Fossil plants, Vol. 1: [A text-book] for students of botany and geology

by A. C. Seward

CHAPTER II.

RELATION OF PALAEOBOTANY TO BOTANY AND GEOLOGY.

“La recherche du plan de la création, voilà le but vers lequel nos efforts peuvent tendre aujourd’hui.” Gaudry, 1883.

Since the greater refinements and thoroughness of scientific methods and the enormous and ever-increasing mass of literature have inevitably led to extreme specialisation, it is more than ever important to look beyond the immediate limits of one’s own subject, and to note its points of contact with other lines of research. A palaeobotanist is primarily concerned with the determination and description of fossil plants, but he must at the same time constantly keep in view the bearing of his work on wider questions of botanical or geological importance. From the nature of the case, we have in due measure to adapt the methods of work to the particular conditions before us. It is impossible to follow in the case of all fossil species precisely the same treatment as with the more complete and perfect recent plants; but it is of the utmost importance for a student of palaeobotany, by adhering to the methods of recent botany, to preserve as far as he is able the continuity of the past and present floras. Palaeontological work has often been undertaken by men who are pure geologists, and whose knowledge of zoology or botany is of the most superficial character, with the result that biologists have not been able to avail themselves, to any considerable extent, of the records of extinct forms of life. They find the literature is often characterised by a special palaeontological phraseology, and by particular methods of treatment, which are unknown to the student of living plants and animals. From this and other causes a purely artificial division has been made between the science of the organic world of to-day and that of the past.

NEGLECT OF FOSSILS BY BOTANISTS.

Fossils are naturally regarded by a stratigraphical geologist as records which enable him to determine the relative age of fossil-bearing rocks. For such a purpose it is superfluous to inquire into the questions of biological interest which centre round the relics of ancient floras. Primarily concerned, therefore, with fixing the age of strata, it is easy to understand how geologists have been content with a special kind of palaeontology which is out of touch with the methods of systematic zoology or botany. On the other hand, the botanist whose observations and researches have not extended beyond the limits of existing plants, sees in the vast majority of fossil forms merely imperfect specimens, which it is impossible to determine with any degree of scientific accuracy. He prefers to wait for perfect material; or in other words, he decides that fossils must be regarded as outside the range of taxonomic botany. It would seem, then, that the unsatisfactory treatment or comparative neglect of fossil plants, has been in a large measure due to the narrowness of view which too often characterises palaeobotanical literature. This has at once repelled those who have made a slight effort to recognise the subject, and has resulted in a one-sided and, from a biological standpoint, unscientific treatment of this branch of science. It must be admitted that palaeobotanists have frequently brought the subject into disrepute by their over-anxiety to institute specific names for fragments which it is quite impossible to identify. This over-eagerness to determine imperfect specimens, and the practice of drawing conclusions as to botanical affinity without any trustworthy evidence, have naturally given rise to considerable scepticism as to the value of palaeobotanical records. Another point, which will be dealt with at greater length in a later chapter, is that geologists have usually shown a distinct prejudice against fossil plants as indices of geological age; this again, is no doubt to a large extent the result of imperfect and inaccurate methods of description, and of the neglect of and consequent imperfect acquaintance with fossil plants as compared with fossil animals.

The student of fossil plants should endeavour to keep before him the fact that the chief object of his work is to deal with the available material in the most natural and scientific manner; and by adopting the methods of modern botany, he should always aim to follow such lines as may best preserve the continuity of past and present types of plants. Descriptions of floras of past ages and lists of fossil species, should be so compiled that they may serve the same purpose to a stratigraphical geologist, who is practically a geographer of former periods of the Earth’s history, as the accounts of existing floras to students of present day physiography. The effect of carrying out researches on some such lines as these, should be to render available to both botanists and geologists the results of the specialist’s work.

In some cases, palaeobotanical investigations may be of the utmost service to botanical science, and of little or no value to geology. The discovery of a completely preserved gametophyte of Lepidodendron or Calamites, or of a petrified Moss plant in Palaeozoic rocks would appeal to most botanists as a matter of primary importance, but for the stratigraphical geologist such discoveries would possess but little value. On the other hand the discovery of some characteristic species of Coal-Measure plants from a deep boring through Mesozoic or Tertiary strata might be a matter of special geological importance, but to the botanist it would be of no scientific value. In very many instances, however, if the palaeobotanist follows such lines as have been briefly suggested, the results of his labours should be at once useful and readily accessible to botanists and geologists. As Humboldt has said in speaking of Palaeontology, “the analytical study of primitive animal and vegetable life has taken a double direction; the one is purely morphological, and embraces especially the natural history and physiology of organisms, filling up the chasms in the series of still living species by the fossil structures of the primitive world. The second is more specially geognostic, considering fossil remains in their relations to the superposition and relative age of the sedimentary formations[38].”

FOSSIL PLANTS AND DISTRIBUTION.

