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Title: Links With the Past in the Plant World
Author: Seward, A. C. (Albert Charles)
Language: English
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The Cambridge Manuals of Science and
Literature
LINKS WITH THE PAST IN
THE PLANT WORLD
CAMBRIDGE UNIVERSITY PRESS
London: FETTER LANE, E.C.
C. F. CLAY, Manager
Edinburgh: 100, PRINCES STREET
London: H. K. LEWIS, 136, GOWER STREET, W.C.
Berlin: A. ASHER AND CO.
Leipzig: F. A. BROCKHAUS
New York: G. P. PUTNAM'S SONS
Bombay and Calcutta: MACMILLAN AND CO., Ltd.
_All rights reserved_
[Illustration: _Sequoia magnifica_ Knowlton, in the Yellowstone
National Park. (From a photograph kindly supplied by Prof. Knowlton.)
See p. 104.]
LINKS WITH THE
PAST IN THE
PLANT WORLD
BY
A. C. SEWARD,
M.A., F.R.S.
Professor of Botany in the
University of Cambridge
Cambridge:
at the University Press
1911
Cambridge:
PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS
_With the exception of the coat of arms at the foot, the design on
the title page is a reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521_
PREFACE
MY object in writing this book is primarily to call attention to some
of the many questions which are raised by an enquiry into the relative
antiquity of existing plants, and to illustrate the nature of the
evidence afforded by the records of the rocks. One may agree with the
dictum, 'There is but one art--to omit,' but to practise this art is
often a difficult task. While fully conscious of the incompleteness of
the treatment of the subjects dealt with in these pages, and of defects
in the method of presentation, I hope that I may succeed in attracting
some of my readers who are already interested in living plants to the
study of plants of former ages.
I am greatly indebted to my colleague Dr C. E. Moss for reading the
proofs and for many valuable suggestions. I wish to thank Mr and Mrs
Clement Reid, Prof. MacDougal of the Arizona Desert Laboratory, Prof
Campbell of Stanford University, Prof F. H. Knowlton of the United
States National Museum, Washington, Mr A. G. Tansley, Prof Yapp, and Mr
W. R. Welch for photographs which they have allowed me to reproduce. As
on many previous occasions, I am indebted to my wife for contributing
drawings.
A. C. SEWARD.
Botany School, Cambridge. _July 1911._
The numbers in brackets interspersed in the text refer to the
Bibliography at the end of the volume.
CONTENTS
PAGE
Preface vii
CHAP.
I. Introductory: the Longevity of Trees, etc. 1
II. The Geographical Distribution of Plants 15
III. The Geological Record 39
IV. Preservation of Plants as Fossils 56
V. Ferns; their Distribution and Antiquity 71
VI. The Redwood and Mammoth Trees of California 95
VII. The Araucaria family 106
VIII. The Maiden Hair Tree 121
Bibliography 134
Index 139
* * * * *
_Links with the Past_
ERRATUM
Page 78. Omit lines 5, 6, and 7, from "One" to "but" inclusive.
Transcriber Note: This correction has been applied. Here is the
original sentence:--
One filmy species of Todea is represented in the British flora
by _Todea radicans_ in the Killarney district of Ireland; but
the maximum development of the genus is in New Zealand.
* * * * *
CHAPTER I
INTRODUCTORY: THE LONGEVITY OF TREES, ETC.
'Believe me who have tried. Thou wilt find something more in
woods than in books. Trees and rocks will teach what thou canst
not hear from a master.'
St Bernard.
The recent publication in the daily press of instances of human
longevity under the heading 'Links with the Past' prompted a comparison
between the length of time represented by the duration of a tree
and the lifetime of a human being. The comparison of single lives
suggested the further step of contrasting the antiquity of the oldest
family-histories with the remoteness of the period to which it is
possible to trace the ancestry of existing members of the plant kingdom.
My primary object in these pages is not to deal with familiar cases
of longevity in trees, but to consider in the first place some of the
problems connected with the origin of the present British flora, and
then to describe a few examples of different types of plants whose
ancestors flourished during periods of the earth's history long ages
before the advent of the human race.
In dealing with plants of former ages we are confronted with the
difficulty of forming an adequate conception of the length of time
embraced by geological periods in comparison with the duration of the
historic era. Some of the 'Selections from the Greek Papyri' recently
edited by Dr Milligan (Cambridge 1910) refer to common-place events
in terms familiar to us in modern letters: we forget the interval of
2000 years which has elapsed since they were written. Similarly, the
close agreement between existing plants and species which lived in
remote epochs speaks of continuity through the ages, and bridges across
an extent of time too great to be expressed by ordinary standards of
measurement. Terms of years when extended beyond the limits to which
our minds are accustomed cease to have any definite meaning. While
there is a certain academic interest in discussions as to the age of
the earth as expressed in years, we are utterly unable to realise the
significance of the chronology employed. After speaking of the futility
of attempting to introduce chronological precision into periods
so recent as those which come into the purview of archaeologists,
Mr Rice Holmes suggests a method better adapted to our powers. He
says--'Ascend the hill on which stands Dover Castle, and gaze upon
Cape Grisnez, let the waters beneath you disappear; across the chalk
that once spanned the channel like a bridge men walked from the white
cliff that marks the horizon to where you stand. No arithmetical
chronology can spur the imagination to flights like these(1).' On the
other hand, the use in some country districts in Britain of spindles
almost identical with instruments used in spinning by the ancient
Egyptians, and similar survivals described by the author of a book
entitled _The Past in the Present_(2), bring within the range of our
vision an early phase of the historic era. The rude implements still
fashioned by the flint-knappers of Brandon in Suffolk connect the
present with the Palaeolithic age. Measured from the standpoint of
historic reckoning, survivals from prehistoric days appeal to us as
persistent types which have remained unchanged in a constantly changing
world.
In one of his essays Weismann quotes an old German saying with regard
to comparative longevity, which asserts that 'a wren lives three years,
a dog three times as long as a wren' and so on in a regularly ascending
series: the life of a deer is estimated at three times that of a crow
and an oak three times that of a deer, which means that, computed
on this basis, an oak lives nearly 20,000 years(3)! This fanciful
illustration of the relative longevity of an oak is the expression of
a truth, namely the superiority of trees over animals in regard to
the duration of life. As a seventeenth-century translator of Pliny's
_Natural History_ writes, 'In old times trees were the very temples
of the gods: and according to that antient manner, the plaine and
simple peasants of the country, savouring still of antiquity, do at
this day consecrate to one God or other, the goodliest and fairest
trees that they can meet withal.' Oaks growing in Pliny's day in the
Hercynian forest are said to have been there 'ever since the creation
of the world(4).' Sir Joseph Hooker, in an account of some Palestine
oaks, gives a drawing of a famous tree at Mamre, known as Abraham's
Oak, which is supposed to mark the spot where the Patriarch pitched
his tent(5). Examples such as these, though of no scientific value,
serve to illustrate the well-founded belief in the extraordinary
longevity of trees. In the absence of evidence to the contrary, it
would be rash to deny the possibility that William the Conqueror's Oak
in Windsor Forest, described by Loudon in his _Arboretum Britannicum_
and mentioned by later writers, may be a survival from the reign of
the king whose name it bears. Although it is seldom possible to state
with confidence the exact age of old oaks and yews famed for length
of days, there can be no doubt as to the enormous antiquity of
many of our trees whose years are 'sacred with many a mystery.' The
section of a trunk of one of the mammoth trees of California (_Sequoia
gigantea_) exhibited in the Natural History department of the British
Museum, shows on its polished surface 1335 concentric rings denoting
successive increments of wood produced by the activity of a cylinder
of cells situated between the hard woody tissue and the bark. It is
generally assumed that each year a tree produces a single ring, though,
as is well known, an estimate of age calculated on this assumption
cannot be regarded as more than an approximation to the truth. If this
giant tree, which was felled in 1890, was then 1335 years old, it had
already reached an age of over two centuries when Charlemagne was
crowned Emperor at Rome. The concentric rings on a tree trunk owe their
existence to certain structural differences between the wood formed
in the spring and in the late summer. In Sequoia, as in other members
of the great class of cone-bearing trees, the wood is composed of
comparatively narrow elements which serve to carry water from the roots
to the branches and leaves. As spring succeeds winter the inactivity of
the plant-machine is followed by a period of energetic life; opening
buds and elongating shoots create a demand for a plentiful supply of
ascending sap, and in response to this the tree produces a fresh
cylinder of wood composed of relatively wide conducting tubes. After
the first rush of life is succeeded by a phase of more uniform and
gentler activity, the demand for water becomes less exacting and the
wood which is formed during the rest of the growing season consists of
narrower water-pipes. A period of rest ensues, until in the following
spring new layers of larger tubes are laid down in juxtaposition to
the narrower elements of the latest phase of the preceding summer.
This alternation of larger and smaller tubes produces the appearance
of concentric rings on a cross-section of a tree. It is not the pause
in the active life of the plant which is responsible for the effect
of rings, but the fact that the wood produced immediately before and
immediately after the pause is not structurally identical. In trees
grown under the more uniform conditions of certain tropical regions,
the annual rings are either feebly developed or absent; for example, in
some Indian oaks the wood shows no concentric rings of growth.
Stated in general terms, rings of growth in the wood of a tree are
the expression of a power possessed by the plant of regulating the
structure of its component elements in response to the varying nature
of the external stimuli. In certain circumstances, for example after
the destruction of the young buds by caterpillars, the tree makes a
special effort to repair the loss by producing a new set of shoots.
This may be recorded by the occurrence of two concentric rings in one
season. An extreme instance of departure from the normal has recently
been described(6) in which a tree of _Theobroma cacao_ (the cocoa
tree), planted in Ceylon in the summer of 1893 and felled in January
1901, after a life of just over 7 years, was found to have 22 rings in
its stem. In this case the tree shed its leaves three times a year, and
each break in the uniformity of its vital activities was registered
by the apposition of what under ordinary conditions are spoken of as
spring and late-summer wood. At Aden trees stated by natives to be
very old showed only five or six rings of wood, a fact connected with
the almost complete lack of rain and with the uniform conditions of
existence.
The degree of accuracy to be allowed to estimates of age founded on
the number of 'annual' rings is, however, of secondary importance in
comparison with the enormously greater hold on life possessed by trees
as contrasted with the higher animals. Early in the nineteenth century
the Swiss botanist A. P. de Candolle expressed the opinion that trees
do not die from senile decay, but only as the result of injury or
disease. Trees are constructed on a plan fundamentally different from
that underlying the structure of the highly complex human organism,
and are thus endowed with a sort of potential immortality. It has
been suggested that some of the large corals in the Red Sea which
are still tenanted by living polyps may have been growing in the days
of the Pharaohs. The coral polyp represents the growing portion of a
lifeless mass of rock which is constantly extended by the activity of
the organism at the summit of each branch. Between a coral-reef and a
tree there are many essential differences, but a rough analogy may be
recognised. A tree, unlike the higher animals, does not reach a stage
at which the whole of its substance attains a condition of permanence
and fixity. It consists of a complex branching-system in which each
shoot increases in length by virtue of the youthful vigour of its apex:
to a large extent the tree as a whole consists of lifeless material
incapable of further growth, as is the case of the older portions of
a coral-reef; but the regular increase in girth of the trunk and its
branches demonstrates that this comparison is only partially true, and
that the power of growth in a tree is not confined to the extremities
of the youngest shoots. The tip of every twig is composed of minute
cells endowed with a potentiality of development like that which
characterises the embryonic tissues of a seedling just emerged from the
seed. In the course of its growth, each branch, by means of its living
and dividing cells, contributes to the several parts of the complex
mechanism of the tree. While the greater number of cells acquire a
permanent form and lose the power of further development, there
remains a cylinder of cells endowed with perpetual youth. This cylinder
of living cells, known as the cambium, extends between the wood and
bark from one end of the tree to the other: by its periodic activity it
adds new layers of tissue each year and thus, by increasing the amount
of conducting tubes for the transport of water and for the distribution
of elaborated food, it enables the tree to respond to the increasing
demands which are the necessary accompaniment of increasing size. It
has already been pointed out that in the spring when the sap flows most
vigorously the cambial cylinder produces larger tubes, and afterwards
when the tree settles down to its normal life, these are succeeded by
narrower and stronger tubes. These later formed elements serve also
an important mechanical purpose; by the strength of their walls they
increase the supporting power of the tree and enable it to sustain the
added burden of the annual increase in the weight and extent of its
spreading branches.
It is the persistence of permanently juvenile tissue in certain regions
of a tree, together with the remarkable power of repairing injuries
and shedding effete parts, that constitute some of the most striking
contrasts between the higher animals and plants. The embryo oak in the
earlier stages of development consists entirely of actively growing
cells; by degrees differentiation of the embryonic tissues results
in the localisation of regions of cell-production at the tips of
the elongating stem and root. These apical groups of cells are, in
fact, portions of the embryonic organism which persist so long as the
plant lives. This continuity between the growing tip of an old oak
stem and the cells of the undifferentiated embryo affords one of the
most remarkable examples in nature of a link between the past and the
present.
If we pass beyond the stretch of time represented by the life of
a single tree, a few successive generations suffice to carry our
retrospect back to the days when forests of oaks, birches, and other
trees impeded the progress of the Roman invaders, and, a stage farther
back, to the age of Neolithic man whose remains are occasionally
found in our heaths and moors and in the submerged forests round our
coast. The blocks of oak and beech, some of which are as sound as when
first felled, recently discovered below the foundations of parts of
Winchester Cathedral constructed at the end of the twelfth or in the
opening years of the thirteenth century, are relics of Norman forests.
In the course of some excavations at Brigg in Lincolnshire in 1886 a
dug-out boat was found nearly 50 feet long and from 4 to 5 feet in
breadth. The stem of the oak from which the canoe had been fashioned
shows no sign of branching for a length of over 40 feet, a fact which
points to the growth of the tree in a forest where the race for light
induced the development of clean columnar stems. The Brigg 'dug-out,'
now in the Hull Museum, was discovered in an old alluvial valley of the
Ancholme river, formerly connected with the Humber, and it may be that
it was used by Neolithic man as a ferry for river-service(7).
From the period claimed by archaeologists we pass by gradual stages
into the domain of the geologist. As Huxley wrote, 'when even the dim
light of Archaeology fades, there yet remains Palaeontology, which...
has brought to daylight once more the exuvia of ancient populations,
whose world was not our world, who have been buried in river beds
immemorially dry, or carried by the rush of waters into caves,
inaccessible to inundation since the dawn of tradition(8).' The length
of time represented by a succession of long-lived individuals of the
same species becomes enormously extended when we pass to the history
of families, and disinter from the sediments of other ages the remains
of extinct types. As we descend the geological series familiar types
gradually disappear, and through a succession of changing floras we
penetrate to the fragmentary records contained in the older rocks until
the absence of documents sets a limit to our quest.
[Illustration: Fig. 1. _Pinus sylvestris_ Linn, in the Black Wood of
Rannoch. (Photograph by Mr A. G. Tansley.)]
The Scots pine shares with the oak, the beech, the aspen, the yew,
and several other trees the right to be included in the native
flora of Britain. In the peat-beds of Scotland even up to 3000 feet
above sea-level the stumps of pines occur in abundance, and in many
places recent researches have revealed the occurrence of successive
forests of pines, oaks, and spruces separated from one another by
the accumulations of swampy vegetation(9). The spruce fir has long
ceased to be a member of the British flora, but in a few localities in
the Scottish Highlands patches of primeval pine forests remain. The
accompanying photograph (Fig. 1), taken by my friend Mr A. G. Tansley,
in the Black Wood of Rannoch in north-west Perthshire, shows a few
trees of _Pinus sylvestris_ growing in their native soil: the form of
the older tree (_A_) suggests comparison with that of a well-grown
beech such as we are familiar with in English plantations. This
spreading dome-shaped habit seems to be a peculiarity of the Highland
tree, and is one of the characters which have led some botanists to
regard it as a variety (_Pinus sylvestris_ var. _scotica_) of the
ordinary Scots pine. Though it is doubtful if any relics of primeval
pine woods are left in England, abundant evidence of the former
existence of the Scots pine is afforded by the submerged forests
exposed at low-tide on many parts of the English and Welsh coasts and
at the base of some of the English peat moors. During the construction
of the Barry docks on the north coast of the Bristol Channel a
few years ago, the exposed sections of peat and forest beds were
investigated by Dr Strahan and by Mr Clement Reid. There is evidence of
a subsidence of the land to an extent of 55 feet since the formation of
the lower peat-beds containing oak, hazel, willow, and other trees. The
pine, unknown in Wales during the historic period, was recognised in
the Barry cutting. The occurrence of a polished flint implement assigns
a date to the uppermost portion of this old land-surface(10).
It is impossible within the limits of a small volume to discuss in
detail the evidence furnished by the records of the rocks as to the
relative antiquity of the different constituents of the present
vegetation of Britain. In later chapters a few selected plants are
described which are pre-eminently ancient types. Before passing to the
consideration of the data on which the geological history of plants is
based, brief reference may be made to one of the most interesting and
difficult problems of botanical research, namely the history of the
British flora.
CHAPTER II
THE GEOGRAPHICAL DISTRIBUTION OF PLANTS
'No speculation is idle or fruitless that is not opposed to truth
or to probability, and which, whilst it co-ordinates a body of well
established facts, does so without violence to nature, and with a due
regard to the possible results of future discoveries.'
Sir Joseph Hooker.
In the vegetation of the British Isles the leading _rôle_ is played
by that large group to which the term Flowering Plants is frequently
applied. This group, including the two sub-divisions Dicotyledons
and Monocotyledons, is known by the name Angiosperms, a designation
denoting the important fact that the seeds are developed in an ovary
or protective seed-case (ἁγγειον, a vessel or box). The fact that
these highly elaborated products of development made their appearance,
so far as we know, at a comparatively late stage in the history of
the plant-world, attests their efficiency as a class and demonstrates
the rapidity with which they have overspread the surface of the earth
as successful competitors in the struggle for existence. As Darwin
wrote in a letter to Sir Joseph Hooker in 1881, 'Nothing is more
extraordinary in the history of the vegetable kingdom, as it seems to
me, than the _apparently_ very sudden or abrupt development of the
higher plants(11).' In another letter (1879) to the same friend we
read, 'The rapid development as far as we can judge of all the higher
plants within recent geological times is an abominable mystery(12).'
Making allowance for the probability, or indeed certainty, that the
imperfection of the geological record tends to exaggerate the apparent
suddenness of the appearance of this vigorous class, and allowing for
the fact that our knowledge of the records of the rocks in which the
highest plants first occur is very incomplete, we cannot escape from
the conclusion that this recently evolved group spread with amazing
rapidity. Various reasons may be suggested in explanation of the
dominant position which the Angiosperms hold in the floras of the
world. As an instance of their rapid increase during the Cretaceous
epoch[1], the period which has furnished the earliest satisfactory
records of Flowering Plants, the following statement by an American
writer may be quoted:--'The rapidity with which it [_i.e._ the group of
Flowering Plants] advanced, conquering or supplanting all rivals, may
be better understood when we remember that it forms 85% of the flora
of the Dakota group '; that is a series of sedimentary rocks in Dakota
referred by geologists to the middle of the Cretaceous period(13). In
the Wealden rocks of England, which are rich in the remains of Lower
Cretaceous plants, no undoubted Flowering Plant has so far been found.
