Ancient Plants
by Marie Carmichael Stopes · English · Gutenberg #43976 · 361,854 chars · public domain (US)
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Title: Ancient Plants
Author: Marie Carmichael Stopes
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*** START OF THE PROJECT GUTENBERG EBOOK ANCIENT PLANTS ***
ANCIENT PLANTS
[Illustration: Photo. of the specimen in Manchester Museum.
THE STUMP OF A _LEPIDODENDRON_ FROM THE COAL MEASURES]
ANCIENT PLANTS
BEING A SIMPLE ACCOUNT OF THE
PAST VEGETATION OF THE EARTH
AND OF THE RECENT IMPORTANT
DISCOVERIES MADE IN THIS REALM
OF NATURE STUDY
BY
MARIE C. STOPES, D.Sc., Ph.D., F.L.S.
Lecturer in Fossil Botany, Manchester University
Author of “The Study of Plant Life for Young People”
LONDON
BLACKIE & SON, Limited, 50 OLD BAILEY, E.C.
GLASGOW AND BOMBAY
1910
Preface
The number and the importance of the discoveries which have been made in
the course of the last five or six years in the realm of Fossil Botany
have largely altered the aspect of the subject and greatly widened its
horizon. Until comparatively recent times the rather narrow outlook and
the technical difficulties of the study made it one which could only be
appreciated by specialists. This has been gradually changed, owing to the
detailed anatomical work which it was found possible to do on the
carboniferous plants, and which proved to be of great botanical
importance. About ten years ago textbooks in English were written, and
the subject was included in the work of the honours students of Botany at
the Universities. To-day the important bearing of the results of this
branch of Science on several others, as well as its intrinsic value, is
so much greater, that anyone who is at all acquainted with general
science, and more particularly with Botany and Geology, must find much to
interest him in it.
There is no book in the English language which places this really
attractive subject before the non-specialist, and to do so is the aim of
the present volume. The two excellent English books which we possess,
viz. Seward’s _Fossil Plants_ (of which the first volume only has
appeared, and that ten years ago) and Scott’s _Studies in Fossil Botany_,
are ideal for advanced University students. But they are written for
students who are supposed to have a previous knowledge of technical
botany, and prove very hard or impossible reading for those who are
merely acquainted with Science in a general way, or for less advanced
students.
The inclusion of fossil types in the South Kensington syllabus for Botany
indicates the increasing importance attached to palæobotany, and as vital
facts about several of those types are not to be found in a simply
written book, the students preparing for the examination must find some
difficulty in getting their information. Furthermore, Scott’s book, the
only up-to-date one, does not give a complete survey of the subject, but
just selects the more important families to describe in detail.
Hence the present book was attempted for the double purpose of presenting
the most interesting discoveries and general conclusions of recent years,
and bringing together the subject as a whole.
The mass of information which has been collected about fossil plants is
now enormous, and the greatest difficulty in writing this little book has
been the necessity of eliminating much that is of great interest. The
author awaits with fear and trembling the criticisms of specialists, who
will probably find that many things considered by them as particularly
interesting or essential have been left out. It is hoped that they will
bear in mind the scope and aim of the book. I try to present only the
structure raised on the foundation of the accumulated details of
specialists’ work, and not to demonstrate brick by brick the exposed
foundation.
Though the book is not written specially for them, it is probable that
University students may find it useful as a general survey of the whole
subject, for there is much in it that can only be learned otherwise by
reference to innumerable original monographs.
In writing this book all possible sources of information have been
consulted, and though Scott’s _Studies_[1] naturally formed the
foundation of some of the chapters on Pteridophytes, the authorities for
all the general part and the recent discoveries are the numerous memoirs
published by many different learned societies here and abroad.
As these pages are primarily for the use of those who have no very
technical preliminary training, the simplest language possible which is
consistent with a concise style has always been adopted. The necessary
technical terms are either explained in the context or in the glossary at
the end of the book. The list of the more important authorities makes no
pretence of including all the references that might be consulted with
advantage, but merely indicates the more important volumes and papers
which anyone should read who wishes to follow up the subject.
All the illustrations are made for the book itself, and I am much obliged
to Mr. D. M. S. Watson, B.Sc., for the microphotos of plant anatomy which
adorn its pages. The figures and diagram are my own work.
This book is dedicated to college students, to the senior pupils of good
schools where the subject is beginning to find a place in the higher
courses of Botany, but especially to all those who take an interest in
plant evolution because it forms a thread in the web of life whose design
they wish to trace.
M. C. STOPES.
_December, 1909._
Contents
Chap. Page
I. Introductory 1
II. Various Kinds of Fossil Plants 6
III. Coal, the most Important of Plant Remains 22
IV. The Seven Ages of Plant Life 33
V. Stages in Plant Evolution 43
VI. Minute Structure of Fossil Plants—
Likenesses to Living Ones 53
VII. ″ ″ Differences from Living Ones 69
VIII. Past Histories of Plant Families—
(i) Flowering Plants 79
IX. ″ ″ (ii) Higher Gymnosperms 86
X. ″ ″ (iii) Bennettitales 102
XI. ″ ″ (iv) The Cycads 109
XII. ″ ″ (v) Pteridosperms 114
XIII. ″ ″ (vi) The Ferns 124
XIV. ″ ″ (vii) The Lycopods 133
XV. ″ ″ (viii) The Horsetails 145
XVI. ″ ″ (ix) Sphenophyllales 153
XVII. ″ ″ (x) The Lower Plants 161
XVIII. Fossil Plants as Records of Ancient Countries 168
XIX. Conclusion 174
APPENDIX
I. List of Requirements for a Collecting Expedition 183
II. Treatment of Specimens 184
III. Literature 186
Glossary 188
Footnotes 193
Index 193
ANCIENT PLANTS
CHAPTER I
INTRODUCTORY
The lore of the plants which have successively clothed this ancient earth
during the thousands of centuries before men appeared is generally
ignored or tossed on one side with a contemptuous comment on the dullness
and “dryness” of fossil botany.
It is true that all that remains of the once luxuriant vegetation are
fragments preserved in stone, fragments which often show little of beauty
or value to the untrained eye; but nevertheless these fragments can tell
a story of great interest when once we have the clue to their meaning.
The plants which lived when the world was young were not the same as
those which live to-day, yet they filled much the same place in the
economy of nature, and were as vitally important to the animals then
depending on them as are the plants which are now indispensable to man.
To-day the life of the modern plants interests many people, and even
philosophers have examined the structure of their bodies and have
pondered over the great unanswered questions of the cause and the course
of their evolution. But all the plants which are now alive are the
descendants of those which lived a few years ago, and those again came
down through generation after generation from the plants which inhabited
the world before the races of men existed. If, therefore, we wish to know
and understand the vegetation living to-day we must look into the past
histories of the families of plants, and there is no way to do this at
once so simple and so direct (in theory) as to examine the remains of the
plants which actually lived in that past. Yet when we come to do this
practically we encounter many difficulties, which have discouraged all
but enthusiasts from attempting the study hitherto, but which in reality
need not dismay us.
When Lindley and Hutton, in 1831, began to publish their classical book
_The Fossil Flora of Great Britain_, they could give but isolated
fragments of information concerning the fossils they described, and the
results of their work threw but little light on the theoretical problems
of morphology and classification of living plants. Since then great
advance has been made, and now the sum of our knowledge of the subject,
though far from complete, is so considerable and has such a far-reaching
influence that it is becoming the chief inspiration of several branches
of modern botany. Of the many workers who have contributed to this stock
of knowledge the foremost, as he was the pioneer in the investigations on
modern lines, is Williamson, who was a professor at Manchester
University, and whose monographs and specimens are classics to-day. Still
living is Dr. Scott, whose greatness is scarcely less, as well as an
ever-increasing number of specialists in this country, who are
continually making discoveries. Abroad, the chief Continental names are
Renault, Bertrand, Count Solms Laubach, Brongniart, Zeiller; and in
America is Dr. Wieland; while there are innumerable other workers in the
field who have deepened and widened the channels of information. The
literature on fossil plants is now vast; so great that to give merely the
names of the publications would fill a very large volume.
But, like the records left by the plants themselves, most of this
literature is unreadable by any but specialists, and its really vital
interest is enclosed in a petrifying medium of technicalities. It is to
give their results in a more accessible form that the present volume has
been written.
The actual plants that lived and died long ago have left either no trace
of their form and character, or but imperfect fragments of some of their
parts embedded in hard rock and often hidden deep in the earth. That such
difficulties lie in our way should not discourage us from attempting to
learn all the fossils can teach. Many an old manuscript which is torn and
partly destroyed bears a record, the fragments of which are more
interesting and important than a tale told by a complete new book. The
very difficulty of the subject of fossil botany is in itself an incentive
to study, and the obstacles to be surmounted before a view of the ancient
plants can be seen increase the fascination of the journey.
The world of to-day has been nearly explored; but the world, or rather
the innumerable world-phases of the past, lie before us practically
unknown, bewilderingly enticing in their mystery. These untrodden regions
are revealed to us only by the fossils lying scattered through the rocks
at our feet, which give us the clues to guide us along an adventurous
path.
Fables of flying dragons and wondrous sea monsters have been shown by the
students of animal fossils to be no more marvellous than were the actual
creatures which once inhabited the globe; and among the plants such
wonderful monsters have their parallels in the floras of the past. The
trees which are living to-day are very recent in comparison with the
ancestors of the families of lowlier plants, and most of the modern
forest trees have usurped a position which once belonged to the monster
members of such families as the Lycopods and Equisetums, which are now
humble and dwindling. An ancient giant of the past is seen in the
frontispiece, and the great girth of its stem offers a striking contrast
to the feeble trailing branches of its living relatives, the Club-mosses.
As we follow their histories we shall see how family after family has
risen to dominate the forest, and has in its turn given place to a
succeeding group. Some of the families that flourished long since have
living descendants of dwarfed and puny growth, others have died out
completely, so that their very existence would have been unsuspected had
it not been revealed by their broken fragments entombed in the rocks.
From the study of the fossils, also, we can discover something of the
course of the evolution of the different parts of the plant body, from
the changes it has passed through in the countless ages of its existence.
Just as the dominant animals of the past had bodies lacking in many of
the characters which are most important to the living animals, so did the
early plants differ from those around us to-day. It is the comparative
study of living and fossil structures which throws the strongest light on
the facts and factors of evolution.
When the study of fossil organisms goes into minute detail and embraces
the fine subtleties of their internal structure, then the student of
fossil plants has the advantage of the zoological observer, for in many
of the fossil plants the cells themselves are petrified with a perfection
that no fossil animal tissues have yet been found to approach. Under the
microscope the most delicate of plant cells, the patterns on their walls,
and sometimes even their nuclei can be recognized as clearly as if they
were living tissues. The value of this is immense, because the external
appearance of leaves and stems is often very deceptive, and only when
both external appearance and internal structure are known can a real
estimate of the character of the plant be made. In the following chapters
a number of photographs taken through the microscope will show some of
the cell structure from fossil plants. Such figures as fig. 11 and fig.
96, for example, illustrate the excellence of preservation which is often
found in petrified plant tissues. Indeed, the microscope becomes an
essential part of the equipment of a fossil botanist; as it is to a
student of living plants. But for those who are not intending to
specialize on the subject micro-photographs will illustrate sufficient
detail, while in most modern museums some excellently preserved specimens
are exhibited which show their structure if examined with a magnifying
glass.
We recognize to-day the effect the vegetation of a district has on its
scenery, even on its more fundamental nature; and we see how the plants
keep in close harmony with the lands and waters, the climates and soils
of the places they inhabit. So was it in the past. Hence the fossil
plants of a district will throw much light on its physical characters
during the epoch when they were living, and from their evidence it is
possible to build up a picture of the conditions of a region during the
epochs of its unwritten history.
From every point of view a student of living plants will find his
knowledge and understanding of them greatly increased by a study of the
fossils. Not only to the botanist is the subject of value, the geologist
is equally concerned with it, though from a slightly different viewpoint,
and all students of the past history of the earth will gain from it a
wider knowledge of their specialty.
To all observers of life, to all philosophers, the whole history of
plants, which only approaches completion when the fossils are studied,
and compared or contrasted with living forms, affords a wonderful
illustration of the laws of evolution on which are based most of the
modern conceptions of life. Even to those whose profession necessitates
purely practical lines of thought, fossil botany has something to teach;
the study of coal, for instance, comes within its boundaries. While to
all who think on the world at all, the story told by the fossil plants is
a chapter in the Book of Life which is as well worth reading as any in
that mystical volume.
CHAPTER II
VARIOUS KINDS OF FOSSIL PLANTS
Of the rocks which form the solid earth of to-day, a very large
proportion have been built up from the deposits at the bottom of ancient
oceans and lakes. The earth is very old, and in the course of its history
dry land and sea, mountains and valleys have been formed and again
destroyed on the same spot, and it is from the silt at the bottom of an
ocean that the hills of the future are built.
The chief key we have to the processes that were in operation in the past
is the course of events passing under our eyes to-day. Hence, if we would
understand the formation of the rocks in the ancient seas, we must go to
the shores of the modern ones and see what is taking place there. One of
the most noticeable characters of a shore is the line of flotsam that is
left by the edge of the waves; here you may find all kinds of land plants
mixed with the sea shells and general rubbish, plants that may have
drifted far. Much of the débris (outside towns) is brought down by the
rivers, and may be carried some distance out to sea; then part becomes
waterlogged and sinks, and part floats in to shore, perhaps to be carried
out again, or to be buried under the coarse sand of the beach. When we
examine sandstone rock, or the finer grained stones which are hardened
mud, we find in them the remains of shells, sometimes of bones, and also
of plant leaves and stems, which in their time had formed the flotsam of
a shore. Indeed, one may say that nearly every rock which has not been
formed in ancient volcanoes, or been altered by their heat, carries in it
_some_ trace of plant or animal. These remains are often very fragmentary
and difficult to recognize, but sometimes they are wellnigh as perfect as
dried specimens of living things. When they are recognizable as plant or
animal remains they are commonly called “fossils”, and it is from their
testimony that we must learn all we can know about the life of the past.
[Illustration: Fig. 1.—The Face of a Quarry, showing layers or “beds”
of different rock, _a_, _b_, and _c_. The top gravel and soil _s_ has
been disintegrated by the growing plants and atmosphere.]
If we would find such stones for ourselves, the quarries offer the best
hunting ground, for there several layers of rock are exposed, and we can
reach fresh surfaces which have not been decayed by rain and storm. Fig.
1 shows a diagram of a quarry, and illustrates the almost universal fact
that the beds of rock when undisturbed lie parallel to each other. Rock
_a_ in the figure is fine-grained limestone, _b_ black friable shale
mixed with sand, and _c_ purer shale. In such a series of rocks the best
fossils will be found in the limestone; its harder and finer structure
acting as a better preservative of organisms than the others. In
limestone one finds both plant and animal fossils, very often mixed
together as the flotsam on the shore is mixed. Many limestones split
along parallel planes, and may break into quite thin sheets on whose
surfaces the flattened fossils show particularly well.
It is, however, with the plant fossils that we must concern ourselves,
and among them we find great variety of form. Some are more or less
complete, and give an immediate idea of the size and appearance of the
plant to which they had belonged; but such are rare. One of the
best-known examples of this type is the base of a great tree trunk
illustrated in the frontispiece. With such a fossil there is no shadow of
doubt that it is part of a giant tree, and its spreading roots running so
far horizontally along the ground suggest the picture of a large crown of
branches. Most fossils, however, are much less illuminating, and it is
usually only by the careful piecing together of fragments that we can
obtain a mental picture of a fossil plant.
A fossil such as that illustrated in the frontispiece—and on a smaller
scale this type of preservation is one of the commonest—does not actually
consist of the plant body itself. Although from the outside it looks as
though it were a stem base covered with bark, the whole of the inner
portion is composed of fine hard rock with no trace of woody tissue. In
such specimens we have the shape, size, and form of the plant preserved,
but none of its actual structure or cells. It is, in fact, a Cast. Fossil
casts appear to have been formed by fine sand or mud silting round a
submerged stump and enclosing it as completely as if it had been set in
plaster of Paris; then the wood and soft tissue decayed and the hollow
was filled up with more fine silt; gradually all the bark also decayed
and the mud hardened into stone. Thus the stone mould round the outside
of the plant enclosed a stone casting. When, after lying for ages
undisturbed, these fossils are unearthed, they are so hard and “set” that
the surrounding stone peels away from the inner part, just as a plaster
cast comes away from an object and retains its shape. There are many
varieties of casts among fossil plants. Sometimes on breaking a rock it
will split so as to show the perfect form of the surface of a stem, while
its reverse is left on the stone as is shown in fig. 2. Had we only the
reverse we should still have been able to see the form of the leaf bases
by taking a wax impression from it; although there is nothing of the
actual tissue of the plant in such a fossil. Sometimes casts of leaf
bases show the detail preserved with wonderful sharpness, as in fig. 3.
This is an illustration of the leaf scars of _Lepidodendron_, which often
form particularly good casts.
[Illustration: Fig. 2.—A, Cast of the Surface showing the Shape of Leaf
Bases of _Sigillaria_; B, the reverse of the impression left on the
adjacent layer of rock. (Photo.)]
In other instances the cast may simply represent the internal hollows of
the plant. This happens most commonly in the case of stems which
contained soft pith cells which quickly decayed, or with naturally hollow
stems like the Horse-tails (_Equisetum_) of to-day. Fine mud or sand
silted into such hollows completely filling them up, and then, whether
the rest of the plant were preserved or not, the shape of the inside of
the stem remains as a solid stone. Where this has happened, and the outer
part of the plant has decayed so as to leave no trace, the solid plug of
stone from the centre may look very much like an actual stem itself, as
it is cylindrical and may have surface markings like those on the
outsides of stems. Some of the casts of this type were for long a puzzle
to the older fossil botanists, particularly that illustrated in fig. 4,
where the whole looks like a pile of discs.
[Illustration: Fig. 3.—Cast of the Leaf Bases of _Lepidodendron_,
showing finely marked detail. (Photo.)]
[Illustration: Fig. 4.—“_Sternbergia._” Internal cast of the stem of
_Cordaites_.]
The true nature of this fossil was recognized when casts of the plan were
found with some of the wood preserved outside the castings; and it was
then known that the plant had a hollow pith, with transverse bands of
tissue across it at intervals which caused the curious constrictions in
the cast.
[Illustration: Fig. 5.—Leaf Impressions of “Fern” _Sphenopteris_ on
Shale. (Photo.)]
Another form of cast which is common in some rocks is that of seeds. As a
rule these casts are not connected with any actually preserved tissue,
but they show the external form, or the form of the stony part of the
seed. Well-known seeds of this type are those of _Trigonocarpon_, which
has three characteristic ridges down the stone. Sometimes in the fine
sandstone in which they occur embedded, the _internal_ cast lies embedded
_in the external_ cast, and between them there is a slight space, now
empty, but which once contained the actual shell of the seed, now
decayed. Thus we may rattle the “stone” of a fossil fruit as we do the
dried nuts of to-day—the external resemblance between the living and the
fossil is very striking, but of the actual tissues of the fossil seed
nothing is left.
Casts have been of great service to the fossil botanists, for they often
give clear indications of the external appearance of the parts they
represent; particularly of stems, leaf scars, and large seeds. But all
such fossils are very imperfect records of the past plants, for none of
the actual plant tissues, no minute anatomy or cell structure, is
preserved in that way.
A type of fossil which often shows more detail, and which usually retains
something of the actual tissues of the plant, is that known technically
as the Impression. These fossils are the most attractive of all the many
kinds we have scattered through the rocks, for they often show with
marvellous perfection the most delicate and beautiful fern leaves, such
as in fig. 5. Here the plant shows up as a black silhouette against the
grey stone, and the very veins of the midrib and leaves are quite
visible.
