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diff --git a/43976-0.txt b/43976-0.txt index 1a25b68..64c511d 100644 --- a/43976-0.txt +++ b/43976-0.txt @@ -1,41 +1,4 @@ -The Project Gutenberg EBook of Ancient Plants, by Marie C. Stopes - -This eBook is for the use of anyone anywhere at no cost and with -almost no restrictions whatsoever. You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - - -Title: 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 - -Author: Marie C. 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You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - - -Title: 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 - -Author: Marie C. Stopes - -Release Date: October 18, 2013 [EBook #43976] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK ANCIENT PLANTS *** - - - - -Produced by Stephen Hutcheson, Chris Curnow and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - - 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 palobotany, 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 dbris (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 dbris 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 palobotanists. 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 dbris. 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 dbris, have been found in the coal or immediately -above it, and are of Palozoic 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 palozoic 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 palontologist. - -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 dbris 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 dbris. 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 dbris -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 dbris 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 dbris 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 dbris 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 ons -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 Palozoic, including - Permian. - Carboniferous. - Devonian. - Palozoic - VI. Older Palozoic. - Eozoic - VII. Archan. - -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 Archan, and even the Older Palozoic 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 on--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 dbris 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 dbris 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 ons -under one heading, the Older Palozoic. 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 Palozoic 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 _Cordaite_) was in full swing long -before the Carboniferous period began. Hence, though of Older Palozoic -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 Palozoic 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 dbris, -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 - Alg Seaweeds. - Fungi Moulds and toadstools. - Bryophyta - Hepatic Liverworts. - Musci Mosses. - Pteridophyta - Equisetales Horsetails. - Sphenophyllales* fossil only, _Sphenophyllum_. - Lycopodales Club-moss. - Filicales Bracken fern. - Pteridosperm - Lyginodendr* 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 Conifer, 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 Protococcoide among the -Alg. 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 Protococcoide 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 Alg 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 Protococcoide 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 Conifer and -Angiosperms the male cell does not swim, but is passive (or largely so), -and is brought to the egg cell. One might almost say that the whole -evolution of the complex structures found in fruiting cones and flowers -is a result of the need of protection of the delicate, simple -reproductive cells and the embryonic tissues resulting from their fusion. -The lower plants scatter these delicate cells broadcast in enormous -numbers, the higher plants protect each single egg cell by an elaborate -series of tissues, and actually bring the male cell to it without ever -allowing either of them to be exposed. - -It must be assumed that the reader possesses a general acquaintance with -the living families tabulated on p. 44; those of the fossil groups will -be given in some detail in succeeding chapters which deal with the -histories of the various families. It is premature to attempt any general -discussion of the evolution of the various groups till all have been -studied, so that this will be reserved for the concluding chapters. - - - - - CHAPTER VI - MINUTE STRUCTURE OF FOSSIL PLANTS--LIKENESSES TO LIVING ONES - - -The individual plants of the Coal Measure period differed entirely from -those now living; they were more than merely distinct species, for in the -main even the families were largely different from the present ones. -Nevertheless, when we come to examine the minute anatomy of the fossils, -and the cells of which they are composed, we find that between the living -and the fossil cell types the closest similarity exists. - -From the earliest times of which we have any knowledge the elements of -the plant body have been the same, though the types of structures which -they built have varied in plan. Individual _cells_ of nearly every type -from the Coal Measure period can be identically matched with those of -to-day. In the way the walls thickened, in the shapes of the wood, -strengthening or epidermal cells, in the form of the various tissues -adapted to specific purposes, there is a unity of organization which it -is reasonable to suppose depends on the fundamental qualities inherent in -plant life. - -This will be illustrated best, perhaps, by tabulating the chief -modifications of cells which are found in plant tissues. The -illustrations of these types in the following table are taken from living -plants, because from them figures of more diagrammatic clearness can be -made, and the salient characters of the cells more easily recognized. -Comparison of these typical cells with those illustrated from the fossil -plants reveals their identity in essential structure, and most of them -will be found in the photos of fossils in these pages, though they are -better recognized in the actual fossils themselves. - - - Principal Types of Plant Cells - -Epidermal. - - [Illustration: Fig. 21 - - _Epidermis._--Protecting layer or skin. Cells with outer wall thickened - in many cases (fig. 21, _a_ and _b_). Compare fossil epidermis in fig. - 34, _e_.] - - [Illustration: Fig. 22 _Hairs._--Extensions of epidermis cells. Single - cells, or complex, as fig. 22, _h_, where _e_ is epidermis and _p_ - parenchyma. Compare fossil hairs in figs. 79 and 120.] - - [Illustration: Fig. 23 - - _Stomates._--Breathing pores in the epidermis. Seen in surface view as - two-lipped structures (fig. 23). _s_, Stomates; _e_, epidermis cells. - Compare fossil stomates in fig. 8.] - -Ground Tissue - - [Illustration: Fig. 24 - - _Parenchyma._--Simple soft cells, either closely packed, as in fig. 24, - or with air spaces between them. Compare 78, B, for fossil.] - - [Illustration: Fig. 25 - - _Palisade._--Elongated, closely packed cells, _p_, chiefly in leaves, - lying below the epidermis, _e_, fig. 25. Compare fig. 34, _p_, for - fossil palisade.] - - [Illustration: Fig. 26 - - _Endodermis._--Cells with specially thickened walls, _en_, lying as - sheath between the parenchyma, _c_, of ground tissue, and the vascular - tissue, _s_, fig. 26. Compare fig. 108 for fossil endodermis.] - - [Illustration: Fig. 27 - - _Latex cells._--Large, often much elongated cells, _m_, lying in the - parenchyma, _p_, fig. 27, which are packed with contents. Compare fig. - 107, _s_.] - - [Illustration: Fig. 28 - - _Sclerenchyma._--Thick-walled cells among parenchyma for strengthening, - fig. 28. Compare fig. 34, _s_.] - - [Illustration: Fig. 29 - - _Cork._--Layers of cells replacing the epidermis in old stems. Outer - cells, _o_, crushed; _k_, closely packed cork cells; stone cells, _s_, - fig. 29. Compare fig. 95, _k_. - - _Cork cambium._--Narrow, actively dividing cells, _c_ in fig. 29, - giving rise to new cork cells in consecutive rows.] - - [Illustration: Fig. 30 - - _Tracheides._--Specially thickened cells in the parenchyma, usually for - water storage, _t_, fig. 30. Compare fig. 95, _t_.] - -Vascular Tissue - - [Illustration: Fig. 31 - - _Wood._--_Protoxylem_, tracheids and vessels, long, narrow elements, - with spiral or ring-like thickenings, _s_^1 and _s_^2, fig. 31. Compare - fig. 81, A, _px_, for fossil. - - _Metaxylem_, long elements, tracheids and vessels. Some with narrow - pits, as _t_ in fig. 31; others with various kinds of pits. In - transverse section seen in fig. 33, w, fossil in fig. 114, _w_. - - _Wood parenchyma._--Soft cells associated with the wood, _p_ in fig. - 31. Fossil in fig. 81, B, _p_. - - _Wood sclerenchyma._--Hard thickened cells in the wood.] - - [Illustration: Fig. 32 - - _Bast._--_Sieve tubes_, long cells which carry foodstuffs, cross walls - pitted like sieves, _s_, fig. 32. In transverse section in fig. 33. - - _Companion cells_, narrow cells with rich proteid contents, _c_, fig. - 32. In transverse section at _c_, fig. 33. - - _Bast parenchyma._--Soft unspecialized cells mixed with the sieve - tubes, _p_, fig. 32. - - _Bast fibres._--Thick-walled sclerenchymatous cells mixed with, or - outside, the soft bast.] - - [Illustration: Fig. 33 - - _Cambium._--Narrow cells, like those of the cork cambium, which lie - between the wood and bast, and give rise to new tissues of each kind, - _cb_, fig. 33. Compare fig. 114, fossil.] - -There are, of course, many minor varieties of cells, but these illustrate -all the main types. - -Among the early fossils, however, one type of wood cell and one type of -bast cell, so far as we know, are not present. These cells are the true -_vessels_ of the wood of flowering plants, and the long bast cells with -their companion proteid cells. The figure of a metaxylem wood cell, shown -in fig. 31, _t_, shows the more primitive type of wood cell, which has an -oblique cross wall. This type of wood cell is found in all the fossil -trees, and all the living plants except the flowering plants. The vessel -type, which is that in the big wood vessels of the flowering plants, and -has no cross wall, is seen in fig. 20, _x_. - -The similarity between the living cells and those of the Coal Measure -fossils is sufficiently illustrated to need no further comment. This -similarity is an extremely helpful point when we come to an -interpretation of the fossils. In living plants we can study the -physiology of the various kinds of cells, and can deduce from experiment -exactly the part they play in the economy of the whole plant. From a -study of the tissues in any plant structure we know what function it -performed, and can very often estimate the nature of the surrounding -conditions under which the plant was growing. To take a single example, -the palisade tissue, illustrated in fig. 25, _p_, in living plants always -contains green colouring matter, and lies just below the epidermis, -usually of leaves, but sometimes also of green stems. These cells do most -of the starch manufacture for the plant, and are found best developed -when exposed to a good light. In very shady places the leaves seldom have -this type of cell. Now, when cells just like these are found in fossils -(as is illustrated in fig. 34), we can assume all the physiological facts -mentioned above, and rest assured that that leaf was growing under normal -conditions of light and was actively engaged in starch-building when it -was alive. From the physiological standpoint the fossil leaf is entirely -the same as a normal living one. - - [Illustration: Fig. 34.--From a Photo of a Fossil Lea - - _e_, Epidermis; _p_, palisade cells; _pr_, soft parenchyma cells - (poorly preserved); _s_, sclerenchyma above the vascular bundle.] - -From the morphological standpoint, also, the features of the plant body -from the Coal Measure period fall into the same divisions as those of the -present. Roots, stems, leaves, and reproductive organs, the essentially -distinct parts of a plant, are to be found in a form entirely -recognizable, or sufficiently like that now in vogue to be interpreted -without great difficulty. In the detailed structure of the reproductive -organs more changes have taken place than in any others, both in internal -organization and external appearance. - -Already, in the Early Palozoic period, the distinction between leaves, -stems, roots, and reproductive organs was as clearly marked as it is -to-day, and, judging by their structure, they must each have performed -the physiological functions they now do. Roots have changed least in the -course of time, probably because, in the earth, they live under -comparatively uniform conditions in whatever period of the world's -history they are growing. Naturally, between the roots of different -species there are slight differences; but the likeness between fern roots -from the Palozoic and from a living fern is absolutely complete. This is -illustrated in fig. 35, which shows the microscopic structure of the two -roots when cut in transverse direction. The various tissues will be -recognized as coming into the table on p. 54, so that both in the details -of individual cells and in the general arrangement of the cell groups or -tissues the roots of these fossil and living ferns agree. - - [Illustration: Fig. 35.--A, Root of Living Fern. B, Root of Palozoic - Fossil Fern. _c_, Cortex; _px_, protoxylem in two groups; _m_, - metaxylem; _s_, space in fossil due to decay of soft cells.] - -Among stems there has been at all periods more variety than among the -roots of the corresponding plants, and in the following chapter, when the -differences between living and fossil plants will be considered, there -will be several important structures to notice. Nevertheless, there are -very many characters in which the stems from such widely different epochs -agree. The plants in the palozoic forests were of many kinds, and among -them were those with weak trailing stems which climbed over and supported -themselves on other plants, and also tall, sturdy shafts of woody trees, -many of which were covered with a corky bark. Leaves were attached to the -stems, either directly, as in the case of some living plants, or by leaf -stalks. In external appearance and in general function the stems then -were as stems are now. In the details of the individual cells also the -likeness is complete; it is in the grouping of the cells, the anatomy of -the tissues, that the important differences lie. It has been remarked -already that increase in complexity of the plant form usually goes with -an increase in complexity of the cells and variety of the tissues. The -general ground tissue in nearly all plants is very similar; it is -principally in the vascular system that the advance and variety lie. - -Plant anatomists lay particular stress on the vascular system, which, in -comparison with animal anatomy, holds an even more important position -than does the skeleton. To understand the essential characters of stems, -both living and fossil, and to appreciate their points of likeness or -difference, it is necessary to have some knowledge of the general facts -of anatomy; hence the main points on which stress is laid will be given -now in brief outline. - -Leaving aside consideration of the more rudimentary and less defined -structure of the alg and mosses, all plants may be said to possess a -"vascular system". This is typically composed of elongated wood (or -xylem) with accessory cells (see p. 57, table), and bast (phloem), also -with accessory cells. These specialized conducting elements lie in the -ground tissue, and in nearly all cases are cut off from direct contact -with it by a definite sheath, called the endodermis (see p. 55, fig. 26). -Very often there are also groups or rings of hard thick-walled cells -associated with the vascular tissues, which protect them and play an -important part in the consolidation of the whole stem. - - [Illustration: Fig. 36.--Diagram of Simplest Arrangement of Complete - Stele in a Stem - - W, Central solid wood; P, ring of bast; E, enclosing sheath of - endodermis; C, ground tissue or cortex.] - -The simplest, and probably evolutionally the most primitive form which is -taken by the vascular tissues, is that of a single central strand, with -the wood in the middle, the bast round it, and a circular endodermis -enclosing all, as in fig. 36, which shows a diagram of this arrangement. -Such a mass of wood and bast surrounded by an endodermis, is technically -known as a _stele_, a very convenient term which is much used by -anatomists. In its simplest form (as in fig. 36) it is called a -_protostele_, and is to be found in both living and fossil plants. A -number of plants which get more complex steles later on, have protosteles -in the early stages of their development, as in _Pteris aurita_ for -example, a species allied to the bracken fern, which has a hollow ring -stele when mature. - - [Illustration: Fig. 37.--Diagram of a Stele with a few Cells of Pith - _p_ in the Middle of the Wood. Lettering as in fig. 36] - - [Illustration: Fig. 38.--Diagram showing Extensive Pith _p_ in the - Wood. Lettering as in fig. 36] - -The next type of stele is quite similar to the protostele, but with the -addition of a few large unspecialized cells in the middle of the wood -(_p_, fig. 37); these are the commencement of the hollowing process which -goes on in the wood, resulting later in the formation of a considerable -pith, as is seen in fig. 38, where the wood is now a hollow cylinder, as -the phloem has been from the first. When this is the case, a second -sheath or endodermis generally develops on the inner side of the wood, -outside the pith, and cuts the vascular tissues off from the inner -parenchyma. A further step is the development of an inner cylinder of -bast so that the vascular ring is completely double, with endodermis on -both sides of the cylinder, as is seen in fig. 39. - - [Illustration: Fig. 39.--A Cylindrical Stele, with _e_, inner - endodermis, and _ph_, inner phloem; W, wood; P, outer phloem; E, outer - endodermis. L, part of the stele going out to supply a large leaf, thus - breaking what would otherwise appear as a closed ring stele] - -In all these cases there is but one strand or cylinder, of vascular -tissue in the stem, but one stele, and this type of anatomy is known as -the _monostelic_ or single-steled type. - - [Illustration: Fig. 40.--A Ring Stele apparently broken up into a - Number of Protosteles by many Leaf Gaps] - -When from the double cylinder just described a strand of tissue goes off -to supply a large leaf, a considerable part of the stele goes out and -breaks the ring. This is shown in fig. 39, where L is the part of the -stele going to a leaf, and the rest the broken central cylinder. When the -stem is short, and leaves grow thickly so that bundles are constantly -going out from the main cylinder, this gets permanently broken, and its -appearance when cut across at any given point is that of a group of -several steles arranged in a ring, each separate stele being like the -simple protostele in its structure. See fig. 40. This type of stem has -long been known as _polystelic_ (_i.e._ many-steled), and it is still a -convenient term to describe it by. There has been much theoretical -discussion about the true meaning of such a "polystelic" stem, which -cannot be entered into here; it may be noted, however, that the various -strands of the broken ring join up and form a meshwork when we consider -the stem as a whole, it is only in a single section that they appear as -quite independent protosteles. Nevertheless, as we generally consider the -anatomy of stems in terms of single sections, and as the descriptive word -"polystelic" is a very convenient and widely understood term, it will be -used throughout the book when speaking of this type of stem anatomy. - -Such a type as this, shown in fig. 40, is already complex, but it often -happens that the steles branch and divide still further, until there is a -highly complicated and sometimes bewildering system of vascular strands -running through the ground tissue in many directions, but cut off from it -by their protective endodermal sheaths. Such complex systems are to be -found both in living and fossil plants, more especially in many of the -larger ferns (see fig. 88). - -Higher plants in general, however, and in particular flowering plants, do -not have a polystelic vascular arrangement, but a specialized type of -monostele. - - [Illustration: Fig. 41.--Monostele in which the Central Pith is - Star-shaped, and the Wood breaking up into Separate Groups - - _p_, Pith; W, wood; P, phloem; E, endodermis; C, cortex.] - -Referring again to fig. 37 as a starting-point, imagine the pith in the -centre to spread in a star-shaped form till the points of the star -touched the edges of the ring, and thus to break the wood ring into -groups. A stage in this process (which is not yet completed) is shown in -fig. 41, while in fig. 42 the wood and bast groups are entirely distinct. -In the flowering plants the cells of the endodermis are frequently poorly -characterized, and the pith cells resemble those of the cortical ground -tissue, so that the separate groups of wood and bast (usually known as -"vascular bundles", in distinction from the "steles" of fig. 40) appear -to lie independently in the ground tissue. These strands, however, must -not be confused with steles, they are only fragments of the single -apparently broken up stele which runs in the stem. - - [Illustration: Fig. 42.--Monostele in which the Pith has invaded all - the Tissues as far as the Endodermis, and broken the Wood and Phloem up - into Separate Bundles. These are usually called "vascular bundles" in - the flowering plants] - - [Illustration: Fig. 43.--Showing actively growing Zone _c_ (Cambium) in - the Vascular Bundles, and joining across the ground tissue between them] - -The vascular bundle, of all except the Monocotyledons, has a potentiality -for continued growth and expansion which places it far above the stele in -value for a plant of long life and considerable growth. The cells lying -between the wood and the bast, the soft parenchyma cells always -accompanying such tissues, retain their vitality and continue to divide -with great regularity, and to give rise to a continuous succession of new -cells of wood on the one side and bast on the other; see fig. 33, _c_, -_b_. In this way the primary, distinct vascular bundles are joined by a -ring of wood, see fig. 43, to which are added further rings every season, -till the mass of wood becomes a strong solid shaft. This ever-recurring -activity of the cambium gives rise to what are known as "annual rings" in -stems, see fig. 44, in which the wood shows both primary distinct groups -in the centre, and the rings of growth of later years. - -Cambium with this power of long-continued activity is found in nearly all -the higher plants of to-day (except the Monocotyledons), but in the fern -and lycopod groups it is in abeyance. Certain cases from nearly every -family of the Pteridophytes are known, where some slight development of -cambium with its secondary thickening takes place, but in the groups -below the Gymnosperms cambium has almost no part to play. On the other -hand, so far back as the Carboniferous period, the masses of wood in the -Pteridophyte trees were formed by cambium in just the same way as they -are now in the higher forms. Its presence was almost universal at that -time in the lower groups where to-day there are hardly any traces of it -to be found. - - [Illustration: Fig. 44.--Stem with Solid Cylinder of Wood developed - from the Cambium, showing three "annual rings". In the centre may still - be seen the separate groups of the wood of the primary "vascular - bundles"] - -It will be seen from this short outline of the vascular system of plants, -that there is much variety possible from modifications of the fundamental -protostele. It is also to be noted that the plants of the Coal Measures -had already evolved all the main varieties of steles which are known to -us even now,[6] and that the development of secondary thickening was very -widespread. In several cases the complexity of type exceeds that of -modern plants (see Chap. VII), and there are to be found vascular -arrangements no longer extant. - -When we turn to the _Reproductive Organs_, we find that the points of -likeness between the living and the fossil forms are not so numerous or -so direct as they are in the case of the vegetative system. - - [Illustration: Fig. 45.--Fern Sporangia - - A, fossil; B, living.] - -As has been indicated, the families of plants typical of the Coal -Measures were not those which are the most prominent to-day, but belonged -to the lower series of Pteridophytes. In their simpler forms the -fructifications then and now resemble each other very closely, but in the -more elaborate developments the points of variety are more striking, so -that they will be dealt with in the following chapter. Cases of likeness -are seen in the sporangia of ferns, some of which appear to have been -practically identical with those now living. This is illustrated in fig. -45, which shows the outline of the cells of the sporangia of living and -fossil side by side. - - [Illustration: Fig. 46.--A, Living Lycopod cone; B, _Lepidodendron_ - (fossil) cone. _a_, Axis; _s_, scale; S, sporangium with spores. One - side of a longitudinal section] - -In the general structure also of the cones of the simpler types of -_Lepidodendron_ (fossil, see frontispiece) there is a close agreement -with the living Lycopods, though as regards size and output of spores -there was a considerable difference in favour of the fossils. The plan of -each is that round the axis of the cone simple scales are arranged, on -each of which, on its upper side, is seated a large sporangium bearing -numerous spores all of one kind (see fig. 46). - -Equally similar are the cones of the living Equisetum and some of the -simple members of the fossil family Calamite, but the more interesting -cases are those where differences of an important morphological nature -are to be seen. - -As regards the second[7] generation there is some very important -evidence, from extremely young stages, which has recently been given to -the world. In a fern sporangium _germinating spores_ were fossilized so -as to show the first divisions of the spore cell. These seem to be -identical with the first divisions of some recent ferns (see fig. 47). -This is not only of interest as showing the close similarity in detail -between plants of such widely different ages, but is a remarkable case of -delicate preservation of soft and most perishable structures in the "coal -balls". - - [Illustration: Fig. 47.--Germinating Fern Spores - - A and B, from carboniferous fossils; C, living fern. (A and B after - Scott.)] - -While these few cases illustrate points of likeness between the -fructifications of the Coal Measures and of to-day, the large size and -successful character of the primitive Coal Measure plants was accompanied -by many developments on the part of their reproductive organs which are -no longer seen in living forms, and the greater number of palozoic -fructifications must be considered in the next chapter. - - - - - CHAPTER VII - MINUTE STRUCTURE OF FOSSIL PLANTS--DIFFERENCES FROM LIVING ONES - - -We have seen in the last chapter that the main morphological divisions, -roots, stems, leaves, and fructifications, were as distinct in the Coal -Measure period as they are now. There is one structure, however, found in -the Coal Measure fossils, which is hardly paralleled by anything similar -in the living plants, and that is the fossil known as _Stigmaria_. -_Stigmaria_ is the name given, not to a distinct species of plant, but to -the large rootlike organs which we know to have belonged to all the -species of _Lepidodendron_ and of _Sigillaria_. In the frontispiece these -organs are well seen, and branch away at the foot of the trunk, spreading -horizontally, to all appearance merely large roots. They are especially -regularly developed, however, the main trunk giving rise always to four -primary branches, these each dividing into two equal branches, and so -on--in this they are unlike the usual roots of trees. They bore numerous -rootlets, of which we know the structure very well, as they are the -commonest of all fossils, but in their internal anatomy the main "roots" -had not the structure which is characteristic of roots, but were like -_stems_. In living plants there are many examples of stems which run -underground, but they always have at least the rudiments of leaves in the -form of scales, while the fossil structures have apparently no trace of -even the smallest scales, but bear only rootlets, thus resembling true -roots. The questions of morphology these structures raise are too complex -to be discussed here, and Stigmaria is only introduced as an example, one -of the very few available, of a palozoic structure which seems to be of -a nature not clearly determinable as either root, stem, leaf, or -fructification. Among living plants the fine rootlike rhizophores of -Selaginella bear some resemblance to Stigmaria in essentials, though so -widely different from them in many ways, and they are probably the -closest analogy to be found among the plants of to-day. - -The individual cells, we have already seen, are strikingly similar in the -case of fossil and living plants. There are, of course, specific -varieties peculiar to the fossils, of which perhaps the most striking -seem to be some forms of _hair_ cells. For example, in a species of fern -from the French rocks there were multicellular hairs which looked like -little stems of Equisetum owing to regular bands of teeth at the -junctions of the cells. These hairs were quite characteristic of the -species--but hairs of all sorts have always abounded in variety, so that -such distinction has but minor significance. - - [Illustration: Fig. 48.--Stele of _Lepidodendron_ W, surrounded by a - small ring of secondary wood S] - -As was noted in the table (p. 58) the only cell types of prime importance -which were not evolved by the Palozoic plants were the wood vessels, -phloem and accompanying cells which are characteristic of the flowering -plants. - -Among the fossils the vascular arrangements are most interesting, and, as -well as all the types of stele development noted in the previous chapter -as common to both living and fossil plants, there are further varieties -found only among the fossils (see fig. 50). - -The simple protostele described (on p. 61) is still found, particularly -in the very young stages of living ferns, but it is a type of vascular -arrangement which is not common in the mature plants of the present day. -In the Coal Measure period, however, the protostele was characteristic of -one of the two main groups of ferns. In different species of these ferns, -the protostele assumed a large variety of shapes and forms as well as the -simple cylindrical type. The central mass of wood became five-rayed in -some, star-shaped, and even very deeply lobed, with slightly irregular -arms, but in all these cases it remained fundamentally monostelic. -Frequently secondary tissue developed round the protosteles of plants -whose living relatives have no such tissue. A case of this kind is -illustrated in fig. 48, which shows a simple circular stele surrounded by -a zone of secondary woody tissue in a species of _Lepidodendron_. - - [Illustration: Fig. 49.--_Lepidodendron_, showing Part of the Hollow - Ring of Primary Wood W, with a relatively large amount of Secondary - Tissue S, surrounding it] - -In many species of _Lepidodendron_ the quantity of secondary wood formed -round the primary stele was very great, so that (as is the case in higher -plants) the primary wood became relatively insignificant compared with -it. In most species of _Lepidodendron_ the primary stele is a hollow ring -of wood (cf. fig. 38, p. 62) round which the secondary wood developed, as -is seen in fig. 49. These two cases illustrate a peculiarity of fossil -plants. Among living ones the solid and the simple ring stele are almost -confined to the Pteridophytes, where secondary wood does not develop, but -the palozoic Pteridophytes, while having the simple primary types of -steles, had quantities of secondary tissue, which was correlated with -their large size and dominant position. - - [Illustration: Fig. 50.--Diagram of Steles of the English _Medullosa_, - showing three irregular, solid, steles A, with secondary thickenings S, - all round each. _a_, Small accessory steles] - -Among _polystelic_ types (see p. 63) we find interesting examples in the -fossil group of the _Medullose_, which are much more complex than any -known at present, both owing to their primary structure and also to the -peculiar fact that all the steles developed secondary tissue towards the -inner as well as the outer side. One of the simpler members of this -family found in the English Coal Measures is illustrated in fig. 50. Here -there are three principal protosteles (and several irregular minor ones) -each of which has a considerable quantity of secondary tissue all round -it, so that a portion of the secondary wood is growing in towards the -actual centre of the stem as a whole--a very anomalous state of affairs. - -In the more complex Continental type of _Medullosa_ there are _very_ -large numbers of steles. In the one figured from the Continent in fig. 51 -but a few are represented. There is a large outer double-ring stele, with -secondary wood on both sides of it, and within these a number of small -steles, all scattered through the ground tissue, and each surrounded by -secondary wood. In actual specimens the number of these central steles is -much greater than that indicated in the diagram. - -No plant exists to-day which has such an arrangement of its vascular -cylinder. It almost appears as though at the early period, when the -Medullose flourished, steles were experimenting in various directions. -Such types as are illustrated in figs. 50 and 51 are obviously wasteful -(for secondary wood developing towards the centre of a stem is bound to -finally meet), and complex, but apparently inefficient, which may partly -account for the fact that this type of structure has not survived to the -present, though simpler and equally ancient types have done so. - - [Illustration: Fig. 51.--Continental _Medullosa_, showing R, outer - double-ring stele with secondary wood all round it; S, inner stellate - steles, also surrounded in each case by secondary tissue] - -Further details of the anatomy of fossils will be mentioned when we come -to consider the individual families; those now illustrated suffice to -show that in the Coal Measures very different arrangements of steles were -to be found, as well as those which were similar to those existing now. -The significance of these differences will become apparent when their -relation to the other characters of the plants is considered. - -The fructifications, always the most important parts of the plant, offer -a wide field, and the divergence between the commoner palozoic and -recent types seems at first to be very great. Indeed, when palozoic -reproductive bodies have to be described, it is often necessary to use -the common descriptive terms in an altered and wider sense. - -Among the plants of to-day there are many varieties of the simple -single-celled reproductive masses which are called _spores_, and which -are usually formed in large numbers inside a spore case or sporangium. -Among the higher plants _seeds_ are also known in endless variety, all of -which, compared with spores, are very complex, for they are many-celled -structures, consisting essentially of an embryo or young plant enclosed -in various protective coats. The distinction between the two is sharp and -well defined, and for the student of living plants there exists no -difficulty in separating and describing seeds and spores. - -But when we look back through the past eras to palozoic plants the -subject is not so easy, and the two main types of potentially -reproductive masses are not sharply distinct. The seed, as we know it -among recent plants, and as it is generally defined, had not fully -evolved; while the spores were of great variety and had evolved in -several directions, some of which seem to have been intermediate stages -between simple spores and true seeds. These seedlike spores served to -reproduce the plants of the period, but their type has since died out and -left but two main methods among living plants, namely the essentially -simple spores, the very simplicity of whose organization gives them a -secure position, and the complex seeds with their infinite variety of -methods for protecting and scattering the young embryos they contain. - -Among the Coal Measure fossils we can pick up some of the early stages in -the evolution of the seed from the spore, or at least we can examine -intermediate stages between them which give some idea of the possible -course of events. Hence, though the differences from our modern -reproductive structures are so noticeable a feature of the palozoic -ones, it will be seen that they are really such differences as exist -between the members at the two ends of a series, not such as exist -between unrelated objects. - -Very few types can be mentioned here, and to make their relations clear a -short series of diagrams with explanations will be found more helpful -than a detailed account of the structures. - - [Illustration: Fig. 52.--Spores - - Each spore a single cell which develops with three others in tetrads - (groups of four). Very numerous tetrads enclosed in a spore case or - sporangium which develops on a leaflike segment called the sporophyll. - Each spore germinates independently of the others after being - scattered, all being of the same size. Common in fossils and living - Pteridophytes.] - - [Illustration: Fig. 53.--Spores - - Each a single cell like the preceding, but here only one tetrad in a - sporangium ripens, so that each contains only four spores. Compared - with the preceding types these spores are very large. Otherwise details - similar to above. Some fossils have such sporangia with eight spores, - or some other small number; living Selaginellas have four. In the same - cone sporangia with small spores are developed and give rise to the - male organs.] - - [Illustration: Fig. 54.--"Spores" of Seedlike Structure - - Out of a tetrad in each sporangium only one spore ripens, S in figure, - the others, _s_, abort. The wall of the sporangium, _w_, is more - massive than in the preceding cases, and from the sporophyll, flaps, - _sp f_, grow up on each side and enclose and protect the sporangium. - The one big spore appears to germinate inside these protective coats, - and not to be scattered separately from them. Only found in fossils, - one of the methods of reproduction in _Lepidodendron_. Other sporangia - with small spores were developed which gave rise to the male organs.] - - [Illustration: Fig. 55.--"Seed" - - In appearance this is like a seed, but differs from a true seed in - having no embryo, and is like the preceding structure in having a very - large spore, S, though there is no trace of the three aborting ones. - The spore develops in a special mass of tissue known as the nucellus, - _n_, which partly corresponds to the sporangium wall of the previous - types. In it a cavity, _p c_, the pollen chamber, receives the pollen - grains which enter at the apex of the "seed". There is a complex coat, - C, which stands round the nucellus but is not joined to it, leaving the - space _l_ between them. Only in fossils; _Trigonocarpus_ (see p. 122) - is similarly organized. Small spores in fern-like sporangia, called - pollen grains.] - - [Illustration: Fig. 56.--"Seed" - - Very similarly organized to the above, but the coat is joined to the - nucellus about two-thirds of its extent, and up to the level _l_. In - the pollen chamber, _p c_, a cone of nucellar tissue projects, and the - upper part of the coat is fluted, but these complexities are not of - primary importance. The large spore S germinated and was fertilized - within the "seed", but apparently produced no embryo before it ripened. - Small "spores" in fern-like sporangia form the pollen grains. Only in - fossils, _e.g._ Lagenostoma. (See p. 119.)] - - [Illustration: Fig. 57.--Seed - - Essentially similar to the preceding, except in the possession of an - embryo _e_, which is, however, small in comparison with the endosperm - which fills the spore S. The whole organization is simpler than in the - fossil _Lagenostoma_, but the coat is fused to the nucellus further up - (see _l_). Small "spores" form the pollen grains. Living and fossil - type, Cycads and Ginkgo.] - - [Illustration: Fig. 58.--Seed - - In the ripe seed the large embryo _e_ practically fills up all the - space within the two seed coats _c^1_ and _c^2_; endosperm, pollen - chamber, &c., have been eliminated, and the young ovule is very simple - and small as a result of the protection and active service of the - carpels in which it is enclosed. Small "spores" form the pollen grains. - Typical of living Dicotyledons.] - -These few illustrations represent only the main divisions of an army of -structures with an almost unimaginable wealth of variety which must be -left out of consideration. - -For the structures illustrated in figs. 54, 55, and 56 we have no name, -for their possible existence was not conceived of when our terminology -was invented, and no one has yet christened them anew with distinct -names. They are evidently too complex in organization and too similar to -seeds in several ways to be called spores, yet they lack the essential -element in a seed, namely, an embryo. The term "ovule" (usually given to -the young seed which has not yet developed an embryo) does not fit them -any better, for their tissues are ripened and hard, and they were of -large size and apparently fully grown and mature. - -For the present a name is not essential; the one thing that is important -is to recognize their intermediate character and the light they throw on -the possible evolution of modern seeds. - -A further point of great interest is the manner in which these "seeds" -were borne on the plant. To-day seeds are always developed (with the -exception of Cycas) in cones or flowers, or at least special -inflorescences. But the "seed" of _Lagenostoma_ (fig. 56), as well as a -number of others in the group it represents, were not borne on a special -structure, but directly on the green foliage leaves. They were in this on -a level with the simple sporangia of ferns which appear on the backs of -the fronds, a fact which is of great significance both for our views on -the evolution of seeds as such, and for the bearing it has on the -relationships of the various groups of allied plants. This will be -referred to subsequently (Chapter XI), and is mentioned now only as an -example of the difference between some of the characters of early fossils -and those of the present day. - -It is true that botanists have long recognized the organ which bears -seeds as a modified leaf. The carpels of all the higher plants are looked -on as _homologous_ with leaves, although they do not appear to be like -them externally. Sometimes among living plants curious diseases cause the -carpels to become foliar, and when this happens the diseased carpel -reverts more or less to the supposed ancestral leaf-like condition. It is -only among the ancient (but recently discovered) fossils, however, that -seeds are known to be borne normally on foliage leaves. - -From Mesozoic plants we shall learn new conceptions about flowers and -reproductive inflorescences in general, but these must be deferred to the -consideration of the family as a whole (Chapter XIII). - -Enough has been illustrated to show that though the individual cells, the -bricks, so to speak, of plant construction, were so similar in the past -and present, yet the organs built up by them have been continually -varying, as a child builds increasingly ambitious palaces with the same -set of bricks. - - - - - CHAPTER VIII - PAST HISTORIES OF PLANT FAMILIES - I. Flowering Plants, Angiosperms - - -In comparison with the other groups of plants the flowering families are -of recent origin, yet in the sense in which the word is usually used they -are ancient indeed, and the earliest records of them must date at least -to periods hundreds of thousands of years ago. - -Through all the Tertiary period (see p. 34) there were numerous flowering -plants, and there is evidence that many families of both Monocotyledons -and Dicotyledons existed in the Upper Cretaceous times. Further back than -this we have little reliable testimony, for the few specimens of -so-called flowering plants from the Lower Mesozoic are for the most part -of a doubtful nature. - -The flowering plants seem to stand much isolated from the rest of the -plant world; there is no _direct_ evidence of connection between their -oldest representatives and any of the more primitive families. So far as -our actual knowledge goes, they might have sprung into being at the -middle of the Mesozoic period quite independently of the other plants -then living; though there are not wanting elaborate and almost convincing -theories of their connection with more than one group of their -predecessors (see p. 108). - -It is a peculiarly unfortunate fact that although the rocks of the -Cretaceous and Tertiary are so much less ancient than those of the Coal -Measures, they have preserved for us far less well the plants which were -living when they were formed. Hitherto no one has found in Mesozoic -strata masses of exquisitely mineralized Angiosperm fragments[8] like -those found in the Coal Measures, which tell us so much about the more -ancient plants. Cases are known of more or less isolated fragments with -their microscopical tissues mineralized. For example, there are some -palms and ferns from South America which show their anatomical structure -very clearly preserved in silica, and which seem to resemble closely the -living species of their genera. The bulk of the plants preserved from -these periods are found in the form of casts or impressions (see p. 10), -which, as has been pointed out already, are much less satisfactory to -deal with, and give much less reliable results than specimens which have -also their internal structure petrified. The quantity of material, -however, is great, and impressions of single leaves innumerable, and of -specimens of leaves attached to stems, and even of flowers and fruits, -are to be found in the later beds of rock. These are generally clearly -recognizable as belonging to one or other of the living families of -flowering plants. Leaf impressions are by far the most frequent, and our -knowledge of the Tertiary flora is principally derived from a study of -them. Their outline and their veins are generally preserved, often also -their petioles and some indication of the thickness and character of the -fleshy part of the leaf. From the outline and veins alone an expert is -generally able to determine the species to which the plant belongs, -though it is not always quite safe to trust to these determinations or to -draw wide-reaching conclusions from them. - -In fig. 59 is shown a photograph of the impression of a Tertiary leaf, -which illustrates the condition of an average good specimen from rocks of -the period. Its shape and the character of the veins are sufficient to -mark it out immediately as belonging to the Dicotyledonous group of the -flowering plants. - -Seeds and fruits are also to be found; and in some very finely preserved -specimens from Japan stamens from a flower and delicate seeds are seen -clearly impressed on the light stone. In fig. 60 is illustrated a couple -of such seeds, which show not only their wings but also the small -antenn-like stigmas. Specimens so perfectly preserved are practically as -good as herbarium material of recent plants, and in this way the -externals of the Tertiary plants are pretty well known to us. - - [Illustration: Fig. 59.--Dicotyledonous Leaf Impression from Tertiary - Rocks] - - [Illustration: Fig. 60.--Seeds from Japanese Tertiary Rocks; at _a_ are - seen the two stigmas still preserved] - -A problem which has long been discussed, and which has aroused much -interest, is the relative antiquity of the Monocotyledonous and the -Dicotyledonous branches of the flowering plants. A peculiar fascination -seems to hang over this still unsolved riddle, and a battle of flowers -may be said to rage between the lily and the rose for priority. Recent -work has thrown no decisive light on the question, but it has undoubtedly -demolished the old view which supposed that the Monocotyledons (the lily -group) appeared at a far earlier date upon this earth than the -Dicotyledons. The old writers based their contention on incorrectly -determined fossils. For instance, seeds from the Palozoic rocks were -described as Monocotyledons because of the three or six ribs which were -so characteristic of their shell; we know now that these seeds -(_Trigonocarpus_) belong to a family already mentioned in another -connection (p. 72), the Medullose (see p. 122), the affinity of which -lies between the cycads and the ferns. Leaves of _Cordaites_, again, -which are broad and long with well-marked parallel veins, were described -as those of a Monocotyledonous plant like the Yucca of to-day; but we now -know them to belong to a family of true Gymnosperms possibly distantly -related to _Taxus_ (the Yew tree). - -Recent work, which has carefully sifted the fossil evidence, can only say -that no true Monocotyledons have yet been found below the Lower -Cretaceous rocks, and that at that period we see also the sudden inrush -of Dicotyledons. Hence, so far as palontology can show, the two parallel -groups of the flowering plants arose about the same time. It is of -interest to note, however, that the only petrifaction of a flower known -from any part of the world is an ovary which seems to be that of one of -the Liliace. In the same nodules, however, there are several specimens -of Dicotyledonous woods, so that it does not throw any light on the -question of priority. - -With the evidence derived from the comparative study of the anatomy of -recent flowering plants we cannot concern ourselves here, beyond noting -that the results weigh in favour of the Dicotyledons as being the more -primitive, though not necessarily developed much earlier in point of -time. Until very much more is discovered than is yet known of the origin -of the flowering plants as a whole, it is impossible to come to a more -definite conclusion about this much-discussed subject. - -Let us now attempt to picture the vegetable communities since the -appearance of the flowering plants. The facts which form the bases of the -following conceptions have been gathered from many lands by numerous -workers in the field of fossil botany, from scattered plant remains such -as have been described. - -When the flowering plants were heralded in they appeared in large -numbers, and already by the Cretaceous period there were very many -different species. Of these a number seem to belong to genera which are -still living, and many of them are extremely like living species. It -would be wearisome and of little value to give a list of all the recorded -species from this period, but a few of the commoner ones may be mentioned -to illustrate the nature of the plants then flourishing. - -Several species of _Quercus_ (the Oak) appeared early, particularly -_Quercus Ilex_; leaves of the _Juglandace_ (Walnut family) were very -common, and among the Tertiary fossils appear its fruits. Both _Populus_ -(the Poplar) and _Salix_ (the Willow) date from the early rocks, while -_Ficus_ (the Fig) was very common, and _Casuarina_ (the Switch Plant) -seems to have been widely spread. Magnolias also were common, and it -appears that _Platanus_ (the Plane) and _Eucalyptus_ coexisted with them. - -It will be immediately recognized that the above plants have all living -representatives, either wild or cultivated, growing in this country at -the present day, so that they are more or less familiar objects, and -there appears to have been no striking difference between the early -flowering plants and those of the present day. Between the ancient -Lycopods, for example, and those now living the differences are very -noteworthy; but the earliest of the known flowering plants seem to have -been essentially like those now flourishing. It must be remembered in -this connection that the existing flowering plants are immensely nearer -in point of time to their origin than are the existing Lycopods, and that -when such ons have passed as divide the present from the Palozoic, the -flowering plants of the future may have dwindled to a subordinate -position corresponding to that held by the Lycopods now. - -A noticeable character of the early flowering-plant flora, when taken as -a whole, is the relatively large proportion of plants in it which belong -to the family _Amentifer_ (oaks, willows, poplars, &c.). This is -supposed by some to indicate that the family is one of the most primitive -stocks of the Angiosperms. This view, however, hardly bears very close -scrutiny, because it derives its main support from the large numbers of -the Amentifer as compared with other groups. Now, the Amentifer were -(and are) largely woody resistant plants, whose very nature would render -them more liable to be preserved as impressions than delicate trees or -herbs, which would more readily decay and leave no trace. Similarly based -on uncertain evidence is the surmise that the group of flowers classed as -_Gamopetal_ (flowers with petals joined up in a tube, like convolvulus) -did not flourish in early times, but are the higher and later development -of the flower type. Now, _Viburnum_ (allied to the honeysuckle) belongs -to this group, and it is found right down in the Cretaceous, and -_Sambucus_ (Elder, of the same family) is known in the early Tertiary. -These two plants are woody shrubs or small trees, while many others of -the family are herbs, and it is noteworthy that it is just these woody, -resistant forms which are preserved as fossils; their presence -demonstrates the antiquity of the group as a whole, and the absence of -other members of it may be reasonably attributed to accidents of -preservation. In the Tertiary also we get a member of the heath family, -viz. _Andromeda_, and another tube-flower, _Bignonia_, as well as several -more _woody_ gamopetalous flowers. - -Hence it is wise to be very cautious about drawing any important -conclusions from the relative numbers of the different species, or the -absence of any type of plant from the lists of those as yet known from -the Cretaceous. When quantities of structurally preserved material can be -examined containing the flowering plants in petrifactions, then it will -be possible to speak with some security of the nature of the Mesozoic -flora as a whole. - -The positive evidence which is already accumulated, however, is of great -value, and from it certain deductions may be safely made. Specimens of -Cretaceous plants from various parts of the world seem to indicate that -there was a very striking uniformity in the flora of that period all over -the globe. In America and in Central Europe, for example, the same types -of plants were growing. We shall see that, as time advanced, the various -types became separated out, dying away in different places, until each -great continent and division of land had a special set of plants of its -own. At the commencement of the reign of flowering plants, however, they -seem to have lived together in the way we are told the beasts first lived -in the garden of Eden. - -At the beginning of the Tertiary period there were still many tropical -forms, such as Palms, Cycads, _Nipa_, various _Artocarpace_, _Laurace_, -_Araliace_, and others, growing side by side with such temperate forms -as _Quercus_, _Alnus_, _Betula_, _Populus_, _Viburnum_, and others of the -same kind. Before the middle of the Tertiary was reached the last Cycads -died in what is now known as Europe; and soon after the middle Tertiary -all the tropical types died out of this zone. - -At the same time those plants whose leaves appear to have fallen at the -end of the warm season began to become common, which is taken as an -indication of a climatic influence at work. Some writers consider that in -the Cretaceous times there was no cold season, and therefore no regular -period of leaf fall, but as the climate became temperate the deciduous -trees increased in numbers; yet the Gymnospermic and Angiospermic woods -which are found with petrified structure show well-marked annual rings -and seem to contradict this view. - -Toward the end of the Tertiary times there were practically no more -tropical forms in the European flora, though there still remained a -number of plants which are now found either only in America or only in -Asia. - -The Glacial epoch at the close of the Tertiary appears to have driven all -the plants before it, and afterwards, when its glaciers retreated, -shrinking up to the North and up the sides of the high mountains, the -plant species that returned to take possession of the land in the -Quaternary or present period were those which are still inhabiting it, -and the floras of the tropics, Asia, and America were no longer mixed -with that of Europe.[9] - - - - - CHAPTER IX - PAST HISTORIES OF PLANT FAMILIES - II. Higher Gymnosperms - - -The more recent history of the higher Gymnosperms, in the Upper -Cretaceous and Tertiary periods, much resembles that of the flowering -plants as sketched in the previous chapter. Many of the genera appear to -have been those still living, and some of the species even may have come -very close to or have been identical with those of to-day. The forms now -characteristic of the different continents were growing together, and -appear to have been widely distributed over the globe. For example, -_Sequoia_ and _Taxodium_, two types now characteristic of America, and -_Glyptostrobus_, at present found in Asia, were still growing with the -other European types in Europe so late as middle Tertiary times. - -As in the case of the Angiosperms, the fossils we have of Cretaceous and -Tertiary Gymnosperms are nearly all impressions and casts, though some -more or less isolated stems have their structure preserved. Hence our -knowledge of these later Gymnosperms is far from complete. From the older -rocks, however, we have both impressions and microscopically preserved -material, and are more fully acquainted with them than with those which -lived nearer our own time. Hard, resistant leaves, which are so -characteristic of most of the living genera of Gymnosperms, seem to have -been also developed in the past members of the group, and these tend to -leave clear impressions in the rocks, so that we have reliable data for -reconstructing the external appearance of the fossil forms from the -Palozoic period. - -The resinous character of Gymnosperm wood probably greatly assisted its -preservation, and fragments of it are very common in rocks of all ages, -generally preserved in silica so as to show microscopic structure. The -isolated wood of Gymnosperms, however, is not very instructive, for from -the wood alone (and usually it is just fragments of the secondary wood -which are preserved) but little of either physiological or evolutional -value can be learned. When twigs with primary tissues and bark and leaves -attached are preserved, then the specimens are of importance, for their -true character can be recognized. Fortunately among the coal balls there -are many such fragments, some of which are accompanied by fruits and male -cones, so that we know much of the Palozoic Gymnosperms, and find that -in some respects they differ widely from those now living. - -There is, therefore, much more to be said about the fossil Gymnosperms -than about the Angiosperms, both because of the better quality of their -preservation and because their history dates back to a very much earlier -period than does the Angiospermic record. Indeed, we do not know when the -Gymnosperms began; the well-developed and ancient group of _Cordaite_ -was flourishing before the Carboniferous period, and must therefore date -back to the rocks of which we have no reliable information from this -point of view, and the origin of the Gymnosperms must lie in the -pre-Carboniferous period. - -The group of Gymnosperms includes a number of genera of different types, -most of which may be arranged under seven principal families. In a sketch -of this nature it is, of course, quite impossible to deal with all the -less-important families and genera. Those that will be considered here -are the following:-- - - Coniferales (see p. 90). - _Araucare_, _e.g._ Monkey-puzzle - Genera both living and fossil. - Fossil forms undoubted so far back as the Jurassic, and - presumably further. - _Abietine_, _e.g._ Pine and Larch - Genera both living and fossil. - Fossils recognized as far back as the Lower Cretaceous. - _Cupresse_, _e.g._ Juniper, Cypress - Genera both living and fossil. - Fossils recognized as far back as the Jurassic. - _Taxe_, _e.g._ Yew - Genera living and fossil. - Fossils recognized as far back as the Cretaceous. - Cordaitales (see p. 92). - _Cordaite_, _e.g._ Cordaites - Fossil only. - Characteristic of Devonian, Carboniferous, and Permian periods. - _Poroxyle_, _e.g._ Poroxylon - Fossil only. - Characteristic of the Carboniferous and Permian. - Ginkgoales (see p. 98). - _Ginkgoace_, _e.g._ Ginkgo - Fossil and living, dating back, apparently with little change, - to Palozoic times. - -We must pay the most attention to the two last groups, as they are so -important as fossils, and the _Cordaite_ were a very numerous family in -Coal Measure times. They had their period of principal development so -long ago that it is probable that no direct descendants remain to the -present time, though some botanists consider that the _Taxe_ are allied -to them. - -Of the groups still living it is difficult, almost impossible, to say -which is the highest, the most evolved type. In the consideration of the -Gymnosperm family it is brought home with great emphasis how incomplete -and partial our knowledge is as yet. Many hold that the _Araucare_ are -the most primitive of the higher Gymnosperms. In support of this view the -following facts are noted. They have a simple type of fructification, -with a single seed on a simple scale, and many scales arranged round an -axis to form a cone. In the microscopic structure of their wood they have -double rows of bordered pits, a kind of wood cell which comes closer to -the old fossil types than does the wood of any of the other living -genera. Further than this, wood which is almost indistinguishable from -the wood of recent Araucarias is found very far back in the rocks, while -their leaves are broad and simple, and attached directly to the stem in a -way similar to the leaves of the fossil _Cordaite_, and very different -from the needle leaves on the secondary stems of the Pine family; so that -there appears good ground for considering the group an ancient and -probably a primitive one.