[1] For the position of the Cretaceous and other systems in the
geological series, see the table on page 42.
The more efficient protection of the ovules, the germs which, after
fertilisation, become the seeds, the extraordinary variety in the
floral mechanisms for assisting cross-pollination, the arrangements
for nursing the embryo, and the structural features of the wood in
relation both to rapid transport of water and to the storage of food,
are factors which have probably contributed to the success of the
Angiosperms. The degree of weight to assign to each contributing cause
cannot as yet be satisfactorily determined, but the general question
raised by the recent origin of these latest products of evolution
offers a promising field for work. While admitting our inability
at present to do more than suggest possibilities, we may encourage
research by speculation.
The members of the Vegetable Kingdom placed next to the Flowering
Plants are the Gymnosperms or naked-seeded (γυμνὁς, naked) plants,
including (i) the Conifers, _e.g._ pines, firs, larches, the yew,
etc., (ii) a small group of plants known as the Cycads, whose
existing members, now almost confined to a few tropical regions, are
the descendants of a vigorous race represented by many species in the
floras of the Mesozoic epoch. A third sub-division of the Gymnosperms,
the Ginkgoales, is represented by a single survivor, which is described
in a later chapter as one of the most remarkable links with the past in
the plant kingdom.
The Gymnosperms are geologically very much older than the Angiosperms.
Members of this class played a prominent part in the vegetation of
the Coal age and it is certain that they existed in the still older
Devonian period. The only other group to which reference is made in
later chapters is that of the Ferns, one of the sub-divisions of a
large class known as the Vascular Cryptogams or Pteridophyta. These
plants, like the Gymnosperms, are represented in the oldest floras of
which recognisable remains have been preserved. The main groups of the
vegetable kingdom, founded on existing plants, are distinguished by
well-defined differences; they are comparable with separate twigs of a
tree springing from larger branches and these again uniting below in
a common trunk. The vegetation of to-day represents only the terminal
portions of the upper branches. As we descend the geological series,
records of extinct types are found which enable us either to trace
the separate branches to a common origin or to recognise a convergence
towards a common stock. Were a botanist to find himself in a forest of
the Coal age he would experience great difficulty in assigning some
of the plants to their systematic position: characters now regarded
as distinguishing features of distinct groups would be met with in
combination in a single individual. It is by the discovery of such
generalised types, which serve as finger-posts pointing the way to
lines of evolution, that the student of pre-existing plants has been
able to throw light on the relative antiquity of existing forms, and
to trace towards a common ancestry plants which now show but little
indication of consanguinity.
Confining our attention to the dominant group of plants in the British
flora, namely the Flowering Plants, we may profitably consider the
question, though we cannot satisfactorily answer it,--which members of
this group are entitled to be regarded as the most ancient inhabitants?
The past history of our native plants, and their geographical range,
not only in the British Isles but on the Continent of Europe, are
subjects well worthy of the attention of field-botanists whose
interests are apt to be confined within too narrow bounds. There are
numerous problems relating to the composition of the present vegetation
of Britain which might be discussed in reference to the relative
antiquity of plants; but in a single chapter it is impossible to do
more than call attention to certain considerations which are frequently
overlooked by students of British species.
It is customary to speak of the British flora as consisting for the
most part of species introduced into this country by natural means,
while some plants owe their introduction to human agency or are
'escapes' from cultivation. It is by no means an easy task in some
instances to decide whether a species is native or introduced, but
in some cases, a few of which are mentioned, there is no doubt as to
the alien nature of the plants. The term 'native' needs a word of
explanation. It is not intended to convey the idea that a plant so
designated came into existence on British soil and spread thence to
other regions; but by native species we mean such as have reached this
country by migration from other lands, or it may be in some instances
have actually originated in this part of Europe. One of the best known
aliens in Britain is the American water-weed, _Elodea canadensis_
(or _Anacharis alsinastrum_), which was discovered about sixty years
ago in a canal near Market Harborough in Leicestershire(14). In all
probability this North American species was introduced into England
with timber. Once established, it spread through the waterways with
alarming rapidity and became a serious pest. Elodea affords an
admirable instance of the serious interference with the balance of
Nature by the introduction of a new competitor into an environment
conducive to vigorous development. Another foreign water-plant,
_Naias graminea_, for the importation of which Egyptian cotton
may be responsible, has been recorded from the Reddish canal near
Manchester(15). This African and Asiatic species occurs in Europe
only as a colonist; it is said to have been introduced into Italy
with East Indian rice. A more recent case of alien immigration due to
unintentional human agency is that of _Potamogeton pennsylvanicus_,
a pond-weed of Canada, the United States, Jamaica, and elsewhere.
Specimens of this species were first noticed in 1907 in a canal near
Halifax close to the effluent from a cotton mill. Since its discovery
the plant has slightly extended its range. It is suggested by Mr
Bennett, who first identified the alien, that its fruits were brought
to this country in goods from the United States(16).
Of the introduction of these and other foreign plants we have
satisfactory records; but there are many others which may owe their
presence to man's agency, though we have no information as to their
arrival.
It has long been recognised that several members of the British flora
are related to Scandinavian species. The Scandinavian flora, as Sir
Joseph Hooker says in his well-known paper on the 'Outlines of the
Distribution of Arctic plants,' 'not only girdles the globe in the
Arctic Circle, and dominates over all others in the North Temperate
Zone of the Old World, but intrudes conspicuously into every other
temperate flora, whether in the northern or southern hemisphere, or on
the Alps of tropical countries'(17). The view generally held is that
during the Glacial period this Arctic flora was driven South, and aided
by land-bridges, which were afterwards submerged, many of the northern
migrants found a more congenial home in Britain. It is however by no
means improbable that this conclusion may have to be considerably
modified. Mr and Mrs Reid, as the result of their careful analysis
of the Pre-Glacial Flora of Britain, express the opinion that 'the
pre-glacial plants suggest climatic conditions almost identical with
those now existing, though slightly warmer' (27, 2). It is noteworthy
that the list of plants given in their paper does not include any
typical Arctic species. The occurrence on the mountains of Scotland
and elsewhere of such plants as _Silene acaulis_, _Dryas octopetala_,
_Saxifraga oppositifolia_ and other Saxifrages, _Rubus chamaemorus_
(the Cloudberry), and the dwarf Birch illustrate the Arctic-Alpine
element in our flora.
The opinion is held by many Swiss botanists that their Alpine species
have in large measure been derived from non-glaciated parts of the
Pyrenees, that is from a region which was presumably able to retain its
flora at a time when more northern lands were exposed to extreme Arctic
conditions. My friend Dr Moss believes that some of the so-called
Scandinavian plants came to Britain from Central Europe after the
retreat of the ice; if this view is correct it means that some at least
of our Arctic-Alpine plants reached these islands by a southern rather
than by a northern route.
Interesting examples of far-travelled northern plants recently
described by Professor Engler of Berlin afford additional illustrations
of the general principles enunciated many years ago by Sir Joseph
Hooker. A species of flowering Rush, _Luzula spicata_ var. _simensis_,
occurs at an altitude of 3600 metres in Abyssinia and on Kilimanjaro.
_Luzula spicata_ is found in the whole of the Arctic and Subarctic belt
in Scotland, Auvergne, the Jura mountains, and elsewhere. The species
probably began its career in the northern hemisphere where it grew
abundantly on the higher ground in the Arctic Circle: it eventually
travelled along the North American Andes and appeared in Mexico under a
guise sufficiently distinct to warrant the use of another name, _Luzula
racemosa_. In an eastern direction it reached the Himalayas and is
represented in Abyssinia by a closely allied form. From Abyssinia to
Kilimanjaro _Luzula spicata_ 'had to travel a long distance; but it is
not impossible that it either still exists or has existed previously on
a few of the high mountains between Abyssinia and Kenia, from which,
having advanced to the Kilimanjaro, it again produced new forms....
At any rate, it is impossible to do without distribution of seeds of
alpine plants by air-currents or by birds from one mountain to the
other in explaining the history of distribution'(18).
The majority of British plants are identical with species in Central
and Northern Europe: of these, some are among the most widely spread
English species, _e.g._ the Daisy and Primrose, while others, such as
the Oxlip (_Primula elatior_), are confined to the Eastern counties,
and others, such as the Cheddar Pink (_Dianthus caesius_), are
restricted to Western counties.
Before considering a small section of the British flora which is the
most interesting from the point of view of origin, a short digression
may be allowed in order to call attention to the importance of a
branch of science which Darwin spoke of as 'that grand subject, that
almost keystone of the laws of creation, geographical distribution,'
and in 1847 referred to as 'that noble subject of which we as yet
but dimly see the full bearing.' It was largely as the results
of his study of distribution in the Galapagos Islands that Darwin
determined to 'collect blindly every sort of fact which bears anyway
on what are species.' The acceptance of the view 'that each species
was first produced within a single region'(19), raises the subject of
geographical distribution to a far higher plane than it occupied in
pre-Darwinian days. Although most people are familiar with some of
the commoner means by which plants are able to colonise new ground
through the adaptation of their fruits and seeds to various methods of
transport, the conception of a plant as a stationary organism tends
to prevent due allowance being made for the comparative facility with
which, in the course of successive generations, a species is able to
wander from place to place. The individual animal is endowed with
powers of locomotion enabling it to seek new feeding-grounds and to
avoid enemies; but with the exception of some of the simplest forms a
plant cannot move--'le matin la laisse où la trouve le soir.'
The rate of travel may or may not be rapid, but in a comparatively
short time, if the conditions are favourable, a tree may spread over
a wide area. Mr Ridley, Director of the Botanic Gardens, Singapore,
writes as follows in reference to the rate of travel of one of the
common Malayan trees (_Shorea leprosula_), which bears winged fruits
particularly well adapted to wind-transport: 'If we assume that a tree
flowers and fruits at 30 years of age and the fruits are disseminated
to a distance of 100 yards, that the furthest fruits always germinate
and so continue in one direction, it will be seen that under such most
favourable circumstances the species can only spread 800 yards in 100
years, and would take 58,666 years to migrate 100 miles'(20).
There is, however, one type of distribution--what is called
discontinuous distribution--to which special attention should be
directed on account of its intimate association with questions
relating to the past history of living organisms. Many examples might
be quoted from both the animal and plant kingdoms in support of the
view that discontinuous distribution is a criterion of antiquity. When
identical or very nearly identical plants occur in regions separated
from one another by areas in which the particular species is unknown,
the inference is either that the surviving individuals are remnants
of a large number formerly distributed over a wider continuous area,
or that in the course of evolution similar conditions in widely
separated areas led to the production of identical types. The former
view is much the more probable: it is consistent with the conclusions
arrived at on other grounds as to the connexion between discontinuous
distribution and ancient lineage. The explanations of the widespread
occurrence among different races of similar objects or legends afford
an analogous case. As Dr Andrew Lang points out in _Custom and Myth_,
it is held by some students that the use of the bull roarer--to cite a
specific instance--by different peoples denotes descent from a common
stock, though he considers the more probable explanation to be that
similar minds, working with simple means towards similar ends, might
evolve the bull roarer and its mystic uses anywhere.
The Cedars of Lebanon afford an interesting example of discontinuous
distribution. They illustrate how a species, which may be assumed to
have originated in one region, in the course of its wanderings may
undergo slight changes until, at a later stage when the plants have
disappeared from parts of the once-continuous area, the remaining
outlying groups of individuals are spoken of under different specific
names. The cedars of Lebanon, known as _Cedrus libani_, occur as
isolated groups on the Lebanon hills as outliers of the larger
forests of the Taunus 250 miles distant. The African cedar, _Cedrus
atlantica_, is separated from the Lebanon cedar by a distance of 1400
miles. Approximately the same distance divides the Lebanon cedar from
the deodar, _Cedrus deodara_, which extends from Afghanistan along
the Himalayas almost to the confines of Nepal. Sir Joseph Hooker
regards the three cedars as varieties of one species which once
formed a continuous forest: he attributes the present discontinuous
distribution, in part at least, to the effects of a warmer succeeding
a colder climate. The less favourable conditions drove the vegetation
of the lowlands to seek more congenial habitats at higher altitudes.
In this connexion it is interesting to find that in Algeria the cedar
is confined to the higher ground where the snow lies long in the
spring(21).
The Tulip Tree of North America and Central China affords one of
many examples of existing flowering plants which illustrate the
close connexion between present distribution and past history. The
genus Liriodendron, often cultivated in the south of England, is
now represented by two species, the best known of which--the Tulip
Tree, _Liriodendron tulipera_--extends from Vermont to Florida and
westwards to Lake Michigan and Arkansas. The leaves bear a superficial
resemblance to those of the Sycamore, but are as a rule easily
distinguished by the truncated form of the apex; the specific name
was suggested by the tulip-like form of the flowers. Fossil leaves
of Liriodendron are not uncommon in the Cretaceous rocks of Disco
Island in latitude 70° N., where they occur with other flowering
plants which bear striking testimony to the mildness of the Cretaceous
climate in high northern latitudes. One of the associated flowering
plants is a species of Artocarpus, described by Dr Nathorst as
_Artocarpus Dicksoni_ which bears a close resemblance to _A. incisa_
the bread-fruit tree of the southern tropics of the Old World. Without
attempting to deal fully with the past history of Liriodendron, it may
be confidently stated that the records of the rocks are consistent with
the idea of antiquity suggested by the present distribution of the two
surviving species.
Islands such as Great Britain and Ireland, situated a short distance
from a continent, contain many plants which are widely spread in
different parts of the world, together with a very small number
peculiar to the British Isles though closely allied to species on the
neighbouring continent or to plants farther afield. The occasional
recognition of species previously believed to be confined to Britain
tends to reduce the short list of our endemic types.
An enquiry into the origin of an island flora involves a consideration
of the data in regard to changes in level and relative distribution of
land and water in the course of geological evolution. It is generally
agreed that at no distant date, in a geological sense. Great Britain
and Ireland were united to the continent. There is, however, another
fact to reckon with, namely the prevalence of Arctic conditions in
northern Europe when a thick sheet of ice spread over the greater
part of the British Isles. There can be no doubt that the severity of
the climate during the Glacial period was such as to destroy a large
proportion of the vegetation. The question is, were all the flowering
plants destroyed or were some of the hardier species able to survive,
either on the higher peaks which kept their heads above the level of
the ice or on the southern fringe of England beyond the ice-covered
region? It is impossible to give a definite answer: the probability
is that nearly all the pre-Glacial species were destroyed, but it is
not impossible that some Alpine-Arctic plants escaped extinction,
while others retreated to more southern and less Arctic areas by means
of a land-connexion with France or crossed the intervening sea by
ocean-currents, by animal agency, or by wind.
Although we possess but imperfect information as to the extent and
duration of land-bridges between Britain and the continent, there are
no special difficulties in the way of accounting for the presence
of Scandinavian, Germanic, and other elements in the British flora.
There are, however, other and more difficult problems to consider in
reference to a small group of flowering plants which are met with in
the west and south of Ireland, also, to a less extent, in Cornwall and
in a few other localities in the south-west of England. In Connemara
in the west of Ireland, where hard frosts are unknown and winter snows
are rare, there are three kinds of Heath, St Dabeoc's Heath (_Daboecia
polifolia_), the Mediterranean Heath (_Erica mediterranea_) and
_Erica Mackaii_ which are not found elsewhere in the British Isles or
in the whole of northern Europe, but reappear in the Pyrenees. The
London Pride (_Saxifraga umbrosa_), another Pyrenean plant, grows
on the south and west coast of Ireland from Waterford to Donegal.
_Arbutus Unedo_, the Strawberry tree, which flourishes in the Killarney
district of County Kerry and occurs in neighbouring localities, has
a wide distribution in the Mediterranean region. Devonshire and
Cornwall possess two other Heaths, _Erica ciliaris_, which extends into
Dorsetshire and occurs in north Brittany, and _Erica vagans_, both
Pyrenean species, while a Mediterranean plant. _Gladiolus illyricus_,
grows in the New Forest.
In 1846 Edward Forbes dealt with the problems presented by the
distribution of British plants in an essay which has exercised a
far-reaching influence. When Forbes published his work, comparatively
little was known as to the possibilities of transport of seeds and
fruits across barriers of water(22). His conviction that the known
means of dispersal were insufficient to account for the presence of
Mediterranean or Lusitanian plants in Ireland led him to turn to
geology for a solution of the problem. He was thus led to put forward
the view that in the course of the Tertiary period when, as we know
from palaeontological evidence, the climate of north and west Europe
was much warmer than it is now, and long-before the beginning of the
climatic changes which culminated in the Glacial period, there was a
land-connexion between the west of Ireland and the south-west of the
continent. Mr Praeger, whose work on the Irish flora is well known to
systematic botanists, agrees with the conclusions of Forbes, and sees
in the Portuguese and Mediterranean plants 'relics of a vegetation
which once spread along a bygone European coast-line which stretched
unbroken from Ireland to Spain'(23). If this explanation is correct it
entitles Arbutus, St Dabeoc's heath and other members of this southern
group to be regarded as a very old section of our flora. There is,
however, another side to the question: granting that a certain number
of Irish plants were able to withstand the rigours of an Ice age, it
is hardly likely that the strawberry tree and other southern types,
which it is admitted flourish in the south-west of Ireland because of
the mildness of the climate, were of the number of those which endured
an extreme Arctic phase. Moreover, if these Mediterranean species
are survivals from the Tertiary period, if they have been isolated
since pre-Glacial days as an outlier of a southern flora, we might
fairly expect that during the long interval between their arrival and
the present day new forms would have been produced closely related
to, though not identical with, the parent types. This, however,
has not been proved to be the case. Darwin in speaking of Forbes'
Essay in a letter to de Candolle in 1863 says that he differs from
most of his contemporaries 'in thinking that the vast continental
extensions of Forbes, Heer, and others are not only advanced without
sufficient evidence, but are opposed to much weighty evidence'(12).
The alternative view is to regard Arbutus and its compatriots as
post-Glacial arrivals and not as survivals from a widely spread
Tertiary flora.