Fig. 6 shows another fernlike leaf in an impression, not quite flat like
that shown in fig. 5, but with a slight natural curvature of the leaves
similar to what would have been their form in life. Though an impression,
this specimen is not of the “pressed plant” type, it almost might be
described as a _bas-relief_.
Sometimes impressions of fern foliage are very large, and show highly
branched and complex leaves like those of tree ferns, and they may cover
large sheets of stone. They are particularly common in the fine shales
above coal seams, and are best seen in the mines, for they are often too
big to bring to the surface complete.
In most impressions the black colour is due to a film of carbon which
represents the partly decomposed tissues of the plant. Sometimes this
film is cohesive enough to be detached from the stone without damage.
Beautiful specimens of this kind are to be seen in the Royal Scottish
Museum, Edinburgh where the coiled bud of a young fern leaf has been
separated from the rock on which it was pressed, and mounted on glass.
Such specimens might be called mummy plants, for they are the actual
plant material, but so decayed and withered that the internal cells are
no longer intact. In really well preserved ones it is sometimes possible
to peel off the plant film, and then treat it with strong chemical agents
to clear the black carbon atoms away, and mount it for microscopic
examination, when the actual outline of the epidermis cells can be seen.
[Illustration: Fig. 6.—Impression of _Neuropteris_ Leaf, showing
details of veins, the leaves in partial relief. (Photo.)]
[Illustration: Fig. 7.—Leaf Impression of _Ginkgo_, of which the film
was strong enough to peel off complete]
In fig. 7, the impression is that of a _Ginkgo_ leaf, and after treatment
the cells of the epidermis were perfectly recognizable under the
microscope, with the stomates (breathing pores) also well preserved. This
is shown in fig. 8, where the outline of the cells was drawn from the
microscope. In such specimens, however, it is only the outer skin which
is preserved, the inner soft tissue, the vital anatomy of the plant, is
crushed and carbonized.
Leaves, stems, roots, even flowers (in the more recent rocks) and seeds
may all be preserved as impressions; and very often those from the more
recently formed rocks are so sharply defined and perfect that they seem
to be actual dried leaves laid on the stone.
[Illustration: Fig. 8.—Outline of the Cells from Specimen of Leaf shown
in fig. 7
_c_, Ordinary cells; _s_, stomates; _v_, elongated cells above the
vein.]
Much evidence has been accumulated that goes to show that the rocks which
contain the best impressions were originally deposited under tranquil
conditions in water. It might have been in a pool or quiet lake with
overshadowing trees, or a landlocked inlet of the sea where silt quietly
accumulated, and as the plant fragments fell or drifted into the spot
they were covered by fine-grained mud without disturbance. In the case of
those which are very well preserved this must have taken place with
considerable rapidity, so that they were shut away from contact with the
air and from the decay which it induces.
Impressions in the thin sheets of fine rock may be compared to dried
specimens pressed between sheets of blotting paper; they are flattened,
preserved from decay, and their detailed outline is retained. Fossils of
this kind are most valuable, for they give a clear picture of the form of
the foliage, and when, as sometimes happens, large masses of leaves, or
branches with several leaves attached to them, are preserved together, it
is possible to reconstruct the plant from them. It is chiefly from such
impressions that the inspiration is drawn for those semi-imaginary
pictures of the forests of long ago. From them also are drawn many facts
of prime importance to scientists about the nature and appearance of
plants, of which the internal anatomy is known from other specimens, and
also about the connection of various parts with each other.
Sometimes isolated impressions are found in clay balls or nodules. When
the latter are split open they may show as a centre or nucleus a leaf or
cone, round which the nodule has collected. In such cases the plant is
often preserved without compression, and may show something of the minute
details of organization. The preservation, however, is generally far from
perfect when viewed from a microscopical standpoint. Fig. 9 shows one of
these smooth, clayey nodules split open, and within it the cone which
formed its centre, also split into two, and standing in high relief, with
its scales showing clearly. Similar nodules or balls of clay are found
to-day, forming in slowly running water, and it may be generally observed
that they collect round some rubbish, shell, or plant fragment. These
nodules are particularly well seen nowadays in the mouth of the Clyde,
where they are formed with great rapidity.
[Illustration: Fig. 9.—Clay Nodule split open, showing the two halves
of the cone which was its centre. (Photo.)]
Another kind of preservation is that which coats over the whole plant
surface with mineral matter, which hardens, and thus preserves the _form_
of the plant. This process can be observed going on to-day in the
neighbourhood of hot volcanic streams where the water is heavily charged
with minerals. In most cases such fossils have proved of little
importance to science, though there are some interesting specimens in the
French museums which have not yet been fully examined. A noteworthy
fossil of this type is the _Chara_, which, growing in masses together,
has sometimes been preserved in this way in large quantities, indicating
the existence of an ancient pond in the locality.
There is quite a variety of other types of preservation among fossil
plants, but they are of minor interest and importance, and hardly justify
detailed consideration. One example that should be mentioned is Amber.
This is the gum of old resinous trees, and is a well-known substance
which may rank as a “fossil”. Jet, too, is formed from plants, while coal
is so important that the whole of the next chapter will be devoted to its
consideration. Even the black lead of pencils possibly represents plants
that were once alive on this globe.
Though such remains tell us of the existence of plants at the place they
were found at a known period in the past, yet they tell very little about
the actual structure of the plants themselves, and therefore very little
that is of real use to the botanist. Fortunately, however, there are
fossils which preserve every cell of the plant tissues, each one perfect,
distended as in life, and yet replaced by stone so as to be hard and to
allow of the preparation of thin sections which can be studied with the
microscope. These are the vegetable fossils which are of prime importance
to the botanist and the scientific enquirer into the evolution of plants.
Such specimens are commonly known as Petrifactions.
Sometimes small isolated stumps of wood or branches have been completely
permeated by silica, which replaces the cell walls and completely
preserves and hardens the tissues. This silicified wood is found in a
number of different beds of rock, and may be seen washed out on the shore
in Yorkshire, Sutherland, and other places where such rocks occur. When
such a block is cut and polished the annual rings and all the fine
structure or “grain” of the wood become as apparent as in recent wood.
From these fossils, too, microscopic sections can be cut, and then the
individual wood cells can be studied almost as well as those of living
trees. A particularly notable example of fossil tree trunks is the
Tertiary forest of the Yellowstone Park. Here the petrified trunks are
weathered out and stand together much as they must have stood when alive;
they are of course bereft of their foliage branches.
Such specimens, however, are usually only isolated blocks of wood, often
fragments from large stumps which show nothing but the rings of
late-formed wood. It is impossible to connect them with the impressions
of leaves or fruits in most cases, so that of the plants they represent
we know only the anatomical structure of the secondary wood and nothing
of the foliage or general appearance of the plant as a whole. Hence these
specimens also give a very partial representation of the plants to which
they belonged.
Fortunately, however, there is still another type of preservation of
fossils, a type more perfect than any of the others and sometimes
combining the advantages of all of them. This is the special type of
petrifaction which includes, not a single piece of wood, but a whole mass
of vegetation consisting of fragments of stems, roots, leaves, and even
seeds, sometimes all together. These petrifactions are those of masses of
forest débris which were lying as they dropped from the trees, or had
drifted together as such fragments do. The plant tissues in such masses
are preserved so that the most delicate soft tissue cells are perfect,
and in many cases the sections are so distinct that one might well be
deluded into the belief that it is a living plant at which one looks.
Very important and well-known specimens have been found in France and
described by the French palæobotanists. As a rule these specimens are
preserved in silica, and are found now in irregular masses of the nature
of chert. Of still greater importance, however, owing partly to their
greater abundance and partly to the quantity of scientific work that has
been done on them, are the masses of stone found in the English coal
seams and commonly called “coal balls”.
The “coal balls” are best known from Lancashire and Yorkshire, where they
are extremely common in some of the mines, but they also occur in
Westphalia and other places on the Continent.
[Illustration: Fig. 10.—Mass of Coal with many “coal balls” embedded in
it
_a a_, In surface view; _b b_, cut across. All washed with acid to make
the coal balls show up against the black coal. (Photo by Lomax.)]
In external appearance the “coal balls” are slightly irregular roundish
masses, most generally about the size of potatoes, and black on the
outside from films of adhering coal. Their size varies greatly, and they
have been found from that of peas up to masses with a diameter of a foot
and a half. They lie embedded in the coal and are not very easily
recognizable in it at first, because they are black also, but when washed
with acid they turn greyish-white and then can be recognized clearly.
Fig. 10 shows a block of coal with an exceptionally large number of the
“coal balls” embedded in it. This figure illustrates their slightly
irregular rounded form in a typical manner. By chemical analysis they are
found to consist of a nearly pure mixture of the carbonates of lime and
magnesia; though in some specimens there is a considerable quantity of
iron sulphide, and in all there is at least 5 per cent of various
impurities and some quantity of carbon.
The important mineral compounds, CaCO_3 and MgCO_3, are mixed in very
different quantities, and even in coal balls lying quite close to each
other there is often much dissimilarity in this respect. In whatever
proportion these minerals are combined, it seems to make but little
difference to their preservative power, and in good “coal balls” they may
completely replace and petrify each individual cell of the plants in
them.
[Illustration: Fig. 11.—Photograph of Section across Stem of
_Sphenophyllum_ from a Lancashire “coal ball”, showing perfect
preservation of woody tissue
W, wood; _c_, cortex.]
Fig. 11 shows a section across the wood of a stem preserved in a “coal
ball”, and illustrates a degree of perfection which is not uncommon. In
the course of the succeeding chapters constant reference will be made to
tissues preserved in “coal balls”, and it may be noticed that not only
the relatively hard woody cells are preserved but the very softest and
youngest tissues also appear equally unharmed by their long sojourn in
the rocks.
[Illustration: Fig. 12.—Photograph of Section through a Bud of
_Lepidodendron_, showing many small leaves tightly packed round the
axis. From a “coal ball”]
The particular value of the coal balls as records of past vegetation lies
in the fact that they are petrifactions, not of individual plants alone,
but of masses of plant débris. Hence in one of these stony concretions
may lie twigs with leaves attached, bits of stems with their fruits, and
fine rootlets growing through the mass. A careful study and comparison of
these fragments has led to the connection, piece by piece, of the various
parts of many plants. Such a specimen as that figured in fig. 12 shows
how the soft tissues of young leaves are preserved, and how their
relation to each other and to the axis is indicated.
Hitherto the only concretions of the nature of “coal balls” containing
well preserved plant débris, have been found in the coal or immediately
above it, and are of Palæozoic age (see p. 34). Recent exploration,
however, has resulted in the discovery of similar concretions of Mesozoic
age, from which much may be hoped in the future. Still, at present, it is
to the palæozoic specimens we must turn for nearly all valuable knowledge
about ancient plants, and primarily to that form of preservation of the
specimens known as structural petrifactions, of which the “coal balls”
are both the commonest and the most perfect examples.
CHAPTER III
COAL, THE MOST IMPORTANT OF PLANT REMAINS
Some of the many forms which are taken by fossil plants were shortly
described in the last chapter, but the most important of all, namely
coal, must now be considered. Of the fossils hitherto mentioned many are
difficult to recognize without examining them very closely, and one might
say that all have but little influence on human life, for they are of
little practical or commercial use, and their scientific value is not yet
very widely known. Of all fossil plants, the great exception is coal. Its
commercial importance all over the world needs no illustration, and its
appearance needs no description for it is in use in nearly every
household. Quite apart from its economic importance, coal has a unique
place among fossils in the eyes of the scientist, and is of special
interest to the palæontologist.
In England nearly all the coal lies in rocks of a great age, belonging to
a period very remote in the world’s history. The rocks bearing the coal
contain other fossils, principally those of marine animals, which are
characteristic of them and of the period during which they were formed,
which is generally known as the “Coal Measure period”. There is
geological proof that at one time the coal seams were much more widely
spread over England than they are at present; they have been broken up
and destroyed in the course of ages, by the natural movements among the
rocks and by the many changes and processes of disintegration and decay
which have gone on ever since they were deposited. To-day there are but
relatively small coal-bearing areas, which have been preserved in the
hollows of the synclines.[2]
The seams of coal are extremely numerous, and even the same seam may vary
greatly in thickness. From a quarter of an inch to five or six feet is
the commonest thickness for coal in this country, but there are many beds
abroad of very much greater size. Thin seams often lie irregularly in
coarse sandstone; for example, they may be commonly seen in the Millstone
Grit; but typical coal seams are found embedded between rocks of a more
or less definite character known as the “roof” and “floor”.
[Illustration: Fig. 13.—Diagram of a Series of Parallel Coal Seams with
Underclays and Shale Roofs of varying thicknesses]
Basalts, granites, and such rocks do not contain coal; the coal measures
in which the seams of coal occur are, generally speaking, limestones,
fine sandstones, and shales, that is to say, rocks which in their origin
were deposited under water. In detail almost every seam has some
individual peculiarity, but the following represents two types of typical
seams. In many cases, below the coal, the limestone or sandstone rocks
give place to fine, yellow-coloured layers of clay, which varies from a
few inches to many feet in thickness and is called the “underclay”. This
fine clay is generally free from pebbles and coarse débris of all kinds,
and is often supposed to be the soil in which the plants forming the coal
had been growing. The line of demarcation between the coal and the clay
is usually very sharp, and the compact black layers of hard coal stop
almost as abruptly on the upper side and give place to a shale or
limestone “roof”; see fig. 13, layers 5, 6, and 7. Very frequently a
number of small seams come together, lying parallel, and sometimes
succeeding each other so rapidly that the “roof” is eliminated, and a
clay floor followed by a coal seam, is succeeded immediately by another
clay floor and another coal seam, as in fig. 13, layers 10, 11, and 12.
The relative thickness of these beds also varies very greatly, and over
an underclay of seven or eight feet the coal seam may only reach a couple
of inches, while a thick seam may have a floor of very slight dimensions.
These relations depend on such a variety of local circumstances from the
day they were forming, that it is only possible to unravel the causes
when an individual case is closely studied. The main sequence, however,
is constant and is that illustrated in fig. 13.
The second type of seam is that in which the underclay floor is not
present, and is replaced either by shales or by a special very hard rock
of a finely granular nature called “gannister”. In the gannister floor it
is usual to find traces of rootlets and basal stumps of plants, which
seem to indicate that the gannister was the ground in which the plants
forming the coal were rooted. The coal itself is generally very pure
plant remains, though between its layers are often found bands of shaly
stone which are called “dirt bands”. These are particularly noticeable in
thick seams, and they may be looked on as corresponding to the roof
shales; as though, in fact, the roof had started to form but had only
reached a slight development when the coal formation began again.
[Illustration: Fig. 14.—Diagram of Coal Seam with Gannister Floor, in
which are traces of rootlets _r_, and of stumps of root-like organs _s_]
That the coal is strikingly different from the rocks in which it lies is
very obvious, but that alone is no indication of its origin. It is now so
universally known and accepted that coal is the remains of vegetables
that no proofs are usually offered for the statement. It is, however, of
both interest and importance to marshal the evidence for this belief. The
grounds for recognizing coal as consisting of practically pure plant
remains are many and various, so that only the more important of them
will be considered now. The most direct suggestion lies in the
impressions of leaves and stems which are found between its layers; this,
however, is confronted by the parallel case of plant impressions found in
shales and limestones which are not of vegetable origin, so that it might
be argued that those plants in the coal drifted in as did those in the
limestone. But when we examine the black impressions on limestone or
sandstone, an item of value is noticeable; it is often possible to peel
off a film, lying between the upper and lower impression, of black coaly
substance, sometimes an eighth of an inch thick, and hard and shining
like coal. This follows the outline of the plant form of the impression,
and it is certain that this minute “coal seam” was formed from the plant
tissues. It is, in fact, a coal seam bearing the clearest possible
evidence of its plant nature. We have only to imagine this multiplied by
many plants lying tightly packed together, with no mineral impurities
between, to see that it would yield a coal seam like those we find
actually existing.
In some cases in the coal itself a certain amount of the structure of the
plants which formed it remains, though usually, in the process of their
decay the tissues have entirely decomposed, and left only their
carbonized elements. Chemical analysis reveals that, beyond the
percentage of mineral ash which is found in living plants, there is
little in a pure sample of coal that is not carbonaceous. All the
deposits of carbon found in any form in nature can be traced to some
animal or vegetable remains, so that it is logical to assume that coal
also arose from either animal or plant débris. But were coal of an animal
origin, the amount of mineral matter in it would be much larger as well
as being of a different nature; for almost all animals have skeletons,
even the simplest single-celled protozoa often own calcareous shells,
sponges have siliceous spicules, molluscs hard shells, and the higher
animals bones and teeth. These things are of a very permanent nature, and
would certainly be found in quantities in the coal had animals formed it.
Further, the peat of to-day, which collects in thick compact masses of
vegetable, shows how plants may form a material consisting of carbonized
remains. By certain experiments in which peat was subjected to pressure
and heat, practically normal coal was made from it.
[Illustration: Fig. 15.—Part of a Coal Ball, showing the concentric
bandings in it which are characteristic of concretions]
[Illustration: Fig. 16.—Mass of Coal with Coal Balls, A and B both
enclosing part of the same stem L]
Still a further witness may be found in the structure of the “coal balls”
described in the last chapter. These stony masses, lying in the pure
coal, might well be considered as apart from it and bearing no relation
to its structure; but recent work has shown that they were actually
formed at the same time as the coal, developing in its mass as mineral
concretions round some of the plants in the soft, saturated, peaty mass
which was to be hardened into coal later on.[3] All “coal balls” do not
show their concretionary structure so clearly, but sometimes it can be
seen that they are made with concentric bands or markings like those
characteristic of ordinary mineral concretions (see fig. 15). Concretions
are formed by the crystallization of minerals round some centre, and it
must have happened that in the coal seams in which the coal-ball
concretions are found that this process took place in the soft plant mass
before it hardened. Recent research has found that there is good evidence
that those seams[4] resulted from the slow accumulation of plant débris
under the salt or brackish water in whose swamps the plants were growing,
and that as they were collecting the ground slowly sank till they were
quite below the level of the sea and were covered by marine silt. At the
same time some of the minerals present in the sea water, which must have
saturated the mass, crystallized partly and deposited themselves round
centres in the plant tissues, and by enclosing them and penetrating them
preserved them from decay till the mineral structure entirely replaced
the cells, molecule by molecule. Evidence is not wanting that this
process went on without disturbance, for in fig. 16 is shown a mass of
coal in which lie several coal balls, two of which enclose parts of the
same plant. This means that round different centres in the same stem two
of the concretions were forming and preserving the tissues; the two stone
masses, however, did not enlarge enough to unite, but left a part of the
tissue unmineralized, which is now seen as a streak of coal. We have here
the most important proof that the coal balls are actually formed in the
coal and of the plants making the coal, for had those coal balls come in
as pebbles, or in any way from the outside into the coal, they could not
have remained in such a position as to lie side by side enclosing part of
the same stem. There are many other details which may be used in this
proof, but this one illustration serves to show the importance of coal
balls when dealing with the theories of the origin of coal, for they are
perfectly preserved samples of what the whole coal mass was at one time.
There are but few seams, however, which contain coal balls, and about
those in which they do not occur our knowledge is very scanty. It is
often assumed that the plant impressions in the shales above the coal
seams can be taken as fair samples of those which formed the coal itself;
but this has been recently shown to be a fallacious argument in some
cases, so that it is impossible to rely on it in general. The truth is,
that though coal is one of the most studied of all the geological
deposits, we are still profoundly ignorant of the details of its
formation except in a few cases.