[10] - -On the other hand, there are not wanting scientists who consider the -_Abietine_ the living representatives of the most primitive and ancient -stock, though on the whole the evidence seems to indicate more clearly -that the Pine-tree group is specialized and highly modified. Their double -series of foliage leaves, their complex cones (whose structures are not -yet fully understood), and their wood all support the latter view. - -Some, again, consider the _Taxe_ as a very primitive group, and would -place them near the Cordaite, with which they may be related. Their -fleshy seeds, growing not in cones but on short special axes, support -this view, and it is certainly true that in many ways the large seeds, -with their succulent coats and big endosperm, are much like those of the -lower Gymnosperms and of several fossil types. Those, however, who hold -to the view that the Abietine are primitive, see in the _Taxe_ the -latest and most modified type of Gymnosperm. - -It will be seen from this that there is no lack of variety regarding the -interpretation of Gymnosperm structures. - -The Gymnosperms do not stand in such an isolated position as do the -Angiosperms. Whatever the variety of views held about the details of the -relative placing of the families within the group, all agree in -recognizing the evidence which enables us to trace with confidence the -connection between the lower Gymnosperms and the families of ferns. There -are many indications of the intimate connection between higher and lower -Gymnosperms. Between the series exist what might be described as -different degrees of cousinship, and in the lower groups lie unmistakable -clues to their connection with more ancient groups in the past which -bridge over the gaps between them and the ferns. - -For the present, however, let us confine ourselves to the history of the -more important Gymnosperms, the discussion of their origin and the groups -from which they may have arisen must be postponed until the necessary -details about those groups have been mentioned. - -To a consideration of the living families of _Araucare_, _Abietine_, -_Cupresse_, and _Taxe_ we can allow but a short space; their general -characters and appearance are likely to be known to the reader, and their -details can be studied from living specimens if they are not. For -purposes of comparison with the fossils, however, it will be necessary to -mention a few of the principal features which are of special importance -in discussing phylogeny. - -The Araucariace are woody trees which attain a considerable size, with -broad-based, large leaves attached directly to the stem. In the leaves -are a series of numerous parallel vascular bundles. The wood cells in -microscopic section show two rows or more of round bordered pits. The -cones are very large, but the male and female are different in size and -organization. The female cone is composed of series of simple scales -arranged spirally round the axis, and each scale bears a single seed and -a small ligule. - -The pollen grains from the male cone are caught on the ligule and the -pollen tubes enter the micropyle of the ovule, bringing in passive male -cells which may develop in large numbers in each grain. The seeds when -ripe are stony, and some are provided with a wing from part of the tissue -of the scale. In the ripe cones the scales separate from the cone axis. - -The Abietine are woody trees, some reaching a great height, all with a -strong main stem. The leaves are of two kinds: primary ones borne -directly attached to the stem (as in first-year shoots of the Larch), and -secondary ones borne in tufts of two (in Pine) or a large number (in -older branches of Larch) on special short branches, the primary leaves -only developing as brown scales closely attached to the stems. Leaves -generally very fine and needlelike, and with a central vascular bundle. -The wood in microscopic section shows a single row of round bordered pits -on the narrow trache. - -The female cones are large, male and female differing greatly in size and -organization. The female cone, composed of a spiral series of pairs of -scales, which often fuse together as the cone ripens. Each upper scale of -the pair bears two seeds. The pollen grains from the male cone enter the -micropyle of the seed and are caught in the tissue (apex of nucellus) -there; the pollen tubes discharge passive male cells, only two of which -develop in each grain. The seeds when ripe are stony and provided with a -wing from the tissue of the scale on which they were borne. - -The Cupresse are woody trees reaching no great height, and of a bushy, -branching growth. The leaves are attached directly to the main stem, and -arrange themselves in alternating pairs of very small leaves, closely -pressed to the stem. The wood in microscopic section shows a single row -of round bordered pits on the trache. - -The cones are small, and the scales forming them arranged in cycles. The -female scales bear a varying number of seeds. The pollen grain has two -passive male cells. The seeds when ripe are stony, with wings, though in -some cases (species of Juniper) the cone scales close up and become -fleshy, so that the whole fruit resembles a berry. - -The Taxe are woody, though not great trees, bushily branched. The leaves -are attached spirally all round the stem, but place themselves so as to -appear to lie in pairs arranged in one horizontal direction. The wood in -microscopic section shows a single row of round bordered pits on the -trache. - -There are small male cones, but the seeds are not borne on cones, growing -instead on special short axes, where there may be several young ovules, -but on which usually two seeds ripen. The seeds are big, and have an -inner stone and outer fleshy covering. Some have special outer fleshy -structures known as "arils", _e.g._ the red outer cup round the yew -"berry" (which is not a berry at all, but a single unenclosed seed with a -fleshy coat). - -When we turn to the Cordaite we come to a group of plants which bears -distinct relationship to the preceding, but which has a number of -individual characters. It is a group of which we should know nothing were -it not for the fossils preserved in the Palozoic rocks; yet, -notwithstanding the fact that it flourished so long ago, it is a family -of which we know much. At the time of the Coal Measures and the -succeeding Permo-carboniferous period, it was of great importance, and, -indeed, in some of the French deposits it would seem as though whole -layers of coal were composed entirely of its leaves. - -Among the fossil remains of this family there are impressions, casts, and -true petrifactions, so that we know both its external appearance and the -internal anatomy of nearly every part of several species of the genus. -For a long time the various fossil remains of the plant were not -recognized as belonging to each other and together forming the records of -one and the same plant--the broad, long leaves with their parallel veins -were looked on as Monocotyledons (see fig. 61); the pith casts (see fig. -63) were thought to be peculiar constricted stems, and were called -_Sternbergia_; while the wood, which was known from its microscopic -structure, was called _Araucarioxylon_--but the careful work of many -masters of fossil botany, whose laborious studies we cannot describe in -detail here, brought all these fragments together and proved them to -belong to _Cordaites_. - - [Illustration: Fig. 61.--Leaf of _Cordaites_, _l_, attached by its - broad base to a Stem, _s_] - -We now know that _Cordaites_ were large trees, with strong upright shafts -of wood, to whose branches large simple leaves were attached. The leaves -were much bigger than those of any living Gymnosperm, even than those of -the Kauri Pine (a member of the Araucariace), and seem in some species -to have exceeded 3 ft. in length. The trees branched only at the top of -the main shaft, and with their huge sword-like leaves must have differed -greatly in appearance from any plant now living. The leaves had many -parallel veins, as can be seen in fig. 61, and were attached by a broad -base directly to the main stem; thus coming closer to the Araucarias than -the other groups of Gymnosperms in their leaf characters. - - [Illustration: Fig. 62A.--Microscopic Section of Part of a Leaf of - _Cordaites_ - - V, Vascular bundle; W, wood of bundle; _sh_, its sheath; S^1, large - sclerenchyma mass alternating with bundles; S^2 and S^3, sclerenchyma - caps of bundle; P, soft tissue of leaf.] - -The internal anatomy is often well preserved, and there is a number of -species of leaves whose anatomy is known. As will be expected from the -parallel veins, in each section there are many vascular bundles running -equidistantly through the tissue. Fig. 62A shows the microscopic details -from a well-preserved leaf. In all the species patches of sclerenchyma -were developed, and everything indicates that they were tough and well -protected against loss of water, even to a greater extent than are most -of the leaves of living Gymnosperms. - -In the stems the pith was much larger than that in living Gymnosperms -(where the wood is generally very solid), and it was hollow in older -stems, except for discs of tissue across the cavity. The internal cast -from these stems has been described before, and is seen in fig. 63. - - [Illustration: Fig. 62B.--Much-magnified Wood Elements from _Cordaites_ - Stem seen in longitudinal section, the type known as _Araucarioxylon_. - Note the hexagonal outlines of the bordered pits, which lie in several - rows] - -The wood was formed in closely packed radiating rows by a normal cambium -(see p. 66), and the trache so formed had characteristic rows of -bordered pits (see fig. 62B). The wood comes nearer to that of the living -Araucarias than any other, and indeed the numerous pieces of fossil wood -of this type which are known from all the geological periods are called -_Araucarioxylon_.[11] A double strand goes out from the main mass of -wood, which afterwards divides and subdivides to provide the numerous -bundles of the leaf. - - [Illustration: Fig. 63.--Cast of Hollow Pith of _Cordaites_, the - constrictions corresponding to discs of solid tissue across the cavity] - -In the case of these fossils we are fortunate enough to have the -fructifications, both male and female, in a good state of preservation. -As in other Gymnosperms, the male and female cones are separate, but they -differed less from each other in their arrangement than do those of any -of the living types hitherto mentioned. They can hardly be described as -true cones, though they had something of that nature; the seeds seem to -be borne on special short stems, round which are also sterile scales. In -the seed and the way it is borne perhaps the Cordaite may be compared -more nearly with the Taxe than with the other groups. A seed, not yet -ripe, is shown in slightly diagrammatic form in fig. 64, where the -essential details are illustrated. The seeds of this family sometimes -reached a considerable size, and had a fleshy layer which was thick in -comparison with the stone, and externally comparable with a -cherry--though, of course, of very different nature in reality, for -_Cordaites_, like _Taxus_, is a Gymnosperm, with simple naked seeds, -while a cherry is the fruit of an Angiosperm. - -In a few words, these are the main characters of the large group of -_Cordaites_, which held the dominant position among Gymnosperms in the -Palozoic era. They have relationships, or perhaps one should say -likenesses, to many groups. Their stem- and root-anatomy is similar to -the Conifer of the present day, the position of the ovules is like that -in the Taxace, the male cones in some measure recall those of _Ginkgo_, -the anatomy of their leaves has points which are comparable with those of -the Cycads, to which group also the large pith in the stem and the -structure of some details in the seeds unite them. Their own specially -distinctive characters lie in their crown of huge leaves, and unbranched -shaft of stem, the similarity of their male and female inflorescences, -and some points in their pollen grains which have not been mentioned. The -type is a very complex one, possibly coming near the stock which, having -branched out in various directions, gave rise to several of the living -families. - - [Illustration: Fig. 64.--Representation of _Cordaites_ Seed and its - Axis with Scales, slightly diagrammatic, modified from Renault. - - A, Axis with _s_, scales; _c_, coat of the seed, from which the inner - parts have shrunk away; _n_, nucellus; _p.c_, pollen chamber containing - pollen grains which enter through _m_.] - -Plants which come very near to the Cordaite are the Poroxyle. Of this -group we have unfortunately no remains of fructifications in organic -connection, so that its actual position must remain a little doubtful -till they are discovered. There seems no doubt that they must have borne -seeds. - -Still, it has been abundantly demonstrated in recent years that the -anatomy of the root, stem, and leaves indicates with considerable -exactness the position of any plant, so that, as these are known, we can -deduce from them, with a feeling of safety, the position that _Poroxylon_ -takes in the natural system. In its anatomy the characters are those of -the Cordaite, with certain details which show a more primitive nature -and seem to be characteristic of the groups below it in organization. - -_Poroxylon_ is not common, and until recently had not been found in the -Lower Coal Measures of England. The plants appear to have been much -smaller than _Cordaites_, with delicate stems which bore relatively large -simple leaves. The anatomy of the root was that common in Gymnosperms, -but the stem had a very large pith, and the leaves were much like those -of _Cordaites_ in having parallel veins. An important character in the -anatomy of the stem was the presence of what is known as _centripetal -wood_. This must be shortly explained. In all the stems hitherto -considered, the first-formed wood cells (protoxylems, see p. 57) -developed at the central point of the wood, towards the pith (see fig. -19, _px_, p. 49). This is characteristic of all Angiosperms and the -higher Gymnosperms (except in a couple of recently investigated Pines), -but among the lower plants we find that part of the later wood develops -to the inner side of these protoxylem masses. The distinction is shown in -fig. 65. - - [Illustration: Fig. 65.--A, Normal bundle of higher plant; _x_, - protoxylem on inner side next the pith _p_, and the older wood _w_ - outside it, _centrifugal_ wood. B, Bundle with wood cells _c_ developed - on inner side of protoxylem, _centripetal_ wood; the arrow indicates - the direction of the centre of the stem.] - -This point is one to which botanists have given much attention, and on -which they have laid much weight in considering the affinities of the -lower Gymnosperms and the intermediate groups between them and the ferns, -which are found among the fossils. In _Cordaites_ this point of -connection with the lower types is not seen, but in _Poroxylon_, which -has otherwise a stem anatomy very similar to _Cordaites_, we find groups -of _centripetal_ wood developed inside the protoxylem of primary bundles. -For this reason, principally, is _Poroxylon_ of interest at present, as -in its stem anatomy it seems to connect the _Cordaites_ type with that of -the group below it in general organization. - - -Ginkgoales.--Reference to p. 44 shows that _Ginkgo_, the Maidenhair tree, -belongs to the Ginkgoales, a group taking equal rank with the large and -complex series of the Coniferales. The Ginkgoales of the present day, -however, have but one living representative. _Ginkgo_ stands alone, the -single living species of its genus, representing a family so different -from any other living family that it forms a prime group by itself. - -Had the tree not been held sacred in China and Japan, it is probable that -it would long since have been extinct, for it is now known only in -cultivation. It is indeed a relic from the past which has been -fortunately preserved alive for our examination. It belongs to the fossil -world, as a belated November rose belongs to the summer. - -Because of its beauty and interest the plant is now widely distributed -under cultivation, and is available for study almost as freely as the -other types of living Gymnosperms already mentioned, so that but a short -summary of its more important features is needed here. - -Old plants, such as can be seen growing freely in Japan (in Kew Gardens -there is also a fine specimen), are very tall handsome woody trees, with -noble shafts and many branches. The leaves grow on little side shoots and -are the most characteristic external feature of the tree; their living -form is illustrated in fig. 66, which shows the typical simple shape as -well as the lobed form of the leaf which are to be found, with all -intermediate stages, on the same tree. No other plant (save a few ferns, -which can generally be distinguished from it without difficulty) has -leaves at all like these, so that it is particularly easy to identify the -fossil remains, of which there are many. - - [Illustration: Fig. 66.--A, Tuft of _Ginkgo_ Leaves, showing their - "maidenhair"-like shape. B, Single deeply-divided Leaf to be found on - the same tree, usually on young branches.] - -The wood is compact and fine grained, the rings of secondary tissue being -developed from a normal cambium as in the case of the higher Gymnosperms, -and the individual trache have round bordered pits. There are small male -cones, but the seeds are not borne in cones. They develop on special -stalks on which are no scales, but a small mass of tissue at the base of -the seed called the "collar". Usually there are two young ovules, of -which often only one ripens to a fleshy seed, though both may mature. - - [Illustration: Fig. 67.--Ripe Stage of _Ginkgo_ Seeds attached to their - Stalk. _c_, "Collar" of seed.] - -The ripe seed reaches the size shown in the diagram, and is orange -coloured and very fleshy; within it is a stone encasing the endosperm, -which is large, _green_, and starchy, and contains the embryo with two -cotyledons. This embryo is small compared with the endosperm, cf. fig. -57, p. 76, which is somewhat similar to that of _Ginkgo_ in this stage. - -Of the microscopic characters of the reproductive organs the most -remarkable is the male cell. This is not a passive nucleus, as in the -plants hitherto considered, but is an _actively swimming_ cell of some -size, provided with a spiral of cilia (hairlike structures) whose -movements propel it through the water. In the cavity of the unripe seed -these swim towards the female cell, and actively penetrate it. The -arrangements of the seed are diagrammatically shown in fig. 68, which -should be compared with that of _Cycas_, fig. 76, with which it has many -points in common. - - [Illustration: Fig. 68.--Section through Seed of _Ginkgo_ - - _p.c_, Pollen chamber in the nucellus _n_, which is fused to the coat - _c_ to the level _l_; _sc_, stony layer in coat; S, the big spore, - filled with endosperm tissue (in this case green in colour); _e_, egg - cells, one of which will produce the embryo after fertilization.] - -The nature of the male cell in _Cordaites_ is not yet known, but there is -reason to suspect it may have been actively swimming also. As this is -uncertain, however, we may consider _Ginkgo_ the most highly organized -plant which has such a primitive feature, a feature which is a bond of -union between it and the ferns, and which, when it was discovered about a -dozen years ago, caused a considerable sensation in the botanical world. - -To turn now to the fossil records of this family. Leaf impressions of -_Ginkgo_ are found in rocks of nearly all ages back even to the Upper -Palozoic. They show a considerable variety of form, and it is certain -that they do not all belong to the same _species_ as the living plant, -but probably they are closely allied. Fig. 69 shows a typical impression -from the Lower Mesozoic rocks. In this specimen, the cells of the -epidermis were fortunately sufficiently well preserved to be seen with -the microscope, and there is a distinct difference in the size and shape -of the cells of living and fossil species, see fig. 70; but this -difference is slight as compared with the great similarity of form and -appearance, as can be seen on comparing figs. 69 and 66, B, so that the -fossil is at the most a different species of the genus _Ginkgo_. Among -the fossil leaves there is greater variety than among the living ones, -and some which are very deeply lobed so as to form a divided palm-like -leaf go by different names, e.g. _Baiera_, but they are supposed to -belong to the same family. Fossil seeds and male cones are also known as -impressions, and are found far back in the Mesozoic rocks. From the -fossil impressions it is certain that _Ginkgo_ and plants closely allied -to it were very widespread in the past, as they are found all over Europe -as well as the other continents. Particularly in the Lower Mesozoic rocks -_Ginkgo_ seems to have been a world-wide type growing in great abundance. - - [Illustration: Fig. 69.--Leaf Impression of _Ginkgo_ from Mesozoic - Rocks of Scotland] - - [Illustration: Fig. 70.--Showing Epidermis with Stomates from the lower - side of the Leaf seen in fig. 69 - - _e_, Epidermis cells; _s_, stomates; _v_, long cells of epidermis lying - over the veins.] - -In the Palozoic the records are not so undoubted, but there is strong -evidence which leads us to suppose that if the genus now living were not -then extant, at least other closely related genera were, and there seems -to be good grounds for supposing that _Ginkgo_ and _Cordaites_ may have -both arisen from some ancient common stock. - - - - - CHAPTER X - PAST HISTORIES OF PLANT FAMILIES - III. The Bennettitales - - -This fascinating family is known only from the fossils, and is so remote -in its organization from any common living forms that it may perhaps be a -little difficult for those who do not know the Cycads to appreciate the -position of Bennettites. It would probably be better for one studying -fossil plants for the first time to read the chapters on the Cycads, -Pteridosperms, and Ferns before this chapter on the present group, which -has characters connecting it with that series. - -Until recently the bulk of the fossils which are found as impressions of -stems and foliage of this family were very naturally classed as Cycads. -They are extremely common in the Mesozoic rocks (the so-called Age of -Cycads), and in the external appearance of both stems and leaves they are -practically identical with the Cycads. - -A few incomplete fructifications of some species have been known in -Europe for many years, but it is only recently that they have been fully -known. This is owing to Wieland's[12] work on the American species, which -has made known the complete organization of the fructifications from a -mass of rich and well-petrified material. - -In the Lower Cretaceous and Upper Jurassic rocks of America these plants -abound, with their microscopic structure well preserved, and their -fructifications show an organization of a different nature from that of -any past or present Cycad. - -Probably owing to their external appearance, Wieland describes the plants -as "Cycads" in the title of his big book on them; but the generic name he -uses, _Cycadeoidea_, seems less known in this country than the equally -well-established name of _Bennettites_, which has long been used to -denote the European specimens of this family, and which will be used in -the following short account of the group. - -At the present time no family of fossils is exciting more interest. Their -completely Cycadean appearance and their unique type of fructification -have led many botanists to see in them the forerunners of the -Angiosperms, to look on them as the key to that mystery--the origin of -the flowering plants. This position will be discussed and the many facts -in its favour noted, but we must not forget that the _Bennettitales_ have -only recently been realized fully by botanists, and that a new toy is -ever particularly charming, a new cure particularly efficacious, and a -new theory all-persuasive. - -From their detailed study of the flowering plants botanists have leaned -toward different groups as the present representatives of the primitive -types. The various claims of the different families to this position -cannot be considered here; probably that of the Ranales (the group of -families round Ranunculace as a central type) is the best supported. Yet -these plants are most frequently delicate herbs, which would have stood -relatively less chance of fossilization than the other families which may -be considered primitive. They are peculiarly remote from the group of -Bennettite in their vegetative structure, a fact the importance of which -seems to have been underrated, for in the same breath we are assured that -the Bennettites are a kind of cousin to the ancient Angiosperms, and that -the Ranales are among the most primitive living Angiosperms, and -therefore presumably nearest the ancient ones. - -However, let us leave the charms of controversy on one side and look at -the actual structure of the group. They were widely spread in Lower -Mesozoic times, the plants being preserved as casts, impressions, and -with structure in great numbers. The bulk of the described structural -specimens have been obtained from the rocks of England, France, Italy, -and America, although leaf impressions are almost universally known. The -genus _Williamsonia_ belongs to this family, and is one of the best known -of Mesozoic plant impressions. - -Externally the Bennettite were identical in appearance with stumpy -Cycads, and their leaves it is which gave rise to the surmise, so long -prevalent, that the Lower Mesozoic was the "Age of Cycads", just as it -was the Pteridosperm leaves that gave the Palozoic the credit of being -the "Age of Ferns". In the anatomy of both stem and leaf, also, the -characters are entirely Cycadean; the outgoing leaf trace is indeed -simpler in its course than that of the Cycads. - - [Illustration: Fig. 71.--Half of a Longitudinal Section through a - Mature Cone of _Bennettites_ - - A, Short conical axis; _s_, enclosing bracts; S, seeds; _sc_, sterile - scales between the seeds.] - -The fructifications, however, differ fundamentally from those of the -Cycads, as indeed they do from those of any known family. They took the -form of compact cones, which occurred in very large numbers in the mature -plants hidden by the leaf bases. In _Williamsonia_, of which we know much -less detail, the fructifications stood away from the main axis on long -pedicels. - -In _Bennettites_ the cones were composed of series of sheathing scales -surrounding a short conical axis on which stood thin radiating stalks, -each bearing a seed. Between them were long-stalked sterile scales with -expanded ends. A part of a cone is illustrated diagrammatically in fig. -71. The whole had much the appearance of a complex fruit. In some -specimens these features alone are present in the cones, but in younger -cones from the American plants further structures are found attached. -Below the main axis of the seed-bearing part of the cone was a series of -large complex leaflike structures closely resembling fern leaves in their -much-divided nature. On the pinn of these leaves were crowded -innumerable large sporangia, similar to those of a fern, which provided -the pollen grains. The fossils are particularly well preserved, and have -been found with these male (pollen-bearing) organs in the young unopened -stages, and also in the mature unfolded condition, as well as the -ripening seed cones from which they have faded, just as the stamens fade -from a flower when the seeds enlarge. - - [Illustration: Fig. 72.--Diagram of Complete Cone of _Bennettites_ - - A, Central axis of conical shape terminating in the seed-bearing cone - S. (After Wieland), and bearing successively Br., bracts, comparable - with floral leaves; M, large complex leaves with pollen sacs.] - -It appears that these huge complex leaflike structures were really -stamens, but nevertheless they were rolled up in the circinate form as -are young fern leaves, and as they unrolled and spread out round the -central cone they must have had the appearance of a whorl of leaves (see -fig. 72). - -This, in a few words, is the main general character of the -fructification. The most important features, on which stress is laid, are -the following. The association of the male and female structures on the -same axis, with the female part _above_ the male. This arrangement is -found only in the flowering plants; the lower plants, which have male and -female on the same cone, have them mixed, or the female below, and are in -any case much simpler in their entire organization. The conical form of -the axis is also important, as is the fact that it terminates in the -seed-bearing structures. - - [Illustration: Fig. 73.--Diagram of Cross Section of _Bennettites_, - Seed, with Embryo - - _c_, Double-layered seed coat; _n_, crushed nucellus; _cot._, two - cotyledons which practically fill the seed.] - -The position of the individual seeds, each on the end of a single stalk, -is remarkable, as are the long-stalked bracts whose shield-like ends join -in the protection of the seeds. These structures together give the cone -much of the appearance of a complex fruit of a flowering plant, but the -structure of the seeds themselves is that of a simple Gymnosperm. - -In the seeds, however, was an _embryo_. In this they differ from all -known seeds of an earlier date, which, as has been already noted (see p. -77), are always devoid of one. This embryo is one of the most important -features of the plant. It had two cotyledons which filled the seed space -(see fig. 73), and left almost no trace of the endosperm. Reference to p. -112 will show that this is an advance on the Cycad seed, which has a -small embryo embedded in a large mass of endosperm, and that it -practically coincides with the Dicotyledonous type. - -The seed with its embryo suggested comparison with the Angiosperms long -before the complete structure of the fructification was known. - -The fern-like nature of the pollen-bearing structures is another very -important point. Were any one of these leaflike "stamens" found isolated -its fern-like nature would not have been questioned a year or two ago, -and their presence in the "flower" of _Bennettites_ is a strong argument -in favour of the Fern-Pteridosperm affinities of the group. - -Had the parts of this remarkable fructification developed on separate -trees, or on separate branches or distinct cones of the same one, they -would have been much less suggestive than they are at present, and the -fructifications might well have been included among those of the -Gymnosperms, differing little more (apart from the embryo) from the other -Gymnosperm genera than they do from each other. In fact, the extremely -fern-like nature of the male organs is almost more suggestive of a -Pteridosperm affinity, for even the simplest Cycads have well-marked -scaly cones as their male organs. The female cone, again, considered as -an isolated structure, can be interpreted as being not vitally different -from _Cordaites_, where the seeds are borne on special short stalks -amidst scales. - -The embryo would, in any case, point to a position among advanced types; -but it is so common for one organ of a plant to evolve along lines of its -own independently, or in advance of the other organs, that the embryo -structure alone could not have been held to counterbalance the Cycadean -stems and leaves, the Pteridosperm-like male organs, and the Gymnospermic -seeds. - -But all these parts occur on the same axis, arranged in the manner -typical of Angiosperms. The seed-bearing structures at the apex, the -"stamens" below them, and a series of expanded scales below these again, -which it takes little imagination to picture as incipient petals and -sepals; and behold--the thing is a flower! - -And being a "flower", is in closest connection with the ancestors of the -modern flowering plants, which must consequently have evolved from some -Cycadean-like ancestor which also gave rise to the Bennettitales. Thus -can the flowering plants be linked on to the series that runs through the -Cycads directly to the primitive ferns! - -It is evident that this group, of all those known among the fossils, -comes most closely to an approximation of Angiospermic structure and -arrangement. Enough has been said to show that in their actual nature -they are not Angiosperms, though they have some of their characters, -while at the same time they are not Cycads, though they have their -appearance. They stand somewhere between the two. Though many botanists -at present hold that this mixture of characters indicates a relationship -equivalent to a kind of cousinship with the Angiosperms, and both groups -may be supposed to have originated from a Cycadean stock, this theory has -not yet stood the test of time, nor is it supported by other evidence -from the fossils. We will go so far as to say that it appears as though -_some_ Angiosperms arose in that way; but flowering plants show so many -points utterly differing from the whole Cycadean stock that a little -scepticism may not be unwholesome. - -It is well to remember the Lycopods, where (as we shall see, p. 141) -structures very like seeds were developed at the time when the Lycopods -were the dominant plants, and we do not find any evidence to prove that -they led on to the main line of seed plants. Similarly, Cycads may have -got what practically amounted to flowers at the time when they were the -dominant group, and it is very conceivable that they did not lead on to -the main line of flowering plants. - -Whatever view may be held, however, and whatever may be the future -discoveries relating to this group of plants, we can see in the -Bennettitales points which throw much light on the potentialities of the -Cycadean stock, and structures which have given rise to some most -interesting speculations on the subject of the Angiosperms. This group is -another of the jewels in the crown of fossil botany, for the whole of its -structures have been reconstructed from the stones that hold all that -remains of this once extensive and now extinct family of plants. - - - - - CHAPTER XI - PAST HISTORIES OF PLANT FAMILIES - IV. The Cycads - - -The group of the _Cycadales_, which has a systematic value equivalent to -the _Ginkgoales_, contains a much larger variety of genera and species -than does the latter. There are still living nine genera, with more than -a hundred and fifty species, which form (though a small one compared with -most of the prime groups) a well-defined family. They are the most -primitive Gymnosperms, the most primitive seed-bearing plants now living, -and in their appearance and characters are very different from any other -modern type. Their external resemblance to the group of the -Bennettitales, however, is very striking, and indeed, without the -fructifications it would be impossible to distinguish them. - -The best known of the genera is that of _Cycas_, of which an illustration -is given in fig. 74. The thick, stumpy stem and crown of "palm"-like -leaves give it a very different appearance from any other Gymnosperm. -Commonly the plants reach only a few feet in height, but very old -specimens may grow to the height of 30 ft. or more. The other genera are -smaller, and some have short stems and a very fern-like appearance, as, -for example, the genus _Stangeria_, which was supposed to be a fern when -it was first discovered and before fruiting specimens had been seen. - -The large compound leaves are all borne directly on the main stem, -generally in a single rosette at its apex, and as they die off they leave -their fleshy leaf bases, which cover the stem and remain for an almost -indefinite number of years. - -The wood of the main trunks differs from that of the other Gymnosperms in -being very loosely built, with a large pith and much soft tissue between -the radiating bands of wood. There is a cambium which adds zones of -secondary tissue, but it does not do its work regularly, and the cross -section of an old Cycad stem shows disconnected rings of wood, -accompanied by much soft tissue. The cells of the wood have bordered pits -on their walls, and in the main axis the wood is usually all developed in -a centrifugal direction, but in the axis of the cones some centripetal -wood is found (refer to _c_, fig. 65, p. 97). - - [Illustration: Fig. 74.--Plant of _Cycas_, showing the main stem with - the crown of leaves and the irregular branches which come on an old - plant] - -In their fructifications the Cycads stand even further apart from the -rest of the Gymnosperms. One striking point is the enormous size of their -_male_ cones. The male cones consist of a stout axis, round which are -spiral series of closely packed simple scales covered with pollen-bearing -sacs (which bear no inconsiderable likeness to fern sporangia), the whole -cone reaching 1-1/2 ft. in length in some genera, and weighing several -pounds. All the other Gymnosperms, except the Araucare, where they are -an inch or two long, have male cones but a fraction of an inch in length. - - [Illustration: Fig. 75.--Seed-bearing Scale of _Cycas_, showing its - lobed and leaflike character - - _s_, Seeds attached on either side below the divisions of the - sporophyll.] - -In all the members of the family, excepting _Cycas_ itself, the female -fructifications also consist of similarly organized cones bearing a -couple of seeds on each scale instead of the numerous pollen sacs. In -_Cycas_ the male cones are like those of the other genera, and reach an -enormous size; but there are no female cones, for the seeds are borne on -special leaflike scales. These are illustrated in fig. 75, which shows -also that there are not two seeds (as in the other genera with cones) to -each scale, but an indefinite number. - -The leafy nature of the seed-bearing scale is an important and -interesting feature. Although theoretically botanists are accustomed to -accept the view that seeds are always borne on specially modified leaves -(so that to a botanist even the "shell" of a pea-pod and the box of a -poppy capsule are leaves), yet in _Cycas_ alone among living plants are -seeds really found growing on a large structure which has the appearance -of a leaf. Hence, from this point of view (see p. 45, however, for a -caution against concluding that the whole plant is similarly lowly -organized), _Cycas_ is the most primitive of all the living plants that -bear seeds, and hence presumably the likest to the fossil ancestors of -the seed-bearing types. In this character it is more primitive than the -fossil group of the _Cordaite_, and comes very close to an intermediate -group of fossils to be considered in the next chapter. - - [Illustration: Fig. 76.--Seed of _Cycas_ cut open - - _n_, The nucellus, fused at the level _l_ to the coat _c_; _sc_, stony - layer of coat; _p.c_, pollen chamber in apex of the nucellus; S, - "spore", filled with endosperm, in which lies the embryo _e_.] - -To enter into the detailed anatomy of the seeds would lead us too far -into the realms of the specialist, but we must notice one or two points -about them. Firstly, their very large size, for ripe seeds of _Cycas_ are -as large as peaches (and peaches, it is to be noted, are fruits, not -seeds), and particularly the large size they attain _before_ they are -fertilized and have an embryo. Among the higher plants the young seeds -remain very minute until an embryo is secured by the act of -fertilization, but in the Cycads the seeds enlarge and lay in a big store -of starch in the endosperm before the embryo appears, so that in the -cases in which fertilization is prevented large, sterile "seeds" are -nevertheless produced. This must be looked on as a want of precision in -the mechanism, and as a wasteful arrangement which is undeniably -primitive. An even more wasteful arrangement appears to have been common -to the "seeds" of the Palozoic period, for, though many fossil "seeds" -are known in detail from the old rocks, not one is known to have any -trace of an embryo. A general plan of the _Cycas_ seed is shown in fig. -76, which should be compared with that of _Ginkgo_ (fig. 68). The large -size of the endosperm and the thick and complex seed-coats are -characteristic features of both these structures. Another point that -makes the Cycad seeds of special interest is the fact that the male cells -(as in _Ginkgo_) are developed as active, free-swimming sperms, which -swim towards the female cell in the space provided for them in the seed -(see _p.c_, fig. 76). - -The characters of the Cycads as they are now living prove them to be an -extremely primitive group, and therefore presumably well represented -among the fossils; and indeed among the Mesozoic rocks there is no lack -of impressions which have been described as the leaves of Cycads. There -is, however, very little reliable material, and practically none which -shows good microscopic structure. Leaf impressions alone are most -unsafe--more unsafe in this group, perhaps, than in any other--for -reasons that will be apparent later on, and the conclusions that used to -be drawn about the vast number of Cycads which inhabited the globe in the -early Mesozoic must be looked on with caution, resulting from the -experience of recent discoveries proving many of these leaves to belong -to a different family. - -There remain, however, many authentic specimens which show that _Cycas_ -certainly goes back very far in history, and specimens of this genus are -known from the older Mesozoic rocks. We cannot say, however, as securely -as used to be said, that the Mesozoic was the "Age of Cycads", although -it was doubtless the age of plants which had much of the external -appearance of Cycads. - -From the Palozoic we have no reliable evidence of the existence of -Cycads, though the plants of that time included a group which has an -undoubted connection with them. - -Indeed, so far as fossil evidence goes, we must suppose that the Cycads, -since their appearance, possibly at the close of the Palozoic, have -never been a dominant or very extensive family, though they grew in the -past all over the world, and in Europe seem to have remained till the -middle of the Tertiary epoch. - - - - - CHAPTER XII - PAST HISTORIES OF PLANT FAMILIES - V. Pteridosperms - - -This group consists entirely of plants which are extinct, and which were -in the height of their development in the Coal Measure period. As a group -they are the most recently discovered in the plant world, and but a few -years ago the name "Pteridosperm" was unknown. They form, however, both -one of the most interesting of plant families and one of the most -numerous of those which flourished in the Carboniferous period. - -To mention first the vital point of interest in their structure, they -show _leaves which in all respects appear like ordinary foliage leaves, -and yet bear seeds_. These leaves, which we now know bore the seeds, had -long been considered as typical fern leaves, and had been named and -described as fern leaves. There are two extremely important results from -the discovery of this fossil group, viz. that leaves, to all appearance -like ordinary foliage, can directly bear seeds, and that the leaves, -though like fern leaves, bore seeds like those of a Cycad. - -As the name _Pteridosperm_ indicates, the group is a link between the -ferns and the seed-bearing plants, and as such is of special interest and -value to botanists. - -The gradual recognition of this group from among the numerous plant -fragments of Palozoic age is one of the most interesting of the -accumulative discoveries of fossil botany. Ever since fossil remains -attracted the attention of enquiring minds many "ferns" have been -recognized among the rich impressions of the Coal Measures. Most of them, -however, were not connected with any structural material, and were given -many different names of specific value. So numerous were these fern -"species" that it was supposed that in the Coal Measure period the ferns -must have been the dominant class, and it is often spoken of even yet as -the "Age of Ferns". From the rocks of the same age, preserved with their -microscopical structure perfect, were stems which were called -_Lyginodendron_. In the coal balls associated with these stems (which -were the commonest of the stems so preserved) were also roots, petioles, -and leaflets, but they were isolated, like the most of the fragments in a -coal ball, and to each was given its name, with no thought of the various -fragments having any connection with each other. Gradually, however, -various fragments from the coal balls had been recognized as belonging -together; one specimen of a petiole attached to a stem sufficed to prove -that all the scattered petioles of the same type belonged also to that -kind of stem, and when leaves were found attached to an isolated fragment -of the petiole, the chain of proof was complete that the leaves belonged -to the stem, and so on. By a series of lengthy and painstaking -investigations all the parts of the plant now called _Lyginodendron_ have -been brought together, and the impressions of its leaves have been -connected with it, these being of the fernlike type so long called -_Sphenopteris_, illustrated in fig. 77. - - [Illustration: Fig. 77.--_Sphenopteris_ Leaf Impression, the fernlike - foliage of _Lyginodendron_] - - [Illustration: Fig. 78A.--Diagram of the Transverse Section of Stem of - the _Lyginodendron_ - - _p_, Pith; P, primary wood groups; W, secondary wood; _l.t_, leaf - trace; _s_, sclerized bands in the cortex; S, longitudinal view of wood - elements to show the rows of bordered pits.] - -The anatomy of the main stem is very suggestive of that of a Cycad. The -zones of secondary wood are loosely built, the quantity of soft tissue -between the radiating bands of wood, and the size of the pith being -large, while from the main axis double strands of wood run out to the -leaf base. The primary bundles, however, are not like those of a Cycad -stem, but have groups of _centripetal_ wood within the protoxylem, and -thus resemble the primary bundles of _Poroxylon_ (see p. 97), which are -more primitive in this respect than those of the Cycads. - -The roots of _Lyginodendron_, when young, were like those of the -Marattiaceous ferns, their five-rayed mass of wood being characteristic -of that family, and different from the type of root found in most other -ferns (cf. fig. 78B with fig. 35 on p. 60). Unlike fern roots of any -kind, however, they have well-developed zones of secondary wood, in which -they approach the Gymnospermic roots (see fig. 78B, _s_). - - [Illustration: Fig. 78B.--Transverse Section of Root of _Lyginodendron_ - - _w_, Five-rayed mass of primary wood; _s_, zone of secondary wood; _c_, - cortical and other soft tissues.] - -A further mixture of characters is seen in the vascular bundles of the -petioles. A double strand, like that in the lower Gymnosperms, goes off -to the leaf base from the main axis, but in the petiole itself the bundle -is like a normal fern stele, and shows no characters in transverse -section which would separate it from the ferns. Such a petiole is -illustrated in fig. 79, with its V-shaped fernlike stele. On the petioles -and stems were certain rough, spiny structures of the nature of complex -hairs. In some cases they are glandular, as is seen in _g_ in fig. 79, -and as they seem to be unique in their appearance they have been of great -service in the identification of the various isolated organs of the -plant. - -As is seen from fig. 77, the leaves were quite fern-like, but in -structural specimens they have been found with the characteristic -glandular hairs of the plant. - -The seeds were so long known under the name of _Lagenostoma_ that they -are still called by it, though they have been identified as belonging to -_Lyginodendron_. They were small (about 1/4 in. in maximum length) when -compared with those of most other plants of the group, or of the Cycads, -with which they show considerable affinity. They are too complex to -describe fully, and have been mentioned already (see p. 76), so that they -will not be described in much detail here. The diagrammatic figure (fig. -56) shows the essential characters of their longitudinal section, and -their transverse section, as illustrated in fig. 80, shows the complex -and elaborate mechanism of the apex. - - [Illustration: Fig. 79.--Transverse Section through Petiole of - _Lyginodendron_ - - _v_, Fern-like stele; _c_, cortex; _g_, glandular hairlike - protuberances.] - -Round the "seed" was a sheath, something like the husk round a hazel nut, -which appears to have had the function of a protective organ, though what -its real morphological nature may have been is as yet an unsolved -problem. On the sheath were glandular hairs like those found on the -petiole and leaves, which were, indeed, the first clues that led to the -discovery of the connection between the seed and the plant -_Lyginodendron_. - -The pollen grains seem to have been produced in sacs very like fern -sporangia developed on normal foliage leaves, each grain entered the -cavity _pc_ in the seed (see fig. 56), but of the nature of the male cell -we are ignorant. In none of the fossils has any embryo been found in the -"seeds", so that presumably they ripened, or at least matured their -tissues, before fertilization. - -These, in a few words, are the essentials of the structures of -_Lyginodendron_. But this plant is only one of a group, and at least two -other of the Pteridosperms deserve notice, viz. _Medullosa_, which is -more complex, and _Heterangium_, which is simpler than the central type. - - [Illustration: Fig. 80.--Diagram of Transverse Section of _Lagenostoma_ - Seed near the Apex, showing the nine flutings _f_ of the coat _c_; _v_, - the vascular strand in each; _nc_, cone of nucellar tissue standing up - in the fluted apex of the nucellus _n_; _pc_, the pollen chamber with a - few pollen grains; _s_, space between nucellus and coat. Compare with - diagram 56.] - -_Heterangium_ is found also in rocks rather older than the coal series of -England, though of Carboniferous age, viz. in the Calciferous sandstone -series of Scotland, it occurs also in the ordinary Coal Measure nodules. -It is in several respects more primitive than _Lyginodendron_, and in -particular in the structure of its stele comes nearer to that of ferns. -The stele is, in fact, a solid mass of primary wood and wood parenchyma, -corresponding in some degree to the protostele of a simple type (see p. -61, fig. 36), but it has towards the outside groups of protoxylem -surrounded by wood in both centripetal and centrifugal directions, which -are just like the primary bundles in _Lyginodendron_. Outside the primary -mass of wood is a zone of secondary wood, but the quantity is not large -in proportion to it (see fig. 81), as is common in Lyginodendron. - -Though the primary mass is so fernlike in appearance the larger tracheids -show series of bordered pits, as do most of the tracheids of the -Pteridosperms, in which they show a Gymnosperm-like character. - - [Illustration: Fig. 81.--_Heterangium_ - - A, Half of the stele of a stem, showing the central mass of wood S - mixed with parenchyma _p_. The protoxylem groups _p. x._ lie towards - the outside of the stele. Surrounding it is the narrow zone of - small-celled secondary wood W. B, A few of the wood cells in - longitudinal view: _p. x._, Protoxylem; _p_, parenchyma. S, Large - vessels with rows of bordered pits.] - -The foliage of _Heterangium_ was fernlike, with much-divided leaves -similar to those of _Lyginodendron_. We have reason to suspect, though -actual proof is wanting as yet, that small Gymnosperm-like seeds were -borne directly on these leaves. - -_Medullosa_ has been mentioned already (see p. 72) because of the -interesting and unusually complex type of its vascular anatomy. Each -individual stele of the group of three in the stem, however, is -essentially similar to the stele of a Heterangium. - -Though the whole arrangement appears to differ so widely from other stems -in the plant world, careful comparison with young stages of recent Cycads -has indicated a possible remote connection with that group, while in the -primary arrangements of the protosteles a likeness may be traced to the -ferns. The roots, even in their primary tissues, were like those of -Gymnosperms, but the foliage with its compound leaves was quite fern-like -externally. A small part of a leaf is shown in fig. 83, and is clearly -like a fern in superficial appearance. The leaves were large, and the -leaf bases strong and well supplied with very numerous branching vascular -bundles. - - [Illustration: Fig. 82.--Steles of _Medullosa_ in Cross Section of the - Stem - - A, Primary solid wood; S, surrounding secondary wood.] - - [Illustration: Fig. 83.--Part of a Leaf of _Medullosa_, known as - _Alethopteris_, for long supposed to be a Fern] - -The connection between this plant and certain large three-ribbed seeds -known as _Trigonocarpus_ is strongly suspected, though actual continuity -is not yet established in any of the specimens hitherto discovered. These -seeds have been mentioned before (p. 76 and p. 82). They were larger than -the other fossil seeds which we have mentioned, and, with their fleshy -coat, were similar in general organization to the Cycads, though the fact -that the seed coat stood free from the inner tissues right down to the -base seems to mark them as being more primitive (cf. fig. 55, p. 76). - -Of impressions of the Pteridosperms the most striking is, perhaps, the -foliage known as _Neuropteris_ (see fig. 6, p. 13), to which the large -seeds are found actually attached (cf. fig. 85). - - [Illustration: Fig. 84.--Diagrammatic Section of a Transverse Section - of a Seed of _Trigonocarpus_ - - S, Stone of coat with three main ridges and six minor ones. F, Flesh of - coat: _i f_, inner flesh; _n_, nucellus, crushed and free from coat; - _s_, spore wall.] - - [Illustration: Fig. 85.--Fragment of Foliage of _Neuropteris_ with Seed - attached, showing the manner in which the seeds grew on the normal - foliage leaves in the Pteridosperms] - -Ever-increasing numbers of the "ferns" are being recognized as belonging -to the Pteridosperms, but _Heterangium_, _Lyginodendron_, and _Medullosa_ -form the three principal genera, and are in themselves a series -indicating the connection between the fernlike and Cycadean characters. - -Before the fructifications were suspected of being seeds the anatomy of -these plants was known, and their nature was partly recognized from it -alone, though at that time they were supposed to have only fernlike -spores. - -The very numerous impressions of their fernlike foliage from the -Palozoic rocks indicate that the plants which bore such leaves must have -existed at that time in great quantity. They must have been, in fact, one -of the dominant types of the vegetation of the period. The recent -discovery that so large a proportion of them were not ferns, but were -seed-bearing plants, alters the long-established belief that the ferns -reached their high-water mark of prosperity in the Coal Measure period. -Indeed, the fossils of this age which remain undoubtedly true ferns are -far from numerous. It is the seed-bearing Pteridosperms which had their -day in Palozoic times. Whether they led directly on to the Cycads is as -yet uncertain, the probability being rather that they and the Cycads -sprang from a common stock which had in some measure the tendencies of -both groups. - -That the Pteridosperms in themselves combined many of the most important -features of both Ferns and Gymnosperms is illustrated in the account of -them given above, which may be summarized as follows:-- - - - Salient Characters of the Pteridosperms - _G_=_Gymnospermic_ _F_=_Fernlike_ - - _F_ Primary structure of root. - _G_ Secondary thickening of root. - _F_ In _Heterangium_ and _Medullosa_ the - _F_ solid centripetal primary wood of stele. - _G_ Pits on trache of primary wood. - _G_ Secondary thickening of stem. - _G_ Double leaf trace. - _F_ Fernlike stele in petiole. - _F_ Fernlike leaves. - _F_ Sporangia pollen-sac-like. - _F_ Reproductive organs borne directly on ordinary foliage leaves. - _G_ General organization of the seed. - -Thus it can be seen at a glance, without entering into minuti, that the -characters are divided between the two groups with approximate equality. -The connection with Ferns is clear, and the connection with Gymnosperms -is clear. The point which is not yet determined, and about which -discussion will probably long rage, is the position of this group in the -whole scheme of the plant world. Do they stand as a connecting link -between the ferns on one hand and the whole train of higher plants on the -other, or do they lead so far as the Cycads and there stop? - - - - - CHAPTER XIII - PAST HISTORIES OF PLANT FAMILIES - VI. The Ferns - - -Unfortunately the records in the rocks do not go back so far as to touch -what must have been the most interesting period in the history of the -ferns, namely, the point where they diverged from some simple ancestral -type, or at least were sufficiently primitive to give indications of -their origin from some lower group. - -Before the Devonian period all plant impressions are of little value, and -by that early pre-Carboniferous time there are preserved complex leaves, -which are to all appearance highly organized ferns. - -To-day the dominant family in this group is the _Polypodiace_. It -includes nearly all our British ferns, and the majority of species for -the whole world. This family does not appear to be very old, however, and -it cannot be recognized with certainty beyond Mesozoic times. - -From the later Mesozoic we have only material in the form of impressions, -from which it is impossible to draw accurate conclusions unless the -specimens have sporangia attached to them, and this is not often the -case. The cuticle of the epidermis or the spores can sometimes be studied -under the microscope after special treatment, but on the whole we have -very little information about the later Mesozoic ferns. - -A couple of specimens from the older Mesozoic have been recently -described, with well-preserved structure, and they belong to the family -of the Osmundas (the so-called "flowering ferns", because of the -appearance of special leaves on which all the sporangia are crowded), and -show in the anatomical characters of their stems indications that they -may be related to an old group, the _Botryopteride_, in which are the -most important of the Palozoic ferns. - -In the Palozoic rocks there are numerous impressions as well as fern -petrifactions, but in the majority of cases the connection between the -two is not yet established. There were two main series of ferns, which -may be classed as belonging to - - I. Marattiace. - II. Botryopteride. - -Of these the former has still living representatives, though the group is -small and unimportant compared with what it once was; the latter is -entirely extinct, and is chiefly developed in the Carboniferous and -succeeding Permian periods. - -The latter group is also the more interesting, for its members show great -variety, and series may be made of them which seem to indicate the course -taken in the advance towards the Pteridosperm type. For this reason the -group will be considered first, while the structure of the Pteridosperms -is still fresh in our minds. - -The Botryopteride formed an extensive and elaborate family, with its -numerous members of different degrees of complexity. There is, -unfortunately, but little known as to their external appearance, and -almost no definite information about their foliage. They are principally -known by the anatomy of their stems and petioles. Some of them had -upright trunks like small tree ferns (living tree ferns belong to quite a -different family, however), others appear to have had underground stems, -and many were slender climbers. - - [Illustration: Fig. 86.--Stele of _Asterochlaena_, showing its deeply - lobed nature] - -In their anatomy all the members of the family have monostelic structure -(see p. 62). This is noteworthy, for at the present time though a number -of genera are monostelic, no family whose members reach any considerable -size or steady growth is exclusively monostelic. In the shape of the -single stele, there is much variety in the different genera, some having -it so deeply lobed that only a careful examination enables one to -recognize its essentially monostelic nature. In fig. 86 a radiating -star-shaped type is illustrated. Between this elaborate type of -protostele in _Asterochlaena_, and the simple solid circular mass seen in -_Botryopteris_ itself (fig. 88) are all possible gradations of structure. - - [Illustration: Fig. 87.--The Stele of a Botryopteridean Stem, showing - soft tissue in the centre of the solid wood of the protostele. - (Microphoto.)] - -In several of the genera the centre of the wood is not entirely solid, -but has cells of soft tissue, an incipient pith, mixed with scattered -tracheids, as in fig. 87. - -In most of the genera numerous petioles are given off from the main axis, -and these are often of a large size compared with it, and may sometimes -be thicker than the axis itself. Together with the petiole usually come -off adventitious roots, as is seen in fig. 88, which shows the main axis -of a _Botryopteris_. The petioles of the group show much variety in their -structure, and some are extremely complex. A few of the shapes assumed by -the steles of the petioles are seen in fig. 89; they are not divided into -separate bundles in any of the known forms, as are many of the petiole -steles of other families. - - [Illustration: Fig. 88.