A recently published account of the New Flora of the volcanic island
of Krakatau furnishes an instructive and remarkable demonstration of
the facility with which a completely sterilised island, separated
by several miles of ocean from neighbouring lands, may be restocked
with vegetation(24). In 1883 the island of Krakatau, then densely
covered with a luxuriant tropical vegetation, was partially destroyed
by a series of exceptionally violent volcanic explosions. After this
catastrophe only a third of the island was left: the surface was deeply
covered by pumice and volcanic ash and no vestige of life remained. In
1906 a party of botanists who spent a few hours on Krakatau collected
137 species of plants: the vegetation was in places so dense that it
was with the greatest difficulty they penetrated beyond the coastal
belt, and some of the trees had reached a height of 50 feet. The seeds
and fruits of this new flora have been carried by ocean-currents,
by wind, and by the agency of birds from other islands in the Malay
Archipelago. The nearest islands, except the small island of Sebesi,
about 12 miles distant, are Java and Sumatra, separated from Krakatau
by a stretch of water about 25 miles in breadth. It is reasonable to
wonder whether, had Forbes known of this and similar modern instances
of the capabilities of plants as travellers, he would have adopted the
view he did. In this connexion it may be added that in recent years the
glaciation of Ireland has been shown to be more extensive than it was
believed to be when Forbes wrote his essay.
There would seem to be no insuperable objection to the conclusion
that the Mediterranean plants in Ireland and in the south of England
reached their present home after the retreat of the ice at the end
of the Glacial period, and after Ireland became an island. A full
consideration of the problem is beyond the scope of this book, but
I have briefly stated the case, not with the authority of an expert
but in order to draw attention to a particularly fascinating study in
plant-migration.
[Illustration: Fig. 2. _Eriocaulon septangulare_ With. West Connemara.
(Photograph by Mr W. R. Welch.) ]
In a volume by W. Canton entitled _A Child's book of Saints_ a story is
told in which the presence in Ireland of Mediterranean species receives
a more picturesque explanation. The Monk Bresal was sent to teach the
brethren in a Spanish monastery the music of Irish choirs. In later
years Bresal longed for a sight of his native land, though he loved his
home and 'every rock, tree, and flower' in his adopted country. After
returning to Ireland, his thoughts reverted to Spain; 'it appeared
to him as though he was once again in a granite nook among the rocks
beside the Priory'; he saw the ice-plant with its little stars of white
flowers sprinkled with red (the London Pride) and a small evergreen
tree from which he had often gathered the orange-scarlet berries
(Arbutus). The Prior of the Spanish monastery 'with heavenly vision
saw Bresal gazing at the evergreen tree and the ice-plant, and turning
to the trees blessed them and commended them to go and make real his
dream. As Bresal brushed away his tears he saw with amazement at his
feet the ice-plant and hard by the evergreen tree.'
The plant represented in Fig. 2 is another British species which tasks
the ingenuity of students of plant-geography. This is the Pipe Wort
(_Eriocaulon septangulare_), the sole representative in Europe of a
certain family of Monocotyledons: it flourishes in the west of Ireland
and in the western islands of Scotland but nowhere else in Europe; it
is native on the other side of the Atlantic in Canada and the northern
United States of America. Mr Praeger in describing the striking mixture
of species in the west of Ireland writes, 'The pool from which we
gather the American Pipe Wort is fringed with Pyrenean Heathers. The
cracks which are filled with the delicate green foam of the maiden hair
are set in Bearberry and Spring Gentian; _Habenaria intacta_, far from
its Mediterranean home, sends up its flower-spikes through carpets of
mountain Avens; and St Dabeoc's Heath and the dwarf Juniper straggle
together over the rocky knolls'(25).
The presence of Eriocaulon on the western edge of Europe may be
attributed to migration in pre-Glacial days from North America by way
of a land-connexion, of which Greenland and Iceland represent surviving
portions. The opinion held by Forbes, and advocated by some later
naturalists, that the southern companions of Eriocaulon in the west of
Ireland are survivors from a Tertiary flora which have lived through
the Ice Age, is consistently extended to the Pipe Wort. On the other
hand, before yielding to the temptation to regard these American and
Mediterranean species as links with the Tertiary period, we must be
convinced that the possibilities of post-Glacial introduction, even
without the aid of land-bridges, have been exhausted. The Pipe Wort is
a botanical puzzle which affords a good example of the accession of
interest to field-botany effected by a knowledge of the distribution
of the component members of the British flora. The problem of its past
history suggests an experimental enquiry into the adaptability of its
seeds to dispersal, and emphasises the importance of the co-operation
of botanists and geologists in a common endeavour to trace the origin
of British plants.
In addition to the Pipe Wort, mention may be made of three other
American flowering plants recognised in the Irish flora. _Sisyrinchium
angustifolium_ recorded from the west of Ireland is native in temperate
North America; the orchid, _Spiranthes romanzoffiana_, met with in the
south and north of Ireland, is widely spread in Canada and the northern
States, while _Sisyrinchium californicum_, a native of California
and Oregon, was discovered by Mr Marshall in marshy meadow-land near
Wexford(26). In the case of the more recently discovered American
immigrants, the possibility of human introduction must be borne in
mind, though there are no special reasons for doubting that some, as in
the case of Eriocaulon, reached the Irish coast by natural agencies.
CHAPTER III
THE GEOLOGICAL RECORD
'All the Epochs of the Past are only a few of the front carriages,
and probably the least wonderful, in the van of an interminable
procession.'
J. B. Bury (_The Science of History_).
The portion of the earth's surface accessible to investigation is made
up in part of accumulations of old sediments, some indistinguishable
from the shingle, sand, and mud now in process of formation by the
ceaseless action of denudation; others have been hardened, gently
folded or violently contorted and so far altered by crust-movements
as to render their sedimentary origin well nigh unrecognisable. It is
these sediments of former ages, the dust of lost continents, in which
are preserved the majority of the fragmentary remains of plants and
animals, the flotsam and jetsam of successive phases of evolution.
The crust of the earth, as Darwin wrote, 'with its imbedded remains
must not be looked at as a well filled museum, but as a poor collection
made at hazard and at rare intervals'(19). It is from this imperfect
record that we seek to discover the relative antiquity of the several
groups or genera of living plants, and in the structure of extinct
types we endeavour to discover connecting links between divisions of
the plant kingdom which in the course of evolution have retained little
or no signs of a common descent.
Sir Joseph Hooker in a letter to Darwin in 1859 speaks of his
'conviction that we have not in a fossilised condition a fraction of
the plants that have existed, and that not a fraction of those we have
are recognisable specifically'(12). Considering the nature of the
palaeontological documents the wonder is how much they have taught us,
and we may look with confidence to the results of future research in
a field of which the importance has only recently been appreciated.
With the strata of sedimentary origin are frequently associated igneous
rocks, and in many continental regions, as in the majority of oceanic
islands, the crust of the earth consists wholly of volcanic material or
of rocks produced by the gradual solidification of molten magmas. Rocks
composed mainly of carbonate of lime, such as limestones and chalk,
bear witness to ocean beds or to sediments deposited on the floors of
inland seas beyond the reach of land detritus where coral reefs were
reared or the shells and other calcareous skeletons of animals supplied
the material for future land. In such rocks the remains of calcareous
seaweeds are frequently recognisable and occasionally, as in the
English chalk, fragments of wood testify to transport from a distant
land.
While there is little difficulty in explaining the nature of much of
the earth's crust, in several parts of the world the strata are totally
unfossiliferous and closely simulate crystalline rocks. In many cases
it is believed that such strata represent ancient sediments which in
the course of ages have been reduced by metamorphic agencies to a
condition which has obscured or entirely obliterated all traces of
their pristine state.
Since the pioneer work of William Smith, who in the early days of the
nineteenth century first realised the importance of fossils as aids to
the determination of relative age, geologists have devoted themselves
to the task of correlating the sedimentary rocks of the world, using
as criteria the order of superposition of the strata and the nature of
their organic remains. The result has been to classify portions of the
earth's crust into periods or chapters, which together constitute a
record of geological evolution as complete as it is possible to obtain
from the available data. The accompanying table shows the order of
sequence of the epochs, which stand for terms of years of a magnitude
beyond our powers to grasp.
The division of geological history into larger and smaller periods does
not imply the recurrence of sudden revolutions; it is in some measure
dictated by considerations of convenience, but more particularly by
our ignorance of certain stages in the history of the world due to the
imperfection of the record.
GEOLOGICAL TABLE.
Showing the position in the Geological Series of the strata referred to
in this volume.
Tertiary (Cainozoic)
{ =Recent= { Superficial accumulations containing human
{ =Pleistocene= { remains (Metal age, Neolithic and
{ { Palaeolithic ages, Glacial deposits)
{
{ =Pliocene= Cromer Forest-bed, etc.
{
{ =Miocene= Absent from Britain.
{
{ =Oligocene= Bovey Tracey beds, etc.
{
{ =Eocene= London Clay, etc.
Secondary (Mesozoic)
{ =Cretaceous= { Chalk
{ { Wealden beds
{
{ { Purbeck and Portland beds (Upper Jurassic)
{ =Jurassic= { Oolites (Middle Jurassic)
{ { Lias (Lower Jurassic)
{
{ { Rhaetic beds
{ =Triassic= { Keuper " (Marls with rock-salt, etc.)
{ { Bunter "
Primary (Palaeozoic)
{ =Permian= { Red Sandstones, etc.
{ { Magnesian limestone
{
{ { Coal Measures
{ =Carboniferous= { Millstone Grit
{ { Carboniferous limestone
{
{ =Devonian= { Devonian limestones, etc.
{ { Old Red Sandstones
{
{ =Silurian= { Sandstones, shales,
{ { some limestone
{
{ =Ordovician= { Slates, sandstones,
{ { Volcanic rocks, etc.
{
{ =Cambrian= { Slates, Sandstones, etc.
{
{ =Pre-Cambrian= { Slates, Volcanic rocks, etc.
{ =or Archean=
In certain parts of the world, as for example the north-west Highlands
of Scotland, the Malvern Hills, Scandinavia, and in many other regions
in Europe and North America, geologists have recognised what they
believe to be the foundation stones of the world. These Archaean
rocks, which underlie the oldest fossiliferous strata, belong to a
period of geological evolution from which it appears to be hopeless to
obtain any light as to the nature of the contemporary organic world.
The earliest vestiges of life so far discovered exhibit a high degree
of organisation, which unmistakably points to their being links in a
chain extending far beyond the limits of the oldest Cambrian strata
in which recognisable fossils first occur. The rocks of the Cambrian
and Ordovician epochs, as represented by the grits, shales, slates and
other sedimentary strata in Wales, Shropshire, the Lake district and
elsewhere, though in places rich in the remains of animals, afford
no information in regard to the land vegetation. From the succeeding
Silurian epoch very little evidence has been gleaned as to the nature
of the flora, and it is not until we come to the sedimentary rocks of
the Devonian era that records of plant-life occur in any abundance. The
almost complete lack of botanical data in the pre-Devonian formations
is in part due to the fact that these older rocks consist to a large
extent of marine deposits formed under conditions unfavourable to the
preservation of plants. That the land-surfaces of the older Palaeozoic
eras supported an abundant vegetation there can be little doubt.
The relics of plant-life furnished by the Devonian and succeeding
formations represent the upper branching-systems of a deeply rooted and
spreading tree, the lowest portions of which have been destroyed or
have left no sign of their existence.
In descending the Geological series, we begin with superficial
deposits, such as peat and river-gravels found subsequently to the
underlying boulder-clay of the Glacial period. The remains of forest
trees preserved in the peat and in submerged forests round the
coast connect the vegetation of the historic period with that of the
Neolithic age. At the base of the Pleistocene series, the name given
to the latest chapter of geological history, we find evidence of the
prevalence of arctic conditions in the widely spread boulder-clays and
other deposits of the Glacial period.
From deposits of post-Glacial date abundant plant remains have
been obtained, but we cannot say with any degree of certainty what
proportion of these plants remained in Britain during the Ice age,
and whether the greater part of the vegetation, the relics of which
have been discovered in pre-Glacial beds, was destroyed or driven
south by the advancing ice. We may briefly consider some of the more
interesting facts brought to light by the investigation of the fossil
plants in the Lower Pleistocene and Upper Tertiary beds. It is mainly
to the researches of Mr Clement Reid into the vegetation of Britain
immediately preceding the Glacial period, that our knowledge of this
phase of the history of the British flora is due.
[Illustration: Fig. 3. Pre-Glacial plants from Mundesley (A), Norfolk
and Pakefield (B, C), Suffolk. (Photographs by Mr Clement Reid and Mrs
E. M. Reid.) A. _Bidens tripartita_ Linn. (× 6); B. _Picea excelsa_
Linn. (nat. size). C. _Stellaria holostea_ Linn. (× 12).]
On the coast of Norfolk in the neighbourhood of Cromer the sections
of the cliffs reveal the existence of a succession of sands, clays,
and gravels underlying Glacial deposits; this material was probably
laid down near the mouth of the ancient Rhine, which in the latter
part of the Tertiary period flowed across a low area, which is now
occupied by the shallow southern half of the North Sea(27). The
plant-fragments found in these river-sediments indicate a temperate
climate. Among the plants of this pre-Glacial flora are many familiar
British species, such as _Caltha palustris_ (marsh marigold), species
of butter-cup, _Stellaria holostea_ (greater stichwort) (Fig. 3, C),
_Bidens tripartita_ (bur-marigold) (Fig. 3, A), maple, hawthorn, the
alder, hazel, the yew, Scots pine and numerous others. If, as is not
improbable, these pre-Glacial plants were swept away by the subsequent
arctic conditions, the great majority of them returned to their old
home when a warmer climate ensued. There are however some pre-Glacial
plants, such as the spruce fir (_Picea excelsa_), a cone of which
is shown in Fig. 3, B, the water chestnut, _Trapa natans_ (Fig. 4),
and a few other species no longer represented in the British flora.
The genus _Trapa_ is a striking example of a flowering plant which
has disappeared since the Tertiary period from many parts of Europe,
including England, most of Sweden, and from several regions in northern
Europe. It still grows in a few localities in Switzerland and in some
of the Italian lakes. In pre-Glacial times the water chestnut was
widely spread from Portugal and England in the west to Siberia in the
east, and its hard four-pronged nuts have been recorded from many
post-Glacial peat-moors in the north of Europe.
[Illustration: Fig. 4. _Trapa natans_ Linn. (nat. size). From
Mundesley. (Photographs by Mr and Mrs Reid.)]
From the so-called Cromer forest-bed and associated deposits on the
Norfolk coast several pre-Glacial plants have been obtained, indicating
a temperate climate during this phase of the Pleistocene period. A few
arctic species, such as the dwarf birch and arctic willow obtained from
the deposits next above the Cromer forest-bed, herald the near approach
of glacial conditions.
It may be remarked in passing that no satisfactory evidence has been
discovered in Britain of the existence of man in this part of Europe in
pre-Glacial days: it is, however, believed that flints from Tertiary
strata on the continent show traces of human workmanship. As Sir
Edwin Ray Lankester said in 1905, 'It is not improbable that it was in
the remote period known as the Lower Miocene--remote as compared with
the gravels in which Eoliths [primitive stone implements] occur--that
Natural Selection began to favour that increase in the size of the
brain of a large and not very powerful semi-erect ape'(28).
Though comparatively recent in terms of geological chronology, the
remoteness, according to ordinary conceptions of time, of the Tertiary
period is brought home to us when we endeavour to grasp the fact
that it was during this chapter in the earth's history that some of
our highest mountain-ranges, such as the Alps, the Carpathians, and
Himalayas were formed by the uplifting of piles of marine sediments.
From Tertiary strata in the Isle of Wight, on the Hampshire coast, and
in the London basin numerous fossil plants have been obtained, which
afford convincing evidence of climatic conditions much more genial than
those of the present day. The presence of palm leaves and of a wealth
of other sub-tropical plants in Lower Tertiary beds in England reveals
the existence of a flora differing considerably both from that in the
uppermost Tertiary beds of Norfolk and from the modern British flora,
but closely allied to the present Mediterranean flora.
The basaltic columns of the Giants' Causeway and of the Staffa Cave,
and the terraced rocks which form so characteristic a feature in the
contours of the Inner Hebrides, are portions of lava-flows, which in
the early days of the Tertiary period were poured out over a wide area
of land stretching from the north-east of Ireland, through the Western
isles of Scotland, the Faroë islands, to Iceland and Greenland. While
in this northern region volcanic activity was being manifested on a
stupendous scale, a shallow sea extended over part of what is now the
south-east of England in which was deposited a considerable thickness
of sedimentary material derived from the neighbouring land. In this
upraised sea-floor, known as the London clay, which is exposed in the
Isle of Sheppey and in many other localities, numerous fossil fruits
and fragments of wood occur in association with marine shells. The
fact that many of the fruits were ripe at the time of their entombment
led some eighteenth century writers to assign an autumn date to the
universal deluge. One of the Sheppey fruits may be mentioned as an
especially interesting sample of the early Tertiary flora, namely
the genus Nipadites, so named from the very close resemblance of the
fossils to the fruits of the existing tropical plant Nipa. _Nipa
fruticans_, sometimes described as a stemless palm because of the
absence of the erect stem which is usually a characteristic feature of
palms, grows in brackish estuaries of many tropical countries (Fig.
5, A): it has long leaves not unlike those of the date-palm and bears
clusters of fruits as large as a man's head; a single fruit is two or
three inches long and its hard fibrous shell is characterised by four
or five longitudinal ribs (Fig. 5, B). The fruits of Nipa, which may be
carried a considerable distance by ocean-currents without losing the
power of germination, are constantly found with other vegetable drift
on the beaches of tropical islands. The discovery of fruits of Nipa (or
Nipadites), hardly distinguishable from those of the existing species,
in Tertiary beds in England, Belgium, in the Paris basin, and in Egypt
affords a striking instance of changes in the geographical distribution
of an ancient plant now restricted to warmer regions.
[Illustration: Fig. 5. _Nipa fruticans_, Thunb. A. On the coast of the
Malay Peninsula. (Photograph by Prof. Yapp.) B. Head of fruits (1/5
nat. size). From a specimen in the British Museum.]
While the higher members of the Cretaceous system, as seen in the chalk
cliffs and downs, represent the upraised calcareous accumulations on
the floor of a fairly deep and clear sea, the lower members testify to
shallower water within reach of river-borne sand and mud. 'During the
Chalk period,' as Huxley wrote, 'not one of the present great physical
features of the globe was in existence. Our great mountain ranges,
Pyrenees, Alps, Himalayas, Andes, have all been upheaved since the
chalk was deposited, and the Cretaceous sea flowed over the sites of
Sinai and Ararat'(29).
The Wealden strata, at the base of the Cretaceous system, as seen on
the Sussex coast, in parts of the Isle of Wight, in the Weald district
of Kent and neighbouring counties, point to the existence of a lake
over a portion of the south of England and of the English Channel. The
remains of a rich Wealden flora have been collected from these Wealden
sediments, notably from the plant-beds of Ecclesbourne near Hastings,
in which, so far as we know, flowering plants played no part or at most
occupied a very subordinate position. A few fossil leaves have been
described from rocks assigned to a Wealden age,--and from the older
Stonesfield Slate, of Jurassic age, a single leaf is recorded,--which
seem to be those of Dicotyledons; but it is certain that even in the
early days of the Cretaceous period the present dominant group in
the plant kingdom was in its infancy and in many regions probably
unrepresented. When we glance at the geological table and consider
that in all the floras from the Wealden down to the Devonian period,
flowering plants played no part, we are able to appreciate the fact of
their rapid development, referred to in a previous chapter, when once
this highest type had become established.