The way in which coal seams were formed has been described often and
variously, and for many years there were heated discussions between the
upholders of the different views as to the merits of their various
theories. It is now certain that there must have been at least four
principal ways in which coal was formed, and the different seams are
illustrations of the products of different methods. In all cases more or
less water is required, for coal is what is known as a sedimentary
deposit, that is, one which collects under water, like the fine mud and
silt and débris in a lake. It will be understood, however, that if the
plant remains were collecting at any spot, and the water brought in sand
and mud as well, then the deposit could not have resulted in pure coal,
but would have been a sandy mixture with many plant remains, and would
have resulted in the formation of a rock, such as parts of the millstone
grit, where there are many streaks of coal through the stone.
Among various coal seams, evidence for the following modes of coal
formation can be found:—
(_a_) _In fresh water._—In still freshwater lakes or pools, with
overhanging plants growing on the banks, twigs and leaves which fell or
were blown into the water became waterlogged and sank to the bottom. With
a luxuriant growth of plants rapidly collecting under water, and there
preserved from contact with the air and its decaying influence, enough
plant remains would collect to form a seam. After that some change in the
local conditions took place, and other deposits covered the plants and
began the accumulations which finally pressed the vegetable mass into
coal.
To freshwater lakes of large size plants might also have been brought by
rivers and streams; they would have become waterlogged in time, after
floating farther than the sand and stones with which they came, and would
thus settle and form a deposit practically free from anything but plant
remains.
(_b_) _As peat._—Peat commonly forms on our heather moors and bogs to-day
to a considerable thickness. This also took place long ago in all
probability, and when the level of the land altered it would have been
covered by other deposits, pressed, and finally changed into coal.
(_c_) _In salt or brackish water, growing in situ._—Trees and undergrowth
growing thickly together in a salt or brackish marsh supplied a large
quantity of débris which fell into the mud or water below them, and were
thus shut off from the air and partly preserved. When conditions favoured
the formation of a coal seam the land level was slowly sinking, and so,
though the débris collected in large quantities, it was always kept just
beneath the water level. Finally the land sank more rapidly, till the
vegetable mass was quite under sea water, then mud was deposited over it,
and the materials which were afterwards hardened to form the roof rocks
were deposited. This was the case in those seams in which “coal balls”
occur, and the evidence of the sea water covering the coal soon after it
was deposited lies in the numerous sea shells found in the roof
immediately above it.
(_d_) _In salt water, drifted material._—Tree trunks and large tangled
masses of vegetation drifted out to sea by the rivers just as they do
to-day. These became waterlogged, and finally sank some distance from the
shore. (Those sinking near the shore would not form pure coal, for sand
and mud would be mixed with them, also brought down by rivers and stirred
up from the bottom by waves.) The currents would bring numbers of such
plants to the same area until a large mass was deposited on the sea
floor. Finally the local conditions would have changed, the currents then
bringing mud or sand, which covered the vegetable mass and formed the
mineral roof of the resulting coal seam. There is a variety of what might
be called the “drifted coals”, which appears to have been formed of
nothing but the _spores_ of plants of a resinous nature. These structures
must have been very light, and possibly floated a long distance before
sinking.
If we could but obtain enough evidence to understand each case fully we
should probably find that every coal seam represents some slightly
different mode of formation, that in each case there was some local
peculiarity in the plants themselves and the way they accumulated in
coal-forming masses, but the above four methods will be found to cover
the principal ways in which coal has arisen.
Coal, as we now know it, has a great variety of qualities. The
differences probably depend only to a small extent on the varieties among
the plants forming it, and are almost entirely due to the many later
conditions which have affected the coal after its original formation.
Some such conditions are the various upheavals and depressions to which
the rocks containing the coal have been subjected, the weight of the beds
lying over the coal seams, and the high temperatures to which they may
have been subjected when lying under a considerable depth of
later-deposited rocks. The influence on the coal of these and many other
physical factors has been enormous, but they are purely cosmical and
belong to the special realm of geological study, and so cannot be
considered in detail now.
To return to our special subject, namely, the plants themselves which are
now preserved in the coal. Their nature and appearance, their affinities
and minute structure, can only be ascertained by a detailed study, to
which the following chapters will be devoted, though in their limited
space but an outline sketch of the subject can be drawn.
It has been stated by some writers that in the Coal Measure period plants
were more numerous and luxuriant than they ever were before or ever have
been since. This view could only have been brought forward by one who was
considering the geology of England alone, and in any case there appears
to be very little real evidence for such a view. Certainly in Europe a
large proportion of the coal is of this age, and to supply the enormous
masses of vegetation it represents a great growth of plants must have
existed. But it is evident that just at the Carboniferous period in what
is now called Europe the physical conditions of the land which roughly
corresponded to the present Continent were such as favoured the
accumulation of plants, and the gradual sinking of the land level also
favoured their preservation under rapidly succeeding deposits. Of the
countless plants growing in Europe to-day very few stand any chance of
being preserved as coal for the future; so that, unless the physical
conditions were suitable, plants might have been growing in great
quantity at any given period without ever forming coal. But now that the
geology of the whole world is becoming better known, it is found that
coal is by no means specially confined to the Coal Measure age. Even in
Europe coals of a much later date are worked, while abroad, especially in
Asia and Australia, the later coals are very important. For example, in
Japan, seams of coal 14, 20, and even more feet in thickness are worked
which belong to the Tertiary period (see p. 34), while in Manchuria coal
100 feet thick is reported of the same age. When these facts are
considered it is soon found that all the statements made about the unique
vegetative luxuriance of the Coal Measure period are founded either on
insufficient evidence or on no evidence at all.
The plants forming the later coals must have had in their own structure
much that differed from those forming the old coals of Britain, and the
gradual change in the character of the vegetation in the course of the
succeeding ages is a point of first-rate importance and interest which
will be considered shortly in the next chapter.
CHAPTER IV
THE SEVEN AGES OF PLANT LIFE
Life has played its important part on the earth for countless series of
years, of the length of whose periods no one has any exact knowledge.
Many guesses have been made, and many scientific theories have been used
to estimate their duration, but they remain inscrutable. When numbers are
immense they cease to hold any meaning for us, for the human mind cannot
comprehend the significance of vast numbers, of immense space, or of æons
of time. Hence when we look back on the history of the world we cannot
attempt to give even approximate dates for its events, and the best we
can do is to speak only of great periods as units whose relative position
and whose relative duration we can estimate to some extent.
Those who have studied geology, which is the science of the world’s
history since its beginning, have given names to the great epochs and to
their chief subdivisions. With the smaller periods and the subdivisions
of the greater ones we will not concern ourselves, for our study of the
plants it will suffice if we recognize the main sequence of past time.
The main divisions are practically universal, and evidence of their
existence and of the character of the creatures living in them can be
found all over the world; the smaller divisions, however, may often be
local, or only of value in one continent. To the specialist even the
smallest of them is of importance, and is a link in the chain of evidence
with which he cannot dispense; but we are at present concerned only with
the broad outlines of the history of the plants of these periods, so will
not trouble ourselves with unnecessary details.[5] Corresponding to
certain marked changes in the character of the vegetation, we find seven
important divisions of geological time which we will take as our unit
periods, and which are tabulated as follows:—
Cainozoic
I. Present Day.
II. Tertiary.
Mesozoic
III. Upper Cretaceous (or Chalk).
IV. The rest of the Mesozoic.
V. Newer Palæozoic, including
Permian.
Carboniferous.
Devonian.
Palæozoic
VI. Older Palæozoic.
Eozoic
VII. Archæan.
Now the actual length of these various periods was very different. The
epoch of the Present Day is only in its commencement, and is like a thin
line if compared with the broad bands of the past epochs. By far the
greatest of the periods is the Archæan, and even the Older Palæozoic is
probably longer than all the others taken together. It is, however, so
remote, and the rocks which were formed in it retain so little plant
structure that is decipherable, so few specimens which are more than mere
fragments, that we know very little about it from the point of view of
the plant life of the time. It includes the immense indefinite epochs
when plants began to evolve, and the later ones when animals of many
kinds flourished, and when plants, too, were of great size and
importance, though we are ignorant of their structure. Of all the seven
divisions of time, we can say least about the two earliest, simply for
want of anything to say which is founded on fact rather than on
theoretical conclusions.
Although these periods seem clearly marked off from one another when
looked at from a great distance, they are, of course, but arbitrary
divisions of one long, continuous series of slow changes. It is not in
the way of nature to make an abrupt change and suddenly shut off one
period—be it a day or an æon—from another, and just as the seasons glide
almost imperceptibly into one another, so did the great periods of the
past. Thus, though there is a strong and very evident contrast between
the plants typical of the Carboniferous period and of the Mesozoic, those
of the Permian are to some extent intermediate, and between the beginning
of the Permian and the end of the Carboniferous—if judged by the flora—it
is often hard to decide.
It must be realized that almost any given spot of land—the north of
England, for example—has been beneath the sea, and again elevated into
the air, at least more than once. That the hard rocks which make its
present-day hills have been built up from the silt and débris under an
ocean, and after being formed have seen daylight on a land surface long
ago, and sunk again to be covered by newer deposits, perhaps even a
second or a third time, before they rose for the time that is the
present. Yet all these profound changes took place so slowly that had we
been living then we could have felt no motion, just as we feel no motion
to-day, though the land is continuing to change all around us. The great
alternations between land and water over large areas mark out to some
extent the main periods tabulated on p. 34, for after each great
submersion the rising land seems to have harboured plants and animals
with somewhat different characters from those which inhabited it before.
Similarly, when the next submersion laid down more rocks of limestone and
sandstone, they enclosed the shells of some creatures different from
those which had inhabited the seas of the region previously.
Through all the periods the actual rocks formed are very similar—shales,
limestones, sandstones, clays. When any rocks happen to have preserved
neither plant nor animal remains it is almost impossible to tell to which
epoch they belong, except from a comparative study of their position as
regards other rocks which do retain fossils. This depends on the fact
that the physical processes of rock building have gone on throughout the
history of the globe on very much the same lines as they are following at
present. By the sifting power of water, fine mud, sand, pebbles, and
other débris are separated from each other and collected in masses like
to like. The fine mud will harden into shales, sandgrains massed together
harden into sandstones, and so on, and when, after being raised once more
to form dry land, they are broken up by wind and rain and brought down
again to the sea, they settle out once again in a similar way and form
new shales and sandstones; and so on indefinitely. But meantime the
living things, both plant and animal, have been changing, growing,
evolving, and the leafy twig brought down with the sandgrains in the
flooded river of one epoch differs from that brought down by the river of
a succeeding epoch—though it might chance that the sandgrains were the
same identical ones. And hence it is by the remains of the plants and
animals in a rock that we can tell to which epoch it belonged. Unless, of
course, ready-formed fossils from an earlier epoch get mixed with it,
coming as pebbles in the river in flood—but that is a subtle point of
geological importance which we cannot consider here. Such cases are
almost always recognizable, and do not affect the main proposition.
From the various epochs, the plants which have been preserved as fossils
are in nearly all cases those which had lived on the land, or at least on
swamps and marshes by the land. Of water plants in the wide sense,
including both those growing in fresh water and those in the sea, we have
comparatively few. This lack is particularly remarkable in the case of
the seaweeds, because they were actually growing in the very medium in
which the bulk of the rocks were formed, and which we know from recent
experiments acts as a preservative for the tissues of land plants
submerged in it. It must be remembered, however, that almost all the
plants growing in water have very soft tissues, and are usually of small
size and delicate structure as compared with land plants, and thus would
stand less chance of being preserved, and would also stand less chance of
being recognized to-day were they preserved. The mark on a stone of the
impression of a soft film of a waterweed would be very slight as compared
with that left by a leathery leaf or the woody twig of a land plant.
There are, of course, exceptions, and, as will be noted later on (see
Chapter XVII), there are fossil seaweeds and fossil freshwater plants,
but we may take it on the whole that the fossils we shall have to deal
with and that give important evidence, are those of the land which had
drifted out to sea, in the many cases when they are found in rocks
together with sea shells.
Let us now consider very shortly the salient features of the seven epochs
we have named as the chief divisions of time. The vegetation of the
Carboniferous Period is better known to us than that of any other period
except that of the present day, so that it will form the best
starting-point for our consideration.
At this period there were, as there are to-day, oceans and continents,
high lands, low lands, rivers and lakes, in fact, all the physical
features of the present-day world, but they were all in different places
from those of to-day. If we confine our attention to Britain, we find
that at that period the far north, Scotland, Wales, and Charnwood were
higher land, but the bulk of the southern area was covered by flat swamps
or shallow inlets, where the land level gradually changed, slowly sinking
in one place and slowly rising in others, which later began also to sink.
Growing on this area wherever they could get a foothold were many plants,
all different from any now living. Among them none bore flowers. A few
families bore seeds in a peculiar way, differing widely from most
seed-bearing plants of to-day. The most prevalent type of tree was that
of which a stump is represented in the frontispiece, and of which there
were many different species. These plants, though in size and some other
ways similar to the great trees of to-day, were fundamentally different
from them, and belonged to a very primitive family, of which but few and
small representatives now exist, namely the Lycopods. Many other great
trees were like hugely magnified “horsetails” or Equisetums; and there
were also seed-bearing Gymnosperms of a type now extinct. There were
ferns of many kinds, of which the principal ones belong to quite extinct
families, as well as several other plants which have no parallel among
living ones. Hence one may judge that the vegetation was rich and
various, and that, as there were tall trees with seeds, the plants were
already very highly evolved. Indeed, except for the highest group of all,
the flowering plants, practically all the main groups now known were
represented. The flora of the Devonian was very similar in essentials.
If that be so, it may seem unsatisfactory to place all the preceding æons
under one heading, the Older Palæozoic. And, indeed, it is very
unsatisfactory to be forced to do so. We know from the study of animal
fossils that this time was vast, and that there were several well-defined
periods in it during which many groups of animals evolved, and became
extinct after reaching their highest development; but of the plants we
know so little that we cannot make any divisions of time which would be
of real value in helping us to understand them.
Fossil plants from the Early Palæozoic there are, but extremely few as
compared with the succeeding period, and those few but little
illuminative. In the later divisions of the Pre-Carboniferous some of the
plants seem to belong to the same genera as those of the Carboniferous
period. There is a fern which is characteristic of one of the earlier
divisions, and there are several rather indefinite impressions which may
be considered as seaweeds. There is evidence also that even one of the
higher groups bearing seeds (the _Cordaiteæ_) was in full swing long
before the Carboniferous period began. Hence, though of Older Palæozoic
plants we know little of actual fact, we can surmise the salient truths;
viz., that in that period those plants must have been evolving which were
important in the Devonian and Carboniferous periods; that in the earlier
part of that period they did not exist, and the simpler types only
clothed the earth; and that further back still, even the simpler types
had not yet evolved.
Names have been given to many fragmentary bits of fossils, but for
practical purposes we might as well be without them. For the present the
actual plants of the Older Palæozoic must remain in a misty obscurity,
their forms we can imagine, but not know.
On the other hand, of the more recent periods, those succeeding the
Carboniferous, we have a little more knowledge. Yet for all these
periods, even the Tertiary immediately preceding the present day, our
knowledge is far less exact and far less detailed than it is for that
unique period, the Carboniferous itself.
The characteristic plants of the Carboniferous period are all very
different from those of the present, and every plant of that date is now
extinct. In the succeeding periods the main types of vegetation changed,
and with each succeeding change advanced a step towards the stage now
reached.
The Permian, geologically speaking, was a period of transition. Toward
the close of the Carboniferous there were many important earth movements
which raised the level of the land and tended to enclose the area of
water in what is now Eastern Europe, and to make a continental area with
inland seas. Many of the Carboniferous genera are found to extend through
the Permian and then die out, while at the same time others became quite
extinct as the physical conditions changed. The seed-bearing plants
became relatively more important, and though the genus _Cordaites_ died
out at the end of the period it was succeeded by an increasing number of
others of more advanced type.
When we come to the older Mesozoic rocks, we have in England at any rate
an area which was slowly submerging again. The more important of the
plants which are preserved, and they are unfortunately all too few, are
of a type which has not yet appeared in the earlier rocks, and are in
some ways like the living _Cycas_, though they have many characters
fundamentally different from any living type. In the vegetation of this
time, plants of Cycad-like appearance seem to have largely predominated,
and may certainly be taken as the characteristic feature of the period.
The great Lycopod and Equisetum-like trees of the Carboniferous are
represented now only by smaller individuals of the same groups, and
practically all the genera which were flourishing in the Carboniferous
times have become extinct.
The Cycad-like plants, however, were far more numerous and varied in
character and widely spread than they ever were in any succeeding time.
Still, no flowers (as we understand the word to-day) had appeared, or at
least we have no indication in any fossil hitherto discovered, that true
flowers were evolved until towards the end of the period (see, however,
Chapter X).
The newer Mesozoic or Upper Cretaceous period represents a relatively
deep sea area over England, and the rocks then formed are now known as
the chalk, which was all deposited under an ocean of some size whose
water must have been clear, and on the whole free from ordinary débris,
for the chalk is a remarkably homogeneous deposit. From the point of view
of plant history, the Upper Mesozoic is notable, because in it the
flowering plants take a suddenly important position. Beds of this age
(though of very different physical nature) are known all over the world,
and in them impressions of leaves and fruits, or their casts, are well
represented. The leaves are those of both Monocotyledons and
Dicotyledons, and the genera are usually directly comparable with those
now living, and sometimes so similar that they appear to belong to the
same genus. The cone-bearing groups of the Gymnosperms are still present
and are represented by a number of forms, but they are far fewer in
varieties than are the groups of flowering plants—while the Cycad-like
plants, so important in the Lower Mesozoic, have relatively few
representatives. There is, it almost seems, a sudden jump from the
flowerless type of vegetation of the Lower Mesozoic, to a flora in the
Upper Mesozoic which is strikingly like that of the present day.
The Tertiary period is a short one (geologically speaking, and compared
with those going before it), and during it the land level rose again
gradually, suffering many great series of earth movements which built
most of the mountain chains in Europe which are standing to the present
day. In the many plant-containing deposits of this age, we find specimens
indicating that the flora was very similar to the plants now living, and
that flowering plants held the dominant position in the forests, as they
do to-day. In fact, from the point of view of plant evolution, it is
almost an arbitrary and unnecessary distinction to separate the Tertiary
epoch from the present, because the main features of the vegetation are
so similar. There are, however, such important differences in the
distribution of the plants of the Tertiary and those of the present
times, that the distinction is advisable; but it must always be
remembered that it is not comparable with the wide differences between
the other epochs.
Among the plants now living we find representatives of most, though not
of all, of the great _groups_ of plants which have flourished in the
past, though in the course of time all the species have altered and those
of the earliest earth periods have become extinct. The relative
importance of the different groups changes greatly in the various
periods, and as we proceed through the ages of time we see the dominant
place in the plant world held successively by increasingly advanced
types, while the plants which dominated earlier epochs dwindle and take a
subordinate position. For example, the great trees of the Carboniferous
period belonged to the Lycopod family, which to-day are represented by
small herbs creeping along the ground. The Cycad-like plants of the
Mesozoic, which grew in such luxuriance and in such variety, are now
restricted to a small number of types scattered over the world in
isolated localities.
During all the periods of which we have any knowledge there existed a
rich and luxuriant vegetation composed of trees, large ferns, and small
herbs of various kinds, but the members of this vegetation have changed
fundamentally with the changing earth, and unlike the earth in her
rock-forming they have never repeated themselves.
