent mass of the ordinary plant.
We are now in a position to see how another fundamental cause of disease
or predisposition to disease exists in the deprivation of the protoplasm
of any of the elements needed to supply--in the food-materials--the
place of those which have been permanently put aside in the form of
cell-walls, or burnt off in respiration, passed out as excretions, or in
other ways lost.
It is clear that the indispensability of an element must mean that the
protoplasmic molecule cannot be completed without it: the same
conclusion is supported by the experimental proof that these elements
cannot be replaced by chemically similar elements.
It does not follow, however, that the protoplasm molecule must always
have the same number of atoms of these elements, and grouped always in
the same atom-complexes before being assimilated; nor that the
protoplasm molecule, when once built up, always breaks down in exactly
the same way. On the contrary, while the protoplasm of corresponding
parts of a daisy and of a rose must contain all the elements named, we
must believe that the atom groupings are different in the protoplasm
molecule in each case; and though the molecules of the cell-protoplasm,
of the nucleus, of the chlorophyll-corpuscles, etc., of one and the
same plant must have all these elements, the atom groupings and modes of
building up and breaking down may be very different in each case.
Again, the cell-protoplasm, bathed by the sap taken in by roots from the
soil or fed directly by that derived from the leaves, must be exposed to
very different stimuli and modes of nourishment, etc., from those
incurred by the protoplasm of the nucleus which it encloses: and similar
conclusions must apply in turn to the protoplasm of the root in the dark
moist soil and of the leaf in the light dry air, or to that of the
superficial epidermis cells as contrasted with that of the deeply
immersed pith, and so on.
It is no doubt in these directions that we must seek for the explanation
of many life-phenomena at present quite beyond explanation. Thus, it is
tolerably easy to modify the action of the cell-protoplasm of a plant,
by exposing it to differences of illumination, temperature, moisture,
and so forth, within certain limits; at least, since the changes in
stature, tissue differentiation, cell-secretions, flowering capacity,
etc., of plants affected by such factors of the environment--_e.g._
alpine plants brought into the plains--_must_ be due to changes in the
mode of activity of the protoplasm, we must assume that the above
factors affect the latter. But it is extremely difficult to reach the
nuclear-protoplasm directly by such stimuli, as proved by the experience
that even where we allow the factors to act for a long time, no
permanent change can be detected in the behaviour of the
nuclear-protoplasm--the essential material in the reproductive organs
and reproductive process. At least we must infer that no change has been
permanently stamped on this nucleo-plasm from such facts as the
characters of the seedlings of the progeny of the plain-raised plants:
if they are again sown in an alpine situation they forthwith behave
again as alpines.
Must we not conclude, then, that this difficulty of reaching the
nuclear-protoplasm is owing to the fact that it is nourished and
influenced directly only by the cell-protoplasm? That the
cell-protoplasm is its environment, and not so directly the outer world?
We may influence the cell-protoplasm--we may make it work harder or less
actively, respire vigorously or slowly, build up and break down in
various different ways, or at different rates, and so forth, _within
limits_; but it is nevertheless cell-protoplasm of its specific kind,
with its own range of molecular variations and activities within these
limits, and it supplies the nuclear-protoplasm with what it wants so
long as these limits are not exceeded. Consequently, while it is very
easy to make the cell-protoplasm vary within the limits of its range, it
is not easy to induce it to vary its effects on the nuclear-protoplasm
to such an extent or in such a way that the latter is permanently or
materially altered in constitution.
Nevertheless it would appear that cases do occur where the
nuclear-protoplasm _is_ reached and affected by external stimuli, as
evinced by some of the phenomena of hybridisation and of cross-and
self-fertilisation, because we find the results expressed in the
mingling of the characters of parents, in strengthened or enfeebled
progeny, and even in the appearance of unexpected properties, which,
from the facts of Reproduction, we know must have taken their origin in
some alteration of the nuclear substance of the embryo.
Here, however, we know in most cases that the principal agent
which has reached the nuclear-protoplasm, is another portion of
nuclear-protoplasm. In hybridisation, one which has been fed and
influenced by cell-protoplasm of a very different plant; in
cross-fertilisation, one fed and influenced by the cell-protoplasm of a
different plant of the same species, and in self-fertilisation, one fed
and influenced by the same cell-protoplasm.
That somewhere, and somehow, such nuclear-protoplasm as induces the
changes in the characters of hybrids, etc., has been influenced by its
immediate environment--the cell-protoplasm of the plant--appears to be a
conclusion from which there is no escape. We may obtain similar evidence
from the experience of grafting. It is relatively easy to influence the
cell-protoplasm of a scion by a suitable stock, obviously because the
latter, while handing on to the former all necessary materials from the
soil, presents the indispensable elements and compounds in somewhat
different proportions, dilutions, etc., from those which its own roots
would have done, and probably mingles with them a certain amount of its
own peculiar products, as well as affects the modes of working and
interaction of both by the molecular impetus impressed on them.
Consequently the cell-protoplasm of the scion, while obtaining from the
stock all it needs within the limits of its own variations of structure
and activity, nevertheless builds up and breaks down in ways or at rates
slightly different from those hitherto normal to it, and perceptible
variations result when the sequences and correlations of these material
and mechanical changes have affected a sufficiently large mass for the
accumulation of visible effects. The limits to grafting suggest not that
an inappropriate stock does not offer to the protoplasm of the scion the
right materials, but that it presents them in proportions and in forms
which are unsuitable for the assimilable powers of the latter, or,
possibly, mingled with substances poisonous in themselves or capable of
becoming so in conjunction with bodies in the scion.
What has been said of the action of stock on scion, will also be true,
_mutatis mutandis_, of the reciprocal action of scion on stock. Here
again we may have causes for disease, or predisposition to disease.
It occasionally happens, however, that the nuclear protoplasm
of the stock or scion _is_ affected in grafting, and we infer
from the difficulty of modifying it in any other way in ordinary
reproduction than by means of other nuclear protoplasm--_e.g._ in
hybridisation--that in such cases a fusion of the nuclei of stock and
scion has occurred during the grafting, and a graft-hybrid has
resulted--_e.g._ _Cytisus Adami_.
It is not impossible however that the nuclear protoplasm has in such
graft-hybrids been subsequently modified by the differences in nutrition
to which it has been subjected, in the modified cell-protoplasm affected
by the mingling of the juices, etc., of scion and stock; for it is quite
conceivable that such materials may affect the protoplasm far more
profoundly than anything derived directly from the environment.
If Daniel's researches are confirmed, however, it appears that in some
cases, at any rate, the nuclear-protoplasm is so altered by the grafting
that when the new embryo is developed, after fusion with nuclear
substance from another plant of the same species, the results are
apparent only in the progeny, and _the effects of alteration in the
cell-protoplasm have been transmitted to the nuclear protoplasm of the
germ-cells_--_i.e._ acquired characters have been transmitted and fixed
by heredity. Should this prove true the importance of the results can
hardly be over-estimated. The matter is too problematical for further
discussion here, but we see that any such action may profoundly affect
the "constitution" of the resulting plant.