The rocks comprised in the Jurassic system extend from East Yorkshire
to the coast of Dorsetshire; they consist of a succession of
limestones, clays, sandstones, and a few thin beds of impure coal.
Sediments of this age also occur, though to a much less extent, on
the north-east coast of Scotland and in a few places in the Inner
Hebrides. Many of the Jurassic strata contain only marine shells,
and corals are occasionally abundant, though in the lower members of
the system in the cliffs near Lyme Regis and at Whitby fossil plants
are fairly common. It is, however, from the middle Jurassic beds,
in the cliffs between Whitby and Scarborough, and in some inland
quarries in East Yorkshire, that we have obtained the richest Jurassic
flora. Rivers from a northern land laden with sediment and carrying
drift-wood, leaves and other plant fragments, deposited their burden in
an estuary which occupied the eastern edge of Yorkshire. Sedimentary
rocks laid down towards the close of the Jurassic period in the island
of Portland in the south and on the Sutherland coast in the north have
furnished valuable records of plant-life.
The passage from the Jurassic to the underlying Triassic system is
formed by some shales and limestones in South Wales containing remains
of fish and other marine organisms. These so-called Rhaetic beds are
poorly represented in the British area, but on the continent of Europe
and in other regions the sediments of this age bulk much more largely
and have yielded a rich collection of plants. The rocks of the upper
division of the Triassic system, as seen in the Midlands, point to
the prevalence of desert conditions; and in the grooved sand-polished
surfaces of granite in Charnwood forest we have a glimpse of a
Triassic landscape. The salt-bearing strata of this period in Cheshire
and Worcestershire suggest conditions paralleled at the present day
in the Caspian and Dead-Sea regions. The vegetation of Britain, and
indeed of the world as a whole, seems to have undergone but little
change during the enormous lapse of time represented by the sediments
comprised between the Wealden and Triassic periods. The Lower Triassic
flora affords evidence of a change in the facies of the vegetation and
prepares us for the still greater differences revealed by a study of
the Permian and Carboniferous floras. To the student of evolution these
Palaeozoic floras are of special interest on account of the facts they
have contributed in regard to the descent and inter-relationship of
different branches of the vegetable kingdom.
It is by a patient study of the waifs and strays of the vegetation
of successive phases of the world's history preserved in sedimentary
strata, that it has been possible to follow the history of many
existing plants and to establish links between the present and the
past.
CHAPTER IV
PRESERVATION OF PLANTS AS FOSSILS
'Some whim of Nature locked them fast in stone for us afterthoughts
of Creation.'
Lowell.
The failure of the earlier naturalists to grasp the true significance
of fossils or even to appreciate their nature is an extraordinary fact
when we consider the pioneer work which they accomplished in biological
and geological science. The following extract from the writings of so
enlightened a man as John Ray serves to illustrate an almost incredible
disinclination to admit what seems to us the obvious. He wrote:--'Yet
I must not dissemble that there is a Phenomenon in Nature, which doth
somewhat puzzle us to reconcile with the prudence observable in all its
work, and seems strongly to prove, that Nature doth sometimes _ludere_,
and delineates figures, for no other end, but for the ornament of some
stone, and to entertain or gratify our curiosity, and exercise our
wits. This is, those elegant impressions of leaves and plants upon
cole-slate, the knowledge of which, I must confess myself to leave to
my learned and ingenious friend Mr Edward Lhwyd of Oxford.... He told
me that Mr Woodward, a Londoner, shewed him very good draughts of the
common female fern, naturally formed in cole.... But these figures are
more diligently to be observed and considered... Dr Woodward will have
them to be the impressions of the leaves of plants which were there
lodged at the time of the Deluge'(31).
The Mr Woodward alluded to by Ray thus expressed his views on fossils
in an _Essay towards the Natural History of the Earth_:--'The whole
terrestrial globe was taken all to pieces and dissolved at the Deluge,
the particles of stone, marble, and all solid fossils dissevered, taken
up into the water, and then sustained together with sea shells and
other animal and vegetable bodies; the present earth consists and was
formed out of that promiscuous mass of sand, earth, shells, and the
rest falling down again, and subsiding from the water'(32).
In the later part of the seventeenth-century Steno, a Dane by birth and
Professor of Anatomy at Padua, by his recognition of the identity of
the teeth in a shark's head, which he had dissected, with some fossils
from Malta known as Glossopetrae, established the true nature of
fossils. He also recognised a certain orderly sequence in fossiliferous
strata, and in the opinion of Professor Sollas he is entitled to be
considered the 'Father and Founder' of Geology(33).
It was by slow degrees that the early observers freed themselves from
the obsession that the remains of animals and plants in the earth's
crust bear witness to a Universal Deluge and are all identical with
existing species. The possibility that some of the fossil plants in
English strata might be more clearly related to forms now met with in
warmer regions was gradually realised. The publication of the _Origin
of Species_ stimulated palaeontological research, and botanists as well
as zoologists turned to the investigation of extinct genera in search
of proofs of the doctrine of evolution.
The common occurrence of petrified wood in rocks of different ages
is well known. Fossil stems are occasionally found in their natural
position of growth, the structural details being rendered permanent by
the deposition of siliceous or calcareous material from water drawn by
capillarity into the dead but still sound tissues. Petrified wood from
Upper Jurassic beds is abundant in the Island of Purbeck; an unusually
long piece of stem may be seen in the small town of Portland fixed
to the wall of a house. Some of these stems have been referred by an
American author to the Araucarian family of Conifers, but the structure
is as a rule hardly well enough preserved to afford satisfactory
evidence for identification. In his _Testimony of the Rocks_, Hugh
Miller speaks of fossil wood from the upper beds of the Jurassic system
in sufficient abundance on the beach at Helmsdale in Sutherlandshire to
be collected in cart-loads; it is still easy to pick up good specimens
on the shingle beach a short distance north of Helmsdale, and a recent
microscopical examination showed that some specimens are pieces of an
Araucarian tree.
Impressive examples of petrified trees on a large scale are to be
seen in the United States, in Arizona and the Yellowstone Park.
(Frontispiece.) In the northern part of Arizona the country for over
an area of 10 square miles is covered with tree trunks, some reaching
200 feet in length and a diameter of 10 feet. The nature of the
mineralising substance has given rise to the name Chalcedony Park for
this Triassic forest(34). A striking example of one of the Arizona
trees is exhibited in the British Museum and in a neighbouring case is
a splendid petrified stem, 9 ft. in height, of a conifer discovered in
Tertiary lavas in Tasmania(35).
[Illustration: Fig. 6. Section of the north face of Amethyst Mountain,
Yellowstone Park, including upwards of 2000 ft. of strata. The
steepness of the slope is exaggerated. (After W. H. Holmes.)]
Figure 6 illustrates the preservation of a series of forests of
Tertiary age in the mass of volcanic sediments, 2000 feet in thickness,
known as Amethyst mountain, in the Yellowstone Park district. By the
weathering away of the surrounding volcanic material the tall stems of
the trees are exposed in places on the mountain sides like the 'columns
of a ruined temple.' The height of the river at the foot of the cliff
is 6700 ft. above sea-level and the mountain rises to a height of
9400 ft. above the sea. In the lower part of the section the volcanic
strata are seen to rest on a foundation of older rocks _A_, and these
in turn were laid down on the eroded surface of a still more ancient
foundation, _B_(36).
The section as a whole affords a striking demonstration of the
magnitude of earth-movements since the last of these forests was
buried below the surface of a sea in which the volcanic material
was deposited. The account of the Yellowstone Park section recalls
Darwin's description in the _Naturalist's Voyage_(37) of snow-white
columns projecting from a bare slope, at an altitude of 7000 ft. in the
Cordillera.
The abundance of drift-wood on the coasts of some countries at the
present day helps us to picture the conditions under which the remains
of former forests have been preserved. In his _Letters from High
Latitudes_, Lord Dufferin gives the following description of drift-wood
on the shores of Spitzbergen:--'A little to the northward, I observed,
lying on the sea-shore innumerable logs of drift-wood. This wood is
floated all the way from America by the Gulf Stream, and as I walked
from one huge bole to another, I could not help wondering in what
primeval forest each had grown, what chance had originally cast them on
the waters, and piloted them to this desert shore'(38). A photograph
reproduced in Amundsen's book on _The North West Passage_ shows the
beach on the Alaskan coast strewn with drifted timber(39). For the
accompanying photograph (Fig. 7) of the flood-plain of the Colorado
River(40), I am indebted to Professor MacDougal of the Desert Research
Laboratory at Tucson, Arizona, who in a recent letter writes, 'During
times of high-water a thin sheet of flood covers the flat for many
miles and bears drift-wood so thickly that it is difficult to push a
boat through it.' The drift-wood consists of poplar, willow, pine, and
juniper, 'the last two have been brought from the upper river, from
as far away as a thousand miles.' A picture such as this affords an
admirable example of the wealth of material available for preservation
in a fossil state.
It is only in the minority of cases that the accidents of preservation
of fragments of ancient floras have given us the means of investigating
the internal structure of the plant organs. It is far more frequently
the case that fossil plants are represented only by a carbonised film
on the surface of a piece of shale or other rock: the actual substance
of the plant has been converted into a thin layer of coal, and though
the venation and other surface-features may be clearly revealed, the
internal tissues have been destroyed. If a lump of clay containing a
piece of fern frond is heated, the result is an impression of the leaf
on the hardened matrix and a coaly substance in place of the plant
substance. It is occasionally possible by detaching a piece of the
black film from a fossil, and heating it with nitric acid and chlorate
of potash and then dipping it in ammonia, to obtain a transparent
preparation suitable for microscopical examination of the cell-outlines
of the superficial layer of the leaf or other plant-fragment. This
method of examination, used by several students of fossil plants and
with conspicuous success by Professor Nathorst of Stockholm, often
affords valuable aids to identification.
[Illustration: Fig. 7. Flood-plain of the delta of Rio Colorado. The
hills in the background are 25 miles distant. (From a photograph by
Prof. MacDougal.)]
Pieces of plants embedded in sandy sediment, if not preserved by
petrifaction, that is by the introduction into the tissues of some
siliceous or calcareous solution, gradually decay and their fragmentary
remains may be washed away by percolating water, leaving a hollow mould
in the gradually hardening sediment, which is afterwards filled with
sand or other material. The plant itself is destroyed, but a cast is
taken which in the case of fine-grained sediments reproduces the form
and surface-pattern of the original specimen. The incrustation of
plants by the falsely named petrifying springs of Knaresborough and
other places illustrate another method of fossilisation.
[Illustration: Fig. 8. Flower of _Cinnamomum prototypum_ Conw.
preserved in amber. × 10. (After Conwentz.)]
Plants which owe their preservation to amber occur both as
incrustations and petrifactions. This fossil resin occurs in Tertiary,
Cretaceous, and Jurassic rocks; the amber found in abundance on the
Baltic coast near Danzig and occasionally picked up on the beach
in Norfolk and Suffolk comes from beds of Tertiary age. Pieces of
Pine-wood have been described from the Baltic beds in which the tissues
are perfectly preserved as the result of the conversion into amber
of the resinous secretion which permeates their cells: in this case
the amber is a petrifying agent. More frequently the preservation is
due to incrustation; as resin trickled down the stems of the Tertiary
pines from an open wound, flowers and leaves, blown by the wind on to
the sticky surface, were eventually sealed up in a translucent case
of amber. Though the actual substance may have gone, the mould which
remains exhibits in wonderful perfection each separate organ of a
flower or the delicate hair-clusters on the surface of a leaf. The
flower represented in Fig. 8, a species of Cinnamon, is one of several
specimens described by the authors of a monograph of Tertiary plants in
the Baltic amber(41).
The fragments of plants preserved in nodules of calcareous rock
occasionally met with in some of the Lancashire and Yorkshire
coal-seams are perhaps the most striking examples of the possibilities
of petrifaction. By cutting sections of these nodules and grinding them
to a transparent thinness, the most delicate tissues of Carboniferous
plants are rendered accessible to investigation under the high power of
a microscope. As our attention is absorbed by the examination of the
details of cell-structure it is easy to forget that the section has not
been cut from a living plant, but from the twig of a tree which grew in
the forests of the Coal age. The preservation is such as to enable us
not only to describe the anatomy of these extinct types of vegetation,
but, by the application of the knowledge of the relation between the
structure of the plant-machine and its functions gained by a study of
living species, it is possible in some degree to picture the plants
of the Coal period as living organisms and to see in the structural
framework a reflection of external environment. The recognition in the
general architectural plan of the Palaeozoic plants, as in many of the
finer anatomical features, of the closest resemblance to plants of
the modern world produces an almost overwhelming sense of continuity
between the past and the present.
The plants of the Palaeozoic period, though often differing
considerably from those of the same class in the floras of to-day,
exhibit a remarkably high type of organisation. Some of the most
abundant trees in the forest of the Coal age are decidedly superior in
the complexity of their structure, as also in size, to modern survivals
of the same stock. On the other hand, it must be remembered that
Monocotyledons and Dicotyledons which now occupy the highest place in
the hierarchy of plants have left no sign of their existence in any
of the Palaeozoic strata. The greater size of some of the Palaeozoic
plants, and in some respects the more advanced stage of evolution which
they represent as compared with their nearest relatives of the present
era, must be considered in relation to their more important and
relatively higher position in the plant-world than that which is now
held by their diminutive descendants. It is, however, impossible to get
away from the conclusion that the oldest Palaeozoic flora of which we
have an intimate knowledge must be the product of development of an age
which is represented by a chapter in the history of the plant kingdom
at least as far removed from the beginning as it is separated from the
chapter now being written. Examples might be quoted in illustration of
the risks attending the determination of fossils by means of external
features alone, but it may suffice to mention the case of a specimen
originally described as a fragment of a Cretaceous Dinosaur under the
name _Aachenosaurus multidens_. By the examination of thin sections
this supposed bone was shown to be a piece of Dicotyledonous wood(42).
The methods of preservation of plants as fossils are numerous and
varied and the few examples selected give but an incomplete idea of the
subject: for a fuller treatment of fossilisation the reader is referred
to more technical treatises (48 vol. I.).
The employment of fossil plants as 'Thermometers of the ages' is a
branch of Palaeobotany to which a passing allusion may be permitted
though it is only indirectly connected with the main question. As
one of the most interesting examples of changed climatic conditions
revealed by a study of fossil plants, reference may be made to the
wealth of material collected within the Arctic circle. The problems
suggested by the discovery of plants in rocks of various ages in
North Siberia, Spitzbergen, Franz Josef Land, Bear Island, Greenland,
and in many other localities in the far north are too difficult and
far-reaching to be discussed in these pages. In the Cretaceous and
Tertiary strata of the west coast of Greenland and Disco Island from
69° to 72° north latitude, to refer only to one case, a great number of
plants have been obtained by several of the earlier Arctic explorers
and more recently by members of one of the Peary Expeditions. At the
present day on the fringe of land on the western edge of Greenland
which is not permanently covered with ice, a considerable number of
herbaceous plants are able to exist and to produce seed during their
concentrated period of development; while trees are represented only
by a few low-growing shrubs such as the dwarf Juniper. In places
accessible to investigation beyond the ice-covered hills of northern
Greenland the rocks have been shown to consist of Cretaceous and
Tertiary sediments containing fossil plants associated with seams
of coal. From these beds numerous Dicotyledons have been obtained,
some of them almost identical with living species characteristic of
sub-tropical or tropical countries. In the lowest of the Cretaceous
series no Dicotyledons have been found, but flowering plants are
abundant in the higher Cretaceous rocks. Allowing for the fact that
closely allied species are often able to live under very different
climatic conditions, there can be no doubt that the Cretaceous
and Tertiary floras of Greenland indicate an average temperature
considerably higher than that which now prevails in the warmest parts
of the British Isles.
In the far south a fairly rich Jurassic flora has recently been
discovered by the members of a Swedish Antarctic expedition in Graham's
Land in latitude 63°·15 S. and longitude 57° W., which in its general
facies bears a close resemblance to the Jurassic flora of Yorkshire.
Although the great majority of the records of ancient plants are
difficult to interpret by reason of imperfect preservation and because
of the frequent separation of leaves, stems, and reproductive organs,
the student who tries to piece together the _disjecta membra_ of the
floras of the past shares the opinion expressed by the late Marquis of
Saporta,--'Si l'on s'attache à les déchiffrer, on oublie bien vite la
singularité des caractères, et le mauvais état des pages. La pensée se
lève, les ideés se développent, le manuscrit se déroule; c'est la tombe
qui parle et livre son secret.'
CHAPTER V
FERNS; THEIR DISTRIBUTION AND ANTIQUITY
'It has been shown that certain forms persist with very little
change, from the oldest to the newest fossiliferous formations; and
thus show that progressive development is a contingent, and not
a necessary, result of the nature of living matter.'
Huxley.
The Ferns as a whole represent a section of the vegetable kingdom
which traces its ancestry as far into the past as any group of plants.
Impressions of leaves on the shales of the Coal-measures and on rocks
of the earlier Devonian period are hardly distinguishable in form and
in the venation and shape of the leaflets from the finely divided
fronds of modern ferns. Until a few years ago these Palaeozoic fossils
were generally regarded as true ferns, and it was believed that ferns
played a conspicuous part in the vegetation of the earliest periods, of
which we have any botanical knowledge. Conclusions based on external
form must frequently be revised in the light of more trustworthy
evidence. It was shown in the later part of the nineteenth century by
the late Professor Williamson of Manchester, whose researches into
the plants of the Coal age shed a flood of light on the ancestry
and inter-relationship of many existing plants, that some of the
fern-like leaves which have long been familiar to those who search
among the shales of the refuse heaps of collieries, were borne on
stems differing in anatomical features from those of any known fern.
The investigation of the structure of the leaves and their supporting
stems led to the recognition of certain extinct genera of Palaeozoic
plants of exceptional interest, to which the term generalised type is
aptly applied. Associated with anatomical and other characters such
as we now regard as the attributes of ferns, these plants exhibit
other features not met with in modern ferns but characteristic of a
group of seed-bearing plants known as the Cycads. Recent research has
revealed the existence of several such generalised types which, by
their combination of characters now met with in distinct sub-divisions
of the plant-kingdom, clearly indicate the derivation of Ferns, and
Cycads as we know them to-day, from a common stock. It was in the
first instance by means of anatomical evidence--obtained by the
microscopical examination of sections of petrified fragments of stems
and leaves--that the generalised nature of these Palaeozoic plants was
recognised. Nothing was known as to the reproductive organs. Ferns as
now represented in the floras of the world are essentially seed-less
plants. As the author of _Hudibras_ wrote:
'Who would believe what strange bugbears
Mankind creates itself, of fears?
That spring like fern, that insect weed,
Equivocally, without seed.'
