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
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.
From the morphological standpoint, also, the features of the plant body from the Coal Measure period fall into the same divisions as those of the present. Roots, stems, leaves, and reproductive organs, the essentially distinct parts of a plant, are to be found in a form entirely recognizable, or sufficiently like that now in vogue to be interpreted without great difficulty. In the detailed structure of the reproductive organs more changes have taken place than in any others, both in internal organization and external appearance.
Already, in the Early Palæozoic period, the distinction between leaves, stems, roots, and reproductive organs was as clearly marked as it is to-day, and, judging by their structure, they must each have performed the physiological functions they now do. Roots have changed least in the course of time, probably because, in the earth, they live under comparatively uniform conditions in whatever period of the world’s history they are growing. Naturally, between the roots of different species there are slight differences; but the likeness between fern roots from the Palæozoic and from a living fern is absolutely complete. This is illustrated in fig. 35, which shows the microscopic structure of the two roots when cut in transverse direction. The various tissues will be recognized as coming into the table on p. 54, so that both in the details of individual cells and in the general arrangement of the cell groups or tissues the roots of these fossil and living ferns agree.
Among stems there has been at all periods more variety than among the roots of the corresponding plants, and in the following chapter, when the differences between living and fossil plants will be considered, there will be several important structures to notice. Nevertheless, there are very many characters in which the stems from such widely different epochs agree. The plants in the palæozoic forests were of many kinds, and among them were those with weak trailing stems which climbed over and supported themselves on other plants, and also tall, sturdy shafts of woody trees, many of which were covered with a corky bark. Leaves were attached to the stems, either directly, as in the case of some living plants, or by leaf stalks. In external appearance and in general function the stems then were as stems are now. In the details of the individual cells also the likeness is complete; it is in the grouping of the cells, the anatomy of the tissues, that the important differences lie. It has been remarked already that increase in complexity of the plant form usually goes with an increase in complexity of the cells and variety of the tissues. The general ground tissue in nearly all plants is very similar; it is principally in the vascular system that the advance and variety lie.
Plant anatomists lay particular stress on the vascular system, which, in comparison with animal anatomy, holds an even more important position than does the skeleton. To understand the essential characters of stems, both living and fossil, and to appreciate their points of likeness or difference, it is necessary to have some knowledge of the general facts of anatomy; hence the main points on which stress is laid will be given now in brief outline.
Leaving aside consideration of the more rudimentary and less defined structure of the algæ and mosses, all plants may be said to possess a “vascular system”. This is typically composed of elongated wood (or xylem) with accessory cells, 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. 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.
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 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.
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.
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.
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.
Higher plants in general, however, and in particular flowering plants, do not have a polystelic vascular arrangement, but a specialized type of monostele.
Referring again to fig. 37 as a starting-point, imagine the pith in the center 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.
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 center, 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.
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, and that the development of secondary thickening was very widespread. In several cases the complexity of type exceeds that of modern plants, 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.
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.
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.
Equally similar are the cones of the living Equisetum and some of the simple members of the fossil family Calamiteæ, but the more interesting cases are those where differences of an important morphological nature are to be seen.
As regards the second 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. 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”.
While these few cases illustrate points of likeness between the fructifications of the Coal Measures and of to-day, the large size and successful character of the primitive Coal Measure plants was accompanied by many developments on the part of their reproductive organs which are no longer seen in living forms, and the greater number of palæozoic fructifications must be considered in the next chapter.