Turning now to the case of fungi or other organisms which obtain access
to the cell-protoplasm. At the one extreme we have cases where the
protoplasm of the diseased plant is rapidly and directly poisoned and
destroyed, as in the killing off of seedlings in "Damping Off": near the
other extreme we have cases where the foreign protoplasm of the
parasite, although it gains complete access to that of the host, merely
stimulates the latter to greater activity and itself works for its own
ends in conjunction with it--_e.g._ _Plasmodiophora_. In such instances
we must figure to ourselves the cells of the root of the Crucifer
handing on food-materials to both masses of protoplasm--that of the
_Plasmodiophora_ and that of the cell into which it penetrates; and it
is immaterial whether both obtain the food-materials directly, or, what
seems more likely, the fungus only at second hand and by the medium of
the host's protoplasm. In any case, the latter is for a long time at
least not poisoned or maimed, or in any perceptible way injured by
excreta from the fungus-protoplasm, although it is evident that each
must excrete various metabolites which may soak into and be taken up by
the other: on the contrary the host-protoplasm grows larger, attracts
more food supplies, makes larger cells, and is evidently stimulated to
greater activity for the time being, its behaviour reminding us of the
stimulation of cells by means of slight doses of poison referred to
previously. We must therefore assume that the general course of building
up and breaking down of its protoplasm-molecules go on as usual--or
nearly so--in both the host cell and the invader; and that the
assimilatory, respiratory, excretory and other functions are carried on
in the former as in the normal cell, or are but slightly modified to an
extent which does no immediate injury to its life. But we must further
assume that the same is also true of the invading protoplasm, and that
the _Plasmodiophora_ is also supplied with suitable atom-complexes to
build up its protoplasm molecules, as fast as they are shattered and the
rejecta burnt off in respiration.
A step further, and we come to instances of _Symbiosis_, where the
commingled masses of protoplasm of host and invader continue this
harmonious action during life. Clearly there are resemblances between
these latter cases and successful grafts, and between both and
successful sexual unions where the resulting embryo-cell gives rises to
a vigorous and healthy plant; and the more these resemblances are
examined in the light of what we know of symbiosis the more they support
our contention.
Such considerations as the foregoing suggest, then, that life consists
in the regular and progressive building up and breaking down of the
complex protoplasm molecules, and is necessarily accompanied by the
influx of the indispensable food-elements in certain combinations and
atom-complexes for assimilation, and by the combustion of some of the
débris of the shattered molecules, which combine with the oxygen in
respiration and so afford explosions which raise the temperature and
enhance the lability of existing molecules, and act as stimuli to the
shattering of further molecules. The results of these rhythmical
buildings up (assimilation) and shatterings (dis-assimilation) of the
protoplasm molecules are the growth of the protoplasm, with further
intercalations of water and new food-supplies, etc., on the one hand,
and the formation of metabolic products (proteids, cellulose, sugars,
fats, etc.), some of which are again used up, others respired, others
deposited as stores, cell-walls, etc., on the other.
That the building-up process depends on the action of molecular forces
comparable to those by which a growing crystal goes on selecting
atom-complexes of its particular kind from the solution around seems
highly probable, and this being the case we can understand how under
certain circumstances _substitutive_ selections may occur. That is to
say, just as a crystal will sometimes build up into its structure
atom-complexes of a kind different from its normal molecules, so, given
the proper conditions, a protoplasmic molecular unit will build up into
its structure atom-complexes somewhat different from those it had
hitherto taken up--_i.e._ assimilated--with consequent modifications of
its behaviour. If this occurs, the modes of further building up and
breaking down will be affected by the subsequent action of these
slightly modified protoplasm units, _and it may well be that the whole
significance of variation turns on this_. Whether the resulting
variation makes for the welfare or otherwise of the organism will then
be decided by the struggle for existence, and the natural selection
which ensues. Such a view also implies that the energy concerned is
primarily what is usually termed chemical energy, and that every
compound entering into the protoplasm carries in a supply of this,
available in various ways.
_Death_, on the contrary, is the cessation of these rhythmical processes
of building up and breaking down of the protoplasm molecules. It does
not imply the cessation of chemical changes of other kinds, but that
these rhythmical constructions of the complex and labile protoplasm
molecules breaking down on stimulation to bodies partly re-assimilable,
partly combustible in respiration, and partly excretory, etc., have
ceased, and that further chemical changes in the material are
thenceforth simpler and different in kind and degree, eventually leading
to total disintegration so that no units are left capable of restoring
the rhythm.
If these ideas are correct, we may define _Disease_ as dangerous
disturbances in the regularity, or interference with the completeness or
range of the molecular activities constituting normal Life--_i.e._
Health--and it is evident that every degree of transition may be
realised between the two extremes. Now, if we further assume, as I think
we must do, that a considerable range or "play" must exist in the
molecular activities of the protoplasm constituting life, we obtain a
sort of expression of what we mean by limits of variation. The fact that
life can go on in a given plant at temperatures between from 1°-5° and
35°-40° C., or in lights of different intensity, or within considerable
ranges of water supply, concentration of salts, partial pressure of
oxygen, etc., implies that the molecular activities of the protoplasm
are of the normal _kind_ all the time, though they may differ in
rapidity, and even in _quantitative_ and _qualitative_ respects within
certain limits; and the meaning of the _optimum_ temperature,
illumination, oxygen pressure, etc., is, from this point of view, not
that the molecular activities differ in kind from those nearer the
minima and maxima, so much as that they are running at the best rates
for the welfare of the plant--_i.e._ for permanent health.
If we transcend the cardinal points limiting the range of this play,
however, and we get variations in the _kind_ as well as _rates_ of
molecular constructions and disruptions, then we pass by imperceptible
gradations into ill-health--_i.e._ _Disease_.
And similarly in relation to other protoplasm. That of the right kind of
pollen grain from another plant of its own species, stimulates the
contents of the ovule to produce a vigorous embryo and healthy seedling:
that of a similar pollen grain in its own flower either does no positive
harm, but has a feebler effect, or it may act like a poison. That of
another pollen grain again may refuse to unite at all; while that of a
fungus hypha--_e.g._ of _Sclerotinia_ on _Vaccinium_--may run down the
style as does the pollen tube and produce death and destruction
throughout the ovule.
Or again, in Clover, we may have the hypha of a _Botrytis_ with its
protoplasm unable to do more than penetrate into the cellulose walls
and diffuse a poison into the adjacent cells, being utterly incapable of
directly facing, or mingling with the living protoplasm of such cells,
whereas the protoplasm of another organism--_e.g._ _Rhizobium_--will
penetrate directly into the cells, live in them for weeks or months
without injury--nay even with advantage to their life. And hundreds of
similar cases can be selected.
We may, therefore, conclude that _Variation_ depends fundamentally on
alterations in the structure or mode of building up and disintegration
of the protoplasmic molecular unit, brought about either by direct
modifying action of the inorganic environment--nutrition, temperature,
oxygen supply, light, etc., etc.--or by the mingling with it of other
protoplasm, the molecules of which since they have already a slightly
different composition, configuration, mode of breaking down and building
up, etc., affect its molecules by supplying them with altered nutritive
atom-complexes, by competing with them for oxygen, etc., etc. Once these
molecules are affected, we must assume that long sequences of other
chemical and molecular changes will be also modified; and although we
have no conception of _how_ these changes bring about changes in form,
that they do so is only a conclusion of the same order as that which we
hold regarding the much simpler changes concerned in the formation of
crystals.
That such variations may be of every degree as regards profundity,
permanence, kind, etc., may well be imagined; and there is nothing
surprising in our being able to induce them more easily by the action of
external factors _in the readily accessible cell-protoplasm_ than in the
_less exposed nuclear-protoplasm_; because the latter is only accessible
through the former, or through the agency of _other nuclear protoplasm
already modified_. On these and similar phenomena depend the relative
permanency and transmissibility of the variations. Our measure of the
latter only begins when the effects referred to have become manifest in
large masses of cells, because only then do they become appreciable to
our senses.