The reproductive organs or spores borne on the fronds of a fern
produce, on germination, a thin green structure, known as the
prothallus, less than an inch in length: this bears the sexual organs,
and as the result of the union of the male and female cells, the embryo
fern-plant begins its existence as a parasite on the inconspicuous
prothallus, until after unfolding its first green leaf and thrusting a
slender root into the ground, it starts its career as an independent
organism[2]. In this life-cycle the seed plays no part.
[2] The life-history of a Fern is clearly described by Prof. Bower in a
recent volume in this series.
It is noteworthy that the absence of any indication of spore-capsules
and spores, in the case of some of the supposed fern leaves from
the Coal-measures, caused some suspicion in the mind of an Austrian
Palaeobotanist as to the right of such specimens to be classed among
the ferns. This opinion, based in the first place on negative evidence
and but little regarded by other authors, has in recent years been
proved correct. In 1904 a paper was read before the Royal Society
by Professor Oliver and Dr Scott(43) in which evidence was brought
forward pointing to the conclusion that one of these generalised plants
bore true seeds. Subsequently Dr Kidston published an account of some
specimens of another of these Palaeozoic plants in which was actually
shown an organic connexion between undoubted seeds and pieces of a
fern-like frond(44). Without entering into further details, these and
similar discoveries may be summarised as follows:--Many of the supposed
Fern-fronds of Palaeozoic age, particularly those characteristic of
the Coal-measures, are the leaves of plants which in their anatomical
characters combined features now shared by true Ferns and by the
Cycads. The reproductive organs of these Palaeozoic genera differed
widely from those of existing ferns; the male organs, while not unlike
the spore-capsules and spores of certain ferns, recall the male organs
of living Conifers and Cycads, and the female organs were represented
by seeds of a highly complex form. These seed-bearing plants have
been called Pteridosperms, a name which expresses the combination
of fern-like features with one of the distinguishing attributes of
the higher plants, namely the possession of seeds. The ancestors of
Pteridosperms are as yet unknown; it is, however, reasonable to assume
that there existed in some pre-Carboniferous epoch a group of simple
plants from which both Ferns and Pteridosperms were derived. In the
forests of the Coal age true Ferns probably occupied a subordinate
position in relation to the Pteridosperms.
The question of the relationship between different families of recent
ferns and the older known fossil members of the group is beyond the
scope of this book. Evidence has been discovered in recent years which
warrants the statement that, although none of those Carboniferous ferns
were generically identical with existing forms, they very clearly
foreshadowed some of those structural features which characterise more
than one family of present-day Ferns. The records of the older Mesozoic
formations afford abundant evidence of the existence of certain types
of Ferns showing a very close resemblance to recent species.
[Illustration: Fig. 9. _Osmunda regalis_ Linn. Fertile frond, (2/5 nat.
size.)]
An enquiry into the geographical distribution of living Ferns reveals
facts of special interest in connexion with the relative antiquity of
different genera and families. The wide distribution of the Bracken
fern has already been referred to: it is abundant in Tasmania; its
vigour in the island is well illustrated by Mr Geoffrey Smith's
statement that constant attention is necessary to keep it from invading
newly opened country(45). On Mount Ophir in the Malay Peninsula the
cosmopolitan bracken occurs in association with the two genera Matonia
and Dipteris, ferns which are among the most striking examples of links
with a remote past and have a restricted geographical range. With
_Osmunda regalis_, the Royal Fern, the Bracken is conspicuous in the
marsh vegetation of the Bermudas; it flourishes on the Atlas Mountains,
in the Canary Islands, in Abyssinia, on Mt Kenia, in British East
Africa, in the Himalayas, and is in fact generally distributed in the
tropics in both the north and south temperate zones.
The Royal Fern (Fig. 9) is another British species with a wide
distribution; it occurs in Northern Asia and in North America; it is
common in the Siberian forests and lives in several tropical countries,
extending to Southern India and Cape Colony, and in South America it
is represented by a closely allied species. Though at the present day
_Osmunda regalis_ is one of the rare English Ferns, its occurrence in
the submerged forest-beds round our coasts and in pre-Glacial beds
points to its former abundance in the British area generally. The Royal
Fern is a member of a family now represented by two genera, Osmunda and
Todea.
With the exception of _Todea barbara_, with its large spreading
fronds and a short root-covered stem, which occurs in Australia and
Cape Colony, all the species of this genus are filmy ferns with
semitransparent fronds adapted to a moisture-laden atmosphere. The
maximum development of the genus is in New Zealand.
_Todea barbara_ affords an instance of discontinuous distribution; it
was no doubt once widely spread in circumpolar regions and now survives
only in South Africa and in Australia.
There are satisfactory reasons for regarding the Bracken Fern, with
its world-wide range in present-day floras, as a comparatively modern
species now in full vigour. Its anatomical and other features are
consistent with the view that it is a late product of evolution, and
as yet no indication has been given by the records of the rocks of an
ancient lineage. The Osmunda family, on the other hand, is undoubtedly
an extremely old branch of the fern group. A comparison of the Royal
Fern with the Bracken shows that their stems are constructed on
very different plans, and we have good reasons for speaking of the
structural peculiarities of the former as those of a more primitive
type. Moreover, the discontinuous geographical range of some members of
the Osmunda family is in itself an indication of antiquity. There is
another point which may have a bearing in this question of antiquity,
namely the fact that the spores of Osmunda are green and do not possess
the powers of indurance inherent in the spores of the majority of
ferns which are not green. It has recently been contended by Professor
Campbell of Stanford University that the delicate green spores of the
Liverworts, plants closely allied to the Mosses, constitute an argument
in favour of the antiquity of these plants(46). Certain Liverworts are
cosmopolitan in their range, _e.g._ the genera Riccia and Marchantia.
If certain genera are widely distributed, notwithstanding the fact
that their reproductive cells, by which dispersal is effected, are
ill-adapted to withstand unfavourable conditions or to endure prolonged
desiccation, it would seem reasonable to conclude that their emigration
has been accomplished slowly and with difficulty. Ferns such as
Osmunda, with green and short-lived spores, would thus be handicapped
in competition with other genera provided with more efficient means of
dispersal and better equipped for the vicissitudes of travel.
The inferences as to antiquity deduced from a study of the existing
species of Osmunda and Todea receive striking confirmation from the
testimony of fossils. Some of the oldest known Palaeozoic ferns,
though differing too widely from the existing Osmundas and Todeas
to be included in the same family, afford distinct glimmerings of
Osmundaceous characters, which at a later period became individualised
in the direct ancestors of the modern forms. Our knowledge of the
past history of the Osmunda family has recently been considerably
extended and placed on a firmer basis by the researches of Dr Kidston
and Professor Gwynne-Vaughan. These authors have recognised in some
exceptionally well-preserved fern-stems from Permian rocks in Russia,
anatomical features which point unmistakably to close relationship with
the recent members of the family(47)(48).
Passing higher up the geological series, fertile fern fronds with
spore-capsules and spores practically identical with those of Osmunda
have been found in the Jurassic plant-beds of Yorkshire and in rocks of
approximately the same age in many parts of the world. From Jurassic
strata in New Zealand a petrified fern-stem has been described
(_Osmundites Dunlopi_), almost identical in structure with the
surviving species. Cretaceous and Tertiary examples of similar ferns
might be quoted; but enough has been said to establish the claim of the
Royal Fern and other members of the Osmunda-family to an ancestry which
possibly extends even farther back than that of any other existing
family of Ferns.
[Illustration: Fig. 10. _Gleichenia dicarpa_ Br. (1/2 nat. size.)]
A brief reference may be made to another fern now represented by
several species widely disseminated in tropical and sub-tropical
countries. The genus Gleichenia occurs abundantly in the warmer regions
of both the Old and New World. The fronds may usually be recognised
by their habit of growth (Fig. 10); in several species the main axis
is repeatedly forked and a small bud between the divergent branches
of the forks forms a characteristic feature. The leaflets are either
long and narrow like the teeth of a comb or short and bluntly rounded.
Moreover the anatomy of the creeping stem affords a ready means of
identification. We have satisfactory evidence of the occurrence of
Gleichenia in European floras during both the Jurassic and Cretaceous
periods. Numerous fragments of plants were obtained some years ago,
not far from Brussels, from the Wealden strata in which the famous
skeletons of Iguanodon were discovered. Visitors to the Natural
History Museum in Brussels are no doubt familiar with the skeletons of
this enormous herbivorous animal: in the same gallery are exhibited
the remains of the fossil plants from the Iguanodon beds. Some of
these fragments are pieces of fern fronds identical in form with
those of existing Gleichenias. The microscopical examination of some
exceptionally well preserved fragments of Wealden stems discovered by
Prof. Bommer of Brussels enabled him to recognise the Gleichenia type
of structure and thus to confirm the inconclusive evidence furnished
by fragmentary leaves. The most interesting records in regard to the
former occurrence of Gleichenia in Northern Europe we owe to the late
Oswald Heer of Zurich, who has described many examples of Gleichenia
fronds from rocks of Lower Cretaceous age in Disco Island on the west
coast of Greenland in latitude 70° N. The same type of fern is recorded
also from upper Jurassic beds in the north-east of Scotland, in the
Wealden rocks of Sussex, as well as from other European localities. It
is clear that the Gleichenia-family, no longer represented in north
temperate floras, was in the Jurassic period, and especially in the
early days of the Cretaceous period, widely spread in Europe, extending
well within the Arctic circle. It may be that the original home of
Gleichenia was in the far North at a time when climatic conditions were
very different from those which now prevail. Gleichenia, like many
other northern plants, retreated to more southern regions where, in
the warmer countries of the world, many species still flourish widely
separated in space and time from the place of their birth.
The ferns so far mentioned have a more or less extended distribution
at the present day. In the case of _Pteridium aquilinum_, the
cosmopolitan Bracken Fern, wide range would seem to be correlated
with comparatively recent origin; on the other hand, the facts of
palaeobotany show that the wide distribution of Osmunda, a type of fern
which differs in many important respects from members of the family
(Polypodiaceae) to which the Bracken belongs, is not inconsistent
with an exceptionally ancient family-history. There are, however,
certain genera of ferns which afford remarkable examples of restricted
geographical distribution associated with great antiquity. The island
of Juan Fernandez, 420 miles off the coast of Chili, the home for
four years of Alexander Selkirk (to whose adventures we owe Defoe's
creation of Robinson Crusoe), is interesting also from a botanical
point of view. The vegetation of this oceanic island, 20 square miles
in area with basaltic cliffs rising to a height of 3000 ft. above the
sea, includes more than 40 species of ferns, eight of which occur
nowhere else. One of these endemic ferns is _Thyrsopteris elegans_, the
only representative of the genus; it is readily distinguished by its
large and graceful fertile fronds, examples of which may occasionally
be seen on a plant of this species in the Royal Gardens at Kew: the
sporangia are produced in circular cups which replace the ordinary
leaflets on the lower branches of the frond and hang from the short
axis like miniature clusters of grapes. It is noteworthy that among
the fragmentary remains of the fern vegetation of the Jurassic flora
in England and in other parts of Europe specimens occur with fertile
segments practically identical with those of the Juan Fernandez
species. Students of fossil plants are occasionally led away by the
temptation to identify imperfect specimens with rare existing species
to which they exhibit a superficial resemblance, and this is well
illustrated by the frequent use of the generic name Thyrsopteris for
Jurassic and Lower Cretaceous ferns which are too imperfect to be
determined with any degree of certainty We have, however, satisfactory
grounds for the assertion that the Juan Fernandez fern affords a
striking confirmation of the truth of Darwin's dictum that 'Rarity,
as geology tells us, is the precursor to extinction.' In this remote
oceanic island, for reasons which we cannot explain, there lingers an
isolated type which belongs to another age.
The following passage, which forms a fitting introduction to an account
of two other genera of ancient ferns, is taken from a description of
an ascent of Mount Ophir in the Malay Peninsula by Dr A. R. Wallace
in his well-known book on the Malay Archipelago:--'After passing a
little tangled jungle and swampy thickets, we emerged into a fine
lofty forest.... We ascended steadily up a moderate slope for several
miles, having a deep ravine on the left. We then had a level plateau or
shoulder to cross, after which the ascent was steeper and the forest
denser till we came out upon the Padang-Batu, or stone-field.... We
found it to be a steep slope of even rock, extending along the mountain
side farther than we could see. Parts of it were quite bare, but where
it was cracked and fissured there grew a most luxuriant vegetation,
among which the pitcher plants were the most remarkable.... A few
coniferae of the genus Dacrydium here first appeared, and in the
thickets, just above the rocky surface, we walked through groves of
those splendid ferns, _Dipteris Horsfieldii_ and _Matonia pectinata_,
which bear large spreading fronds on slender stems, 6 or 8 feet
high'(49).
[Illustration: Fig. 11. _Matonia pectinata._ A group of plants in a
wood on Gunong Tundok, Mount Ophir. (Photograph by Mr A. G. Tansley.)]
The two genera Matonia and Dipteris afford exceptionally striking
examples of survivals from the past. Matonia is represented by two
species, _Matonia pectinata_ (Fig. 11), which grows abundantly on the
upper slopes of Padang-Batu in dense thickets on the rock faces where,
as Mr Tansley states, its associates are a species of Gleichenia,
Dipteris, and a little _Pteridium aquilinum_ (Bracken Fern). _Matonia
pectinata_ occurs also on Bornean mountains at an altitude of over
3000 ft. and descends to the coast on some of the Malay islands. The
other species of the genus, _Matonia sarmentosa_, has so far been
found in one locality only, Niak, Sarawak, where it was discovered by
Mr Charles Hose. _Matonia pectinata_ has a creeping stem covered with
a thick felt of brown hairs bearing tall fan-shaped fronds divided
into numerous comb-like branches thickly set with narrow linear
leaflets on which circular clusters of spore-capsules are sparsely
scattered. In some respects Matonia is unlike other ferns; the fronds
constitute a striking feature, and the anatomy of the stem is still
more distinctive. In the form, development, and arrangement of the
sporangia (spore-capsules)--organs which from the constancy of their
characters have long been recognised as the most useful basis for
classification--Matonia exhibits distinctive features.
In order to emphasise the isolated position of the genus it has
recently been placed in a separate family, the Matonineae, of which
it is the sole living representative. The restricted geographical
range of Matonia, considered in connexion with the clearly marked
peculiarities in structure and form, leads us to expect other evidence
in support of the natural inference that the genus is a survivor of
a once more vigorous and widely spread family. If Matonia were a
recently evolved type which has not spread far from its original home,
we should expect it to conform more closely than it does to other
ferns in the Malay region. Even assuming for the sake of argument
that variation may occur _per saltum_, and new forms may be produced
differing in more than the finer shades of small variation from their
parents, the peculiar features of Matonia are too pronounced and its
individual characteristics too obvious to warrant the assumption of
recent production. It is, however, from the testimony of the rocks that
we obtain confirmation of the opinion that these Malayan species are
plants on the verge of extinction. In shales of Jurassic age exposed
on the Yorkshire coast at Gristhorpe Bay and in iron-stained rocks of
the same age between Whitby and Scarborough, well preserved leaves have
been found agreeing in the shape of the frond, as also in the form of
the leaflets and of the groups of sporangia which they bare, with those
of _Matonia pectinata_.
The exposure by a stroke of the hammer, on the fractured surface of
a rock picked up on the beach at Hayburn Wyke (a few miles south of
Whitby), of a piece of fern frond which is unmistakably closely allied
to the species described by Wallace on Mount Ophir, establishes a link
between the Jurassic and the present era and presents a fascinating
problem in geographical distribution. These fossil Matonias are known
to students of ancient plants as species of the genus Matonidium, a
name adopted by a German botanist for specimens apparently identical
with those from the Yorkshire coast discovered in slightly younger
rocks (Wealden) in North Germany. The same type has been found also in
sediments of Wealden age on the Sussex coast. Other leaf-impressions
agreeing closely with those of Matonidium have been obtained from
the Yorkshire Jurassic rocks and these are assigned to another genus
Laccopteris, an extinct member of the family Matonineae. It is not
merely in the habit of the fronds and in the shape and venation of the
leaflets that these fossil ferns resemble the existing species, but
the more important features exhibited by the spore-capsules supply
additional evidence. It has already been pointed out that the stems
of Matonia are characterised by a type of structure unknown in an
identical form in any other recent fern.
A few years ago Prof. Bommer discovered fragments of leaves and
stems in Wealden beds a few miles from Brussels sufficiently well
preserved to reveal the details of internal organisation. Some of
these fossils were found to possess structural features identical
with those of the Malayan species of Matonia. A full account of the
fossil representatives of the Matonia family would be out of place in
a general essay on Links with the Past, but brief reference may be
made to some of the data which throw light on the geological history
of the family. In strata classed by geologists as Rhaetic, a phase
of earth-history between the Triassic and Jurassic eras (see p. 42),
species of Laccopteris and allied forms have been described from
several other countries; from Jurassic and Wealden strata examples of
both Laccopteris and Matonia have been found in Germany, Portugal,
Belgium, Austria, and elsewhere. From rocks of Cretaceous age, higher
in the series than the Wealden strata, well preserved impressions of a
Matonidium have been discovered in Moravia. The Matonineae were widely
distributed in Europe during the Rhaetic and Jurassic periods, but, so
far as we know, the family did not survive in the northern hemisphere
beyond the limits of the Cretaceous period. It is noteworthy that, in
spite of the preservation of the remains of Jurassic and Cretaceous
floras in many extra-European regions, notably in India, South Africa,
Australia, China, and Tonkin, no specimens have been found which can
with confidence be assigned to the Matonineae. A single fossil has,
however, been described from Queensland which may be a piece of a
Laccopteris frond.
There is some evidence that ferns very similar to Matonia existed
in North America during the Mesozoic period. It would be in the
highest degree rash to assume that the Matonineae played no part in
the Jurassic vegetation of India, South Africa, and other southern
lands, but there can be little doubt that the family was especially
characteristic of European floras during a portion of the Mesozoic era.
It would seem that subsequent to the Wealden period the ancestors of
Matonia dwindled in numbers and their geographical range became much
more restricted.
The records of Tertiary rocks have hitherto added nothing to our
knowledge of the distribution of the family subsequent to the
Cretaceous period. All we can say is that the existing species of
Matonia are the last survivors of a family which in the Jurassic period
overspread a wide area in Europe and probably extended to the other
side of the Atlantic. Exposed to unfavourable climatic conditions and
possibly affected by the revolution in the plant world consequent
on the appearance of the Flowering Plants, the Matonineae gradually
retreated beyond the equator until the two surviving species found a
last retreat in the Malayan region.
[Illustration: Fig. 12. _Dipteris conjugata_ Rein. and, in the middle
of the upper part of the photograph, a frond of _Matonia pectinata_ R.
Brown. Mount Ophir. (Photograph by Mr A. G. Tansley.)]