--Main Axis of _Botryopteris_ with simple solid - Protostele _x_. A petiole about to detach itself _p_ and the strand - going out to an adventitious root _r_ are also seen. - (Micro-photograph.)] - - [Illustration: Fig. 89.--Diagrams showing the Shapes of the Steles in - some of the Petioles of different Genera of Botryopteride - - A, _Zygopteris_; B, _Botryopteris_; C, _Tubicaulis_; D, - _Asterochlaena_.] - -In one genus of the family secondary wood has been observed. This is -highly suggestive of the condition of the stele in _Heterangium_, where -the large mass of the primary wood is surrounded by a relatively small -quantity of secondary thickening, developed in normal radial rows from a -cambium. - -Another noteworthy point in the wood of these plants is the thickening of -the walls of the wood cells. Many of them have several rows of bordered -pits, and are, individually, practically indistinguishable from those of -the Pteridosperms, cf. fig. 81 and fig. 90. These are unlike the -characteristic wood cells of modern ferns and of the other family of -Palozoic ferns. - -The foliage of most members of the family is unknown, or at least, of the -many impressions which possibly belong to the different genera, the most -part have not yet been connected with their corresponding structural -material. There are indications, however, that the leaves were large and -complexly divided. - -The fructifications were presumably fern sporangia of normal but rather -massive type. Of most genera they are not known, though in a few they -have been found in connection with recognizable parts of their tissue. -The best known of the sporangia are large, in comparison with living -sporangia (actually about 2.5 millimetres long), oval sacs clustered -together on little pedicels. The spores within them seem in no way -essentially different from normal fern spores. - -The coexistence of the Botryopteride and Pteridosperms, and the several -points in the structure of the former which seem to lead up to the -characters of the latter group, are significant. The Botryopteride, even -were they an entirely isolated group, would be interesting from the -variety of structures and the variations of the monostele in their -anatomy; and the prominent place they held in the Palozoic flora, as the -greatest family of ferns of that period, gives them an important position -in fossil botany. - - [Illustration: Fig. 90.--Trache of Wood of Botryopteridean Fern in - Longitudinal Section, showing the rows of pits on the walls. - (Microphoto.)] - -The other family of importance in Palozoic times, the Marattiace, has -descendants living at the present day, though the family is now -represented by a small number of species belonging to but five genera -which are confined to the tropics. Perhaps the best known of these is the -giant "Elephant Fern", which sends up from its underground stock huge -complex fronds ten or a dozen feet high. Other species are of the more -usual size and appearance of ferns, while some have sturdy trunks -above-ground supporting a crown of leaves. The members of this family -have a very complex anatomy, with several series of steles of large size -and irregular shape. Their fructifications are characteristic, the -sporangia being placed in groups of about five to a dozen, and fused -together instead of ripening as separate sacs as in the other fern -families. - -Impressions of leaves with this type of sorus (group of fern sporangia) -are found in the Mesozoic rocks, and these bridge over the interval -between the living members of the family and those which lived in -Palozoic times. - -In the Coal Measure and Permian periods these plants flourished greatly, -and there are remains of very numerous species from that time. The family -was much more extensive then than it is now, and the individual members -also seem to have reached much greater dimensions, for many of them had -the habit of large tree ferns with massive trunks. Up till Triassic times -half of the ferns appear to have belonged to this family; since then, -however, they seem to have dwindled gradually down to the few genera now -existing. - -On the Continent fossils of this type with well-preserved structure have -long been known to the general public, as their anatomy gave the stones a -very beautiful appearance when polished, so that they were used for -decorative purposes by lapidaries before their scientific interest was -recognized. - -The members of the Palozoic Marattiace which have structure preserved -generally go by the generic name _Psaronius_, in which there is a great -number of species. They show considerable uniformity in their essential -structure (in which they differ noticeably from the group of ferns just -described), so that but one type will be considered. - -In external appearance they probably resembled the "tree ferns" of the -present day (though these belong to an entirely different family), with -massive stumps, some of which reached a height of 60 ft. The large -spreading leaves were arranged in various ways on the stem, some in a -double row along it, as is seen by the impressions of the leaf scars, and -others in complex spirals. On the leaves were the spore sacs, which were -in groups, some completely fused like those of the modern members of the -family, and others with independent sporangia massed in well-defined -groups. In their microscopic structure also they appear to have been -closely similar to those of the living Marattiace. - -The transverse section of a stem shows the most characteristic and -best-known view of the plant. This is shown in fig. 91, in somewhat -diagrammatic form. - -The mass of rootlets which entirely permeate and surround the outer -tissues of the stem is a very striking and characteristic feature of all -the species of _Psaronius_. Though such a mass of roots is not found in -the living species, yet the microscopic structure of an individual fossil -root is almost identical with that of a living _Marattia_. - -Though these plants were so successful and so important in Palozoic -times, the group even then seems to have possessed little variety and -little potentiality for advance in new directions. They stand apart from -the other fossils, and the few forms which now compose the living -Marattiace are isolated from the present successful types of modern -ferns. From the _Psaronie_ we can trace no development towards a modern -series of plants, no connection with another important group in the past. -They appear to have culminated in the later Palozoic and to have slowly -dwindled ever since. It has been suggested that the male fructifications -of the Bennettite and the Pteridosperms show some likeness to the -Marattiace, but there does not seem much to support any view of -phylogenetic connection between them. - - [Illustration: Fig. 91.--Transverse Section of Stem of _Psaronius_ - - _v_, Numerous irregularly-shaped steles; _s_, irregular patches of - sclerenchyma; _l_, leaf trace going out as a horseshoe-shaped stele; - _c_, zone of cortex with numerous adventitious roots _r_ running - through it; _sc_, sclerized cortical zone of roots; _w_, vascular - strand of roots.] - -Before leaving the palozoic ferns, mention should be made of the very -numerous leaf impressions which seem to show true fern characters, though -they have not been connected with material showing their internal -structure. Among them it is rare to get impressions with the _sori_ or -sporangia, but such are known and are in themselves enough to prove the -contention that true ferns existed in the Palozoic epoch. For it might -be mentioned as a scientific curiosity, that after the discovery that so -many of the leaf impressions which had always been supposed to be ferns, -really belonged to the seed-bearing Pteridosperms, there was a period of -panic among some botanists, who brought forward the startling idea that -there were _no_ ferns at all in the Palozoic periods, and that modern -ferns were degenerated seed-bearing plants! - - [Illustration: Fig. 92.--Impression of Palozoic Fern, showing _sori_ - on the pinnules. (Photo.)] - -These two big groups from the Palozoic include practically all the ferns -that then flourished. They have been spoken of (together with a few other -types of which little is known) as the _Primofilices_, a name which -emphasizes their primitive characters. As can be seen by the complex -organization of the genera, however, they themselves had advanced far -beyond their really primitive ancestors. There is clear indication that -the Botryopteride were in a period of change, what might almost be -termed a condition of flux, and that from their central types various -families separated and specialized. Behind the Botryopteride, however, -we have no specimens to show us the connection between them and the -simpler groups from which they must have sprung. From a detailed -comparative study of plant anatomy we can deduce some of the essential -characters of such ancestral plants, but here the realm of fossil botany -ceases, to give place to theoretical speculation. As a fact, there is a -deep abyss between the ferns and the other families of the Pteridophytes, -which is not yet bridged firmly enough for any but specialists, used to -the hazardous footing on such structures, to attempt to cross it. Until -the buttresses and pillars of the bridge are built of the strong stone of -fossil structures we must beware of setting out on what would prove a -perilous journey. - -In the Coal Measures and previous periods we see the ferns already -represented by two large families, differing greatly from each other, and -from the main families of modern ferns which sprung at a later date from -some stock which we have not yet recognized. But though their past is so -obscure, the palozoic ferns and their allies throw a brilliant light on -the course of evolution of the higher groups of plants, and the gulf -between ferns and seed-bearing types may be said to be securely bridged -by the Botryopteride and the Pteridosperms. - - - - - CHAPTER XIV - PAST HISTORIES OF PLANT FAMILIES - VII. The Lycopods - - -The present-day members of this family are not at all impressive, and in -their lowliness may well be overlooked by one who is not interested in -unpretending plants. The fresh green mosslike _Selaginella_ grown by -florists as ornamental borders in greenhouses and the creeping "club -moss" twining among the heather on a Highland moor are probably the best -known of the living representatives of the Lycopods. In the past the -group held a very different position, and in the distant era of the Coal -Measures it held a dominant one. Many of the giants of the forest -belonged to the family (see frontispiece), and the number of species it -contained was very great. - -Let us turn at once to this halcyon period of the group. The history of -the times intervening between it and the present is but the tale of the -dying out of the large species, and the gradual shrinking of the family -and dwarfing of its representative genera. - -It is difficult to give the characters of a scientific family in a few -simple words; but perhaps we may describe the living Lycopods as plants -with creeping stems which divide and subdivide into two with great -regularity, and which bear large numbers of very small pointed leaves -closely arranged round the stem. The fruiting organs come at the tips of -the branches, and sometimes themselves divide into two, and in these -cone-like axes the spore cases are arranged, a single one on the upper -side of each of the scales (see p. 67, fig. 46, A). In the Lycopods the -spores are all alike, in the Selaginellas there are larger spores borne -in a small number (four) in some sporangia (see fig. 53, p. 75), and -others in large numbers and of smaller size on the scales above them. The -stems are all very slender, and have no zones of secondary wood. They -generally creep or climb, and from them are put out long structures -something like roots in appearance, which are specially modified -stem-like organs giving rise to roots. - -From the fossils of the Coal Measures _Lepidodendron_ must be chosen as -the example for comparison. The different species of this genus are very -numerous, and the various fossilized remains of it are among the -commonest and best known of palontological specimens. The huge stems are -objects of public interest, and have been preserved in the Victoria Park -in Glasgow in their original position in the rocks, apparently as they -grew with their spreading rootlike organs running horizontally. A great -stump is also preserved in the Manchester Museum, and is figured in the -frontispiece. While among the casts and impressions the leaf bases of the -plant are among the best preserved and the most beautiful (see fig. 93). -The cone has already been illustrated (see fig. 46 and fig. 9), and is -one of the best known of fossil fructifications. - - [Illustration: Fig. 93.--Photo of Leaf Bases of _Lepidodendron_ - - C, Scar of leaf; S, leaf base. In the scar: _v_, mark of severed - vascular bundle, and _p_, of parichnos. _l_, Ligule scar.] - -From the abundant, though scattered material, fossil botanists have -reconstructed the plants in all their detail. The trunks were lofty and -of great thickness, bearing towards the apex a much-branched crown, the -branches, even down to the finest twigs, all dividing into two equal -parts. The leaves, as would be expected from the great size of the -plants, were much bigger than those of the recent species (fig. 93 shows -the actual size of the leaf bases), but they were of the same -_relatively_ small size as compared with the stems, and of the same -simple pointed shape. A transverse section across the apex of a fertile -branch shows these closely packed leaves arranged in series round the -axis, those towards the outside show the central vascular strand which -runs through each. - - [Illustration: Fig. 94.--Section across an Axis surrounded by many - Leaves, which shows their simple shape and single central vascular - bundle _v_] - -The markings left on the well-preserved leaf-scars indicate the main -features of the internal anatomy of the leaves. They had a single central -vascular strand (_v_, fig. 93), on either side of which ran a strand of -soft tissue _p_ called the parichnos, which is characteristic of the -plants of this group. While another similarly obscure structure -associated with the leaf is the little scale-like ligule _l_ on its upper -surface. - -The anatomy of the stems is interesting, for in the different species -different stages of advance are to be found, from the simple solid -protostele with a uniform mass of wood to hollow ring steles with a pith. -An interesting intermediate stage between these two is found in -_Lepidodendron selaginoides_ (see fig. 95), where the central cells of -the wood are not true water-conducting cells, but short irregular -water-storage tracheides (see p. 56), which are mixed with parenchyma. -All the genera of these fossils have a single central stele, round which -it is usual to find a zone of secondary wood of greater or less extent -according to the age of the plant. - - [Illustration: Fig. 95.--Transverse Section of _Lepidodendron - selaginoides_, showing the circular mass of primary wood, the central - cells of which are irregular water-storage tracheides - - _s_, Zone of secondary wood; _c_, inner cortical tissues; _r_, - intrusive burrowing rootlet; _oc_, outer cortical tissues with corky - external layers _k_. (Microphoto.)] - -Some stems instead of this compact central stele have a ring of wood with -an extensive pith. Such a type is illustrated in fig. 96, which shows but -a part of the circle of wood, and the zone of the secondary wood outside -it, which greatly exceeds the primary mass in thickness. This zone of -secondary wood became very extensive in old stems, for, as will be -imagined, the primary wood was not sufficient to supply the large trunks. -The method of its development from a normal cambium in radiating rows of -uniform tracheides is quite similar to that which is found in the pines -to-day. This is the most important difference between the living and the -fossil stems of the family, for no living plants of the family have such -secondary wood. On the other hand, the individual elements of this wood -are different from those of the higher families hitherto considered, and -have narrow slit-like pits separated by bands of thickening on the -longitudinal walls. Such tracheides are found commonly in the -Pteridophytes, both living and fossil. Their type is seen in fig. 96, B, -which should be compared with that in figs. 78, A and 62, B to see the -contrast with the higher groups. - - [Illustration: Fig. 96.--A, _Lepidodendron_ Stem with Hollow Ring of - Wood W and Zone of Secondary Wood S. B, Longitudinal View of the Narrow - Pits of the Wood Elements.] - -To supply the vascular tissues of the leaf traces, simple strands come -off from the outer part of the primary wood, where groups of small-celled -protoxylem project (see _px_ in fig. 97). The leaf strands _lt_ move out -through the cortex in considerable numbers to supply the many leaves, -into each of which a single one enters. - - [Illustration: Fig. 97.--Transverse Section of Outer Part of Primary - Wood of _Lepidodendron_, showing _px_, projecting protoxylem groups; - _lt_, leaf trace coming from the stele and passing (as _lt_^1) through - the cortex] - -As regards the fructifications of _Lepidodendron_ much could be said were -there space. The many genera of _Lepidodendron_ bore several distinct -types of cones of different degrees of complexity. In several of the -genera the cones were simple in organization, directly comparable with -those of the living Lycopods, though on a much larger scale (see p. 67). -In some the spores were uniform, all developing equally in numerous -tetrads. The sporophyll was radially extended, and along it the large -sausage-shaped sporangia were attached (see fig. 98). The tips of the -sporophylls overlapped and afforded protection to the sporangia. The axis -of the cone had a central stele with wood elements like those in the -stem. The appearance of a transverse section of an actual cone is shown -in fig. 99. Here the sporangia are irregular in shape, owing to their -contraction after ripeness and during fossilization. Other cones had -sporangia similar in size and shape, but which produced spores of two -kinds, large ones resulting from the ripening of only two or three -tetrads in the lower sporangia, and numerous small ones in the sporangia -above. - - [Illustration: Fig. 98.--Longitudinal Diagram, showing the arrangement - of the elongated sporangia on the sporophylls - - _a_, Main axis, round which the sporophylls are inserted; S, - sporangium; _s_, leaflike end of sporophyll.] - -The similarity between the _Lepidodendron_ and the modern Lycopod cone -has been pointed out already (p. 67), and it is this which forms the -principal guarantee that they belong to the same family, though the size -and wood development of the palozoic and the modern plants differ so -greatly. - -The large group of the Lepidodendra included some members whose -fructifications had advanced so far beyond the simple sporangial cones -described above as to approach very closely to seeds in their -construction. This type was described on p. 75, fig. 54, in a series of -female fructifications, so that its essential structure need not be -recapitulated. - - [Illustration: Fig. 99.--Transverse Section through Cone of - _Lepidodendron_ - - A, Main axis with woody tissue; _st_, stalks of sporophylls cut in - oblique longitudinal direction; _s_, tips of sporophylls cut across; S, - sporangia with a few groups of spores. (Microphoto.)] - -The section shown in fig. 100 is that cut at right angles to that in -which the sporangia are shown in fig. 98, viz. tangential to the axis. A -remarkable feature of the plant is that there were also round those -sporangia which bore the numerous small spores (corresponding to pollen -grains) enclosing integument-like flaps similar to those shown in fig. -100, _sp. f_. - - [Illustration: Fig. 100.--Section through one Sporangium of - _Lepidocarpon_ - - _sp_, Sporophyll; _sp.f._, flaps of sporophyll protecting sporangium; - S, large spore within the sporangium wall _w_; _s_, the three aborted - spores of the tetrad to which S belongs.] - -This type of fructification is the nearest approach to seed and pollen -grains reached by any of the Pteridophytes, and its appearance at a time -when the Lycopods were one of the dominant families is suggestive of the -effect that such a position has on the families occupying it, however -lowly they may be. The simple Pteridophyte Lycopods had not only the tall -trunks and solid woody structure of a modern tree, but also a semblance -of its seeds. Whether this line of development ever led on to any of the -higher families is still uncertain. The feeling of most specialists is -that it did not; but there are not wanting men who support the view that -the lycopod affinity evolved in time and entered the ranks of the higher -plants, and indeed there are many points of superficial likeness between -the palozoic Lycopods and the Conifer. Judged from their internal -structure, however, the series through the ferns and Pteridosperms leads -much more convincingly to the seed plants. - -In their roots, or rather in the underground structures commonly called -roots, the Lepidodendrons were also remarkable. Even more symmetrically -than in their above-ground branching, the base of their trunks divided; -there were four main large divisions, each of which branched into two and -these into two again. These structures were called _Stigmaria_, and were -common to all species of _Lepidodendron_ and also the group of -_Sigillaria_ (see fig. 102). On these horizontally running structures -(well shown in the frontispiece) small appendages were borne all over -their surface in great profusion, which were, both in their function and -microscopic structure, rootlets. They left circular scars of a -characteristic appearance on the big trunks, of which they were the only -appendages. These scars show clearly on the fragments along the ledge to -the left of the photograph. The exact morphological nature of the big -axes is not known; their anatomy is not like that of roots, but is that -of a stem, yet they do not bear what practically every stem, whether -underground or not, has developed, namely leaves, or scales representing -reduced leaves. Their nature has been commented on previously (p. 69), -and we cannot discuss the point further, but must be content to consider -them as a form of root-bearing stem, practically confined to the Lycopods -and principally developed among the palozoic fossils of that group. - - [Illustration: Fig. 101.--Transverse Section through a Rootlet of - _Stigmaria_ - - _oc_, Outer cortex; _s_, space; _ic_, inner cortex; _w_, wood of - vascular strand (wood only preserved); _px_, protoxylem group.] - -In microscopic structure the rootlets are extremely well known, because -in their growth they have penetrated the masses of the tissues of other -plants which were being petrified and have become petrified with them. -The mass of decaying vegetable tissue on which the living plants of the -period flourished were everywhere pierced by these intrusive rootlets, -and they are found petrified inside otherwise perfect seeds, in the -hearts of woody stems, in leaves and sporangia, and sometimes even inside -each other! Fig. 95 shows such a root _r_ lying in the space left by the -decay of the soft tissue of the inner cortex in an otherwise excellently -preserved _Lepidodendron_ stem (see also fig. 101). In fig. 101 their -simple structure is seen. They are often extremely irregular in shape, -owing to the way they seem to have twisted and flattened themselves in -order to fit into the tissues they were penetrating. No root hairs seem -to have been developed in these rootlets, but otherwise their structure -is that of a typical simple root, and very like the swamp-penetrating -rootlets of the living Isoetes. - -The Stigmarian axes and their rootlets are very commonly found in the -"underclays" and "gannister" beds which lie below the coal seams (see p. -25), and they may sometimes be seen attached to a bit of the trunk -growing upwards through the layers. They and the aerial stems of -Lepidodendron are perhaps the commonest and most widely known of fossil -plants. - -Before leaving the palozoic Lycopods another genus must be mentioned, -which is also a widely spread and important one, though it is less well -known than its contemporary. The genus _Sigillaria_ is best known by its -impressions and casts of stems covered by leaf scars. The stems were -sometimes deeply ribbed, and the leaf scars were arranged in rows and -were more or less hexagonal in outline, as is seen in fig. 102, which -shows a cast and its reverse of the stem of a typical _Sigillaria_. - - [Illustration: Fig. 102.--Cast and Reverse of Leaf Scars of - _Sigillaria_. In A the shape of the leaf bases is clearly shown, the - central markings in each being the scar of the vascular bundle and - parichnos] - -In its primary wood _Sigillaria_ differed from _Lepidodendron_ in being -more remote from the type with a primary solid stele. Its woody structure -was that of a ring, in some cases irregularly broken up into -crescent-shaped bundles. The secondary wood was quite similar to that of -_Lepidodendron_. - -_Stigmaria_ and its rootlets belong equally to the two plants, and -hitherto it has been impossible to tell whether any given specimen of -_Stigmaria_ had belonged to a _Lepidodendron_ or a _Sigillaria_. Between -the two genera there certainly existed the closest affinity and -similarity in general appearance. - -These two genera represent the climax of development of the Lycopod -family. In the Lower Mesozoic some large forms are still found, but all -through the Mesozoic periods the group dwindled, and in the Tertiary -little is known of it, and it seems to have taken the retiring position -it occupies to-day. - - - - - CHAPTER XV - PAST HISTORIES OF PLANT FAMILIES - VIII. The Horsetails - - -The horsetails of to-day all belong to the one genus, _Equisetum_, among -the different species of which there is a remarkably close similarity. -Most of the species love swampy land, and even grow standing up through -water; but some live on the dry clay of ploughed fields. Wherever they -grow they usually congregate in large numbers, and form little groves -together. They are easily recognized by their delicate stems, branching -in bottle-brush fashion, and the small leaves arranged round them in -whorls, with their narrow teeth joined to a ring at the base. At the end -of some of the branches come the cones, with compactly arranged and -simple sporophylls all of one kind. In England most plants of this family -are but a few inches or a foot in height, though one species sometimes -reaches 6 ft., while in South America there are groves of -delicate-stemmed plants 20 ft. high. - -The ribbed stems and the whorls of small, finely toothed leaves are the -most important external characteristics of the plants, while in their -internal anatomy the hollow stems have very little wood, which is -arranged in a series of small bundles, each associated with a hollow -canal in the ground tissue. - -The family stands apart from all others, and even between it and the -group of Lycopods there seems to be a big gap across which stretch no -bonds of affinity. Has the group always been in a similar position, and -stood isolated in a backwater of the stream of plant life? - - [Illustration: Fig. 103.--Impression of Leaf Whorl of _Equisetites_ - from the Mesozoic Rocks, showing the narrow toothed form of the leaves. - (Photo.)] - -In the late Tertiary period they seem to have held much the same position -as they do now, and we learn nothing new of them from rocks of that age. -When, however, we come to the Mesozoic, the members of the family are of -greater size, though they appear (to judge from their external -appearance) to have been practically identical with those now living in -all their arrangements. In some beds their impressions are very numerous, -but unfortunately most are without any indication of internal structure. -Fossils from the Mesozoic are called _Equisetites_, a name which -indicates that they come very close to the living ones in their -characters. In the Lower Mesozoic some of these stems seem to have -reached the great size of a couple of feet in circumference, but to have -no essential difference from the others of the group. - -When, however, we come to the Palozoic rocks we find many specimens with -their structure preserved, and we are at once in a very different -position as regards the family. - -First in the Permian we meet with the important genus of plant called -_Calamites_, which were very abundant in the Coal Measures. Many of the -Calamites were of great size, for specimens with large trunks have been -found 30 ft. and more long, which when growing must certainly have been -much taller than that. The number of individuals must also have been very -great, for casts and impressions of the genus are among the commonest -fossils. They were, in fact, one of the dominant groups of the period. -Like the Lycopods, the Equisetace reached their high-water mark of -development in the Carboniferous period; at that time the plants were -most numerous, and of the largest size and most complicated structure -that they ever attained. - - [Illustration: Fig. 104.--Small Branches attached to stouter Axis of - _Calamites_. Photo of Impression] - -As will be immediately suspected from analogy with the Lycopods, they -differed from the modern members of the family in their strongly -developed anatomy, and in the strength and quantity of their secondary -wood. Yet in their external appearance they probably resembled the living -genus in all essentials, and the groves of the larger ones of to-day -growing in the marshes probably have the appearance that the palozoic -plants would have had if looked at through a reversed opera glass. - -Fig. 104 is a photograph of some of the small branches of a Calamite, in -which the ribbed stem can be seen, and on the small side twigs the fine, -pointed leaves lying in whorls. - -In most of the fossil specimens, however, particularly the larger ones, -the ribs are not those of the true surface, but are those marked on the -_internal cast_ of the pith. - - [Illustration: Fig. 105.--Transverse Section of _Calamites_ Stem with - Secondary Wood _w_ formed in Regular Radial Rows in a Solid Ring - - _c_, Canals associated with the primary bundles; _p_, cells of the - pith, which is hollow with a cavity _l_, _cor_, Cortex and outer - tissues well preserved. (Microphoto.)] - -Among tissue petrifactions there are many Calamite stems of various -stages of growth. In the very young ones there are only primary bundles, -and these little stems are like those of a living Equisetum in their -anatomy, and have a hollow pith and small vascular bundles with canals -associated. The fossil forms, however, soon began to grow secondary wood, -which developed in regular radial rows from a cambium behind the primary -bundles and joined to a complete ring. - -A stem in this stage of development is seen in fig. 105, where only the -wood and internal tissues are preserved. The very characteristic canals -associated with the primary bundles are clearly shown. The amount of -secondary wood steadily increased as the stems grew (there appear to have -been no "annual rings") till there was a very large quantity of secondary -tissue of regular texture, through which ran small medullary rays, so -that the stems became increasingly like those of the higher plants as -they grew older. It is the primary structure which is the important -factor in considering their affinity, and that is essentially the same as -in the other members of the family in which secondary thickening is not -developed. As we have seen already in other groups of fossils, secondary -wood appears to develop on similar lines whenever it is needed in any -group, and therefore has but little value as an indication of systematic -position. This important fact is one, however, which has only been -realized as a result of the study of fossil plants. - - [Illustration: Fig. 106.--Diagram of the Arrangement of the Bundles at - the Node of a _Calamite_, showing how those of consecutive internodes - alternate - - _n_, Region of node] - - [Illustration: Fig. 107.--Leaf of _Calamites_ in Cross Section - - _v_, Vascular bundles; _s_, cells of sheath, filled with blackened - contents; _p_, palisade cells; _e_, epidermis.] - -The longitudinal section of the stems, when cut tangentially, is very -characteristic, as the bundles run straight down to each node and there -divide, the neighbouring halves joining so that the bundles of each node -alternate with those of the ones above and below it (see fig. 106). - -The leaves which were attached at the nodes were naturally much larger -than those of the present Equisetums, though they were similarly simple -and undivided. Their anatomy is preserved in a number of cases (see fig. -107), and was simple, with a single small strand of vascular tissue lying -in the centre. They had certain large cells, sometimes very black in the -fossils, which may have been filled with mucilage. - - [Illustration: Fig. 108.--Transverse Section of Young Root of - _Calamites_ - - _w_, Wood of axis; _l_, spaces in the lacunar cortex, whose radiating - strands _r_ are somewhat crushed; _ex_, outermost cells of cortex with - thickened wall.] - - [Illustration: Fig. 109.--Diagram of Cone of _Calamites_ - - A, Main axis; _br_, sterile bracts; _sp_, sporophylls with four - sporangia S attached to each, of which two only are seen.] - -The young roots of these plants have a very characteristic cortex, which -consists of cells loosely built together in a lacelike fashion, with -large air spaces, so that they are much like water plants in their -appearance (see fig. 108). Indeed, so unlike the old roots and the stems -are they, that for long they were called by another name and supposed to -be submerged stems, but their connection with _Calamites_ is now quite -certain. As their woody axis develops, the secondary tissue increases and -pushes off the lacelike cortex, and the roots become very similar in -their anatomy to the stems. Both have similar zones of secondary wood, -but the roots do not have those primary canals which are so -characteristic of the stems, and thereby they can be readily -distinguished from them. - -The fructifications of the Calamites were not unlike those of the living -types of the family, though in some respects slightly more complex. Round -each cone axis developed rings of sporophylls which alternated with -sterile sheathing bracts. Each sporophyll was shaped like a small -umbrella with four spokes, and stood at right angles to the axis, bearing -a sporangium at each of the spokes. A diagram of this arrangement is seen -in fig. 109. - - [Illustration: Fig. 110.--Longitudinal Section of Part of _Calamites_ - Cone - - _br_, Sterile bracts attached to axis; _sp_, attachment of sporophylls; - S, sporangia. At X a group of four sporangia is seen round the - sporophyll, which is seen at _a_. (Microphoto.)] - -A photograph of an actual section of such a cone, cut slightly obliquely -through the length of the axis, is seen in fig. 110, where the upper -groups of sporangia are cut tangentially, and show their grouping round -the sporophyll to which they are attached. - -A few single tetrads of spores are enlarged in fig. 111, where it will be -seen that the large spores are of a similar size, but that the small ones -of the tetrads are very irregular. They are aborting members of the -tetrad, and appear to have been used as food by the other spores. In each -sporangium large numbers of these tetrads develop and all the ripe spores -seem to have been of one size. - -In a species of _Calamites_ (_C. casheana_), otherwise very similar to -the common one we have been considering, there is a distinct difference -in the sizes of the spores from different sporangia. The small ones, -however, were only about one-third of the diameter of the large ones, so -that the difference was very much less marked than it was between the -small and large spores of the Lycopods. - -Among the palozoic members of the group are other genera closely allied -to, but differing from _Calamites_ in some particulars. One of these is -_Archocalamites_, which has a cone almost identical with that of the -living Equisetums, as it has no sterile bracts mingled with the -umbrella-like sporophylls. Other genera are more complex than those -described for _Calamites_, and even in the simple coned _Archocalamites_ -itself the leaves are finely branched and divided instead of being simple -scales. - -But no genus is so completely known as is _Calamites_, which will itself -suffice as an illustration of the palozoic Equisetace. Though the -genus, as was pointed out above, shows several important characters -differing from those of Equisetum, and parallel to some extent to those -of the palozoic Lycopods, yet these features are more of a physiological -nature than a systematic one, and they throw no light on the origin of -the family or on its connection with the other Pteridophytes. It is in -the extinct family dealt with in the next chapter that we find what some -consider as a clue to the solution of these problems. - - [Illustration: Fig. 111.--Tetrads of Spores of _Calamites_ - - S, Normal-sized spores; _a_, _b_, &c., aborting spores.] - - - - - CHAPTER XVI - PAST HISTORIES OF PLANT FAMILIES - IX. Sphenophyllales - - -The group to which _Sphenophyllum_ belongs is of considerable interest -and importance, and is, further, one of those extinct families whose very -existence would never have been suspected had it not been discovered by -fossil botanists. Not only is the family as a whole extinct, it also -shows features in its anatomy which are not to be paralleled among living -stems. _Sphenophyllum_ became extinct in the Palozoic period, but its -interest is very real and living to-day, and in the peculiar features of -its structure we see the first clue that suggests a common ancestor for -the still living groups of Lycopods and Equisetace, which now stand so -isolated and far apart. - -Before, however, we can consider the affinities of the group, we must -describe the structure of a typical plant belonging to it. The genus -_Sphenophyllum_ includes several species (for which there are no common -English names, as they are only known to science) whose differences are -of less importance than their points of similarity, so that one species -only, _S. plurifoliatum_, will be described. - -We have a general knowledge of the external appearance of _Sphenophyllum_ -from the numerous impressions of leaves attached to twigs which are found -in the rocks of the Carboniferous period. These impressions present a -good deal of variety, but all have rather delicate stems with whorls of -leaves attached at regular intervals. The specimens are generally easy to -recognize from the shape of the leaves, which are like broad wedges -attached at the point (see fig. 112). In some cases the leaves are more -finely divided and less fanlike, and it may even happen that on the same -branch some may be wedge-shaped like those in fig. 112, and others almost -hairlike. This naturally suggests comparison with water plants, which -have finely divided submerged leaves and expanded aerial ones. In the -case of _Sphenophyllum_, however, the divided leaves sometimes come at -the upper ends of the stems, quite near the cones, and so can hardly have -been those of a submerged part. The very delicate stems and some points -in their internal anatomy suggest that the plant was a trailing creeper -which supported itself on the stouter stems of other plants. - - [Illustration: Fig. 112.--Impression of _Sphenophyllum_ Leaves attached - to the Stem, showing the wedge-shaped leaflets arranged in whorls] - -The stems were ribbed, but unlike those of the Calamites the ribs ran -straight down the stem through the nodes, and did not alternate there, so -that the bundles at the node did not branch and fuse as they did in -_Calamites_. - -The external appearance of the long slender cones was not unlike that of -the Calamite cones, though their internal details showed important -distinctions. - -In one noticeable external feature the plants differed from those of the -last two groups considered, and that was in their _size_. Palozoic -Lycopods and Equisetace reached the dimensions of great trees, but -hitherto no treelike form of _Sphenophyllum_ has been discovered, and in -the structure-petrifactions the largest stems we know were less than an -inch in diameter. - -In the internal anatomy of these stems lies one of the chief interests -and peculiarities of the plants. In the very young stage there was a -sharply pointed solid triangle of wood in the centre (fig. 113), at each -of the corners of which was a group of small cells, the protoxylems. The -structure of such a stem is like that of a root, in which the primary -wood all grows inwards from the protoxylems towards the centre, and had -we had nothing but these isolated young stems it would have been -impossible to recognize their true nature. - - [Illustration: Fig. 113.--_Sphenophyllum_, Transverse Section of Young - Stem - - _c_, Cortex, the soft tissue within which has decayed and left a space, - in which lies the solid triangle of wood, with the small protoxylem - groups _px_ at each corner. (Microphoto.)] - -Such very young stems are rare, for the development of secondary wood -began early, and it soon greatly exceeded the primary wood in amount. -Fig. 114 shows a photograph of a stem in which the secondary wood is well -developed. The primary triangle of wood is still to be seen in the -centre, and corresponds to that in fig. 113, while closely fitting to it -are the bays of the first-formed secondary wood, which makes the wood -mass roughly circular. Outside this the secondary wood forms a regular -cylinder round the axis, which shows no sign of annual rings. The cells -of the wood are large and approximately square in shape, while at the -angles formed at the junction of every four cells is a group of small, -thin-walled parenchyma, see fig. 115. There are no medullary rays going -out radially through the wood, such as are found in all other zones of -secondary wood, and in this arrangement of soft tissue the plants are -unique. - - [Illustration: Fig. 114.--_Sphenophyllum_, Transverse Section with - Secondary Wood W. At _c_ the cork formation is to be seen. - (Microphoto.)] - -Beyond the wood was a zone of soft tissue and phloem, which is not often -preserved, while outside that was the cork, which added to the cortical -tissues as the stem grew (see fig. 114, _c_). - - [Illustration: Fig. 115.--Group of Wood Cells _w_, showing their shape - and the small soft-walled cells at the angles between them _p_] - -Petrified material of leaves and roots is rare, and both are chiefly -known through the work of the French palobotanist Renault. The leaves -are chiefly remarkable for the bands of sclerized strengthening tissue, -and generally had the structure of aerial, not submerged leaves. The -roots were simple in structure, and, as in _Calamites_, had secondary -tissue like that in the stems. - -In the case of the fructifications it is the English material which has -yielded the most illuminating specimens. The cones were long and slender, -externally covered by the closely packed tips of the scales, which -overlapped deeply. Between the whorls of scales lay the sporangia, -attached to their upper sides by slender stalks. A diagram will best -explain how they were arranged (see fig. 116). Two sporangia were -attached to each bract, but their stalks were of different lengths, so -that one sporangium lay near the axis and one lay outside it toward the -tip of the bract. - - [Illustration: Fig. 116.--Diagram of Arrangement of Scales and - Sporangia in Cones of _Sphenophyllum_ - - A, Axis; _br_, bract; S, sporangium, with stalk _st_.] - -In its anatomy the stalk of the cone has certain features similar to -those in the stem proper, which were among the first indications that led -to the discovery that the cone belonged to _Sphenophyllum_. There were -numerous spores in each of the sporangia, which had coats ornamented with -little spines when they were ripe (fig. 117, if examined with a -magnifying glass, will show this). Hitherto the only spores known are of -uniform size, and there is no evidence that there was any differentiation -into small (male) and large (female) spores such as were found in some of -the Lepidodendrons. In this respect _Sphenophyllum_ was less specialized -than either _Lepidodendron_ or _Calamites_. - -In the actual sections of _Sphenophyllum_ cones the numerous sporangia -seem massed together in confusion, but usually some are cut so as to show -the attachment of the stalk, as in fig. 117, _st_. As the stalk was long -and slender, but a short length of it is usually cut through in any one -section, and to realize their mode of attachment to the axis (as shown in -fig. 116) it is necessary to study a series of sections. - - [Illustration: Fig. 117.--Part of Cone of _Sphenophyllum_, showing - sporangia _sp_, some of which are cut so as to show a part of their - stalks _st_. B, Bract. (Microphoto.)] - - -Of the other plants belonging to the group, _Bowmanites Rmeri_ is -specially interesting. Its sporangia were borne on stalks similar to -those of _Sphenophyllum_, but each stalk had two sporangia attached to -it. Two sporangia are also borne on each stalk in _S. fertile_. These -plants help in elucidating the nature of the stalked sporangia of -_Sphenophyllum_, for they seem to indicate a direct comparison between -them and the sporophylls of the Equisetales. - - -There is, further, another plant, of which we only know the cone, of -still greater importance. This cone (_Cheirostrobus_) is, however, so -complex that it would take far too much space to describe it in detail. -Even a diagram of its arrangements is extraordinarily elaborate. To the -specialist the cone is peculiarly fascinating, for its very complexity -gives him great scope for weaving theories about it; but for our purposes -most of these are too abstruse. - - [Illustration: Fig. 118.--A, Diagram of Three-lobed Bract from Cone of - _Cheirostrobus_. _a_, Axis; _br_, the three sterile lower lobes of the - bract; _sp_, the three upper sporophyll-like lobes, to each of which - were attached four sporangia S. B, Part of the above seen in section - longitudinal to the axis. (Modified from Scott.)] - -Its most important features are the following. Round the axis were series -of scales, twelve in each whorl, and each scale was divided into an upper -and a lower portion, each of which again divided into three lobes. The -lower three of each of these scale groups were sterile and bractlike, -comparable, perhaps, with the bracts in fig. 116; while the upper three -divisions were stalks round each of which were four sporangia. Each -sporophyll segment thus resembled the sporophyll of _Calamites_, while -the long sausage-shaped sporangia themselves were more like those of -_Lepidodendron_. In fig. 118 is a diagram of a trilobed bract with its -three attached sporophylls. Round the axis were very numerous whorls of -such bracts, and as the cone was large there were enormous numbers of -spore sacs. - -A point of interest is the character of the wood of the main axis, which -is similar to that of Lepidodendron in many respects, being a ring of -centripetally developed wood with twelve projecting external points of -protoxylem. - -This cone[13] is the most complex fructification of any of the known -Pteridophytes, whether living or fossil, which alone ensures it a special -importance, though for our purpose the mixed affinities it shows are of -greater interest. - -To mention some of its characters:--The individual segments of the -sporophylls, each bearing four sporangia, are comparable with those of -_Calamites_, while the individual sporangia and the length of the -sporophyll stalk are similar in appearance to those of _Lepidodendron_. -The wood of the main axis also resembles that of a typical -_Lepidodendron_. The way the vascular bundles of the bract pass out from -the axis, and the way the stalks bearing the sporangia are attached to -the sterile part of the bracts, are like the corresponding features in -_Sphenophyllum_, and still more like _Bowmanites_. - -Many other points of comparison are to be found in these plants, but -without going into further detail enough has been indicated to support -the conclusion that _Cheirostrobus_ is a very important clue to the -affinities of the Sphenophyllales and early Pteridophytes. It is indeed -considered to have belonged to an ancient stock of plants, from which the -Equisetace, and _Sphenophylla_, and possibly also the Lycopods all -sprang. - -_Sphenophyllum_, _Bowmanites_, and _Cheirostrobus_, a series of forms -that became extinct in the Palozoic, remote in their structure from any -living types, whose existence would have been entirely unsuspected but -for the work of fossil botany, are yet the clues which have led to a -partial solution of the mysteries surrounding the present-day Lycopods -and Equisetums, and which help to bridge the chasm between these remote -and degenerate families. - - - - - CHAPTER XVII - PAST HISTORIES OF PLANT FAMILIES - X. The Lower Plants - - -In the plant world of to-day there are many families including immense -numbers of species whose organization is simpler than that of the groups -hitherto considered. Taken all together they form, in fact, a very large -proportion of the total number of living species, though the bulk of them -are of small size, and many are microscopic. - -These "lower plants" include all the mosses, and the flat green -liverworts, the lichens, the toadstools, and all the innumerable moulds -and parasites causing plant diseases, the green weeds growing in water, -and all the seaweeds, large and small, in the sea, the minute green cells -growing in crevices of the bark of trees, and all the similar ones living -by millions in water. Truly a host of forms with an endless variety of -structures. - -Yet when we turn to the fossil representatives of this formidable -multitude, we find but few. Indeed, of the fossil members of all these -groups taken together we know less that is of importance and real -interest than we do of any single family of those hitherto considered. -The reasons for this dearth of fossils of the lower types are not quite -apparent, but one which may have some bearing on it is the difficulty of -mineralization. It is self-evident that the more delicate and soft-walled -any structure is the less chance has it of being preserved without decay -long enough to be fossilized. As will have been understood from Chapter -II, even when the process of fossilization took place, geologically -speaking, rapidly, it can never have been actually accomplished quickly -as compared with the counter processes of decay. Hence all the lower -plants, with their soft tissue and lack of wood and strengthening cells, -seem on the face of it to stand but little chance of petrifaction. - -There is much in this argument, but it is not a sufficient explanation of -the rarity of lower plant fossils. All through the preceding chapters -mention has been made of very delicate cells, such as pith, spores, and -even germinating spores (see fig. 47, p. 68), with their most delicate -outgrowing cells. If then such small and delicate elements from the -higher plants are preserved, why should not many of the lower plants -(some of which are large and sturdy) be found in the rocks? - -As regards the first group, the mosses, it is probable that they did not -exist in the Palozoic period, whence our most delicately preserved -fossils are derived. There seems much to support the view that they have -evolved comparatively recently although they are less highly organized -than the ferns. Quite recently experiments have been made with their near -allies the liverworts, and those which were placed for one year under -conditions similar to those under which plant petrifaction took place, -were found to be perfectly preserved at the end of the period; though -they would naturally decay rapidly under usual conditions. This shows -that Bryophyte cells are not peculiarly incapable of preservation as -fossils, and adds weight to the negative evidence of the rocks, -strengthening the presumption of their late origin. - -That some of the lower plants, among the very lowest and simplest, can be -well preserved is shown in the case of the fossil fungi which often occur -in microscopic sections of palozoic leaves, where they infest the higher -plants as similar parasitic species do to-day. - -We must now bring forward the more important of the facts known about the -fossils of the various groups of lower plants. - - -Bryophytes.--_Mosses._ Of this family there are no specimens of any age -which are so preserved as to show their microscopical structure. Of -impressions there are a few from various beds which show, with more or -less uncertainty in most cases, stems and leaves of what appear to be -mosses similar to those now extant, but they nearly all lack the -fructifications which would determine them with certainty. These -impressions go by the name of _Muscites_, which is a dignified cloak for -ignorance in most cases. The few which are quite satisfactory as -impressions belong to comparatively recent rocks. - -_Liverworts_ are similarly scanty, and there is nothing among them which -could throw any light on the living forms or their evolution. The more -common are of the same types as the recent ones, and are called -_Marchantites_, specimens of which have been found in beds of various -ages, chiefly, however, in the more recent periods of the earth's -history. - -It is of interest to note that among all the delicate tissue which is so -well preserved in the "coal balls" and other palozoic petrifactions, -there are no specimens which give evidence of the existence of mosses at -that time. It is not unlikely that they may have evolved more recently -than the other groups of the "lower" plants. - -Charace.--Members of this somewhat isolated family (Stoneworts) are -better known, as they frequently occur as fossil casts. This is probably -due to their character, for even while alive they tend to cover their -delicate stems and leaves, and even fruits, with a limy incrustation. -This assists fossilization to some degree, and fossil Charas are not -uncommon. Usually they are from the recently deposited rocks, and the -earliest true Charas date only to the middle of the Mesozoic. - -An interesting occurrence is the petrifaction of masses of these plants -together, which indicate the existence of an ancient pool in which they -must have grown in abundance at one time. A case has been described where -masses of _Chara_ are petrified where they seem to have been growing, and -in their accumulations had gradually filled up the pond till they had -accumulated to a height of 8 feet. - -The plants, however, have little importance from our present point of -view. - - -Fungi.--Of the higher fungi, namely, "toadstools", we have no true -fossils. Some indications of them have been found in amber, but such -specimens are so unsatisfactory that they can hardly afford much -interest. - - [Illustration: Fig. 119.--The Hyph of Fungi Parasitic on a Woody Tree - - _c_, Cells of host; _h_, hyph of fungus, with dividing cell walls.] - -The lower fungi, however, and in particular the microscopic and parasitic -forms, occur very frequently, and are found in the Coal Measure fossils. -Penetrating the tissues of the higher plants, their hosts, the parasitic -cells are often excellently preserved, and we may see their delicate -hyph wandering from cell to cell as in fig. 119, while sometimes there -are attached swollen cells which seem to be sporangia. From the Palozoic -we get leaves with nests of spores of the fungus which had attacked and -spotted them as so many do to leaves to-day (see fig. 120). What is -specially noticeable about these plants is their similarity to the living -forms infesting the higher plants of the present day. Already in the -Palozoic the sharp distinction existed between the highly organized -independent higher plants and their simple parasites. The higher plants -have changed profoundly since that time, stimulated by ever-changing -surroundings, but the parasites living within them are now much as they -were then, just sufficiently highly organized to rob and reproduce. - -A form of fungus inhabitant which seems to be useful to the higher plant -appears also to have existed in Palozoic times, viz. _Mycorhiza_. In the -roots of many living trees, particularly such as the Beech and its -allies, the cells of the outer layers are penetrated by many fungal forms -which live in association with the tree and do it some service at the -same time as gaining something for themselves. This curious, and as yet -incompletely understood physiological relation between the higher plants -and the fungi, existed so far back as the Palozoic period, from which -roots have been described whose cells were packed with minute organisms -apparently identical with _Mycorhiza_. - - [Illustration: Fig. 120.--Fossil Leaf _l_ with Nests of Infesting - Fungal Spores _f_ on its lower side] - - -Alg.--_Green Alg_ (pond weeds). Many impressions have been described as -alg from time to time, numbers of which have since been shown to be a -variety of other things, sometimes not plants at all. Other impressions -may really be those of alg, but hitherto they have added practically -nothing to our knowledge of the group. - -Several genera of alg coat themselves with calcareous matter while they -are alive, much in the same way as do the Charas, and of these, as is -natural, there are quite a number of fossil remains from Tertiary and -Mesozoic rocks. This is still more the case in the group of the _Red -Alg_ (seaweeds), of which the calcareous-coated genera, such as -_Corallina_ and others, have many fossil representatives. These plants -appear so like corals in many cases that they were long held to be of -animal nature. The genus _Lithothamnion_ now grows attached to rocks, and -is thickly encrusted with calcareous matter. A good many species of this -genus have been described among fossils, particularly from the Tertiary -and Cretaceous rocks. As the plant grew in association with animal -corals, it is not always very easy to separate it from them. - - -Brown Alg (seaweeds) have often been described as fossils. This is very -natural, as so many fossils have been found in marine deposits, and when -among them there is anything showing a dark, wavy impression, it is -usually described as a seaweed. And possibly it may be one, but such an -impression does not lead to much advance in knowledge. From the early -Palozoic rocks of both Europe and America a large fossil plant is known -from the partially petrified structure of its stem. There seem to be -several species, or at least different varieties of this, known under the -generic name _Nematophycus_. Specimens of this genus are found to have -several anatomical characters common to the big living seaweeds of the -_Laminaria_ type, and it is very possible that the fossils represent an -early member of that group. In none of these petrified specimens, -however, is there any indication of the microscopic structure of -reproductive organs, so that the exact nature of the fossils is not -determinable. It is probable that though perhaps allied to the Laminarias -they belong to an entirely extinct group. - -An interesting and even amusing chapter might be written on all the -fossils which look like alg and even have been described as such. The -minute river systems that form in the moist mud of a foreshore, if -preserved in the rocks (as they often are, with the ripples and raindrops -of the past), look extraordinarily like seaweeds--as do also countless -impressions and trails of animals. In this portion of the study of -fossils it is better to have a healthy scepticism than an illuminating -imagination. - - -Diatoms, with their hard siliceous shells, are naturally well preserved -as fossils (see fig. 121), for even if the protoplasm decays the mineral -coats remain practically unchanged. - -Diatoms to-day exist in great numbers, both in the cold water of the -polar regions and in the heat of hot springs. Often, in the latter, one -can see them actually being turned into fossils. In the Yellowstone Park -they are accumulating in vast numbers over large areas, and in some -places have collected to a thickness of 6 feet. At the bottoms of -freshwater lakes they may form an almost pure mud of fine texture, while -on the floor of deep oceans there is an ooze of diatoms which have been -separated from the calcareous shells by their greater powers of -resistance to solution by salt water. - - [Illustration: Fig. 121.