Further, variations thus induced may be of advantage to the continued
life of the plant, or in all degrees disadvantageous or threatening to
its existence. These latter variations are _Disease_, and if their
interference with the normal rhythmical play of the building up and
breaking down of the protoplasm molecules proceeds beyond certain
limits, life ceases, and we have death supervening on disease.
NOTES TO CHAPTER XXX.
It appears probable that calcium is not always needed by
living cells, and may not enter into the composition of
protoplasm; on the other hand traces of iron are perhaps
necessary.
The criticisms and summary of facts on which the hypothesis
regarding protoplasm here adopted is based are developed at
length in Kassowitz, _Allgemeine Biologie_, Wien, 1899, B. I.
and II., where the collected literature may be found, and the
reader introduced to the huge mass of controversial writings
put forward since Darwin and associated with the names of
Weismann and others.
It will probably be noticed that I have employed the term
molecular unit of protoplasm, and have not discussed the
question of organised structure in the latter: this is because
it seems clear to me that living protoplasm as such does not
possess "organised structure" in the true sense of that
term--it is, rather, busy preparing and making "organised
structure," and a molecular constitution would have to be
ascribed to all "physiological units" of the nature of
micellæ, pangens, ids, etc., as truly as to the structural
units of a starch-grain or cell-wall, or even of a crystal. In
this connection, the student will find the necessary points of
view put forward in Pfeffer, _Physiology_, pp. 37-83.
INDEX.
Absorption by roots, 49.
Absorption of energy, 23.
Absorption of light, 27.
Absorption of water, 50.
_Abutilon_, 183.
_Acarus_, 88.
Accessory buds, 259.
_Acer_, 214.
Acid gases, 181, 191.
Acids, 130, 136.
Acquired characters, 283.
_Acrostalagmus_, 238.
Action of the environment, 271.
Adaptation, 176.
Adapted races, 177.
_Adonis_, 220.
Adventitious buds, 224, 225, 257, 260.
_Æcidium_, 88, 114, 116, 187, 188, 189, 217, 223, 225, 232, 247, 252.
Aeration, 104.
Aerobic organisms, 57.
Aetiology, 89, 100.
_Agaricus melleus_, 115, 143, 145, 234.
Agents of disease, 113.
_Aglaospora_, 223.
Agriculture, 65.
Agricultural Chemistry, 2.
_Ajuga_, 217.
Albinism, 179, 182, 183, 186.
Alder, 207, 219.
Aleurone layer, 173.
Algae, 215.
_Allium_, 258.
Almond, 168.
_Alnus_, 214.
_Aloe_, 134, 161.
Alpine plants, 250, 279.
American blight, 164, 219.
American vines, 155, 169, 172.
Amides, 31.
Amoeba, 144.
Amount of energy stored, 25.
Amygdalin, 173.
_Anabaena_, 128.
Anaerobic bacteria, 58, 237.
_Anagallis_, 261.
Analyses, 65.
Analyses of waters, 58.
Anemone, 247.
Animals, 99, 108, 142, 207.
_Antennaria_, 232.
_Anthonomos_, 249.
Anthrax, 144.
Antiseptics, 162.
Ants, 232.
_Aphis_, 88, 109, 161, 165, 188, 213, 214, 232, 241, 253.
_Aphrophora_, 233.
Apogamy, 257, 261.
_Aporia Crataegi_, 187.
Apospory, 257, 261.
Apple, 170, 171, 187, 189, 192, 206, 217, 218, 219, 223, 226, 231,
233, 248, 249, 253, 254.
Apricot, 188, 206.
Apricots, 234.
Area of root-surface, 37, 39.
_Arisarum_, 188.
_Aristolochia_, 259.
Aroids, 113.
Arrest of growth, 246.
Arsenic, 162.
Artificial wounds, 194.
Ascomycetes, 189, 217, 269.
_Ascochyta_, 190.
Ash, 182, 223, 225, 251.
_Asparagus_, 180, 230, 251, 252.
_Aspergillus_, 231.
_Aspergillus niger_, 58.
_Aspidiotus_, 187.
Assimilation, 8, 21, 133, 271, 275, 277, 285, 286.
Assimilates, 274.
Atmosphere, 1, 99.
Atmospheric influences, 101.
Atrophy, 246, 247, 266.
Attractive substances, 136.
_Aucuba_, 264.
Autumnal colouring, 191.
Autumnal fall, 93.
Avalanches, 106.
Bacteria, 102, 133, 143, 168, 173, 176, 182, 190, 200, 216, 219, 223,
227, 231, 236, 237.
Bacteriosis, 227.
Barberry, 176.
Bark boring, 204, 205.
Bark-beetles, 205.
Barley, 176, 248.
Barrenness, 246, 249.
Bats, 244.
Bean, 188, 190, 191, 268.
Beech, 192, 222, 223, 225, 233, 240, 242, 254, 269.
Beech Miner, 208.
Bees, 142, 143, 164.
Beet, 192, 216, 219, 230.
Beet-rot, 230.
Beetles, 110, 143, 145, 205, 206, 207, 248, 254.
Berkeley, 85.
Bilberries, 116, 142, 217, 218, 248.
Biology of soil, 56, 102.
Birch, 207, 218, 224.
Birds, 108, 144, 164, 166.
Bird's-eye Maple, 224.
Black spots on leaves, 186, 189, 191.
Bladders, 218.
Blemish, 198.
Blights, 86, 104, 179.
Blisters, 230.
Blue rays, 21.
_Bombyx_, 187, 218.
Bordeaux mixture, 162.
Boring, 204.
_Botrytis_, 131, 132, 136, 175, 230, 231, 243, 288.
Boussingault, 5, 10.
Bouvardia, 260.
Bramble, 112.
Branch stumps, 194, 199.
Brand, 240.
Breeding, 78.
Briars, 113.
Broom-rapes, 265.
Browning, 122, 186, 235.
Brown spots, 186, 189, 190, 191.
Browsing, 244.
_Bruchus_, 248.
Bruises, 194, 203, 240, 241.
Bryony, 112.
_Bryophyllum_, 260.
Bud galls, 219.
Bud variations, 92, 93.
Bulb diseases, 227.
Buried objects, 211, 269.
Burning, 191.
Burning-glass effect, 192.
Burrows, 204, 205.
Burrs, 222, 223, 224.
Bursting of fruits, 227, 230.
Butterflies, 145.
Bye-products, 276.
Cabbage, 253.
Cabbage moth, 208.
_Caeoma_, 252.
_Caesalpinia_, 233.
Calcium, 272.
Calcium oxalate, 138.
_Calla_, 183.
_Calliandra_, 233.
Callus, 119, 120, 124, 139, 140, 196, 197, 199, 201, 202, 210, 241,
260, 263, 269.
_Calyptospora_, 116, 217.
Cambium, 120, 196, 199, 222.
Camellia, 187.
Cancer, 127.
Canker, 87, 222, 223, 241.
_Capnodium_, 232.
_Capsella_, 116, 175, 252.
Carbohydrates, 16, 17, 20, 34, 122, 184, 272, 273, 277.
Carbolic acid, 162.
Carbon, 272.
Carbon assimilation, 8, 10, 28, 106.