To turn for a moment to some of the most obvious connections between palaeobotany and the wider sciences of botany and geology. The records of fossil species must occupy a prominent position in the data by which we may hope to solve some at least of the problems of plant evolution. From the point of view of distribution, palaeobotany is of considerable value, not only to the student of geographical botany, but to the geologist, who endeavours to map out the positions of ancient continents with the help of palaeontological evidence. The present distribution of plants and animals represents but one chapter in the history of life on the Earth; and to understand or appreciate the facts which it records, we have to look back through such pages as have been deciphered in the earlier chapters of the volume. The distribution of fossil plants lies at the foundation of the principles of the present grouping of floras on the Earth’s surface. Those who have confined their study of distribution to the plant geography of the present age, must supplement their investigations by reference to the work of palaeobotanical writers. If the lists of plant species drawn up by specialists in fossil botany, have been prepared with a due sense of the important conclusions which botanists may draw from them from the standpoint of distribution, they will be readily accepted as sound links in the chain of evidence. Unfortunately, however, if many of the lists of ancient floras were made use of in such investigations, the conclusions arrived at would frequently be of little value on account of the untrustworthy determinations of many of the species. In the case of particular genera the study of the distribution of the former species both in time and space, that is geologically and geographically, points to rational explanations of, or gives added significance to, the facts of present day distribution. That isolated conifer, Ginkgo biloba L. now restricted to Japan and China, was in former times abundant in Europe and in other parts of the world. It is clearly an exceedingly ancient type, isolated not only in geographical distribution but in botanical affinities, which has reached the last stage in its natural life. The Mammoth trees of California (Sequoia sempervirens Endl., and S. gigantea Lindl. and Gord.) afford other examples of a parallel case. The Tulip tree of North America and China and other allied forms are fairly common in the Tertiary plant beds of Europe, but the living representatives are now exclusively North American. Such differences in distribution as are illustrated by these dicotyledonous forest trees in Tertiary times and at the present day, have been clearly explained with the help of the geological record. Forbes, Darwin, Asa Gray[39] and others have been able to explain many apparent anomalies in the distribution of existing plants, and to reconcile the differences between the past and present distribution of many genera by taking account of the effect on plant life of the glacial period. As the ice gradually crept down from the polar regions and spread over the northern parts of Europe, many plants were driven further south in search of the necessary warmth. In the American continent such migration was rendered possible by the southern land extension; in Europe on the other hand the southerly retreat was cut off by impassable barriers, and the extinction of several genera was the natural result.

The comparatively abundant information which we possess as to the past vegetation of polar regions and the value of such knowledge to geologists and botanists alike is in striking contrast to the absence of similar data as regards Antarctic fossils. Darwin in an exceedingly interesting letter to Hooker à propos of a forthcoming British Association address, referring to this subject writes as follows:—

“The extreme importance of the Arctic fossil plants is self-evident. Take the opportunity of groaning over our ignorance of the Lignite plants of Kerguelen Land, or any Antarctic land. It might do good[40].”

In working out any collection of fossil plants, it would be well, therefore, to bear in mind that our aim should be rather to reproduce an accurate fragment of botanical history, than to perform feats of determination with hopelessly inadequate specimens. Had this principle been generally followed, the number of fossil plant species would be enormously reduced, but the value of the records would be considerably raised.

FOSSIL PLANTS AND CLIMATE.

Our knowledge of plant anatomy, and of those laws of growth which govern certain classes of plants to-day and in past time, has been very materially widened and extended by the facts revealed to us by the detailed study of Coal-Measure species. The modern science of Plant Biology, refounded by Charles Darwin, has thrown considerable light on the laws of plant life, and it enables us to correlate structural characteristics with physiological conditions of growth. Applying the knowledge gained from living plants to the study of such extinct types as permit of close microscopic examination, we may obtain a glimpse into the secrets of the botanical binomics of Palaeozoic times. The wider questions of climatic conditions depend very largely upon the evidence of fossil botany for a rational solution. As an instance of the best authenticated and most striking alternation in climatic conditions in comparatively recent times, we may cite the glacial period or Ice-Age. The existence of Arctic conditions has been proved by purely geological evidence, but it receives additional confirmation, and derives a wider importance from the testimony of fossil plants. In rocks deposited before the spread of ice from high northern latitudes, we find indubitable proofs of a widely distributed subtropical flora in Central and Northern Europe. Passing from these rocks to more recent beds there are found indications of a fall in temperature, and such northern plants as the dwarf Birch, the Arctic Willow and others reveal the southern extension of Arctic cold to our own latitudes.

The distribution of plants in time, that is the range of classes, families, genera and species of plants through the series of strata which make up the crust of the earth, is a matter of primary importance from a botanical as well as from a geological point of view.

Among the earlier writers, Brongniart recognised the marked differences between the earlier and later floras, and attempted to correlate the periods of maximum development of certain classes of plants with definite epochs of geological history. He gives the following classification in which are represented the general outlines of plant development from Palaeozoic to Tertiary times[41].