--Diatom showing the Double Siliceous Coat] - -There are enormous numbers of species now living, and of fossils from the -Tertiary and Upper Mesozoic rocks; but, strangely enough, though so -numerous and so widely distributed, both now and in these past periods, -they have not been found in the earlier rocks. - -In one way the diatoms differ from ordinary fossils. In the latter the -soft tissues of the plant have been replaced by stone, while in the -former the living cell was enclosed in a siliceous case which does not -decompose, thus resembling more the fossils of animal shells. - - -Bacteria are so very minute that it is impossible to recognize them in -ordinary cases. In the matrix of the best-preserved fossils are always -minute crystals and granules that may simulate bacterial shapes -perfectly. _Bacillus_ and _Micrococcus_ of various species have been -described by French writers, but they do not carry conviction. - -As was stated at the beginning of the chapter, from all the fossils of -all the lower-plant families we cannot learn much of prime importance for -the present purpose. Yet, as the history of plants would be incomplete -without mention of the little that is known, the foregoing pages have -been added. - - - - - CHAPTER XVIII - FOSSIL PLANTS AS RECORDS OF ANCIENT COUNTRIES - - -The land which to-day appears so firm and unchanging has been under the -sea many times, and in many different ways has been united to other land -masses to form continents. At each period, doubtless, the solid earth -appeared as stable as it is now, while the country was as well -characterized, and had its typical scenery, plants, and animals. We know -what an important feature of the character of any present country is its -flora; and we have no reason to suspect that it was ever less so than it -is to-day. Indeed, in the ages before men interfered with forest growth, -and built their cities, with their destructive influences, the plants -were relatively more important in the world landscape than they are -to-day. - -As we go back in the periods of geological history we find the plants had -an ever-increasing area of distribution. To-day most individual species -and many genera are limited to islands or parts of continents, but before -the Glacial epoch many were distributed over both America and Europe. In -the Mesozoic _Ginkgo_ was spread all over the world, and in the present -epoch it was confined to China and Japan till it was distributed again by -cultivation; while in the Palozoic period _Lepidodendron_ seemed to -stretch wellnigh from pole to pole. - -The importance of the relation of plant structure to the climate and -local physical conditions under which it was growing cannot be too much -insisted upon. Modern biology and ecology are continually enlarging and -rendering more precise our views of this interrelation, so that we can -safely search the details of anatomical structure of the fossil plants -for sidelights on the character of the countries they inhabited and their -climates. - -It has been remarked already that most of the fossils which we have well -preserved, whether of plants or animals, were fossilized in rocks which -collected under sea water; yet it was also noted that of marine plants we -have almost no reliable fossils at all. How comes this seeming -contradiction? - -The lack of marine plant fossils probably depends on their easily -decomposable nature, while the presence of the numerous land plants -resulted from their drifting out to sea in streams and rivers, or -dropping into the still salt marshes where they grew. Hence, in the rocks -deposited in a sea, we have the plants preserved which grew on adjacent -lands. In fresh water, also, the plants of the neighbourhood were often -fossilized; but actually on the land itself but little was preserved. The -winds and rains and decay that are always at work on a land area tend to -break down and wash away its surface, not to build it up. - -There are many different details which are used in determining the -evidence of a fossil plant. Where leaf impressions are preserved which -exhibit a close similarity to living species (as often happens in the -Tertiary period), it is directly assumed that they lived under conditions -like those under which the present plants of that kind are living; while, -if the anatomy is well preserved (as in the Palozoic and several -Mesozoic types), we can compare its details with that of similar plants -growing under known conditions, and judge of the climate that had -nurtured the fossil plant while it grew. - -Previous to the present period there was what is so well known as the -Glacial epoch. In the earthy deposits of this age in which fossils are -found plants are not uncommon. They are of the same kind as those now -growing in the cold regions of the Arctic circle, and on the heights of -hills whose temperature is much lower than that of the surrounding -lowlands. Glacial epochs occurred in other parts of the world at -different times; for example, in South Africa, in the Permo-Carboniferous -period, during which time the fossils indicate that the warmth-loving -plants were driven much farther north than is now the case. - -It is largely from the nature of the plant fossils that we know the -climate of England at the time preceding the Glacial epoch. Impressions -of leaves and stems, and even of fruits, are abundant from the various -periods of the Tertiary. Many of them were Angiosperms (see Chap. VIII), -and were of the families and even genera which are now living, of which -not a few belong to the warm regions of the earth, and are subtropical. -It is generally assumed that the fossils related to, or identical with, -these plants must therefore have found in Tertiary Northern Europe a much -warmer climate than now exists. Not only in Northern Europe, but right up -into the Arctic circle, such plants occur in Tertiary rocks, and even if -we had not their living representatives with which to compare them, the -large size and thin texture of their leaves, their smoothness, and a -number of other characteristics would make it certain that the climate -was very much milder than it is at present, though the value of some of -the evidence has been overestimated. - -From the Tertiary we are dependent chiefly on impressions of fossils; -anatomical structure would doubtless yield more details, but even as it -is we have quite enough evidence to throw much light on the physiography -of the Tertiary period. The causes for such marked changes of climate -must be left for the consideration of geologists and astronomers. Plants -are passive, driven before great climatic changes, though they have a -considerable influence on rainfall, as has been proved repeatedly in -India in recent times. - -From the more distant periods it is the plants of the Carboniferous, -whose structure we know so well, that teach us most. Although there is -still very much to be done before knowledge is as complete as we should -wish, there are sufficient facts now discovered to correct several -popular illusions concerning the Palozoic period. The "deep, -all-enveloping mists, through which the sun's rays could scarcely -penetrate", which have taken the popular imagination, appear to have no -foundation in fact. There is nothing in the actual structure of the -plants to indicate that the light intensity of the climate in which they -grew was any less than it is in a smoke-free atmosphere to-day. - -Look at the "shade leaves" of any ordinary tree, such as a Lime or Maple, -and compare them with those growing in the sunlight, even on the same -tree. They are larger and softer and thinner. To absorb the same amount -of energy as the more brilliantly lighted leaves, they must expose a -larger surface to the light. Hence if the Coal Measure plants grew in -very great shade, to supply their large growth with the necessary sun -energy we should expect to find enormous spreading leaves. But what is -the fact? No such large leaves are known. _Calamites_ and -_Lepidodendron_, the commonest and most successful plants of the period, -had narrow simple leaves with but a small area of surface. They were, in -fact, leaves of the type we now find growing in exposed places. The ferns -had large divided leaves, but they were finely lobed and did not expose a -large continuous area as a true "shade leaf" does; while the height of -their stems indicates that they were growing in partial shade--at least, -the shade cast by the small-leaved Calamites and Lepidodendrons which -overtopped them. - -Indeed there is no indication from geological evidence that so late as -Palozoic times there was any great abnormality of atmosphere, and from -the internal evidence of the plants then growing there is everything to -indicate a dry or physiologically dry[14] sunny condition. - -Of the plant fossils from the Coal Measures we have at least two types. -One, those commonly found in nodules _in_ the coal itself; and the other, -nodules in the rocks above the coal which had drifted from high lands -into the sea. - -The former are the plants which actually formed the coal itself, and from -their internal organization we see that these plants were growing with -partly submerged roots in brackish swamps. Their roots are those of water -plants (see p. 150, young root of Calamite), but their leaves are those -of the "protected" type with narrow surface and various devices for -preventing a loss of water by rapid transpiration. If the water they grew -in had been fresh they would not have had such leaves, for there would -have been no need for them to economize their water, but, as we see in -bogs and brackish or salt water to-day (which is physiologically usable -in only small quantities by the plant), plants even partly submerged -protect their exposed leaves from transpiring largely. - -There are details too numerous to mention in connection with these -coal-forming plants which go to prove that there were large regions of -swampy ground near the sea where they were growing in a bright atmosphere -and uniform climate. Extensive areas of coal, and geological evidence of -still more extensive deposits, show that in Europe in the Coal Measure -period there were vast flats, so near the sea level that they were -constantly being submerged and appearing again as dbris drifted and -collected over them. Such a land area must have differed greatly from the -Europe now existing, in all its features. But the whole continent did not -consist of these flats; there were hills and higher ground, largely to -the north-east, on which a dry land flora grew, a flora where several of -the Pteridosperms and _Cordaites_ with its allies were the principal -plants. These plants have leaves so organized as to suggest that they -grew in a region where the climate was bright and dry. - -A fossil flora which has aroused much interest, particularly among -geologists, is that known as the Glossopteris flora. This Palozoic flora -has in general characters similar to those of the European -Permo-Carboniferous, but it has special features of its own, in -particular the genus _Glossopteris_ and also the genera _Phyllotheca_ and -_Schizoneura_. - -These genera, with a few others, are characteristic of the -Permo-Carboniferous period in the regions in the Southern Hemisphere now -known by the names of Australasia, South Africa, and South America, and -in India. These regions, at that date, formed what is called by -geologists "Gondwanaland". In the rocks below those containing the plants -there is evidence of glacial conditions, and it is not impossible that -this great difference in climate accounts for the differences which exist -between the flora of the Gondwanaland region and the Northern Hemisphere. -Unfortunately we have not microscopically preserved specimens of the -Glossopteris flora, which could be compared with those of our own -Palozoic.[15] - -To describe in detail the series of changes through which the seas and -continents have passed belongs to the realm of pure geology. Here it is -only necessary to point out how the evidence from the fossil plants may -afford much information concerning these continents, and as our knowledge -of fossil anatomy and of recent ecology increases, their evidence will -become still more weighty. Even now, had we no other sources of -information, we could tell from the plants alone where in the past -continents were snow and ice, heat and drought, swamps and hilly land. -However different in their systematic position or scale of evolutional -development, plants have always had similar minute structure and similar -physiological response to the conditions of climate and land surface, so -that in their petrified cells are preserved the histories of countries -and conditions long past. - - - - - CHAPTER XIX - CONCLUSION - - -In the stupendous pageant of living things which moves through creation, -the plants have a place unique and vitally important. Yet so quietly and -so slowly do they live and move that we in our hasty motion often forget -that they, equally with ourselves, belong to the living and evolving -organisms. When we look at the relative structures of plants divided by -long intervals of time we can recognize the progress they make; and this -is what we do in the study of fossil botany. We can place the salient -features of the flora of Palozoic and Mesozoic eras in a few pages of -print, and the contrast becomes surprising. But the actual distance in -time between these two types of plants is immense, and must have extended -over several million years; indeed to speak of years becomes meaningless, -for the duration of the periods must have been so vast that they pass -beyond our mental grasp. In these periods we find a contrast in the -characters of the plants as striking as that in the characters of the -animals. Whole families died out, and new ones arose of more complex and -advanced organization. But in height and girth there is little difference -between the earliest and the latest trees; there seems a limit to the -possible size of plants on this planet, as there is to that of animals, -the height of mountains, or the depth of the sea. The "higher plants" are -often less massive and less in height than the lower--Man is less in -stature than was the Dinosaur--and though by no legitimate stretch of the -imagination can we speak of brain in plants, there is an unconscious -superiority of adaptation by which the more highly organized plants -capture the soil they dominate. - -It has been noted in the previous chapters that so far back as the Coal -Measure period the vegetative parts of plants were in many respects -similar to those of the present, it was in the reproductive organs that -the essential differences lay. Naturally, when a race (as all races do) -depends for its very existence on the chain of individuals leading from -generation to generation, the most important items in the plant -structures must be those mechanisms concerned with reproduction. It is -here that we see the most fundamental differences between living and -fossil plants, between the higher and the lower of those now living, -between the forest trees of the present and the forest trees of the past. -The wood of the palozoic Lycopods was in the quality and extent and -origin of its secondary growth comparable with that of higher plants -still living to-day--yet in the fruiting organs how vast is the contrast! -The Lycopods, with simple cones composed of scales in whose huge -sporangia were simple single-celled spores; the flowering plants, with -male and female sharply contrasted yet growing in the same cone (one can -legitimately compare a flower with a cone), surrounded by specially -coloured and protective scales, and with the "spore" in the tissue of the -young seed so modified and changed that it is only in a technical sense -that comparison with the Lycopod spore is possible. - -To study the minute details of fossil plants it is necessary to have an -elaborate training in the structure of living ones. In the preceding -chapters only the salient features have been considered, so that from -them we can only glean a knowledge similar to the picture of a house by a -Japanese artist--a thing of few lines. - -Even from the facts brought together in these short chapters, however, it -cannot fail to be evident how large a field fossil botany covers, and -with how many subjects it comes in touch. From the minute details of -plant anatomy and evolution pure and simple to the climate of departed -continents, and from the determination of the geological age of a piece -of rock by means of a blackened fern impression on it to the chemical -questions of the preservative properties of sea water, all is a part of -the study of "fossil botany". - -To bring together the main results of the study in a graphic form is not -an easy task, but it is possible to construct a rough diagram giving some -indication of the distribution of the chief groups of plants in the main -periods of time (see fig. 122). - -Such a diagram can only represent the present state of our imperfect -knowledge; any day discoveries may extend the line of any group up or -down in the series, or may connect the groups together. - -It becomes evident that so early as the Palozoic there are nearly as -many types represented as in the present day, and that in fact -everything, up to the higher Gymnosperms, was well developed (for it is -hard indeed to prove that _Cordaites_ is less highly organized than some -of the present Gymnosperm types), but flowering plants and also the true -cycads are wanting, as well as the intermediate Mesozoic Bennettitales. -The peculiar groups of the period were the Pteridosperm series, -connecting links between fern and cycad, and the Sphenophyllums, -connecting in some measure the Lycopods and Calamites. With them some of -the still living groups of ferns, Lycopods, and Equisetace were -flourishing, though all the species differed from those now extant. This -shows us how very far from the beginning our earliest information is, for -already in the Palozoic we have a flora as diversified as that now -living, though with more primitive characters. - - [Illustration: Fig. 122.--Diagram showing the relative distribution of - the main groups of plants through the geological eras. The dotted lines - connecting the groups and those in the pre-Carboniferous are entirely - theoretical, and merely indicate the conclusions reached at present. - The size of the surface of each group roughly indicates the part it - played in the flora of each period. Those with dotted surface bore - seeds, the others spores.] - -In Mesozoic times the most striking group is that of the Cycads and -Bennettitales, the latter branch suggesting a direct connection between -the fern-cycad series and the flowering plants. This view, so recently -published and upheld by various eminent botanists, is fast gaining -ground. Indeed, so popular has it become among the specialists that there -is a danger of overlooking the real difficulties of the case. The -morphological leap from the leaves and stems of cycads to those of the -flowering plants seems a much more serious matter to presuppose than is -at present recognized. - -As is indicated in the diagram, the groups do not appear isolated by -great unbridged gaps, as they did even twenty years ago. By means of the -fossils either direct connections or probable lines of connection are -discovered which link up the series of families. At present the greatest -gap now lies hedging in the Moss family, and, as was mentioned (p. 163), -fossil botany cannot as yet throw much light on that problem owing to the -lack of fossil mosses. - - -This glimpse into the past suggests a prophecy for the future. Evolution -having proceeded steadily for such vast periods is not likely to stop at -the stage reached by the plants of to-day. What will be the main line of -advance of the plants of the future, and how will they differ from those -of the present? - -We have seen in the past how the differentiation of size in the spores -resulted in sex, and in the higher plants in the modifications along -widely different lines of the male and female; how the large spore -(female) became enclosed in protecting tissues, which finally led up to -true seeds (see p. 75), while the male being so temporary had no such -elaboration. As the seed advances it becomes more and more complex, and -when we reach still higher plants further surrounding tissues are pressed -into its service and it becomes enclosed in the carpel of the highest -flowering plants. After that the seed itself has fewer general duties, -and instead of those of the Gymnosperms with large endosperms collecting -food before the embryo appears, small ovules suffice, which only develop -after fertilization is assured. The various families of flowering plants -have gone further, and the whole complex series of bracts and fertile -parts which make up a flower is adapted to ensure the crossing of male -and female of different individuals. The complex mechanisms which seem -adapted for "cross fertilization" are innumerable, and are found in the -highest groups of the flowering plants. But some have gone beyond the -stage when the individual flowers had each its device, and accomplished -its seed-bearing independently of the other flowers on the same branch. -These have a combination of many flowers crowded together into one -community, in which there is specialization of different flowers for -different duties. In such a composite flower, the Daisy for example, some -are large petalled and brightly coloured to attract the pollen-carrying -insects, some bear the male organs only, and others the female or -seed-producing. Here, then, in the most advanced type of flowering plant -we get back again to the separation of the sexes in separate flowers; but -these flowers are combined in an organized community much more complex -than the cones of the Gymnosperms, for example, where the sexes are -separate on a lower plane of development. - -It seems possible that an important group, if not the dominant group, of -flowering plants in the future will be so organized that the individual -flowers are very simple, with fewer parts than those of to-day, but that -they will be combined in communities of highly specialized individuals in -each flower head or cluster. - -As well as this, in other species the minute structure of the vital -organs may show a development in a direction contrary to what has -hitherto seemed advance. Until recently flowers and their organs have -appeared to us to be specialized in the more advanced groups on such -lines as encourage "cross fertilization". In "cross fertilization", in -fact, has appeared to lie the secret of the strength and advance of the -races of plants. But modern cytologists have found that many of the -plants long believed to depend on cross fertilization are either -self-fertilized or not fertilized at all! They have passed through the -period when their complex structures for ensuring cross fertilization -were used, and though they retain these external structures they have -taken to a simpler method of seed production, and in some cases have even -dispensed with fertilization of the egg cell altogether. The female -vitality increased, the male becomes superfluous. It is simpler and more -direct to breed with only one sex, or to use the pollen of the same -individual. Many flowers are doing this which until recently had not been -suspected of it. We cannot yet tell whether it will work successfully for -centuries to come or is an indication of "race senility". - -Whether in the epochs to come flowering plants will continue to hold the -dominant position which they now do is an interesting theoretical -problem. Flowers were evolved in correlation with insect pollination. One -can conceive of a future, when all the earth is under dominion of man, in -which fruits will be sterilized for man's use, as the banana is now, and -seed formation largely replaced by gardeners' "cuttings". - -In those plants which are now living where the complex mechanisms for -cross-fertilization have been superseded by simple self-fertilization, -the external parts of the more elaborate method are still produced, -though they are apparently futile. In the future these vestigial organs -will be discarded, or developed in a more rudimentary form (for it is -remarkable how organs that were once used by the race reappear in members -of it that have long outgrown their use), and the morphology of the -flower will be greatly simplified. - -Thus we can foresee on both sides much simplified individual flowers--in -the one group the reduced individuals associating together in communities -the members of which are highly specialized, and in the other the -solitary flowers becoming less elaborate and conspicuous, as they no -longer need the assistance of insects (the cleistogamic flowers of the -Violet, for example, even in the present day bend toward the earth, and -lack all the bright attractiveness of ordinary flowers), and perhaps -finally developing underground, where the seeds could directly germinate. - -In the vegetative organs less change is to be expected, the examples from -the past lead us to foresee no great difference in size or general -organization of the essential parts, though the internal anatomy has -varied, and probably will vary, greatly with the whole evolution of the -plant. - -But one more point and we must have done. Why do plants evolve at all? -Why did they do so through the geological ages of the past, and why -should we expect them to do so in the future? The answer to this question -must be less assured than it might have been even twenty years ago, when -the magnetism of Darwin's discoveries and elucidations seemed to obsess -his disciples. "Response to environment" is undoubtedly a potent factor -in the course of evolution, but it is not the cause of it. There seems to -be something inherent in life, something apparently (though that may be -due to our incomplete powers of observation) apart from observable -factors of environment which causes slight spontaneous changes, -_mutations_, and some individuals of a species will suddenly develop in a -new direction in one or other of their parts. If, then, this places them -in a superior position as regards their environment or neighbours, it -persists, but if not, those individuals die out. The work of a special -branch of modern botany seems clearly to indicate the great importance of -this seemingly inexplicable spontaneity of life. In environment alone the -thoughtful student of the present cannot find incentive enough for the -great changes and advances made by organisms in the course of the world's -history. The climate and purely physical conditions of the Coal Measure -period were probably but little different from those in some parts of the -world to-day, but the plants themselves have fundamentally changed. True, -their effect upon each other must be taken into account, but this is a -less active factor with plants than with men, for we can imagine nothing -equivalent to citizenship, society, and education in the plant -communities, which are so vital in human development. - -It seems to have been proved that plants and animals may, at certain -unknown intervals, "mutate"; and mutation is a fine word to express our -recent view of one of the essential factors in evolution. But it is a -cloak for an ignorance avowedly less mitigated than when we thought to -have found a complete explanation of the causes of evolution in -"environment". - - -In a sketch such as the present, outlines alone are possible, detail -cannot be elaborated. If it has suggested enough of atmosphere to show -the vastness of the landscape spreading out before our eyes back into the -past and on into the future, the task has been accomplished. There are -many detailed volumes which follow out one or other special line of -enquiry along the highroads and by-ways of this long traverse in -creation. If the bird's-eye view of the country given in this book -entices some to foot it yard by yard under the guidance of specialists -for each district, it will have done its part. While to those who will -make no intimate acquaintance with so far off a land it presents a short -account by a traveller, so that they may know something of the main -features and a little of the romance of the fossil world. - - - - - APPENDIX I - LIST OF REQUIREMENTS FOR A COLLECTING EXPEDITION - - -In order to obtain the best possible results from an expedition, it is -well to go fossil hunting in a party of two, four, or six persons. Large -parties tend to split up into detachments, or to waste time in trying to -keep together. - -Each individual should have strong suitable clothes, with as many pockets -arranged in them as possible. The weight of the stones can thus be -distributed over the body, and is not felt so much as if they were all -carried in a knapsack. Each collector should also provide himself with-- - - A satchel or knapsack, preferably of leather or strong canvas, but not - of large size, for when the space is limited selection of the specimens - is likely to be made carefully. - - One or two hammers. If only one is carried, it should be of a fair size - with a square head and strong straight edge. - - One chisel, entirely of metal, and with a strong straight cutting edge. - - Soft paper to wrap up the more delicate fossils, in order to prevent - them from scraping each other's surfaces; and one or two small - cardboard boxes for very fragile specimens. - - A map of the district (preferably geologically coloured). Localities - should be noted in pencil on this, indicating the exact spot of finds. - For general work the one-inch survey map suffices, but for detailed - work it is necessary to have the six-inch maps of important districts. - - A small notebook. Few notes are needed, but those few _must_ be taken - on the spot to be reliable. - - A pencil or fountain pen, preferably both. - - A penknife, which, among other things, will be found useful for working - out very delicate fossils. - - - - - APPENDIX II - TREATMENT OF SPECIMENS - - -1. The commonest form in which fossils are collected is that which has -been described as _impression material_ (see p. 12). In many cases these -will need no further attention after the block of stone on which they lie -has been chipped into shape. - -In chipping a block down to the size required it is best to hold it -freely in the left hand, protecting the actual specimen with the palm -where possible, and taking the surplus edges away by means of short sharp -blows from the hammer, striking so that only small pieces come away with -each blow. For delicate specimens it is wise to leave a good margin of -the matrix round the specimen, and to do the final clearing with a -thin-bladed penknife, taking away small flakes of the stone with delicate -taps on the handle of the knife. - -Specimens from fine sandstones, shales, and limestones are usually -thoroughly hard and resistant, and are then much better if left without -treatment; by varnishing and polishing them many amateur collectors spoil -their specimens, for a coat of shiny varnish often conceals the details -of the fossil itself. Impressions of plants on friable shales, on the -other hand, or those which have a tendency to peel off as they dry, will -require some treatment. In such cases the best substance to use is a -dilute solution of size, in which the specimen should soak for a short -period while the liquid is warm (not hot), after which it should be -slightly drained and the size allowed to dry in. The congealed substance -then holds the plant film on to the rock surface and prevents the rock -from crumbling away, while it is almost invisible and does not spoil the -plant with any excessive glaze. - -2. For specimens of _casts_ the same treatment generally applies, though -they are more apt to separate completely from the matrix after one or two -sharp blows, and thus save one the work of picking out the details of -their structure. - -3. Those blocks which contain _petrifactions_, and can therefore be made -to show microscopic details, will require much more treatment. In some -cases mere polishing reveals much of the structure--such, for instance, -were the "Staarsteine" of the German lapidaries, where the axis and -rootlets of a fossil like a treefern show their very characteristic -pattern distinctly. - -As a rule, however, it is better, and for any detailed work it is -essential, to cut thin sections transversely across and longitudinally -through the axis of the specimen and to grind them down till they are so -transparent that they can be studied through the microscope. The cutting -can be done on a lapidary's wheel, where a revolving metal disc set with -diamond powder acts as a knife. The comparatively thin slice thus -obtained is fastened on to glass by means of hard Canada balsam, and -rubbed down with carborundum powder till it is thin enough. - -The process, however, is very slow, and an amateur cannot get good -results without spending a large amount of time and patience over the -work which would be better spent over the study of the plant structures -themselves. Therefore it is usually more economical to send specimens to -be cut by a professional, if they are good enough to be worth cutting at -all, though it is often advisable to cut through an unpromising block to -see whether its preservation is such as would justify the expense. - -In the case of true "coal balls" much can be seen on the cut surface of a -block, particularly if it be washed for a minute in dilute hydrochloric -acid and then in water, and then dried thoroughly. The acid acts on the -carbonates of which the stone is largely composed, and the treatment -accentuates the black-and-white contrast in the petrified tissues (see -fig. 10). After lying about for a few months the sharpness of the surface -gets rubbed off, as the acid eats it into very delicate irregularities -which break and form a smearing powder; but in such a case all that is -needed to bring back the original perfection of definition is a quick -wash of dilute acid and water. If the specimens are not rubbed at all the -surface is practically permanent. Blocks so treated reveal a remarkable -amount of detail when examined with a strong hand lens, and form very -valuable museum specimens. - -The microscope slides should be covered with glass slips (as they would -naturally be if purchased), and studied under the microscope as sections -of living plants would be. - -Microscopic slides of fossils make excellent museum specimens when -mounted as transparencies against a window or strong light, when a -magnifying glass will reveal all but the last minuti of their structure. - -4. _Labelling_ and numbering of specimens is very important, even if the -collection be but a small one. As well as the paper label giving full -details, there should be a reference number on every specimen itself. On -the microscope slides this can be cut with a diamond pencil, and on the -stones sealing wax dissolved in alcohol painted on with a brush is -perhaps the best medium. On light-coloured close-textured stones ink is -good, and when quite dry can even be washed without blurring. - -The importance of marking the stone itself will be brought home to one on -going through an old collection where the paper labels have peeled or -rubbed off, or their wording been obliterated by age or mould. - -A notebook should be kept in which the numbers are entered, with a note -of all the items on the paper label, and any additional details of -interest. - - - - - APPENDIX III - LITERATURE - - -A short list of a few of the more important papers and books to which a -student should refer. The innumerable papers of the specialists will be -found cited in these, so that, as they would be read only by advanced -students, there is no attempt to catalogue them here. - - -Carruthers, W., "On Fossil Cycadean Stems from the Secondary Rocks of -Britain," published in the _Transactions of the Linnean Society_, vol. -xxvi, 1870. - - -*Geikie, A., _A Text-Book of Geology_, vols. i and ii, London, 1903. - - -Grand'Eury, C., "Flore Carbonifre du dpartement de la Loire et du -centre de la France", published in the _Mmoirs de l'Acadmie des -Sciences_, Paris, vol. xxiv, 1877. - - -*Kidston, R., _Catalogue of the Palozoic Plants in the Department of -Geology and Palontology of the British Museum_, London, 1886. - - -*Lapworth, C., _An Intermediate Text-Book of Geology_, twelfth edition, -London, 1888. - - -Laurent, L., "Les Progrs de la palobotanique angiospermique dans la -dernire decade", _Progressus Rei Botanic_, vol. i, Heft 2, pp. 319-68, -Jena, 1907. - - -Lindley, J., and Hutton, W., _The Fossil Flora of Great Britain_, 3 -vols., published in London, 1831-7. - - -Lyell, C., _Principles of Geology_ and _The Student's Lyell_, edited by -J. W. Judd, London, 1896. - - -Oliver, F. W., and Scott, D. H., "On the Structure of the Palozoic Seed, -_Lagenostoma Lomaxi_", published in the _Transactions of the Royal -Society_, series B, vol. cxcvii, London, 1904. - - -Renault, B., _Cours de Botanique fossile_, Paris, 1882, 4 vols. - - -Renault, B., _Bassin Houiller et Permien d'Autun et d'Epinac_, Atlas and -Text, 1893-6, Paris. - - -*Scott, D. H., _Studies in Fossil Botany_, London, second edition, 1909. - - -Scott, D. H., "On the Structure and Affinities of Fossil Plants from the -Palozoic Rocks. On _Cheirostrobus_, a New Type of Fossil Cone from the -Lower Carboniferous Strata." Published in the _Philosophical Transactions -of the Royal Society_, vol. clxxxix, B, 1897. - - -*Seward, A. C., _Fossil Plants_, vol. i, Cambridge, 1898. - - -Seward, A. C., _Catalogue of the Mesozoic Plants in the Department of -Geology of the British Museum_, Parts I and II, London, 1894-5. - - -*Solms-Laubach, Graf zu, _Fossil Botany_ (translation from the German), -Oxford, 1891. - - -Stopes, M. C., and Watson, D. M. S., "On the Structure and Affinities of -the Calcareous Concretions known as 'Coal Balls'", published in the -_Philosophical Transactions of the Royal Society_, vol. cc. - - -*Stopes, M. C., _The Study of Plant Life for Young People_, London, 1906. - - -*Watts, W. W., _Geology for Beginners_, London, 1905 (second edition). - - -Wieland, G. R., _American Fossil Cycads_, Carnegie Institute, 1906. - - -Williamson, W. C., A whole series of publications in the _Philosophical -Transactions of the Royal Society_ from 1871 to 1891, and three later -ones jointly with Dr. Scott; the series entitled "On the Organization of -the Fossil Plants of the Coal Measures", Memoir I, II, &c. - - -Zeiller, R., _lments de Palobotanique_, Paris, 1900. - - -*Zittel, K., _Handbuch der Palontologie_, vol. ii; _Palophytologie_, by -Schimper & Schenk, Mnchen and Leipzig, 1900. - - Those marked * would be found the most useful for one beginning the - subject. - - - - - GLOSSARY - - -Some of the more technical terms about which there might be some doubt, -as they are not always accompanied by explanations in the text, are here -briefly defined. - - -Anatomy.--The study of the details and relative arrangements of the -internal features of plants; in particular, the relations of the -different tissue systems. - - -Bracts.--Organs of the nature of leaves, though not usual foliage leaves. -They often surround fructifications, and are generally brown and scaly, -though they may be brightly coloured or merely green. - - -Calcareous.--Containing earthy carbonates, particularly calcium carbonate -(chalk). - - -Cambium.--Narrow living cells, which are constantly dividing and giving -rise to new tissues (see fig. 33, p. 57). - - -Carbonates, as used in this book, refer to the combinations of some -earthy mineral, such as calcium or magnesium, combined with carbonic acid -gas and oxygen, formula CaCO_3, MgCO_3, &c. - - -Carpel.--The closed structure covering the seeds which grow attached to -it. The "husk" of a peapod is a carpel. - - -Cell.--The unit of a plant body. Fundamentally a mass of living -protoplasm with its nucleus, surrounded in most cases by a wall. Mature -cells show many varieties of shape and organization. See Chapter VI, p. -54. - - -Centrifugal.--Wood or other tissues developed away from the centre of the -stem. See fig. 65, p. 97. - - -Centripetal.--Wood or other tissues developed towards the centre of the -stem. See fig. 65, p. 97. - - -Chloroplast.--The microscopic coloured masses, usually round, green -bodies, in the cells of plants which are actively assimilating. - - -Coal Balls.--Masses of carbonate of calcium, magnesium, &c., generally of -roundish form, which are found embedded in the coal, and contain -petrified plant tissues. See p. 28. - - -Concretions.--Roundish mineral masses, formed in concentric layers, like -the coats of an onion. See p. 27. - - -Cotyledons.--The first leaves of an embryo. In many cases packed with -food and filling the seed. See fig. 58. - - -Cross Fertilization.--The fusion of male and female cells from different -plants. - - -Cuticle.--A skin of a special chemical nature which forms on the outer -wall of the epidermis cells. See p. 54, fig. 21. - - -Earth Movements.--The gradual shifting of the level of the land, and the -bending and contortions of rocks which result from the slow shrinking of -the earth's surface, and give rise to earthquakes and volcanic action. - - -Embryo.--The very young plant, sometimes consisting of only a few -delicate cells, which results from the divisions of the fertilized egg -cell. The embryo is an essential part of modern seeds, and often fills -the whole seed, as in a bean, where the two fleshy masses filling it are -the two first leaves of the embryo. See fig. 58, p. 77. - - -Endodermis.--The specialized layer of cells forming a sheath round the -vascular tissue. See p. 55. - - -Endosperm.--The many-celled tissue which fills the large "spore" in the -Gymnosperm seed, into which the embryo finally grows. See fig. 57. - - -Epidermis.--Outer layer of cells, which forms a skin, in the -multicellular plants. See fig. 21, p. 54. - - -Fruit.--Essentially consisting of a seed or seeds, enclosed in some -surrounding tissues, which may be only those of the carpel, or may also -be other parts of the flower fused to it. Thus a peapod is a _fruit_, -containing the peas, which are seeds. - - -Gannister.--A very hard, gritty rock found below some coal seams. See p. -25. - - -Genus.--A small group within a family which includes all the plants very -like each other, to which are all given the same "surname"; e.g. _Pinus -montana_, _Pinus sylvestris_, _Pinus Pinaster_, &c. &c., are all members -of the genus _Pinus_, and would be called "pine trees" in general (see -"Species"). - - -Hyph.--The delicate elongated cells of Fungi. - - -Molecule.--The group of chemical elements, in a definite proportion, -which is the basis of any compound substance; _e.g._ two atoms of -hydrogen and one atom of oxygen form a molecule of water, H_2O. A lime -carbonate molecule (see definition of "Carbonate") is represented as -CaCO_3. - - -Monostelic.--A type of stem that contains only one stele. - - -Morphology.--The study of the features of plants, their shapes and -relations, and the theories regarding the origin of the organs. - - -Nucellus.--The tissue in a Gymnosperm seed in which the large "spore" -develops. See figs. 55 and 56, p. 76. - - -Nucleus.--The more compact mass of protoplasm in the centre of each -living cell, which controls its growth and division. See fig. 17, _n._ - - -Palobotany.--The study of fossil plants. - - -Palontology.--The study of fossil organisms, both plants and animals. - - -Petiole.--The stalk of a leaf, which attaches it to the stem. - - -Phloem.--Commonly called "bast". The elongated vessel-like cells which -conduct the manufactured food. See p. 57. - - -Pollen Chamber.--The cavity inside a Gymnosperm seed in which the pollen -grains rest for some time before giving out the male cells which -fertilize the egg-cell in the seed. See p. 76. - - -Polystelic.--A type of stem that appears, in any transverse section, to -contain several steles. See note on the use of the word on p. 63. - - -Protoplasm.--The colourless, constantly moving mass of finely granulated, -jelly-like substance, which is the essentially living part of both plants -and animals. - - -Rock.--Used by a geologist for all kinds of earth layers. Clay, and even -gravel, are "rocks" in a geological sense. - - -Roof, of a coal seam. The layers of rock--usually shale, limestone, or -sandstone--which lie just above the coal. See p. 24. - - -Sclerenchyma.--Cells with very thick walls, specially modified for -strengthening the tissues. See fig. 28, p. 56. - - -Seed.--Essentially consisting of a young embryo and the tissues round it, -which are enclosed in a double coat. See definition of "Fruit". - - -Shale.--A fine-grained soft rock, formed of dried and pressed mud or -silt, which tends to split into thin sheets, on the surface of which -fossils are often found. - - -Species.--Individuals which in all essentials are identical are said to -be of the same species. As there are many variations which are not -essential, it is sometimes far from easy to draw the boundary between -actual species. The specific name comes after that of the genus, e.g. -_Pinus montana_ is a species of the genus _Pinus_, as is also _Pinus -sylvestris_. See "Genus". - - -Sporangium.--The saclike case which contains the spores. See figs. 52 and -53, p. 75. - - -Spore.--A single cell (generally protected by a cell wall) which has the -power of germinating and reproducing the plant of which it is the -reproductive body. See p. 75. - - -Sporophyll.--A leaf or part of a leaf which bears spores or seeds, and -which may be much or little modified. - - -Stele.--A strand of vascular tissue completely enclosed in an endodermis. -See p. 62. - - -Stigma.--A special protuberance of the carpel in flowering plants which -catches the pollen grains. - - -Stomates.--Breathing pores in the epidermis, which form as a space -between two curved liplike cells. See fig. 23, p. 54. - - -Tetrads.--Groups of four cells which develop by the division of a single -cell called the "mother cell". Spores and pollen grains are nearly always -formed in this way. See p. 75. - - -Tracheid.--A cell specially modified for conducting or storing of water, -often much elongated. The long wood cells of Ferns and Gymnosperms are -tracheids. - - -Underclay.--The fine clay found immediately below some coal seams. See p. -24. - - -Vascular Tissue.--The elongated cells which are specialized for -conduction of water and semifluid foodstuffs. - - - - - FOOTNOTES - - -[1]My book was entirely written before the second edition of Scott's - _Studies_ appeared, which, had it been available, would have tempted - me to escape some of the labour several of the chapters of this - little book involved. - -[2]The student would do well to read up the general geology of this very - interesting subject. Such books as Lyell's _Principles of Geology_, - Geikie's textbooks, and many others, provide information about the - process of "mountain building" on which the form of our coalfields - depends. A good elementary account is to be found in Watt's _Geology - for Beginners_, p. 96 _et seq._ - -[3]See note on p. 28. - -[4]This refers only to the "coal-ball"-bearing seams; there are many - other coals which have certainly collected in other ways. See Stopes - & Watson, Appendix, p. 187. - -[5]For a detailed list of the strata refer to Watts, p. 219 (see - Appendix). - -[6]Though the Angiosperm was not then evolved, the Gymnosperm stem has - distinct vascular bundles arranged as are those of the Angiosperm, - the difference here lies in the type of wood cells. - -[7]The gametophyte generation (represented in the ferns by the - prothallium on which the sexual organs develop) alternates with the - large, leafy sporophyte. Refer to Scott's volume on _Flowerless - Plants_ (see Appendix) for an account of this alternation of - generations. - -[8]Material recently obtained by the author and Dr. Fujii in Japan does - contain some true petrifactions of Angiosperms and other plant - debris. The account of these discoveries has not yet been published. - -[9]A fuller account of the Angiospermic flora can be had in French, in M. - Laurent's paper in _Progressus Rei Botanic_. See Appendix for - reference. - -[10]From the Cretaceous deposits of North America several fossil forms - (_Brachyphyllum_, _Protodammara_) are described which show clear - affinities with the family as it is now constituted. (See Hollick and - Jeffrey; reference in the Appendix.) - -[11]The addition of _-oxylon_ to the generic name of any living type - indicates that we are dealing with a fossil which closely resembles - the living type so far as we have information from the petrified - material. - -[12]See reference in the Appendix to this richly illustrated volume. - -[13]For fuller description of this interesting cone, see Scott's - _Studies_, p. 114 _et seq._ - -[14]A brackish swampy land is physiologically dry, as the plants cannot - use the water. See Warming's _Oecology of Plants_, English edition, - for a detailed account of such conditions. For a simple account see - Stopes' _The Study of Plant Life_, p. 170. - -[15]The student interested in this special flora should refer to Arber's - British Museum _Catalogue of the Fossil Plants of the Glossopteris - Flora_. - - - - - INDEX - - - (_Italicized numbers refer to illustrations_) - - - Abietine, 88, 89, 90. - -- family characters of, 91. - Alg, 44, 47, 165. - -- brown, 166. - -- green, 165. - -- red, 165. - _Alnus_, 85. - Amber, 17. - Amentifer, 84. - Anatomy of fossil plants, likeness in detail to that of living plants, - 53 et seq. - -- -- -- -- differences in detail from that of living plants, 69 et - seq. - _Andromeda_, 84. - Angiosperms, comparison with Bennettitales, 103. - -- early history of, 79 et seq. - -- general distribution of in time, 177. - -- later evolution of, 178. - -- male cell of, 52. - Araliace, 85. - Araucare, 88, 90, 111. - -- description of, 90. - -- primitive characters of, 89. - _Araucarioxylon_, 93, 95. - Arber, 173. - _Archocalamites_, 152. - Artocarpace, 85. - _Asterochlaena_, 126, 127, _86_, _89_. - - - _Bacillus_, 167. - Bacteria, 167. - _Baiera_, 101. - Bast, 57, _32_. - Bennettitales, 44, 102, 131. - -- general distribution of in time, 177. - _Bennettites_, 103 et seq. - -- external appearance of, 103. - -- flower-like nature of fructification, 108. - -- fructification of, 104, _71_, 105, _72_. - -- seed of, 106, _73_. - Bertrand, 2. - _Betula_, 85. - _Bignonia_, 84. - Botryopteride, 125, 132. - -- description of group, 125. - -- fructifications of, 128. - -- petioles of, 127, _89_. - -- stem anatomy of, 126, _86_, _87_. - -- wood of, 128, _90_. - _Botryopteris_, 126, 127. - -- axis with petiole, 127, _88_, _89_. - _Bowmanites Rmeri_, 158, 160. - _Brachyphyllum_, 89. - Brongniart, 2. - Bryophytes, 163. - - - _Calamites_, 147, 154, 157, 159, 160, 171. - -- branch of, 147, _104_. - -- _casheana_, 152. - -- cone of, 150, _109_, 151, _110_. - -- leaf of, 149, _107_. - -- node of, 149, _106_. - -- spores of, 152, _111_. - -- young roots of, 150, _108_. - -- young stem of, 148, _105_. - Cambium, 57, _33_, 65, _43_, 66, _44_. - Carbon, film of representing decayed plant, 12. - Carbonate of magnesium, 20. - Carbonates of lime, 19. - Carpels, modified leaves, 78. - Carruthers, 186. - Casts of fossil plants, 8, 9, _2_, 10, _3_, _4_, 11, 12. - -- of seeds, 11, 12. - -- treatment of specimens of, 184. - _Casuarina_, 83. - Cells, similarity of living and fossil types of, 53. - -- principal types of, 53 et seq., _22_-_33_. - Cell wall, 47, _17_. - Centrifugal wood, 97, _65_. - Centripetal wood, 97, _65_, 116. - _Chara_, 16. - Charace, 163. - _Cheirostrobus_, 159, _118_, 160. - Chloroplast, 47, _17_. - Coal, origin of, 29 et seq. - -- of different ages, 33. - -- seams in the rocks, 24, _13_, _14_. - -- vegetable nature of, 25 et seq. - -- importance of, 17, Chap. III, p. 22 et seq. - "Coal balls", 18, 19, _10_, 20, 21, 22, 27, _15_, 163, 185. - -- -- mass of, in coal, 28, _16_. - Coal Measures, climate of, 172. - Companion cells, 57, _32_. - Concretions, 21, 22, 27, _15_, 28. - -- concentric banding in, 27, _15_. - Conducting tissue in higher plants, 49, _19_, 50, _20_. - Coniferales, conflicting observations among, 46. - -- general distribution in time, 177. - -- male cell of, 52. - _Corallina_, 166. - Cordaite, 39-88, 112. - -- comparison of fructifications with those of Taxe, 95. - -- description of family, 92. - -- general distribution in time, 177. - _Cordaites_, 40, 107, 176. - -- fructification of, 95, 96, _64_. - -- internal cast of stem, 10, _4_, 93, 94, 95. - -- leaves of, once considered to be Monocotyledons, 82, 93. - -- leaves of, 93, _61_, 94, _62A_. - -- possible common origin with Ginkgo, 102. - -- wood of, 94, _62B_. - Cork, 56, _29_. - -- cambium, 56, _29_. - Cross fertilization, 179. - Cupresse, 88, 90. - -- description of, 91. - _Cycadeoidea_, 103. - Cycads in the Mesozoic period, 40, 41, 42, 113. - -- description of group, 109. - -- general distribution of in time, 177. - -- in Tertiary period, 85. - -- large size of male cones of, 110. - -- seeds of, 112. - -- type of seed of, 76, _57_. - -- wood of, 110. - _Cycas_, 109, 110, _74_. - -- seed-bearing sporophyll of, 111. - -- seeds of, 112, _76_. - -- comparison with Ginkgo seeds, 112. - Darwin, 181. - Diatoms, 167, _121_. - Dicotyledons, 41, 44, 79. - -- relative antiquity of, 81, 82. - -- seed type of, 77, _58_. - Differentiation, commencement of in simple plants, 48. - -- of tissues in higher plants, 49, _19_, 50 et seq., _20_. - - - Embryo of _Ginkgo_, 100. - -- in seeds, 76, _57_, 77, _58_. - -- of _Bennettites_, 106, _73_. - Endodermis, 55, _26_, 61. - Environment, 181, 182. - Epidermal tissues, 54, _21_, _22_, _23_, 125. - Epidermis cells, fossil impressions of, 13, 14, _8_, 59, _34_, 125. - Equisetales, 44. - -- general distribution of in time, 177. - _Equisetites_, 146, _103_. - _Equisetum_, 9, 38, 40, 44, 145, 149, 152. - -- underground rhizomes of, 43. - _Eucalyptus_, 83. - Europe, 87, 102. - -- ancient climates of, 170. - Evolution, 43. - -- in plants, various degrees of in the organs of the same plant, 45 et - seq. - Evolution in plants, cause of, 181. - -- -- -- suggestions as to possible future lines of, 178. - Expedition, requirements for collecting, 183. - Extinct families, 44. - - - Ferns, sporangia of, 67, _45_. - -- connection with Pteridosperms, 123. - -- description of group, 124. - -- fructifications of among fossils, 131, 132, _92_. - -- general distribution of in time, 177. - -- germinating spores of, 68, _47_. - _Ficus_, 83. - Flotsam, 6. - Flowering plant, anatomy of stem, 49, _19_. - Formation of rocks, key to processes, 6. - Fossil plants, indications of ancient climates and conditions, 168. - -- -- diagram illustration the distribution of, 177, _122_. - Fungi, fossils of, 164. - -- parasitic, _119_, 164, 165, _120_. - - - Gamopetal, 84. - Gannister, 25, _14_. - Geikie, 186. - _Ginkgo_, leaf impression, 14, 7, 100, _69_. - -- comparison with _Cycas_ seeds, 112. - -- distribution in the past, 168. - -- embryo of, 100. - -- epidermis of fossil, 14, _8_, 100. - -- foliage of, 99, _66_. - -- only living species of genus, 98, _70_. - -- possible common origin with _Cordaites_, 102. - -- ripe seed of, 99, _67_. - -- section of seed of, 100, _68_. - -- seed structure of, 76, _57_. - -- similarity to _Cordaites_, 96. - Ginkgoace, 88. - Ginkgoales, 88, 98. - -- description of group, 98. - -- general distribution in time, 177. - Glacial epoch, 170. - _Glossopteris_, 173. - _Glyptostrobus_, 86. - Gondwanaland, 173. - Grand'Eury, 186. - Gum, 17. - Gymnosperms, 38, 41, 44, 86, 176, 179. - -- connection with Pteridosperms, 124. - -- general distribution in time, 177. - -- relations between the groups of, 88, 89, 90. - - - Hairs, 54, _22_, 70. - -- special forms among fossils, 70. - _Heterangium_, 119, 122, 123, 127. - -- foliage of, 120. - -- stem of, 120, _81_. - Hollick and Jeffrey, 89. - Horsetails, description of group, 145. - Hutton, 2. - - - Impression, form of fossil, _5_, 12, 13, _6_, 14, _7_, 15, 80, 81, - _59_, 60. - -- treatment of specimens of, 184. - Investigators of fossil plants, 2. - Iron sulphide, 20. - - - Jet, 17. - Juglandace, 83. - - - Kauri pine, 93. - Kew Gardens, 98. - Kidston, 186. - - - Labelling of specimens, 185. - _Lagenostoma_, 76, _56_, 118, 119, _80_. - _Laminaria_, 166. - Lapworth, 186. - Latex cells, 55, _27_. - Laurace, 85. - Laurent, 186. - Leaves, starch manufacture in cells of, 58. - -- fossil leaf anatomy, 59, _34_. - -- general similarity of living and fossil, 58. - _Lepidocarpon_, 141, _100_. - _Lepidodendron_, 9, 10, _3_, 21, _12_, 67, _46_, 72, 75, 134, 144, 145, - 157, 160, 171. - -- anatomy of stem of, 136, 137, _95_, 138, _96_, 139, _97_. - -- comparison of reproductive organs with those of living lycopods, 67, - _46_. - _Lepidodendron_, description of, 134. - -- distribution in the past, 177. - -- fructification of, 139, 140, _98_, 141, _99_. - -- huge stumps of, 134, frontispiece. - -- leaf bases, 10, _3_, 135, _93_. - -- leaf traces of, 139, _97_. - -- peculiar fructification of, 75, _54_. - -- petrifaction of leaves, 21, _12_. - -- rootlike organs of, 69. - -- secondary thickening in, 70, _48_, 71, _49_. - -- _selaginoides_, stem of, 137, _95_. - -- wood of, 70, _48_, 71, _49_. - Liliace, 82. - Limestone, 7, _1_, 24, 25, 36. - Lindley, 2, 186. - Literature on fossil plants, 186. - _Lithothamnion_, 166. - Liverworts, 163. - Lycopods, 38, 40, 42, 44, 67, 133, 175. - -- description of group, 133. - -- general distribution in time, 177. - -- reproductive organs of, 67, _46_. - -- secondary wood in fossil, 70, _48_, 71, _49_. - Lyell, 186. - _Lyginodendron_, 115, 116, 122. - -- anatomy of stem of, 116, _78A_. - -- petioles of, 117, 118, _79_. - -- roots of, 117, _78B_. - -- seeds of, 118, 119, _80_. - - - _Magnolia_, 83. - _Marattia_, 130. - Marattiace, 125, 129. - -- appearance of, 130. - -- description of group, 129. - _Marchantites_, 163. - _Medullosa_, 72, 73, 119, 120, 121, _82_, _83_, 122, 123. - -- foliage of, 121, _83_. - -- probable seeds of, 121. - -- steles of, 72, _50_, 73, _51_, 120. - Mesozoic, character of flora, 40. - Metaxylem, 57, _31_. - _Mycorhiza_, 165. - _Micrococcus_, 167. - Monocotyledons, 41, 44, 79. - -- relative antiquity of, 81, 82. - Monostelic anatomy, 63, 126. - Mosses, scarcity of fossils of, 162. - Mosses, fossils of, 163. - Mountain building, from deposits under water, 6. - -- -- slow and continuous changes, 35. - _Muscites_, 163. - Mutation, 181. - - - _Nematophycus_, 166. - _Neuropteris_, leaf impression, _6_, 13. - -- foliage of _Medullosa_, 122. - -- with seed attached, 122, _85_. - _Nipa_, 85. - Nodules, 15, 16, _9_. - Nucleus, 47, _17_. - - - Oliver, 187. - _Osmunda_, 125. - Ovule, word unsuitable for palaeozoic "seeds", 77. - - - Palisade cells, 55, _25_. - -- tissue in leaves, 58. - -- -- -- fossil leaf, 59, _34_. - Palms, 85. - Parenchyma, 55, _24_. - Petrifaction of cells, 4. - Petrifactions, 17. - -- of forest dbris, 18. - -- treatment of specimens of, 184. - _Phyllotheca_, 173. - Plant, parts of, the same in living and fossil, 59. - -- world, main families in, 44. - _Platanus_, 83. - Polypodiace, 124. - Polystelic anatomy, 63, 72. - _Populus_, 83, 85. - Poroxyle, 88. - -- description of group of, 96. - _Poroxylon_, anatomy of, 97, 116. - Primitive plants, 46. - Primofilices, 132. - Protococcoide, 47, _17_. - _Protodammara_, 89. - Protoplasm, 47. - Protostele, 62, 70. - Protoxylem, 57, _31_. - _Psaronius_, 129, 130. - -- stem anatomy of, 131, _91_. - Pteridophytes, development of secondary wood in fossil forms of, 72. - Pteridosperms, 44, 104, 114, 131. - -- description of group, 114 et seq. - -- general distribution of in time, 177. - -- summary of characters of, 123. - _Pteris aurita_, 62. - - - Quarries, 7, _1_. - _Quercus_, 83, 85. - - - Race senility, 180. - Ranales, 103. - Renault, 2, 156, 187. - Reproductive organs, likeness between those of living and fossil - plants, 67, _45_, _46_. - -- -- peculiar characters of some from the Palozoic, 74. - -- -- simplicity of essential cells of, 52. - Rocks, persistence of mineral constituents, 36. - -- fossils varying in according to the geological age, 37 et seq. - Roof of coal seam, 24, _13_, 25, _14_. - Roots, likeness of structure in living and fossil, 60, _35_. - - - _Salix_, 83. - _Sambucus_, 84. - _Schizoneura_, 173. - Sclerenchyma, 56, _26_, 59, _34_. - Scott, 2, 160, 187. - Secondary wood, development of in fossil members of families now - lacking it, 72. - Seeds, series of types from spores to seeds, 75, 76, _52_-_58_. - -- position on the plant, 77, 78. - -- Tertiary impressions of, 80, 81, _60_. - _Selaginella_, 75, 133, 134. - -- with four spores in a sporangium, 75, _53_. - _Sequoia_, 86. - Seward, 187. - "Shade leaves", 171. - Shale, 7, _1_, 11, 24, 25, 36. - Sieve tubes, 57, _32_. - _Sigillaria_, 142, 145. - _Sigillaria_, cast of leaf bases, 9, _2_, 144, _102_. - -- description of, 144. - Silica, 17. - Silicified wood, 17, 80, 87. - Solms Laubach, 2, 187. - Specimens, treatment of, 184. - Sphenophyllales, 44, 153. - -- description of, 153. - -- general distribution in time, 177. - _Sphenophyllum_, 44, 153, 154, 160. - -- cone of, 157, 116. - -- _fertile_, 158. - -- impression of foliage, 154, _112_. - -- _plurifoliatum_, 153. - -- sporangia of, 158, _117_. - -- stem anatomy, 155, _113_, 156, _114_. - -- stem in coal ball, 20. - -- wood of, 156, _114_, _115_. - _Sphenopteris_, leaf impression, 11, _5_. - -- foliage of Pteridosperms, 115, _77_. - Sporangium of ferns, 67, _45_. - -- of lycopods, 67, _46_. - -- of pteridophytes, 75, _52_, _53_, _54_. - Spores, germinating, in fossil sporangia, 68, _47_. - -- peculiar structures among palozoic examples of, 74. - -- series of types from "spores" to "seeds", 75, 76, _52_-_58_. - -- tetrads of, 75, _52_, _53_, _54_. - Sporophyll, 75, _52_, _53_, _54_. - _Stangeria_, 110. - Stele, modifications of, 62, _36_-_42_. - Stems, external similarity in living and fossil, 60. - _Sternbergia_, cast of, 10, _4_. - -- pith cast of _Cordaites_, 93. - _Stigmaria_, 69, 142, 143, 144, 145. - -- rootlet of, 143, _101_. - Stomates, 54, _23_. - -- in fossil epidermis, 14, _8_. - Stoneworts, 163. - Synclines, 23. - - - Taxe, 88, 90. - -- comparison of fructification with that of Cordaite, 95. - -- description of, 92. - Taxe, fleshy seeds of, 89. - _Taxodium_, 86. - _Taxus_, 82. - Time, divisions of geological time, 34. - Tracheides for water storage, 56, 30. - Tree-ferns, 130. - _Trigonocarpus_, 11, 76, 82, 122, _84_. - -- once supposed to be a Monocotyledon, 82. - -- probably the seed of Medullosa, 121. - _Tubicaulis_, 127, _89_. - - - Unexplored world, 3. - Unicellular plants, 47, _17_. - -- -- division of cells in, 47, 48, _18_. - - - "Vascular bundles", relation of to steles, 65, _42_. - -- tissue, 57, _31_, _32_, _33_, 59. - -- -- continued growth of, 65, _43_. - -- -- importance in plant anatomy, 61 et seq. - _Viburnum_, 84, 85. - - - Watts, 187. - Westphalia, 19. - Wieland, 2, 102, 187. - Williamson, 2, 187. - _Williamsonia_, 104. - Wood, cells composing, 57, _31_. - -- centrifugal development of, 97, _65_. - -- centripetal development of, 97, _65_. - -- parenchyma, 57, _31_. - -- silicified, 17, 80, 87. - -- solid rings of formed by cambium, 66, _44_. - -- vessels of Angiosperms, 58. - - - Yellowstone Park, 17, 167. - Yew, 82. - _Yucca_, 82. - - - Zeiller, 2, 187. - Zittel, 187. - _Zygopteris_, 127. - - - _By the Same Author_ - - The Study of Plant Life for Young People - - Demy 8vo, fully illustrated, 2s. 6d. net - - -Nature: "Of the various books written for children in elementary schools, -_The Study of Plant Life_ is quite the most logical and intelligent that -we have seen". - - -Manchester Guardian: "With great skill the knowledge gained from simple -experiments in the classroom or laboratory is made effective.... 