Carbon-bisulphide, 163.
Cardinal points, 288.
Carrot, 164.
_Carpocapsa_, 207.
Cast branches, 123.
Castor oil, 172.
Caterpillars, 109, 164, 207, 208, 244, 254, 259.
Cats, 164.
Cattle, 108.
Cauliflowers, 247, 250.
Causes of disease, 89, 99, 108, 159, 278, 282.
_Cecidia_, 212.
_Cecidomyia_, 182, 213, 214, 218, 219, 254.
Celery, 180, 230.
Cell contents, 168.
Cell-protoplasm, 279, 280, 290.
Cellulose, 132, 277, 286.
_Celosia_, 250.
_Centaurea_, 188.
_Centhorhynchus_, 219.
_Cephaleuros_, 188.
_Cephus_, 248.
_Cercis_, 259.
_Cercospora_, 190.
Cereals, 248.
Change of conditions, 78.
Charlock, 165.
Checks to disease, 166.
Chemical analysis, 32, 64, 103, 272.
Chemical antiseptics, 159.
Chemical energy, 29, 287.
Chemotactic phenomena, 72, 130, 135, 137.
_Chermes_, 153, 223.
Cherry, 208, 209, 231, 234, 235, 247, 248.
Chestnut, 190.
Chlorine, 181.
Chlorophyll, 19, 106, 122.
Chlorophyll action, 184, 192.
Chlorophyll corpuscles, 9, 18, 22.
Chlorosis, 122, 165, 179, 180, 181.
Chrysanthemum, 243, 252.
Chytridiaceae, 127, 136, 189, 208.
_Cicada_, 235.
Cicatrix, 123.
_Cinchona_, 168, 172, 173.
Circulation of carbon, 62.
Circulation of nitrogen, 62.
_Citrus_, 168.
_Clasterosporium_, 188, 209.
Classification of diseases, 99, 101, 120.
_Claviceps_, 232.
Climate, 1.
Climbing plants, 112, 113, 210.
_Clostridium_, 236, 237.
Clothes, 142.
Clover, 164, 187, 249, 252, 288.
Cluster-cups, 188.
Coal gas, 104, 182.
Coccideae, 164, 232.
_Coccus_, 223.
Coffee leaf-disease, 114, 146, 166, 169, 242.
_Coleophora_, 153, 206.
_Coleosporium_, 169.
_Coleus_, 192, 220.
Competition of fungi, 61.
Complex interactions, 91, 99.
Conifers, 125, 205, 223, 225, 234, 258.
Constitution, 156, 283.
Consumption, 248.
Contact irritability, 125, 135.
_Contagium fluidum vivum_, 183.
Contortions, 252.
_Convallaria_, 175.
_Convolvulus_, 112.
_Copaifera_, 234.
Copper sulphate, 162, 165.
Coppery leaves, 191.
Cork, 119, 123, 194, 199, 216, 222.
Cork wings, 217.
Corky warts, 212.
Corn, 248.
Corrosion of marble, 46.
_Cossus_, 206.
Cost of epidemics, 146, 147.
Cotton, 172.
_Crassula_, 253.
Creeping of mycelia, 142.
_Crepis_, 252.
Crimson spots, 189.
Cross-breeding, 266.
Cross-fertilisation, 69, 74, 77, 281.
Cross-graining, 124.
Crucifers, 219, 284.
Cryptogams, 87, 108, 111, 113.
Cuckoo-spit, 233.
Cucullate leaves, 253.
Cucumber, 219.
_Cucurbitaria_, 217, 243.
Cultivation of pest and host plant, 168.
_Curculio_, 248.
Curling, 235, 246.
_Cuscuta_, 134.
Cuts, 119, 143, 194.
Cuttings, 194, 198, 262, 263.
Cyanide of potassium, 165.
Cycads, 128.
_Cynips_, 110, 213, 219.
_Cystopus_, 116, 136, 175, 187, 217, 247, 252.
Cytases, 132.
_Cytisus Adami_, 264, 283.
Daisy, 278.
Damping off, 114, 144, 160, 229, 284.
Dandelion, 247, 252.
Daniel's researches, 283.
Dark heat rays, 27.
Darwin, 72, 125.
_Dasyscypha Willkommii_, 152, 223.
Death, 271, 272, 287, 290.
De Bary, 85, 151.
Deficiency of iron, 180.
Defoliation, 109, 240, 244.
Deformation, 132.
_Dematium_, 135.
_Dematophora_, 143, 145.
Denitrification, 62.
Derivation of Phytopathology, 85.
Destruction, 275.
Development of root-hairs, 40.
Dextrine, 173.
Diagnosis, 85, 89.
Diastases, 132.
Diffusion, 53.
Digestion, 133.
_Digraphis_, 175.
_Dilophia_, 188.
_Dionaea_, 125.
_Dipsacus_, 252.
_Diptera_, 207.
Dis-assimilation, 275, 277, 286.
Discolorations, 179, 186, 192.
Disease, 64, 91, 271, 272, 277, 287, 288, 290.
Disease dodging, 168.
Disease-fungi, 189.
Disease of organs, 119.
"Disease-proof" varieties, 168, 169, 171, 173, 177.
Disease-resisting varieties, 177.
Diseases of absorptive organs, 121.
Diseases of assimilatory organs, 119.
Diseases of bark, 120.
Diseases of cambium, 120.
Diseases of parenchyma, 120.
Diseases of respiratory organs, 119, 121.
Disintegration, 275.
Distortions, 140, 246, 251, 252, 253.
Dissemination of fungi, 142.
Division, 127.
Dodder, 113.
_Dolium_, 134.
Dormant buds, 224, 225, 257, 259, 260.
Double flowers, 247, 256.
Double ideals in selection, 168.
_Dracaena_, 192.
Drainage, 103.
Drawing, 106, 180.
Drip, 103.
Drooping, 43, 179.
Drops of water, 192.
Dropsy, 228.
Drought, 121, 183, 190, 191, 245, 248, 249.
Dry-rot, 143, 237.
Ducks, 144.
Dutrochet, 7.
Dwarfing, 246, 249.
"Dying back," 190, 240, 242, 243, 244.
Earwigs, 164, 207.
_Eau Céleste_, 162.
_Edelfäule_, 230.
Eelworms, 111, 248.
Effects of environment, 262.
Eggs of insects, 187.
Elaborated sap, 94.
Elm, 218, 224, 225, 233, 260.
_Empusa_, 163.
Endemic diseases, 153, 160, 166.
Endive, 180.
Endophytes, 130.
Endophytic algae, 128.
Endophytic fungi, 193.
Energy in plants, 15, 25, 287.
Engelmann, 20, 27.
_Entyloma_, 187.
Enzymes, 10, 130, 132, 136.
_Epichloë_, 218.
Epicormic shoots, 224, 257, 260.
Epidemics, 108, 109, 113, 115, 142, 153, 160, 163, 166.
Epiphytes, 113, 130, 135, 137.
Epiphytic algae, 188.
Epiphytic fungi, 161, 193, 232.
_Equisetum_, 113.
Ergot, 131, 142, 144.
_Erineum_, 88, 212, 214, 215.
Erosions, 204, 207.
_Erysipheae_, 135, 142, 161, 187, 268.
Essentials of fertilisation, 69.
Estimates of loss, 146.
Etiolation, 106, 179, 180, 229.
_Euphorbia_, 116, 134, 247, 266.
