Read Ebook: 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 by Stopes Marie Carmichael

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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.

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".

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 d?bris 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.

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.

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. 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 . 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 resulted from the slow accumulation of plant d?bris 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.

Among various coal seams, evidence for the following modes of coal formation can be found:--

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.

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 , 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.

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. 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:--

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 d?bris 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.

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 , 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 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 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 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 .

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 d?bris, 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 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 , 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.

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.

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 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.

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.

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 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.

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.

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