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.