Excavations, 204.
Excess of food, 229.
Excess of minerals, 102.
Excess of water, 100.
Excessive growth, 246.
Excessive nutrition, 250.
Excrescences, 114, 212, 222.
Excreta, 45, 130, 133.
_Exobasidium_, 128, 218.
_Exoascus_, 116, 128, 188, 208, 214, 218, 225, 247, 253.
Expense of materials, 161.
Experiments necessary, 168.
Exposure of roots, 179, 184.
External causes of disease, 99.
Extinction of species, 91.
Exudations, 227.
Exudation under pressure, 51.
Factors of an epidemic, 149, 165.
Falling of fruit, 206.
Falling leaves, 123.
False chlorosis, 181.
False etiolation, 180.
_Farfugium_, 188.
Fasciations, 230, 246, 251.
Fats, 272, 286.
Feeding, 14, 16.
Fermentation, 58, 102, 130, 233.
Ferns, 113, 247, 260, 261.
Fertilisation, 71.
Field-mice, 164.
Figs, 113.
Finger and toe, 114, 127, 163.
Fire, 240.
Flaming, 164.
Flattened roots, 246, 252.
Fleshiness, 228.
Flies, 86, 110, 142, 143, 145.
Flux, 227, 231.
Flying foxes, 244.
Focussing of solar rays, 192.
Foliage, 110.
_Fontaria_, 134.
Food, 18.
Forest-fires, 241.
Formic-aldehyde, 20.
Foul products, 100.
Foxy leaves, 191.
Freezing, 121, 183.
Frit fly, 182.
Frost, 153, 160, 225, 229, 248, 249.
Frost-beds, 243.
Frost-blisters, 212, 218.
Frost canker, 222.
Frost-cracks, 204, 209, 242.
Frost-patches, 240.
Frost-ridge, 209.
_Fumago_, 190, 232.
Fumes, 104.
Functions of roots, 43, 45.
Functional depression, 96.
Fungi, 89, 108, 143, 174, 189, 200, 205, 207, 208, 212, 216, 219, 223,
229, 231, 233, 238, 240, 241, 243, 248, 251, 255, 258, 265, 267,
283, 284, 288.
Fungus attacks, 139.
Fungus galls, 219.
_Fusarium_, 143, 238.
_Fusicladium_, 189.
_Fusisporium_, 237.
_Gagea_, 258.
Gall-apple, 218.
Gall-flies, 219.
Gall-insect, 139.
Gall-like swellings, 128.
Galls, 86, 110, 120, 130, 138, 212, 214, 218, 255.
Gangrene, 231.
_Garreya_, 264.
Gas, 160.
Gases in soil, 104.
_Gastropacha_, 225.
Gelatine, 163.
General death, 116.
General disease, 119, 120.
Germ-plasm, 267.
_Gesneria_, 260.
_Glechoma_, 218.
_Gloeosporium_, 189, 190, 208.
_Gloxinia_, 260.
Goats, 164.
Gooseberry, 217.
Graft-hybrids, 262, 267, 271, 283.
Grafting, 78, 155, 169, 183, 250, 262, 271, 281.
Grain-rust, 146.
Grapes, 192, 230, 231.
_Grapholitha_, 109, 207.
Grass, 111, 189, 190, 205, 218, 233.
Green fly, 161.
Grew, 85.
Greyish spots, 187.
Growth, 271, 274, 275, 286.
Grubs, 110, 207.
Gumming, 235.
Gummosis, 227, 234, 235.
_Gymnosporangium_, 114, 176, 223.
Hail, 106, 240, 241.
Hales, 85.
_Haltica_, 209.
Hardy varieties, 168, 170, 177.
Haustoria, 134, 135, 136.
Healing, 194, 196.
Healing by cork, 123.
Health, 272, 287.
Health and disease, 91, 97, 287.
Heliotropism, 126.
_Hemileia_, 146, 169.
Heredity, 72, 283.
_Herpotrichia_, 135, 190.
Hessian Fly, 182.
_Heterodora_, 219, 220.
_Hieracium_, 112.
History of Phytopathology, 85.
Holdfast of roots, 42.
Hollyhock disease, 143.
Holly, 217.
Honey dew, 144, 227, 232, 233.
Hops, 162, 187, 191, 232, 253.
Hop-aphis, 146.
Hop-disease, 166.
Hop mildew, 161.
_Hormomyia_, 219.
Hornbeam, 224, 233, 242.
Horse-radish, 260.
Host, 284, 285.
Hyacinth, 231, 261.
Hyacinth disease, 143.
Hybrids, 69, 156, 281.
Hybridisation, 69, 75, 169, 266, 281.
Hydrochloric acid, 181.
Hydrogen, 272.
Hymenomycetes, 206.
Hypertrophy, 119, 127, 139, 213, 215, 247, 266.
_Hypochaeris_, 112.
_Hypomyces_, 237.
_Hyponomeuta_, 254.
Ice, 184, 209.
Ichneumon-flies, 165.
_Icterus_, 181.
Illegitimate unions, 265.
Immunity, 155, 156, 165, 168, 169.
Impervious subsoil, 181.
Inarching, 269.
Increase in dry weight, 23.
Indian agriculture, 172.
Indian wheats, 168.
Indispensability of elements, 278.
Infection, 262, 265, 267.
Ingredients of protoplasm, 272.
Insect bites, 225.
Insect diseases, 145, 146, 154, 189.
Insect punctures, 88.
Insects, 89, 98, 108, 109, 120, 138, 142, 153, 174, 187, 194, 203,
205, 206, 207, 208, 212, 223, 229, 241, 244, 248, 251, 254, 255,
258, 259, 269.
Insolation, 180, 242.
Intercellular endophytes, 136, 137.
Intercellular mycelium, 128.
Interference, 91.
Internal causes of disease, 99, 101.
Intracellular parasites, 127, 136.
Intramolecular respiration, 277.
Intumescences, 212, 215.
Inulin, 11, 17.
Invertebrata, 108.
Irritability, 125, 127.
Irritation, 119, 139.
_Isaria_, 163.
Ivy, 113, 165.
Japanese trees, 250.
Jerusalem Artichoke, 264.
_Juncus_, 219.
Juniper, 114.
Kidney bean, 192.
Knauers, 223.
Knife wounds, 194, 195.
Labour, 161.
Lace-wings, 165.
_Lachnus_, 223.
Lady-birds, 164, 165.
Lammas shoots, 257, 259.
Larch, 168, 171.
Larch disease, 115, 149, 152, 166, 171, 223, 241.
Larvae, 110.
Lateral wounds, 132.
Lawns, 112.
Laying of wheat, 179, 180.
Leaf-curl, 236, 253.
Leaf-diseases, 114, 119, 120, 242.
Leaf-galls, 217, 218.
Leaf-miner, 86, 109, 204.
Leaf perforations, 208.
Leaf rolling, 214, 246, 254.
Leaf-spots, 114, 190.
Leguminoseae, 137, 219.
Lemons, 235.
Lenticels, 217.
Lepidoptera, 187.
_Leptosphaeria_, 249.
Lichens, 137.
Liebig, 4.
Life, 271, 285, 287.
Life and death, 271.
Light, 27, 106.
Lily disease, 143.
Lime, 163, 215, 218, 232, 253, 254, 260, 269.
Limes, 172.
Limits of variation, 287.
_Linaria_, 252.
Liquid antiseptics, 160, 161, 162.
Living environment, 99, 108.