I. Reign of Acrogens		1.	Carboniferous epoch
		2.	Permian epoch.
II. Reign of Gymnosperms		3.	Triassic epoch.
		4.	Jurassic epoch (including the Wealden).
III. Reign of Angiosperms		5.	Cretaceous epoch.
		6.	Tertiary epoch.

Since Brongniart’s time this method of classification has been extended to many of the smaller subdivisions of the geological epochs, and species of fossil plants are often of the greatest value in questions of correlation. In recent years the systematic treatment of Coal-Measure and other plants in the hands of various Continental and English writers has clearly demonstrated their capabilities for the purpose of subdividing a series of strata into stages and zones[42]. The more complete becomes our knowledge of any flora, the greater possibility there is of making use of the plants as indices of geological age[43].

FOSSIL PLANTS AND PHYLOGENY.

Not only is it possible to derive valuable aid in the correlation of strata from the facts of plant distribution, but we may often follow the various stages in the history of a particular genus as we trace the records of its occurrence through the geologic series. In studying the march of plant life through past ages, the botanist may sometimes follow the progress of a genus from its first appearance, through the time of maximum development, to its decline or extinction. In the Palaeozoic forests there was perhaps no more conspicuous or common tree than the genus long known under the name of Calamites. This plant attained a height of fifty or a hundred feet, with a proportionate girth, and increased in thickness in a manner precisely similar to that in which our forest trees grow in diameter. The exceptionally favourable conditions under which specimens of calamitean plants have been preserved, have enabled us to become almost as familiar with the minute structure of their stems and roots, as well as with their spore-producing organs, as with those of a living species. In short, it is thoroughly established that Calamites agrees in most essential respects with our well known Equisetum, and must be included in the same order, or at least sub-class, as the recent genus of Equisetaceae. As we ascend the geologic series from the Coal-Measures, a marked numerical decline of Calamites is obvious in the Permian period, and in the red sandstones of the Vosges, which belong to the same series of rocks as the Triassic strata of the Cheshire plain, the true Calamites is replaced by a large Equisetum apparently identical in external appearance and habit of growth with the species living to-day. In the more recent strata the Horse-tails are still represented, but the size of the Tertiary species agrees more closely with the comparatively small forms which have such a wide geographical distribution at the present time. Thus we are able to trace out the history of a recent genus of Vascular Cryptogams, and to follow a particular type of organisation from the time of its maximum development, through its gradual transition to those structural characters which are represented in the living descendants of the arborescent Calamites of the coal-period forests. The pages of such a history are frequently imperfect and occasionally missing, but others, again, are written in characters as clear as those which we decipher by a microscopical examination of the tissues of a recent plant.

As one of the most striking instances in which the microscopic study of fossil plants has shown the way to a satisfactory solution of the problems of development, we may mention such extinct genera as Lyginodendron, Myeloxylon and others. Each of these genera will be dealt with at some length in the systematic part of the book, and we shall afterwards discuss the importance of such types, from the point of view of plant evolution.

The botanist who would trace out the phylogeny of any existing class or family, makes it his chief aim to discover points of contact between the particular type of structure which he is investigating, and that of other more or less closely related classes or families.

Confining himself to recent forms, he may discover, here and there, certain anatomical or embryological facts, which suggest promising lines of inquiry in the quest after such affinities as point to a common descent. Without recourse to the evidence afforded by the plants of past ages, we must always admit that our existing classification of the vegetable kingdom is an expression of real gaps which separate the several classes of plants from one another. On the other hand our recently acquired and more accurate knowledge of such genera as have been alluded to, has made us acquainted with types of plant structure which enable us to fill in some of the lacunae in our existing classification. In certain instances we find merged in a single species morphological characteristics which, in the case of recent plants, are regarded as distinctive features of different subdivisions. It has been clearly demonstrated that in Lyginodendron, we have anatomical peculiarities typical of recent cycads, combined with structural characteristics always associated with existing ferns. In rare cases, it happens that the remarkably perfect fossilisation of the tissues of fossil plants, enables us not only to give a complete description of the histology of extinct forms, but also to speak with confidence as to some of those physiological processes which governed their life.

GEOLOGICAL HISTORY.

So far, palaeobotany has been considered in its bearings on the study of recent plants. From a geological point of view the records of ancient floras have scarcely less importance. In recent years, facts have been brought to light, which show that plants have played a more conspicuous part than has usually been supposed as agents of rock-building. As tests of geologic age, there are good grounds for believing that the inferiority of plants to animals is more apparent than real. This question, however, must be discussed at greater length in a later chapter.

Enough has been said to show the many-sided nature of the science of Fossil Plants, and the wide range of the problems which the geologist or botanist may reasonably expect to solve, by means of trustworthy data afforded by scientific palaeobotanical methods.

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