The fern spoken of by Dr Wallace as _Dipteris Horsfieldii_ (perhaps
better known as _Dipteris conjugata_ (Fig. 12)), which grows with
_Matonia pectinata_ on Mount Ophir and in the Malay region generally,
is one of seven species of a genus characterised by a somewhat wider
geographical range than Matonia. _Dipteris conjugata_ extends to the
Philippines, Samoa, Fiji, New Caledonia, New Guinea and Central China;
its fronds, like those of Matonia, are borne on long slender stalks
attached to a creeping stem; they have a broad lamina divided by a deep
median sinus into two symmetrical halves and each half is cut up into
segments with a saw-like edge. Several stout ribs spread through the
lamina from the apex of the long stalk like the open fingers of a hand;
from these ribs smaller veins are given off at a wide angle, and these
in turn give rise to a reticulum of finer veins forming a skeletal
system like that in the leaves of an oak and many other flowering
plants.
Numerous groups of spore-capsules are borne on the lower surface of
the broad lobed frond. The leaves of other species of Dipteris have
the same type of structure, but in some the segmentation of the lamina
is carried further and the leaf consists of numerous long and narrow
segments with one or two main ribs. Dipteris is represented in the
flora of Assam, and it is interesting to find that a species recently
discovered in Borneo is more closely connected with the Assam type
than with those of the Malay region. Until a few years ago the genus
Dipteris was included in the large family Polypodiaceae of which
nearly all our British ferns are members, but the discovery of certain
distinguishing features in the structure of the sporangia showed that
these Eastern and Southern species form a fairly well-defined group
worthy of family rank.
In the Rhaetic plant-beds of Northern and Central Europe, of North
America, Tonkin, and elsewhere, numerous fossil leaves have been
discovered which in shape, venation, and in the manner of occurrence of
the sporangia bear a close resemblance to species of Dipteris. Ferns
of this type were abundant in the Jurassic floras of the northern
hemisphere, and it is interesting to find impressions of Dipteris-like
leaves both in the Jurassic rocks of the Yorkshire coast as well as in
slightly newer beds of the same geological period on the north-east
coast of Sutherland.
It is impossible to say with confidence how nearly these Rhaetic and
Jurassic ferns were related to the existing species, as our knowledge
of them is less complete than in the case of the fossil representatives
of the Matonineae, but there can be no reasonable doubt that in
Dipteris as in Matonia we have a connecting link between the present
and a remote past.
CHAPTER VI
THE REDWOOD AND MAMMOTH TREES OF CALIFORNIA
'Your sense is sealed, or you should hear them tell
The tale of their dim life, with all
Its compost of experience....'
W. E. Henley.
Since their introduction into England about the middle of the
nineteenth century, the two Californian species _Sequoia sempervirens_
(the Redwood) and _Sequoia gigantea_ (the Mammoth tree) have become
familiar as cultivated trees. The name Sequoia, said to be taken
from Sequoiah, the inventor of the Cherokee alphabet, was instituted
in 1847, while the name Wellingtonia, often used in horticulture
though discarded by botanists in favour of the older designation
Sequoia, was proposed in 1853. Both species are now confined to a
comparatively small area in California: their restricted geographical
range, considered as an isolated fact, might be regarded as a sign of
recent origin. The records of the rocks, however, afford ample proof
that rarity in this as in many other instances is the precursor of
extinction. The famous groves of Mariposa and Calaveras represent the
last resting-place of giant survivors of a race which formerly held its
own in Europe and in other parts of the world.
The Redwood, _Sequoia sempervirens_, occupies a narrow belt of country,
rarely more than 20 or 30 miles from the coast, three hundred miles
long from Monterey in the south to the frontiers of Oregon; it has
a stronger hold on existence than Sequoia gigantea. In Northern
California it still forms pure forests on the sides of ravines and on
the banks of streams. The tapering trunk, rising from a broad base to
over 300 ft., gives off short horizontal branches thickly set with
narrow spirally disposed leaves 1/4-1/2 inch in length arranged in
two ranks like the similar leaves of the Yew. The lower edge of each
leaf is decurrent, that is it runs a short distance down the axis of
the branch instead of terminating at the point of attachment. It is by
paying attention to such details as this as well as to more important
features, that we are able to connect fragmentary fossil twigs with
those of existing species. The female 'flowers' have the form of oblong
cones from 3/4 to 1 inch long: each consists of a central axis bearing
crowded wedge-shaped, woody appendages or cone-scales, which gradually
increase in breadth towards the exposed distal end characterised by its
four sloping sides and by a median transverse groove. Several small
seeds are borne on the upper surface of the cone-scales. The smaller
and short-lived male flowers need not be described.
[Illustration: Fig. 13, _Sequoia gigantea._ King's Co., California.
(From Prof. D. H. Campbell)]
The other and better known species _Sequoia gigantea_ (Fig. 13) has an
even more restricted range and is confined to groves on the western
slopes of the Sierra Nevada between 3000 to 9000 ft. above sea-level.
This tree is at once distinguished from the Redwood by its ovate,
sharply pointed and stiffer leaves which retain their spiral
disposition and closely surround the axis of the twigs like
obliquely-set needles. The cones are of the same type as those of
_Sequoia sempervirens_, but are broader and may attain a length of
3-1/2 inches (9·5 cm.) (Fig. 14).
Reference has already been made to Sequoia as a striking illustration
of longevity. It is also selected as an equally impressive example of
a type verging on extinction, which played a prominent part in the
vegetation of both west and east during the Cretaceous and Tertiary
periods.
[Illustration: Fig. 14. _Sequoia gigantea_ Torr. (7/8 nat. size.)]
Scraps of branches with leaves hardly distinguishable from those of
the existing Californian trees are frequently met with in Tertiary
and Mesozoic sediments, and with them occasionally occur cones too
imperfectly preserved to afford satisfactory evidence of more than
superficial agreement with those of the recent species. The task of
deciphering the past history of plants, particularly of the Conifers,
is accompanied by many difficulties and insidious temptations. It is
clear from a critical examination of many of the recorded instances
of fossil Sequoias that the generic name has been frequently used
by writers without adequate grounds. The fragmentary specimens
available to the botanical historian cannot as a rule be subjected
to microscopical investigation, and even a partial acquaintance with
the similarity of the foliage of different types of living Conifers
is sufficient to convince the student of the need of self-control in
the identification of the fossils. It is, however, easy to point out
obvious pitfalls, though difficult to maintain a judicial attitude in
the excitement of endeavouring to interpret documents which are too
incomplete to be identified with certainty. If we put on one side all
records of supposed fossil Sequoias not based on satisfactory data,
there remains a wealth of material testifying to the antiquity of the
surviving species.
It is by no means improbable that Conifers closely allied to the
Redwoods and Mammoth trees of California were represented in Jurassic
floras; but hitherto no proof has been obtained of the occurrence of
a Sequoia among the rich material afforded by the Jurassic plant-beds
of Yorkshire and by beds of the same age in other countries. A small
cone has recently been described from strata near Boulogne belonging
to the latest phase of the Jurassic period, which presents a strong
resemblance in shape and size and in the form of the cone-scales to
those of the recent species. This specimen, though not conclusive,
is the most satisfactory indication of a Jurassic Sequoia so far
discovered. From Lower Jurassic rocks in Madagascar similar cones
have been recorded in association with foliage-shoots like those of
_Sequoia gigantea_, but here too the evidence is not beyond suspicion.
In plant-bearing strata of Wealden age, such as are exposed in the
cliff near Hastings and in deposits of the same age in North Germany,
Portugal, and elsewhere, twigs and cones have been found which may be
those of trees nearly allied to the genus Sequoia.
[Illustration: Fig. 15. _Sequoia Couttsiae_ Heer. Twigs (A) and
cone-scales (B) from Bovey Tracey. (× 3.) (Photographs by Mr and Mrs
Clement Reid.)]
It is, however, in the sedimentary rocks of Cretaceous age, rather
higher in the series than those in the Hastings cliffs, and in the
succeeding Tertiary rocks, that undoubted Sequoias are met with in
abundance. At Bovey Tracey in Devonshire there is a basin-shaped
depression in the granitic rocks of Dartmoor filled with clay, gravel
and sand--the flood-deposits of a Tertiary lake containing waifs
and strays of the vegetation on the surrounding hills. Among the
commonest plants is one to which the late Oswald Heer gave the name
_Sequoia Couttsiae_, and his reference of the specimens to the genus
Sequoia has been confirmed by the recent researches of Air and Mrs
Clement Reid(50). This Tertiary (Oligocene) species is represented by
slender twigs almost identical with those of _Sequoia gigantea_ and by
well-preserved cone-scales and seeds (Fig. 15). Moreover, it has been
possible to examine microscopically the structure of the carbonised
outer skin of the leaves and to demonstrate its agreement with that
of the superficial tissue in the leaves of the Mammoth tree. With the
Bovey Tracey Sequoia are associated fragments of Magnolia, Vitis, and
_Taxodium distichum_, the swamp Cypress of North America, together
with other types which have long ceased to exist in Western Europe.
Other British examples of Sequoia have been described from Tertiary
beds at Bournemouth, the Isle of Wight, Sheppey, and Antrim, but the
material from these localities is inferior in preservation and cannot
be identified with the same degree of certainty as in the case of the
Devonshire specimens. The occurrence of twigs and cones of several
species of Sequoia in both Cretaceous and Tertiary rocks in Austria,
Germany, Italy, France, and elsewhere, shows that the ancestors of the
Californian trees were common in the European region.
The exploration of Cretaceous and Tertiary rocks in Arctic Europe
has revealed the former existence in Greenland, Spitzbergen, and
other more or less ice-covered lands of plants which clearly denote
a mild climate. Cones and branches of Sequoias have been found in
abundance in Lower Tertiary beds on Disco Island off the west coast
of Greenland, and similar evidence of the northern extension of the
genus has been obtained from Spitzbergen. Dr Nathorst of Stockholm
speaks of twigs of Sequoia in the Tertiary clays of Ellesmere Land
almost as perfect as herbarium specimens. In Tertiary beds on the banks
of the Mackenzie River, in Alaska, Saghalien Island and Vancouver
Island, and in Upper Cretaceous rocks in the Queen Charlotte Islands,
remains of Sequoia have been discovered. One of the most remarkable
instances of the preservation of trees of a bygone age is supplied
by the volcanic deposits of Lower Tertiary age exposed on the slopes
of Amethyst mountain in the Yellowstone Park district. At different
levels in the volcanic and sedimentary material, which is piled up to
a height of over 2000 ft. above the valley, as many as fifteen forests
are represented by erect and prostrate limbs of petrified trees (Fig.
6). The microscopical examination of some of these trees has shown
that they bear a striking resemblance to _Sequoia sempervirens_. In a
photograph of these petrified forests by the U.S. Geological Survey
(36, 2) one sees living Conifers side by side with the lichen-covered
and weathered trunks of the fossil species (_Sequoia magnifica_),
living and extinct being at a distance hardly distinguishable.
(Frontispiece.)
In concluding this brief survey of the fossil records of Sequoia,
reference may be made to the discovery of petrified wood in Cretaceous
rocks in South Nevada, possessing the anatomical features of _Sequoia
gigantea_, which shows that close to the present home of the big trees
their ancestors flourished during a period of the earth's history too
remote to be measured by human reckoning.
The distribution of the Tertiary and Cretaceous Sequoias would appear
to have been mainly in the northern hemisphere, extending well within
the Arctic circle. It is, however, by no means improbable that the
ancestors of Sequoia flourished far south of the equator. Reference has
been made to Jurassic fossils from Madagascar which have been compared
with the existing species, and from Lower Tertiary beds in New Zealand
the late Baron Ettingshausen described some cones and twigs as _Sequoia
novae zeelandicae_ which bear a close resemblance to the existing
type. The available evidence would seem to point to a northern origin
of the genus, though allowance must be made for erroneous conclusions
based on negative evidence. Further research may well extend the past
distribution of Sequoia in southern lands, but the data to hand point
to the conclusion that the Californian trees represent the survivors
of a type which flourished in the Cretaceous and Tertiary periods over
a wide area in North America and in what we now call the Continent of
Europe.
CHAPTER VII
THE ARAUCARIA FAMILY
'And so the grandeur of the Forest-tree
Comes not by casting in a formal mould,
But from its own divine vitality.'
Wordsworth.
As an additional illustration of existing cone-bearing trees which form
links with the past we may briefly consider the genera Araucaria and
Agathis, the two members of the family Araucarieae. It is generally
agreed that the branches of the genealogical tree of this family
extended farther back into the past than in the case of the majority
of Conifers. By some authors the surviving representatives of the
Araucarian stock are considered to have a strong claim to be regarded
as the most primitive as well as the oldest of cone-bearing trees,
though this opinion, like many others, is not held by botanists as
a whole. This is not the place to discuss matters of controversy,
and I shall confine myself to a general consideration of Araucaria
and Agathis from the point of view of their present distribution
and the part they played in the vegetation of the Mesozoic and
Tertiary epochs. In 1741 a plant from Amboyna, one of the Moluccas,
was described under the name _Dammara alba_. For this tree, known
as the Amboyna Fine, the English botanist Salisbury instituted the
generic name Agathis, from a Greek word (αγαθις) meaning a ball of
string and probably suggested by the form of the cones, which is the
designation usually adopted in botanical literature instead of the
pre-Linnean term Dammara. The best known species of the genus is the
Kauri Pine, probably the finest forest tree in New Zealand where it
still flourishes from the North Cape to latitude 38° S., though the
occurrence of sub-fossil trunks and pieces of buried resin shows that
the Kauri forests are gradually dwindling. The stems of this species,
_Agathis australis_, rise like massive grey columns to a height of 160
ft., terminating in a succession of spreading branches given off in
tiers from the main trunk. The thick narrow lanceolate leaves, with
several parallel veins, reach a length of 2 to 3 inches. The female
shoots have the form of small and almost spherical cones consisting of
a central axis bearing overlapping spiral series of broadly triangular
scales (Fig. 16). Each scale carries a single seed with a large
wing attached to one side which facilitates disposal by wind. Other
species of Agathis occur in the Malay Archipelago, the Philippines,
in Queensland, in the New Hebrides, New Caledonia, the Fiji Islands,
and elsewhere. With the exception of the Australian Kauri (_Agathis
robusta_), with leaves larger and broader than those of the New Zealand
Kauri, the genus is essentially an island type. With the exception of
some species of the southern hemisphere genus Podocarpus, there are no
Conifers with foliage like that of Agathis. It is, however, the broad
and thin single-seeded scales and the spherical cones, in some species
six inches in length, which furnish the most trustworthy means of
identifying the genus.
[Illustration: Fig. 16. A. _Agathis robusta_ Muell. (much reduced). B.
_Agathis Moorei_ Lind. (1/2 nat. size).]
[Illustration: Fig. 17. _Araucaria excelsa._ The upper part of a small
tree in the Cambridge Botanic Garden. (Much reduced.)]
The allied genus Araucaria, with the exception of two South American
species, the familiar Monkey Puzzle, _Araucaria imbricata_, and
a Brazilian tree, _Araucaria brasiliana_, is confined within the
geographical area occupied by Agathis. The name Araucaria was first
used by de Jussieu in 1789 for a plant previously referred to the
genus Pinus and described as one of the most beautiful trees of Chili.
This species, _A. imbricata_, introduced into England in 1796, grows
on the southern slopes of the Andes and, as in the case of the Kauri
forests of New Zealand, buried stems point to a wider extension of
the forests in earlier days. The sharp and thick leaves of the Monkey
Puzzle distinguish it from all other Conifers; its large almost
spherical seed-bearing cones, more than half a foot in length, which
may occasionally be seen on well-grown British trees, are unlike those
of other genera. Each of the deep and narrow scales bears a single
seed embedded in the substance of the scale and terminates distally
in a narrow upturned process. Some species of Araucaria, differing
considerably in the form of the leaves and in the shape and structure
of the seed-scales from the Chilian species, are conveniently placed
in a distinct sub-division of the genus Araucaria. Of this type the
Norfolk Island Pine, _Araucaria excelsa_, is the best-known example
(Fig. 17). It was introduced to Kew by Sir Joseph Banks in 1793, soon
after its discovery by Captain Cook, who describes the stems of the
Norfolk Island trees as resembling basaltic columns, and relates how on
approaching the island everyone was satisfied that the columnar objects
were trees, 'except our Philosophers, who still maintained they were
basaltes.' The leaves are short, about half an inch long, laterally
compressed and slightly spreading and sickle-shaped--sometimes shorter
and broader and overlapping--arranged in crowded spirals. The scales
of the broadly oval cones are single-seeded, but differ from those
of _Araucaria imbricata_ in having the seed exposed on the surface
and in the greater breadth and thinner borders of the scales. In both
Araucaria and Agathis the nature of the seed-scales constitutes a
distinguishing feature. The leaves of _Araucaria imbricata_ differ in
form from those of other Conifers. The foliage shoots of _Araucaria
excelsa_ and other species, _e.g._ the very closely allied _A. Cookii_
of the New Hebrides and New Caledonia, though not unlike the branches
of a Japanese Conifer (_Cryptomeria japonica_), often cultivated in
England, afford fairly trustworthy characters for identification
purposes.
The minute structure of the wood of both Araucaria and Agathis
constitutes an important distinguishing feature and enables us to
recognise on microscopical examination even a fragment of wood of
either of these genera. The small elongated cells or water-conducting
elements of the wood of the Araucarieae are characterised by one or
two, and occasionally as many as three or four, contiguous rows
of pits on their radial walls, and these appear in surface view as
flattened circles or polygonal areas.
These details have been mentioned in order to show that Araucaria and
Agathis are sufficiently distinct in many respects from other Conifers
to render their identification in a fossil state comparatively easy, at
least much easier than the recognition of the majority of the members
of the Coniferae. It would be going too far to state definitely that
Araucarieae, as defined by reference to existing species, existed
during the Palaeozoic period; on the other hand it would seem in a
high degree probable that the vegetation of the Coal age and of the
succeeding Permian period included trees in which certain Araucarian
characters were clearly foreshadowed. The name Araucarioxylon was
formerly applied to petrified wood, obtained from Palaeozoic as well
as from later formations, which agrees anatomically with that of
Araucaria and Agathis. It has been shown in recent years that much of
the Palaeozoic wood of this type of structure belongs to the extinct
genus Cordaites, a tree which played a prominent part in the earlier
floras. Cordaites affords a good example of a generalised type: in its
wood-structure it resembles very closely the existing Araucarieae;
its long strap-like leaves are not unlike those of some species of
Agathis; its male flowers have often been compared with those of the
Maiden Hair tree, _Ginkgo biloba_, and certain anatomical features form
connecting links between this Palaeozoic genus and the Cycads.
It is noteworthy that in another Palaeozoic genus, Walchia, the
leaf-bearing branches are identical in appearance with those of the
Norfolk Island Pine (Fig. 17) and some other species of Araucaria.