CHAPTER V
STAGES IN PLANT EVOLUTION
To attempt any discussion of the _causes_ of evolution is far beyond the
scope of the present work. At present we must accept life as we find it,
endowed with an endless capacity for change and a continuous impulse to
advance. We can but study in some degree the _course_ taken by its
changes.
From the most primitive beginnings of the earliest periods, enormous
advance had been made before we have any detailed records of the forms.
Yet there remain in the world of to-day numerous places where the types
with the simplest structure can still flourish, and successfully compete
with higher forms. Many places which, from the point of view of the
higher plants, are undesirable, are well suited to the lower. Such
places, for example, as the sea, and on land the small nooks and crannies
where water drops collect, which are useless for the higher plants,
suffice for the minute forms. In some cases the lower plants may grow in
such masses together as to capture a district and keep the higher plants
from it. Equisetum (the horsetail) does this by means of an extensive
system of underground rhizomes which give the plant a very strong hold on
a piece of land which favours it, so that the flowering plants may be
quite kept from growing there.
In such places, by a variety of means, plants are now flourishing on the
earth which represent practically all the main stages of development of
plant life as a whole. It is to the study of the simpler of the living
forms that we owe most of our conceptions of the course taken by
evolution. Had we to depend on fossil evidence alone, we should be in
almost complete ignorance of the earliest types of vegetation and all the
simpler cohorts of plants, because their minute size and very delicate
structure have always rendered them unsuitable for preservation in stone.
At the same time, had we none of the knowledge of the numerous fossil
forms which we now possess, there would be great gaps in the series which
no study of living forms could supply. It is only by a study and
comparison of both living and fossil plants of all kinds and from beds of
all ages that we can get any true conception of the whole scheme of plant
life.
Grouping together all the main families of plants at present known to us
to exist or to have existed, we get the following series:—
Group. Common examples of typical families in the group.
Thallophyta
Algæ Seaweeds.
Fungi Moulds and toadstools.
Bryophyta
Hepaticæ Liverworts.
Musci Mosses.
Pteridophyta
Equisetales Horsetails.
Sphenophyllales* fossil only, _Sphenophyllum_.
Lycopodales Club-moss.
Filicales Bracken fern.
Pteridospermæ
Lyginodendræ* fossil only, _Sphenopteris_.
Gymnosperms
Cycadales Cycads.
Bennettitales* fossil only, _Bennettites_.
Ginkgoales Maidenhair Tree.
Cordaitales* fossil only, _Cordaites_.
Coniferales Pine, Yew.
Gnetales Welwitschia.
Angiosperms
Monocotyledons Lily, Palm, Grass.
Dicotyledons Rose, Oak, Daisy.
In this table the different groups have not a strictly equivalent
scientific value, but each of those in the second column represents a
large and well-defined series of primary importance, whose members could
not possibly be included along with any of the other groups.
Those marked with an asterisk are known only as fossils, and it will be
seen that of the seventeen groups, so many as four are known only in the
fossil state. This indicates, however, but a part of their importance,
for in nearly every other group are many families or genera which are
only known as fossils, though there are living representatives of the
group as a whole.
In this table the individual families are not mentioned, because for the
present we need only the main outline of classification to illustrate the
principal facts about the course of evolution. As the table is given, the
simplest families come first, the succeeding ones gradually increasing in
complexity till the last group represents the most advanced type with
which we are acquainted, and the one which is the dominant group of the
present day.
This must not be taken as a suggestion that the members of this series
have evolved directly one from the other in the order in which they stand
in the table. That is indeed far from the case, and the relations between
the groups are highly complex.
It must be remarked here that it is often difficult, even impossible, to
decide which are the most highly evolved members of any group of plants.
Each individual of the higher families is a very complicated organism
consisting of many parts, each of which has evolved more or less
independently of the others in response to some special quality of the
surroundings. For instance, one plant may require, and therefore evolve,
a very complex and well-developed water-carriage system while retaining a
simple type of flower; another may grow where the water problem does not
trouble it, but where it needs to develop special methods for getting its
ovules pollinated; and so on, in infinite variety. As a result of this,
in almost all plants we have some organs highly evolved and specialized,
and others still in a primitive or relatively primitive condition. It is
only possible to determine the relative positions of plants on the scale
of development by making an average conclusion from the study of the
details of all their parts. This, however, is beset with difficulties,
and in most cases the scientist, weighed by personal inclinations,
arbitrarily decides on one or other character to which he pays much
attention as a criterion, while another scientist tends to lay stress on
different characters which may point in another direction.
In no group is this better illustrated than among the Coniferæ, where the
relative arrangement of the different families included in it is still
very uncertain, and where the observations of different workers, each
dealing mainly with different characters in the plants, tend to
contradict each other.
This, however, as a byword. Notwithstanding these difficulties, which it
would be unfair to ignore, the main scheme of evolution stands out
clearly before the scientist of to-day, and his views are largely
supported by many important facts from both fossil and living plants.
Very strong evidence points to the conclusion that the most primitive
plants of early time were, like the simplest plants of to-day, water
dwellers. Whether in fresh water or the sea is an undecided point, though
opinion seems to incline in general to the view that the sea was the
first home of plant life. It can, however, be equally well, and perhaps
even more successfully argued, that the freshwater lakes and streams were
the homes of the first families from which the higher plants have
gradually been evolved.
For this there is no direct evidence in the rocks, for the minute forms
of the single soft cells assumed by the most primitive types were just
such as one could not expect to be successfully fossilized. Hence the
earliest stages must be deduced from a comparative study of the simplest
plants now living. Fortunately there is much material for this in the
numerous waters of the earth, where swarms of minute types in many stages
of complexity are to be found.
The simplest type of plants now living, which appears to be capable of
evolution on lines which might have led to the higher plants, is that
found in various members of the group of the Protococcoideæ among the
Algæ. The claim of bacteria and other primitive organisms of various
kinds to the absolute priority of existence is one which is entirely
beyond the scope of a book dealing with fossil plants. The early
evolution of the simple types of the Protococcoideæ is also somewhat
beyond its scope, but as they appear to lie on the most direct “line of
descent” of the majority of the higher plants it cannot be entirely
ignored. From the simpler groups of the green Algæ other types have
specialized and advanced along various directions, but among them there
seems an inherent limitation, and none but the protococcoid forms seem to
indicate the possibility of really high development.
[Illustration: Fig. 17.—A Protococcoid Plant consisting of one cell
_p_, Protoplasm; _n_, nucleus; _g_, colouring body or chloroplast; _w_,
cell wall.]
In a few words, a typical example of one of the simple Protococcoideæ may
be described as consisting of a mass of protoplasm in which lie a
recognizable nucleus and a green colouring body or chloroplast, with a
cell wall or skin surrounding these vital structures, a cell wall that
may at times be dispensed with or unusually thickened according as the
need arises. This plant is represented in fig. 17 in a somewhat
diagrammatic form.
In such a case the whole plant consists of one single cell, living
surrounded by the water, which supplies it with the necessary food
materials, and also protects it from drying up and from immediate contact
with any hard or injurious object. When these plants propagate they
divide into four parts, each one similar to the original cell, which all
remain together within the main cell wall for a short time before they
separate.
If now we imagine that the four cells do not separate, but remain
together permanently, we can see the possibility of a beginning of
specialization in the different parts of the cell. The single living cell
is equally acted on from all sides, and in itself it must perform all the
life functions; but where four lie together, each of the four cells is no
longer equally acted on from all sides. This shows clearly in the diagram
of a divided cell given in fig. 18. Here it is obvious that one side of
each of the four cells, viz. that named _a_ in the diagram, is on the
outside and in direct contact with the water and external things; but
walls _b_ and _c_ touch only the corresponding walls of the neighbouring
cells. Through walls _b_ and _c_ no food and water can enter directly,
but at the same time they are protected from injury and external
stimulus. Hence, in this group of four cells there is a slight
differentiation of the sides of the cells. If now we imagine that each of
the four cells, still remaining in contact, divides once more into four
members, each of which reaches mature size while all remain together,
then we have a group of sixteen cells, some of which will be entirely
inside, and some of which will have walls exposed to the environment.
[Illustration: Fig. 18.—Diagram of Protococcoid Cell divided into four
daughter cells. Walls _a_ are external, and walls _b_ and _c_ in
contact with each other.]
If the cells of the group all divide again, in the manner shown the mass
will become more than one cell thick, and the inner cells will be more
completely differentiated, for they will be entirely cut off from the
outside and all direct contact with water and food materials, and will
depend on what the outer cells transmit to them. The outer cells will
become specialized for protection and also for the absorption of the
water and salts and air for the whole mass. From such a plastic group of
green cells it is probable that the higher and increasingly complex forms
of plants have evolved. There are still living plants which correspond
with the groups of four, sixteen, &c., cells just now theoretically
stipulated.
[Illustration: Fig. 19.—A, Details of Part of the Tissues in a Stem of
a Flowering Plant. B, Diagram of the Whole Arrangement of Cross Section
of a Stem: _e_, Outer protecting skin; _g_, green cells; _s_,
thick-walled strengthening cells; _p_, general ground tissue cells. V,
Groups of special conducting tissues: _x_, vessels for water carriage;
_px_, first formed of the water vessels; _c_, growing cells to add to
the tissues; _b_, food-conducting cells; _ss_, strengthening cells.]
The higher plants of to-day all consist of very large numbers of cells
forming tissues of different kinds, each of which is specialized more or
less, some very elaborately, for the performance of certain functions of
importance for the plant body as a whole. With the increase in the number
of cells forming the solid plant body, the number of those living wholly
cut off from the outside becomes increasingly great in comparison with
those forming the external layer. Some idea of the complexity and
differentiation of this cell mass is given in fig. 19, A, which shows the
relative sizes and shapes of the cells composing a small part of the stem
of a common flowering plant. The complete section would be circular and
the groups V would be repeated round it symmetrically, and the whole
would be enclosed by an unbroken layer of the cells marked _e_, as in the
diagram B.
[Illustration: Fig. 20.—Conducting Cells and Surrounding Tissue seen in
fig. 19, A, cut lengthways. _px_, First formed vessels for water
conduction; _x_, larger vessel; _b_, food-conducting cells; _ss_,
strengthening cells; _p_, general ground tissue.]
In the tissues of the higher plants the most important feature is the
complex system of conducting tissues, shown in the young condition in V
in fig. 19, A. In them the food and water conducting elements are very
much elongated and highly specialized cells, which run between the others
much like a system of pipes in the brickwork of a house. These cells are
shown cut longitudinally in fig. 20, where they are lettered to
correspond with the cells in fig. 19, A, with which they should be
compared. In such a view the great difference between the highly
specialized cells _x_, _px_, _b_, &c., and those of the main mass of
ground tissue _p_ becomes apparent.
Even in the comparatively simply organized groups of the Equisetales and
Lycopodiales the differentiation of tissues is complete. In the mosses,
and still more in the liverworts, it is rudimentary; but they grow in
very damp situations, where the conduction of water and the protection
from too much drying is not a difficult problem for them. As plants grow
higher into the air, or inhabit drier situations, the need of
specialization of tissues becomes increasingly great, for they are
increasingly liable to be dried, and therefore need a better flow of
water and a more perfect protective coat.
It is needless to point out how the individual cells of a plant, such as
that figured in figs. 19 and 20, have specialized away from the simple
type of the protococcoid cell in their mature form. In the young growing
parts of a plant, however, they are essentially like protococcoid cells
of squarish outline, fitting closely to each other to make a solid mass,
from which the individual types will differentiate later and take on the
form suitable for the special part they have to play in the economy of
the whole plant.
To trace the specialization not only of the tissues but of the various
parts of the whole plant which have become elaborate organs, such as
leaves, stems, and flowers, is a task quite beyond the present work to
attempt. From the illustrations given of tissue structure from plants at
the two ends of the series much can be imagined of the inevitable
intermediate stages in tissue evolution.
As regards the elaboration of organs, and particularly of the
reproductive organs, details will be found throughout the book. In
judging of the place of any plant in the scale of evolution it is to the
reproductive organs that we look for the principal criteria, for the
reproductive organs tend to be influenced less by their physical
surroundings than the vegetative organs, and are therefore truer guides
to natural relationships.
In the essential cells of the reproductive organs, viz. the egg cell and
the male cell, we get the most primitively organized cells in the plant
body. In the simpler families both male and female cells return to the
condition of a free-swimming protococcoid cell, and in all but the
highest families the male cell requires a liquid environment, in which it
_swims_ to the egg cell. In the higher families the necessary water is
provided within the structure of the seed, and the male cell does not
swim, a naked, solitary cell, out into the wide world, as it does in all
the families up to and including the Filicales. In the Coniferæ and
Angiosperms the male cell does not swim, but is passive (or largely so),
and is brought to the egg cell. One might almost say that the whole
evolution of the complex structures found in fruiting cones and flowers
is a result of the need of protection of the delicate, simple
reproductive cells and the embryonic tissues resulting from their fusion.
The lower plants scatter these delicate cells broadcast in enormous
numbers, the higher plants protect each single egg cell by an elaborate
series of tissues, and actually bring the male cell to it without ever
allowing either of them to be exposed.
It must be assumed that the reader possesses a general acquaintance with
the living families tabulated on p. 44; those of the fossil groups will
be given in some detail in succeeding chapters which deal with the
histories of the various families. It is premature to attempt any general
discussion of the evolution of the various groups till all have been
studied, so that this will be reserved for the concluding chapters.
CHAPTER VI
MINUTE STRUCTURE OF FOSSIL PLANTS—LIKENESSES TO LIVING ONES
The individual plants of the Coal Measure period differed entirely from
those now living; they were more than merely distinct species, for in the
main even the families were largely different from the present ones.
Nevertheless, when we come to examine the minute anatomy of the fossils,
and the cells of which they are composed, we find that between the living
and the fossil cell types the closest similarity exists.
From the earliest times of which we have any knowledge the elements of
the plant body have been the same, though the types of structures which
they built have varied in plan. Individual _cells_ of nearly every type
from the Coal Measure period can be identically matched with those of
to-day. In the way the walls thickened, in the shapes of the wood,
strengthening or epidermal cells, in the form of the various tissues
adapted to specific purposes, there is a unity of organization which it
is reasonable to suppose depends on the fundamental qualities inherent in
plant life.
This will be illustrated best, perhaps, by tabulating the chief
modifications of cells which are found in plant tissues. The
illustrations of these types in the following table are taken from living
plants, because from them figures of more diagrammatic clearness can be
made, and the salient characters of the cells more easily recognized.
Comparison of these typical cells with those illustrated from the fossil
plants reveals their identity in essential structure, and most of them
will be found in the photos of fossils in these pages, though they are
better recognized in the actual fossils themselves.
Principal Types of Plant Cells
Epidermal.
[Illustration: Fig. 21
_Epidermis._—Protecting layer or skin. Cells with outer wall thickened
in many cases (fig. 21, _a_ and _b_). Compare fossil epidermis in fig.
34, _e_.]
[Illustration: Fig. 22 _Hairs._—Extensions of epidermis cells. Single
cells, or complex, as fig. 22, _h_, where _e_ is epidermis and _p_
parenchyma. Compare fossil hairs in figs. 79 and 120.]
[Illustration: Fig. 23
_Stomates._—Breathing pores in the epidermis. Seen in surface view as
two-lipped structures (fig. 23). _s_, Stomates; _e_, epidermis cells.
Compare fossil stomates in fig. 8.]
Ground Tissue
[Illustration: Fig. 24
_Parenchyma._—Simple soft cells, either closely packed, as in fig. 24,
or with air spaces between them. Compare 78, B, for fossil.]
[Illustration: Fig. 25
_Palisade._—Elongated, closely packed cells, _p_, chiefly in leaves,
lying below the epidermis, _e_, fig. 25. Compare fig. 34, _p_, for
fossil palisade.]
[Illustration: Fig. 26
_Endodermis._—Cells with specially thickened walls, _en_, lying as
sheath between the parenchyma, _c_, of ground tissue, and the vascular
tissue, _s_, fig. 26. Compare fig. 108 for fossil endodermis.]
[Illustration: Fig. 27
_Latex cells._—Large, often much elongated cells, _m_, lying in the
parenchyma, _p_, fig. 27, which are packed with contents. Compare fig.
107, _s_.]
[Illustration: Fig. 28
_Sclerenchyma._—Thick-walled cells among parenchyma for strengthening,
fig. 28. Compare fig. 34, _s_.]
[Illustration: Fig. 29
_Cork._—Layers of cells replacing the epidermis in old stems. Outer
cells, _o_, crushed; _k_, closely packed cork cells; stone cells, _s_,
fig. 29. Compare fig. 95, _k_.
_Cork cambium._—Narrow, actively dividing cells, _c_ in fig. 29, giving
rise to new cork cells in consecutive rows.]
[Illustration: Fig. 30
_Tracheides._—Specially thickened cells in the parenchyma, usually for
water storage, _t_, fig. 30. Compare fig. 95, _t_.]
Vascular Tissue
[Illustration: Fig. 31
_Wood._—_Protoxylem_, tracheids and vessels, long, narrow elements,
with spiral or ring-like thickenings, _s_^1 and _s_^2, fig. 31. Compare
fig. 81, A, _px_, for fossil.
_Metaxylem_, long elements, tracheids and vessels. Some with narrow
pits, as _t_ in fig. 31; others with various kinds of pits. In
transverse section seen in fig. 33, w, fossil in fig. 114, _w_.
_Wood parenchyma._—Soft cells associated with the wood, _p_ in fig. 31.
Fossil in fig. 81, B, _p_.
_Wood sclerenchyma._—Hard thickened cells in the wood.]
[Illustration: Fig. 32
_Bast._—_Sieve tubes_, long cells which carry foodstuffs, cross walls
pitted like sieves, _s_, fig. 32. In transverse section in fig. 33.
_Companion cells_, narrow cells with rich proteid contents, _c_, fig.
32. In transverse section at _c_, fig. 33.
_Bast parenchyma._—Soft unspecialized cells mixed with the sieve tubes,
_p_, fig. 32.
_Bast fibres._—Thick-walled sclerenchymatous cells mixed with, or
outside, the soft bast.]
[Illustration: Fig. 33
_Cambium._—Narrow cells, like those of the cork cambium, which lie
between the wood and bast, and give rise to new tissues of each kind,
_cb_, fig. 33. Compare fig. 114, fossil.]
There are, of course, many minor varieties of cells, but these illustrate
all the main types.
Among the early fossils, however, one type of wood cell and one type of
bast cell, so far as we know, are not present. These cells are the true
_vessels_ of the wood of flowering plants, and the long bast cells with
their companion proteid cells. The figure of a metaxylem wood cell, shown
in fig. 31, _t_, shows the more primitive type of wood cell, which has an
oblique cross wall. This type of wood cell is found in all the fossil
trees, and all the living plants except the flowering plants. The vessel
type, which is that in the big wood vessels of the flowering plants, and
has no cross wall, is seen in fig. 20, _x_.
The similarity between the living cells and those of the Coal Measure
fossils is sufficiently illustrated to need no further comment. This
similarity is an extremely helpful point when we come to an
interpretation of the fossils. In living plants we can study the
physiology of the various kinds of cells, and can deduce from experiment
exactly the part they play in the economy of the whole plant. From a
study of the tissues in any plant structure we know what function it
performed, and can very often estimate the nature of the surrounding
conditions under which the plant was growing. To take a single example,
the palisade tissue, illustrated in fig. 25, _p_, in living plants always
contains green colouring matter, and lies just below the epidermis,
usually of leaves, but sometimes also of green stems. These cells do most
of the starch manufacture for the plant, and are found best developed
when exposed to a good light. In very shady places the leaves seldom have
this type of cell. Now, when cells just like these are found in fossils
(as is illustrated in fig. 34), we can assume all the physiological facts
mentioned above, and rest assured that that leaf was growing under normal
conditions of light and was actively engaged in starch-building when it
was alive. From the physiological standpoint the fossil leaf is entirely
the same as a normal living one.