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You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - - -Title: 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 - -Author: Marie C. Stopes - -Release Date: October 18, 2013 [EBook #43976] - -Language: English - -Character set encoding: ASCII - -*** START OF THIS PROJECT GUTENBERG EBOOK ANCIENT PLANTS *** - - - - -Produced by Stephen Hutcheson, Chris Curnow and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - - 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 palaeobotany, 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 debris (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 debris 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 palaeobotanists. 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 debris. 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 debris, have been found in the coal or immediately -above it, and are of Palaeozoic 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 palaeozoic 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 palaeontologist. - -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 debris 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 debris. 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 debris -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 debris 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 debris 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 debris 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 aeons -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 Palaeozoic, including - Permian. - Carboniferous. - Devonian. - Palaeozoic - VI. Older Palaeozoic. - Eozoic - VII. Archaean. - -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 Archaean, and even the Older Palaeozoic 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 aeon--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 debris 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 debris 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 aeons -under one heading, the Older Palaeozoic. 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 Palaeozoic 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 _Cordaiteae_) was in full swing long -before the Carboniferous period began. Hence, though of Older Palaeozoic -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 Palaeozoic 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 debris, -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 - Algae Seaweeds. - Fungi Moulds and toadstools. - Bryophyta - Hepaticae Liverworts. - Musci Mosses. - Pteridophyta - Equisetales Horsetails. - Sphenophyllales* fossil only, _Sphenophyllum_. - Lycopodales Club-moss. - Filicales Bracken fern. - Pteridospermae - Lyginodendrae* 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 Coniferae, 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 Protococcoideae among the -Algae. 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 Protococcoideae 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 Algae 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 Protococcoideae 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 Coniferae and -Angiosperms the male cell does not swim, but is passive (or largely so), -and is brought to the egg cell. One might almost say that the whole -evolution of the complex structures found in fruiting cones and flowers -is a result of the need of protection of the delicate, simple -reproductive cells and the embryonic tissues resulting from their fusion. -The lower plants scatter these delicate cells broadcast in enormous -numbers, the higher plants protect each single egg cell by an elaborate -series of tissues, and actually bring the male cell to it without ever -allowing either of them to be exposed. - -It must be assumed that the reader possesses a general acquaintance with -the living families tabulated on p. 44; those of the fossil groups will -be given in some detail in succeeding chapters which deal with the -histories of the various families. It is premature to attempt any general -discussion of the evolution of the various groups till all have been -studied, so that this will be reserved for the concluding chapters. - - - - - CHAPTER VI - MINUTE STRUCTURE OF FOSSIL PLANTS--LIKENESSES TO LIVING ONES - - -The individual plants of the Coal Measure period differed entirely from -those now living; they were more than merely distinct species, for in the -main even the families were largely different from the present ones. -Nevertheless, when we come to examine the minute anatomy of the fossils, -and the cells of which they are composed, we find that between the living -and the fossil cell types the closest similarity exists. - -From the earliest times of which we have any knowledge the elements of -the plant body have been the same, though the types of structures which -they built have varied in plan. Individual _cells_ of nearly every type -from the Coal Measure period can be identically matched with those of -to-day. In the way the walls thickened, in the shapes of the wood, -strengthening or epidermal cells, in the form of the various tissues -adapted to specific purposes, there is a unity of organization which it -is reasonable to suppose depends on the fundamental qualities inherent in -plant life. - -This will be illustrated best, perhaps, by tabulating the chief -modifications of cells which are found in plant tissues. The -illustrations of these types in the following table are taken from living -plants, because from them figures of more diagrammatic clearness can be -made, and the salient characters of the cells more easily recognized. -Comparison of these typical cells with those illustrated from the fossil -plants reveals their identity in essential structure, and most of them -will be found in the photos of fossils in these pages, though they are -better recognized in the actual fossils themselves. - - - Principal Types of Plant Cells - -Epidermal. - - [Illustration: Fig. 21 - - _Epidermis._--Protecting layer or skin. Cells with outer wall thickened - in many cases (fig. 21, _a_ and _b_). Compare fossil epidermis in fig. - 34, _e_.] - - [Illustration: Fig. 22 _Hairs._--Extensions of epidermis cells. Single - cells, or complex, as fig. 22, _h_, where _e_ is epidermis and _p_ - parenchyma. Compare fossil hairs in figs. 79 and 120.] - - [Illustration: Fig. 23 - - _Stomates._--Breathing pores in the epidermis. Seen in surface view as - two-lipped structures (fig. 23). _s_, Stomates; _e_, epidermis cells. - Compare fossil stomates in fig. 8.] - -Ground Tissue - - [Illustration: Fig. 24 - - _Parenchyma._--Simple soft cells, either closely packed, as in fig. 24, - or with air spaces between them. Compare 78, B, for fossil.] - - [Illustration: Fig. 25 - - _Palisade._--Elongated, closely packed cells, _p_, chiefly in leaves, - lying below the epidermis, _e_, fig. 25. Compare fig. 34, _p_, for - fossil palisade.] - - [Illustration: Fig. 26 - - _Endodermis._--Cells with specially thickened walls, _en_, lying as - sheath between the parenchyma, _c_, of ground tissue, and the vascular - tissue, _s_, fig. 26. Compare fig. 108 for fossil endodermis.] - - [Illustration: Fig. 27 - - _Latex cells._--Large, often much elongated cells, _m_, lying in the - parenchyma, _p_, fig. 27, which are packed with contents. Compare fig. - 107, _s_.] - - [Illustration: Fig. 28 - - _Sclerenchyma._--Thick-walled cells among parenchyma for strengthening, - fig. 28. Compare fig. 34, _s_.] - - [Illustration: Fig. 29 - - _Cork._--Layers of cells replacing the epidermis in old stems. Outer - cells, _o_, crushed; _k_, closely packed cork cells; stone cells, _s_, - fig. 29. Compare fig. 95, _k_. - - _Cork cambium._--Narrow, actively dividing cells, _c_ in fig. 29, - giving rise to new cork cells in consecutive rows.] - - [Illustration: Fig. 30 - - _Tracheides._--Specially thickened cells in the parenchyma, usually for - water storage, _t_, fig. 30. Compare fig. 95, _t_.] - -Vascular Tissue - - [Illustration: Fig. 31 - - _Wood._--_Protoxylem_, tracheids and vessels, long, narrow elements, - with spiral or ring-like thickenings, _s_^1 and _s_^2, fig. 31. Compare - fig. 81, A, _px_, for fossil. - - _Metaxylem_, long elements, tracheids and vessels. Some with narrow - pits, as _t_ in fig. 31; others with various kinds of pits. In - transverse section seen in fig. 33, w, fossil in fig. 114, _w_. - - _Wood parenchyma._--Soft cells associated with the wood, _p_ in fig. - 31. Fossil in fig. 81, B, _p_. - - _Wood sclerenchyma._--Hard thickened cells in the wood.] - - [Illustration: Fig. 32 - - _Bast._--_Sieve tubes_, long cells which carry foodstuffs, cross walls - pitted like sieves, _s_, fig. 32. In transverse section in fig. 33. - - _Companion cells_, narrow cells with rich proteid contents, _c_, fig. - 32. In transverse section at _c_, fig. 33. - - _Bast parenchyma._--Soft unspecialized cells mixed with the sieve - tubes, _p_, fig. 32. - - _Bast fibres._--Thick-walled sclerenchymatous cells mixed with, or - outside, the soft bast.] - - [Illustration: Fig. 33 - - _Cambium._--Narrow cells, like those of the cork cambium, which lie - between the wood and bast, and give rise to new tissues of each kind, - _cb_, fig. 33. Compare fig. 114, fossil.] - -There are, of course, many minor varieties of cells, but these illustrate -all the main types. - -Among the early fossils, however, one type of wood cell and one type of -bast cell, so far as we know, are not present. These cells are the true -_vessels_ of the wood of flowering plants, and the long bast cells with -their companion proteid cells. The figure of a metaxylem wood cell, shown -in fig. 31, _t_, shows the more primitive type of wood cell, which has an -oblique cross wall. This type of wood cell is found in all the fossil -trees, and all the living plants except the flowering plants. The vessel -type, which is that in the big wood vessels of the flowering plants, and -has no cross wall, is seen in fig. 20, _x_. - -The similarity between the living cells and those of the Coal Measure -fossils is sufficiently illustrated to need no further comment. This -similarity is an extremely helpful point when we come to an -interpretation of the fossils. In living plants we can study the -physiology of the various kinds of cells, and can deduce from experiment -exactly the part they play in the economy of the whole plant. From a -study of the tissues in any plant structure we know what function it -performed, and can very often estimate the nature of the surrounding -conditions under which the plant was growing. To take a single example, -the palisade tissue, illustrated in fig. 25, _p_, in living plants always -contains green colouring matter, and lies just below the epidermis, -usually of leaves, but sometimes also of green stems. These cells do most -of the starch manufacture for the plant, and are found best developed -when exposed to a good light. In very shady places the leaves seldom have -this type of cell. Now, when cells just like these are found in fossils -(as is illustrated in fig. 34), we can assume all the physiological facts -mentioned above, and rest assured that that leaf was growing under normal -conditions of light and was actively engaged in starch-building when it -was alive. From the physiological standpoint the fossil leaf is entirely -the same as a normal living one. - - [Illustration: Fig. 34.--From a Photo of a Fossil Lea - - _e_, Epidermis; _p_, palisade cells; _pr_, soft parenchyma cells - (poorly preserved); _s_, sclerenchyma above the vascular bundle.] - -From the morphological standpoint, also, the features of the plant body -from the Coal Measure period fall into the same divisions as those of the -present. Roots, stems, leaves, and reproductive organs, the essentially -distinct parts of a plant, are to be found in a form entirely -recognizable, or sufficiently like that now in vogue to be interpreted -without great difficulty. In the detailed structure of the reproductive -organs more changes have taken place than in any others, both in internal -organization and external appearance. - -Already, in the Early Palaeozoic period, the distinction between leaves, -stems, roots, and reproductive organs was as clearly marked as it is -to-day, and, judging by their structure, they must each have performed -the physiological functions they now do. Roots have changed least in the -course of time, probably because, in the earth, they live under -comparatively uniform conditions in whatever period of the world's -history they are growing. Naturally, between the roots of different -species there are slight differences; but the likeness between fern roots -from the Palaeozoic and from a living fern is absolutely complete. This is -illustrated in fig. 35, which shows the microscopic structure of the two -roots when cut in transverse direction. The various tissues will be -recognized as coming into the table on p. 54, so that both in the details -of individual cells and in the general arrangement of the cell groups or -tissues the roots of these fossil and living ferns agree. - - [Illustration: Fig. 35.--A, Root of Living Fern. B, Root of Palaeozoic - Fossil Fern. _c_, Cortex; _px_, protoxylem in two groups; _m_, - metaxylem; _s_, space in fossil due to decay of soft cells.] - -Among stems there has been at all periods more variety than among the -roots of the corresponding plants, and in the following chapter, when the -differences between living and fossil plants will be considered, there -will be several important structures to notice. Nevertheless, there are -very many characters in which the stems from such widely different epochs -agree. The plants in the palaeozoic forests were of many kinds, and among -them were those with weak trailing stems which climbed over and supported -themselves on other plants, and also tall, sturdy shafts of woody trees, -many of which were covered with a corky bark. Leaves were attached to the -stems, either directly, as in the case of some living plants, or by leaf -stalks. In external appearance and in general function the stems then -were as stems are now. In the details of the individual cells also the -likeness is complete; it is in the grouping of the cells, the anatomy of -the tissues, that the important differences lie. It has been remarked -already that increase in complexity of the plant form usually goes with -an increase in complexity of the cells and variety of the tissues. The -general ground tissue in nearly all plants is very similar; it is -principally in the vascular system that the advance and variety lie. - -Plant anatomists lay particular stress on the vascular system, which, in -comparison with animal anatomy, holds an even more important position -than does the skeleton. To understand the essential characters of stems, -both living and fossil, and to appreciate their points of likeness or -difference, it is necessary to have some knowledge of the general facts -of anatomy; hence the main points on which stress is laid will be given -now in brief outline. - -Leaving aside consideration of the more rudimentary and less defined -structure of the algae and mosses, all plants may be said to possess a -"vascular system". This is typically composed of elongated wood (or -xylem) with accessory cells (see p. 57, table), and bast (phloem), also -with accessory cells. These specialized conducting elements lie in the -ground tissue, and in nearly all cases are cut off from direct contact -with it by a definite sheath, called the endodermis (see p. 55, fig. 26). -Very often there are also groups or rings of hard thick-walled cells -associated with the vascular tissues, which protect them and play an -important part in the consolidation of the whole stem. - - [Illustration: Fig. 36.--Diagram of Simplest Arrangement of Complete - Stele in a Stem - - W, Central solid wood; P, ring of bast; E, enclosing sheath of - endodermis; C, ground tissue or cortex.] - -The simplest, and probably evolutionally the most primitive form which is -taken by the vascular tissues, is that of a single central strand, with -the wood in the middle, the bast round it, and a circular endodermis -enclosing all, as in fig. 36, which shows a diagram of this arrangement. -Such a mass of wood and bast surrounded by an endodermis, is technically -known as a _stele_, a very convenient term which is much used by -anatomists. In its simplest form (as in fig. 36) it is called a -_protostele_, and is to be found in both living and fossil plants. A -number of plants which get more complex steles later on, have protosteles -in the early stages of their development, as in _Pteris aurita_ for -example, a species allied to the bracken fern, which has a hollow ring -stele when mature. - - [Illustration: Fig. 37.--Diagram of a Stele with a few Cells of Pith - _p_ in the Middle of the Wood. Lettering as in fig. 36] - - [Illustration: Fig. 38.--Diagram showing Extensive Pith _p_ in the - Wood. Lettering as in fig. 36] - -The next type of stele is quite similar to the protostele, but with the -addition of a few large unspecialized cells in the middle of the wood -(_p_, fig. 37); these are the commencement of the hollowing process which -goes on in the wood, resulting later in the formation of a considerable -pith, as is seen in fig. 38, where the wood is now a hollow cylinder, as -the phloem has been from the first. When this is the case, a second -sheath or endodermis generally develops on the inner side of the wood, -outside the pith, and cuts the vascular tissues off from the inner -parenchyma. A further step is the development of an inner cylinder of -bast so that the vascular ring is completely double, with endodermis on -both sides of the cylinder, as is seen in fig. 39. - - [Illustration: Fig. 39.--A Cylindrical Stele, with _e_, inner - endodermis, and _ph_, inner phloem; W, wood; P, outer phloem; E, outer - endodermis. L, part of the stele going out to supply a large leaf, thus - breaking what would otherwise appear as a closed ring stele] - -In all these cases there is but one strand or cylinder, of vascular -tissue in the stem, but one stele, and this type of anatomy is known as -the _monostelic_ or single-steled type. - - [Illustration: Fig. 40.--A Ring Stele apparently broken up into a - Number of Protosteles by many Leaf Gaps] - -When from the double cylinder just described a strand of tissue goes off -to supply a large leaf, a considerable part of the stele goes out and -breaks the ring. This is shown in fig. 39, where L is the part of the -stele going to a leaf, and the rest the broken central cylinder. When the -stem is short, and leaves grow thickly so that bundles are constantly -going out from the main cylinder, this gets permanently broken, and its -appearance when cut across at any given point is that of a group of -several steles arranged in a ring, each separate stele being like the -simple protostele in its structure. See fig. 40. This type of stem has -long been known as _polystelic_ (_i.e._ many-steled), and it is still a -convenient term to describe it by. There has been much theoretical -discussion about the true meaning of such a "polystelic" stem, which -cannot be entered into here; it may be noted, however, that the various -strands of the broken ring join up and form a meshwork when we consider -the stem as a whole, it is only in a single section that they appear as -quite independent protosteles. Nevertheless, as we generally consider the -anatomy of stems in terms of single sections, and as the descriptive word -"polystelic" is a very convenient and widely understood term, it will be -used throughout the book when speaking of this type of stem anatomy. - -Such a type as this, shown in fig. 40, is already complex, but it often -happens that the steles branch and divide still further, until there is a -highly complicated and sometimes bewildering system of vascular strands -running through the ground tissue in many directions, but cut off from it -by their protective endodermal sheaths. Such complex systems are to be -found both in living and fossil plants, more especially in many of the -larger ferns (see fig. 88). - -Higher plants in general, however, and in particular flowering plants, do -not have a polystelic vascular arrangement, but a specialized type of -monostele. - - [Illustration: Fig. 41.--Monostele in which the Central Pith is - Star-shaped, and the Wood breaking up into Separate Groups - - _p_, Pith; W, wood; P, phloem; E, endodermis; C, cortex.] - -Referring again to fig. 37 as a starting-point, imagine the pith in the -centre to spread in a star-shaped form till the points of the star -touched the edges of the ring, and thus to break the wood ring into -groups. A stage in this process (which is not yet completed) is shown in -fig. 41, while in fig. 42 the wood and bast groups are entirely distinct. -In the flowering plants the cells of the endodermis are frequently poorly -characterized, and the pith cells resemble those of the cortical ground -tissue, so that the separate groups of wood and bast (usually known as -"vascular bundles", in distinction from the "steles" of fig. 40) appear -to lie independently in the ground tissue. These strands, however, must -not be confused with steles, they are only fragments of the single -apparently broken up stele which runs in the stem. - - [Illustration: Fig. 42.--Monostele in which the Pith has invaded all - the Tissues as far as the Endodermis, and broken the Wood and Phloem up - into Separate Bundles. These are usually called "vascular bundles" in - the flowering plants] - - [Illustration: Fig. 43.--Showing actively growing Zone _c_ (Cambium) in - the Vascular Bundles, and joining across the ground tissue between them] - -The vascular bundle, of all except the Monocotyledons, has a potentiality -for continued growth and expansion which places it far above the stele in -value for a plant of long life and considerable growth. The cells lying -between the wood and the bast, the soft parenchyma cells always -accompanying such tissues, retain their vitality and continue to divide -with great regularity, and to give rise to a continuous succession of new -cells of wood on the one side and bast on the other; see fig. 33, _c_, -_b_. In this way the primary, distinct vascular bundles are joined by a -ring of wood, see fig. 43, to which are added further rings every season, -till the mass of wood becomes a strong solid shaft. This ever-recurring -activity of the cambium gives rise to what are known as "annual rings" in -stems, see fig. 44, in which the wood shows both primary distinct groups -in the centre, and the rings of growth of later years. - -Cambium with this power of long-continued activity is found in nearly all -the higher plants of to-day (except the Monocotyledons), but in the fern -and lycopod groups it is in abeyance. Certain cases from nearly every -family of the Pteridophytes are known, where some slight development of -cambium with its secondary thickening takes place, but in the groups -below the Gymnosperms cambium has almost no part to play. On the other -hand, so far back as the Carboniferous period, the masses of wood in the -Pteridophyte trees were formed by cambium in just the same way as they -are now in the higher forms. Its presence was almost universal at that -time in the lower groups where to-day there are hardly any traces of it -to be found. - - [Illustration: Fig. 44.--Stem with Solid Cylinder of Wood developed - from the Cambium, showing three "annual rings". In the centre may still - be seen the separate groups of the wood of the primary "vascular - bundles"] - -It will be seen from this short outline of the vascular system of plants, -that there is much variety possible from modifications of the fundamental -protostele. It is also to be noted that the plants of the Coal Measures -had already evolved all the main varieties of steles which are known to -us even now,[6] and that the development of secondary thickening was very -widespread. In several cases the complexity of type exceeds that of -modern plants (see Chap. VII), and there are to be found vascular -arrangements no longer extant. - -When we turn to the _Reproductive Organs_, we find that the points of -likeness between the living and the fossil forms are not so numerous or -so direct as they are in the case of the vegetative system. - - [Illustration: Fig. 45.--Fern Sporangia - - A, fossil; B, living.] - -As has been indicated, the families of plants typical of the Coal -Measures were not those which are the most prominent to-day, but belonged -to the lower series of Pteridophytes. In their simpler forms the -fructifications then and now resemble each other very closely, but in the -more elaborate developments the points of variety are more striking, so -that they will be dealt with in the following chapter. Cases of likeness -are seen in the sporangia of ferns, some of which appear to have been -practically identical with those now living. This is illustrated in fig. -45, which shows the outline of the cells of the sporangia of living and -fossil side by side. - - [Illustration: Fig. 46.--A, Living Lycopod cone; B, _Lepidodendron_ - (fossil) cone. _a_, Axis; _s_, scale; S, sporangium with spores. One - side of a longitudinal section] - -In the general structure also of the cones of the simpler types of -_Lepidodendron_ (fossil, see frontispiece) there is a close agreement -with the living Lycopods, though as regards size and output of spores -there was a considerable difference in favour of the fossils. The plan of -each is that round the axis of the cone simple scales are arranged, on -each of which, on its upper side, is seated a large sporangium bearing -numerous spores all of one kind (see fig. 46). - -Equally similar are the cones of the living Equisetum and some of the -simple members of the fossil family Calamiteae, but the more interesting -cases are those where differences of an important morphological nature -are to be seen. - -As regards the second[7] generation there is some very important -evidence, from extremely young stages, which has recently been given to -the world. In a fern sporangium _germinating spores_ were fossilized so -as to show the first divisions of the spore cell. These seem to be -identical with the first divisions of some recent ferns (see fig. 47). -This is not only of interest as showing the close similarity in detail -between plants of such widely different ages, but is a remarkable case of -delicate preservation of soft and most perishable structures in the "coal -balls". - - [Illustration: Fig. 47.--Germinating Fern Spores - - A and B, from carboniferous fossils; C, living fern. (A and B after - Scott.)] - -While these few cases illustrate points of likeness between the -fructifications of the Coal Measures and of to-day, the large size and -successful character of the primitive Coal Measure plants was accompanied -by many developments on the part of their reproductive organs which are -no longer seen in living forms, and the greater number of palaeozoic -fructifications must be considered in the next chapter. - - - - - CHAPTER VII - MINUTE STRUCTURE OF FOSSIL PLANTS--DIFFERENCES FROM LIVING ONES - - -We have seen in the last chapter that the main morphological divisions, -roots, stems, leaves, and fructifications, were as distinct in the Coal -Measure period as they are now. There is one structure, however, found in -the Coal Measure fossils, which is hardly paralleled by anything similar -in the living plants, and that is the fossil known as _Stigmaria_. -_Stigmaria_ is the name given, not to a distinct species of plant, but to -the large rootlike organs which we know to have belonged to all the -species of _Lepidodendron_ and of _Sigillaria_. In the frontispiece these -organs are well seen, and branch away at the foot of the trunk, spreading -horizontally, to all appearance merely large roots. They are especially -regularly developed, however, the main trunk giving rise always to four -primary branches, these each dividing into two equal branches, and so -on--in this they are unlike the usual roots of trees. They bore numerous -rootlets, of which we know the structure very well, as they are the -commonest of all fossils, but in their internal anatomy the main "roots" -had not the structure which is characteristic of roots, but were like -_stems_. In living plants there are many examples of stems which run -underground, but they always have at least the rudiments of leaves in the -form of scales, while the fossil structures have apparently no trace of -even the smallest scales, but bear only rootlets, thus resembling true -roots. The questions of morphology these structures raise are too complex -to be discussed here, and Stigmaria is only introduced as an example, one -of the very few available, of a palaeozoic structure which seems to be of -a nature not clearly determinable as either root, stem, leaf, or -fructification. Among living plants the fine rootlike rhizophores of -Selaginella bear some resemblance to Stigmaria in essentials, though so -widely different from them in many ways, and they are probably the -closest analogy to be found among the plants of to-day. - -The individual cells, we have already seen, are strikingly similar in the -case of fossil and living plants. There are, of course, specific -varieties peculiar to the fossils, of which perhaps the most striking -seem to be some forms of _hair_ cells. For example, in a species of fern -from the French rocks there were multicellular hairs which looked like -little stems of Equisetum owing to regular bands of teeth at the -junctions of the cells. These hairs were quite characteristic of the -species--but hairs of all sorts have always abounded in variety, so that -such distinction has but minor significance. - - [Illustration: Fig. 48.--Stele of _Lepidodendron_ W, surrounded by a - small ring of secondary wood S] - -As was noted in the table (p. 58) the only cell types of prime importance -which were not evolved by the Palaeozoic plants were the wood vessels, -phloem and accompanying cells which are characteristic of the flowering -plants. - -Among the fossils the vascular arrangements are most interesting, and, as -well as all the types of stele development noted in the previous chapter -as common to both living and fossil plants, there are further varieties -found only among the fossils (see fig. 50). - -The simple protostele described (on p. 61) is still found, particularly -in the very young stages of living ferns, but it is a type of vascular -arrangement which is not common in the mature plants of the present day. -In the Coal Measure period, however, the protostele was characteristic of -one of the two main groups of ferns. In different species of these ferns, -the protostele assumed a large variety of shapes and forms as well as the -simple cylindrical type. The central mass of wood became five-rayed in -some, star-shaped, and even very deeply lobed, with slightly irregular -arms, but in all these cases it remained fundamentally monostelic. -Frequently secondary tissue developed round the protosteles of plants -whose living relatives have no such tissue. A case of this kind is -illustrated in fig. 48, which shows a simple circular stele surrounded by -a zone of secondary woody tissue in a species of _Lepidodendron_. - - [Illustration: Fig. 49.--_Lepidodendron_, showing Part of the Hollow - Ring of Primary Wood W, with a relatively large amount of Secondary - Tissue S, surrounding it] - -In many species of _Lepidodendron_ the quantity of secondary wood formed -round the primary stele was very great, so that (as is the case in higher -plants) the primary wood became relatively insignificant compared with -it. In most species of _Lepidodendron_ the primary stele is a hollow ring -of wood (cf. fig. 38, p. 62) round which the secondary wood developed, as -is seen in fig. 49. These two cases illustrate a peculiarity of fossil -plants. Among living ones the solid and the simple ring stele are almost -confined to the Pteridophytes, where secondary wood does not develop, but -the palaeozoic Pteridophytes, while having the simple primary types of -steles, had quantities of secondary tissue, which was correlated with -their large size and dominant position. - - [Illustration: Fig. 50.--Diagram of Steles of the English _Medullosa_, - showing three irregular, solid, steles A, with secondary thickenings S, - all round each. _a_, Small accessory steles] - -Among _polystelic_ types (see p. 63) we find interesting examples in the -fossil group of the _Medulloseae_, which are much more complex than any -known at present, both owing to their primary structure and also to the -peculiar fact that all the steles developed secondary tissue towards the -inner as well as the outer side. One of the simpler members of this -family found in the English Coal Measures is illustrated in fig. 50. Here -there are three principal protosteles (and several irregular minor ones) -each of which has a considerable quantity of secondary tissue all round -it, so that a portion of the secondary wood is growing in towards the -actual centre of the stem as a whole--a very anomalous state of affairs. - -In the more complex Continental type of _Medullosa_ there are _very_ -large numbers of steles. In the one figured from the Continent in fig. 51 -but a few are represented. There is a large outer double-ring stele, with -secondary wood on both sides of it, and within these a number of small -steles, all scattered through the ground tissue, and each surrounded by -secondary wood. In actual specimens the number of these central steles is -much greater than that indicated in the diagram. - -No plant exists to-day which has such an arrangement of its vascular -cylinder. It almost appears as though at the early period, when the -Medulloseae flourished, steles were experimenting in various directions. -Such types as are illustrated in figs. 50 and 51 are obviously wasteful -(for secondary wood developing towards the centre of a stem is bound to -finally meet), and complex, but apparently inefficient, which may partly -account for the fact that this type of structure has not survived to the -present, though simpler and equally ancient types have done so. - - [Illustration: Fig. 51.--Continental _Medullosa_, showing R, outer - double-ring stele with secondary wood all round it; S, inner stellate - steles, also surrounded in each case by secondary tissue] - -Further details of the anatomy of fossils will be mentioned when we come -to consider the individual families; those now illustrated suffice to -show that in the Coal Measures very different arrangements of steles were -to be found, as well as those which were similar to those existing now. -The significance of these differences will become apparent when their -relation to the other characters of the plants is considered. - -The fructifications, always the most important parts of the plant, offer -a wide field, and the divergence between the commoner palaeozoic and -recent types seems at first to be very great. Indeed, when palaeozoic -reproductive bodies have to be described, it is often necessary to use -the common descriptive terms in an altered and wider sense. - -Among the plants of to-day there are many varieties of the simple -single-celled reproductive masses which are called _spores_, and which -are usually formed in large numbers inside a spore case or sporangium. -Among the higher plants _seeds_ are also known in endless variety, all of -which, compared with spores, are very complex, for they are many-celled -structures, consisting essentially of an embryo or young plant enclosed -in various protective coats. The distinction between the two is sharp and -well defined, and for the student of living plants there exists no -difficulty in separating and describing seeds and spores. - -But when we look back through the past eras to palaeozoic plants the -subject is not so easy, and the two main types of potentially -reproductive masses are not sharply distinct. The seed, as we know it -among recent plants, and as it is generally defined, had not fully -evolved; while the spores were of great variety and had evolved in -several directions, some of which seem to have been intermediate stages -between simple spores and true seeds. These seedlike spores served to -reproduce the plants of the period, but their type has since died out and -left but two main methods among living plants, namely the essentially -simple spores, the very simplicity of whose organization gives them a -secure position, and the complex seeds with their infinite variety of -methods for protecting and scattering the young embryos they contain. - -Among the Coal Measure fossils we can pick up some of the early stages in -the evolution of the seed from the spore, or at least we can examine -intermediate stages between them which give some idea of the possible -course of events. Hence, though the differences from our modern -reproductive structures are so noticeable a feature of the palaeozoic -ones, it will be seen that they are really such differences as exist -between the members at the two ends of a series, not such as exist -between unrelated objects. - -Very few types can be mentioned here, and to make their relations clear a -short series of diagrams with explanations will be found more helpful -than a detailed account of the structures. - - [Illustration: Fig. 52.--Spores - - Each spore a single cell which develops with three others in tetrads - (groups of four). Very numerous tetrads enclosed in a spore case or - sporangium which develops on a leaflike segment called the sporophyll. - Each spore germinates independently of the others after being - scattered, all being of the same size. Common in fossils and living - Pteridophytes.] - - [Illustration: Fig. 53.--Spores - - Each a single cell like the preceding, but here only one tetrad in a - sporangium ripens, so that each contains only four spores. Compared - with the preceding types these spores are very large. Otherwise details - similar to above. Some fossils have such sporangia with eight spores, - or some other small number; living Selaginellas have four. In the same - cone sporangia with small spores are developed and give rise to the - male organs.] - - [Illustration: Fig. 54.--"Spores" of Seedlike Structure - - Out of a tetrad in each sporangium only one spore ripens, S in figure, - the others, _s_, abort. The wall of the sporangium, _w_, is more - massive than in the preceding cases, and from the sporophyll, flaps, - _sp f_, grow up on each side and enclose and protect the sporangium. - The one big spore appears to germinate inside these protective coats, - and not to be scattered separately from them. Only found in fossils, - one of the methods of reproduction in _Lepidodendron_. Other sporangia - with small spores were developed which gave rise to the male organs.] - - [Illustration: Fig. 55.--"Seed" - - In appearance this is like a seed, but differs from a true seed in - having no embryo, and is like the preceding structure in having a very - large spore, S, though there is no trace of the three aborting ones. - The spore develops in a special mass of tissue known as the nucellus, - _n_, which partly corresponds to the sporangium wall of the previous - types. In it a cavity, _p c_, the pollen chamber, receives the pollen - grains which enter at the apex of the "seed". There is a complex coat, - C, which stands round the nucellus but is not joined to it, leaving the - space _l_ between them. Only in fossils; _Trigonocarpus_ (see p. 122) - is similarly organized. Small spores in fern-like sporangia, called - pollen grains.] - - [Illustration: Fig. 56.--"Seed" - - Very similarly organized to the above, but the coat is joined to the - nucellus about two-thirds of its extent, and up to the level _l_. In - the pollen chamber, _p c_, a cone of nucellar tissue projects, and the - upper part of the coat is fluted, but these complexities are not of - primary importance. The large spore S germinated and was fertilized - within the "seed", but apparently produced no embryo before it ripened. - Small "spores" in fern-like sporangia form the pollen grains. Only in - fossils, _e.g._ Lagenostoma. (See p. 119.)] - - [Illustration: Fig. 57.--Seed - - Essentially similar to the preceding, except in the possession of an - embryo _e_, which is, however, small in comparison with the endosperm - which fills the spore S. The whole organization is simpler than in the - fossil _Lagenostoma_, but the coat is fused to the nucellus further up - (see _l_). Small "spores" form the pollen grains. Living and fossil - type, Cycads and Ginkgo.] - - [Illustration: Fig. 58.--Seed - - In the ripe seed the large embryo _e_ practically fills up all the - space within the two seed coats _c^1_ and _c^2_; endosperm, pollen - chamber, &c., have been eliminated, and the young ovule is very simple - and small as a result of the protection and active service of the - carpels in which it is enclosed. Small "spores" form the pollen grains. - Typical of living Dicotyledons.] - -These few illustrations represent only the main divisions of an army of -structures with an almost unimaginable wealth of variety which must be -left out of consideration. - -For the structures illustrated in figs. 54, 55, and 56 we have no name, -for their possible existence was not conceived of when our terminology -was invented, and no one has yet christened them anew with distinct -names. They are evidently too complex in organization and too similar to -seeds in several ways to be called spores, yet they lack the essential -element in a seed, namely, an embryo. The term "ovule" (usually given to -the young seed which has not yet developed an embryo) does not fit them -any better, for their tissues are ripened and hard, and they were of -large size and apparently fully grown and mature. - -For the present a name is not essential; the one thing that is important -is to recognize their intermediate character and the light they throw on -the possible evolution of modern seeds. - -A further point of great interest is the manner in which these "seeds" -were borne on the plant. To-day seeds are always developed (with the -exception of Cycas) in cones or flowers, or at least special -inflorescences. But the "seed" of _Lagenostoma_ (fig. 56), as well as a -number of others in the group it represents, were not borne on a special -structure, but directly on the green foliage leaves. They were in this on -a level with the simple sporangia of ferns which appear on the backs of -the fronds, a fact which is of great significance both for our views on -the evolution of seeds as such, and for the bearing it has on the -relationships of the various groups of allied plants. This will be -referred to subsequently (Chapter XI), and is mentioned now only as an -example of the difference between some of the characters of early fossils -and those of the present day. - -It is true that botanists have long recognized the organ which bears -seeds as a modified leaf. The carpels of all the higher plants are looked -on as _homologous_ with leaves, although they do not appear to be like -them externally. Sometimes among living plants curious diseases cause the -carpels to become foliar, and when this happens the diseased carpel -reverts more or less to the supposed ancestral leaf-like condition. It is -only among the ancient (but recently discovered) fossils, however, that -seeds are known to be borne normally on foliage leaves. - -From Mesozoic plants we shall learn new conceptions about flowers and -reproductive inflorescences in general, but these must be deferred to the -consideration of the family as a whole (Chapter XIII). - -Enough has been illustrated to show that though the individual cells, the -bricks, so to speak, of plant construction, were so similar in the past -and present, yet the organs built up by them have been continually -varying, as a child builds increasingly ambitious palaces with the same -set of bricks. - - - - - CHAPTER VIII - PAST HISTORIES OF PLANT FAMILIES - I. Flowering Plants, Angiosperms - - -In comparison with the other groups of plants the flowering families are -of recent origin, yet in the sense in which the word is usually used they -are ancient indeed, and the earliest records of them must date at least -to periods hundreds of thousands of years ago. - -Through all the Tertiary period (see p. 34) there were numerous flowering -plants, and there is evidence that many families of both Monocotyledons -and Dicotyledons existed in the Upper Cretaceous times. Further back than -this we have little reliable testimony, for the few specimens of -so-called flowering plants from the Lower Mesozoic are for the most part -of a doubtful nature. - -The flowering plants seem to stand much isolated from the rest of the -plant world; there is no _direct_ evidence of connection between their -oldest representatives and any of the more primitive families. So far as -our actual knowledge goes, they might have sprung into being at the -middle of the Mesozoic period quite independently of the other plants -then living; though there are not wanting elaborate and almost convincing -theories of their connection with more than one group of their -predecessors (see p. 108). - -It is a peculiarly unfortunate fact that although the rocks of the -Cretaceous and Tertiary are so much less ancient than those of the Coal -Measures, they have preserved for us far less well the plants which were -living when they were formed. Hitherto no one has found in Mesozoic -strata masses of exquisitely mineralized Angiosperm fragments[8] like -those found in the Coal Measures, which tell us so much about the more -ancient plants. Cases are known of more or less isolated fragments with -their microscopical tissues mineralized. For example, there are some -palms and ferns from South America which show their anatomical structure -very clearly preserved in silica, and which seem to resemble closely the -living species of their genera. The bulk of the plants preserved from -these periods are found in the form of casts or impressions (see p. 10), -which, as has been pointed out already, are much less satisfactory to -deal with, and give much less reliable results than specimens which have -also their internal structure petrified. The quantity of material, -however, is great, and impressions of single leaves innumerable, and of -specimens of leaves attached to stems, and even of flowers and fruits, -are to be found in the later beds of rock. These are generally clearly -recognizable as belonging to one or other of the living families of -flowering plants. Leaf impressions are by far the most frequent, and our -knowledge of the Tertiary flora is principally derived from a study of -them. Their outline and their veins are generally preserved, often also -their petioles and some indication of the thickness and character of the -fleshy part of the leaf. From the outline and veins alone an expert is -generally able to determine the species to which the plant belongs, -though it is not always quite safe to trust to these determinations or to -draw wide-reaching conclusions from them. - -In fig. 59 is shown a photograph of the impression of a Tertiary leaf, -which illustrates the condition of an average good specimen from rocks of -the period. Its shape and the character of the veins are sufficient to -mark it out immediately as belonging to the Dicotyledonous group of the -flowering plants. - -Seeds and fruits are also to be found; and in some very finely preserved -specimens from Japan stamens from a flower and delicate seeds are seen -clearly impressed on the light stone. In fig. 60 is illustrated a couple -of such seeds, which show not only their wings but also the small -antennae-like stigmas. Specimens so perfectly preserved are practically as -good as herbarium material of recent plants, and in this way the -externals of the Tertiary plants are pretty well known to us. - - [Illustration: Fig. 59.--Dicotyledonous Leaf Impression from Tertiary - Rocks] - - [Illustration: Fig. 60.--Seeds from Japanese Tertiary Rocks; at _a_ are - seen the two stigmas still preserved] - -A problem which has long been discussed, and which has aroused much -interest, is the relative antiquity of the Monocotyledonous and the -Dicotyledonous branches of the flowering plants. A peculiar fascination -seems to hang over this still unsolved riddle, and a battle of flowers -may be said to rage between the lily and the rose for priority. Recent -work has thrown no decisive light on the question, but it has undoubtedly -demolished the old view which supposed that the Monocotyledons (the lily -group) appeared at a far earlier date upon this earth than the -Dicotyledons. The old writers based their contention on incorrectly -determined fossils. For instance, seeds from the Palaeozoic rocks were -described as Monocotyledons because of the three or six ribs which were -so characteristic of their shell; we know now that these seeds -(_Trigonocarpus_) belong to a family already mentioned in another -connection (p. 72), the Medulloseae (see p. 122), the affinity of which -lies between the cycads and the ferns. Leaves of _Cordaites_, again, -which are broad and long with well-marked parallel veins, were described -as those of a Monocotyledonous plant like the Yucca of to-day; but we now -know them to belong to a family of true Gymnosperms possibly distantly -related to _Taxus_ (the Yew tree). - -Recent work, which has carefully sifted the fossil evidence, can only say -that no true Monocotyledons have yet been found below the Lower -Cretaceous rocks, and that at that period we see also the sudden inrush -of Dicotyledons. Hence, so far as palaeontology can show, the two parallel -groups of the flowering plants arose about the same time. It is of -interest to note, however, that the only petrifaction of a flower known -from any part of the world is an ovary which seems to be that of one of -the Liliaceae. In the same nodules, however, there are several specimens -of Dicotyledonous woods, so that it does not throw any light on the -question of priority. - -With the evidence derived from the comparative study of the anatomy of -recent flowering plants we cannot concern ourselves here, beyond noting -that the results weigh in favour of the Dicotyledons as being the more -primitive, though not necessarily developed much earlier in point of -time. Until very much more is discovered than is yet known of the origin -of the flowering plants as a whole, it is impossible to come to a more -definite conclusion about this much-discussed subject. - -Let us now attempt to picture the vegetable communities since the -appearance of the flowering plants. The facts which form the bases of the -following conceptions have been gathered from many lands by numerous -workers in the field of fossil botany, from scattered plant remains such -as have been described. - -When the flowering plants were heralded in they appeared in large -numbers, and already by the Cretaceous period there were very many -different species. Of these a number seem to belong to genera which are -still living, and many of them are extremely like living species. It -would be wearisome and of little value to give a list of all the recorded -species from this period, but a few of the commoner ones may be mentioned -to illustrate the nature of the plants then flourishing. - -Several species of _Quercus_ (the Oak) appeared early, particularly -_Quercus Ilex_; leaves of the _Juglandaceae_ (Walnut family) were very -common, and among the Tertiary fossils appear its fruits. Both _Populus_ -(the Poplar) and _Salix_ (the Willow) date from the early rocks, while -_Ficus_ (the Fig) was very common, and _Casuarina_ (the Switch Plant) -seems to have been widely spread. Magnolias also were common, and it -appears that _Platanus_ (the Plane) and _Eucalyptus_ coexisted with them. - -It will be immediately recognized that the above plants have all living -representatives, either wild or cultivated, growing in this country at -the present day, so that they are more or less familiar objects, and -there appears to have been no striking difference between the early -flowering plants and those of the present day. Between the ancient -Lycopods, for example, and those now living the differences are very -noteworthy; but the earliest of the known flowering plants seem to have -been essentially like those now flourishing. It must be remembered in -this connection that the existing flowering plants are immensely nearer -in point of time to their origin than are the existing Lycopods, and that -when such aeons have passed as divide the present from the Palaeozoic, the -flowering plants of the future may have dwindled to a subordinate -position corresponding to that held by the Lycopods now. - -A noticeable character of the early flowering-plant flora, when taken as -a whole, is the relatively large proportion of plants in it which belong -to the family _Amentiferae_ (oaks, willows, poplars, &c.). This is -supposed by some to indicate that the family is one of the most primitive -stocks of the Angiosperms. This view, however, hardly bears very close -scrutiny, because it derives its main support from the large numbers of -the Amentiferae as compared with other groups. Now, the Amentiferae were -(and are) largely woody resistant plants, whose very nature would render -them more liable to be preserved as impressions than delicate trees or -herbs, which would more readily decay and leave no trace. Similarly based -on uncertain evidence is the surmise that the group of flowers classed as -_Gamopetalae_ (flowers with petals joined up in a tube, like convolvulus) -did not flourish in early times, but are the higher and later development -of the flower type. Now, _Viburnum_ (allied to the honeysuckle) belongs -to this group, and it is found right down in the Cretaceous, and -_Sambucus_ (Elder, of the same family) is known in the early Tertiary. -These two plants are woody shrubs or small trees, while many others of -the family are herbs, and it is noteworthy that it is just these woody, -resistant forms which are preserved as fossils; their presence -demonstrates the antiquity of the group as a whole, and the absence of -other members of it may be reasonably attributed to accidents of -preservation. In the Tertiary also we get a member of the heath family, -viz. _Andromeda_, and another tube-flower, _Bignonia_, as well as several -more _woody_ gamopetalous flowers. - -Hence it is wise to be very cautious about drawing any important -conclusions from the relative numbers of the different species, or the -absence of any type of plant from the lists of those as yet known from -the Cretaceous. When quantities of structurally preserved material can be -examined containing the flowering plants in petrifactions, then it will -be possible to speak with some security of the nature of the Mesozoic -flora as a whole. - -The positive evidence which is already accumulated, however, is of great -value, and from it certain deductions may be safely made. Specimens of -Cretaceous plants from various parts of the world seem to indicate that -there was a very striking uniformity in the flora of that period all over -the globe. In America and in Central Europe, for example, the same types -of plants were growing. We shall see that, as time advanced, the various -types became separated out, dying away in different places, until each -great continent and division of land had a special set of plants of its -own. At the commencement of the reign of flowering plants, however, they -seem to have lived together in the way we are told the beasts first lived -in the garden of Eden. - -At the beginning of the Tertiary period there were still many tropical -forms, such as Palms, Cycads, _Nipa_, various _Artocarpaceae_, _Lauraceae_, -_Araliaceae_, and others, growing side by side with such temperate forms -as _Quercus_, _Alnus_, _Betula_, _Populus_, _Viburnum_, and others of the -same kind. Before the middle of the Tertiary was reached the last Cycads -died in what is now known as Europe; and soon after the middle Tertiary -all the tropical types died out of this zone. - -At the same time those plants whose leaves appear to have fallen at the -end of the warm season began to become common, which is taken as an -indication of a climatic influence at work. Some writers consider that in -the Cretaceous times there was no cold season, and therefore no regular -period of leaf fall, but as the climate became temperate the deciduous -trees increased in numbers; yet the Gymnospermic and Angiospermic woods -which are found with petrified structure show well-marked annual rings -and seem to contradict this view. - -Toward the end of the Tertiary times there were practically no more -tropical forms in the European flora, though there still remained a -number of plants which are now found either only in America or only in -Asia. - -The Glacial epoch at the close of the Tertiary appears to have driven all -the plants before it, and afterwards, when its glaciers retreated, -shrinking up to the North and up the sides of the high mountains, the -plant species that returned to take possession of the land in the -Quaternary or present period were those which are still inhabiting it, -and the floras of the tropics, Asia, and America were no longer mixed -with that of Europe.[9] - - - - - CHAPTER IX - PAST HISTORIES OF PLANT FAMILIES - II. Higher Gymnosperms - - -The more recent history of the higher Gymnosperms, in the Upper -Cretaceous and Tertiary periods, much resembles that of the flowering -plants as sketched in the previous chapter. Many of the genera appear to -have been those still living, and some of the species even may have come -very close to or have been identical with those of to-day. The forms now -characteristic of the different continents were growing together, and -appear to have been widely distributed over the globe. For example, -_Sequoia_ and _Taxodium_, two types now characteristic of America, and -_Glyptostrobus_, at present found in Asia, were still growing with the -other European types in Europe so late as middle Tertiary times. - -As in the case of the Angiosperms, the fossils we have of Cretaceous and -Tertiary Gymnosperms are nearly all impressions and casts, though some -more or less isolated stems have their structure preserved. Hence our -knowledge of these later Gymnosperms is far from complete. From the older -rocks, however, we have both impressions and microscopically preserved -material, and are more fully acquainted with them than with those which -lived nearer our own time. Hard, resistant leaves, which are so -characteristic of most of the living genera of Gymnosperms, seem to have -been also developed in the past members of the group, and these tend to -leave clear impressions in the rocks, so that we have reliable data for -reconstructing the external appearance of the fossil forms from the -Palaeozoic period. - -The resinous character of Gymnosperm wood probably greatly assisted its -preservation, and fragments of it are very common in rocks of all ages, -generally preserved in silica so as to show microscopic structure. The -isolated wood of Gymnosperms, however, is not very instructive, for from -the wood alone (and usually it is just fragments of the secondary wood -which are preserved) but little of either physiological or evolutional -value can be learned. When twigs with primary tissues and bark and leaves -attached are preserved, then the specimens are of importance, for their -true character can be recognized. Fortunately among the coal balls there -are many such fragments, some of which are accompanied by fruits and male -cones, so that we know much of the Palaeozoic Gymnosperms, and find that -in some respects they differ widely from those now living. - -There is, therefore, much more to be said about the fossil Gymnosperms -than about the Angiosperms, both because of the better quality of their -preservation and because their history dates back to a very much earlier -period than does the Angiospermic record. Indeed, we do not know when the -Gymnosperms began; the well-developed and ancient group of _Cordaiteae_ -was flourishing before the Carboniferous period, and must therefore date -back to the rocks of which we have no reliable information from this -point of view, and the origin of the Gymnosperms must lie in the -pre-Carboniferous period. - -The group of Gymnosperms includes a number of genera of different types, -most of which may be arranged under seven principal families. In a sketch -of this nature it is, of course, quite impossible to deal with all the -less-important families and genera. Those that will be considered here -are the following:-- - - Coniferales (see p. 90). - _Araucareae_, _e.g._ Monkey-puzzle - Genera both living and fossil. - Fossil forms undoubted so far back as the Jurassic, and - presumably further. - _Abietineae_, _e.g._ Pine and Larch - Genera both living and fossil. - Fossils recognized as far back as the Lower Cretaceous. - _Cupresseae_, _e.g._ Juniper, Cypress - Genera both living and fossil. - Fossils recognized as far back as the Jurassic. - _Taxeae_, _e.g._ Yew - Genera living and fossil. - Fossils recognized as far back as the Cretaceous. - Cordaitales (see p. 92). - _Cordaiteae_, _e.g._ Cordaites - Fossil only. - Characteristic of Devonian, Carboniferous, and Permian periods. - _Poroxyleae_, _e.g._ Poroxylon - Fossil only. - Characteristic of the Carboniferous and Permian. - Ginkgoales (see p. 98). - _Ginkgoaceae_, _e.g._ Ginkgo - Fossil and living, dating back, apparently with little change, - to Palaeozoic times. - -We must pay the most attention to the two last groups, as they are so -important as fossils, and the _Cordaiteae_ were a very numerous family in -Coal Measure times. They had their period of principal development so -long ago that it is probable that no direct descendants remain to the -present time, though some botanists consider that the _Taxeae_ are allied -to them. - -Of the groups still living it is difficult, almost impossible, to say -which is the highest, the most evolved type. In the consideration of the -Gymnosperm family it is brought home with great emphasis how incomplete -and partial our knowledge is as yet. Many hold that the _Araucareae_ are -the most primitive of the higher Gymnosperms. In support of this view the -following facts are noted. They have a simple type of fructification, -with a single seed on a simple scale, and many scales arranged round an -axis to form a cone. In the microscopic structure of their wood they have -double rows of bordered pits, a kind of wood cell which comes closer to -the old fossil types than does the wood of any of the other living -genera. Further than this, wood which is almost indistinguishable from -the wood of recent Araucarias is found very far back in the rocks, while -their leaves are broad and simple, and attached directly to the stem in a -way similar to the leaves of the fossil _Cordaiteae_, and very different -from the needle leaves on the secondary stems of the Pine family; so that -there appears good ground for considering the group an ancient and -probably a primitive one.