Local action, 114.
Local disease, 119, 121.
Locusts, 109, 145, 163, 164.
Longicorns, 205.
_Loranthus_, 113, 245, 265.
Losses due to epidemics, 142.
Lowering of temperature, 100.
Lucerne, 249.
Lurking parasites, 142, 145.
Lychnis, 232.
_Lyonetra_, 206.
_Lysimachia_, 217.
Machine, plant compared to a, 79.
Magnesium, 272.
Maize, 116, 173, 219, 267.
_Majanthemum_, 175.
Malformations, 116, 130, 131, 246, 251.
_Mal nero_, 190.
Mallow, 252.
Malpighi, 85.
Mammals, 142.
Man and plants, 108, 142, 143.
Manna, 227, 235.
Manna Ash, 235.
Maple, 259.
Maximum, 288.
Maximum absorption, 19.
Maximum assimilation, 19.
Maximum temperature, 105.
Mealy bug, 164.
_Melampsora_, 176.
Melon, 220.
Messmates, 63.
Metabolic products, 274.
Metabolism, 23, 127, 271.
Metabolites, 278.
Metallic compounds, 162.
Mice, 108, 163.
Microbes, 227.
Micro-organisms, 183.
Mildew, 86, 164.
Millardet, 169.
Mineral salts, 101.
Miniature trees, 250.
Minimum, 288.
Minimum temperature, 105.
Misconceptions, 12.
Mistletoe, 113, 265.
Mites, 192, 214, 255.
Mixed species, 166.
Molecular structure of protoplasm, 273, 274.
Mongrel forms, 74.
_Monilia_, 217, 231.
Monstrosities, 246.
Moraine plants, 250.
Moths, 110, 145, 206.
Moulds, 230, 231, 237, 243.
_Mucor_, 230, 231.
Mulberry, 244.
Mutilations, 252.
Mycelial strands, 145.
Mycelium, 188.
Mycocecidia, 219.
Mycorrhiza, 137.
Myrtaceae, 258.
_Mytilaspis_, 187.
Natural checks, 159.
Natural demise, 91, 93.
Natural Grafts, 269.
Natural Selection, 72, 99, 167, 286.
Natural Wounds, 204.
Nature of soil, 57.
Necrosis, 240, 241, 243.
_Nectria_, 145, 217, 223, 241, 243, 269.
Nematodes, 111, 134, 139, 219, 220.
Nettle, 116, 252.
_Neurotus_, 219.
New formations, 255.
Nitrate, 273.
Nitrification, 62, 102.
Nitrogen, 272.
Nodosities, 219.
Nodules on roots, 63, 137.
Non-living environment, 99.
_Notommata_, 140.
Nuclear fusion, 267.
Nuclear protoplasm, 271, 279, 280, 290.
Nuclear substance, 71.
Nucleo-plasm, 280.
Nuts, 248.
Oak, 110, 188, 215, 218, 219, 223, 233.
Oak leaf-roller, 254.
Oat, 176.
Occlusion, 200, 201, 222, 223.
Odours, 144.
Oedema, 228.
Olive, 223.
Onion, 231.
_Oniscus_, 182.
_Oospora_, 216.
Optimum temperature, 105, 288.
Orange, 173, 187, 235, 247.
Orange-coloured spots, 187.
Orchard trees, 163.
_Orchestes_, 206.
Orchids, 113, 266.
Organic acids, 50.
Organisation, 89.
Organised structure, 13.
Organisms in soil, 60.
_Orobanche_, 112.
Osmosis, 26, 29, 46.
Osmotic pressures, 18, 41, 52.
Over-crowding, 104, 111.
Over-feeding, 102.
Over-watering, 97.
Oxalic acid, 134, 136.
Oxidation, 124.
Oxygen, 104, 272.
Oxygen-respiration, 12, 64.
Pallor, 179, 180.
Palms, 192.
_Pangium_, 134, 165.
Parasites, 61, 113, 119, 130, 139, 174, 187, 230, 265, 269, 284.
Parasitic algae, 188, 217, 219.
Parasitic bacteria, 163.
Parasitic diseases, 88, 119, 121.
Parasitic epiphyte, 136.
Parasitic fungi, 87, 97.
Parasitism, 262, 264, 268, 271.
_Paris_, 175.
"Paris green," 162.
Parti-coloured leaves, 191.
Parti-coloured spots, 186.
Pasture grasses, 69.
Pathology, 121, 257.
Pathology of cell, 119.
Pathological conditions, 168, 170, 246.
Pea, 190, 191, 206, 208, 248, 268.
Peach, 170, 253.
Pear, 179, 187, 189, 191, 216, 218, 231, 240, 248, 249, 253, 257.
Pedigree wheats, 69.
_Pelargonium_, 198, 253.
Peloria, 252.
_Penicillium_, 231.
_Peridermium Pini_, 223, 234.
_Periola_, 238.
Permanganate, 162.
_Peronospora_, 136, 160, 175, 187, 189, 208.
_Petasites_, 188.
Petroleum, 162.
_Peziza_, 115, 144, 152.
Phanerogams, 108, 111.
_Phellomyces_, 238.
_Phoma_, 217, 243.
Phosphorus, 272.
Photo-synthesis, 11, 16.
_Phragmidium_, 189.
_Phyllachora_, 189.
_Phyllereum_, 253.
_Phyllobium_, 217.
_Phyllosiphon_, 188.
_Phyllosticta_, 188, 209.
_Phylloxera_, 110, 145, 149, 154, 155, 163, 166, 172, 188, 219, 220, 268.
Physiology, 1, 66, 85.
Physiological diseases, 119, 121.
_Phytomyza_, 206.
Phytopathology, 85.
_Phytophthora_, 115, 136, 144, 150, 151, 235, 236.
_Phytophysa_, 219.
_Phytoptus_, 189, 213, 214, 215, 218, 219, 253, 254.
_Pilea_, 219.
_Pilobolus_, 126, 140.
Pines, 183, 223, 234, 251, 252.
Pine-apple, 258.
Pith flecks, 204, 207.
Plant as agent of disease, 99, 108.
Plant, agricultural chemistry of, 1.
Plant and its food, 7.
Plant and its surroundings, 1.
Plant, a machine, 1, 15.
Plant, central object of study, 1.
Plant, physiology, 1.
_Plantago_, 257.
Plantain, 112, 257.
Plants, dying out of, 93.
Plasmodia, 163.
_Plasmodiophora_, 114, 126, 127, 144, 163, 219, 284, 285.
Plasmolysis, 47.
_Pleospora_, 236.
_Pleotrachelus_, 126, 140.
Plum, 171, 189, 192, 209, 214, 206, 231, 235, 248, 249, 260.
_Poa_, 258.
Pocket-like galls, 155, 214, 218.
Pocket-plums, 214.
Pockets, 253.
Poison, 102, 130, 136, 163, 216.
Poisonous gases, 181, 248.
Pollen grain, 288.
Pollination, 248, 262, 265, 266, 271.
_Polydesmus_, 236.
_Polygonatum_, 175.
_Polygonum_, 258.
Polymorphism, 174.
Polyporei, 142.
_Polyporus_, 143, 206.
_Polystigma_, 189.
Poplar, 188, 206, 215, 218, 254.
Post and epidemics, 142.
Potassium, 272.
Potassium sulphite, 162.
Potato, 162, 171, 194, 209, 216, 236, 237, 258.