[Illustration: Fig. 34.—From a Photo of a Fossil Lea
_e_, Epidermis; _p_, palisade cells; _pr_, soft parenchyma cells
(poorly preserved); _s_, sclerenchyma above the vascular bundle.]
From the morphological standpoint, also, the features of the plant body
from the Coal Measure period fall into the same divisions as those of the
present. Roots, stems, leaves, and reproductive organs, the essentially
distinct parts of a plant, are to be found in a form entirely
recognizable, or sufficiently like that now in vogue to be interpreted
without great difficulty. In the detailed structure of the reproductive
organs more changes have taken place than in any others, both in internal
organization and external appearance.
Already, in the Early Palæozoic period, the distinction between leaves,
stems, roots, and reproductive organs was as clearly marked as it is
to-day, and, judging by their structure, they must each have performed
the physiological functions they now do. Roots have changed least in the
course of time, probably because, in the earth, they live under
comparatively uniform conditions in whatever period of the world’s
history they are growing. Naturally, between the roots of different
species there are slight differences; but the likeness between fern roots
from the Palæozoic and from a living fern is absolutely complete. This is
illustrated in fig. 35, which shows the microscopic structure of the two
roots when cut in transverse direction. The various tissues will be
recognized as coming into the table on p. 54, so that both in the details
of individual cells and in the general arrangement of the cell groups or
tissues the roots of these fossil and living ferns agree.
[Illustration: Fig. 35.—A, Root of Living Fern. B, Root of Palæozoic
Fossil Fern. _c_, Cortex; _px_, protoxylem in two groups; _m_,
metaxylem; _s_, space in fossil due to decay of soft cells.]
Among stems there has been at all periods more variety than among the
roots of the corresponding plants, and in the following chapter, when the
differences between living and fossil plants will be considered, there
will be several important structures to notice. Nevertheless, there are
very many characters in which the stems from such widely different epochs
agree. The plants in the palæozoic forests were of many kinds, and among
them were those with weak trailing stems which climbed over and supported
themselves on other plants, and also tall, sturdy shafts of woody trees,
many of which were covered with a corky bark. Leaves were attached to the
stems, either directly, as in the case of some living plants, or by leaf
stalks. In external appearance and in general function the stems then
were as stems are now. In the details of the individual cells also the
likeness is complete; it is in the grouping of the cells, the anatomy of
the tissues, that the important differences lie. It has been remarked
already that increase in complexity of the plant form usually goes with
an increase in complexity of the cells and variety of the tissues. The
general ground tissue in nearly all plants is very similar; it is
principally in the vascular system that the advance and variety lie.
Plant anatomists lay particular stress on the vascular system, which, in
comparison with animal anatomy, holds an even more important position
than does the skeleton. To understand the essential characters of stems,
both living and fossil, and to appreciate their points of likeness or
difference, it is necessary to have some knowledge of the general facts
of anatomy; hence the main points on which stress is laid will be given
now in brief outline.
Leaving aside consideration of the more rudimentary and less defined
structure of the algæ and mosses, all plants may be said to possess a
“vascular system”. This is typically composed of elongated wood (or
xylem) with accessory cells (see p. 57, table), and bast (phloem), also
with accessory cells. These specialized conducting elements lie in the
ground tissue, and in nearly all cases are cut off from direct contact
with it by a definite sheath, called the endodermis (see p. 55, fig. 26).
Very often there are also groups or rings of hard thick-walled cells
associated with the vascular tissues, which protect them and play an
important part in the consolidation of the whole stem.
[Illustration: Fig. 36.—Diagram of Simplest Arrangement of Complete
Stele in a Stem
W, Central solid wood; P, ring of bast; E, enclosing sheath of
endodermis; C, ground tissue or cortex.]
The simplest, and probably evolutionally the most primitive form which is
taken by the vascular tissues, is that of a single central strand, with
the wood in the middle, the bast round it, and a circular endodermis
enclosing all, as in fig. 36, which shows a diagram of this arrangement.
Such a mass of wood and bast surrounded by an endodermis, is technically
known as a _stele_, a very convenient term which is much used by
anatomists. In its simplest form (as in fig. 36) it is called a
_protostele_, and is to be found in both living and fossil plants. A
number of plants which get more complex steles later on, have protosteles
in the early stages of their development, as in _Pteris aurita_ for
example, a species allied to the bracken fern, which has a hollow ring
stele when mature.
[Illustration: Fig. 37.—Diagram of a Stele with a few Cells of Pith _p_
in the Middle of the Wood. Lettering as in fig. 36]
[Illustration: Fig. 38.—Diagram showing Extensive Pith _p_ in the Wood.
Lettering as in fig. 36]
The next type of stele is quite similar to the protostele, but with the
addition of a few large unspecialized cells in the middle of the wood
(_p_, fig. 37); these are the commencement of the hollowing process which
goes on in the wood, resulting later in the formation of a considerable
pith, as is seen in fig. 38, where the wood is now a hollow cylinder, as
the phloem has been from the first. When this is the case, a second
sheath or endodermis generally develops on the inner side of the wood,
outside the pith, and cuts the vascular tissues off from the inner
parenchyma. A further step is the development of an inner cylinder of
bast so that the vascular ring is completely double, with endodermis on
both sides of the cylinder, as is seen in fig. 39.
[Illustration: Fig. 39.—A Cylindrical Stele, with _e_, inner
endodermis, and _ph_, inner phloem; W, wood; P, outer phloem; E, outer
endodermis. L, part of the stele going out to supply a large leaf, thus
breaking what would otherwise appear as a closed ring stele]
In all these cases there is but one strand or cylinder, of vascular
tissue in the stem, but one stele, and this type of anatomy is known as
the _monostelic_ or single-steled type.
[Illustration: Fig. 40.—A Ring Stele apparently broken up into a Number
of Protosteles by many Leaf Gaps]
When from the double cylinder just described a strand of tissue goes off
to supply a large leaf, a considerable part of the stele goes out and
breaks the ring. This is shown in fig. 39, where L is the part of the
stele going to a leaf, and the rest the broken central cylinder. When the
stem is short, and leaves grow thickly so that bundles are constantly
going out from the main cylinder, this gets permanently broken, and its
appearance when cut across at any given point is that of a group of
several steles arranged in a ring, each separate stele being like the
simple protostele in its structure. See fig. 40. This type of stem has
long been known as _polystelic_ (_i.e._ many-steled), and it is still a
convenient term to describe it by. There has been much theoretical
discussion about the true meaning of such a “polystelic” stem, which
cannot be entered into here; it may be noted, however, that the various
strands of the broken ring join up and form a meshwork when we consider
the stem as a whole, it is only in a single section that they appear as
quite independent protosteles. Nevertheless, as we generally consider the
anatomy of stems in terms of single sections, and as the descriptive word
“polystelic” is a very convenient and widely understood term, it will be
used throughout the book when speaking of this type of stem anatomy.
Such a type as this, shown in fig. 40, is already complex, but it often
happens that the steles branch and divide still further, until there is a
highly complicated and sometimes bewildering system of vascular strands
running through the ground tissue in many directions, but cut off from it
by their protective endodermal sheaths. Such complex systems are to be
found both in living and fossil plants, more especially in many of the
larger ferns (see fig. 88).
Higher plants in general, however, and in particular flowering plants, do
not have a polystelic vascular arrangement, but a specialized type of
monostele.
[Illustration: Fig. 41.—Monostele in which the Central Pith is
Star-shaped, and the Wood breaking up into Separate Groups
_p_, Pith; W, wood; P, phloem; E, endodermis; C, cortex.]
Referring again to fig. 37 as a starting-point, imagine the pith in the
centre to spread in a star-shaped form till the points of the star
touched the edges of the ring, and thus to break the wood ring into
groups. A stage in this process (which is not yet completed) is shown in
fig. 41, while in fig. 42 the wood and bast groups are entirely distinct.
In the flowering plants the cells of the endodermis are frequently poorly
characterized, and the pith cells resemble those of the cortical ground
tissue, so that the separate groups of wood and bast (usually known as
“vascular bundles”, in distinction from the “steles” of fig. 40) appear
to lie independently in the ground tissue. These strands, however, must
not be confused with steles, they are only fragments of the single
apparently broken up stele which runs in the stem.
[Illustration: Fig. 42.—Monostele in which the Pith has invaded all the
Tissues as far as the Endodermis, and broken the Wood and Phloem up
into Separate Bundles. These are usually called “vascular bundles” in
the flowering plants]
[Illustration: Fig. 43.—Showing actively growing Zone _c_ (Cambium) in
the Vascular Bundles, and joining across the ground tissue between them]
The vascular bundle, of all except the Monocotyledons, has a potentiality
for continued growth and expansion which places it far above the stele in
value for a plant of long life and considerable growth. The cells lying
between the wood and the bast, the soft parenchyma cells always
accompanying such tissues, retain their vitality and continue to divide
with great regularity, and to give rise to a continuous succession of new
cells of wood on the one side and bast on the other; see fig. 33, _c_,
_b_. In this way the primary, distinct vascular bundles are joined by a
ring of wood, see fig. 43, to which are added further rings every season,
till the mass of wood becomes a strong solid shaft. This ever-recurring
activity of the cambium gives rise to what are known as “annual rings” in
stems, see fig. 44, in which the wood shows both primary distinct groups
in the centre, and the rings of growth of later years.
Cambium with this power of long-continued activity is found in nearly all
the higher plants of to-day (except the Monocotyledons), but in the fern
and lycopod groups it is in abeyance. Certain cases from nearly every
family of the Pteridophytes are known, where some slight development of
cambium with its secondary thickening takes place, but in the groups
below the Gymnosperms cambium has almost no part to play. On the other
hand, so far back as the Carboniferous period, the masses of wood in the
Pteridophyte trees were formed by cambium in just the same way as they
are now in the higher forms. Its presence was almost universal at that
time in the lower groups where to-day there are hardly any traces of it
to be found.
[Illustration: Fig. 44.—Stem with Solid Cylinder of Wood developed from
the Cambium, showing three “annual rings”. In the centre may still be
seen the separate groups of the wood of the primary “vascular bundles”]
It will be seen from this short outline of the vascular system of plants,
that there is much variety possible from modifications of the fundamental
protostele. It is also to be noted that the plants of the Coal Measures
had already evolved all the main varieties of steles which are known to
us even now,[6] and that the development of secondary thickening was very
widespread. In several cases the complexity of type exceeds that of
modern plants (see Chap. VII), and there are to be found vascular
arrangements no longer extant.
When we turn to the _Reproductive Organs_, we find that the points of
likeness between the living and the fossil forms are not so numerous or
so direct as they are in the case of the vegetative system.
[Illustration: Fig. 45.—Fern Sporangia
A, fossil; B, living.]
As has been indicated, the families of plants typical of the Coal
Measures were not those which are the most prominent to-day, but belonged
to the lower series of Pteridophytes. In their simpler forms the
fructifications then and now resemble each other very closely, but in the
more elaborate developments the points of variety are more striking, so
that they will be dealt with in the following chapter. Cases of likeness
are seen in the sporangia of ferns, some of which appear to have been
practically identical with those now living. This is illustrated in fig.
45, which shows the outline of the cells of the sporangia of living and
fossil side by side.
[Illustration: Fig. 46.—A, Living Lycopod cone; B, _Lepidodendron_
(fossil) cone. _a_, Axis; _s_, scale; S, sporangium with spores. One
side of a longitudinal section]
In the general structure also of the cones of the simpler types of
_Lepidodendron_ (fossil, see frontispiece) there is a close agreement
with the living Lycopods, though as regards size and output of spores
there was a considerable difference in favour of the fossils. The plan of
each is that round the axis of the cone simple scales are arranged, on
each of which, on its upper side, is seated a large sporangium bearing
numerous spores all of one kind (see fig. 46).
Equally similar are the cones of the living Equisetum and some of the
simple members of the fossil family Calamiteæ, but the more interesting
cases are those where differences of an important morphological nature
are to be seen.
As regards the second[7] generation there is some very important
evidence, from extremely young stages, which has recently been given to
the world. In a fern sporangium _germinating spores_ were fossilized so
as to show the first divisions of the spore cell. These seem to be
identical with the first divisions of some recent ferns (see fig. 47).
This is not only of interest as showing the close similarity in detail
between plants of such widely different ages, but is a remarkable case of
delicate preservation of soft and most perishable structures in the “coal
balls”.
[Illustration: Fig. 47.—Germinating Fern Spores
A and B, from carboniferous fossils; C, living fern. (A and B after
Scott.)]
While these few cases illustrate points of likeness between the
fructifications of the Coal Measures and of to-day, the large size and
successful character of the primitive Coal Measure plants was accompanied
by many developments on the part of their reproductive organs which are
no longer seen in living forms, and the greater number of palæozoic
fructifications must be considered in the next chapter.
CHAPTER VII
MINUTE STRUCTURE OF FOSSIL PLANTS—DIFFERENCES FROM LIVING ONES
We have seen in the last chapter that the main morphological divisions,
roots, stems, leaves, and fructifications, were as distinct in the Coal
Measure period as they are now. There is one structure, however, found in
the Coal Measure fossils, which is hardly paralleled by anything similar
in the living plants, and that is the fossil known as _Stigmaria_.
_Stigmaria_ is the name given, not to a distinct species of plant, but to
the large rootlike organs which we know to have belonged to all the
species of _Lepidodendron_ and of _Sigillaria_. In the frontispiece these
organs are well seen, and branch away at the foot of the trunk, spreading
horizontally, to all appearance merely large roots. They are especially
regularly developed, however, the main trunk giving rise always to four
primary branches, these each dividing into two equal branches, and so
on—in this they are unlike the usual roots of trees. They bore numerous
rootlets, of which we know the structure very well, as they are the
commonest of all fossils, but in their internal anatomy the main “roots”
had not the structure which is characteristic of roots, but were like
_stems_. In living plants there are many examples of stems which run
underground, but they always have at least the rudiments of leaves in the
form of scales, while the fossil structures have apparently no trace of
even the smallest scales, but bear only rootlets, thus resembling true
roots. The questions of morphology these structures raise are too complex
to be discussed here, and Stigmaria is only introduced as an example, one
of the very few available, of a palæozoic structure which seems to be of
a nature not clearly determinable as either root, stem, leaf, or
fructification. Among living plants the fine rootlike rhizophores of
Selaginella bear some resemblance to Stigmaria in essentials, though so
widely different from them in many ways, and they are probably the
closest analogy to be found among the plants of to-day.
The individual cells, we have already seen, are strikingly similar in the
case of fossil and living plants. There are, of course, specific
varieties peculiar to the fossils, of which perhaps the most striking
seem to be some forms of _hair_ cells. For example, in a species of fern
from the French rocks there were multicellular hairs which looked like
little stems of Equisetum owing to regular bands of teeth at the
junctions of the cells. These hairs were quite characteristic of the
species—but hairs of all sorts have always abounded in variety, so that
such distinction has but minor significance.
[Illustration: Fig. 48.—Stele of _Lepidodendron_ W, surrounded by a
small ring of secondary wood S]
As was noted in the table (p. 58) the only cell types of prime importance
which were not evolved by the Palæozoic plants were the wood vessels,
phloem and accompanying cells which are characteristic of the flowering
plants.
Among the fossils the vascular arrangements are most interesting, and, as
well as all the types of stele development noted in the previous chapter
as common to both living and fossil plants, there are further varieties
found only among the fossils (see fig. 50).
The simple protostele described (on p. 61) is still found, particularly
in the very young stages of living ferns, but it is a type of vascular
arrangement which is not common in the mature plants of the present day.
In the Coal Measure period, however, the protostele was characteristic of
one of the two main groups of ferns. In different species of these ferns,
the protostele assumed a large variety of shapes and forms as well as the
simple cylindrical type. The central mass of wood became five-rayed in
some, star-shaped, and even very deeply lobed, with slightly irregular
arms, but in all these cases it remained fundamentally monostelic.
Frequently secondary tissue developed round the protosteles of plants
whose living relatives have no such tissue. A case of this kind is
illustrated in fig. 48, which shows a simple circular stele surrounded by
a zone of secondary woody tissue in a species of _Lepidodendron_.
[Illustration: Fig. 49.—_Lepidodendron_, showing Part of the Hollow
Ring of Primary Wood W, with a relatively large amount of Secondary
Tissue S, surrounding it]
In many species of _Lepidodendron_ the quantity of secondary wood formed
round the primary stele was very great, so that (as is the case in higher
plants) the primary wood became relatively insignificant compared with
it. In most species of _Lepidodendron_ the primary stele is a hollow ring
of wood (cf. fig. 38, p. 62) round which the secondary wood developed, as
is seen in fig. 49. These two cases illustrate a peculiarity of fossil
plants. Among living ones the solid and the simple ring stele are almost
confined to the Pteridophytes, where secondary wood does not develop, but
the palæozoic Pteridophytes, while having the simple primary types of
steles, had quantities of secondary tissue, which was correlated with
their large size and dominant position.
[Illustration: Fig. 50.—Diagram of Steles of the English _Medullosa_,
showing three irregular, solid, steles A, with secondary thickenings S,
all round each. _a_, Small accessory steles]
Among _polystelic_ types (see p. 63) we find interesting examples in the
fossil group of the _Medulloseæ_, which are much more complex than any
known at present, both owing to their primary structure and also to the
peculiar fact that all the steles developed secondary tissue towards the
inner as well as the outer side. One of the simpler members of this
family found in the English Coal Measures is illustrated in fig. 50. Here
there are three principal protosteles (and several irregular minor ones)
each of which has a considerable quantity of secondary tissue all round
it, so that a portion of the secondary wood is growing in towards the
actual centre of the stem as a whole—a very anomalous state of affairs.
In the more complex Continental type of _Medullosa_ there are _very_
large numbers of steles. In the one figured from the Continent in fig. 51
but a few are represented. There is a large outer double-ring stele, with
secondary wood on both sides of it, and within these a number of small
steles, all scattered through the ground tissue, and each surrounded by
secondary wood. In actual specimens the number of these central steles is
much greater than that indicated in the diagram.
No plant exists to-day which has such an arrangement of its vascular
cylinder. It almost appears as though at the early period, when the
Medulloseæ flourished, steles were experimenting in various directions.
Such types as are illustrated in figs. 50 and 51 are obviously wasteful
(for secondary wood developing towards the centre of a stem is bound to
finally meet), and complex, but apparently inefficient, which may partly
account for the fact that this type of structure has not survived to the
present, though simpler and equally ancient types have done so.
[Illustration: Fig. 51.—Continental _Medullosa_, showing R, outer
double-ring stele with secondary wood all round it; S, inner stellate
steles, also surrounded in each case by secondary tissue]
Further details of the anatomy of fossils will be mentioned when we come
to consider the individual families; those now illustrated suffice to
show that in the Coal Measures very different arrangements of steles were
to be found, as well as those which were similar to those existing now.
The significance of these differences will become apparent when their
relation to the other characters of the plants is considered.
The fructifications, always the most important parts of the plant, offer
a wide field, and the divergence between the commoner palæozoic and
recent types seems at first to be very great. Indeed, when palæozoic
reproductive bodies have to be described, it is often necessary to use
the common descriptive terms in an altered and wider sense.
Among the plants of to-day there are many varieties of the simple
single-celled reproductive masses which are called _spores_, and which
are usually formed in large numbers inside a spore case or sporangium.