[10] - -On the other hand, there are not wanting scientists who consider the -_Abietineae_ the living representatives of the most primitive and ancient -stock, though on the whole the evidence seems to indicate more clearly -that the Pine-tree group is specialized and highly modified. Their double -series of foliage leaves, their complex cones (whose structures are not -yet fully understood), and their wood all support the latter view. - -Some, again, consider the _Taxeae_ as a very primitive group, and would -place them near the Cordaiteae, with which they may be related. Their -fleshy seeds, growing not in cones but on short special axes, support -this view, and it is certainly true that in many ways the large seeds, -with their succulent coats and big endosperm, are much like those of the -lower Gymnosperms and of several fossil types. Those, however, who hold -to the view that the Abietineae are primitive, see in the _Taxeae_ the -latest and most modified type of Gymnosperm. - -It will be seen from this that there is no lack of variety regarding the -interpretation of Gymnosperm structures. - -The Gymnosperms do not stand in such an isolated position as do the -Angiosperms. Whatever the variety of views held about the details of the -relative placing of the families within the group, all agree in -recognizing the evidence which enables us to trace with confidence the -connection between the lower Gymnosperms and the families of ferns. There -are many indications of the intimate connection between higher and lower -Gymnosperms. Between the series exist what might be described as -different degrees of cousinship, and in the lower groups lie unmistakable -clues to their connection with more ancient groups in the past which -bridge over the gaps between them and the ferns. - -For the present, however, let us confine ourselves to the history of the -more important Gymnosperms, the discussion of their origin and the groups -from which they may have arisen must be postponed until the necessary -details about those groups have been mentioned. - -To a consideration of the living families of _Araucareae_, _Abietineae_, -_Cupresseae_, and _Taxeae_ we can allow but a short space; their general -characters and appearance are likely to be known to the reader, and their -details can be studied from living specimens if they are not. For -purposes of comparison with the fossils, however, it will be necessary to -mention a few of the principal features which are of special importance -in discussing phylogeny. - -The Araucariaceae are woody trees which attain a considerable size, with -broad-based, large leaves attached directly to the stem. In the leaves -are a series of numerous parallel vascular bundles. The wood cells in -microscopic section show two rows or more of round bordered pits. The -cones are very large, but the male and female are different in size and -organization. The female cone is composed of series of simple scales -arranged spirally round the axis, and each scale bears a single seed and -a small ligule. - -The pollen grains from the male cone are caught on the ligule and the -pollen tubes enter the micropyle of the ovule, bringing in passive male -cells which may develop in large numbers in each grain. The seeds when -ripe are stony, and some are provided with a wing from part of the tissue -of the scale. In the ripe cones the scales separate from the cone axis. - -The Abietineae are woody trees, some reaching a great height, all with a -strong main stem. The leaves are of two kinds: primary ones borne -directly attached to the stem (as in first-year shoots of the Larch), and -secondary ones borne in tufts of two (in Pine) or a large number (in -older branches of Larch) on special short branches, the primary leaves -only developing as brown scales closely attached to the stems. Leaves -generally very fine and needlelike, and with a central vascular bundle. -The wood in microscopic section shows a single row of round bordered pits -on the narrow tracheae. - -The female cones are large, male and female differing greatly in size and -organization. The female cone, composed of a spiral series of pairs of -scales, which often fuse together as the cone ripens. Each upper scale of -the pair bears two seeds. The pollen grains from the male cone enter the -micropyle of the seed and are caught in the tissue (apex of nucellus) -there; the pollen tubes discharge passive male cells, only two of which -develop in each grain. The seeds when ripe are stony and provided with a -wing from the tissue of the scale on which they were borne. - -The Cupresseae are woody trees reaching no great height, and of a bushy, -branching growth. The leaves are attached directly to the main stem, and -arrange themselves in alternating pairs of very small leaves, closely -pressed to the stem. The wood in microscopic section shows a single row -of round bordered pits on the tracheae. - -The cones are small, and the scales forming them arranged in cycles. The -female scales bear a varying number of seeds. The pollen grain has two -passive male cells. The seeds when ripe are stony, with wings, though in -some cases (species of Juniper) the cone scales close up and become -fleshy, so that the whole fruit resembles a berry. - -The Taxeae are woody, though not great trees, bushily branched. The leaves -are attached spirally all round the stem, but place themselves so as to -appear to lie in pairs arranged in one horizontal direction. The wood in -microscopic section shows a single row of round bordered pits on the -tracheae. - -There are small male cones, but the seeds are not borne on cones, growing -instead on special short axes, where there may be several young ovules, -but on which usually two seeds ripen. The seeds are big, and have an -inner stone and outer fleshy covering. Some have special outer fleshy -structures known as "arils", _e.g._ the red outer cup round the yew -"berry" (which is not a berry at all, but a single unenclosed seed with a -fleshy coat). - -When we turn to the Cordaiteae we come to a group of plants which bears -distinct relationship to the preceding, but which has a number of -individual characters. It is a group of which we should know nothing were -it not for the fossils preserved in the Palaeozoic rocks; yet, -notwithstanding the fact that it flourished so long ago, it is a family -of which we know much. At the time of the Coal Measures and the -succeeding Permo-carboniferous period, it was of great importance, and, -indeed, in some of the French deposits it would seem as though whole -layers of coal were composed entirely of its leaves. - -Among the fossil remains of this family there are impressions, casts, and -true petrifactions, so that we know both its external appearance and the -internal anatomy of nearly every part of several species of the genus. -For a long time the various fossil remains of the plant were not -recognized as belonging to each other and together forming the records of -one and the same plant--the broad, long leaves with their parallel veins -were looked on as Monocotyledons (see fig. 61); the pith casts (see fig. -63) were thought to be peculiar constricted stems, and were called -_Sternbergia_; while the wood, which was known from its microscopic -structure, was called _Araucarioxylon_--but the careful work of many -masters of fossil botany, whose laborious studies we cannot describe in -detail here, brought all these fragments together and proved them to -belong to _Cordaites_. - - [Illustration: Fig. 61.--Leaf of _Cordaites_, _l_, attached by its - broad base to a Stem, _s_] - -We now know that _Cordaites_ were large trees, with strong upright shafts -of wood, to whose branches large simple leaves were attached. The leaves -were much bigger than those of any living Gymnosperm, even than those of -the Kauri Pine (a member of the Araucariaceae), and seem in some species -to have exceeded 3 ft. in length. The trees branched only at the top of -the main shaft, and with their huge sword-like leaves must have differed -greatly in appearance from any plant now living. The leaves had many -parallel veins, as can be seen in fig. 61, and were attached by a broad -base directly to the main stem; thus coming closer to the Araucarias than -the other groups of Gymnosperms in their leaf characters. - - [Illustration: Fig. 62A.--Microscopic Section of Part of a Leaf of - _Cordaites_ - - V, Vascular bundle; W, wood of bundle; _sh_, its sheath; S^1, large - sclerenchyma mass alternating with bundles; S^2 and S^3, sclerenchyma - caps of bundle; P, soft tissue of leaf.] - -The internal anatomy is often well preserved, and there is a number of -species of leaves whose anatomy is known. As will be expected from the -parallel veins, in each section there are many vascular bundles running -equidistantly through the tissue. Fig. 62A shows the microscopic details -from a well-preserved leaf. In all the species patches of sclerenchyma -were developed, and everything indicates that they were tough and well -protected against loss of water, even to a greater extent than are most -of the leaves of living Gymnosperms. - -In the stems the pith was much larger than that in living Gymnosperms -(where the wood is generally very solid), and it was hollow in older -stems, except for discs of tissue across the cavity. The internal cast -from these stems has been described before, and is seen in fig. 63. - - [Illustration: Fig. 62B.--Much-magnified Wood Elements from _Cordaites_ - Stem seen in longitudinal section, the type known as _Araucarioxylon_. - Note the hexagonal outlines of the bordered pits, which lie in several - rows] - -The wood was formed in closely packed radiating rows by a normal cambium -(see p. 66), and the tracheae so formed had characteristic rows of -bordered pits (see fig. 62B). The wood comes nearer to that of the living -Araucarias than any other, and indeed the numerous pieces of fossil wood -of this type which are known from all the geological periods are called -_Araucarioxylon_.[11] A double strand goes out from the main mass of -wood, which afterwards divides and subdivides to provide the numerous -bundles of the leaf. - - [Illustration: Fig. 63.--Cast of Hollow Pith of _Cordaites_, the - constrictions corresponding to discs of solid tissue across the cavity] - -In the case of these fossils we are fortunate enough to have the -fructifications, both male and female, in a good state of preservation. -As in other Gymnosperms, the male and female cones are separate, but they -differed less from each other in their arrangement than do those of any -of the living types hitherto mentioned. They can hardly be described as -true cones, though they had something of that nature; the seeds seem to -be borne on special short stems, round which are also sterile scales. In -the seed and the way it is borne perhaps the Cordaiteae may be compared -more nearly with the Taxeae than with the other groups. A seed, not yet -ripe, is shown in slightly diagrammatic form in fig. 64, where the -essential details are illustrated. The seeds of this family sometimes -reached a considerable size, and had a fleshy layer which was thick in -comparison with the stone, and externally comparable with a -cherry--though, of course, of very different nature in reality, for -_Cordaites_, like _Taxus_, is a Gymnosperm, with simple naked seeds, -while a cherry is the fruit of an Angiosperm. - -In a few words, these are the main characters of the large group of -_Cordaites_, which held the dominant position among Gymnosperms in the -Palaeozoic era. They have relationships, or perhaps one should say -likenesses, to many groups. Their stem- and root-anatomy is similar to -the Coniferae of the present day, the position of the ovules is like that -in the Taxaceae, the male cones in some measure recall those of _Ginkgo_, -the anatomy of their leaves has points which are comparable with those of -the Cycads, to which group also the large pith in the stem and the -structure of some details in the seeds unite them. Their own specially -distinctive characters lie in their crown of huge leaves, and unbranched -shaft of stem, the similarity of their male and female inflorescences, -and some points in their pollen grains which have not been mentioned. The -type is a very complex one, possibly coming near the stock which, having -branched out in various directions, gave rise to several of the living -families. - - [Illustration: Fig. 64.--Representation of _Cordaites_ Seed and its - Axis with Scales, slightly diagrammatic, modified from Renault. - - A, Axis with _s_, scales; _c_, coat of the seed, from which the inner - parts have shrunk away; _n_, nucellus; _p.c_, pollen chamber containing - pollen grains which enter through _m_.] - -Plants which come very near to the Cordaiteae are the Poroxyleae. Of this -group we have unfortunately no remains of fructifications in organic -connection, so that its actual position must remain a little doubtful -till they are discovered. There seems no doubt that they must have borne -seeds. - -Still, it has been abundantly demonstrated in recent years that the -anatomy of the root, stem, and leaves indicates with considerable -exactness the position of any plant, so that, as these are known, we can -deduce from them, with a feeling of safety, the position that _Poroxylon_ -takes in the natural system. In its anatomy the characters are those of -the Cordaiteae, with certain details which show a more primitive nature -and seem to be characteristic of the groups below it in organization. - -_Poroxylon_ is not common, and until recently had not been found in the -Lower Coal Measures of England. The plants appear to have been much -smaller than _Cordaites_, with delicate stems which bore relatively large -simple leaves. The anatomy of the root was that common in Gymnosperms, -but the stem had a very large pith, and the leaves were much like those -of _Cordaites_ in having parallel veins. An important character in the -anatomy of the stem was the presence of what is known as _centripetal -wood_. This must be shortly explained. In all the stems hitherto -considered, the first-formed wood cells (protoxylems, see p. 57) -developed at the central point of the wood, towards the pith (see fig. -19, _px_, p. 49). This is characteristic of all Angiosperms and the -higher Gymnosperms (except in a couple of recently investigated Pines), -but among the lower plants we find that part of the later wood develops -to the inner side of these protoxylem masses. The distinction is shown in -fig. 65. - - [Illustration: Fig. 65.--A, Normal bundle of higher plant; _x_, - protoxylem on inner side next the pith _p_, and the older wood _w_ - outside it, _centrifugal_ wood. B, Bundle with wood cells _c_ developed - on inner side of protoxylem, _centripetal_ wood; the arrow indicates - the direction of the centre of the stem.] - -This point is one to which botanists have given much attention, and on -which they have laid much weight in considering the affinities of the -lower Gymnosperms and the intermediate groups between them and the ferns, -which are found among the fossils. In _Cordaites_ this point of -connection with the lower types is not seen, but in _Poroxylon_, which -has otherwise a stem anatomy very similar to _Cordaites_, we find groups -of _centripetal_ wood developed inside the protoxylem of primary bundles. -For this reason, principally, is _Poroxylon_ of interest at present, as -in its stem anatomy it seems to connect the _Cordaites_ type with that of -the group below it in general organization. - - -Ginkgoales.--Reference to p. 44 shows that _Ginkgo_, the Maidenhair tree, -belongs to the Ginkgoales, a group taking equal rank with the large and -complex series of the Coniferales. The Ginkgoales of the present day, -however, have but one living representative. _Ginkgo_ stands alone, the -single living species of its genus, representing a family so different -from any other living family that it forms a prime group by itself. - -Had the tree not been held sacred in China and Japan, it is probable that -it would long since have been extinct, for it is now known only in -cultivation. It is indeed a relic from the past which has been -fortunately preserved alive for our examination. It belongs to the fossil -world, as a belated November rose belongs to the summer. - -Because of its beauty and interest the plant is now widely distributed -under cultivation, and is available for study almost as freely as the -other types of living Gymnosperms already mentioned, so that but a short -summary of its more important features is needed here. - -Old plants, such as can be seen growing freely in Japan (in Kew Gardens -there is also a fine specimen), are very tall handsome woody trees, with -noble shafts and many branches. The leaves grow on little side shoots and -are the most characteristic external feature of the tree; their living -form is illustrated in fig. 66, which shows the typical simple shape as -well as the lobed form of the leaf which are to be found, with all -intermediate stages, on the same tree. No other plant (save a few ferns, -which can generally be distinguished from it without difficulty) has -leaves at all like these, so that it is particularly easy to identify the -fossil remains, of which there are many. - - [Illustration: Fig. 66.--A, Tuft of _Ginkgo_ Leaves, showing their - "maidenhair"-like shape. B, Single deeply-divided Leaf to be found on - the same tree, usually on young branches.] - -The wood is compact and fine grained, the rings of secondary tissue being -developed from a normal cambium as in the case of the higher Gymnosperms, -and the individual tracheae have round bordered pits. There are small male -cones, but the seeds are not borne in cones. They develop on special -stalks on which are no scales, but a small mass of tissue at the base of -the seed called the "collar". Usually there are two young ovules, of -which often only one ripens to a fleshy seed, though both may mature. - - [Illustration: Fig. 67.--Ripe Stage of _Ginkgo_ Seeds attached to their - Stalk. _c_, "Collar" of seed.] - -The ripe seed reaches the size shown in the diagram, and is orange -coloured and very fleshy; within it is a stone encasing the endosperm, -which is large, _green_, and starchy, and contains the embryo with two -cotyledons. This embryo is small compared with the endosperm, cf. fig. -57, p. 76, which is somewhat similar to that of _Ginkgo_ in this stage. - -Of the microscopic characters of the reproductive organs the most -remarkable is the male cell. This is not a passive nucleus, as in the -plants hitherto considered, but is an _actively swimming_ cell of some -size, provided with a spiral of cilia (hairlike structures) whose -movements propel it through the water. In the cavity of the unripe seed -these swim towards the female cell, and actively penetrate it. The -arrangements of the seed are diagrammatically shown in fig. 68, which -should be compared with that of _Cycas_, fig. 76, with which it has many -points in common. - - [Illustration: Fig. 68.--Section through Seed of _Ginkgo_ - - _p.c_, Pollen chamber in the nucellus _n_, which is fused to the coat - _c_ to the level _l_; _sc_, stony layer in coat; S, the big spore, - filled with endosperm tissue (in this case green in colour); _e_, egg - cells, one of which will produce the embryo after fertilization.] - -The nature of the male cell in _Cordaites_ is not yet known, but there is -reason to suspect it may have been actively swimming also. As this is -uncertain, however, we may consider _Ginkgo_ the most highly organized -plant which has such a primitive feature, a feature which is a bond of -union between it and the ferns, and which, when it was discovered about a -dozen years ago, caused a considerable sensation in the botanical world. - -To turn now to the fossil records of this family. Leaf impressions of -_Ginkgo_ are found in rocks of nearly all ages back even to the Upper -Palaeozoic. They show a considerable variety of form, and it is certain -that they do not all belong to the same _species_ as the living plant, -but probably they are closely allied. Fig. 69 shows a typical impression -from the Lower Mesozoic rocks. In this specimen, the cells of the -epidermis were fortunately sufficiently well preserved to be seen with -the microscope, and there is a distinct difference in the size and shape -of the cells of living and fossil species, see fig. 70; but this -difference is slight as compared with the great similarity of form and -appearance, as can be seen on comparing figs. 69 and 66, B, so that the -fossil is at the most a different species of the genus _Ginkgo_. Among -the fossil leaves there is greater variety than among the living ones, -and some which are very deeply lobed so as to form a divided palm-like -leaf go by different names, e.g. _Baiera_, but they are supposed to -belong to the same family. Fossil seeds and male cones are also known as -impressions, and are found far back in the Mesozoic rocks. From the -fossil impressions it is certain that _Ginkgo_ and plants closely allied -to it were very widespread in the past, as they are found all over Europe -as well as the other continents. Particularly in the Lower Mesozoic rocks -_Ginkgo_ seems to have been a world-wide type growing in great abundance. - - [Illustration: Fig. 69.--Leaf Impression of _Ginkgo_ from Mesozoic - Rocks of Scotland] - - [Illustration: Fig. 70.--Showing Epidermis with Stomates from the lower - side of the Leaf seen in fig. 69 - - _e_, Epidermis cells; _s_, stomates; _v_, long cells of epidermis lying - over the veins.] - -In the Palaeozoic the records are not so undoubted, but there is strong -evidence which leads us to suppose that if the genus now living were not -then extant, at least other closely related genera were, and there seems -to be good grounds for supposing that _Ginkgo_ and _Cordaites_ may have -both arisen from some ancient common stock. - - - - - CHAPTER X - PAST HISTORIES OF PLANT FAMILIES - III. The Bennettitales - - -This fascinating family is known only from the fossils, and is so remote -in its organization from any common living forms that it may perhaps be a -little difficult for those who do not know the Cycads to appreciate the -position of Bennettites. It would probably be better for one studying -fossil plants for the first time to read the chapters on the Cycads, -Pteridosperms, and Ferns before this chapter on the present group, which -has characters connecting it with that series. - -Until recently the bulk of the fossils which are found as impressions of -stems and foliage of this family were very naturally classed as Cycads. -They are extremely common in the Mesozoic rocks (the so-called Age of -Cycads), and in the external appearance of both stems and leaves they are -practically identical with the Cycads. - -A few incomplete fructifications of some species have been known in -Europe for many years, but it is only recently that they have been fully -known. This is owing to Wieland's[12] work on the American species, which -has made known the complete organization of the fructifications from a -mass of rich and well-petrified material. - -In the Lower Cretaceous and Upper Jurassic rocks of America these plants -abound, with their microscopic structure well preserved, and their -fructifications show an organization of a different nature from that of -any past or present Cycad. - -Probably owing to their external appearance, Wieland describes the plants -as "Cycads" in the title of his big book on them; but the generic name he -uses, _Cycadeoidea_, seems less known in this country than the equally -well-established name of _Bennettites_, which has long been used to -denote the European specimens of this family, and which will be used in -the following short account of the group. - -At the present time no family of fossils is exciting more interest. Their -completely Cycadean appearance and their unique type of fructification -have led many botanists to see in them the forerunners of the -Angiosperms, to look on them as the key to that mystery--the origin of -the flowering plants. This position will be discussed and the many facts -in its favour noted, but we must not forget that the _Bennettitales_ have -only recently been realized fully by botanists, and that a new toy is -ever particularly charming, a new cure particularly efficacious, and a -new theory all-persuasive. - -From their detailed study of the flowering plants botanists have leaned -toward different groups as the present representatives of the primitive -types. The various claims of the different families to this position -cannot be considered here; probably that of the Ranales (the group of -families round Ranunculaceae as a central type) is the best supported. Yet -these plants are most frequently delicate herbs, which would have stood -relatively less chance of fossilization than the other families which may -be considered primitive. They are peculiarly remote from the group of -Bennettiteae in their vegetative structure, a fact the importance of which -seems to have been underrated, for in the same breath we are assured that -the Bennettites are a kind of cousin to the ancient Angiosperms, and that -the Ranales are among the most primitive living Angiosperms, and -therefore presumably nearest the ancient ones. - -However, let us leave the charms of controversy on one side and look at -the actual structure of the group. They were widely spread in Lower -Mesozoic times, the plants being preserved as casts, impressions, and -with structure in great numbers. The bulk of the described structural -specimens have been obtained from the rocks of England, France, Italy, -and America, although leaf impressions are almost universally known. The -genus _Williamsonia_ belongs to this family, and is one of the best known -of Mesozoic plant impressions. - -Externally the Bennettiteae were identical in appearance with stumpy -Cycads, and their leaves it is which gave rise to the surmise, so long -prevalent, that the Lower Mesozoic was the "Age of Cycads", just as it -was the Pteridosperm leaves that gave the Palaeozoic the credit of being -the "Age of Ferns". In the anatomy of both stem and leaf, also, the -characters are entirely Cycadean; the outgoing leaf trace is indeed -simpler in its course than that of the Cycads. - - [Illustration: Fig. 71.--Half of a Longitudinal Section through a - Mature Cone of _Bennettites_ - - A, Short conical axis; _s_, enclosing bracts; S, seeds; _sc_, sterile - scales between the seeds.] - -The fructifications, however, differ fundamentally from those of the -Cycads, as indeed they do from those of any known family. They took the -form of compact cones, which occurred in very large numbers in the mature -plants hidden by the leaf bases. In _Williamsonia_, of which we know much -less detail, the fructifications stood away from the main axis on long -pedicels. - -In _Bennettites_ the cones were composed of series of sheathing scales -surrounding a short conical axis on which stood thin radiating stalks, -each bearing a seed. Between them were long-stalked sterile scales with -expanded ends. A part of a cone is illustrated diagrammatically in fig. -71. The whole had much the appearance of a complex fruit. In some -specimens these features alone are present in the cones, but in younger -cones from the American plants further structures are found attached. -Below the main axis of the seed-bearing part of the cone was a series of -large complex leaflike structures closely resembling fern leaves in their -much-divided nature. On the pinnae of these leaves were crowded -innumerable large sporangia, similar to those of a fern, which provided -the pollen grains. The fossils are particularly well preserved, and have -been found with these male (pollen-bearing) organs in the young unopened -stages, and also in the mature unfolded condition, as well as the -ripening seed cones from which they have faded, just as the stamens fade -from a flower when the seeds enlarge. - - [Illustration: Fig. 72.--Diagram of Complete Cone of _Bennettites_ - - A, Central axis of conical shape terminating in the seed-bearing cone - S. (After Wieland), and bearing successively Br., bracts, comparable - with floral leaves; M, large complex leaves with pollen sacs.] - -It appears that these huge complex leaflike structures were really -stamens, but nevertheless they were rolled up in the circinate form as -are young fern leaves, and as they unrolled and spread out round the -central cone they must have had the appearance of a whorl of leaves (see -fig. 72). - -This, in a few words, is the main general character of the -fructification. The most important features, on which stress is laid, are -the following. The association of the male and female structures on the -same axis, with the female part _above_ the male. This arrangement is -found only in the flowering plants; the lower plants, which have male and -female on the same cone, have them mixed, or the female below, and are in -any case much simpler in their entire organization. The conical form of -the axis is also important, as is the fact that it terminates in the -seed-bearing structures. - - [Illustration: Fig. 73.--Diagram of Cross Section of _Bennettites_, - Seed, with Embryo - - _c_, Double-layered seed coat; _n_, crushed nucellus; _cot._, two - cotyledons which practically fill the seed.] - -The position of the individual seeds, each on the end of a single stalk, -is remarkable, as are the long-stalked bracts whose shield-like ends join -in the protection of the seeds. These structures together give the cone -much of the appearance of a complex fruit of a flowering plant, but the -structure of the seeds themselves is that of a simple Gymnosperm. - -In the seeds, however, was an _embryo_. In this they differ from all -known seeds of an earlier date, which, as has been already noted (see p. -77), are always devoid of one. This embryo is one of the most important -features of the plant. It had two cotyledons which filled the seed space -(see fig. 73), and left almost no trace of the endosperm. Reference to p. -112 will show that this is an advance on the Cycad seed, which has a -small embryo embedded in a large mass of endosperm, and that it -practically coincides with the Dicotyledonous type. - -The seed with its embryo suggested comparison with the Angiosperms long -before the complete structure of the fructification was known. - -The fern-like nature of the pollen-bearing structures is another very -important point. Were any one of these leaflike "stamens" found isolated -its fern-like nature would not have been questioned a year or two ago, -and their presence in the "flower" of _Bennettites_ is a strong argument -in favour of the Fern-Pteridosperm affinities of the group. - -Had the parts of this remarkable fructification developed on separate -trees, or on separate branches or distinct cones of the same one, they -would have been much less suggestive than they are at present, and the -fructifications might well have been included among those of the -Gymnosperms, differing little more (apart from the embryo) from the other -Gymnosperm genera than they do from each other. In fact, the extremely -fern-like nature of the male organs is almost more suggestive of a -Pteridosperm affinity, for even the simplest Cycads have well-marked -scaly cones as their male organs. The female cone, again, considered as -an isolated structure, can be interpreted as being not vitally different -from _Cordaites_, where the seeds are borne on special short stalks -amidst scales. - -The embryo would, in any case, point to a position among advanced types; -but it is so common for one organ of a plant to evolve along lines of its -own independently, or in advance of the other organs, that the embryo -structure alone could not have been held to counterbalance the Cycadean -stems and leaves, the Pteridosperm-like male organs, and the Gymnospermic -seeds. - -But all these parts occur on the same axis, arranged in the manner -typical of Angiosperms. The seed-bearing structures at the apex, the -"stamens" below them, and a series of expanded scales below these again, -which it takes little imagination to picture as incipient petals and -sepals; and behold--the thing is a flower! - -And being a "flower", is in closest connection with the ancestors of the -modern flowering plants, which must consequently have evolved from some -Cycadean-like ancestor which also gave rise to the Bennettitales. Thus -can the flowering plants be linked on to the series that runs through the -Cycads directly to the primitive ferns! - -It is evident that this group, of all those known among the fossils, -comes most closely to an approximation of Angiospermic structure and -arrangement. Enough has been said to show that in their actual nature -they are not Angiosperms, though they have some of their characters, -while at the same time they are not Cycads, though they have their -appearance. They stand somewhere between the two. Though many botanists -at present hold that this mixture of characters indicates a relationship -equivalent to a kind of cousinship with the Angiosperms, and both groups -may be supposed to have originated from a Cycadean stock, this theory has -not yet stood the test of time, nor is it supported by other evidence -from the fossils. We will go so far as to say that it appears as though -_some_ Angiosperms arose in that way; but flowering plants show so many -points utterly differing from the whole Cycadean stock that a little -scepticism may not be unwholesome. - -It is well to remember the Lycopods, where (as we shall see, p. 141) -structures very like seeds were developed at the time when the Lycopods -were the dominant plants, and we do not find any evidence to prove that -they led on to the main line of seed plants. Similarly, Cycads may have -got what practically amounted to flowers at the time when they were the -dominant group, and it is very conceivable that they did not lead on to -the main line of flowering plants. - -Whatever view may be held, however, and whatever may be the future -discoveries relating to this group of plants, we can see in the -Bennettitales points which throw much light on the potentialities of the -Cycadean stock, and structures which have given rise to some most -interesting speculations on the subject of the Angiosperms. This group is -another of the jewels in the crown of fossil botany, for the whole of its -structures have been reconstructed from the stones that hold all that -remains of this once extensive and now extinct family of plants. - - - - - CHAPTER XI - PAST HISTORIES OF PLANT FAMILIES - IV. The Cycads - - -The group of the _Cycadales_, which has a systematic value equivalent to -the _Ginkgoales_, contains a much larger variety of genera and species -than does the latter. There are still living nine genera, with more than -a hundred and fifty species, which form (though a small one compared with -most of the prime groups) a well-defined family. They are the most -primitive Gymnosperms, the most primitive seed-bearing plants now living, -and in their appearance and characters are very different from any other -modern type. Their external resemblance to the group of the -Bennettitales, however, is very striking, and indeed, without the -fructifications it would be impossible to distinguish them. - -The best known of the genera is that of _Cycas_, of which an illustration -is given in fig. 74. The thick, stumpy stem and crown of "palm"-like -leaves give it a very different appearance from any other Gymnosperm. -Commonly the plants reach only a few feet in height, but very old -specimens may grow to the height of 30 ft. or more. The other genera are -smaller, and some have short stems and a very fern-like appearance, as, -for example, the genus _Stangeria_, which was supposed to be a fern when -it was first discovered and before fruiting specimens had been seen. - -The large compound leaves are all borne directly on the main stem, -generally in a single rosette at its apex, and as they die off they leave -their fleshy leaf bases, which cover the stem and remain for an almost -indefinite number of years. - -The wood of the main trunks differs from that of the other Gymnosperms in -being very loosely built, with a large pith and much soft tissue between -the radiating bands of wood. There is a cambium which adds zones of -secondary tissue, but it does not do its work regularly, and the cross -section of an old Cycad stem shows disconnected rings of wood, -accompanied by much soft tissue. The cells of the wood have bordered pits -on their walls, and in the main axis the wood is usually all developed in -a centrifugal direction, but in the axis of the cones some centripetal -wood is found (refer to _c_, fig. 65, p. 97). - - [Illustration: Fig. 74.--Plant of _Cycas_, showing the main stem with - the crown of leaves and the irregular branches which come on an old - plant] - -In their fructifications the Cycads stand even further apart from the -rest of the Gymnosperms. One striking point is the enormous size of their -_male_ cones. The male cones consist of a stout axis, round which are -spiral series of closely packed simple scales covered with pollen-bearing -sacs (which bear no inconsiderable likeness to fern sporangia), the whole -cone reaching 1-1/2 ft. in length in some genera, and weighing several -pounds. All the other Gymnosperms, except the Araucareae, where they are -an inch or two long, have male cones but a fraction of an inch in length. - - [Illustration: Fig. 75.--Seed-bearing Scale of _Cycas_, showing its - lobed and leaflike character - - _s_, Seeds attached on either side below the divisions of the - sporophyll.] - -In all the members of the family, excepting _Cycas_ itself, the female -fructifications also consist of similarly organized cones bearing a -couple of seeds on each scale instead of the numerous pollen sacs. In -_Cycas_ the male cones are like those of the other genera, and reach an -enormous size; but there are no female cones, for the seeds are borne on -special leaflike scales. These are illustrated in fig. 75, which shows -also that there are not two seeds (as in the other genera with cones) to -each scale, but an indefinite number. - -The leafy nature of the seed-bearing scale is an important and -interesting feature. Although theoretically botanists are accustomed to -accept the view that seeds are always borne on specially modified leaves -(so that to a botanist even the "shell" of a pea-pod and the box of a -poppy capsule are leaves), yet in _Cycas_ alone among living plants are -seeds really found growing on a large structure which has the appearance -of a leaf. Hence, from this point of view (see p. 45, however, for a -caution against concluding that the whole plant is similarly lowly -organized), _Cycas_ is the most primitive of all the living plants that -bear seeds, and hence presumably the likest to the fossil ancestors of -the seed-bearing types. In this character it is more primitive than the -fossil group of the _Cordaiteae_, and comes very close to an intermediate -group of fossils to be considered in the next chapter. - - [Illustration: Fig. 76.--Seed of _Cycas_ cut open - - _n_, The nucellus, fused at the level _l_ to the coat _c_; _sc_, stony - layer of coat; _p.c_, pollen chamber in apex of the nucellus; S, - "spore", filled with endosperm, in which lies the embryo _e_.] - -To enter into the detailed anatomy of the seeds would lead us too far -into the realms of the specialist, but we must notice one or two points -about them. Firstly, their very large size, for ripe seeds of _Cycas_ are -as large as peaches (and peaches, it is to be noted, are fruits, not -seeds), and particularly the large size they attain _before_ they are -fertilized and have an embryo. Among the higher plants the young seeds -remain very minute until an embryo is secured by the act of -fertilization, but in the Cycads the seeds enlarge and lay in a big store -of starch in the endosperm before the embryo appears, so that in the -cases in which fertilization is prevented large, sterile "seeds" are -nevertheless produced. This must be looked on as a want of precision in -the mechanism, and as a wasteful arrangement which is undeniably -primitive. An even more wasteful arrangement appears to have been common -to the "seeds" of the Palaeozoic period, for, though many fossil "seeds" -are known in detail from the old rocks, not one is known to have any -trace of an embryo. A general plan of the _Cycas_ seed is shown in fig. -76, which should be compared with that of _Ginkgo_ (fig. 68). The large -size of the endosperm and the thick and complex seed-coats are -characteristic features of both these structures. Another point that -makes the Cycad seeds of special interest is the fact that the male cells -(as in _Ginkgo_) are developed as active, free-swimming sperms, which -swim towards the female cell in the space provided for them in the seed -(see _p.c_, fig. 76). - -The characters of the Cycads as they are now living prove them to be an -extremely primitive group, and therefore presumably well represented -among the fossils; and indeed among the Mesozoic rocks there is no lack -of impressions which have been described as the leaves of Cycads. There -is, however, very little reliable material, and practically none which -shows good microscopic structure. Leaf impressions alone are most -unsafe--more unsafe in this group, perhaps, than in any other--for -reasons that will be apparent later on, and the conclusions that used to -be drawn about the vast number of Cycads which inhabited the globe in the -early Mesozoic must be looked on with caution, resulting from the -experience of recent discoveries proving many of these leaves to belong -to a different family. - -There remain, however, many authentic specimens which show that _Cycas_ -certainly goes back very far in history, and specimens of this genus are -known from the older Mesozoic rocks. We cannot say, however, as securely -as used to be said, that the Mesozoic was the "Age of Cycads", although -it was doubtless the age of plants which had much of the external -appearance of Cycads. - -From the Palaeozoic we have no reliable evidence of the existence of -Cycads, though the plants of that time included a group which has an -undoubted connection with them. - -Indeed, so far as fossil evidence goes, we must suppose that the Cycads, -since their appearance, possibly at the close of the Palaeozoic, have -never been a dominant or very extensive family, though they grew in the -past all over the world, and in Europe seem to have remained till the -middle of the Tertiary epoch. - - - - - CHAPTER XII - PAST HISTORIES OF PLANT FAMILIES - V. Pteridosperms - - -This group consists entirely of plants which are extinct, and which were -in the height of their development in the Coal Measure period. As a group -they are the most recently discovered in the plant world, and but a few -years ago the name "Pteridosperm" was unknown. They form, however, both -one of the most interesting of plant families and one of the most -numerous of those which flourished in the Carboniferous period. - -To mention first the vital point of interest in their structure, they -show _leaves which in all respects appear like ordinary foliage leaves, -and yet bear seeds_. These leaves, which we now know bore the seeds, had -long been considered as typical fern leaves, and had been named and -described as fern leaves. There are two extremely important results from -the discovery of this fossil group, viz. that leaves, to all appearance -like ordinary foliage, can directly bear seeds, and that the leaves, -though like fern leaves, bore seeds like those of a Cycad. - -As the name _Pteridosperm_ indicates, the group is a link between the -ferns and the seed-bearing plants, and as such is of special interest and -value to botanists. - -The gradual recognition of this group from among the numerous plant -fragments of Palaeozoic age is one of the most interesting of the -accumulative discoveries of fossil botany. Ever since fossil remains -attracted the attention of enquiring minds many "ferns" have been -recognized among the rich impressions of the Coal Measures. Most of them, -however, were not connected with any structural material, and were given -many different names of specific value. So numerous were these fern -"species" that it was supposed that in the Coal Measure period the ferns -must have been the dominant class, and it is often spoken of even yet as -the "Age of Ferns". From the rocks of the same age, preserved with their -microscopical structure perfect, were stems which were called -_Lyginodendron_. In the coal balls associated with these stems (which -were the commonest of the stems so preserved) were also roots, petioles, -and leaflets, but they were isolated, like the most of the fragments in a -coal ball, and to each was given its name, with no thought of the various -fragments having any connection with each other. Gradually, however, -various fragments from the coal balls had been recognized as belonging -together; one specimen of a petiole attached to a stem sufficed to prove -that all the scattered petioles of the same type belonged also to that -kind of stem, and when leaves were found attached to an isolated fragment -of the petiole, the chain of proof was complete that the leaves belonged -to the stem, and so on. By a series of lengthy and painstaking -investigations all the parts of the plant now called _Lyginodendron_ have -been brought together, and the impressions of its leaves have been -connected with it, these being of the fernlike type so long called -_Sphenopteris_, illustrated in fig. 77. - - [Illustration: Fig. 77.--_Sphenopteris_ Leaf Impression, the fernlike - foliage of _Lyginodendron_] - - [Illustration: Fig. 78A.--Diagram of the Transverse Section of Stem of - the _Lyginodendron_ - - _p_, Pith; P, primary wood groups; W, secondary wood; _l.t_, leaf - trace; _s_, sclerized bands in the cortex; S, longitudinal view of wood - elements to show the rows of bordered pits.] - -The anatomy of the main stem is very suggestive of that of a Cycad. The -zones of secondary wood are loosely built, the quantity of soft tissue -between the radiating bands of wood, and the size of the pith being -large, while from the main axis double strands of wood run out to the -leaf base. The primary bundles, however, are not like those of a Cycad -stem, but have groups of _centripetal_ wood within the protoxylem, and -thus resemble the primary bundles of _Poroxylon_ (see p. 97), which are -more primitive in this respect than those of the Cycads. - -The roots of _Lyginodendron_, when young, were like those of the -Marattiaceous ferns, their five-rayed mass of wood being characteristic -of that family, and different from the type of root found in most other -ferns (cf. fig. 78B with fig. 35 on p. 60). Unlike fern roots of any -kind, however, they have well-developed zones of secondary wood, in which -they approach the Gymnospermic roots (see fig. 78B, _s_). - - [Illustration: Fig. 78B.--Transverse Section of Root of _Lyginodendron_ - - _w_, Five-rayed mass of primary wood; _s_, zone of secondary wood; _c_, - cortical and other soft tissues.] - -A further mixture of characters is seen in the vascular bundles of the -petioles. A double strand, like that in the lower Gymnosperms, goes off -to the leaf base from the main axis, but in the petiole itself the bundle -is like a normal fern stele, and shows no characters in transverse -section which would separate it from the ferns. Such a petiole is -illustrated in fig. 79, with its V-shaped fernlike stele. On the petioles -and stems were certain rough, spiny structures of the nature of complex -hairs. In some cases they are glandular, as is seen in _g_ in fig. 79, -and as they seem to be unique in their appearance they have been of great -service in the identification of the various isolated organs of the -plant. - -As is seen from fig. 77, the leaves were quite fern-like, but in -structural specimens they have been found with the characteristic -glandular hairs of the plant. - -The seeds were so long known under the name of _Lagenostoma_ that they -are still called by it, though they have been identified as belonging to -_Lyginodendron_. They were small (about 1/4 in. in maximum length) when -compared with those of most other plants of the group, or of the Cycads, -with which they show considerable affinity. They are too complex to -describe fully, and have been mentioned already (see p. 76), so that they -will not be described in much detail here. The diagrammatic figure (fig. -56) shows the essential characters of their longitudinal section, and -their transverse section, as illustrated in fig. 80, shows the complex -and elaborate mechanism of the apex. - - [Illustration: Fig. 79.--Transverse Section through Petiole of - _Lyginodendron_ - - _v_, Fern-like stele; _c_, cortex; _g_, glandular hairlike - protuberances.] - -Round the "seed" was a sheath, something like the husk round a hazel nut, -which appears to have had the function of a protective organ, though what -its real morphological nature may have been is as yet an unsolved -problem. On the sheath were glandular hairs like those found on the -petiole and leaves, which were, indeed, the first clues that led to the -discovery of the connection between the seed and the plant -_Lyginodendron_. - -The pollen grains seem to have been produced in sacs very like fern -sporangia developed on normal foliage leaves, each grain entered the -cavity _pc_ in the seed (see fig. 56), but of the nature of the male cell -we are ignorant. In none of the fossils has any embryo been found in the -"seeds", so that presumably they ripened, or at least matured their -tissues, before fertilization. - -These, in a few words, are the essentials of the structures of -_Lyginodendron_. But this plant is only one of a group, and at least two -other of the Pteridosperms deserve notice, viz. _Medullosa_, which is -more complex, and _Heterangium_, which is simpler than the central type. - - [Illustration: Fig. 80.--Diagram of Transverse Section of _Lagenostoma_ - Seed near the Apex, showing the nine flutings _f_ of the coat _c_; _v_, - the vascular strand in each; _nc_, cone of nucellar tissue standing up - in the fluted apex of the nucellus _n_; _pc_, the pollen chamber with a - few pollen grains; _s_, space between nucellus and coat. Compare with - diagram 56.] - -_Heterangium_ is found also in rocks rather older than the coal series of -England, though of Carboniferous age, viz. in the Calciferous sandstone -series of Scotland, it occurs also in the ordinary Coal Measure nodules. -It is in several respects more primitive than _Lyginodendron_, and in -particular in the structure of its stele comes nearer to that of ferns. -The stele is, in fact, a solid mass of primary wood and wood parenchyma, -corresponding in some degree to the protostele of a simple type (see p. -61, fig. 36), but it has towards the outside groups of protoxylem -surrounded by wood in both centripetal and centrifugal directions, which -are just like the primary bundles in _Lyginodendron_. Outside the primary -mass of wood is a zone of secondary wood, but the quantity is not large -in proportion to it (see fig. 81), as is common in Lyginodendron. - -Though the primary mass is so fernlike in appearance the larger tracheids -show series of bordered pits, as do most of the tracheids of the -Pteridosperms, in which they show a Gymnosperm-like character. - - [Illustration: Fig. 81.--_Heterangium_ - - A, Half of the stele of a stem, showing the central mass of wood S - mixed with parenchyma _p_. The protoxylem groups _p. x._ lie towards - the outside of the stele. Surrounding it is the narrow zone of - small-celled secondary wood W. B, A few of the wood cells in - longitudinal view: _p. x._, Protoxylem; _p_, parenchyma. S, Large - vessels with rows of bordered pits.] - -The foliage of _Heterangium_ was fernlike, with much-divided leaves -similar to those of _Lyginodendron_. We have reason to suspect, though -actual proof is wanting as yet, that small Gymnosperm-like seeds were -borne directly on these leaves. - -_Medullosa_ has been mentioned already (see p. 72) because of the -interesting and unusually complex type of its vascular anatomy. Each -individual stele of the group of three in the stem, however, is -essentially similar to the stele of a Heterangium. - -Though the whole arrangement appears to differ so widely from other stems -in the plant world, careful comparison with young stages of recent Cycads -has indicated a possible remote connection with that group, while in the -primary arrangements of the protosteles a likeness may be traced to the -ferns. The roots, even in their primary tissues, were like those of -Gymnosperms, but the foliage with its compound leaves was quite fern-like -externally. A small part of a leaf is shown in fig. 83, and is clearly -like a fern in superficial appearance. The leaves were large, and the -leaf bases strong and well supplied with very numerous branching vascular -bundles. - - [Illustration: Fig. 82.--Steles of _Medullosa_ in Cross Section of the - Stem - - A, Primary solid wood; S, surrounding secondary wood.] - - [Illustration: Fig. 83.--Part of a Leaf of _Medullosa_, known as - _Alethopteris_, for long supposed to be a Fern] - -The connection between this plant and certain large three-ribbed seeds -known as _Trigonocarpus_ is strongly suspected, though actual continuity -is not yet established in any of the specimens hitherto discovered. These -seeds have been mentioned before (p. 76 and p. 82). They were larger than -the other fossil seeds which we have mentioned, and, with their fleshy -coat, were similar in general organization to the Cycads, though the fact -that the seed coat stood free from the inner tissues right down to the -base seems to mark them as being more primitive (cf. fig. 55, p. 76). - -Of impressions of the Pteridosperms the most striking is, perhaps, the -foliage known as _Neuropteris_ (see fig. 6, p. 13), to which the large -seeds are found actually attached (cf. fig. 85). - - [Illustration: Fig. 84.--Diagrammatic Section of a Transverse Section - of a Seed of _Trigonocarpus_ - - S, Stone of coat with three main ridges and six minor ones. F, Flesh of - coat: _i f_, inner flesh; _n_, nucellus, crushed and free from coat; - _s_, spore wall.] - - [Illustration: Fig. 85.--Fragment of Foliage of _Neuropteris_ with Seed - attached, showing the manner in which the seeds grew on the normal - foliage leaves in the Pteridosperms] - -Ever-increasing numbers of the "ferns" are being recognized as belonging -to the Pteridosperms, but _Heterangium_, _Lyginodendron_, and _Medullosa_ -form the three principal genera, and are in themselves a series -indicating the connection between the fernlike and Cycadean characters. - -Before the fructifications were suspected of being seeds the anatomy of -these plants was known, and their nature was partly recognized from it -alone, though at that time they were supposed to have only fernlike -spores. - -The very numerous impressions of their fernlike foliage from the -Palaeozoic rocks indicate that the plants which bore such leaves must have -existed at that time in great quantity. They must have been, in fact, one -of the dominant types of the vegetation of the period. The recent -discovery that so large a proportion of them were not ferns, but were -seed-bearing plants, alters the long-established belief that the ferns -reached their high-water mark of prosperity in the Coal Measure period. -Indeed, the fossils of this age which remain undoubtedly true ferns are -far from numerous. It is the seed-bearing Pteridosperms which had their -day in Palaeozoic times. Whether they led directly on to the Cycads is as -yet uncertain, the probability being rather that they and the Cycads -sprang from a common stock which had in some measure the tendencies of -both groups. - -That the Pteridosperms in themselves combined many of the most important -features of both Ferns and Gymnosperms is illustrated in the account of -them given above, which may be summarized as follows:-- - - - Salient Characters of the Pteridosperms - _G_=_Gymnospermic_ _F_=_Fernlike_ - - _F_ Primary structure of root. - _G_ Secondary thickening of root. - _F_ In _Heterangium_ and _Medullosa_ the - _F_ solid centripetal primary wood of stele. - _G_ Pits on tracheae of primary wood. - _G_ Secondary thickening of stem. - _G_ Double leaf trace. - _F_ Fernlike stele in petiole. - _F_ Fernlike leaves. - _F_ Sporangia pollen-sac-like. - _F_ Reproductive organs borne directly on ordinary foliage leaves. - _G_ General organization of the seed. - -Thus it can be seen at a glance, without entering into minutiae, that the -characters are divided between the two groups with approximate equality. -The connection with Ferns is clear, and the connection with Gymnosperms -is clear. The point which is not yet determined, and about which -discussion will probably long rage, is the position of this group in the -whole scheme of the plant world. Do they stand as a connecting link -between the ferns on one hand and the whole train of higher plants on the -other, or do they lead so far as the Cycads and there stop? - - - - - CHAPTER XIII - PAST HISTORIES OF PLANT FAMILIES - VI. The Ferns - - -Unfortunately the records in the rocks do not go back so far as to touch -what must have been the most interesting period in the history of the -ferns, namely, the point where they diverged from some simple ancestral -type, or at least were sufficiently primitive to give indications of -their origin from some lower group. - -Before the Devonian period all plant impressions are of little value, and -by that early pre-Carboniferous time there are preserved complex leaves, -which are to all appearance highly organized ferns. - -To-day the dominant family in this group is the _Polypodiaceae_. It -includes nearly all our British ferns, and the majority of species for -the whole world. This family does not appear to be very old, however, and -it cannot be recognized with certainty beyond Mesozoic times. - -From the later Mesozoic we have only material in the form of impressions, -from which it is impossible to draw accurate conclusions unless the -specimens have sporangia attached to them, and this is not often the -case. The cuticle of the epidermis or the spores can sometimes be studied -under the microscope after special treatment, but on the whole we have -very little information about the later Mesozoic ferns. - -A couple of specimens from the older Mesozoic have been recently -described, with well-preserved structure, and they belong to the family -of the Osmundas (the so-called "flowering ferns", because of the -appearance of special leaves on which all the sporangia are crowded), and -show in the anatomical characters of their stems indications that they -may be related to an old group, the _Botryopterideae_, in which are the -most important of the Palaeozoic ferns. - -In the Palaeozoic rocks there are numerous impressions as well as fern -petrifactions, but in the majority of cases the connection between the -two is not yet established. There were two main series of ferns, which -may be classed as belonging to - - I. Marattiaceae. - II. Botryopterideae. - -Of these the former has still living representatives, though the group is -small and unimportant compared with what it once was; the latter is -entirely extinct, and is chiefly developed in the Carboniferous and -succeeding Permian periods. - -The latter group is also the more interesting, for its members show great -variety, and series may be made of them which seem to indicate the course -taken in the advance towards the Pteridosperm type. For this reason the -group will be considered first, while the structure of the Pteridosperms -is still fresh in our minds. - -The Botryopterideae formed an extensive and elaborate family, with its -numerous members of different degrees of complexity. There is, -unfortunately, but little known as to their external appearance, and -almost no definite information about their foliage. They are principally -known by the anatomy of their stems and petioles. Some of them had -upright trunks like small tree ferns (living tree ferns belong to quite a -different family, however), others appear to have had underground stems, -and many were slender climbers. - - [Illustration: Fig. 86.--Stele of _Asterochlaena_, showing its deeply - lobed nature] - -In their anatomy all the members of the family have monostelic structure -(see p. 62). This is noteworthy, for at the present time though a number -of genera are monostelic, no family whose members reach any considerable -size or steady growth is exclusively monostelic. In the shape of the -single stele, there is much variety in the different genera, some having -it so deeply lobed that only a careful examination enables one to -recognize its essentially monostelic nature. In fig. 86 a radiating -star-shaped type is illustrated. Between this elaborate type of -protostele in _Asterochlaena_, and the simple solid circular mass seen in -_Botryopteris_ itself (fig. 88) are all possible gradations of structure. - - [Illustration: Fig. 87.--The Stele of a Botryopteridean Stem, showing - soft tissue in the centre of the solid wood of the protostele. - (Microphoto.)] - -In several of the genera the centre of the wood is not entirely solid, -but has cells of soft tissue, an incipient pith, mixed with scattered -tracheids, as in fig. 87. - -In most of the genera numerous petioles are given off from the main axis, -and these are often of a large size compared with it, and may sometimes -be thicker than the axis itself. Together with the petiole usually come -off adventitious roots, as is seen in fig. 88, which shows the main axis -of a _Botryopteris_. The petioles of the group show much variety in their -structure, and some are extremely complex. A few of the shapes assumed by -the steles of the petioles are seen in fig. 89; they are not divided into -separate bundles in any of the known forms, as are many of the petiole -steles of other families. - - [Illustration: Fig. 88.