Potato-disease, 114, 143, 149, 150, 166, 189, 207, 235.
Powders, antiseptic, 159, 160, 161.
Predisposition to disease, 98, 99, 105, 168, 169, 229, 262, 268, 277,
278, 282.
Preventible diseases, 159.
Preventitious buds, 259.
Prolepsis, 257, 259.
Proliferations, 257, 258.
Properties of soil, 57.
Prophylactic measures, 160.
Proteids, 132, 138, 272, 277, 286.
Proteolytic enzymes, 132.
_Protomyces_, 217.
Protoplasmic molecules, 276, 278, 286.
Protoplasm, 33, 41, 271, 272, 274, 276.
Pruning, 105, 143, 194, 225, 250.
Prussic acid, 163, 165, 173.
_Psylla_, 253.
_Puccinia_, 88, 114, 169, 175, 176, 188, 189, 247, 252.
Puckers, 214, 235, 246, 253.
Puffing of spores, 142, 144.
Punctures, 212.
Pure culture, 166.
Purple-black spots, 191.
Pustules, 188, 190, 212, 217.
Putrefaction, 234.
_Pyrethrum_, 161.
_Pyrus_, 214.
_Pythium_, 114, 119, 136, 144, 160, 230.
_Quassia_, 161.
Quinine, 173.
Rabbits, 108, 142, 164, 194.
Rain trees, 233.
Rankness, 97, 227, 228.
Rats, 108, 163.
Rays of light, 18.
Red light, 21.
Red spider, 161, 187, 188, 192.
Red spots, 188, 253.
References in Bible, 85.
Remedial measures, 89.
Repellent substances, 136.
Reproduction, 72, 281.
Reserves, 274.
Resin, 125.
Resin-flux, 234.
Resinosis, 227, 234.
Resistance to disease, 155, 268.
Resistant races, 172.
Respiration, 17, 31, 130, 271, 275, 276, 285, 287.
Reversions, 73.
Rhinanthoideae, 265.
_Rhinanthus_, 112.
_Rhizobium_, 289.
_Rhizoctonia_, 238.
Rhizomorph, 145.
Rhododendron, 218.
Rhubarb, 180, 230.
_Rhynchitis_, 254.
_Rhytisma_, 188.
Ribbon grass, 183.
_Ribes_, 214.
Rice, 172.
Rimpau's experiments, 69, 73, 77.
Ringing, 194, 201, 202, 210.
Ripened wood, 243.
_Robinia_, 259.
Rodents, 109.
_Roestelia_, 217.
Rolled leaves, 86.
Root, 9, 35, 96, 120, 227, 270.
Root-absorption, 181.
Root-diseases, 119, 120.
Root-excretions, 46.
Root-fusions, 262.
Root-galls, 221.
Root-hairs, 34, 102, 163.
Root-nodules, 212, 219.
Root-parasites, 112, 265.
Root-rot, 230.
Roses, 232, 243, 257, 268, 278.
Rosettes, 225.
Rot, 97, 182, 227, 229, 231, 236.
Rotation of crops, 69, 166.
Rotifer, 140.
Rot-organisms, 200.
Rotting of wounds, 87.
Rouen law, 85.
Rushes, 114.
Rust, 122, 142, 171, 172, 175, 191.
Rye, 176, 248.
_Saccharomyces_, 60.
Sachs, 7, 36.
_Salvia_, 214.
San José scale, 187.
Sand-blast action, 184.
Sandy soils, 184.
_Saperda_, 205.
Saprophytes, 135, 137, 175, 234, 243, 244.
_Scab_, 189, 216.
_Scale_, 187.
_Schinzia_, 114.
_Schizoneura_, 223.
Scion, 183, 262, 264, 266, 282.
_Scleroderris_, 223.
Sclerotia, 143.
Schwarz, 39.
_Sclerotinia_, 142, 143, 144, 231, 248, 249, 288.
Scolytidae, 205.
Scorching, 240, 241.
Scurf, 216.
Sea-kale, 261.
_Secale_, 76.
Secretions, 130, 133, 173, 274.
Sedges, 189.
Seedless grapes, 247.
_Selandria_, 208.
Selection, 69, 74, 78, 169.
Selective absorption, 53, 65.
Self-fertilisation, 281.
Semi-parasites, 112.
_Senecio_, 188.
Sensitive plant, 125.
_Septoria_, 114, 187.
Sewage waters, 59.
Sexual act, 72.
Shaded foliage, 113.
Shanking, 246, 249.
Shoots from old wood, 260.
Shot holes, 204, 208, 209.
Silver fir, 224.
Silver leaf, 192.
_Sirex_, 206.
Skeleton leaves, 204, 207.
Slime flux, 227, 233.
Slime fungus, 219.
Slugs, 111, 164, 207, 269.
Smut, 117, 143, 162, 190.
Snails, 111, 142, 207.
Snow, 106.
Soap, as insecticide, 161.
Soil, 1, 42, 99, 102, 142, 163.
Soil-bacteria, 60.
Soil-filtration, 59.
Soil-organisms, 61, 143.
Solar energy, 135.
Somato-plasm, 267.
Sooty moulds, 135, 190, 232.
_Sorbus_, 207.
_Sorosporium_, 216.
Sour-rot, 231.
Sparrows, 164.
Specialised races, 168, 176.
Specific predisposition, 155.
Spectrum, 19, 21, 26.
Spermogonia, 144, 232.
_Sphaerella_, 189.
_Sphaerotheca_, 187.
Sphaeroblasts, 222, 225.
_Spicaria_, 237.
Spiral grooving, 204, 210.
Spiral growth, 252.
_Spongospora_, 216.
Spontaneous variations, 78, 246, 255.
Spores, 144.
Sports, 93, 247.
Spots on leaves, 120, 186.
Spraying, 159, 161, 162.
Spreading of disease, 142.
Squirrels, 108.
Stag-head, 240, 244.
Starch, 9, 16, 17, 20, 23, 138, 173.
Statistics of epidemics, 147.
Steeping, 161.
Stem diseases, 120.
_Stereum_, 206.
Sterility of soil, 61.
Stimulation, 119.
Stimuli, 126, 127, 139.
Stock, 262, 264, 266, 282.
Stomata, 23.
Stool-shoots, 201, 225, 269.
Stool stumps, 194, 201.
Strangulations, 204, 209.
Strawberry, 189, 257.
Stripping, 194, 197.
Stroma, 217.
Structure, 274.
Structure of protoplasm, 271.
Structure of root-hairs, 40.
Struggle for existence, 105, 159, 164, 165, 167, 286.
Study of causes, 85.
Stumps, 194.
Subsoil, 57, 103.
Substitutive selections, 286.
Suckers, 225, 260.
Sugar, 11, 17, 20, 173, 286.
Sugar cane, 172.
Sugar cane disease, 166.
Sulphate, 273.
Sulphur, 161, 163, 272.
Sulphurous acid, 181.
Sun-burn, 240, 241.
Sun-cracks, 240, 242.
Sundew, 232.
Sunflower, 256, 264.
Sun-spots, 192.
Superstitions, 85.
Surface energy, 26.
Surface roots, 112.
Sweet almond, 173.
Symbiosis, 63, 130, 137, 219, 263, 265, 268, 285.
Symptoms of disease, 89, 122, 179, 186.
_Synchytrium_, 127, 188, 217, 247.
Synthesis, 65.
_Syringa_, 259.
Syringing, 161, 164.