Among the higher plants _seeds_ are also known in endless variety, all of
which, compared with spores, are very complex, for they are many-celled
structures, consisting essentially of an embryo or young plant enclosed
in various protective coats. The distinction between the two is sharp and
well defined, and for the student of living plants there exists no
difficulty in separating and describing seeds and spores.
But when we look back through the past eras to palæozoic plants the
subject is not so easy, and the two main types of potentially
reproductive masses are not sharply distinct. The seed, as we know it
among recent plants, and as it is generally defined, had not fully
evolved; while the spores were of great variety and had evolved in
several directions, some of which seem to have been intermediate stages
between simple spores and true seeds. These seedlike spores served to
reproduce the plants of the period, but their type has since died out and
left but two main methods among living plants, namely the essentially
simple spores, the very simplicity of whose organization gives them a
secure position, and the complex seeds with their infinite variety of
methods for protecting and scattering the young embryos they contain.
Among the Coal Measure fossils we can pick up some of the early stages in
the evolution of the seed from the spore, or at least we can examine
intermediate stages between them which give some idea of the possible
course of events. Hence, though the differences from our modern
reproductive structures are so noticeable a feature of the palæozoic
ones, it will be seen that they are really such differences as exist
between the members at the two ends of a series, not such as exist
between unrelated objects.
Very few types can be mentioned here, and to make their relations clear a
short series of diagrams with explanations will be found more helpful
than a detailed account of the structures.
[Illustration: Fig. 52.—Spores
Each spore a single cell which develops with three others in tetrads
(groups of four). Very numerous tetrads enclosed in a spore case or
sporangium which develops on a leaflike segment called the sporophyll.
Each spore germinates independently of the others after being
scattered, all being of the same size. Common in fossils and living
Pteridophytes.]
[Illustration: Fig. 53.—Spores
Each a single cell like the preceding, but here only one tetrad in a
sporangium ripens, so that each contains only four spores. Compared
with the preceding types these spores are very large. Otherwise details
similar to above. Some fossils have such sporangia with eight spores,
or some other small number; living Selaginellas have four. In the same
cone sporangia with small spores are developed and give rise to the
male organs.]
[Illustration: Fig. 54.—“Spores” of Seedlike Structure
Out of a tetrad in each sporangium only one spore ripens, S in figure,
the others, _s_, abort. The wall of the sporangium, _w_, is more
massive than in the preceding cases, and from the sporophyll, flaps,
_sp f_, grow up on each side and enclose and protect the sporangium.
The one big spore appears to germinate inside these protective coats,
and not to be scattered separately from them. Only found in fossils,
one of the methods of reproduction in _Lepidodendron_. Other sporangia
with small spores were developed which gave rise to the male organs.]
[Illustration: Fig. 55.—“Seed”
In appearance this is like a seed, but differs from a true seed in
having no embryo, and is like the preceding structure in having a very
large spore, S, though there is no trace of the three aborting ones.
The spore develops in a special mass of tissue known as the nucellus,
_n_, which partly corresponds to the sporangium wall of the previous
types. In it a cavity, _p c_, the pollen chamber, receives the pollen
grains which enter at the apex of the “seed”. There is a complex coat,
C, which stands round the nucellus but is not joined to it, leaving the
space _l_ between them. Only in fossils; _Trigonocarpus_ (see p. 122)
is similarly organized. Small spores in fern-like sporangia, called
pollen grains.]
[Illustration: Fig. 56.—“Seed”
Very similarly organized to the above, but the coat is joined to the
nucellus about two-thirds of its extent, and up to the level _l_. In
the pollen chamber, _p c_, a cone of nucellar tissue projects, and the
upper part of the coat is fluted, but these complexities are not of
primary importance. The large spore S germinated and was fertilized
within the “seed”, but apparently produced no embryo before it ripened.
Small “spores” in fern-like sporangia form the pollen grains. Only in
fossils, _e.g._ Lagenostoma. (See p. 119.)]
[Illustration: Fig. 57.—Seed
Essentially similar to the preceding, except in the possession of an
embryo _e_, which is, however, small in comparison with the endosperm
which fills the spore S. The whole organization is simpler than in the
fossil _Lagenostoma_, but the coat is fused to the nucellus further up
(see _l_). Small “spores” form the pollen grains. Living and fossil
type, Cycads and Ginkgo.]
[Illustration: Fig. 58.—Seed
In the ripe seed the large embryo _e_ practically fills up all the
space within the two seed coats _c^1_ and _c^2_; endosperm, pollen
chamber, &c., have been eliminated, and the young ovule is very simple
and small as a result of the protection and active service of the
carpels in which it is enclosed. Small “spores” form the pollen grains.
Typical of living Dicotyledons.]
These few illustrations represent only the main divisions of an army of
structures with an almost unimaginable wealth of variety which must be
left out of consideration.
For the structures illustrated in figs. 54, 55, and 56 we have no name,
for their possible existence was not conceived of when our terminology
was invented, and no one has yet christened them anew with distinct
names. They are evidently too complex in organization and too similar to
seeds in several ways to be called spores, yet they lack the essential
element in a seed, namely, an embryo. The term “ovule” (usually given to
the young seed which has not yet developed an embryo) does not fit them
any better, for their tissues are ripened and hard, and they were of
large size and apparently fully grown and mature.
For the present a name is not essential; the one thing that is important
is to recognize their intermediate character and the light they throw on
the possible evolution of modern seeds.
A further point of great interest is the manner in which these “seeds”
were borne on the plant. To-day seeds are always developed (with the
exception of Cycas) in cones or flowers, or at least special
inflorescences. But the “seed” of _Lagenostoma_ (fig. 56), as well as a
number of others in the group it represents, were not borne on a special
structure, but directly on the green foliage leaves. They were in this on
a level with the simple sporangia of ferns which appear on the backs of
the fronds, a fact which is of great significance both for our views on
the evolution of seeds as such, and for the bearing it has on the
relationships of the various groups of allied plants. This will be
referred to subsequently (Chapter XI), and is mentioned now only as an
example of the difference between some of the characters of early fossils
and those of the present day.
It is true that botanists have long recognized the organ which bears
seeds as a modified leaf. The carpels of all the higher plants are looked
on as _homologous_ with leaves, although they do not appear to be like
them externally. Sometimes among living plants curious diseases cause the
carpels to become foliar, and when this happens the diseased carpel
reverts more or less to the supposed ancestral leaf-like condition. It is
only among the ancient (but recently discovered) fossils, however, that
seeds are known to be borne normally on foliage leaves.
From Mesozoic plants we shall learn new conceptions about flowers and
reproductive inflorescences in general, but these must be deferred to the
consideration of the family as a whole (Chapter XIII).
Enough has been illustrated to show that though the individual cells, the
bricks, so to speak, of plant construction, were so similar in the past
and present, yet the organs built up by them have been continually
varying, as a child builds increasingly ambitious palaces with the same
set of bricks.
CHAPTER VIII
PAST HISTORIES OF PLANT FAMILIES
I. Flowering Plants, Angiosperms
In comparison with the other groups of plants the flowering families are
of recent origin, yet in the sense in which the word is usually used they
are ancient indeed, and the earliest records of them must date at least
to periods hundreds of thousands of years ago.
Through all the Tertiary period (see p. 34) there were numerous flowering
plants, and there is evidence that many families of both Monocotyledons
and Dicotyledons existed in the Upper Cretaceous times. Further back than
this we have little reliable testimony, for the few specimens of
so-called flowering plants from the Lower Mesozoic are for the most part
of a doubtful nature.
The flowering plants seem to stand much isolated from the rest of the
plant world; there is no _direct_ evidence of connection between their
oldest representatives and any of the more primitive families. So far as
our actual knowledge goes, they might have sprung into being at the
middle of the Mesozoic period quite independently of the other plants
then living; though there are not wanting elaborate and almost convincing
theories of their connection with more than one group of their
predecessors (see p. 108).
It is a peculiarly unfortunate fact that although the rocks of the
Cretaceous and Tertiary are so much less ancient than those of the Coal
Measures, they have preserved for us far less well the plants which were
living when they were formed. Hitherto no one has found in Mesozoic
strata masses of exquisitely mineralized Angiosperm fragments[8] like
those found in the Coal Measures, which tell us so much about the more
ancient plants. Cases are known of more or less isolated fragments with
their microscopical tissues mineralized. For example, there are some
palms and ferns from South America which show their anatomical structure
very clearly preserved in silica, and which seem to resemble closely the
living species of their genera. The bulk of the plants preserved from
these periods are found in the form of casts or impressions (see p. 10),
which, as has been pointed out already, are much less satisfactory to
deal with, and give much less reliable results than specimens which have
also their internal structure petrified. The quantity of material,
however, is great, and impressions of single leaves innumerable, and of
specimens of leaves attached to stems, and even of flowers and fruits,
are to be found in the later beds of rock. These are generally clearly
recognizable as belonging to one or other of the living families of
flowering plants. Leaf impressions are by far the most frequent, and our
knowledge of the Tertiary flora is principally derived from a study of
them. Their outline and their veins are generally preserved, often also
their petioles and some indication of the thickness and character of the
fleshy part of the leaf. From the outline and veins alone an expert is
generally able to determine the species to which the plant belongs,
though it is not always quite safe to trust to these determinations or to
draw wide-reaching conclusions from them.
In fig. 59 is shown a photograph of the impression of a Tertiary leaf,
which illustrates the condition of an average good specimen from rocks of
the period. Its shape and the character of the veins are sufficient to
mark it out immediately as belonging to the Dicotyledonous group of the
flowering plants.
Seeds and fruits are also to be found; and in some very finely preserved
specimens from Japan stamens from a flower and delicate seeds are seen
clearly impressed on the light stone. In fig. 60 is illustrated a couple
of such seeds, which show not only their wings but also the small
antennæ-like stigmas. Specimens so perfectly preserved are practically as
good as herbarium material of recent plants, and in this way the
externals of the Tertiary plants are pretty well known to us.
[Illustration: Fig. 59.—Dicotyledonous Leaf Impression from Tertiary
Rocks]
[Illustration: Fig. 60.—Seeds from Japanese Tertiary Rocks; at _a_ are
seen the two stigmas still preserved]
A problem which has long been discussed, and which has aroused much
interest, is the relative antiquity of the Monocotyledonous and the
Dicotyledonous branches of the flowering plants. A peculiar fascination
seems to hang over this still unsolved riddle, and a battle of flowers
may be said to rage between the lily and the rose for priority. Recent
work has thrown no decisive light on the question, but it has undoubtedly
demolished the old view which supposed that the Monocotyledons (the lily
group) appeared at a far earlier date upon this earth than the
Dicotyledons. The old writers based their contention on incorrectly
determined fossils. For instance, seeds from the Palæozoic rocks were
described as Monocotyledons because of the three or six ribs which were
so characteristic of their shell; we know now that these seeds
(_Trigonocarpus_) belong to a family already mentioned in another
connection (p. 72), the Medulloseæ (see p. 122), the affinity of which
lies between the cycads and the ferns. Leaves of _Cordaites_, again,
which are broad and long with well-marked parallel veins, were described
as those of a Monocotyledonous plant like the Yucca of to-day; but we now
know them to belong to a family of true Gymnosperms possibly distantly
related to _Taxus_ (the Yew tree).
Recent work, which has carefully sifted the fossil evidence, can only say
that no true Monocotyledons have yet been found below the Lower
Cretaceous rocks, and that at that period we see also the sudden inrush
of Dicotyledons. Hence, so far as palæontology can show, the two parallel
groups of the flowering plants arose about the same time. It is of
interest to note, however, that the only petrifaction of a flower known
from any part of the world is an ovary which seems to be that of one of
the Liliaceæ. In the same nodules, however, there are several specimens
of Dicotyledonous woods, so that it does not throw any light on the
question of priority.
With the evidence derived from the comparative study of the anatomy of
recent flowering plants we cannot concern ourselves here, beyond noting
that the results weigh in favour of the Dicotyledons as being the more
primitive, though not necessarily developed much earlier in point of
time. Until very much more is discovered than is yet known of the origin
of the flowering plants as a whole, it is impossible to come to a more
definite conclusion about this much-discussed subject.
Let us now attempt to picture the vegetable communities since the
appearance of the flowering plants. The facts which form the bases of the
following conceptions have been gathered from many lands by numerous
workers in the field of fossil botany, from scattered plant remains such
as have been described.
When the flowering plants were heralded in they appeared in large
numbers, and already by the Cretaceous period there were very many
different species. Of these a number seem to belong to genera which are
still living, and many of them are extremely like living species. It
would be wearisome and of little value to give a list of all the recorded
species from this period, but a few of the commoner ones may be mentioned
to illustrate the nature of the plants then flourishing.
Several species of _Quercus_ (the Oak) appeared early, particularly
_Quercus Ilex_; leaves of the _Juglandaceæ_ (Walnut family) were very
common, and among the Tertiary fossils appear its fruits. Both _Populus_
(the Poplar) and _Salix_ (the Willow) date from the early rocks, while
_Ficus_ (the Fig) was very common, and _Casuarina_ (the Switch Plant)
seems to have been widely spread. Magnolias also were common, and it
appears that _Platanus_ (the Plane) and _Eucalyptus_ coexisted with them.
It will be immediately recognized that the above plants have all living
representatives, either wild or cultivated, growing in this country at
the present day, so that they are more or less familiar objects, and
there appears to have been no striking difference between the early
flowering plants and those of the present day. Between the ancient
Lycopods, for example, and those now living the differences are very
noteworthy; but the earliest of the known flowering plants seem to have
been essentially like those now flourishing. It must be remembered in
this connection that the existing flowering plants are immensely nearer
in point of time to their origin than are the existing Lycopods, and that
when such æons have passed as divide the present from the Palæozoic, the
flowering plants of the future may have dwindled to a subordinate
position corresponding to that held by the Lycopods now.
A noticeable character of the early flowering-plant flora, when taken as
a whole, is the relatively large proportion of plants in it which belong
to the family _Amentiferæ_ (oaks, willows, poplars, &c.). This is
supposed by some to indicate that the family is one of the most primitive
stocks of the Angiosperms. This view, however, hardly bears very close
scrutiny, because it derives its main support from the large numbers of
the Amentiferæ as compared with other groups. Now, the Amentiferæ were
(and are) largely woody resistant plants, whose very nature would render
them more liable to be preserved as impressions than delicate trees or
herbs, which would more readily decay and leave no trace. Similarly based
on uncertain evidence is the surmise that the group of flowers classed as
_Gamopetalæ_ (flowers with petals joined up in a tube, like convolvulus)
did not flourish in early times, but are the higher and later development
of the flower type. Now, _Viburnum_ (allied to the honeysuckle) belongs
to this group, and it is found right down in the Cretaceous, and
_Sambucus_ (Elder, of the same family) is known in the early Tertiary.
These two plants are woody shrubs or small trees, while many others of
the family are herbs, and it is noteworthy that it is just these woody,
resistant forms which are preserved as fossils; their presence
demonstrates the antiquity of the group as a whole, and the absence of
other members of it may be reasonably attributed to accidents of
preservation. In the Tertiary also we get a member of the heath family,
viz. _Andromeda_, and another tube-flower, _Bignonia_, as well as several
more _woody_ gamopetalous flowers.
Hence it is wise to be very cautious about drawing any important
conclusions from the relative numbers of the different species, or the
absence of any type of plant from the lists of those as yet known from
the Cretaceous. When quantities of structurally preserved material can be
examined containing the flowering plants in petrifactions, then it will
be possible to speak with some security of the nature of the Mesozoic
flora as a whole.
The positive evidence which is already accumulated, however, is of great
value, and from it certain deductions may be safely made. Specimens of
Cretaceous plants from various parts of the world seem to indicate that
there was a very striking uniformity in the flora of that period all over
the globe. In America and in Central Europe, for example, the same types
of plants were growing. We shall see that, as time advanced, the various
types became separated out, dying away in different places, until each
great continent and division of land had a special set of plants of its
own. At the commencement of the reign of flowering plants, however, they
seem to have lived together in the way we are told the beasts first lived
in the garden of Eden.
At the beginning of the Tertiary period there were still many tropical
forms, such as Palms, Cycads, _Nipa_, various _Artocarpaceæ_, _Lauraceæ_,
_Araliaceæ_, and others, growing side by side with such temperate forms
as _Quercus_, _Alnus_, _Betula_, _Populus_, _Viburnum_, and others of the
same kind. Before the middle of the Tertiary was reached the last Cycads
died in what is now known as Europe; and soon after the middle Tertiary
all the tropical types died out of this zone.
At the same time those plants whose leaves appear to have fallen at the
end of the warm season began to become common, which is taken as an
indication of a climatic influence at work. Some writers consider that in
the Cretaceous times there was no cold season, and therefore no regular
period of leaf fall, but as the climate became temperate the deciduous
trees increased in numbers; yet the Gymnospermic and Angiospermic woods
which are found with petrified structure show well-marked annual rings
and seem to contradict this view.
Toward the end of the Tertiary times there were practically no more
tropical forms in the European flora, though there still remained a
number of plants which are now found either only in America or only in
Asia.
The Glacial epoch at the close of the Tertiary appears to have driven all
the plants before it, and afterwards, when its glaciers retreated,
shrinking up to the North and up the sides of the high mountains, the
plant species that returned to take possession of the land in the
Quaternary or present period were those which are still inhabiting it,
and the floras of the tropics, Asia, and America were no longer mixed
with that of Europe.[9]
CHAPTER IX
PAST HISTORIES OF PLANT FAMILIES
II. Higher Gymnosperms
The more recent history of the higher Gymnosperms, in the Upper
Cretaceous and Tertiary periods, much resembles that of the flowering
plants as sketched in the previous chapter. Many of the genera appear to
have been those still living, and some of the species even may have come
very close to or have been identical with those of to-day. The forms now
characteristic of the different continents were growing together, and
appear to have been widely distributed over the globe. For example,
_Sequoia_ and _Taxodium_, two types now characteristic of America, and
_Glyptostrobus_, at present found in Asia, were still growing with the
other European types in Europe so late as middle Tertiary times.
As in the case of the Angiosperms, the fossils we have of Cretaceous and
Tertiary Gymnosperms are nearly all impressions and casts, though some
more or less isolated stems have their structure preserved. Hence our
knowledge of these later Gymnosperms is far from complete. From the older
rocks, however, we have both impressions and microscopically preserved
material, and are more fully acquainted with them than with those which
lived nearer our own time. Hard, resistant leaves, which are so
characteristic of most of the living genera of Gymnosperms, seem to have
been also developed in the past members of the group, and these tend to
leave clear impressions in the rocks, so that we have reliable data for
reconstructing the external appearance of the fossil forms from the
Palæozoic period.
The resinous character of Gymnosperm wood probably greatly assisted its
preservation, and fragments of it are very common in rocks of all ages,
generally preserved in silica so as to show microscopic structure. The
isolated wood of Gymnosperms, however, is not very instructive, for from
the wood alone (and usually it is just fragments of the secondary wood
which are preserved) but little of either physiological or evolutional
value can be learned. When twigs with primary tissues and bark and leaves
attached are preserved, then the specimens are of importance, for their
true character can be recognized. Fortunately among the coal balls there
are many such fragments, some of which are accompanied by fruits and male
cones, so that we know much of the Palæozoic Gymnosperms, and find that
in some respects they differ widely from those now living.
There is, therefore, much more to be said about the fossil Gymnosperms
than about the Angiosperms, both because of the better quality of their
preservation and because their history dates back to a very much earlier
period than does the Angiospermic record. Indeed, we do not know when the
Gymnosperms began; the well-developed and ancient group of _Cordaiteæ_
was flourishing before the Carboniferous period, and must therefore date
back to the rocks of which we have no reliable information from this
point of view, and the origin of the Gymnosperms must lie in the
pre-Carboniferous period.