--Main Axis of _Botryopteris_ with simple solid - Protostele _x_. A petiole about to detach itself _p_ and the strand - going out to an adventitious root _r_ are also seen. - (Micro-photograph.)] - - [Illustration: Fig. 89.--Diagrams showing the Shapes of the Steles in - some of the Petioles of different Genera of Botryopterideae - - A, _Zygopteris_; B, _Botryopteris_; C, _Tubicaulis_; D, - _Asterochlaena_.] - -In one genus of the family secondary wood has been observed. This is -highly suggestive of the condition of the stele in _Heterangium_, where -the large mass of the primary wood is surrounded by a relatively small -quantity of secondary thickening, developed in normal radial rows from a -cambium. - -Another noteworthy point in the wood of these plants is the thickening of -the walls of the wood cells. Many of them have several rows of bordered -pits, and are, individually, practically indistinguishable from those of -the Pteridosperms, cf. fig. 81 and fig. 90. These are unlike the -characteristic wood cells of modern ferns and of the other family of -Palaeozoic ferns. - -The foliage of most members of the family is unknown, or at least, of the -many impressions which possibly belong to the different genera, the most -part have not yet been connected with their corresponding structural -material. There are indications, however, that the leaves were large and -complexly divided. - -The fructifications were presumably fern sporangia of normal but rather -massive type. Of most genera they are not known, though in a few they -have been found in connection with recognizable parts of their tissue. -The best known of the sporangia are large, in comparison with living -sporangia (actually about 2.5 millimetres long), oval sacs clustered -together on little pedicels. The spores within them seem in no way -essentially different from normal fern spores. - -The coexistence of the Botryopterideae and Pteridosperms, and the several -points in the structure of the former which seem to lead up to the -characters of the latter group, are significant. The Botryopterideae, even -were they an entirely isolated group, would be interesting from the -variety of structures and the variations of the monostele in their -anatomy; and the prominent place they held in the Palaeozoic flora, as the -greatest family of ferns of that period, gives them an important position -in fossil botany. - - [Illustration: Fig. 90.--Tracheae of Wood of Botryopteridean Fern in - Longitudinal Section, showing the rows of pits on the walls. - (Microphoto.)] - -The other family of importance in Palaeozoic times, the Marattiaceae, has -descendants living at the present day, though the family is now -represented by a small number of species belonging to but five genera -which are confined to the tropics. Perhaps the best known of these is the -giant "Elephant Fern", which sends up from its underground stock huge -complex fronds ten or a dozen feet high. Other species are of the more -usual size and appearance of ferns, while some have sturdy trunks -above-ground supporting a crown of leaves. The members of this family -have a very complex anatomy, with several series of steles of large size -and irregular shape. Their fructifications are characteristic, the -sporangia being placed in groups of about five to a dozen, and fused -together instead of ripening as separate sacs as in the other fern -families. - -Impressions of leaves with this type of sorus (group of fern sporangia) -are found in the Mesozoic rocks, and these bridge over the interval -between the living members of the family and those which lived in -Palaeozoic times. - -In the Coal Measure and Permian periods these plants flourished greatly, -and there are remains of very numerous species from that time. The family -was much more extensive then than it is now, and the individual members -also seem to have reached much greater dimensions, for many of them had -the habit of large tree ferns with massive trunks. Up till Triassic times -half of the ferns appear to have belonged to this family; since then, -however, they seem to have dwindled gradually down to the few genera now -existing. - -On the Continent fossils of this type with well-preserved structure have -long been known to the general public, as their anatomy gave the stones a -very beautiful appearance when polished, so that they were used for -decorative purposes by lapidaries before their scientific interest was -recognized. - -The members of the Palaeozoic Marattiaceae which have structure preserved -generally go by the generic name _Psaronius_, in which there is a great -number of species. They show considerable uniformity in their essential -structure (in which they differ noticeably from the group of ferns just -described), so that but one type will be considered. - -In external appearance they probably resembled the "tree ferns" of the -present day (though these belong to an entirely different family), with -massive stumps, some of which reached a height of 60 ft. The large -spreading leaves were arranged in various ways on the stem, some in a -double row along it, as is seen by the impressions of the leaf scars, and -others in complex spirals. On the leaves were the spore sacs, which were -in groups, some completely fused like those of the modern members of the -family, and others with independent sporangia massed in well-defined -groups. In their microscopic structure also they appear to have been -closely similar to those of the living Marattiaceae. - -The transverse section of a stem shows the most characteristic and -best-known view of the plant. This is shown in fig. 91, in somewhat -diagrammatic form. - -The mass of rootlets which entirely permeate and surround the outer -tissues of the stem is a very striking and characteristic feature of all -the species of _Psaronius_. Though such a mass of roots is not found in -the living species, yet the microscopic structure of an individual fossil -root is almost identical with that of a living _Marattia_. - -Though these plants were so successful and so important in Palaeozoic -times, the group even then seems to have possessed little variety and -little potentiality for advance in new directions. They stand apart from -the other fossils, and the few forms which now compose the living -Marattiaceae are isolated from the present successful types of modern -ferns. From the _Psaronieae_ we can trace no development towards a modern -series of plants, no connection with another important group in the past. -They appear to have culminated in the later Palaeozoic and to have slowly -dwindled ever since. It has been suggested that the male fructifications -of the Bennettiteae and the Pteridosperms show some likeness to the -Marattiaceae, but there does not seem much to support any view of -phylogenetic connection between them. - - [Illustration: Fig. 91.--Transverse Section of Stem of _Psaronius_ - - _v_, Numerous irregularly-shaped steles; _s_, irregular patches of - sclerenchyma; _l_, leaf trace going out as a horseshoe-shaped stele; - _c_, zone of cortex with numerous adventitious roots _r_ running - through it; _sc_, sclerized cortical zone of roots; _w_, vascular - strand of roots.] - -Before leaving the palaeozoic ferns, mention should be made of the very -numerous leaf impressions which seem to show true fern characters, though -they have not been connected with material showing their internal -structure. Among them it is rare to get impressions with the _sori_ or -sporangia, but such are known and are in themselves enough to prove the -contention that true ferns existed in the Palaeozoic epoch. For it might -be mentioned as a scientific curiosity, that after the discovery that so -many of the leaf impressions which had always been supposed to be ferns, -really belonged to the seed-bearing Pteridosperms, there was a period of -panic among some botanists, who brought forward the startling idea that -there were _no_ ferns at all in the Palaeozoic periods, and that modern -ferns were degenerated seed-bearing plants! - - [Illustration: Fig. 92.--Impression of Palaeozoic Fern, showing _sori_ - on the pinnules. (Photo.)] - -These two big groups from the Palaeozoic include practically all the ferns -that then flourished. They have been spoken of (together with a few other -types of which little is known) as the _Primofilices_, a name which -emphasizes their primitive characters. As can be seen by the complex -organization of the genera, however, they themselves had advanced far -beyond their really primitive ancestors. There is clear indication that -the Botryopterideae were in a period of change, what might almost be -termed a condition of flux, and that from their central types various -families separated and specialized. Behind the Botryopterideae, however, -we have no specimens to show us the connection between them and the -simpler groups from which they must have sprung. From a detailed -comparative study of plant anatomy we can deduce some of the essential -characters of such ancestral plants, but here the realm of fossil botany -ceases, to give place to theoretical speculation. As a fact, there is a -deep abyss between the ferns and the other families of the Pteridophytes, -which is not yet bridged firmly enough for any but specialists, used to -the hazardous footing on such structures, to attempt to cross it. Until -the buttresses and pillars of the bridge are built of the strong stone of -fossil structures we must beware of setting out on what would prove a -perilous journey. - -In the Coal Measures and previous periods we see the ferns already -represented by two large families, differing greatly from each other, and -from the main families of modern ferns which sprung at a later date from -some stock which we have not yet recognized. But though their past is so -obscure, the palaeozoic ferns and their allies throw a brilliant light on -the course of evolution of the higher groups of plants, and the gulf -between ferns and seed-bearing types may be said to be securely bridged -by the Botryopterideae and the Pteridosperms. - - - - - CHAPTER XIV - PAST HISTORIES OF PLANT FAMILIES - VII. The Lycopods - - -The present-day members of this family are not at all impressive, and in -their lowliness may well be overlooked by one who is not interested in -unpretending plants. The fresh green mosslike _Selaginella_ grown by -florists as ornamental borders in greenhouses and the creeping "club -moss" twining among the heather on a Highland moor are probably the best -known of the living representatives of the Lycopods. In the past the -group held a very different position, and in the distant era of the Coal -Measures it held a dominant one. Many of the giants of the forest -belonged to the family (see frontispiece), and the number of species it -contained was very great. - -Let us turn at once to this halcyon period of the group. The history of -the times intervening between it and the present is but the tale of the -dying out of the large species, and the gradual shrinking of the family -and dwarfing of its representative genera. - -It is difficult to give the characters of a scientific family in a few -simple words; but perhaps we may describe the living Lycopods as plants -with creeping stems which divide and subdivide into two with great -regularity, and which bear large numbers of very small pointed leaves -closely arranged round the stem. The fruiting organs come at the tips of -the branches, and sometimes themselves divide into two, and in these -cone-like axes the spore cases are arranged, a single one on the upper -side of each of the scales (see p. 67, fig. 46, A). In the Lycopods the -spores are all alike, in the Selaginellas there are larger spores borne -in a small number (four) in some sporangia (see fig. 53, p. 75), and -others in large numbers and of smaller size on the scales above them. The -stems are all very slender, and have no zones of secondary wood. They -generally creep or climb, and from them are put out long structures -something like roots in appearance, which are specially modified -stem-like organs giving rise to roots. - -From the fossils of the Coal Measures _Lepidodendron_ must be chosen as -the example for comparison. The different species of this genus are very -numerous, and the various fossilized remains of it are among the -commonest and best known of palaeontological specimens. The huge stems are -objects of public interest, and have been preserved in the Victoria Park -in Glasgow in their original position in the rocks, apparently as they -grew with their spreading rootlike organs running horizontally. A great -stump is also preserved in the Manchester Museum, and is figured in the -frontispiece. While among the casts and impressions the leaf bases of the -plant are among the best preserved and the most beautiful (see fig. 93). -The cone has already been illustrated (see fig. 46 and fig. 9), and is -one of the best known of fossil fructifications. - - [Illustration: Fig. 93.--Photo of Leaf Bases of _Lepidodendron_ - - C, Scar of leaf; S, leaf base. In the scar: _v_, mark of severed - vascular bundle, and _p_, of parichnos. _l_, Ligule scar.] - -From the abundant, though scattered material, fossil botanists have -reconstructed the plants in all their detail. The trunks were lofty and -of great thickness, bearing towards the apex a much-branched crown, the -branches, even down to the finest twigs, all dividing into two equal -parts. The leaves, as would be expected from the great size of the -plants, were much bigger than those of the recent species (fig. 93 shows -the actual size of the leaf bases), but they were of the same -_relatively_ small size as compared with the stems, and of the same -simple pointed shape. A transverse section across the apex of a fertile -branch shows these closely packed leaves arranged in series round the -axis, those towards the outside show the central vascular strand which -runs through each. - - [Illustration: Fig. 94.--Section across an Axis surrounded by many - Leaves, which shows their simple shape and single central vascular - bundle _v_] - -The markings left on the well-preserved leaf-scars indicate the main -features of the internal anatomy of the leaves. They had a single central -vascular strand (_v_, fig. 93), on either side of which ran a strand of -soft tissue _p_ called the parichnos, which is characteristic of the -plants of this group. While another similarly obscure structure -associated with the leaf is the little scale-like ligule _l_ on its upper -surface. - -The anatomy of the stems is interesting, for in the different species -different stages of advance are to be found, from the simple solid -protostele with a uniform mass of wood to hollow ring steles with a pith. -An interesting intermediate stage between these two is found in -_Lepidodendron selaginoides_ (see fig. 95), where the central cells of -the wood are not true water-conducting cells, but short irregular -water-storage tracheides (see p. 56), which are mixed with parenchyma. -All the genera of these fossils have a single central stele, round which -it is usual to find a zone of secondary wood of greater or less extent -according to the age of the plant. - - [Illustration: Fig. 95.--Transverse Section of _Lepidodendron - selaginoides_, showing the circular mass of primary wood, the central - cells of which are irregular water-storage tracheides - - _s_, Zone of secondary wood; _c_, inner cortical tissues; _r_, - intrusive burrowing rootlet; _oc_, outer cortical tissues with corky - external layers _k_. (Microphoto.)] - -Some stems instead of this compact central stele have a ring of wood with -an extensive pith. Such a type is illustrated in fig. 96, which shows but -a part of the circle of wood, and the zone of the secondary wood outside -it, which greatly exceeds the primary mass in thickness. This zone of -secondary wood became very extensive in old stems, for, as will be -imagined, the primary wood was not sufficient to supply the large trunks. -The method of its development from a normal cambium in radiating rows of -uniform tracheides is quite similar to that which is found in the pines -to-day. This is the most important difference between the living and the -fossil stems of the family, for no living plants of the family have such -secondary wood. On the other hand, the individual elements of this wood -are different from those of the higher families hitherto considered, and -have narrow slit-like pits separated by bands of thickening on the -longitudinal walls. Such tracheides are found commonly in the -Pteridophytes, both living and fossil. Their type is seen in fig. 96, B, -which should be compared with that in figs. 78, A and 62, B to see the -contrast with the higher groups. - - [Illustration: Fig. 96.--A, _Lepidodendron_ Stem with Hollow Ring of - Wood W and Zone of Secondary Wood S. B, Longitudinal View of the Narrow - Pits of the Wood Elements.] - -To supply the vascular tissues of the leaf traces, simple strands come -off from the outer part of the primary wood, where groups of small-celled -protoxylem project (see _px_ in fig. 97). The leaf strands _lt_ move out -through the cortex in considerable numbers to supply the many leaves, -into each of which a single one enters. - - [Illustration: Fig. 97.--Transverse Section of Outer Part of Primary - Wood of _Lepidodendron_, showing _px_, projecting protoxylem groups; - _lt_, leaf trace coming from the stele and passing (as _lt_^1) through - the cortex] - -As regards the fructifications of _Lepidodendron_ much could be said were -there space. The many genera of _Lepidodendron_ bore several distinct -types of cones of different degrees of complexity. In several of the -genera the cones were simple in organization, directly comparable with -those of the living Lycopods, though on a much larger scale (see p. 67). -In some the spores were uniform, all developing equally in numerous -tetrads. The sporophyll was radially extended, and along it the large -sausage-shaped sporangia were attached (see fig. 98). The tips of the -sporophylls overlapped and afforded protection to the sporangia. The axis -of the cone had a central stele with wood elements like those in the -stem. The appearance of a transverse section of an actual cone is shown -in fig. 99. Here the sporangia are irregular in shape, owing to their -contraction after ripeness and during fossilization. Other cones had -sporangia similar in size and shape, but which produced spores of two -kinds, large ones resulting from the ripening of only two or three -tetrads in the lower sporangia, and numerous small ones in the sporangia -above. - - [Illustration: Fig. 98.--Longitudinal Diagram, showing the arrangement - of the elongated sporangia on the sporophylls - - _a_, Main axis, round which the sporophylls are inserted; S, - sporangium; _s_, leaflike end of sporophyll.] - -The similarity between the _Lepidodendron_ and the modern Lycopod cone -has been pointed out already (p. 67), and it is this which forms the -principal guarantee that they belong to the same family, though the size -and wood development of the palaeozoic and the modern plants differ so -greatly. - -The large group of the Lepidodendra included some members whose -fructifications had advanced so far beyond the simple sporangial cones -described above as to approach very closely to seeds in their -construction. This type was described on p. 75, fig. 54, in a series of -female fructifications, so that its essential structure need not be -recapitulated. - - [Illustration: Fig. 99.--Transverse Section through Cone of - _Lepidodendron_ - - A, Main axis with woody tissue; _st_, stalks of sporophylls cut in - oblique longitudinal direction; _s_, tips of sporophylls cut across; S, - sporangia with a few groups of spores. (Microphoto.)] - -The section shown in fig. 100 is that cut at right angles to that in -which the sporangia are shown in fig. 98, viz. tangential to the axis. A -remarkable feature of the plant is that there were also round those -sporangia which bore the numerous small spores (corresponding to pollen -grains) enclosing integument-like flaps similar to those shown in fig. -100, _sp. f_. - - [Illustration: Fig. 100.--Section through one Sporangium of - _Lepidocarpon_ - - _sp_, Sporophyll; _sp.f._, flaps of sporophyll protecting sporangium; - S, large spore within the sporangium wall _w_; _s_, the three aborted - spores of the tetrad to which S belongs.] - -This type of fructification is the nearest approach to seed and pollen -grains reached by any of the Pteridophytes, and its appearance at a time -when the Lycopods were one of the dominant families is suggestive of the -effect that such a position has on the families occupying it, however -lowly they may be. The simple Pteridophyte Lycopods had not only the tall -trunks and solid woody structure of a modern tree, but also a semblance -of its seeds. Whether this line of development ever led on to any of the -higher families is still uncertain. The feeling of most specialists is -that it did not; but there are not wanting men who support the view that -the lycopod affinity evolved in time and entered the ranks of the higher -plants, and indeed there are many points of superficial likeness between -the palaeozoic Lycopods and the Coniferae. Judged from their internal -structure, however, the series through the ferns and Pteridosperms leads -much more convincingly to the seed plants. - -In their roots, or rather in the underground structures commonly called -roots, the Lepidodendrons were also remarkable. Even more symmetrically -than in their above-ground branching, the base of their trunks divided; -there were four main large divisions, each of which branched into two and -these into two again. These structures were called _Stigmaria_, and were -common to all species of _Lepidodendron_ and also the group of -_Sigillaria_ (see fig. 102). On these horizontally running structures -(well shown in the frontispiece) small appendages were borne all over -their surface in great profusion, which were, both in their function and -microscopic structure, rootlets. They left circular scars of a -characteristic appearance on the big trunks, of which they were the only -appendages. These scars show clearly on the fragments along the ledge to -the left of the photograph. The exact morphological nature of the big -axes is not known; their anatomy is not like that of roots, but is that -of a stem, yet they do not bear what practically every stem, whether -underground or not, has developed, namely leaves, or scales representing -reduced leaves. Their nature has been commented on previously (p. 69), -and we cannot discuss the point further, but must be content to consider -them as a form of root-bearing stem, practically confined to the Lycopods -and principally developed among the palaeozoic fossils of that group. - - [Illustration: Fig. 101.--Transverse Section through a Rootlet of - _Stigmaria_ - - _oc_, Outer cortex; _s_, space; _ic_, inner cortex; _w_, wood of - vascular strand (wood only preserved); _px_, protoxylem group.] - -In microscopic structure the rootlets are extremely well known, because -in their growth they have penetrated the masses of the tissues of other -plants which were being petrified and have become petrified with them. -The mass of decaying vegetable tissue on which the living plants of the -period flourished were everywhere pierced by these intrusive rootlets, -and they are found petrified inside otherwise perfect seeds, in the -hearts of woody stems, in leaves and sporangia, and sometimes even inside -each other! Fig. 95 shows such a root _r_ lying in the space left by the -decay of the soft tissue of the inner cortex in an otherwise excellently -preserved _Lepidodendron_ stem (see also fig. 101). In fig. 101 their -simple structure is seen. They are often extremely irregular in shape, -owing to the way they seem to have twisted and flattened themselves in -order to fit into the tissues they were penetrating. No root hairs seem -to have been developed in these rootlets, but otherwise their structure -is that of a typical simple root, and very like the swamp-penetrating -rootlets of the living Isoetes. - -The Stigmarian axes and their rootlets are very commonly found in the -"underclays" and "gannister" beds which lie below the coal seams (see p. -25), and they may sometimes be seen attached to a bit of the trunk -growing upwards through the layers. They and the aerial stems of -Lepidodendron are perhaps the commonest and most widely known of fossil -plants. - -Before leaving the palaeozoic Lycopods another genus must be mentioned, -which is also a widely spread and important one, though it is less well -known than its contemporary. The genus _Sigillaria_ is best known by its -impressions and casts of stems covered by leaf scars. The stems were -sometimes deeply ribbed, and the leaf scars were arranged in rows and -were more or less hexagonal in outline, as is seen in fig. 102, which -shows a cast and its reverse of the stem of a typical _Sigillaria_. - - [Illustration: Fig. 102.--Cast and Reverse of Leaf Scars of - _Sigillaria_. In A the shape of the leaf bases is clearly shown, the - central markings in each being the scar of the vascular bundle and - parichnos] - -In its primary wood _Sigillaria_ differed from _Lepidodendron_ in being -more remote from the type with a primary solid stele. Its woody structure -was that of a ring, in some cases irregularly broken up into -crescent-shaped bundles. The secondary wood was quite similar to that of -_Lepidodendron_. - -_Stigmaria_ and its rootlets belong equally to the two plants, and -hitherto it has been impossible to tell whether any given specimen of -_Stigmaria_ had belonged to a _Lepidodendron_ or a _Sigillaria_. Between -the two genera there certainly existed the closest affinity and -similarity in general appearance. - -These two genera represent the climax of development of the Lycopod -family. In the Lower Mesozoic some large forms are still found, but all -through the Mesozoic periods the group dwindled, and in the Tertiary -little is known of it, and it seems to have taken the retiring position -it occupies to-day. - - - - - CHAPTER XV - PAST HISTORIES OF PLANT FAMILIES - VIII. The Horsetails - - -The horsetails of to-day all belong to the one genus, _Equisetum_, among -the different species of which there is a remarkably close similarity. -Most of the species love swampy land, and even grow standing up through -water; but some live on the dry clay of ploughed fields. Wherever they -grow they usually congregate in large numbers, and form little groves -together. They are easily recognized by their delicate stems, branching -in bottle-brush fashion, and the small leaves arranged round them in -whorls, with their narrow teeth joined to a ring at the base. At the end -of some of the branches come the cones, with compactly arranged and -simple sporophylls all of one kind. In England most plants of this family -are but a few inches or a foot in height, though one species sometimes -reaches 6 ft., while in South America there are groves of -delicate-stemmed plants 20 ft. high. - -The ribbed stems and the whorls of small, finely toothed leaves are the -most important external characteristics of the plants, while in their -internal anatomy the hollow stems have very little wood, which is -arranged in a series of small bundles, each associated with a hollow -canal in the ground tissue. - -The family stands apart from all others, and even between it and the -group of Lycopods there seems to be a big gap across which stretch no -bonds of affinity. Has the group always been in a similar position, and -stood isolated in a backwater of the stream of plant life? - - [Illustration: Fig. 103.--Impression of Leaf Whorl of _Equisetites_ - from the Mesozoic Rocks, showing the narrow toothed form of the leaves. - (Photo.)] - -In the late Tertiary period they seem to have held much the same position -as they do now, and we learn nothing new of them from rocks of that age. -When, however, we come to the Mesozoic, the members of the family are of -greater size, though they appear (to judge from their external -appearance) to have been practically identical with those now living in -all their arrangements. In some beds their impressions are very numerous, -but unfortunately most are without any indication of internal structure. -Fossils from the Mesozoic are called _Equisetites_, a name which -indicates that they come very close to the living ones in their -characters. In the Lower Mesozoic some of these stems seem to have -reached the great size of a couple of feet in circumference, but to have -no essential difference from the others of the group. - -When, however, we come to the Palaeozoic rocks we find many specimens with -their structure preserved, and we are at once in a very different -position as regards the family. - -First in the Permian we meet with the important genus of plant called -_Calamites_, which were very abundant in the Coal Measures. Many of the -Calamites were of great size, for specimens with large trunks have been -found 30 ft. and more long, which when growing must certainly have been -much taller than that. The number of individuals must also have been very -great, for casts and impressions of the genus are among the commonest -fossils. They were, in fact, one of the dominant groups of the period. -Like the Lycopods, the Equisetaceae reached their high-water mark of -development in the Carboniferous period; at that time the plants were -most numerous, and of the largest size and most complicated structure -that they ever attained. - - [Illustration: Fig. 104.--Small Branches attached to stouter Axis of - _Calamites_. Photo of Impression] - -As will be immediately suspected from analogy with the Lycopods, they -differed from the modern members of the family in their strongly -developed anatomy, and in the strength and quantity of their secondary -wood. Yet in their external appearance they probably resembled the living -genus in all essentials, and the groves of the larger ones of to-day -growing in the marshes probably have the appearance that the palaeozoic -plants would have had if looked at through a reversed opera glass. - -Fig. 104 is a photograph of some of the small branches of a Calamite, in -which the ribbed stem can be seen, and on the small side twigs the fine, -pointed leaves lying in whorls. - -In most of the fossil specimens, however, particularly the larger ones, -the ribs are not those of the true surface, but are those marked on the -_internal cast_ of the pith. - - [Illustration: Fig. 105.--Transverse Section of _Calamites_ Stem with - Secondary Wood _w_ formed in Regular Radial Rows in a Solid Ring - - _c_, Canals associated with the primary bundles; _p_, cells of the - pith, which is hollow with a cavity _l_, _cor_, Cortex and outer - tissues well preserved. (Microphoto.)] - -Among tissue petrifactions there are many Calamite stems of various -stages of growth. In the very young ones there are only primary bundles, -and these little stems are like those of a living Equisetum in their -anatomy, and have a hollow pith and small vascular bundles with canals -associated. The fossil forms, however, soon began to grow secondary wood, -which developed in regular radial rows from a cambium behind the primary -bundles and joined to a complete ring. - -A stem in this stage of development is seen in fig. 105, where only the -wood and internal tissues are preserved. The very characteristic canals -associated with the primary bundles are clearly shown. The amount of -secondary wood steadily increased as the stems grew (there appear to have -been no "annual rings") till there was a very large quantity of secondary -tissue of regular texture, through which ran small medullary rays, so -that the stems became increasingly like those of the higher plants as -they grew older. It is the primary structure which is the important -factor in considering their affinity, and that is essentially the same as -in the other members of the family in which secondary thickening is not -developed. As we have seen already in other groups of fossils, secondary -wood appears to develop on similar lines whenever it is needed in any -group, and therefore has but little value as an indication of systematic -position. This important fact is one, however, which has only been -realized as a result of the study of fossil plants. - - [Illustration: Fig. 106.--Diagram of the Arrangement of the Bundles at - the Node of a _Calamite_, showing how those of consecutive internodes - alternate - - _n_, Region of node] - - [Illustration: Fig. 107.--Leaf of _Calamites_ in Cross Section - - _v_, Vascular bundles; _s_, cells of sheath, filled with blackened - contents; _p_, palisade cells; _e_, epidermis.] - -The longitudinal section of the stems, when cut tangentially, is very -characteristic, as the bundles run straight down to each node and there -divide, the neighbouring halves joining so that the bundles of each node -alternate with those of the ones above and below it (see fig. 106). - -The leaves which were attached at the nodes were naturally much larger -than those of the present Equisetums, though they were similarly simple -and undivided. Their anatomy is preserved in a number of cases (see fig. -107), and was simple, with a single small strand of vascular tissue lying -in the centre. They had certain large cells, sometimes very black in the -fossils, which may have been filled with mucilage. - - [Illustration: Fig. 108.--Transverse Section of Young Root of - _Calamites_ - - _w_, Wood of axis; _l_, spaces in the lacunar cortex, whose radiating - strands _r_ are somewhat crushed; _ex_, outermost cells of cortex with - thickened wall.] - - [Illustration: Fig. 109.--Diagram of Cone of _Calamites_ - - A, Main axis; _br_, sterile bracts; _sp_, sporophylls with four - sporangia S attached to each, of which two only are seen.] - -The young roots of these plants have a very characteristic cortex, which -consists of cells loosely built together in a lacelike fashion, with -large air spaces, so that they are much like water plants in their -appearance (see fig. 108). Indeed, so unlike the old roots and the stems -are they, that for long they were called by another name and supposed to -be submerged stems, but their connection with _Calamites_ is now quite -certain. As their woody axis develops, the secondary tissue increases and -pushes off the lacelike cortex, and the roots become very similar in -their anatomy to the stems. Both have similar zones of secondary wood, -but the roots do not have those primary canals which are so -characteristic of the stems, and thereby they can be readily -distinguished from them. - -The fructifications of the Calamites were not unlike those of the living -types of the family, though in some respects slightly more complex. Round -each cone axis developed rings of sporophylls which alternated with -sterile sheathing bracts. Each sporophyll was shaped like a small -umbrella with four spokes, and stood at right angles to the axis, bearing -a sporangium at each of the spokes. A diagram of this arrangement is seen -in fig. 109. - - [Illustration: Fig. 110.--Longitudinal Section of Part of _Calamites_ - Cone - - _br_, Sterile bracts attached to axis; _sp_, attachment of sporophylls; - S, sporangia. At X a group of four sporangia is seen round the - sporophyll, which is seen at _a_. (Microphoto.)] - -A photograph of an actual section of such a cone, cut slightly obliquely -through the length of the axis, is seen in fig. 110, where the upper -groups of sporangia are cut tangentially, and show their grouping round -the sporophyll to which they are attached. - -A few single tetrads of spores are enlarged in fig. 111, where it will be -seen that the large spores are of a similar size, but that the small ones -of the tetrads are very irregular. They are aborting members of the -tetrad, and appear to have been used as food by the other spores. In each -sporangium large numbers of these tetrads develop and all the ripe spores -seem to have been of one size. - -In a species of _Calamites_ (_C. casheana_), otherwise very similar to -the common one we have been considering, there is a distinct difference -in the sizes of the spores from different sporangia. The small ones, -however, were only about one-third of the diameter of the large ones, so -that the difference was very much less marked than it was between the -small and large spores of the Lycopods. - -Among the palaeozoic members of the group are other genera closely allied -to, but differing from _Calamites_ in some particulars. One of these is -_Archaeocalamites_, which has a cone almost identical with that of the -living Equisetums, as it has no sterile bracts mingled with the -umbrella-like sporophylls. Other genera are more complex than those -described for _Calamites_, and even in the simple coned _Archaeocalamites_ -itself the leaves are finely branched and divided instead of being simple -scales. - -But no genus is so completely known as is _Calamites_, which will itself -suffice as an illustration of the palaeozoic Equisetaceae. Though the -genus, as was pointed out above, shows several important characters -differing from those of Equisetum, and parallel to some extent to those -of the palaeozoic Lycopods, yet these features are more of a physiological -nature than a systematic one, and they throw no light on the origin of -the family or on its connection with the other Pteridophytes. It is in -the extinct family dealt with in the next chapter that we find what some -consider as a clue to the solution of these problems. - - [Illustration: Fig. 111.--Tetrads of Spores of _Calamites_ - - S, Normal-sized spores; _a_, _b_, &c., aborting spores.] - - - - - CHAPTER XVI - PAST HISTORIES OF PLANT FAMILIES - IX. Sphenophyllales - - -The group to which _Sphenophyllum_ belongs is of considerable interest -and importance, and is, further, one of those extinct families whose very -existence would never have been suspected had it not been discovered by -fossil botanists. Not only is the family as a whole extinct, it also -shows features in its anatomy which are not to be paralleled among living -stems. _Sphenophyllum_ became extinct in the Palaeozoic period, but its -interest is very real and living to-day, and in the peculiar features of -its structure we see the first clue that suggests a common ancestor for -the still living groups of Lycopods and Equisetaceae, which now stand so -isolated and far apart. - -Before, however, we can consider the affinities of the group, we must -describe the structure of a typical plant belonging to it. The genus -_Sphenophyllum_ includes several species (for which there are no common -English names, as they are only known to science) whose differences are -of less importance than their points of similarity, so that one species -only, _S. plurifoliatum_, will be described. - -We have a general knowledge of the external appearance of _Sphenophyllum_ -from the numerous impressions of leaves attached to twigs which are found -in the rocks of the Carboniferous period. These impressions present a -good deal of variety, but all have rather delicate stems with whorls of -leaves attached at regular intervals. The specimens are generally easy to -recognize from the shape of the leaves, which are like broad wedges -attached at the point (see fig. 112). In some cases the leaves are more -finely divided and less fanlike, and it may even happen that on the same -branch some may be wedge-shaped like those in fig. 112, and others almost -hairlike. This naturally suggests comparison with water plants, which -have finely divided submerged leaves and expanded aerial ones. In the -case of _Sphenophyllum_, however, the divided leaves sometimes come at -the upper ends of the stems, quite near the cones, and so can hardly have -been those of a submerged part. The very delicate stems and some points -in their internal anatomy suggest that the plant was a trailing creeper -which supported itself on the stouter stems of other plants. - - [Illustration: Fig. 112.--Impression of _Sphenophyllum_ Leaves attached - to the Stem, showing the wedge-shaped leaflets arranged in whorls] - -The stems were ribbed, but unlike those of the Calamites the ribs ran -straight down the stem through the nodes, and did not alternate there, so -that the bundles at the node did not branch and fuse as they did in -_Calamites_. - -The external appearance of the long slender cones was not unlike that of -the Calamite cones, though their internal details showed important -distinctions. - -In one noticeable external feature the plants differed from those of the -last two groups considered, and that was in their _size_. Palaeozoic -Lycopods and Equisetaceae reached the dimensions of great trees, but -hitherto no treelike form of _Sphenophyllum_ has been discovered, and in -the structure-petrifactions the largest stems we know were less than an -inch in diameter. - -In the internal anatomy of these stems lies one of the chief interests -and peculiarities of the plants. In the very young stage there was a -sharply pointed solid triangle of wood in the centre (fig. 113), at each -of the corners of which was a group of small cells, the protoxylems. The -structure of such a stem is like that of a root, in which the primary -wood all grows inwards from the protoxylems towards the centre, and had -we had nothing but these isolated young stems it would have been -impossible to recognize their true nature. - - [Illustration: Fig. 113.--_Sphenophyllum_, Transverse Section of Young - Stem - - _c_, Cortex, the soft tissue within which has decayed and left a space, - in which lies the solid triangle of wood, with the small protoxylem - groups _px_ at each corner. (Microphoto.)] - -Such very young stems are rare, for the development of secondary wood -began early, and it soon greatly exceeded the primary wood in amount. -Fig. 114 shows a photograph of a stem in which the secondary wood is well -developed. The primary triangle of wood is still to be seen in the -centre, and corresponds to that in fig. 113, while closely fitting to it -are the bays of the first-formed secondary wood, which makes the wood -mass roughly circular. Outside this the secondary wood forms a regular -cylinder round the axis, which shows no sign of annual rings. The cells -of the wood are large and approximately square in shape, while at the -angles formed at the junction of every four cells is a group of small, -thin-walled parenchyma, see fig. 115. There are no medullary rays going -out radially through the wood, such as are found in all other zones of -secondary wood, and in this arrangement of soft tissue the plants are -unique. - - [Illustration: Fig. 114.--_Sphenophyllum_, Transverse Section with - Secondary Wood W. At _c_ the cork formation is to be seen. - (Microphoto.)] - -Beyond the wood was a zone of soft tissue and phloem, which is not often -preserved, while outside that was the cork, which added to the cortical -tissues as the stem grew (see fig. 114, _c_). - - [Illustration: Fig. 115.--Group of Wood Cells _w_, showing their shape - and the small soft-walled cells at the angles between them _p_] - -Petrified material of leaves and roots is rare, and both are chiefly -known through the work of the French palaeobotanist Renault. The leaves -are chiefly remarkable for the bands of sclerized strengthening tissue, -and generally had the structure of aerial, not submerged leaves. The -roots were simple in structure, and, as in _Calamites_, had secondary -tissue like that in the stems. - -In the case of the fructifications it is the English material which has -yielded the most illuminating specimens. The cones were long and slender, -externally covered by the closely packed tips of the scales, which -overlapped deeply. Between the whorls of scales lay the sporangia, -attached to their upper sides by slender stalks. A diagram will best -explain how they were arranged (see fig. 116). Two sporangia were -attached to each bract, but their stalks were of different lengths, so -that one sporangium lay near the axis and one lay outside it toward the -tip of the bract. - - [Illustration: Fig. 116.--Diagram of Arrangement of Scales and - Sporangia in Cones of _Sphenophyllum_ - - A, Axis; _br_, bract; S, sporangium, with stalk _st_.] - -In its anatomy the stalk of the cone has certain features similar to -those in the stem proper, which were among the first indications that led -to the discovery that the cone belonged to _Sphenophyllum_. There were -numerous spores in each of the sporangia, which had coats ornamented with -little spines when they were ripe (fig. 117, if examined with a -magnifying glass, will show this). Hitherto the only spores known are of -uniform size, and there is no evidence that there was any differentiation -into small (male) and large (female) spores such as were found in some of -the Lepidodendrons. In this respect _Sphenophyllum_ was less specialized -than either _Lepidodendron_ or _Calamites_. - -In the actual sections of _Sphenophyllum_ cones the numerous sporangia -seem massed together in confusion, but usually some are cut so as to show -the attachment of the stalk, as in fig. 117, _st_. As the stalk was long -and slender, but a short length of it is usually cut through in any one -section, and to realize their mode of attachment to the axis (as shown in -fig. 116) it is necessary to study a series of sections. - - [Illustration: Fig. 117.--Part of Cone of _Sphenophyllum_, showing - sporangia _sp_, some of which are cut so as to show a part of their - stalks _st_. B, Bract. (Microphoto.)] - - -Of the other plants belonging to the group, _Bowmanites Roemeri_ is -specially interesting. Its sporangia were borne on stalks similar to -those of _Sphenophyllum_, but each stalk had two sporangia attached to -it. Two sporangia are also borne on each stalk in _S. fertile_. These -plants help in elucidating the nature of the stalked sporangia of -_Sphenophyllum_, for they seem to indicate a direct comparison between -them and the sporophylls of the Equisetales. - - -There is, further, another plant, of which we only know the cone, of -still greater importance. This cone (_Cheirostrobus_) is, however, so -complex that it would take far too much space to describe it in detail. -Even a diagram of its arrangements is extraordinarily elaborate. To the -specialist the cone is peculiarly fascinating, for its very complexity -gives him great scope for weaving theories about it; but for our purposes -most of these are too abstruse. - - [Illustration: Fig. 118.--A, Diagram of Three-lobed Bract from Cone of - _Cheirostrobus_. _a_, Axis; _br_, the three sterile lower lobes of the - bract; _sp_, the three upper sporophyll-like lobes, to each of which - were attached four sporangia S. B, Part of the above seen in section - longitudinal to the axis. (Modified from Scott.)] - -Its most important features are the following. Round the axis were series -of scales, twelve in each whorl, and each scale was divided into an upper -and a lower portion, each of which again divided into three lobes. The -lower three of each of these scale groups were sterile and bractlike, -comparable, perhaps, with the bracts in fig. 116; while the upper three -divisions were stalks round each of which were four sporangia. Each -sporophyll segment thus resembled the sporophyll of _Calamites_, while -the long sausage-shaped sporangia themselves were more like those of -_Lepidodendron_. In fig. 118 is a diagram of a trilobed bract with its -three attached sporophylls. Round the axis were very numerous whorls of -such bracts, and as the cone was large there were enormous numbers of -spore sacs. - -A point of interest is the character of the wood of the main axis, which -is similar to that of Lepidodendron in many respects, being a ring of -centripetally developed wood with twelve projecting external points of -protoxylem. - -This cone[13] is the most complex fructification of any of the known -Pteridophytes, whether living or fossil, which alone ensures it a special -importance, though for our purpose the mixed affinities it shows are of -greater interest. - -To mention some of its characters:--The individual segments of the -sporophylls, each bearing four sporangia, are comparable with those of -_Calamites_, while the individual sporangia and the length of the -sporophyll stalk are similar in appearance to those of _Lepidodendron_. -The wood of the main axis also resembles that of a typical -_Lepidodendron_. The way the vascular bundles of the bract pass out from -the axis, and the way the stalks bearing the sporangia are attached to -the sterile part of the bracts, are like the corresponding features in -_Sphenophyllum_, and still more like _Bowmanites_. - -Many other points of comparison are to be found in these plants, but -without going into further detail enough has been indicated to support -the conclusion that _Cheirostrobus_ is a very important clue to the -affinities of the Sphenophyllales and early Pteridophytes. It is indeed -considered to have belonged to an ancient stock of plants, from which the -Equisetaceae, and _Sphenophylla_, and possibly also the Lycopods all -sprang. - -_Sphenophyllum_, _Bowmanites_, and _Cheirostrobus_, a series of forms -that became extinct in the Palaeozoic, remote in their structure from any -living types, whose existence would have been entirely unsuspected but -for the work of fossil botany, are yet the clues which have led to a -partial solution of the mysteries surrounding the present-day Lycopods -and Equisetums, and which help to bridge the chasm between these remote -and degenerate families. - - - - - CHAPTER XVII - PAST HISTORIES OF PLANT FAMILIES - X. The Lower Plants - - -In the plant world of to-day there are many families including immense -numbers of species whose organization is simpler than that of the groups -hitherto considered. Taken all together they form, in fact, a very large -proportion of the total number of living species, though the bulk of them -are of small size, and many are microscopic. - -These "lower plants" include all the mosses, and the flat green -liverworts, the lichens, the toadstools, and all the innumerable moulds -and parasites causing plant diseases, the green weeds growing in water, -and all the seaweeds, large and small, in the sea, the minute green cells -growing in crevices of the bark of trees, and all the similar ones living -by millions in water. Truly a host of forms with an endless variety of -structures. - -Yet when we turn to the fossil representatives of this formidable -multitude, we find but few. Indeed, of the fossil members of all these -groups taken together we know less that is of importance and real -interest than we do of any single family of those hitherto considered. -The reasons for this dearth of fossils of the lower types are not quite -apparent, but one which may have some bearing on it is the difficulty of -mineralization. It is self-evident that the more delicate and soft-walled -any structure is the less chance has it of being preserved without decay -long enough to be fossilized. As will have been understood from Chapter -II, even when the process of fossilization took place, geologically -speaking, rapidly, it can never have been actually accomplished quickly -as compared with the counter processes of decay. Hence all the lower -plants, with their soft tissue and lack of wood and strengthening cells, -seem on the face of it to stand but little chance of petrifaction. - -There is much in this argument, but it is not a sufficient explanation of -the rarity of lower plant fossils. All through the preceding chapters -mention has been made of very delicate cells, such as pith, spores, and -even germinating spores (see fig. 47, p. 68), with their most delicate -outgrowing cells. If then such small and delicate elements from the -higher plants are preserved, why should not many of the lower plants -(some of which are large and sturdy) be found in the rocks? - -As regards the first group, the mosses, it is probable that they did not -exist in the Palaeozoic period, whence our most delicately preserved -fossils are derived. There seems much to support the view that they have -evolved comparatively recently although they are less highly organized -than the ferns. Quite recently experiments have been made with their near -allies the liverworts, and those which were placed for one year under -conditions similar to those under which plant petrifaction took place, -were found to be perfectly preserved at the end of the period; though -they would naturally decay rapidly under usual conditions. This shows -that Bryophyte cells are not peculiarly incapable of preservation as -fossils, and adds weight to the negative evidence of the rocks, -strengthening the presumption of their late origin. - -That some of the lower plants, among the very lowest and simplest, can be -well preserved is shown in the case of the fossil fungi which often occur -in microscopic sections of palaeozoic leaves, where they infest the higher -plants as similar parasitic species do to-day. - -We must now bring forward the more important of the facts known about the -fossils of the various groups of lower plants. - - -Bryophytes.--_Mosses._ Of this family there are no specimens of any age -which are so preserved as to show their microscopical structure. Of -impressions there are a few from various beds which show, with more or -less uncertainty in most cases, stems and leaves of what appear to be -mosses similar to those now extant, but they nearly all lack the -fructifications which would determine them with certainty. These -impressions go by the name of _Muscites_, which is a dignified cloak for -ignorance in most cases. The few which are quite satisfactory as -impressions belong to comparatively recent rocks. - -_Liverworts_ are similarly scanty, and there is nothing among them which -could throw any light on the living forms or their evolution. The more -common are of the same types as the recent ones, and are called -_Marchantites_, specimens of which have been found in beds of various -ages, chiefly, however, in the more recent periods of the earth's -history. - -It is of interest to note that among all the delicate tissue which is so -well preserved in the "coal balls" and other palaeozoic petrifactions, -there are no specimens which give evidence of the existence of mosses at -that time. It is not unlikely that they may have evolved more recently -than the other groups of the "lower" plants. - -Characeae.--Members of this somewhat isolated family (Stoneworts) are -better known, as they frequently occur as fossil casts. This is probably -due to their character, for even while alive they tend to cover their -delicate stems and leaves, and even fruits, with a limy incrustation. -This assists fossilization to some degree, and fossil Charas are not -uncommon. Usually they are from the recently deposited rocks, and the -earliest true Charas date only to the middle of the Mesozoic. - -An interesting occurrence is the petrifaction of masses of these plants -together, which indicate the existence of an ancient pool in which they -must have grown in abundance at one time. A case has been described where -masses of _Chara_ are petrified where they seem to have been growing, and -in their accumulations had gradually filled up the pond till they had -accumulated to a height of 8 feet. - -The plants, however, have little importance from our present point of -view. - - -Fungi.--Of the higher fungi, namely, "toadstools", we have no true -fossils. Some indications of them have been found in amber, but such -specimens are so unsatisfactory that they can hardly afford much -interest. - - [Illustration: Fig. 119.--The Hyphae of Fungi Parasitic on a Woody Tree - - _c_, Cells of host; _h_, hyphae of fungus, with dividing cell walls.] - -The lower fungi, however, and in particular the microscopic and parasitic -forms, occur very frequently, and are found in the Coal Measure fossils. -Penetrating the tissues of the higher plants, their hosts, the parasitic -cells are often excellently preserved, and we may see their delicate -hyphae wandering from cell to cell as in fig. 119, while sometimes there -are attached swollen cells which seem to be sporangia. From the Palaeozoic -we get leaves with nests of spores of the fungus which had attacked and -spotted them as so many do to leaves to-day (see fig. 120). What is -specially noticeable about these plants is their similarity to the living -forms infesting the higher plants of the present day. Already in the -Palaeozoic the sharp distinction existed between the highly organized -independent higher plants and their simple parasites. The higher plants -have changed profoundly since that time, stimulated by ever-changing -surroundings, but the parasites living within them are now much as they -were then, just sufficiently highly organized to rob and reproduce. - -A form of fungus inhabitant which seems to be useful to the higher plant -appears also to have existed in Palaeozoic times, viz. _Mycorhiza_. In the -roots of many living trees, particularly such as the Beech and its -allies, the cells of the outer layers are penetrated by many fungal forms -which live in association with the tree and do it some service at the -same time as gaining something for themselves. This curious, and as yet -incompletely understood physiological relation between the higher plants -and the fungi, existed so far back as the Palaeozoic period, from which -roots have been described whose cells were packed with minute organisms -apparently identical with _Mycorhiza_. - - [Illustration: Fig. 120.--Fossil Leaf _l_ with Nests of Infesting - Fungal Spores _f_ on its lower side] - - -Algae.--_Green Algae_ (pond weeds). Many impressions have been described as -algae from time to time, numbers of which have since been shown to be a -variety of other things, sometimes not plants at all. Other impressions -may really be those of algae, but hitherto they have added practically -nothing to our knowledge of the group. - -Several genera of algae coat themselves with calcareous matter while they -are alive, much in the same way as do the Charas, and of these, as is -natural, there are quite a number of fossil remains from Tertiary and -Mesozoic rocks. This is still more the case in the group of the _Red -Algae_ (seaweeds), of which the calcareous-coated genera, such as -_Corallina_ and others, have many fossil representatives. These plants -appear so like corals in many cases that they were long held to be of -animal nature. The genus _Lithothamnion_ now grows attached to rocks, and -is thickly encrusted with calcareous matter. A good many species of this -genus have been described among fossils, particularly from the Tertiary -and Cretaceous rocks. As the plant grew in association with animal -corals, it is not always very easy to separate it from them. - - -Brown Algae (seaweeds) have often been described as fossils. This is very -natural, as so many fossils have been found in marine deposits, and when -among them there is anything showing a dark, wavy impression, it is -usually described as a seaweed. And possibly it may be one, but such an -impression does not lead to much advance in knowledge. From the early -Palaeozoic rocks of both Europe and America a large fossil plant is known -from the partially petrified structure of its stem. There seem to be -several species, or at least different varieties of this, known under the -generic name _Nematophycus_. Specimens of this genus are found to have -several anatomical characters common to the big living seaweeds of the -_Laminaria_ type, and it is very possible that the fossils represent an -early member of that group. In none of these petrified specimens, -however, is there any indication of the microscopic structure of -reproductive organs, so that the exact nature of the fossils is not -determinable. It is probable that though perhaps allied to the Laminarias -they belong to an entirely extinct group. - -An interesting and even amusing chapter might be written on all the -fossils which look like algae and even have been described as such. The -minute river systems that form in the moist mud of a foreshore, if -preserved in the rocks (as they often are, with the ripples and raindrops -of the past), look extraordinarily like seaweeds--as do also countless -impressions and trails of animals. In this portion of the study of -fossils it is better to have a healthy scepticism than an illuminating -imagination. - - -Diatoms, with their hard siliceous shells, are naturally well preserved -as fossils (see fig. 121), for even if the protoplasm decays the mineral -coats remain practically unchanged. - -Diatoms to-day exist in great numbers, both in the cold water of the -polar regions and in the heat of hot springs. Often, in the latter, one -can see them actually being turned into fossils. In the Yellowstone Park -they are accumulating in vast numbers over large areas, and in some -places have collected to a thickness of 6 feet. At the bottoms of -freshwater lakes they may form an almost pure mud of fine texture, while -on the floor of deep oceans there is an ooze of diatoms which have been -separated from the calcareous shells by their greater powers of -resistance to solution by salt water. - - [Illustration: Fig. 121.