CHAPTER V

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.