Unfortunately our knowledge of the reproductive organs of Walchia is
insufficient to warrant any definite statement as to the degree of
consanguinity between this Permian and Upper Carboniferous plant and
the Araucarieae; it is probable that in Walchia we have a type not far
removed from the line of evolution which led to Araucaria. Petrified
wood, identified as that of Walchia, and exhibiting the Araucarian type
of structure, has been recorded from Permian rocks of the Vosges. Other
instances might be quoted in support of the view that the Palaeozoic
floras included a few plants with which the surviving Araucarieae may
fairly claim relationship. Professor Zeiller of Paris has recently
described some fossil shoots from Palaeozoic rocks in India under the
name _Araucarites Oldhami_ on the ground of the similarity of the
leaves to those of _Araucaria imbricata_. Similarly, from Triassic
rocks several fossils have been described as closely allied to
Araucaria, in some cases because of anatomical resemblances and in
others on the less satisfactory evidence furnished by a similarity in
the foliage shoots. Professor Jeffrey of Harvard has recently given an
account of a new type of stem (Woodworthia) from the petrified Triassic
forest of Arizona possessing some Araucarian characters, though
differing from existing species of Araucaria in certain structural
features, a combination of characters regarded by this Author as an
indication of relationship with the family of Conifers, which includes
the Pines, Firs, Larches and other well-known northern genera.
It is, however, from the records of Jurassic rocks that we obtain
the most satisfactory information as to the great antiquity and the
very wide geographical range of the ancestors of the recent genus.
The plant-beds of the Yorkshire coast afford clear evidence of the
occurrence of Araucarian trees in the woodlands of the Jurassic
period. Petrified wood has been found at Whitby, associated with jet,
showing the minute structural characteristics of the surviving species
of Araucarieae, and it is not improbable that some at least of the
Whitby jet has been formed from the wood of Araucarian plants. The
carbonised remains of leafy shoots preserved in the Jurassic shales
near Scarborough and on other parts of the Yorkshire coast include
twigs hardly distinguishable from those of _Araucaria excelsa_, though
the resemblance of external form alone, especially in the case of
foliage shoots, does not amount to proof of generic identity. We have,
however, the much more trustworthy evidence of cones and seed-bearing
scales in which the characteristic features of living species are
clearly shown. Seed-bearing scales almost identical with those of
_Araucaria excelsa_ and other recent species have long been known from
the Jurassic rocks of Yorkshire.
From other parts of England where samples of Jurassic floras are
preserved, as at Stonesfield in Oxfordshire, in Northamptonshire and
elsewhere, equally striking examples of undoubted Araucarias have been
found.
Fig. 18 represents part of a large cone described in 1866 by Mr
Carruthers from Jurassic rocks at Bruton in Somersetshire: this
specimen, now in the British Museum, consists of one side of a
spherical cone about 5 inches long and 5 inches broad; in size, as in
the form of the seed-scales, it shows a striking likeness to the cones
of the Australian species _Araucaria Bidwillii_, the Bunya Bunya of
Queensland. Other equally convincing examples of Jurassic Araucarian
cones and seeds may be seen in the museums of York and Northampton. On
the north-east coast of Sutherland there is a narrow strip of Jurassic
beds forming a low platform between the granitic and Old Red Sandstone
hills and the sea. From these rocks Hugh Miller described several
fossil plants in his _Testimony of the Rocks_, and an examination of a
large collection obtained from this district by the late Dr Marcus Gunn
shows that Miller was justified in speaking of Araucaria as a member of
this northern flora.
[Illustration: Fig. 18. _Araucarites sphaerocarpus_ Carr. From Jurassic
rocks at Bruton, Somersetshire. (British Museum, 2/3 nat. size.)]
There is abundant evidence pointing to the existence in Britain during
the Jurassic period, and in the early days of the Cretaceous epoch, of
Araucarian trees which differed but slightly from the modern species
confined to the southern hemisphere. In several localities in France,
Germany, and other parts of the continent, Araucarian fossils have
been recognised in Jurassic rocks. It is almost certain that some
foliage shoots and imperfectly preserved cones described by Dr Nathorst
from Upper Jurassic rocks in Spitzbergen were borne by a species of
Araucaria. Cone-scales very similar to those from Yorkshire have been
discovered in Wealden beds in Cape Colony, and Araucarian wood of
Jurassic and Cretaceous age has been found in Madagascar. From Jurassic
strata in India and Victoria (Australia), as well as from Upper
Jurassic and Lower Cretaceous rocks in Virginia and elsewhere in the
eastern United States, well preserved Araucarian fossils are recorded.
In a collection of Jurassic plants, obtained a few years ago by the
members of a Swedish Antarctic Expedition in Graham's Land, Dr Nathorst
has recognised some cone-scales of Araucaria, which demonstrate a
former extension of the family beyond the southern limits of South
America.
It is interesting to find that when we ascend higher in the
geological series and pass beyond the Wealden strata to the Middle
and Upper sub-divisions of the Cretaceous period, evidence of the
wide geographical distribution of the Araucarieae is still abundant.
Araucarian wood has been obtained in rocks classed as Upper Cretaceous
in Egypt, in East Africa, in Dakota, and elsewhere. In the sedimentary
rocks of the Tertiary period undoubted examples of Araucaria are less
common, though there can be no doubt that the genus was much more
widely spread then than it is at the present day. The well-known
Tertiary plant-beds of Bournemouth have afforded specimens of foliage
shoots which have been described as a species of Araucaria, though in
the absence of well-preserved cones or petrified wood we must admit
that the data are inconclusive. It is, however, legitimate to regard
the striking similarity of the Bournemouth twigs to those of _Araucaria
excelsa_ and _A. Cookii_ as constituting a fairly strong case in
favour of the persistence of Araucaria in Western Europe up to the
earlier stage of the Tertiary period. Araucarian wood of Tertiary age
is recorded from India, while branches with broad leaves like those
of _Araucaria imbricata_ have been found in Seymour Island and the
Magellan Straits, and specimens of Tertiary wood are described from
Patagonia. At the other end of the world. Tertiary rocks on the west
coast of Greenland have yielded fragments which may be referred with
some hesitation to the genus Araucaria.
A few words must be added in regard to the recent discovery by
Professor Jeffrey and Dr Hollick of some very interesting Cretaceous
specimens in New Jersey of well-preserved cone-scales and foliage
shoots of extinct plants closely related to the existing species of
Agathis(51). The American fossils are particularly valuable because
their preservation admits of microscopical examination of the tissues.
In Cretaceous rocks of Staten Island and in other localities on the
eastern border of the northern United States, kite-shaped seed-bearing
scales almost identical in form with those of recent species of Agathis
are fairly common fossils. Similar specimens have long been known
from Tertiary rocks in western Greenland. In the case of some of the
American examples each scale bore three seeds instead of a single seed
in living species: on account of this difference Prof. Jeffrey and Dr
Hollick have adopted a distinct generic name, _Protodammara_.
The foregoing sketch is necessarily far from complete, but it may serve
as an illustration of the light which is thrown on the past history of
recent plants by the investigation of the relics of ancient floras.
The family Araucarieae now represented by a small number of species
which, with the exception of the Andian and Brazilian Araucarias, are
restricted to a small region in the southern hemisphere, was one of
the most widely spread sections of the seed-bearing plants during the
Mesozoic era. Ancestors of Araucaria must have been common trees in the
European vegetation in Jurassic and Lower Cretaceous periods, and even
as late as the Tertiary period there is evidence that representatives
of the family still lingered in the north. One conclusion which seems
almost unavoidable is that the species of Araucaria and Agathis that
survive, in some cases only in one or two small islands in the South
Pacific, have in the course of successive ages wandered from the other
end of the world. Their migrations can be partially traced by the
fragments embedded in Jurassic and later sediments, but we can only
speculate as to the causes which have contributed to the changes in the
fortunes of the family; how much influence may have been exerted by
changes in physical conditions in the environment, and to what extent
the production of more successful types may have been the dominant
cause of the decline, it is impossible to say. One thing at least is
certain, that few existing plants are better entitled to veneration as
survivals from the past than are the living species of Araucaria.
CHAPTER VIII
THE MAIDEN HAIR TREE
'...the trees
That whisper round a temple become soon
Dear as the temple's self.'
Keats.
The Maiden Hair tree of China and Japan, which was introduced into
Europe early in the eighteenth century, has now become fairly well
known. Though hardy in England, it requires warmer summers for full
development and regular flowering. To botanists this Eastern tree is of
peculiar interest, partly because of the isolated position it occupies
in the plant-kingdom and partly by reason of its great antiquity. There
is probably no other existing tree which has so strong a claim to be
styled a 'living fossil,' to use a term applied by Darwin to survivals
from the past. In 1712 the traveller Kaempfer proposed for this plant
the generic name Ginkgo, and Linnaeus adopted this designation, adding
the specific name _biloba_ to denote the bisection of the wedge-shaped
lamina of the leaf into two divergent segments. In 1777 the English
botanist Sir J. E. Smith expressed his disapproval of what he called
the uncouth name Ginkgo by substituting for _Ginkgo biloba_ the title
Salisburia adiantifolia, but as it is customary to retain names adopted
or proposed by Linnaeus, the founder of the binominal system of
nomenclature, the correct botanical designation of the maiden hair tree
is _Ginkgo biloba_. Mere personal preference such as that of Sir J. E.
Smith for Salisburia is not an adequate reason for rejecting an older
name.
[Illustration: Fig. 19. _Ginkgo biloba_ Linn. (Slightly reduced.)]
In its pyramidal habit Ginkgo agrees generally with the larch and other
Conifers. Like the larch and cedar it possesses two kinds of foliage
shoots, the more rapidly growing long shoots with scattered leaves
and the much shorter dwarf-shoots which elongate slightly each year
and bear several leaves crowded round their apex. The leaves (Fig.
19), which are shed each year, are similar in the cuneate form of the
lamina and in the fan-like distribution of the forked veins, to the
large leaflets of some species of maiden hair ferns: the thin lamina
carried by a slender leaf-stalk is usually about 3 inches across,
though in exceptional cases it may reach a breadth of 8 inches. The
lamina is usually divided by a deep =V=-shaped sinus into two equal
halves; it may be entire with an irregularly crenulate margin, or, on
seedlings and vigorous long shoots, the lamina may be cut into several
wedge-shaped segments.
The male and female flowers are borne on separate trees; the male
consists of a central axis giving off slender branches, each of which
ends in a small terminal knot and two elliptical capsules in which
the pollen is produced. The female flowers have a stouter axis which
normally produces two seeds at the apex. The seed is encased in a
green fleshy substance and, as in the fruit of a cherry or plum, the
kernel is protected by a hard woody shell. In the form of the leaves
and in the structure of the flowers Ginkgo presents features which
clearly distinguish it from the Conifers, the class in which, until
recently, it was included. In 1896 the Japanese botanist Hirase made
the important discovery that the male reproductive cells of Ginkgo are
large motile bodies provided with a spirally coiled band of minute
cilia--delicate hairs which by their rapid lashing-movement propel the
cell through water. In all Flowering Plants and in Conifers the male
reproductive cells have no independent means of locomotion; they are
carried to the female cell by the formation of a slender tube--the
pollen-tube--produced by the pollen-grain. In the Ferns, Lycopods and
Horsetails--in fact in all members of the Pteridophyta--as also in the
Mosses and Liverworts as well as in many of the still lower plants,
the male cells swim to the egg by the lashing of cilia like those on
the male cells of Ginkgo. This difference in regard to the nature of
the male cells was considered to be a fundamental distinction between
the higher seed-bearing plants and all other groups of the vegetable
kingdom. It was, therefore, with no ordinary interest that Hirase's
discovery was received, as it broke down a distinction between the two
great divisions of the plant-world which had been generally accepted
as fundamental; though it is only fair to say that the German botanist
Hofmeister, a man of exceptional originality and power of grasping the
essential, foresaw the possibility that this arbitrary barrier would
eventually be removed. The Ferns and other plants in which the male
cells are motile, represent earlier stages in the progress of plant
development, when the presence of water was essential for the act
of fertilisation, a relic of earlier days when the whole plant-body
was fitted for a life in water. As higher types were produced, the
plant-machinery became less dependent on an aqueous habitat, and the
loss of organs of locomotion in the male cells is an instance of the
kind of change accompanying the gradual adaptation to life on land. The
idea of the gradual emancipation of plants from a watery environment is
expressed in a somewhat extreme form by the author of a book entitled
_The Lessons of Evolution_(52), who states that the ocean is the mother
of plant-life and that plants formed the army which conquered the land.
In Ginkgo we have a type which, though similar in most respects to
the Conifers, possesses in its motile reproductive cells a persistent
inheritance from the past. The recognition of this special feature
afforded a sound reason, especially when other peculiarities are
considered, for removing Ginkgo from the Conifers and instituting a new
class-name, Ginkgoales.
Ginkgo is a generalised type, linked by different characters both
with living members of the two classes of naked-seeded plants and
with certain existing Palaeozoic genera. It is a survivor of a race
which has narrowly escaped extinction; the last of a long line that
has outlived its family and offers by its persistence an impressive
instance of the past in the present. Though Mrs Bishop in her
_Untrodden Paths in Japan_ speaks of forests of Maiden Hair trees
apparently in a wild state, it is generally believed that they were
cultivated specimens. Mr Henry who has an exceptionally wide knowledge
of Chinese vegetation tells us that 'all scientific travellers in
Japan and the leading Japanese botanists and foresters deny its being
indigenous in any part of Japan; and botanical collectors have not
observed it truly wild in China.' Moreover, Mr E. H. Wilson, after
traversing the whole of the district where Ginkgo was supposed to occur
in a wild state, says that he found only cultivated trees. There is no
reason to doubt that China is the last stronghold of this ancient type
which in an earlier period of the earth's history overspread the world.
A brief summary of the past history of Ginkgo and of the Ginkgoales
supplies overwhelming testimony to the tenacity of life with which the
Maiden Hair tree has persisted through the ages.
It was pointed out in the account of the past history of Araucaria that
the records obtained from Palaeozoic rocks, while affording evidence of
the existence of Carboniferous and Permian genera undoubtedly allied
to the living species, do not enable us to speak with certainty as to
the precise degree of affinity. Similarly, Palaeozoic leaves have been
described as representatives of the class of which Ginkgo is the sole
survivor, but the evidence on which this relationship is assumed is by
no means conclusive.
The generic name Psygmophyllum has been applied to some impressions
of Ginkgo-like leaves discovered in the Upper Devonian rocks of
Bear Island, a small remnant of land in the Arctic circle, which
has furnished valuable information as to the composition of one of
the oldest floras of which satisfactory remains have been found.
Other examples of these lobed, wedge-shaped leaves are recorded from
Carboniferous rocks in Germany, France, and elsewhere; from Permian
strata in the east of Russia and from Palaeozoic beds in Cape Colony
and Kashmir. A relationship between Psygmophyllum and Ginkgo is,
however, by no means established and rests solely on a resemblance in
the form of the leaves. The close correspondence in form and venation
between some leaves from Permian rocks in the Ural mountains and
from Lower Permian beds in France, and those of the recent species,
is considered by some authors sufficiently striking to justify the
reference of these fossils to the genus Ginkgo. Similar leaves of
Permian age, which may also be related to the existing species, have
been described under the name Ginkgophyllum. Other specimens of
Palaeozoic age from North America and elsewhere have been assigned to
the Ginkgoales; but in none of these cases, despite the resemblance
in leaf-form, is there sufficiently convincing evidence of close
relationship to warrant a definite assertion that the plants in
question were members of the group of which Ginkgo alone remains.
It is, however, an undoubted fact that the Maiden Hair tree is
connected by a long line of ancestors with the earliest phase of the
Mesozoic era. From many parts of the world large collections of fossil
plants have been obtained from strata referred to the Rhaetic period,
or to the upper division of the Triassic system. A comparison of floras
from these geological horizons in different parts of the world points
to a vegetation extending from Australia, Cape Colony, and South
America, to Tonkin, the south of Sweden and North America, which was
characterised by a greater uniformity than is shown by widely separated
floras at the present day. One of the commonest genera in Rhaetic
floras is that known as Baiera; this name is applied to wedge-shaped
leaves with a slender stalk similar in shape and venation to those of
Ginkgo, but differing in the greater number and smaller breadth of
the segments. Between the deeply dissected leaf of a typical Baiera
with its narrow linear lobes and the entire or broadly lobed leaf of a
Ginkgo there are many connecting links, and to some specimens either
name might be applied with equal fitness. Examples of Baiera leaves,
in some cases associated with fragments of reproductive organs, are
recorded from Rhaetic rocks of France, the south of Sweden, Tonkin,
Chili, the Argentine, North America, South Africa, and from other
regions. There is abundant evidence pointing to the almost world-wide
distribution of the Ginkgoales, as represented more especially by
Baiera, in the older Mesozoic floras. In the later Jurassic rocks of
Yorkshire true Ginkgo leaves as well as those of the Baiera type are
fairly common; with the leaves have been found pieces of male and
female flowers. Ginkgo and Baiera have been described from Jurassic
rocks of Germany, France, Russia, Bornholm, and elsewhere in Europe;
they occur abundantly in Middle Jurassic rocks in northern Siberia, and
are represented in the Jurassic floras of Franz Josef Land, the East
Coast of Greenland, and Spitzbergen (Fig. 20). The abundance of Ginkgo
and Baiera leaves associated with male flowers and seeds discovered in
Jurassic rocks, approximately of the same geological age as those on
the Yorkshire coast, in East Siberia and in the Amur district, has led
to the suggestion that this region may have been a centre where the
Ginkgoales reached their maximum development in the Mesozoic period.
[Illustration: Fig. 20. Fossil Ginkgo leaves. (1/2 nat. size.)
A. Tertiary, Island of Mull.
B. Wealden, North Germany (after Schenk).
C. Jurassic, Japan (after Yokoyama).
D. Jurassic, Australia (after Stirling).
E. Jurassic, Siberia (after Heer).
F. Jurassic, Turkestan.
G. Lower Cretaceous, Greenland (after Heer).
H. Jurassic, California (after Fontaine).
I. Jurassic, Yorkshire.
J. Jurassic, N.E. Scotland (after Stopes).
K. Wealden, Franz Josef Land (after Nathorst).
L. Rhaetic, South Africa.
M. Jurassic, Spitzbergen (after Heer).
]
It should be added that other genera of Jurassic and Rhaetic fossils
in addition to Ginkgo and Baiera have been referred to the Ginkgoales,
though evidence of such affinity is not convincing. There is, however,
good reason to believe that this widespread group was represented by
several genera in the older Mesozoic floras.
The occurrence of the Ginkgoales in Jurassic rocks in King Charles
Land and in the New Siberian Islands (lat. 78° and 75° N.), in Central
China, Japan, Turkestan, California, Oregon, South Africa, Australia,
and Graham's Land demonstrates the cosmopolitan nature of the group.
During the later part of the Jurassic period and in the Wealden floras
both Baiera and Ginkgo were abundant; leaves are recorded from Jurassic
strata in the north-east of Scotland, from Lower Cretaceous or Wealden
rocks in North Germany, Portugal, Vancouver Island, Wyoming, and
Greenland.