--Diatom showing the Double Siliceous Coat] - -There are enormous numbers of species now living, and of fossils from the -Tertiary and Upper Mesozoic rocks; but, strangely enough, though so -numerous and so widely distributed, both now and in these past periods, -they have not been found in the earlier rocks. - -In one way the diatoms differ from ordinary fossils. In the latter the -soft tissues of the plant have been replaced by stone, while in the -former the living cell was enclosed in a siliceous case which does not -decompose, thus resembling more the fossils of animal shells. - - -Bacteria are so very minute that it is impossible to recognize them in -ordinary cases. In the matrix of the best-preserved fossils are always -minute crystals and granules that may simulate bacterial shapes -perfectly. _Bacillus_ and _Micrococcus_ of various species have been -described by French writers, but they do not carry conviction. - -As was stated at the beginning of the chapter, from all the fossils of -all the lower-plant families we cannot learn much of prime importance for -the present purpose. Yet, as the history of plants would be incomplete -without mention of the little that is known, the foregoing pages have -been added. - - - - - CHAPTER XVIII - FOSSIL PLANTS AS RECORDS OF ANCIENT COUNTRIES - - -The land which to-day appears so firm and unchanging has been under the -sea many times, and in many different ways has been united to other land -masses to form continents. At each period, doubtless, the solid earth -appeared as stable as it is now, while the country was as well -characterized, and had its typical scenery, plants, and animals. We know -what an important feature of the character of any present country is its -flora; and we have no reason to suspect that it was ever less so than it -is to-day. Indeed, in the ages before men interfered with forest growth, -and built their cities, with their destructive influences, the plants -were relatively more important in the world landscape than they are -to-day. - -As we go back in the periods of geological history we find the plants had -an ever-increasing area of distribution. To-day most individual species -and many genera are limited to islands or parts of continents, but before -the Glacial epoch many were distributed over both America and Europe. In -the Mesozoic _Ginkgo_ was spread all over the world, and in the present -epoch it was confined to China and Japan till it was distributed again by -cultivation; while in the Palaeozoic period _Lepidodendron_ seemed to -stretch wellnigh from pole to pole. - -The importance of the relation of plant structure to the climate and -local physical conditions under which it was growing cannot be too much -insisted upon. Modern biology and ecology are continually enlarging and -rendering more precise our views of this interrelation, so that we can -safely search the details of anatomical structure of the fossil plants -for sidelights on the character of the countries they inhabited and their -climates. - -It has been remarked already that most of the fossils which we have well -preserved, whether of plants or animals, were fossilized in rocks which -collected under sea water; yet it was also noted that of marine plants we -have almost no reliable fossils at all. How comes this seeming -contradiction? - -The lack of marine plant fossils probably depends on their easily -decomposable nature, while the presence of the numerous land plants -resulted from their drifting out to sea in streams and rivers, or -dropping into the still salt marshes where they grew. Hence, in the rocks -deposited in a sea, we have the plants preserved which grew on adjacent -lands. In fresh water, also, the plants of the neighbourhood were often -fossilized; but actually on the land itself but little was preserved. The -winds and rains and decay that are always at work on a land area tend to -break down and wash away its surface, not to build it up. - -There are many different details which are used in determining the -evidence of a fossil plant. Where leaf impressions are preserved which -exhibit a close similarity to living species (as often happens in the -Tertiary period), it is directly assumed that they lived under conditions -like those under which the present plants of that kind are living; while, -if the anatomy is well preserved (as in the Palaeozoic and several -Mesozoic types), we can compare its details with that of similar plants -growing under known conditions, and judge of the climate that had -nurtured the fossil plant while it grew. - -Previous to the present period there was what is so well known as the -Glacial epoch. In the earthy deposits of this age in which fossils are -found plants are not uncommon. They are of the same kind as those now -growing in the cold regions of the Arctic circle, and on the heights of -hills whose temperature is much lower than that of the surrounding -lowlands. Glacial epochs occurred in other parts of the world at -different times; for example, in South Africa, in the Permo-Carboniferous -period, during which time the fossils indicate that the warmth-loving -plants were driven much farther north than is now the case. - -It is largely from the nature of the plant fossils that we know the -climate of England at the time preceding the Glacial epoch. Impressions -of leaves and stems, and even of fruits, are abundant from the various -periods of the Tertiary. Many of them were Angiosperms (see Chap. VIII), -and were of the families and even genera which are now living, of which -not a few belong to the warm regions of the earth, and are subtropical. -It is generally assumed that the fossils related to, or identical with, -these plants must therefore have found in Tertiary Northern Europe a much -warmer climate than now exists. Not only in Northern Europe, but right up -into the Arctic circle, such plants occur in Tertiary rocks, and even if -we had not their living representatives with which to compare them, the -large size and thin texture of their leaves, their smoothness, and a -number of other characteristics would make it certain that the climate -was very much milder than it is at present, though the value of some of -the evidence has been overestimated. - -From the Tertiary we are dependent chiefly on impressions of fossils; -anatomical structure would doubtless yield more details, but even as it -is we have quite enough evidence to throw much light on the physiography -of the Tertiary period. The causes for such marked changes of climate -must be left for the consideration of geologists and astronomers. Plants -are passive, driven before great climatic changes, though they have a -considerable influence on rainfall, as has been proved repeatedly in -India in recent times. - -From the more distant periods it is the plants of the Carboniferous, -whose structure we know so well, that teach us most. Although there is -still very much to be done before knowledge is as complete as we should -wish, there are sufficient facts now discovered to correct several -popular illusions concerning the Palaeozoic period. The "deep, -all-enveloping mists, through which the sun's rays could scarcely -penetrate", which have taken the popular imagination, appear to have no -foundation in fact. There is nothing in the actual structure of the -plants to indicate that the light intensity of the climate in which they -grew was any less than it is in a smoke-free atmosphere to-day. - -Look at the "shade leaves" of any ordinary tree, such as a Lime or Maple, -and compare them with those growing in the sunlight, even on the same -tree. They are larger and softer and thinner. To absorb the same amount -of energy as the more brilliantly lighted leaves, they must expose a -larger surface to the light. Hence if the Coal Measure plants grew in -very great shade, to supply their large growth with the necessary sun -energy we should expect to find enormous spreading leaves. But what is -the fact? No such large leaves are known. _Calamites_ and -_Lepidodendron_, the commonest and most successful plants of the period, -had narrow simple leaves with but a small area of surface. They were, in -fact, leaves of the type we now find growing in exposed places. The ferns -had large divided leaves, but they were finely lobed and did not expose a -large continuous area as a true "shade leaf" does; while the height of -their stems indicates that they were growing in partial shade--at least, -the shade cast by the small-leaved Calamites and Lepidodendrons which -overtopped them. - -Indeed there is no indication from geological evidence that so late as -Palaeozoic times there was any great abnormality of atmosphere, and from -the internal evidence of the plants then growing there is everything to -indicate a dry or physiologically dry[14] sunny condition. - -Of the plant fossils from the Coal Measures we have at least two types. -One, those commonly found in nodules _in_ the coal itself; and the other, -nodules in the rocks above the coal which had drifted from high lands -into the sea. - -The former are the plants which actually formed the coal itself, and from -their internal organization we see that these plants were growing with -partly submerged roots in brackish swamps. Their roots are those of water -plants (see p. 150, young root of Calamite), but their leaves are those -of the "protected" type with narrow surface and various devices for -preventing a loss of water by rapid transpiration. If the water they grew -in had been fresh they would not have had such leaves, for there would -have been no need for them to economize their water, but, as we see in -bogs and brackish or salt water to-day (which is physiologically usable -in only small quantities by the plant), plants even partly submerged -protect their exposed leaves from transpiring largely. - -There are details too numerous to mention in connection with these -coal-forming plants which go to prove that there were large regions of -swampy ground near the sea where they were growing in a bright atmosphere -and uniform climate. Extensive areas of coal, and geological evidence of -still more extensive deposits, show that in Europe in the Coal Measure -period there were vast flats, so near the sea level that they were -constantly being submerged and appearing again as debris drifted and -collected over them. Such a land area must have differed greatly from the -Europe now existing, in all its features. But the whole continent did not -consist of these flats; there were hills and higher ground, largely to -the north-east, on which a dry land flora grew, a flora where several of -the Pteridosperms and _Cordaites_ with its allies were the principal -plants. These plants have leaves so organized as to suggest that they -grew in a region where the climate was bright and dry. - -A fossil flora which has aroused much interest, particularly among -geologists, is that known as the Glossopteris flora. This Palaeozoic flora -has in general characters similar to those of the European -Permo-Carboniferous, but it has special features of its own, in -particular the genus _Glossopteris_ and also the genera _Phyllotheca_ and -_Schizoneura_. - -These genera, with a few others, are characteristic of the -Permo-Carboniferous period in the regions in the Southern Hemisphere now -known by the names of Australasia, South Africa, and South America, and -in India. These regions, at that date, formed what is called by -geologists "Gondwanaland". In the rocks below those containing the plants -there is evidence of glacial conditions, and it is not impossible that -this great difference in climate accounts for the differences which exist -between the flora of the Gondwanaland region and the Northern Hemisphere. -Unfortunately we have not microscopically preserved specimens of the -Glossopteris flora, which could be compared with those of our own -Palaeozoic.[15] - -To describe in detail the series of changes through which the seas and -continents have passed belongs to the realm of pure geology. Here it is -only necessary to point out how the evidence from the fossil plants may -afford much information concerning these continents, and as our knowledge -of fossil anatomy and of recent ecology increases, their evidence will -become still more weighty. Even now, had we no other sources of -information, we could tell from the plants alone where in the past -continents were snow and ice, heat and drought, swamps and hilly land. -However different in their systematic position or scale of evolutional -development, plants have always had similar minute structure and similar -physiological response to the conditions of climate and land surface, so -that in their petrified cells are preserved the histories of countries -and conditions long past. - - - - - CHAPTER XIX - CONCLUSION - - -In the stupendous pageant of living things which moves through creation, -the plants have a place unique and vitally important. Yet so quietly and -so slowly do they live and move that we in our hasty motion often forget -that they, equally with ourselves, belong to the living and evolving -organisms. When we look at the relative structures of plants divided by -long intervals of time we can recognize the progress they make; and this -is what we do in the study of fossil botany. We can place the salient -features of the flora of Palaeozoic and Mesozoic eras in a few pages of -print, and the contrast becomes surprising. But the actual distance in -time between these two types of plants is immense, and must have extended -over several million years; indeed to speak of years becomes meaningless, -for the duration of the periods must have been so vast that they pass -beyond our mental grasp. In these periods we find a contrast in the -characters of the plants as striking as that in the characters of the -animals. Whole families died out, and new ones arose of more complex and -advanced organization. But in height and girth there is little difference -between the earliest and the latest trees; there seems a limit to the -possible size of plants on this planet, as there is to that of animals, -the height of mountains, or the depth of the sea. The "higher plants" are -often less massive and less in height than the lower--Man is less in -stature than was the Dinosaur--and though by no legitimate stretch of the -imagination can we speak of brain in plants, there is an unconscious -superiority of adaptation by which the more highly organized plants -capture the soil they dominate. - -It has been noted in the previous chapters that so far back as the Coal -Measure period the vegetative parts of plants were in many respects -similar to those of the present, it was in the reproductive organs that -the essential differences lay. Naturally, when a race (as all races do) -depends for its very existence on the chain of individuals leading from -generation to generation, the most important items in the plant -structures must be those mechanisms concerned with reproduction. It is -here that we see the most fundamental differences between living and -fossil plants, between the higher and the lower of those now living, -between the forest trees of the present and the forest trees of the past. -The wood of the palaeozoic Lycopods was in the quality and extent and -origin of its secondary growth comparable with that of higher plants -still living to-day--yet in the fruiting organs how vast is the contrast! -The Lycopods, with simple cones composed of scales in whose huge -sporangia were simple single-celled spores; the flowering plants, with -male and female sharply contrasted yet growing in the same cone (one can -legitimately compare a flower with a cone), surrounded by specially -coloured and protective scales, and with the "spore" in the tissue of the -young seed so modified and changed that it is only in a technical sense -that comparison with the Lycopod spore is possible. - -To study the minute details of fossil plants it is necessary to have an -elaborate training in the structure of living ones. In the preceding -chapters only the salient features have been considered, so that from -them we can only glean a knowledge similar to the picture of a house by a -Japanese artist--a thing of few lines. - -Even from the facts brought together in these short chapters, however, it -cannot fail to be evident how large a field fossil botany covers, and -with how many subjects it comes in touch. From the minute details of -plant anatomy and evolution pure and simple to the climate of departed -continents, and from the determination of the geological age of a piece -of rock by means of a blackened fern impression on it to the chemical -questions of the preservative properties of sea water, all is a part of -the study of "fossil botany". - -To bring together the main results of the study in a graphic form is not -an easy task, but it is possible to construct a rough diagram giving some -indication of the distribution of the chief groups of plants in the main -periods of time (see fig. 122). - -Such a diagram can only represent the present state of our imperfect -knowledge; any day discoveries may extend the line of any group up or -down in the series, or may connect the groups together. - -It becomes evident that so early as the Palaeozoic there are nearly as -many types represented as in the present day, and that in fact -everything, up to the higher Gymnosperms, was well developed (for it is -hard indeed to prove that _Cordaites_ is less highly organized than some -of the present Gymnosperm types), but flowering plants and also the true -cycads are wanting, as well as the intermediate Mesozoic Bennettitales. -The peculiar groups of the period were the Pteridosperm series, -connecting links between fern and cycad, and the Sphenophyllums, -connecting in some measure the Lycopods and Calamites. With them some of -the still living groups of ferns, Lycopods, and Equisetaceae were -flourishing, though all the species differed from those now extant. This -shows us how very far from the beginning our earliest information is, for -already in the Palaeozoic we have a flora as diversified as that now -living, though with more primitive characters. - - [Illustration: Fig. 122.--Diagram showing the relative distribution of - the main groups of plants through the geological eras. The dotted lines - connecting the groups and those in the pre-Carboniferous are entirely - theoretical, and merely indicate the conclusions reached at present. - The size of the surface of each group roughly indicates the part it - played in the flora of each period. Those with dotted surface bore - seeds, the others spores.] - -In Mesozoic times the most striking group is that of the Cycads and -Bennettitales, the latter branch suggesting a direct connection between -the fern-cycad series and the flowering plants. This view, so recently -published and upheld by various eminent botanists, is fast gaining -ground. Indeed, so popular has it become among the specialists that there -is a danger of overlooking the real difficulties of the case. The -morphological leap from the leaves and stems of cycads to those of the -flowering plants seems a much more serious matter to presuppose than is -at present recognized. - -As is indicated in the diagram, the groups do not appear isolated by -great unbridged gaps, as they did even twenty years ago. By means of the -fossils either direct connections or probable lines of connection are -discovered which link up the series of families. At present the greatest -gap now lies hedging in the Moss family, and, as was mentioned (p. 163), -fossil botany cannot as yet throw much light on that problem owing to the -lack of fossil mosses. - - -This glimpse into the past suggests a prophecy for the future. Evolution -having proceeded steadily for such vast periods is not likely to stop at -the stage reached by the plants of to-day. What will be the main line of -advance of the plants of the future, and how will they differ from those -of the present? - -We have seen in the past how the differentiation of size in the spores -resulted in sex, and in the higher plants in the modifications along -widely different lines of the male and female; how the large spore -(female) became enclosed in protecting tissues, which finally led up to -true seeds (see p. 75), while the male being so temporary had no such -elaboration. As the seed advances it becomes more and more complex, and -when we reach still higher plants further surrounding tissues are pressed -into its service and it becomes enclosed in the carpel of the highest -flowering plants. After that the seed itself has fewer general duties, -and instead of those of the Gymnosperms with large endosperms collecting -food before the embryo appears, small ovules suffice, which only develop -after fertilization is assured. The various families of flowering plants -have gone further, and the whole complex series of bracts and fertile -parts which make up a flower is adapted to ensure the crossing of male -and female of different individuals. The complex mechanisms which seem -adapted for "cross fertilization" are innumerable, and are found in the -highest groups of the flowering plants. But some have gone beyond the -stage when the individual flowers had each its device, and accomplished -its seed-bearing independently of the other flowers on the same branch. -These have a combination of many flowers crowded together into one -community, in which there is specialization of different flowers for -different duties. In such a composite flower, the Daisy for example, some -are large petalled and brightly coloured to attract the pollen-carrying -insects, some bear the male organs only, and others the female or -seed-producing. Here, then, in the most advanced type of flowering plant -we get back again to the separation of the sexes in separate flowers; but -these flowers are combined in an organized community much more complex -than the cones of the Gymnosperms, for example, where the sexes are -separate on a lower plane of development. - -It seems possible that an important group, if not the dominant group, of -flowering plants in the future will be so organized that the individual -flowers are very simple, with fewer parts than those of to-day, but that -they will be combined in communities of highly specialized individuals in -each flower head or cluster. - -As well as this, in other species the minute structure of the vital -organs may show a development in a direction contrary to what has -hitherto seemed advance. Until recently flowers and their organs have -appeared to us to be specialized in the more advanced groups on such -lines as encourage "cross fertilization". In "cross fertilization", in -fact, has appeared to lie the secret of the strength and advance of the -races of plants. But modern cytologists have found that many of the -plants long believed to depend on cross fertilization are either -self-fertilized or not fertilized at all! They have passed through the -period when their complex structures for ensuring cross fertilization -were used, and though they retain these external structures they have -taken to a simpler method of seed production, and in some cases have even -dispensed with fertilization of the egg cell altogether. The female -vitality increased, the male becomes superfluous. It is simpler and more -direct to breed with only one sex, or to use the pollen of the same -individual. Many flowers are doing this which until recently had not been -suspected of it. We cannot yet tell whether it will work successfully for -centuries to come or is an indication of "race senility". - -Whether in the epochs to come flowering plants will continue to hold the -dominant position which they now do is an interesting theoretical -problem. Flowers were evolved in correlation with insect pollination. One -can conceive of a future, when all the earth is under dominion of man, in -which fruits will be sterilized for man's use, as the banana is now, and -seed formation largely replaced by gardeners' "cuttings". - -In those plants which are now living where the complex mechanisms for -cross-fertilization have been superseded by simple self-fertilization, -the external parts of the more elaborate method are still produced, -though they are apparently futile. In the future these vestigial organs -will be discarded, or developed in a more rudimentary form (for it is -remarkable how organs that were once used by the race reappear in members -of it that have long outgrown their use), and the morphology of the -flower will be greatly simplified. - -Thus we can foresee on both sides much simplified individual flowers--in -the one group the reduced individuals associating together in communities -the members of which are highly specialized, and in the other the -solitary flowers becoming less elaborate and conspicuous, as they no -longer need the assistance of insects (the cleistogamic flowers of the -Violet, for example, even in the present day bend toward the earth, and -lack all the bright attractiveness of ordinary flowers), and perhaps -finally developing underground, where the seeds could directly germinate. - -In the vegetative organs less change is to be expected, the examples from -the past lead us to foresee no great difference in size or general -organization of the essential parts, though the internal anatomy has -varied, and probably will vary, greatly with the whole evolution of the -plant. - -But one more point and we must have done. Why do plants evolve at all? -Why did they do so through the geological ages of the past, and why -should we expect them to do so in the future? The answer to this question -must be less assured than it might have been even twenty years ago, when -the magnetism of Darwin's discoveries and elucidations seemed to obsess -his disciples. "Response to environment" is undoubtedly a potent factor -in the course of evolution, but it is not the cause of it. There seems to -be something inherent in life, something apparently (though that may be -due to our incomplete powers of observation) apart from observable -factors of environment which causes slight spontaneous changes, -_mutations_, and some individuals of a species will suddenly develop in a -new direction in one or other of their parts. If, then, this places them -in a superior position as regards their environment or neighbours, it -persists, but if not, those individuals die out. The work of a special -branch of modern botany seems clearly to indicate the great importance of -this seemingly inexplicable spontaneity of life. In environment alone the -thoughtful student of the present cannot find incentive enough for the -great changes and advances made by organisms in the course of the world's -history. The climate and purely physical conditions of the Coal Measure -period were probably but little different from those in some parts of the -world to-day, but the plants themselves have fundamentally changed. True, -their effect upon each other must be taken into account, but this is a -less active factor with plants than with men, for we can imagine nothing -equivalent to citizenship, society, and education in the plant -communities, which are so vital in human development. - -It seems to have been proved that plants and animals may, at certain -unknown intervals, "mutate"; and mutation is a fine word to express our -recent view of one of the essential factors in evolution. But it is a -cloak for an ignorance avowedly less mitigated than when we thought to -have found a complete explanation of the causes of evolution in -"environment". - - -In a sketch such as the present, outlines alone are possible, detail -cannot be elaborated. If it has suggested enough of atmosphere to show -the vastness of the landscape spreading out before our eyes back into the -past and on into the future, the task has been accomplished. There are -many detailed volumes which follow out one or other special line of -enquiry along the highroads and by-ways of this long traverse in -creation. If the bird's-eye view of the country given in this book -entices some to foot it yard by yard under the guidance of specialists -for each district, it will have done its part. While to those who will -make no intimate acquaintance with so far off a land it presents a short -account by a traveller, so that they may know something of the main -features and a little of the romance of the fossil world. - - - - - APPENDIX I - LIST OF REQUIREMENTS FOR A COLLECTING EXPEDITION - - -In order to obtain the best possible results from an expedition, it is -well to go fossil hunting in a party of two, four, or six persons. Large -parties tend to split up into detachments, or to waste time in trying to -keep together. - -Each individual should have strong suitable clothes, with as many pockets -arranged in them as possible. The weight of the stones can thus be -distributed over the body, and is not felt so much as if they were all -carried in a knapsack. Each collector should also provide himself with-- - - A satchel or knapsack, preferably of leather or strong canvas, but not - of large size, for when the space is limited selection of the specimens - is likely to be made carefully. - - One or two hammers. If only one is carried, it should be of a fair size - with a square head and strong straight edge. - - One chisel, entirely of metal, and with a strong straight cutting edge. - - Soft paper to wrap up the more delicate fossils, in order to prevent - them from scraping each other's surfaces; and one or two small - cardboard boxes for very fragile specimens. - - A map of the district (preferably geologically coloured). Localities - should be noted in pencil on this, indicating the exact spot of finds. - For general work the one-inch survey map suffices, but for detailed - work it is necessary to have the six-inch maps of important districts. - - A small notebook. Few notes are needed, but those few _must_ be taken - on the spot to be reliable. - - A pencil or fountain pen, preferably both. - - A penknife, which, among other things, will be found useful for working - out very delicate fossils. - - - - - APPENDIX II - TREATMENT OF SPECIMENS - - -1. The commonest form in which fossils are collected is that which has -been described as _impression material_ (see p. 12). In many cases these -will need no further attention after the block of stone on which they lie -has been chipped into shape. - -In chipping a block down to the size required it is best to hold it -freely in the left hand, protecting the actual specimen with the palm -where possible, and taking the surplus edges away by means of short sharp -blows from the hammer, striking so that only small pieces come away with -each blow. For delicate specimens it is wise to leave a good margin of -the matrix round the specimen, and to do the final clearing with a -thin-bladed penknife, taking away small flakes of the stone with delicate -taps on the handle of the knife. - -Specimens from fine sandstones, shales, and limestones are usually -thoroughly hard and resistant, and are then much better if left without -treatment; by varnishing and polishing them many amateur collectors spoil -their specimens, for a coat of shiny varnish often conceals the details -of the fossil itself. Impressions of plants on friable shales, on the -other hand, or those which have a tendency to peel off as they dry, will -require some treatment. In such cases the best substance to use is a -dilute solution of size, in which the specimen should soak for a short -period while the liquid is warm (not hot), after which it should be -slightly drained and the size allowed to dry in. The congealed substance -then holds the plant film on to the rock surface and prevents the rock -from crumbling away, while it is almost invisible and does not spoil the -plant with any excessive glaze. - -2. For specimens of _casts_ the same treatment generally applies, though -they are more apt to separate completely from the matrix after one or two -sharp blows, and thus save one the work of picking out the details of -their structure. - -3. Those blocks which contain _petrifactions_, and can therefore be made -to show microscopic details, will require much more treatment. In some -cases mere polishing reveals much of the structure--such, for instance, -were the "Staarsteine" of the German lapidaries, where the axis and -rootlets of a fossil like a treefern show their very characteristic -pattern distinctly. - -As a rule, however, it is better, and for any detailed work it is -essential, to cut thin sections transversely across and longitudinally -through the axis of the specimen and to grind them down till they are so -transparent that they can be studied through the microscope. The cutting -can be done on a lapidary's wheel, where a revolving metal disc set with -diamond powder acts as a knife. The comparatively thin slice thus -obtained is fastened on to glass by means of hard Canada balsam, and -rubbed down with carborundum powder till it is thin enough. - -The process, however, is very slow, and an amateur cannot get good -results without spending a large amount of time and patience over the -work which would be better spent over the study of the plant structures -themselves. Therefore it is usually more economical to send specimens to -be cut by a professional, if they are good enough to be worth cutting at -all, though it is often advisable to cut through an unpromising block to -see whether its preservation is such as would justify the expense. - -In the case of true "coal balls" much can be seen on the cut surface of a -block, particularly if it be washed for a minute in dilute hydrochloric -acid and then in water, and then dried thoroughly. The acid acts on the -carbonates of which the stone is largely composed, and the treatment -accentuates the black-and-white contrast in the petrified tissues (see -fig. 10). After lying about for a few months the sharpness of the surface -gets rubbed off, as the acid eats it into very delicate irregularities -which break and form a smearing powder; but in such a case all that is -needed to bring back the original perfection of definition is a quick -wash of dilute acid and water. If the specimens are not rubbed at all the -surface is practically permanent. Blocks so treated reveal a remarkable -amount of detail when examined with a strong hand lens, and form very -valuable museum specimens. - -The microscope slides should be covered with glass slips (as they would -naturally be if purchased), and studied under the microscope as sections -of living plants would be. - -Microscopic slides of fossils make excellent museum specimens when -mounted as transparencies against a window or strong light, when a -magnifying glass will reveal all but the last minutiae of their structure. - -4. _Labelling_ and numbering of specimens is very important, even if the -collection be but a small one. As well as the paper label giving full -details, there should be a reference number on every specimen itself. On -the microscope slides this can be cut with a diamond pencil, and on the -stones sealing wax dissolved in alcohol painted on with a brush is -perhaps the best medium. On light-coloured close-textured stones ink is -good, and when quite dry can even be washed without blurring. - -The importance of marking the stone itself will be brought home to one on -going through an old collection where the paper labels have peeled or -rubbed off, or their wording been obliterated by age or mould. - -A notebook should be kept in which the numbers are entered, with a note -of all the items on the paper label, and any additional details of -interest. - - - - - APPENDIX III - LITERATURE - - -A short list of a few of the more important papers and books to which a -student should refer. The innumerable papers of the specialists will be -found cited in these, so that, as they would be read only by advanced -students, there is no attempt to catalogue them here. - - -Carruthers, W., "On Fossil Cycadean Stems from the Secondary Rocks of -Britain," published in the _Transactions of the Linnean Society_, vol. -xxvi, 1870. - - -*Geikie, A., _A Text-Book of Geology_, vols. i and ii, London, 1903. - - -Grand'Eury, C., "Flore Carbonifere du departement de la Loire et du -centre de la France", published in the _Memoirs de l'Academie des -Sciences_, Paris, vol. xxiv, 1877. - - -*Kidston, R., _Catalogue of the Palaeozoic Plants in the Department of -Geology and Palaeontology of the British Museum_, London, 1886. - - -*Lapworth, C., _An Intermediate Text-Book of Geology_, twelfth edition, -London, 1888. - - -Laurent, L., "Les Progres de la paleobotanique angiospermique dans la -derniere decade", _Progressus Rei Botanicae_, vol. i, Heft 2, pp. 319-68, -Jena, 1907. - - -Lindley, J., and Hutton, W., _The Fossil Flora of Great Britain_, 3 -vols., published in London, 1831-7. - - -Lyell, C., _Principles of Geology_ and _The Student's Lyell_, edited by -J. W. Judd, London, 1896. - - -Oliver, F. W., and Scott, D. H., "On the Structure of the Palaeozoic Seed, -_Lagenostoma Lomaxi_", published in the _Transactions of the Royal -Society_, series B, vol. cxcvii, London, 1904. - - -Renault, B., _Cours de Botanique fossile_, Paris, 1882, 4 vols. - - -Renault, B., _Bassin Houiller et Permien d'Autun et d'Epinac_, Atlas and -Text, 1893-6, Paris. - - -*Scott, D. H., _Studies in Fossil Botany_, London, second edition, 1909. - - -Scott, D. H., "On the Structure and Affinities of Fossil Plants from the -Palaeozoic Rocks. On _Cheirostrobus_, a New Type of Fossil Cone from the -Lower Carboniferous Strata." Published in the _Philosophical Transactions -of the Royal Society_, vol. clxxxix, B, 1897. - - -*Seward, A. C., _Fossil Plants_, vol. i, Cambridge, 1898. - - -Seward, A. C., _Catalogue of the Mesozoic Plants in the Department of -Geology of the British Museum_, Parts I and II, London, 1894-5. - - -*Solms-Laubach, Graf zu, _Fossil Botany_ (translation from the German), -Oxford, 1891. - - -Stopes, M. C., and Watson, D. M. S., "On the Structure and Affinities of -the Calcareous Concretions known as 'Coal Balls'", published in the -_Philosophical Transactions of the Royal Society_, vol. cc. - - -*Stopes, M. C., _The Study of Plant Life for Young People_, London, 1906. - - -*Watts, W. W., _Geology for Beginners_, London, 1905 (second edition). - - -Wieland, G. R., _American Fossil Cycads_, Carnegie Institute, 1906. - - -Williamson, W. C., A whole series of publications in the _Philosophical -Transactions of the Royal Society_ from 1871 to 1891, and three later -ones jointly with Dr. Scott; the series entitled "On the Organization of -the Fossil Plants of the Coal Measures", Memoir I, II, &c. - - -Zeiller, R., _Elements de Paleobotanique_, Paris, 1900. - - -*Zittel, K., _Handbuch der Palaeontologie_, vol. ii; _Palaeophytologie_, by -Schimper & Schenk, Muenchen and Leipzig, 1900. - - Those marked * would be found the most useful for one beginning the - subject. - - - - - GLOSSARY - - -Some of the more technical terms about which there might be some doubt, -as they are not always accompanied by explanations in the text, are here -briefly defined. - - -Anatomy.--The study of the details and relative arrangements of the -internal features of plants; in particular, the relations of the -different tissue systems. - - -Bracts.--Organs of the nature of leaves, though not usual foliage leaves. -They often surround fructifications, and are generally brown and scaly, -though they may be brightly coloured or merely green. - - -Calcareous.--Containing earthy carbonates, particularly calcium carbonate -(chalk). - - -Cambium.--Narrow living cells, which are constantly dividing and giving -rise to new tissues (see fig. 33, p. 57). - - -Carbonates, as used in this book, refer to the combinations of some -earthy mineral, such as calcium or magnesium, combined with carbonic acid -gas and oxygen, formula CaCO_3, MgCO_3, &c. - - -Carpel.--The closed structure covering the seeds which grow attached to -it. The "husk" of a peapod is a carpel. - - -Cell.--The unit of a plant body. Fundamentally a mass of living -protoplasm with its nucleus, surrounded in most cases by a wall. Mature -cells show many varieties of shape and organization. See Chapter VI, p. -54. - - -Centrifugal.--Wood or other tissues developed away from the centre of the -stem. See fig. 65, p. 97. - - -Centripetal.--Wood or other tissues developed towards the centre of the -stem. See fig. 65, p. 97. - - -Chloroplast.--The microscopic coloured masses, usually round, green -bodies, in the cells of plants which are actively assimilating. - - -Coal Balls.--Masses of carbonate of calcium, magnesium, &c., generally of -roundish form, which are found embedded in the coal, and contain -petrified plant tissues. See p. 28. - - -Concretions.--Roundish mineral masses, formed in concentric layers, like -the coats of an onion. See p. 27. - - -Cotyledons.--The first leaves of an embryo. In many cases packed with -food and filling the seed. See fig. 58. - - -Cross Fertilization.--The fusion of male and female cells from different -plants. - - -Cuticle.--A skin of a special chemical nature which forms on the outer -wall of the epidermis cells. See p. 54, fig. 21. - - -Earth Movements.--The gradual shifting of the level of the land, and the -bending and contortions of rocks which result from the slow shrinking of -the earth's surface, and give rise to earthquakes and volcanic action. - - -Embryo.--The very young plant, sometimes consisting of only a few -delicate cells, which results from the divisions of the fertilized egg -cell. The embryo is an essential part of modern seeds, and often fills -the whole seed, as in a bean, where the two fleshy masses filling it are -the two first leaves of the embryo. See fig. 58, p. 77. - - -Endodermis.--The specialized layer of cells forming a sheath round the -vascular tissue. See p. 55. - - -Endosperm.--The many-celled tissue which fills the large "spore" in the -Gymnosperm seed, into which the embryo finally grows. See fig. 57. - - -Epidermis.--Outer layer of cells, which forms a skin, in the -multicellular plants. See fig. 21, p. 54. - - -Fruit.--Essentially consisting of a seed or seeds, enclosed in some -surrounding tissues, which may be only those of the carpel, or may also -be other parts of the flower fused to it. Thus a peapod is a _fruit_, -containing the peas, which are seeds. - - -Gannister.--A very hard, gritty rock found below some coal seams. See p. -25. - - -Genus.--A small group within a family which includes all the plants very -like each other, to which are all given the same "surname"; e.g. _Pinus -montana_, _Pinus sylvestris_, _Pinus Pinaster_, &c. &c., are all members -of the genus _Pinus_, and would be called "pine trees" in general (see -"Species"). - - -Hyphae.--The delicate elongated cells of Fungi. - - -Molecule.--The group of chemical elements, in a definite proportion, -which is the basis of any compound substance; _e.g._ two atoms of -hydrogen and one atom of oxygen form a molecule of water, H_2O. A lime -carbonate molecule (see definition of "Carbonate") is represented as -CaCO_3. - - -Monostelic.--A type of stem that contains only one stele. - - -Morphology.--The study of the features of plants, their shapes and -relations, and the theories regarding the origin of the organs. - - -Nucellus.--The tissue in a Gymnosperm seed in which the large "spore" -develops. See figs. 55 and 56, p. 76. - - -Nucleus.--The more compact mass of protoplasm in the centre of each -living cell, which controls its growth and division. See fig. 17, _n._ - - -Palaeobotany.--The study of fossil plants. - - -Palaeontology.--The study of fossil organisms, both plants and animals. - - -Petiole.--The stalk of a leaf, which attaches it to the stem. - - -Phloem.--Commonly called "bast". The elongated vessel-like cells which -conduct the manufactured food. See p. 57. - - -Pollen Chamber.--The cavity inside a Gymnosperm seed in which the pollen -grains rest for some time before giving out the male cells which -fertilize the egg-cell in the seed. See p. 76. - - -Polystelic.--A type of stem that appears, in any transverse section, to -contain several steles. See note on the use of the word on p. 63. - - -Protoplasm.--The colourless, constantly moving mass of finely granulated, -jelly-like substance, which is the essentially living part of both plants -and animals. - - -Rock.--Used by a geologist for all kinds of earth layers. Clay, and even -gravel, are "rocks" in a geological sense. - - -Roof, of a coal seam. The layers of rock--usually shale, limestone, or -sandstone--which lie just above the coal. See p. 24. - - -Sclerenchyma.--Cells with very thick walls, specially modified for -strengthening the tissues. See fig. 28, p. 56. - - -Seed.--Essentially consisting of a young embryo and the tissues round it, -which are enclosed in a double coat. See definition of "Fruit". - - -Shale.--A fine-grained soft rock, formed of dried and pressed mud or -silt, which tends to split into thin sheets, on the surface of which -fossils are often found. - - -Species.--Individuals which in all essentials are identical are said to -be of the same species. As there are many variations which are not -essential, it is sometimes far from easy to draw the boundary between -actual species. The specific name comes after that of the genus, e.g. -_Pinus montana_ is a species of the genus _Pinus_, as is also _Pinus -sylvestris_. See "Genus". - - -Sporangium.--The saclike case which contains the spores. See figs. 52 and -53, p. 75. - - -Spore.--A single cell (generally protected by a cell wall) which has the -power of germinating and reproducing the plant of which it is the -reproductive body. See p. 75. - - -Sporophyll.--A leaf or part of a leaf which bears spores or seeds, and -which may be much or little modified. - - -Stele.--A strand of vascular tissue completely enclosed in an endodermis. -See p. 62. - - -Stigma.--A special protuberance of the carpel in flowering plants which -catches the pollen grains. - - -Stomates.--Breathing pores in the epidermis, which form as a space -between two curved liplike cells. See fig. 23, p. 54. - - -Tetrads.--Groups of four cells which develop by the division of a single -cell called the "mother cell". Spores and pollen grains are nearly always -formed in this way. See p. 75. - - -Tracheid.--A cell specially modified for conducting or storing of water, -often much elongated. The long wood cells of Ferns and Gymnosperms are -tracheids. - - -Underclay.--The fine clay found immediately below some coal seams. See p. -24. - - -Vascular Tissue.--The elongated cells which are specialized for -conduction of water and semifluid foodstuffs. - - - - - FOOTNOTES - - -[1]My book was entirely written before the second edition of Scott's - _Studies_ appeared, which, had it been available, would have tempted - me to escape some of the labour several of the chapters of this - little book involved. - -[2]The student would do well to read up the general geology of this very - interesting subject. Such books as Lyell's _Principles of Geology_, - Geikie's textbooks, and many others, provide information about the - process of "mountain building" on which the form of our coalfields - depends. A good elementary account is to be found in Watt's _Geology - for Beginners_, p. 96 _et seq._ - -[3]See note on p. 28. - -[4]This refers only to the "coal-ball"-bearing seams; there are many - other coals which have certainly collected in other ways. See Stopes - & Watson, Appendix, p. 187. - -[5]For a detailed list of the strata refer to Watts, p. 219 (see - Appendix). - -[6]Though the Angiosperm was not then evolved, the Gymnosperm stem has - distinct vascular bundles arranged as are those of the Angiosperm, - the difference here lies in the type of wood cells. - -[7]The gametophyte generation (represented in the ferns by the - prothallium on which the sexual organs develop) alternates with the - large, leafy sporophyte. Refer to Scott's volume on _Flowerless - Plants_ (see Appendix) for an account of this alternation of - generations. - -[8]Material recently obtained by the author and Dr. Fujii in Japan does - contain some true petrifactions of Angiosperms and other plant - debris. The account of these discoveries has not yet been published. - -[9]A fuller account of the Angiospermic flora can be had in French, in M. - Laurent's paper in _Progressus Rei Botanicae_. See Appendix for - reference. - -[10]From the Cretaceous deposits of North America several fossil forms - (_Brachyphyllum_, _Protodammara_) are described which show clear - affinities with the family as it is now constituted. (See Hollick and - Jeffrey; reference in the Appendix.) - -[11]The addition of _-oxylon_ to the generic name of any living type - indicates that we are dealing with a fossil which closely resembles - the living type so far as we have information from the petrified - material. - -[12]See reference in the Appendix to this richly illustrated volume. - -[13]For fuller description of this interesting cone, see Scott's - _Studies_, p. 114 _et seq._ - -[14]A brackish swampy land is physiologically dry, as the plants cannot - use the water. See Warming's _Oecology of Plants_, English edition, - for a detailed account of such conditions. For a simple account see - Stopes' _The Study of Plant Life_, p. 170. - -[15]The student interested in this special flora should refer to Arber's - British Museum _Catalogue of the Fossil Plants of the Glossopteris - Flora_. - - - - - INDEX - - - (_Italicized numbers refer to illustrations_) - - - Abietineae, 88, 89, 90. - -- family characters of, 91. - Algae, 44, 47, 165. - -- brown, 166. - -- green, 165. - -- red, 165. - _Alnus_, 85. - Amber, 17. - Amentiferae, 84. - Anatomy of fossil plants, likeness in detail to that of living plants, - 53 et seq. - -- -- -- -- differences in detail from that of living plants, 69 et - seq. - _Andromeda_, 84. - Angiosperms, comparison with Bennettitales, 103. - -- early history of, 79 et seq. - -- general distribution of in time, 177. - -- later evolution of, 178. - -- male cell of, 52. - Araliaceae, 85. - Araucareae, 88, 90, 111. - -- description of, 90. - -- primitive characters of, 89. - _Araucarioxylon_, 93, 95. - Arber, 173. - _Archaeocalamites_, 152. - Artocarpaceae, 85. - _Asterochlaena_, 126, 127, _86_, _89_. - - - _Bacillus_, 167. - Bacteria, 167. - _Baiera_, 101. - Bast, 57, _32_. - Bennettitales, 44, 102, 131. - -- general distribution of in time, 177. - _Bennettites_, 103 et seq. - -- external appearance of, 103. - -- flower-like nature of fructification, 108. - -- fructification of, 104, _71_, 105, _72_. - -- seed of, 106, _73_. - Bertrand, 2. - _Betula_, 85. - _Bignonia_, 84. - Botryopterideae, 125, 132. - -- description of group, 125. - -- fructifications of, 128. - -- petioles of, 127, _89_. - -- stem anatomy of, 126, _86_, _87_. - -- wood of, 128, _90_. - _Botryopteris_, 126, 127. - -- axis with petiole, 127, _88_, _89_. - _Bowmanites Roemeri_, 158, 160. - _Brachyphyllum_, 89. - Brongniart, 2. - Bryophytes, 163. - - - _Calamites_, 147, 154, 157, 159, 160, 171. - -- branch of, 147, _104_. - -- _casheana_, 152. - -- cone of, 150, _109_, 151, _110_. - -- leaf of, 149, _107_. - -- node of, 149, _106_. - -- spores of, 152, _111_. - -- young roots of, 150, _108_. - -- young stem of, 148, _105_. - Cambium, 57, _33_, 65, _43_, 66, _44_. - Carbon, film of representing decayed plant, 12. - Carbonate of magnesium, 20. - Carbonates of lime, 19. - Carpels, modified leaves, 78. - Carruthers, 186. - Casts of fossil plants, 8, 9, _2_, 10, _3_, _4_, 11, 12. - -- of seeds, 11, 12. - -- treatment of specimens of, 184. - _Casuarina_, 83. - Cells, similarity of living and fossil types of, 53. - -- principal types of, 53 et seq., _22_-_33_. - Cell wall, 47, _17_. - Centrifugal wood, 97, _65_. - Centripetal wood, 97, _65_, 116. - _Chara_, 16. - Characeae, 163. - _Cheirostrobus_, 159, _118_, 160. - Chloroplast, 47, _17_. - Coal, origin of, 29 et seq. - -- of different ages, 33. - -- seams in the rocks, 24, _13_, _14_. - -- vegetable nature of, 25 et seq. - -- importance of, 17, Chap. III, p. 22 et seq. - "Coal balls", 18, 19, _10_, 20, 21, 22, 27, _15_, 163, 185. - -- -- mass of, in coal, 28, _16_. - Coal Measures, climate of, 172. - Companion cells, 57, _32_. - Concretions, 21, 22, 27, _15_, 28. - -- concentric banding in, 27, _15_. - Conducting tissue in higher plants, 49, _19_, 50, _20_. - Coniferales, conflicting observations among, 46. - -- general distribution in time, 177. - -- male cell of, 52. - _Corallina_, 166. - Cordaiteae, 39-88, 112. - -- comparison of fructifications with those of Taxeae, 95. - -- description of family, 92. - -- general distribution in time, 177. - _Cordaites_, 40, 107, 176. - -- fructification of, 95, 96, _64_. - -- internal cast of stem, 10, _4_, 93, 94, 95. - -- leaves of, once considered to be Monocotyledons, 82, 93. - -- leaves of, 93, _61_, 94, _62A_. - -- possible common origin with Ginkgo, 102. - -- wood of, 94, _62B_. - Cork, 56, _29_. - -- cambium, 56, _29_. - Cross fertilization, 179. - Cupresseae, 88, 90. - -- description of, 91. - _Cycadeoidea_, 103. - Cycads in the Mesozoic period, 40, 41, 42, 113. - -- description of group, 109. - -- general distribution of in time, 177. - -- in Tertiary period, 85. - -- large size of male cones of, 110. - -- seeds of, 112. - -- type of seed of, 76, _57_. - -- wood of, 110. - _Cycas_, 109, 110, _74_. - -- seed-bearing sporophyll of, 111. - -- seeds of, 112, _76_. - -- comparison with Ginkgo seeds, 112. - Darwin, 181. - Diatoms, 167, _121_. - Dicotyledons, 41, 44, 79. - -- relative antiquity of, 81, 82. - -- seed type of, 77, _58_. - Differentiation, commencement of in simple plants, 48. - -- of tissues in higher plants, 49, _19_, 50 et seq., _20_. - - - Embryo of _Ginkgo_, 100. - -- in seeds, 76, _57_, 77, _58_. - -- of _Bennettites_, 106, _73_. - Endodermis, 55, _26_, 61. - Environment, 181, 182. - Epidermal tissues, 54, _21_, _22_, _23_, 125. - Epidermis cells, fossil impressions of, 13, 14, _8_, 59, _34_, 125. - Equisetales, 44. - -- general distribution of in time, 177. - _Equisetites_, 146, _103_. - _Equisetum_, 9, 38, 40, 44, 145, 149, 152. - -- underground rhizomes of, 43. - _Eucalyptus_, 83. - Europe, 87, 102. - -- ancient climates of, 170. - Evolution, 43. - -- in plants, various degrees of in the organs of the same plant, 45 et - seq. - Evolution in plants, cause of, 181. - -- -- -- suggestions as to possible future lines of, 178. - Expedition, requirements for collecting, 183. - Extinct families, 44. - - - Ferns, sporangia of, 67, _45_. - -- connection with Pteridosperms, 123. - -- description of group, 124. - -- fructifications of among fossils, 131, 132, _92_. - -- general distribution of in time, 177. - -- germinating spores of, 68, _47_. - _Ficus_, 83. - Flotsam, 6. - Flowering plant, anatomy of stem, 49, _19_. - Formation of rocks, key to processes, 6. - Fossil plants, indications of ancient climates and conditions, 168. - -- -- diagram illustration the distribution of, 177, _122_. - Fungi, fossils of, 164. - -- parasitic, _119_, 164, 165, _120_. - - - Gamopetalae, 84. - Gannister, 25, _14_. - Geikie, 186. - _Ginkgo_, leaf impression, 14, 7, 100, _69_. - -- comparison with _Cycas_ seeds, 112. - -- distribution in the past, 168. - -- embryo of, 100. - -- epidermis of fossil, 14, _8_, 100. - -- foliage of, 99, _66_. - -- only living species of genus, 98, _70_. - -- possible common origin with _Cordaites_, 102. - -- ripe seed of, 99, _67_. - -- section of seed of, 100, _68_. - -- seed structure of, 76, _57_. - -- similarity to _Cordaites_, 96. - Ginkgoaceae, 88. - Ginkgoales, 88, 98. - -- description of group, 98. - -- general distribution in time, 177. - Glacial epoch, 170. - _Glossopteris_, 173. - _Glyptostrobus_, 86. - Gondwanaland, 173. - Grand'Eury, 186. - Gum, 17. - Gymnosperms, 38, 41, 44, 86, 176, 179. - -- connection with Pteridosperms, 124. - -- general distribution in time, 177. - -- relations between the groups of, 88, 89, 90. - - - Hairs, 54, _22_, 70. - -- special forms among fossils, 70. - _Heterangium_, 119, 122, 123, 127. - -- foliage of, 120. - -- stem of, 120, _81_. - Hollick and Jeffrey, 89. - Horsetails, description of group, 145. - Hutton, 2. - - - Impression, form of fossil, _5_, 12, 13, _6_, 14, _7_, 15, 80, 81, - _59_, 60. - -- treatment of specimens of, 184. - Investigators of fossil plants, 2. - Iron sulphide, 20. - - - Jet, 17. - Juglandaceae, 83. - - - Kauri pine, 93. - Kew Gardens, 98. - Kidston, 186. - - - Labelling of specimens, 185. - _Lagenostoma_, 76, _56_, 118, 119, _80_. - _Laminaria_, 166. - Lapworth, 186. - Latex cells, 55, _27_. - Lauraceae, 85. - Laurent, 186. - Leaves, starch manufacture in cells of, 58. - -- fossil leaf anatomy, 59, _34_. - -- general similarity of living and fossil, 58. - _Lepidocarpon_, 141, _100_. - _Lepidodendron_, 9, 10, _3_, 21, _12_, 67, _46_, 72, 75, 134, 144, 145, - 157, 160, 171. - -- anatomy of stem of, 136, 137, _95_, 138, _96_, 139, _97_. - -- comparison of reproductive organs with those of living lycopods, 67, - _46_. - _Lepidodendron_, description of, 134. - -- distribution in the past, 177. - -- fructification of, 139, 140, _98_, 141, _99_. - -- huge stumps of, 134, frontispiece. - -- leaf bases, 10, _3_, 135, _93_. - -- leaf traces of, 139, _97_. - -- peculiar fructification of, 75, _54_. - -- petrifaction of leaves, 21, _12_. - -- rootlike organs of, 69. - -- secondary thickening in, 70, _48_, 71, _49_. - -- _selaginoides_, stem of, 137, _95_. - -- wood of, 70, _48_, 71, _49_. - Liliaceae, 82. - Limestone, 7, _1_, 24, 25, 36. - Lindley, 2, 186. - Literature on fossil plants, 186. - _Lithothamnion_, 166. - Liverworts, 163. - Lycopods, 38, 40, 42, 44, 67, 133, 175. - -- description of group, 133. - -- general distribution in time, 177. - -- reproductive organs of, 67, _46_. - -- secondary wood in fossil, 70, _48_, 71, _49_. - Lyell, 186. - _Lyginodendron_, 115, 116, 122. - -- anatomy of stem of, 116, _78A_. - -- petioles of, 117, 118, _79_. - -- roots of, 117, _78B_. - -- seeds of, 118, 119, _80_. - - - _Magnolia_, 83. - _Marattia_, 130. - Marattiaceae, 125, 129. - -- appearance of, 130. - -- description of group, 129. - _Marchantites_, 163. - _Medullosa_, 72, 73, 119, 120, 121, _82_, _83_, 122, 123. - -- foliage of, 121, _83_. - -- probable seeds of, 121. - -- steles of, 72, _50_, 73, _51_, 120. - Mesozoic, character of flora, 40. - Metaxylem, 57, _31_. - _Mycorhiza_, 165. - _Micrococcus_, 167. - Monocotyledons, 41, 44, 79. - -- relative antiquity of, 81, 82. - Monostelic anatomy, 63, 126. - Mosses, scarcity of fossils of, 162. - Mosses, fossils of, 163. - Mountain building, from deposits under water, 6. - -- -- slow and continuous changes, 35. - _Muscites_, 163. - Mutation, 181. - - - _Nematophycus_, 166. - _Neuropteris_, leaf impression, _6_, 13. - -- foliage of _Medullosa_, 122. - -- with seed attached, 122, _85_. - _Nipa_, 85. - Nodules, 15, 16, _9_. - Nucleus, 47, _17_. - - - Oliver, 187. - _Osmunda_, 125. - Ovule, word unsuitable for palaeozoic "seeds", 77. - - - Palisade cells, 55, _25_. - -- tissue in leaves, 58. - -- -- -- fossil leaf, 59, _34_. - Palms, 85. - Parenchyma, 55, _24_. - Petrifaction of cells, 4. - Petrifactions, 17. - -- of forest debris, 18. - -- treatment of specimens of, 184. - _Phyllotheca_, 173. - Plant, parts of, the same in living and fossil, 59. - -- world, main families in, 44. - _Platanus_, 83. - Polypodiaceae, 124. - Polystelic anatomy, 63, 72. - _Populus_, 83, 85. - Poroxyleae, 88. - -- description of group of, 96. - _Poroxylon_, anatomy of, 97, 116. - Primitive plants, 46. - Primofilices, 132. - Protococcoideae, 47, _17_. - _Protodammara_, 89. - Protoplasm, 47. - Protostele, 62, 70. - Protoxylem, 57, _31_. - _Psaronius_, 129, 130. - -- stem anatomy of, 131, _91_. - Pteridophytes, development of secondary wood in fossil forms of, 72. - Pteridosperms, 44, 104, 114, 131. - -- description of group, 114 et seq. - -- general distribution of in time, 177. - -- summary of characters of, 123. - _Pteris aurita_, 62. - - - Quarries, 7, _1_. - _Quercus_, 83, 85. - - - Race senility, 180. - Ranales, 103. - Renault, 2, 156, 187. - Reproductive organs, likeness between those of living and fossil - plants, 67, _45_, _46_. - -- -- peculiar characters of some from the Palaeozoic, 74. - -- -- simplicity of essential cells of, 52. - Rocks, persistence of mineral constituents, 36. - -- fossils varying in according to the geological age, 37 et seq. - Roof of coal seam, 24, _13_, 25, _14_. - Roots, likeness of structure in living and fossil, 60, _35_. - - - _Salix_, 83. - _Sambucus_, 84. - _Schizoneura_, 173. - Sclerenchyma, 56, _26_, 59, _34_. - Scott, 2, 160, 187. - Secondary wood, development of in fossil members of families now - lacking it, 72. - Seeds, series of types from spores to seeds, 75, 76, _52_-_58_. - -- position on the plant, 77, 78. - -- Tertiary impressions of, 80, 81, _60_. - _Selaginella_, 75, 133, 134. - -- with four spores in a sporangium, 75, _53_. - _Sequoia_, 86. - Seward, 187. - "Shade leaves", 171. - Shale, 7, _1_, 11, 24, 25, 36. - Sieve tubes, 57, _32_. - _Sigillaria_, 142, 145. - _Sigillaria_, cast of leaf bases, 9, _2_, 144, _102_. - -- description of, 144. - Silica, 17. - Silicified wood, 17, 80, 87. - Solms Laubach, 2, 187. - Specimens, treatment of, 184. - Sphenophyllales, 44, 153. - -- description of, 153. - -- general distribution in time, 177. - _Sphenophyllum_, 44, 153, 154, 160. - -- cone of, 157, 116. - -- _fertile_, 158. - -- impression of foliage, 154, _112_. - -- _plurifoliatum_, 153. - -- sporangia of, 158, _117_. - -- stem anatomy, 155, _113_, 156, _114_. - -- stem in coal ball, 20. - -- wood of, 156, _114_, _115_. - _Sphenopteris_, leaf impression, 11, _5_. - -- foliage of Pteridosperms, 115, _77_. - Sporangium of ferns, 67, _45_. - -- of lycopods, 67, _46_. - -- of pteridophytes, 75, _52_, _53_, _54_. - Spores, germinating, in fossil sporangia, 68, _47_. - -- peculiar structures among palaeozoic examples of, 74. - -- series of types from "spores" to "seeds", 75, 76, _52_-_58_. - -- tetrads of, 75, _52_, _53_, _54_. - Sporophyll, 75, _52_, _53_, _54_. - _Stangeria_, 110. - Stele, modifications of, 62, _36_-_42_. - Stems, external similarity in living and fossil, 60. - _Sternbergia_, cast of, 10, _4_. - -- pith cast of _Cordaites_, 93. - _Stigmaria_, 69, 142, 143, 144, 145. - -- rootlet of, 143, _101_. - Stomates, 54, _23_. - -- in fossil epidermis, 14, _8_. - Stoneworts, 163. - Synclines, 23. - - - Taxeae, 88, 90. - -- comparison of fructification with that of Cordaiteae, 95. - -- description of, 92. - Taxeae, fleshy seeds of, 89. - _Taxodium_, 86. - _Taxus_, 82. - Time, divisions of geological time, 34. - Tracheides for water storage, 56, 30. - Tree-ferns, 130. - _Trigonocarpus_, 11, 76, 82, 122, _84_. - -- once supposed to be a Monocotyledon, 82. - -- probably the seed of Medullosa, 121. - _Tubicaulis_, 127, _89_. - - - Unexplored world, 3. - Unicellular plants, 47, _17_. - -- -- division of cells in, 47, 48, _18_. - - - "Vascular bundles", relation of to steles, 65, _42_. - -- tissue, 57, _31_, _32_, _33_, 59. - -- -- continued growth of, 65, _43_. - -- -- importance in plant anatomy, 61 et seq. - _Viburnum_, 84, 85. - - - Watts, 187. - Westphalia, 19. - Wieland, 2, 102, 187. - Williamson, 2, 187. - _Williamsonia_, 104. - Wood, cells composing, 57, _31_. - -- centrifugal development of, 97, _65_. - -- centripetal development of, 97, _65_. - -- parenchyma, 57, _31_. - -- silicified, 17, 80, 87. - -- solid rings of formed by cambium, 66, _44_. - -- vessels of Angiosperms, 58. - - - Yellowstone Park, 17, 167. - Yew, 82. - _Yucca_, 82. - - - Zeiller, 2, 187. - Zittel, 187. - _Zygopteris_, 127. - - - _By the Same Author_ - - The Study of Plant Life for Young People - - Demy 8vo, fully illustrated, 2s. 6d. net - - -Nature: "Of the various books written for children in elementary schools, -_The Study of Plant Life_ is quite the most logical and intelligent that -we have seen". - - -Manchester Guardian: "With great skill the knowledge gained from simple -experiments in the classroom or laboratory is made effective.... 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