Plant fossil

“ While ferns in great variety, gymnosperms, and hundreds of flowering plants are known quite definitely, they total only a minute fraction of these groups to-day. Even granting the always imperfect nature of the fossil record, and we have seen what an accident the formation of a plant fossil may be, and it is common knowledge how few comparatively have ever been recovered—even granting all this, there still remains a large part of our present flora of which the origin probably dates from comparatively recent times. ”

“ It is too frequently the case that while we are conversant with the most detailed histological structure of a fossil plant stem, its external form is a matter of conjecture. The conditions which have favoured the petrifaction of plant tissues have as a rule not been favourable for the preservation of good casts or impressions of the external features ”

“ Heer [207] from the Tertiary rocks of Switzerland cannot be accepted as a trustworthy example of a fossil plant, much less of a genus of recent algae. The application of recent generic names to fossils which are possibly not even organic must do more harm than good. ”

“ So overwhelmingly true is this that of the Compositæ, or daisy, family, now numbering over 11,000 species, scarcely a handful of fossil species have been found. And in all collections of fossils the woody plants far outnumber the herbs, perhaps because of the greater probability of their being thus preserved rather than to any actual scarcity of herbs in the upper strata. And yet herbs to-day outnumber woody species over two to one. ”

“ While it is true, then, that our present flora must have been derived from preexisting races, it is also true that much of it is apparently derived from plants that do not date very far back into the past. A few main types of flowering plants unquestionably are to be linked with fossil genera, but these types have now branched out into a wealth of detail that may not have existed and is certainly not recorded in the fossil record. ”

“ 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. ”

“ Of this period, with the exception of fossil algæ, we know practically nothing, and, worst of all, the actual transition from a wholly water-inhabiting flora to these certainly land-inhabiting club mosses may never be known. For, added to the difficulty of water plants being preserved as fossils, already mentioned, is the fact that as they are the oldest, they are found in the deepest strata and, consequently, the hardest to find; and due to changes in the earth’s crust, these ancient fossil-bearing strata have often been much disturbed. ”

“ Among fossil fern fronds 105differences in the form of the pinnules and in the shape and extent of the lamina, to which a specific value is assigned, are no doubt in many cases merely the expression either of differences in the state of the leaves at the time of fossilisation or of the different conditions under which they became embedded. Differential decay and disorganisation of plant tissues are factors of considerable importance with regard to the fossilisation of plants. ”

“ The term ‘fossil’ suggests to the lay mind a petrifaction or a replacement by mineral matter of the plant tissues. In the scientific sense, a fossil plant, that is a plant or part of a plant whether in the form of a true petrifaction or a structureless mould or cast, which has been buried in the earth by natural causes, may be indistinguishable from a piece of recent wood lately fallen from the parent tree. ”

“ The mineralisation of the most delicate tissues and the preservation of the various forms of cell-contents are now generally admitted by those at all conversant with the possibilities of plant petrifaction. If we consider what these facts mean—the microscopic investigation of not only the finest framework but even the very life-substance of Palaeozoic plants—we feel that the aeons since the days when these plants lived have been well-nigh obliterated. ”

“ During the latter half of the present century, however, great additions have been made to our knowledge of fossil plants; and although there are still indications of vast gaps in our knowledge, due, no doubt, to the very exceptional conditions required for the preservation of plant remains, we now possess evidence of a more continuous development of the various types of vegetation. ”

“ It was by slow degrees that the early observers freed themselves from the obsession that the remains of animals and plants in the earth's crust bear witness to a Universal Deluge and are all identical with existing species. The possibility that some of the fossil plants in English strata might be more clearly related to forms now met with in warmer regions was gradually realised. The publication of the Origin of Species stimulated palaeontological research, and botanists as well as zoologists turned to the investigation of extinct genera in search of proofs of the doctrine of evolution. ”

“ 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. ”

“ In some of the uppermost Archaean rocks there have been recently discovered a few undoubted traces of fossil animals, but with this exception no fossils are known throughout the great mass of Archaean strata. It is true that some authorities regard the beds of graphite and other rocks as a proof of the abundance of plant life, but this supposition is not supported by any convincing evidence. ”

“ And even when we get back as far as the oldest fossils will take us, we are still a long way from having reached the epoch when life on the earth originated. As we work back{156}ward and study the fossils of older and older rocks, the multitudinous assembly of plants and animals which fill the world to-day are replaced by other and more primitive forms, many groups approaching each other and merging in common ancestral types. But still, the very oldest fossil-bearing strata contain the remains of organisms already far up the ladder of evolution. ”

“ Fossil plants then, and this delving into the dead past of the plant world, reveals to us as nothing else can how much the modern plant kingdom is literally built upon a mighty race of ancestors. Some perished as did the Cordaitales, but left descendants who themselves gave rise to other groups that survive to-day. ”

