Ancient Plants – Conclusion

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 Palæozoic 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 palæozoic 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.

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 Palæozoic 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 Equisetaceæ 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 Palæozoic we have a flora as diversified as that now living, though with more primitive characters.

fig122

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, 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, 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.

Ancient Plants – Fossil Plants as Records of Ancient Countries

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 Palæozoic 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 neighborhood 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 Palæozoic 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, 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 Palæozoic 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 Palæozoic 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 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, 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 Palæozoic 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 Palæozoic.

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.

Ancient Plants – Past Histories of Plant Families: Sphenophyllales

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 Palæozoic 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 Equisetaceæ, 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. 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.

fig112

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. Palæozoic Lycopods and Equisetaceæ 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 center, 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 center, and had we had nothing but these isolated young stems it would have been impossible to recognize their true nature.

fig113

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 center, 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. 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.

fig114

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.

fig115

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 palæobotanist 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. 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.

fig116

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 (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.

fig117

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 Römeri 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.

fig118

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 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 Equisetaceæ, and Sphenophylla, and possibly also the Lycopods all sprang.

Sphenophyllum, Bowmanites, and Cheirostrobus, a series of forms that became extinct in the Palæozoic, 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.

Ancient Plants – Past Histories of Plant Families:The Lycopods

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, 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. In the Lycopods the spores are all alike, in the Selaginellas there are larger spores borne in a small number (four) in some sporangia, 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 palæontological 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. The cone has already been illustrated, and is one of the best known of fossil fructifications.

fig93

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, 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.

fig94

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, where the central cells of the wood are not true water-conducting cells, but short irregular water-storage tracheides, 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.

fig95

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.

fig96

fig96b

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. 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.

fig97

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 lt1) 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. 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. 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.

fig98

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, and it is this which forms the principal guarantee that they belong to the same family, though the size and wood development of the palæozoic 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 fig. 54, in a series of female fructifications, so that its essential structure need not be recapitulated.

fig99

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.

fig100

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 palæozoic Lycopods and the Coniferæ. 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. On these horizontally running structures 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, 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 palæozoic fossils of that group.

fig101

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!nFig. 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. 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, 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 palæozoic 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.

fig102

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.