Tamarisk, 235.
Tannin, 138.
_Taphrina_, 218.
Tar, 164.
Tea, 244.
Teazel, 252.
Teleutospore, 189, 191.
Temperature, 99, 105.
Tendencies to ill-health, 91.
Tendrils, 125.
Teratology, 246, 253, 254, 257.
_Tetraneura_, 218.
_Tetranychus_, 187, 192.
Thawing, 183.
_Thelephora_, 206.
Therapeutics, 85, 89, 159.
Thermotropism, 126.
_Thesium_, 112.
Thick-skinned organs, 168, 171.
Thinning, 96, 105.
Thistle, 247.
Thrips, 88, 191, 208.
Thyloses, 125.
_Tilia_, 214.
Timber diseases, 119, 120.
Timiriazeff, 21.
_Tinea_, 206.
Tissue diseases, 119.
Tobacco, 209.
Tobacco powder, 161.
Tomato, 171, 219, 230.
Top-dry trees, 244.
Topical remedies, 161.
_Tomicus_, 205.
Torsions, 246, 252.
_Tortrix_, 254.
Toxic agents, 130.
Transformation of energy, 25, 28.
Transformation of organs, 254, 255.
Transmission of acquired characters, 264, 283, 290.
Transplanting, 96.
Transpiration, 181, 228.
Trees, 109.
_Trichosphaeria_, 135.
_Triticum_, 76.
Tumescence, 227, 228.
Tunnels, 206.
Turgescence, 47, 228, 230.
Turnip, 126, 162, 230.
Twitch, 113.
_Tylenchus_, 238, 248.
Ulcer, 231.
Unger, 85.
Unsuitable soils, 101.
Upheaval of seedlings, 179, 183.
Uredineae, 114, 134, 136, 145, 169, 188, 189.
_Uredo_, 88, 188, 191.
Uredospores, 191.
_Uromyces_, 116, 188, 191, 266.
_Urocystis_, 220.
Ustilagineae, 145, 190, 217, 248.
_Ustilago_, 116, 117, 175, 190, 219, 255.
_Vaccinium_, 128, 288.
Variability, 174.
Variation, 67, 72, 91, 92, 168, 174, 176, 246, 262, 263, 264, 271,
282, 286, 288, 289.
Variegation, 179, 182, 183, 192.
Varieties, 78, 247.
Varieties of soil, 56.
_Vaucheria_, 139, 140.
Vegetable acids, 48.
Vertebrata, 108.
_Verticillium_, 145, 236.
_Viburnum_, 214.
Vine, 110, 149, 156, 162, 164, 169, 171, 189, 190, 191, 222, 248, 268.
Vine disease, 143.
Vivipary, 257, 258.
Walnut, 190, 209, 253.
Want of air, 100.
Washing leaves, etc., 161.
Wasp-flies, 165.
Wasps, 145.
Water, 272.
Water and insects, 161.
Water-culture, 65.
Water in soil, 103.
Waterlogging, 181.
Weaving of fungi, 190.
Webs, 190, 254.
Weeding, 105.
Weeds, 69, 111, 113, 165, 229, 249.
Weevils, 248.
Wet feet, 181.
Wheat, 169, 171, 172, 176, 179, 180, 182, 183, 230, 248.
Wheat rust, 86, 122, 146, 166, 169, 176.
White spots, 186, 187.
Willow, 206, 207, 219, 223, 233, 259.
Willow beetle, 208.
Wilting, 179, 181, 235, 249.
Wind, 106, 142, 144, 153, 184, 209, 229.
Wire-worms, 109, 181.
Witches' brooms, 116, 222, 224.
Wood, 124.
Wood-ashes, 161.
Woodbine, 112, 210.
Wood-boring, 204, 205.
Woodlice, 164.
Wood-nodules, 225.
Wood-wasps, 206.
Woolly-aphis, 219, 223.
Worms, 109, 142, 144, 194, 238.
Wounds, 108, 139, 194, 204, 207, 213, 260, 263, 269.
Wound-cork, 195.
Wound-fever, 123.
Wound-fungi, 203, 204, 240.
Wound-gum, 125.
Wound-wood, 124.
Wrens, 165.
Wrinkling, 253.
_Xenia_, 267.
_Xyloma_, 88.
Yeasts, 134, 172, 231, 233.
Yellowing, 179, 181, 182, 184.
Yellow leaves, 89.
Yellow spots, 186, 187, 188, 253.
Zoospores, 151.
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Transcriber's Notes:
The word Oedema uses an OE ligature in the original.
The following corrections have been made to the text:
Page vi: be the better for a real knowledge[original has
knowlege]
Page 55: and[original has and and] are too crudely mechanical
Page 117: Prillieux[original has Prilleux], _Maladies des
Plantes Agricoles_
Page 128: the intercellular mycelium of _Exoascus_[original
has Exoacus]
Page 134: subject to attacks of Uredineae[original has
Uredinae]
Page 142: carried[original has carrried] from plant to plant
Page 176: its æcidia[original has æcida] on the Barberry
Page 182: _e.g._ _Oniscus_[original has Oscinis], the Frit
Fly, and _Cecidomyia_[original has Cecidomya]
Page 182: not necessarily less ash constituents[original has
constitutents]
Page 183: nature of a transmissible enzyme[original has
enyzme]
Page 203: _Krankh. d. Pflanzen_, B. I.[original has 1] cap. 2
Page 206: leaves of Apples by _Lyonetia_[original has
Lyonettia]
Page 218: _Epichloë_[original has Epichloe], which clothes the
sheaths
Page 219: beetle which attacks Crucifers[original has
Crucificers]
Page 221: on the green parts of Hibiscus,[comma missing in
original]
Page 221: nodules of the roots of Leguminoseae[original has
Leguminosae]
Page 230: _Edelfäule_[original has Edelfaüle], a rotten
condition of the grapes
Page 235: giving an almost mealy[original has meally]
appearance
Page 243: as its mycelium[original has myceliun] spreads
Page 258: _Prolepsis._[original has Proplesis]--It frequently
Page 293: Aetiology[original has Ætiology], 89, 100.
Page 293: _Anthonomus_[original has Anthonomos], 249.
Page 294: Bird's-eye[original has Birds'-eye] Maple, 224.
Page 295: _Cercospora_,[original has Cereospora] 190.
Page 298: _Eau Céleste_[original has Celeste], 162.
Page 300: _Heterodora_[original has Heterodera], 219, 220.
Page 300: Holly, 217.[period missing in original]
Page 300: _Hypomyces_, 237.[original has comma]
Page 301: _Lyonetia_[original has Lyonetra], 206.
Page 303: Permanganate[original has Permangate], 162.
Page 304: Prophylactic[original has Phophylactic] measures,
160.
Page 304: _Phytomyza_, 206.[period missing in original]
Page 304: _Phyllereum_[original has Phyllereus], 253.
Page 304: Pine-apple[original has Pine apple], 258.
Page 305: _Puccinia_, 88, 114, 169, 175, 176, 188, 189, 247,
252[original has 252, 247].
Page 307: Somato-plasm[original has Somatoplasm], 267.
Page 307: Spermogonia[original has Spermagonia], 144, 232.
Page 308: _Tomicus_[original has Tornicus], 205.
The following index entries were out of alphabetical order and have been
moved to the appropriate locations:
Phylloxera
Plants, dying out of
Poisonous gases
Preventible diseases
Prophylactic measures
Spermogonia
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