“ We constantly find in the several classes of plants a repetition of the same form either in the whole or in the separate members of the vegetative body, and but a slight acquaintance with plant types should lead us to use the test of external resemblance with the greatest possible caution. To emphasize this danger may seem merely the needless reiteration of a self-evident fact, 144but there is, perhaps, no source of error which has been more responsible for the creation of numerous worthless species among fossil plants. ”

“ One of the most striking features in the developmental history of plants and animals is found in the great number of fossil types which have left no descendants.
Both the animal and plant kingdoms furnish a wealth of material with which to demonstrate the aid which paleontology has rendered to morphology. ”

“ 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 17perform 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. ”

“ Though hardy in England, it requires warmer summers for full development and regular flowering. To botanists this Eastern tree is of peculiar interest, partly because of the isolated position it occupies in the plant-kingdom and partly by reason of its great antiquity. There is probably no other existing tree which has so strong a claim to be styled a 'living fossil,' to use a term applied by Darwin to survivals from the past. ”

“ The difficulty of getting any true picture of the beginnings of plant life is great, for those earliest stages of development were unquestionably water-inhabiting plants, whose tissue is mostly too soft and too easily decayed to make fossil impressions of them likely to be preserved. ”

“ Other microfossils, such as fusulinids, ostracodes, spores, and pollens, are also used to identify subsurface formations in many other parts of the State.
Plant fossils are very useful as climatic indicators but are not too reliable for purposes of age determination. They do, however, provide much information about the development of plants throughout geologic time.
33 GEOLOGIC HISTORY
The geologic history of our earth has been recorded primarily in marine sedimentary rocks, and this record indicates that our earth is very old and that life has been present for many millions of years. ”

“ In consequence of this deficiency of the essential characters on which the determinations of the botanist are founded, there exists a necessity for examining more minutely and accurately than has yet been done, the internal structure of recent plants; their habits of growth, the cicatrices or scars left on the stem by the leaves that are spontaneously shed, the different appearances which their fruits exhibit in their various stages of development—all these points must be minutely studied before we can obtain any certainty as to the identity of fossil and living species of plants. ”

“ Unfortunately we cannot always extend our examination of fossil plants beyond the characters of external form and surface markings; but in a few districts there are preserved remnants of ancient floras in which fragments of stems, roots, leaves and other structures have been petrified in such a manner as to retain with wonderful completeness the minute structure of their internal tissues. ”

“ When we inquire what constitutes a good species for any given period, we have reason to believe that many names in our lists represent merely varietal forms or erroneous determinations. This is the case even in the modern flora; and in fossil floras, through the poverty of specimens, their fragmentary condition, and various states « 260 » of preservation, it is still more likely to occur. Every revision of any group of fossils detects numerous synonyms, and of these many are incapable of detection without the comparison of large suites of specimens. ”

“ Perhaps the most important is Schimper’s “Traité de Paléontologie Végétale.” Very useful information is also contained in Renault’s “Cours de Botanique Fossile,” and in Balfour’s “Introduction to Palæontological Botany,” and Nicholson’s “Palæontology.” Unger’s “Genera et Species,” Brongniart’s “Histoire des Végétaux Fossiles,” and Lindley and Button’s “Fossil Flora,” are older though very valuable works. Williamson’s “Memoirs,” in the “Philosophical Transactions,” have greatly advanced our knowledge of the structures of Palæozoic plants. ”

“ Of the accumulated remains of these plants coal seams are really composed, and one cause of the differences amongst them is the different structural composition of the original plants. How far the above fossil flora is to be taken as exhibiting the true proportions of the tribes of plants living on the globe, at the time of the production of the rocks of the carboniferous system, is uncertain: since, when plants are swept down from the land into the sea, it depends on many unknown conditions what sorts of them shall escape the floods, or perish by maceration in the waters. ”

“ Thus the resin which was exuded from trees of long-perished forests is as much a fossil as any portion of the stem, leaves, flowers or fruit, and in some respects is even more valuable to the geologist than more determinable remains of its parent trees, because it has often preserved in admirable perfection the insects which flitted about in the woodlands. The burrows and trails of a worm preserved in sandstone and shale claim recognition as fossils, and indeed are commonly the only indications to be met with of the existence of annelid life among old geological formations. ”

“ Antiquity of Plants as affecting their Distribution.—We have already referred to the importance of great antiquity in enabling us to account for the wide dispersal of some genera and species of insects and land-shells, and recent discoveries in fossil botany show that this cause has also had great influence in the case of plants. Rich floras have been discovered in the Miocene, the Eocene, and the Upper Cretaceous formations, and these consist almost wholly of living genera, and many of them of species very closely allied to existing forms. ”

“ The plants of this subdivision of the vegetable kingdom, from their singular structure and mode of growth, their simple cylindrical stems, and coronets of pinnated foliage, resembling that of certain palms, their usually gyrate vernation like that of the ferns, and their anomalous inflorescence and fructification, are objects of great interest to the scientific botanist; while the abundance of their fossil remains in the secondary formations renders them of the highest importance to the geologist. ”