The group of Gymnosperms includes a number of genera of different types,
most of which may be arranged under seven principal families. In a sketch
of this nature it is, of course, quite impossible to deal with all the
less-important families and genera. Those that will be considered here
are the following:—
Coniferales (see p. 90).
_Araucareæ_, _e.g._ Monkey-puzzle
Genera both living and fossil.
Fossil forms undoubted so far back as the Jurassic, and
presumably further.
_Abietineæ_, _e.g._ Pine and Larch
Genera both living and fossil.
Fossils recognized as far back as the Lower Cretaceous.
_Cupresseæ_, _e.g._ Juniper, Cypress
Genera both living and fossil.
Fossils recognized as far back as the Jurassic.
_Taxeæ_, _e.g._ Yew
Genera living and fossil.
Fossils recognized as far back as the Cretaceous.
Cordaitales (see p. 92).
_Cordaiteæ_, _e.g._ Cordaites
Fossil only.
Characteristic of Devonian, Carboniferous, and Permian periods.
_Poroxyleæ_, _e.g._ Poroxylon
Fossil only.
Characteristic of the Carboniferous and Permian.
Ginkgoales (see p. 98).
_Ginkgoaceæ_, _e.g._ Ginkgo
Fossil and living, dating back, apparently with little change,
to Palæozoic times.
We must pay the most attention to the two last groups, as they are so
important as fossils, and the _Cordaiteæ_ were a very numerous family in
Coal Measure times. They had their period of principal development so
long ago that it is probable that no direct descendants remain to the
present time, though some botanists consider that the _Taxeæ_ are allied
to them.
Of the groups still living it is difficult, almost impossible, to say
which is the highest, the most evolved type. In the consideration of the
Gymnosperm family it is brought home with great emphasis how incomplete
and partial our knowledge is as yet. Many hold that the _Araucareæ_ are
the most primitive of the higher Gymnosperms. In support of this view the
following facts are noted. They have a simple type of fructification,
with a single seed on a simple scale, and many scales arranged round an
axis to form a cone. In the microscopic structure of their wood they have
double rows of bordered pits, a kind of wood cell which comes closer to
the old fossil types than does the wood of any of the other living
genera. Further than this, wood which is almost indistinguishable from
the wood of recent Araucarias is found very far back in the rocks, while
their leaves are broad and simple, and attached directly to the stem in a
way similar to the leaves of the fossil _Cordaiteæ_, and very different
from the needle leaves on the secondary stems of the Pine family; so that
there appears good ground for considering the group an ancient and
probably a primitive one.[10]
On the other hand, there are not wanting scientists who consider the
_Abietineæ_ the living representatives of the most primitive and ancient
stock, though on the whole the evidence seems to indicate more clearly
that the Pine-tree group is specialized and highly modified. Their double
series of foliage leaves, their complex cones (whose structures are not
yet fully understood), and their wood all support the latter view.
Some, again, consider the _Taxeæ_ as a very primitive group, and would
place them near the Cordaiteæ, with which they may be related. Their
fleshy seeds, growing not in cones but on short special axes, support
this view, and it is certainly true that in many ways the large seeds,
with their succulent coats and big endosperm, are much like those of the
lower Gymnosperms and of several fossil types. Those, however, who hold
to the view that the Abietineæ are primitive, see in the _Taxeæ_ the
latest and most modified type of Gymnosperm.
It will be seen from this that there is no lack of variety regarding the
interpretation of Gymnosperm structures.
The Gymnosperms do not stand in such an isolated position as do the
Angiosperms. Whatever the variety of views held about the details of the
relative placing of the families within the group, all agree in
recognizing the evidence which enables us to trace with confidence the
connection between the lower Gymnosperms and the families of ferns. There
are many indications of the intimate connection between higher and lower
Gymnosperms. Between the series exist what might be described as
different degrees of cousinship, and in the lower groups lie unmistakable
clues to their connection with more ancient groups in the past which
bridge over the gaps between them and the ferns.
For the present, however, let us confine ourselves to the history of the
more important Gymnosperms, the discussion of their origin and the groups
from which they may have arisen must be postponed until the necessary
details about those groups have been mentioned.
To a consideration of the living families of _Araucareæ_, _Abietineæ_,
_Cupresseæ_, and _Taxeæ_ we can allow but a short space; their general
characters and appearance are likely to be known to the reader, and their
details can be studied from living specimens if they are not. For
purposes of comparison with the fossils, however, it will be necessary to
mention a few of the principal features which are of special importance
in discussing phylogeny.
The Araucariaceæ are woody trees which attain a considerable size, with
broad-based, large leaves attached directly to the stem. In the leaves
are a series of numerous parallel vascular bundles. The wood cells in
microscopic section show two rows or more of round bordered pits. The
cones are very large, but the male and female are different in size and
organization. The female cone is composed of series of simple scales
arranged spirally round the axis, and each scale bears a single seed and
a small ligule.
The pollen grains from the male cone are caught on the ligule and the
pollen tubes enter the micropyle of the ovule, bringing in passive male
cells which may develop in large numbers in each grain. The seeds when
ripe are stony, and some are provided with a wing from part of the tissue
of the scale. In the ripe cones the scales separate from the cone axis.
The Abietineæ are woody trees, some reaching a great height, all with a
strong main stem. The leaves are of two kinds: primary ones borne
directly attached to the stem (as in first-year shoots of the Larch), and
secondary ones borne in tufts of two (in Pine) or a large number (in
older branches of Larch) on special short branches, the primary leaves
only developing as brown scales closely attached to the stems. Leaves
generally very fine and needlelike, and with a central vascular bundle.
The wood in microscopic section shows a single row of round bordered pits
on the narrow tracheæ.
The female cones are large, male and female differing greatly in size and
organization. The female cone, composed of a spiral series of pairs of
scales, which often fuse together as the cone ripens. Each upper scale of
the pair bears two seeds. The pollen grains from the male cone enter the
micropyle of the seed and are caught in the tissue (apex of nucellus)
there; the pollen tubes discharge passive male cells, only two of which
develop in each grain. The seeds when ripe are stony and provided with a
wing from the tissue of the scale on which they were borne.
The Cupresseæ are woody trees reaching no great height, and of a bushy,
branching growth. The leaves are attached directly to the main stem, and
arrange themselves in alternating pairs of very small leaves, closely
pressed to the stem. The wood in microscopic section shows a single row
of round bordered pits on the tracheæ.
The cones are small, and the scales forming them arranged in cycles. The
female scales bear a varying number of seeds. The pollen grain has two
passive male cells. The seeds when ripe are stony, with wings, though in
some cases (species of Juniper) the cone scales close up and become
fleshy, so that the whole fruit resembles a berry.
The Taxeæ are woody, though not great trees, bushily branched. The leaves
are attached spirally all round the stem, but place themselves so as to
appear to lie in pairs arranged in one horizontal direction. The wood in
microscopic section shows a single row of round bordered pits on the
tracheæ.
There are small male cones, but the seeds are not borne on cones, growing
instead on special short axes, where there may be several young ovules,
but on which usually two seeds ripen. The seeds are big, and have an
inner stone and outer fleshy covering. Some have special outer fleshy
structures known as “arils”, _e.g._ the red outer cup round the yew
“berry” (which is not a berry at all, but a single unenclosed seed with a
fleshy coat).
When we turn to the Cordaiteæ we come to a group of plants which bears
distinct relationship to the preceding, but which has a number of
individual characters. It is a group of which we should know nothing were
it not for the fossils preserved in the Palæozoic rocks; yet,
notwithstanding the fact that it flourished so long ago, it is a family
of which we know much. At the time of the Coal Measures and the
succeeding Permo-carboniferous period, it was of great importance, and,
indeed, in some of the French deposits it would seem as though whole
layers of coal were composed entirely of its leaves.
Among the fossil remains of this family there are impressions, casts, and
true petrifactions, so that we know both its external appearance and the
internal anatomy of nearly every part of several species of the genus.
For a long time the various fossil remains of the plant were not
recognized as belonging to each other and together forming the records of
one and the same plant—the broad, long leaves with their parallel veins
were looked on as Monocotyledons (see fig. 61); the pith casts (see fig.
63) were thought to be peculiar constricted stems, and were called
_Sternbergia_; while the wood, which was known from its microscopic
structure, was called _Araucarioxylon_—but the careful work of many
masters of fossil botany, whose laborious studies we cannot describe in
detail here, brought all these fragments together and proved them to
belong to _Cordaites_.
[Illustration: Fig. 61.—Leaf of _Cordaites_, _l_, attached by its broad
base to a Stem, _s_]
We now know that _Cordaites_ were large trees, with strong upright shafts
of wood, to whose branches large simple leaves were attached. The leaves
were much bigger than those of any living Gymnosperm, even than those of
the Kauri Pine (a member of the Araucariaceæ), and seem in some species
to have exceeded 3 ft. in length. The trees branched only at the top of
the main shaft, and with their huge sword-like leaves must have differed
greatly in appearance from any plant now living. The leaves had many
parallel veins, as can be seen in fig. 61, and were attached by a broad
base directly to the main stem; thus coming closer to the Araucarias than
the other groups of Gymnosperms in their leaf characters.
[Illustration: Fig. 62A.—Microscopic Section of Part of a Leaf of
_Cordaites_
V, Vascular bundle; W, wood of bundle; _sh_, its sheath; S^1, large
sclerenchyma mass alternating with bundles; S^2 and S^3, sclerenchyma
caps of bundle; P, soft tissue of leaf.]
The internal anatomy is often well preserved, and there is a number of
species of leaves whose anatomy is known. As will be expected from the
parallel veins, in each section there are many vascular bundles running
equidistantly through the tissue. Fig. 62A shows the microscopic details
from a well-preserved leaf. In all the species patches of sclerenchyma
were developed, and everything indicates that they were tough and well
protected against loss of water, even to a greater extent than are most
of the leaves of living Gymnosperms.
In the stems the pith was much larger than that in living Gymnosperms
(where the wood is generally very solid), and it was hollow in older
stems, except for discs of tissue across the cavity. The internal cast
from these stems has been described before, and is seen in fig. 63.
[Illustration: Fig. 62B.—Much-magnified Wood Elements from _Cordaites_
Stem seen in longitudinal section, the type known as _Araucarioxylon_.
Note the hexagonal outlines of the bordered pits, which lie in several
rows]
The wood was formed in closely packed radiating rows by a normal cambium
(see p. 66), and the tracheæ so formed had characteristic rows of
bordered pits (see fig. 62B). The wood comes nearer to that of the living
Araucarias than any other, and indeed the numerous pieces of fossil wood
of this type which are known from all the geological periods are called
_Araucarioxylon_.[11] A double strand goes out from the main mass of
wood, which afterwards divides and subdivides to provide the numerous
bundles of the leaf.
[Illustration: Fig. 63.—Cast of Hollow Pith of _Cordaites_, the
constrictions corresponding to discs of solid tissue across the cavity]
In the case of these fossils we are fortunate enough to have the
fructifications, both male and female, in a good state of preservation.
As in other Gymnosperms, the male and female cones are separate, but they
differed less from each other in their arrangement than do those of any
of the living types hitherto mentioned. They can hardly be described as
true cones, though they had something of that nature; the seeds seem to
be borne on special short stems, round which are also sterile scales. In
the seed and the way it is borne perhaps the Cordaiteæ may be compared
more nearly with the Taxeæ than with the other groups. A seed, not yet
ripe, is shown in slightly diagrammatic form in fig. 64, where the
essential details are illustrated. The seeds of this family sometimes
reached a considerable size, and had a fleshy layer which was thick in
comparison with the stone, and externally comparable with a
cherry—though, of course, of very different nature in reality, for
_Cordaites_, like _Taxus_, is a Gymnosperm, with simple naked seeds,
while a cherry is the fruit of an Angiosperm.
In a few words, these are the main characters of the large group of
_Cordaites_, which held the dominant position among Gymnosperms in the
Palæozoic era. They have relationships, or perhaps one should say
likenesses, to many groups. Their stem- and root-anatomy is similar to
the Coniferæ of the present day, the position of the ovules is like that
in the Taxaceæ, the male cones in some measure recall those of _Ginkgo_,
the anatomy of their leaves has points which are comparable with those of
the Cycads, to which group also the large pith in the stem and the
structure of some details in the seeds unite them. Their own specially
distinctive characters lie in their crown of huge leaves, and unbranched
shaft of stem, the similarity of their male and female inflorescences,
and some points in their pollen grains which have not been mentioned. The
type is a very complex one, possibly coming near the stock which, having
branched out in various directions, gave rise to several of the living
families.
[Illustration: Fig. 64.—Representation of _Cordaites_ Seed and its Axis
with Scales, slightly diagrammatic, modified from Renault.
A, Axis with _s_, scales; _c_, coat of the seed, from which the inner
parts have shrunk away; _n_, nucellus; _p.c_, pollen chamber containing
pollen grains which enter through _m_.]
Plants which come very near to the Cordaiteæ are the Poroxyleæ. Of this
group we have unfortunately no remains of fructifications in organic
connection, so that its actual position must remain a little doubtful
till they are discovered. There seems no doubt that they must have borne
seeds.
Still, it has been abundantly demonstrated in recent years that the
anatomy of the root, stem, and leaves indicates with considerable
exactness the position of any plant, so that, as these are known, we can
deduce from them, with a feeling of safety, the position that _Poroxylon_
takes in the natural system. In its anatomy the characters are those of
the Cordaiteæ, with certain details which show a more primitive nature
and seem to be characteristic of the groups below it in organization.
_Poroxylon_ is not common, and until recently had not been found in the
Lower Coal Measures of England. The plants appear to have been much
smaller than _Cordaites_, with delicate stems which bore relatively large
simple leaves. The anatomy of the root was that common in Gymnosperms,
but the stem had a very large pith, and the leaves were much like those
of _Cordaites_ in having parallel veins. An important character in the
anatomy of the stem was the presence of what is known as _centripetal
wood_. This must be shortly explained. In all the stems hitherto
considered, the first-formed wood cells (protoxylems, see p. 57)
developed at the central point of the wood, towards the pith (see fig.
19, _px_, p. 49). This is characteristic of all Angiosperms and the
higher Gymnosperms (except in a couple of recently investigated Pines),
but among the lower plants we find that part of the later wood develops
to the inner side of these protoxylem masses. The distinction is shown in
fig. 65.
[Illustration: Fig. 65.—A, Normal bundle of higher plant; _x_,
protoxylem on inner side next the pith _p_, and the older wood _w_
outside it, _centrifugal_ wood. B, Bundle with wood cells _c_ developed
on inner side of protoxylem, _centripetal_ wood; the arrow indicates
the direction of the centre of the stem.]
This point is one to which botanists have given much attention, and on
which they have laid much weight in considering the affinities of the
lower Gymnosperms and the intermediate groups between them and the ferns,
which are found among the fossils. In _Cordaites_ this point of
connection with the lower types is not seen, but in _Poroxylon_, which
has otherwise a stem anatomy very similar to _Cordaites_, we find groups
of _centripetal_ wood developed inside the protoxylem of primary bundles.
For this reason, principally, is _Poroxylon_ of interest at present, as
in its stem anatomy it seems to connect the _Cordaites_ type with that of
the group below it in general organization.
Ginkgoales.—Reference to p. 44 shows that _Ginkgo_, the Maidenhair tree,
belongs to the Ginkgoales, a group taking equal rank with the large and
complex series of the Coniferales. The Ginkgoales of the present day,
however, have but one living representative. _Ginkgo_ stands alone, the
single living species of its genus, representing a family so different
from any other living family that it forms a prime group by itself.
Had the tree not been held sacred in China and Japan, it is probable that
it would long since have been extinct, for it is now known only in
cultivation. It is indeed a relic from the past which has been
fortunately preserved alive for our examination. It belongs to the fossil
world, as a belated November rose belongs to the summer.
Because of its beauty and interest the plant is now widely distributed
under cultivation, and is available for study almost as freely as the
other types of living Gymnosperms already mentioned, so that but a short
summary of its more important features is needed here.
Old plants, such as can be seen growing freely in Japan (in Kew Gardens
there is also a fine specimen), are very tall handsome woody trees, with
noble shafts and many branches. The leaves grow on little side shoots and
are the most characteristic external feature of the tree; their living
form is illustrated in fig. 66, which shows the typical simple shape as
well as the lobed form of the leaf which are to be found, with all
intermediate stages, on the same tree. No other plant (save a few ferns,
which can generally be distinguished from it without difficulty) has
leaves at all like these, so that it is particularly easy to identify the
fossil remains, of which there are many.
[Illustration: Fig. 66.—A, Tuft of _Ginkgo_ Leaves, showing their
“maidenhair”-like shape. B, Single deeply-divided Leaf to be found on
the same tree, usually on young branches.]
The wood is compact and fine grained, the rings of secondary tissue being
developed from a normal cambium as in the case of the higher Gymnosperms,
and the individual tracheæ have round bordered pits. There are small male
cones, but the seeds are not borne in cones. They develop on special
stalks on which are no scales, but a small mass of tissue at the base of
the seed called the “collar”. Usually there are two young ovules, of
which often only one ripens to a fleshy seed, though both may mature.
[Illustration: Fig. 67.—Ripe Stage of _Ginkgo_ Seeds attached to their
Stalk. _c_, “Collar” of seed.]
The ripe seed reaches the size shown in the diagram, and is orange
coloured and very fleshy; within it is a stone encasing the endosperm,
which is large, _green_, and starchy, and contains the embryo with two
cotyledons. This embryo is small compared with the endosperm, cf. fig.
57, p. 76, which is somewhat similar to that of _Ginkgo_ in this stage.
Of the microscopic characters of the reproductive organs the most
remarkable is the male cell. This is not a passive nucleus, as in the
plants hitherto considered, but is an _actively swimming_ cell of some
size, provided with a spiral of cilia (hairlike structures) whose
movements propel it through the water. In the cavity of the unripe seed
these swim towards the female cell, and actively penetrate it. The
arrangements of the seed are diagrammatically shown in fig. 68, which
should be compared with that of _Cycas_, fig. 76, with which it has many
points in common.
[Illustration: Fig. 68.—Section through Seed of _Ginkgo_
_p.c_, Pollen chamber in the nucellus _n_, which is fused to the coat
_c_ to the level _l_; _sc_, stony layer in coat; S, the big spore,
filled with endosperm tissue (in this case green in colour); _e_, egg
cells, one of which will produce the embryo after fertilization.]
The nature of the male cell in _Cordaites_ is not yet known, but there is
reason to suspect it may have been actively swimming also. As this is
uncertain, however, we may consider _Ginkgo_ the most highly organized
plant which has such a primitive feature, a feature which is a bond of
union between it and the ferns, and which, when it was discovered about a
dozen years ago, caused a considerable sensation in the botanical world.
To turn now to the fossil records of this family. Leaf impressions of
_Ginkgo_ are found in rocks of nearly all ages back even to the Upper
Palæozoic. They show a considerable variety of form, and it is certain
that they do not all belong to the same _species_ as the living plant,
but probably they are closely allied. Fig. 69 shows a typical impression
from the Lower Mesozoic rocks. In this specimen, the cells of the
epidermis were fortunately sufficiently well preserved to be seen with
the microscope, and there is a distinct difference in the size and shape
of the cells of living and fossil species, see fig. 70; but this
difference is slight as compared with the great similarity of form and
appearance, as can be seen on comparing figs. 69 and 66, B, so that the
fossil is at the most a different species of the genus _Ginkgo_. Among
the fossil leaves there is greater variety than among the living ones,
and some which are very deeply lobed so as to form a divided palm-like
leaf go by different names, e.g. _Baiera_, but they are supposed to
belong to the same family. Fossil seeds and male cones are also known as
impressions, and are found far back in the Mesozoic rocks. From the
fossil impressions it is certain that _Ginkgo_ and plants closely allied
to it were very widespread in the past, as they are found all over Europe
as well as the other continents. Particularly in the Lower Mesozoic rocks
_Ginkgo_ seems to have been a world-wide type growing in great abundance.