During the Tertiary period, or probably in the earlier days of
that era. Ginkgo flourished in North America, in Alaska and in the
Mackenzie River district, Greenland, Saghalien Island, and in several
European regions. In Chapter III reference was made to the volcanic
activity which characterised the north-west European area in the
early Tertiary period and resulted in the formation of the thick
sheets of basalt on the north-east coast of Ireland and in the Inner
Hebrides. There were occasional pauses in the volcanic activity,
during which vegetation established itself on the weathered surface
of the lava, and left traces of its existence in the leaves and twigs
preserved in the sedimentary material enclosed between successive
lava-floras. At Ardtun Head in the Isle of Mull beautifully preserved
leaves of Ginkgo, 2-4 inches in breadth, with the median sinus and
the venation characteristic of the leaves of the existing plant,
have been discovered in a bed of clay which marks the site of a
lake in a depression on the lava-plateau. The resemblance of these
Tertiary leaves from Mull to those of the surviving Maiden Hair tree
is so close as to suggest specific identity. Mr Starkie Gardner and
Baron Ettingshausen have described some seeds from the London clay
(Lower Tertiary) in the Isle of Sheppey as those of Ginkgo, but this
identification rests on data too insufficient to be accepted without
hesitation.
The recent cultivation of _Ginkgo biloba_ in Britain may therefore
be spoken of as the re-introduction of a plant which in the earlier
part or in the middle of the Tertiary period flourished in the west of
Scotland, and was abundant in England in the earlier Jurassic period.
It is impossible to say with any confidence where the Ginkgoales first
made their appearance, whether in the far north or in the south, nor
are we able to explain the gradual decline of so venerable and vigorous
a race.
As we search among the fragmentary herbaria scattered through the
sedimentary rocks in that comparatively small portion of the earth's
crust which is accessible to investigation, we discover evidence of a
shifting of the balance of power among different classes of plants in
the course of our survey of successive floras. Plants now insignificant
and few in number are found to be descendants of a long line of
ancestors stretching back to a remote antiquity when they formed the
dominant class. Others which flourished in a former period no longer
survive, either themselves or in direct descendants. 'The extinction of
species has been involved in the most gratuitous mystery.' We can only
speculate vaguely as to the cause of success or failure. Certain types
were better armed for the struggle for life, and produced descendants
able to hold their own and to perpetuate the race through the ages in
an unbroken line. Others had a shorter life and fell out of the ranks
of the advancing and ever changing army. To quote Darwin's words:'We
need not marvel at extinction; if we must marvel, let it be at our
own presumption in imagining for a moment that we understand the many
complex contingencies on which the existence of each species depends.'
BIBLIOGRAPHY
Many of the books and papers dealing with subjects touched upon in
this volume are not included in the following list. For reference to
a more complete bibliography the reader should consult more technical
treatises.
1. Holmes, T. Rice. Ancient Britain and the invasions of Julius
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2. Mitchell, A. The Past in the Present; what is Civilisation?
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3. Weismann, A. Essays upon Heredity and Kindred Biological
Problems, (I. The Duration of Life.) Vol. I. Edited by E. B.
Poulton, S. Schönland, and A. E. Shipley. (Second Edition.)
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4. The Historie of the World, commonly called the Naturall
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Philemon Holland. London, 1634.
5. Hooker, J. D. On Three Oaks of Palestine. _Trans. Linnean
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6. Holtermann, Carl. Der Einfluss des Klimas auf den Bau der
Pflanzengewebe. Leipzig, 1907.
7. Atkinson, A. Notes on an Ancient Boat found at Brigg.
_Archaeologia_, Vol. I. p. 361. 1887.
8. Huxley, T. H. Man's Place in Nature and other Anthropological
Essays. _Collected Essays_, Vol. VII. (On the methods and
results of Ethnology.) London, 1901.
9. Lewis, F. J. The sequence of Plant Remains in the British Peat
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With notes on the Fauna and Flora by Clement Reid; etc. _Quart.
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11. The Life and Letters of Charles Darwin. Edited by Francis
Darwin. 3 Vols. London, 1887.
12. More Letters of Charles Darwin. Edited by Francis Darwin and A.
C. Seward. 2 Vols. London, 1903.
13. Ward, Lester F. The Course of Biologic Evolution. Anniversary
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1890.
14. Marshall, W. _Anacharis alsinastrum_, a new water weed.
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{ Bailey, C. Notes on the structure, the occurrence in
{ Lancashire, and the source of origin, of _Naias gramineus_
{ Delile, var. _Delilei magnus_. _Journal of Botany_,
{ Vol. XXII. p. 305. 1884.
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{ Weiss, F. H. and H. Murray. On the occurrence and
{ distribution of some alien aquatic plants in the Reddish
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{ Vol. LIII. Pt. II. 1909.
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October, 1908.
17. Hooker, J. D. Outlines of the Distribution of Arctic Plants.
_Trans. Linn. Soc._ Vol. XXIII. p. 251. 1862.
18. Engler, A. Plants of the Northern Temperate Zone in their
transition to the High Mountains of Tropical Africa. _Annals of
Botany_, Vol. XVIII. p. 523. 1904.
19. Darwin, C. The Origin of Species. London, 1900.
20. Ridley, H. N. On the dispersal of seeds by wind. _Annals of
Botany_, Vol. XIX. p. 351. 1905.
21. Hooker, J. D. On the Cedars of Lebanon, Taunus, Algeria, and
India. _The Natural History Review_, 1862, p. 11.
22. Forbes, E. On the connection between the distribution of
the existing Fauna and Flora of the British Isles, and the
geological changes which have affected their area.... _Memoirs,
Geological Survey_, Vol. I. p. 336. 1846.
23. Praeger, R. L. The Wild Flowers of the West of Ireland and their
history. _Journ. R. Hort. Soc._ Vol. XXXVI. p. 299. 1910.
24. Ernst, A. The New Flora of the Volcanic Island of Krakatau.
Translated by A. C. Seward. Cambridge, 1908.
25. Praeger, R. L. A Tourist's Flora of the West of Ireland. Dublin,
1909.
{ Rendle, A. B. _Sisyrinchium californicum_ Dryand.
26. { _Journal of Botany_, Vol XXXIV. p. 494. 1896.
{ Marshall, E. S. _Sisyrinchium californicum_ in Ireland.
{ _Ibid._ p. 366.
{ Reid, Clement. The Origin of the British Flora. London, 1899.
27. { Reid, Clement and Eleanor M. On the Pre-glacial
{ Flora of Britain _Journ. Linn. Soc._ Vol. XXVIII. p. 206.
{ 1908.
28. Lankester, Sir Edwin Ray. Mature and Man. The Romanes Lecture.
Oxford, 1905.
29. Huxley, T. H. On a Piece of Chalk. _Collected Essays_, Vol.
VIII. London, 1896.
30. Jukes-Browne, A. J. The Building of the British Isles. London,
1911.
31. Ray, J. Three Physico-Theological Discourses, etc. (2nd
Edition.) London, 1693.
32. Woodward, J. An Essay toward a Natural History of the Earth.
London, 1695.
33. Sollas, W. J. The Age of the Earth, and other geological
studies. London, 1905.
34. Ward, L. F. Status of the Mesozoic Floras of the United States.
_Monograph 48, U. S. Geol. Surv._ 1905.
35. Arber, E. A. Newell. _Cupressinoxylon Hookeri_ sp. nov., a large
silicified tree from Tasmania. _Geological Magazine_, Vol. I.
[V.], p. 7. 1904.
{ 1. Holmes, W. H. Fossil Forests of the Volcanic Tertiary
{ Formations of the Yellowstone National Park. Ann.
{ Rep. Geol. and Geogr. Surv. (U.S.A.), 1878, Pt. II. p. 47.
36. { 2. Knowlton, F. H. Fossil Flora of the Yellowstone
{ National Park. _Monograph 32, U. S. Geol. Survey_,
{ Pt. II. 1899.
37. Darwin, C. Journal of Researches into the Natural History and
Geology of the countries visited during the voyage round the
world of H.M.S. 'Beagle.' London, 1902.
38. Dufferin, Lord. Letters from High Latitudes. London. N.D.
39. Amundsen, R. The North-West Passage. 2 Vols. London,. 1908.
40. MacDougal, D. T. Botanical Explorations in the South-west.
_Journ. New York Botanical Garden_, Vol. V. p. 89. 1904.
{ Goeppert, H. R. and A. Menge. Die Flora des Bernsteins.
41. { Danzig, 1883.
{ Conwentz, H. Monographie der baltischen Bernsteinbäume.
{ Danzig, 1890.
42. Hovelacque, M. Sur la Nature végétale de l'_Aachenosaurus
multidens._ _Bull. Soc. Belge de Géol._ etc. Tome IV. p. 59.
1890.
43. Oliver, F. W. and D. H. Scott. On the structure of the
Palaeozoic seed _Lagenostoma Lomaxi_. _Phil. Trans. R. Soc._
Vol. CXCVII. p. 193. 1904.
44. Kidston, R. On the fructification of _Neuropteris heterophylla_.
_Phil. Trans. Royal Soc. London_, Vol. CXCVII. p. 1.
45. Smith, Geoffrey. A Naturalist in Tasmania. Oxford, 1909.
46. Campbell, D. H. On the Distribution of the Hepaticae, and its
significance. _New Phytologist_, Vol. VI. p. 203. 1907.
47. Kidston, R. and D. T. Gwynne-Vaughan. On the Fossil Osmundaceae.
Phil. Trans. R. Soc. Edinburgh, Vols. XLV., XLVI. 1907-09.
48. Seward, A. C. Fossil Plants. 2 Vols. Cambridge, 1898-1910.
49. Wallace, A. R. The Malay Archipelago. London, 1886.
50. Reid, C. and Eleanor M. The Lignite of Bovey Tracey. _Phil.
Trans. Royal Soc. London_, Vol. 201, p. 161. 1910.
51. Hollick, A. and E. C. Jeffrey. Studies of Cretaceous Coniferous
remains from Kreischerville, New York. _Mem. New York Bot.
Garden_, Vol. III. 1909.
52. Hutton, F. W. The Lessons of Evolution. London, 1902.
INDEX
_Aachenosaurus multidens_, 68
_Agathis australis_, 107
_A. Moorei_, 108
_A. robusta_, 108, 109
Amber, 65, 66
Amethyst Mountain, 59, 60, 104
Amundsen, E., 62
_Anacharis alsinastrum_, 20
Angiosperms, history of, 16, 53, 69, 70
Annual rings, 5-9
Antarctic fossil plants, 70, 117
_Araucaria Bidwillii_, 115
_A. brasiliana_, 109
_A. Cookii_, 111, 118
_A. excelsa_, 110, 111, 114, 115, 118
_A. imbricata_, 109, 111, 113, 118
Araucarieae, 106-120
Araucarioxylon, 112
_Araucarites Oldhami_, 113
_A. sphaerocarpus_, 116
Arbutus Unedo, 31-33, 36
Archaean rocks, 43
Arctic plants, 22, 23, 30, 69, 127
Arctic-Alpine plants, 22, 23
Arizona, fossil forests of, 59
Artocarpus, 28, 29
Baiera, 129, 131
Banks, Sir Joseph, 110
Barry, forest beds at, 13
Bennett, A., 21
_Bidens tripartita_, 46, 47
Bishop, Mrs, 126
Bommer, C, 73, 82, 89
Bovey Tracey, fossil plants from, 101-103
Bower, Prof., 73
Bracken Fern, 75, 77, 83, 87
Brandon, flint-knappers of, 3
Brigg, dug-out boat from, 10, 11
British flora, 19-38
_Caltha palustris_, 47
Campbell, Prof. D. H., 79
Candolle, A. P. de, 7, 33
Canton, W., 34
Carboniferous plants, 66-68, 72-75, 127
Carruthers, Dr W., 115
Cedars, 27
Cheddar Pink, 24
_Cinnamomum prototypum_, 65, 66
Climate, fossil plants and, 68, 69
Coal age, plants of the, 19, 66, 67
Connemara, 30, 31
Cook, Capt., 111
Coral polyps compared with plants, 8
Cordaites, 112
Cornwall, Pyrenean Heaths in, 31
Cretaceous plants, 16, 17, 53, 82, 117
Cromer Forest bed, 48
_Cryptomeria japonica_, 111
Cycads, 11, 72-74
_Daboecia polifolia_, 30
Dacrydium, 85
Dakota group, flora of, 17
_Dammara alba_, 107
Darwin, C, 16, 24, 25, 33, 39, 40, 61, 133
Devonshire, Pyrenean Heaths in, 31
_Dianthus caesius_, 24
Dipteris, 77, 92-94
_D. conjugata_, 92, 93
_D. Horsfieldii_, 86, 93
Disco Island, Fossil plants from, 28, 69, 82, 103
Discontinuous distribution, 26
Dispersal of plants, rate of, 25, 26
Distribution of plants, 15-38
Drift-wood, 61-63
_Dryas octopetala_, 22
Dufferin, Lord, 61
_Elodea canadensis_, 20, 21
Engler, Prof., 23
_Erica ciliaris_, 31
_E. Mackaii_, 31
_E. mediterranea_, 30, 31
_E. vagans_, 31
_Eriocaulon septangulare_, 35-38
Ettingshausen, Baron, 105, 132
Ferns, 71-94
Flowering plants, see Angiosperms
Forbes, E., 31-34, 37
Fossil plants, as thermometers, 68;
preservation of, 56-70
Gardner, J. S., 132
Geographical distribution of plants, 15-38
Geological evolution of Britain, 29
Geological record, 39-55
Geological table, 42, 43
Geological time, 2, 3, 49
_Ginkgo biloba_, 113, 120-133
Ginkgoales, 18, 120-133
Ginkgophyllum, 128
Glacial period, effect on vegetation of, 29-32, 45, 47
_Gladiolus illyricus_, 31
Gleichenia, 81-83
Graham's Land, 70, 117
Gunn, Dr Marcus, 117
Gwynne-Vaughan, Prof., 80
Gymnosperms, 17, 18
_Habenaria intacta_, 37
Halifax, plants from canal near, 21
Heer, O.,33, 82, 101
Henry, A., 126
Hirase, Prof., 124, 125
Hofmeister, W. F. B., 125
Hollick, A., 119
Holmes, T. Rice, 2
Hooker, Sir J. D., 4, 15, 16, 22, 23, 27, 40
Hose, C, 87
Huxley, T. H., 11, 52
Ireland, Mediterranean plants in, 30-38
Jeffrey, Prof. E. C, 114, 119
Jet, 114
Juan Fernandez, 84
Jurassic flora, 53, 54, 70, 117
Jussieu, A. de, 109
Kaempfer, E., 121
Kauri Pine, 107
Kidston, Dr E., 74, 80
Knaresborough, petrifying spring at, 64
Krakatau, new flora of, 33, 31
Laccopteris, 90, 91
Lang, Dr A. , 27
Lankester, Sir Edwin Ray, 49
Lhwyd, E., 57
Linnaeus, C, 121, 123
Liriodendron, 28, 29
Liverworts, antiquity of, 79
London Pride, 31
Longevity of trees, 1-10
_Luzula racemosa_, 23
_L. spicata_, 23, 24
MacDougal, Prof., 62
Maiden Hair Tree, 120-133
Mammoth trees of California, 95-105
Man, first appearance of, 49
Marchantia, 79
Market Harborough, Elodea discovered near, 20
Marshall, E. S., 38
Matonia, 77, 86-94
_M. pectinata_, 86-88, 92, 93
_M. sarmentosa_, 87
Matonidium, 89
Mediterranean plants in Ireland, 30-38
Miller, Hugh, 59, 115, 117
Milligan, Dr, 2
Moss, Dr C. E., 23
Mull, Fossil plants in Isle of, 132
_Naias graminea_, 21
Nathorst, A. G., 64, 103, 117
Native plants, 20
New forest, 31
_Nipa fruticans_, 50-52
Nipadites, 50, 52
Norfolk Island Pine, see _Araucaria excelsa_
Oaks, longevity of, 4
Oliver, Prof. F. W., 74
_Osmunda regalis_, 76-79, 83
_Osmundites Dunlopi_, 80
Palaeozoic plants, 55, 66-68, 71, 127, 128
Peat, trees in, 12, 13
Permian floras, 55
Petrifaction, 64-67
_Picea excelsa_, 46, 48
Pines, Tertiary, 65, 66
_Pinus sylvestris_, 11-13, 47
Pipewort, 36-38
Pliny's Natural History, 4
Podocarpus, 109
_Potamogeton pennsylvanicus_, 21
Praeger, E. L., 32, 36, 37
Pre-Glacial plants, 22, 45-48
_Primula elatior_, 24
Protodammara, 119
Psygmophyllum, 127
_Pteridium aquilinum_, 75, 77, 83, 87
Pteridophyta, 18, 124
Pteridosperms, 74, 75
Pyrenean plants in Ireland, 31-38
Ray, John, 56
Reddish Canal, plants from the, 21
Redwoods of California, 95-105
Reid, Clement, 13, 22, 45, 102
Reid, Mrs, 22, 102
Rhaetic plants, 54, 94
Riccia, 79
Ridley, H. N., 25
Royal Fern, 76-79, 83
_Rubus chamaemorus_, 22
St Dabeoc's Heath, 30, 32, 37
_Salisburia adiantifolia_, 122
Saporta, the Marquis of, 70
_Saxifraga oppositifolia_, 22
_S. umbrosa_, 31
Scandinavian plants in Britain, 21-23, 30
Scots Pine, see _Pinus sylvestris_
Scott, Dr D. H., 74
Sequoia, 5, 95-105
_Sequoia Couttsiae_, 101
_S. gigantea_, 5, 95, 96, 98-104
_S. magnifica_, 104
_S. novae zeelandicae_, 105
_S. sempervirens_, 45, 96, 98, 104
Sheppey, fossil plants from, 50, 132
_Shorea leprosula_, 25
_Silene acaulis_, 22
_Sisyrinchium angustifolium_, 38
_S. californicum_, 38
Smith, G., 75
Smith, Sir J. E., 122, 123
Smith, W., 41
Sollas, Prof., 58
_Spiranthes romanzoffiana_, 38
_Stellaria holostea_, 46, 47
Steno, 57
Strahan, A., 13
Strawberry tree, 31, 32
Tansley, A. G., 12, 13, 87
_Taxodium distichum_, 103
Tertiary plants, 31-33, 37, 49, 50, 103
_Theobroma cacao_, 7
_Thyrsopteris elegans_, 84
_Todea barbara_, 11, 78
_T. radicans_, 78
_Trapa natans_, 47, 48
Triassic period, 54, 55
Tulip tree, 28
Walchia, 113
Wallace, Dr A. R., 85
Wealden flora, 17, 53
Weismann, A., 3
Wexford, American plant from, 38
Williamson, Prof. W. C, 72
Wilson, E. H., 120
Winchester Cathedral, wood from foundations of, 10
Woodward, Dr J., 57
Woodworthia, 114
Yellowstone Park, fossil trees in the, 59-61, 104
Zeiller, R., 113
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