[Illustration: Fig. 69.—Leaf Impression of _Ginkgo_ from Mesozoic Rocks
of Scotland]
[Illustration: Fig. 70.—Showing Epidermis with Stomates from the lower
side of the Leaf seen in fig. 69
_e_, Epidermis cells; _s_, stomates; _v_, long cells of epidermis lying
over the veins.]
In the Palæozoic the records are not so undoubted, but there is strong
evidence which leads us to suppose that if the genus now living were not
then extant, at least other closely related genera were, and there seems
to be good grounds for supposing that _Ginkgo_ and _Cordaites_ may have
both arisen from some ancient common stock.
CHAPTER X
PAST HISTORIES OF PLANT FAMILIES
III. The Bennettitales
This fascinating family is known only from the fossils, and is so remote
in its organization from any common living forms that it may perhaps be a
little difficult for those who do not know the Cycads to appreciate the
position of Bennettites. It would probably be better for one studying
fossil plants for the first time to read the chapters on the Cycads,
Pteridosperms, and Ferns before this chapter on the present group, which
has characters connecting it with that series.
Until recently the bulk of the fossils which are found as impressions of
stems and foliage of this family were very naturally classed as Cycads.
They are extremely common in the Mesozoic rocks (the so-called Age of
Cycads), and in the external appearance of both stems and leaves they are
practically identical with the Cycads.
A few incomplete fructifications of some species have been known in
Europe for many years, but it is only recently that they have been fully
known. This is owing to Wieland’s[12] work on the American species, which
has made known the complete organization of the fructifications from a
mass of rich and well-petrified material.
In the Lower Cretaceous and Upper Jurassic rocks of America these plants
abound, with their microscopic structure well preserved, and their
fructifications show an organization of a different nature from that of
any past or present Cycad.
Probably owing to their external appearance, Wieland describes the plants
as “Cycads” in the title of his big book on them; but the generic name he
uses, _Cycadeoidea_, seems less known in this country than the equally
well-established name of _Bennettites_, which has long been used to
denote the European specimens of this family, and which will be used in
the following short account of the group.
At the present time no family of fossils is exciting more interest. Their
completely Cycadean appearance and their unique type of fructification
have led many botanists to see in them the forerunners of the
Angiosperms, to look on them as the key to that mystery—the origin of the
flowering plants. This position will be discussed and the many facts in
its favour noted, but we must not forget that the _Bennettitales_ have
only recently been realized fully by botanists, and that a new toy is
ever particularly charming, a new cure particularly efficacious, and a
new theory all-persuasive.
From their detailed study of the flowering plants botanists have leaned
toward different groups as the present representatives of the primitive
types. The various claims of the different families to this position
cannot be considered here; probably that of the Ranales (the group of
families round Ranunculaceæ as a central type) is the best supported. Yet
these plants are most frequently delicate herbs, which would have stood
relatively less chance of fossilization than the other families which may
be considered primitive. They are peculiarly remote from the group of
Bennettiteæ in their vegetative structure, a fact the importance of which
seems to have been underrated, for in the same breath we are assured that
the Bennettites are a kind of cousin to the ancient Angiosperms, and that
the Ranales are among the most primitive living Angiosperms, and
therefore presumably nearest the ancient ones.
However, let us leave the charms of controversy on one side and look at
the actual structure of the group. They were widely spread in Lower
Mesozoic times, the plants being preserved as casts, impressions, and
with structure in great numbers. The bulk of the described structural
specimens have been obtained from the rocks of England, France, Italy,
and America, although leaf impressions are almost universally known. The
genus _Williamsonia_ belongs to this family, and is one of the best known
of Mesozoic plant impressions.
Externally the Bennettiteæ were identical in appearance with stumpy
Cycads, and their leaves it is which gave rise to the surmise, so long
prevalent, that the Lower Mesozoic was the “Age of Cycads”, just as it
was the Pteridosperm leaves that gave the Palæozoic the credit of being
the “Age of Ferns”. In the anatomy of both stem and leaf, also, the
characters are entirely Cycadean; the outgoing leaf trace is indeed
simpler in its course than that of the Cycads.
[Illustration: Fig. 71.—Half of a Longitudinal Section through a Mature
Cone of _Bennettites_
A, Short conical axis; _s_, enclosing bracts; S, seeds; _sc_, sterile
scales between the seeds.]
The fructifications, however, differ fundamentally from those of the
Cycads, as indeed they do from those of any known family. They took the
form of compact cones, which occurred in very large numbers in the mature
plants hidden by the leaf bases. In _Williamsonia_, of which we know much
less detail, the fructifications stood away from the main axis on long
pedicels.
In _Bennettites_ the cones were composed of series of sheathing scales
surrounding a short conical axis on which stood thin radiating stalks,
each bearing a seed. Between them were long-stalked sterile scales with
expanded ends. A part of a cone is illustrated diagrammatically in fig.
71. The whole had much the appearance of a complex fruit. In some
specimens these features alone are present in the cones, but in younger
cones from the American plants further structures are found attached.
Below the main axis of the seed-bearing part of the cone was a series of
large complex leaflike structures closely resembling fern leaves in their
much-divided nature. On the pinnæ of these leaves were crowded
innumerable large sporangia, similar to those of a fern, which provided
the pollen grains. The fossils are particularly well preserved, and have
been found with these male (pollen-bearing) organs in the young unopened
stages, and also in the mature unfolded condition, as well as the
ripening seed cones from which they have faded, just as the stamens fade
from a flower when the seeds enlarge.
[Illustration: Fig. 72.—Diagram of Complete Cone of _Bennettites_
A, Central axis of conical shape terminating in the seed-bearing cone
S. (After Wieland), and bearing successively Br., bracts, comparable
with floral leaves; M, large complex leaves with pollen sacs.]
It appears that these huge complex leaflike structures were really
stamens, but nevertheless they were rolled up in the circinate form as
are young fern leaves, and as they unrolled and spread out round the
central cone they must have had the appearance of a whorl of leaves (see
fig. 72).
This, in a few words, is the main general character of the
fructification. The most important features, on which stress is laid, are
the following. The association of the male and female structures on the
same axis, with the female part _above_ the male. This arrangement is
found only in the flowering plants; the lower plants, which have male and
female on the same cone, have them mixed, or the female below, and are in
any case much simpler in their entire organization. The conical form of
the axis is also important, as is the fact that it terminates in the
seed-bearing structures.
[Illustration: Fig. 73.—Diagram of Cross Section of _Bennettites_,
Seed, with Embryo
_c_, Double-layered seed coat; _n_, crushed nucellus; _cot._, two
cotyledons which practically fill the seed.]
The position of the individual seeds, each on the end of a single stalk,
is remarkable, as are the long-stalked bracts whose shield-like ends join
in the protection of the seeds. These structures together give the cone
much of the appearance of a complex fruit of a flowering plant, but the
structure of the seeds themselves is that of a simple Gymnosperm.
In the seeds, however, was an _embryo_. In this they differ from all
known seeds of an earlier date, which, as has been already noted (see p.
77), are always devoid of one. This embryo is one of the most important
features of the plant. It had two cotyledons which filled the seed space
(see fig. 73), and left almost no trace of the endosperm. Reference to p.
112 will show that this is an advance on the Cycad seed, which has a
small embryo embedded in a large mass of endosperm, and that it
practically coincides with the Dicotyledonous type.
The seed with its embryo suggested comparison with the Angiosperms long
before the complete structure of the fructification was known.
The fern-like nature of the pollen-bearing structures is another very
important point. Were any one of these leaflike “stamens” found isolated
its fern-like nature would not have been questioned a year or two ago,
and their presence in the “flower” of _Bennettites_ is a strong argument
in favour of the Fern-Pteridosperm affinities of the group.
Had the parts of this remarkable fructification developed on separate
trees, or on separate branches or distinct cones of the same one, they
would have been much less suggestive than they are at present, and the
fructifications might well have been included among those of the
Gymnosperms, differing little more (apart from the embryo) from the other
Gymnosperm genera than they do from each other. In fact, the extremely
fern-like nature of the male organs is almost more suggestive of a
Pteridosperm affinity, for even the simplest Cycads have well-marked
scaly cones as their male organs. The female cone, again, considered as
an isolated structure, can be interpreted as being not vitally different
from _Cordaites_, where the seeds are borne on special short stalks
amidst scales.
The embryo would, in any case, point to a position among advanced types;
but it is so common for one organ of a plant to evolve along lines of its
own independently, or in advance of the other organs, that the embryo
structure alone could not have been held to counterbalance the Cycadean
stems and leaves, the Pteridosperm-like male organs, and the Gymnospermic
seeds.
But all these parts occur on the same axis, arranged in the manner
typical of Angiosperms. The seed-bearing structures at the apex, the
“stamens” below them, and a series of expanded scales below these again,
which it takes little imagination to picture as incipient petals and
sepals; and behold—the thing is a flower!
And being a “flower”, is in closest connection with the ancestors of the
modern flowering plants, which must consequently have evolved from some
Cycadean-like ancestor which also gave rise to the Bennettitales. Thus
can the flowering plants be linked on to the series that runs through the
Cycads directly to the primitive ferns!
It is evident that this group, of all those known among the fossils,
comes most closely to an approximation of Angiospermic structure and
arrangement. Enough has been said to show that in their actual nature
they are not Angiosperms, though they have some of their characters,
while at the same time they are not Cycads, though they have their
appearance. They stand somewhere between the two. Though many botanists
at present hold that this mixture of characters indicates a relationship
equivalent to a kind of cousinship with the Angiosperms, and both groups
may be supposed to have originated from a Cycadean stock, this theory has
not yet stood the test of time, nor is it supported by other evidence
from the fossils. We will go so far as to say that it appears as though
_some_ Angiosperms arose in that way; but flowering plants show so many
points utterly differing from the whole Cycadean stock that a little
scepticism may not be unwholesome.
It is well to remember the Lycopods, where (as we shall see, p. 141)
structures very like seeds were developed at the time when the Lycopods
were the dominant plants, and we do not find any evidence to prove that
they led on to the main line of seed plants. Similarly, Cycads may have
got what practically amounted to flowers at the time when they were the
dominant group, and it is very conceivable that they did not lead on to
the main line of flowering plants.
Whatever view may be held, however, and whatever may be the future
discoveries relating to this group of plants, we can see in the
Bennettitales points which throw much light on the potentialities of the
Cycadean stock, and structures which have given rise to some most
interesting speculations on the subject of the Angiosperms. This group is
another of the jewels in the crown of fossil botany, for the whole of its
structures have been reconstructed from the stones that hold all that
remains of this once extensive and now extinct family of plants.
CHAPTER XI
PAST HISTORIES OF PLANT FAMILIES
IV. The Cycads
The group of the _Cycadales_, which has a systematic value equivalent to
the _Ginkgoales_, contains a much larger variety of genera and species
than does the latter. There are still living nine genera, with more than
a hundred and fifty species, which form (though a small one compared with
most of the prime groups) a well-defined family. They are the most
primitive Gymnosperms, the most primitive seed-bearing plants now living,
and in their appearance and characters are very different from any other
modern type. Their external resemblance to the group of the
Bennettitales, however, is very striking, and indeed, without the
fructifications it would be impossible to distinguish them.
The best known of the genera is that of _Cycas_, of which an illustration
is given in fig. 74. The thick, stumpy stem and crown of “palm”-like
leaves give it a very different appearance from any other Gymnosperm.
Commonly the plants reach only a few feet in height, but very old
specimens may grow to the height of 30 ft. or more. The other genera are
smaller, and some have short stems and a very fern-like appearance, as,
for example, the genus _Stangeria_, which was supposed to be a fern when
it was first discovered and before fruiting specimens had been seen.
The large compound leaves are all borne directly on the main stem,
generally in a single rosette at its apex, and as they die off they leave
their fleshy leaf bases, which cover the stem and remain for an almost
indefinite number of years.
The wood of the main trunks differs from that of the other Gymnosperms in
being very loosely built, with a large pith and much soft tissue between
the radiating bands of wood. There is a cambium which adds zones of
secondary tissue, but it does not do its work regularly, and the cross
section of an old Cycad stem shows disconnected rings of wood,
accompanied by much soft tissue. The cells of the wood have bordered pits
on their walls, and in the main axis the wood is usually all developed in
a centrifugal direction, but in the axis of the cones some centripetal
wood is found (refer to _c_, fig. 65, p. 97).
[Illustration: Fig. 74.—Plant of _Cycas_, showing the main stem with
the crown of leaves and the irregular branches which come on an old
plant]
In their fructifications the Cycads stand even further apart from the
rest of the Gymnosperms. One striking point is the enormous size of their
_male_ cones. The male cones consist of a stout axis, round which are
spiral series of closely packed simple scales covered with pollen-bearing
sacs (which bear no inconsiderable likeness to fern sporangia), the whole
cone reaching 1½ ft. in length in some genera, and weighing several
pounds. All the other Gymnosperms, except the Araucareæ, where they are
an inch or two long, have male cones but a fraction of an inch in length.
[Illustration: Fig. 75.—Seed-bearing Scale of _Cycas_, showing its
lobed and leaflike character
_s_, Seeds attached on either side below the divisions of the
sporophyll.]
In all the members of the family, excepting _Cycas_ itself, the female
fructifications also consist of similarly organized cones bearing a
couple of seeds on each scale instead of the numerous pollen sacs. In
_Cycas_ the male cones are like those of the other genera, and reach an
enormous size; but there are no female cones, for the seeds are borne on
special leaflike scales. These are illustrated in fig. 75, which shows
also that there are not two seeds (as in the other genera with cones) to
each scale, but an indefinite number.
The leafy nature of the seed-bearing scale is an important and
interesting feature. Although theoretically botanists are accustomed to
accept the view that seeds are always borne on specially modified leaves
(so that to a botanist even the “shell” of a pea-pod and the box of a
poppy capsule are leaves), yet in _Cycas_ alone among living plants are
seeds really found growing on a large structure which has the appearance
of a leaf. Hence, from this point of view (see p. 45, however, for a
caution against concluding that the whole plant is similarly lowly
organized), _Cycas_ is the most primitive of all the living plants that
bear seeds, and hence presumably the likest to the fossil ancestors of
the seed-bearing types. In this character it is more primitive than the
fossil group of the _Cordaiteæ_, and comes very close to an intermediate
group of fossils to be considered in the next chapter.
[Illustration: Fig. 76.—Seed of _Cycas_ cut open
_n_, The nucellus, fused at the level _l_ to the coat _c_; _sc_, stony
layer of coat; _p.c_, pollen chamber in apex of the nucellus; S,
“spore”, filled with endosperm, in which lies the embryo _e_.]
To enter into the detailed anatomy of the seeds would lead us too far
into the realms of the specialist, but we must notice one or two points
about them. Firstly, their very large size, for ripe seeds of _Cycas_ are
as large as peaches (and peaches, it is to be noted, are fruits, not
seeds), and particularly the large size they attain _before_ they are
fertilized and have an embryo. Among the higher plants the young seeds
remain very minute until an embryo is secured by the act of
fertilization, but in the Cycads the seeds enlarge and lay in a big store
of starch in the endosperm before the embryo appears, so that in the
cases in which fertilization is prevented large, sterile “seeds” are
nevertheless produced. This must be looked on as a want of precision in
the mechanism, and as a wasteful arrangement which is undeniably
primitive. An even more wasteful arrangement appears to have been common
to the “seeds” of the Palæozoic period, for, though many fossil “seeds”
are known in detail from the old rocks, not one is known to have any
trace of an embryo. A general plan of the _Cycas_ seed is shown in fig.
76, which should be compared with that of _Ginkgo_ (fig. 68). The large
size of the endosperm and the thick and complex seed-coats are
characteristic features of both these structures. Another point that
makes the Cycad seeds of special interest is the fact that the male cells
(as in _Ginkgo_) are developed as active, free-swimming sperms, which
swim towards the female cell in the space provided for them in the seed
(see _p.c_, fig. 76).
The characters of the Cycads as they are now living prove them to be an
extremely primitive group, and therefore presumably well represented
among the fossils; and indeed among the Mesozoic rocks there is no lack
of impressions which have been described as the leaves of Cycads. There
is, however, very little reliable material, and practically none which
shows good microscopic structure. Leaf impressions alone are most
unsafe—more unsafe in this group, perhaps, than in any other—for reasons
that will be apparent later on, and the conclusions that used to be drawn
about the vast number of Cycads which inhabited the globe in the early
Mesozoic must be looked on with caution, resulting from the experience of
recent discoveries proving many of these leaves to belong to a different
family.
There remain, however, many authentic specimens which show that _Cycas_
certainly goes back very far in history, and specimens of this genus are
known from the older Mesozoic rocks. We cannot say, however, as securely
as used to be said, that the Mesozoic was the “Age of Cycads”, although
it was doubtless the age of plants which had much of the external
appearance of Cycads.
From the Palæozoic we have no reliable evidence of the existence of
Cycads, though the plants of that time included a group which has an
undoubted connection with them.
Indeed, so far as fossil evidence goes, we must suppose that the Cycads,
since their appearance, possibly at the close of the Palæozoic, have
never been a dominant or very extensive family, though they grew in the
past all over the world, and in Europe seem to have remained till the
middle of the Tertiary epoch.
CHAPTER XII
PAST HISTORIES OF PLANT FAMILIES
V. Pteridosperms
This group consists entirely of plants which are extinct, and which were
in the height of their development in the Coal Measure period. As a group
they are the most recently discovered in the plant world, and but a few
years ago the name “Pteridosperm” was unknown. They form, however, both
one of the most interesting of plant families and one of the most
numerous of those which flourished in the Carboniferous period.
To mention first the vital point of interest in their structure, they
show _leaves which in all respects appear like ordinary foliage leaves,
and yet bear seeds_. These leaves, which we now know bore the seeds, had
long been considered as typical fern leaves, and had been named and
described as fern leaves. There are two extremely important results from
the discovery of this fossil group, viz. that leaves, to all appearance
like ordinary foliage, can directly bear seeds, and that the leaves,
though like fern leaves, bore seeds like those of a Cycad.
As the name _Pteridosperm_ indicates, the group is a link between the
ferns and the seed-bearing plants, and as such is of special interest and
value to botanists.
The gradual recognition of this group from among the numerous plant
fragments of Palæozoic age is one of the most interesting of the
accumulative discoveries of fossil botany. Ever since fossil remains
attracted the attention of enquiring minds many “ferns” have been
recognized among the rich impressions of the Coal Measures. Most of them,
however, were not connected with any structural material, and were given
many different names of specific value. So numerous were these fern
“species” that it was supposed that in the Coal Measure period the ferns
must have been the dominant class, and it is often spoken of even yet as
the “Age of Ferns”. From the rocks of the same age, preserved with their
microscopical structure perfect, were stems which were called
_Lyginodendron_. In the coal balls associated with these stems (which
were the commonest of the stems so preserved) were also roots, petioles,
and leaflets, but they were isolated, like the most of the fragments in a
coal ball, and to each was given its name, with no thought of the various
fragments having any connection with each other. Gradually, however,
various fragments from the coal balls had been recognized as belonging
together; one specimen of a petiole attached to a stem sufficed to prove
that all the scattered petioles of the same type belonged also to that
kind of stem, and when leaves were found attached to an isolated fragment
of the petiole, the chain of proof was complete that the leaves belonged
to the stem, a
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