[PALÆONTOLOGICAL BOTANY].
The study of the changes which have taken place in the nature of living beings since their first appearance on the globe till the period when the surface of the earth, having assumed its present form, has been covered by the creation which now occupies it, constitutes one of the most important departments in Geology. It is, as Brongniart remarks, the history of life and its metamorphoses. The researches of geologists show clearly that the globe has undergone various alterations since that "beginning" when "God created the heavens and the earth." These alterations are exhibited in the different stratified rocks which form the outer crust of the earth, and which were chiefly sedimentary deposits produced by the weathering of the exposed rocks. Remains of the plants and animals living on the globe at the time of the formation of the different beds are preserved in them. Elevations and depressions of the surface of the earth affected the organisms on its surface, and gave to successive deposits new faunas and floras. Some of these epochs have been marked by great changes in the physical state of our planet, and they have been accompanied with equally great modifications in the nature of the living beings which inhabited it. The study of the fossil remains of animals is called Palæozoology (παλαιός, ancient, and ζῷον, animal), while the consideration of those of vegetables is denominated Palæophytology (παλαιός and φυτόν, a plant). Both are departments of the science of Palæontology, which has been the means of bringing geology to its present state of advancement. The study of these extinct forms has afforded valuable indications as to the physical state of the earth, and as to its climate at different epochs. This study requires the conjunct labours of the Zoologist, the Botanist, and the Petralogist.
The vegetation of the globe, during the different stages of its formation, has undergone very evident changes. At the same time there is no reason to doubt that the plants may all be referred to the great classes distinguished at the present day—namely, Thallogens, including such plants as Lichens, Algæ, and Fungi; Acrogens, such as Ferns and Lycopods; Gymnosperms, such as Cone-bearing plants and Cycads; Endogens, such as Palms, Lilies, and Grasses; and Exogens, such as the common trees of Britain (excluding the Fir), and the great mass of ordinary flowering plants. The relative proportion of these classes, however, has been different, and the predominance of certain forms has given a character to the vegetation of different epochs. The farther we recede in geological history from the present day, the greater is the difference between the fossil plants and those which now occupy the surface. At the time when the coal-beds were formed, the plants covering the earth belonged to genera and species not existing at the present day. As we ascend higher, the similarity between the ancient and the modern flora increases, and in the latest stratified rocks we have in certain instances an identity in species and a considerable number of existing genera. At early epochs the flora appears to have been uniform, to have presented less diversity of forms than at present, and to have been similar in the different quarters of the globe. The vegetation also indicates that the nature of the climate was different from that which characterises the countries in which these early fossil plants are now found.
[Determination of Fossil Plants.]
Fig. 1.
Fig. 1. Section of Peuce Withami, after Lindley and Hutton, a fossil Conifer of the coal epoch. Punctated woody tissue seen.
Fossil plants are by no means so easily examined as recent species. They are seldom found in a complete state. Fragments of stems, leaves, and fruits, are the data by which the plant is to be determined. It is very rare to find any traces of the flowers. The parts of fossil plants are usually separated from each other, and it is difficult to ascertain what are the portions which should be associated together so as to complete an individual plant. Specimens are sometimes preserved, so that the anatomical structure of the organs, especially of the stem, can be detected by very thin slices placed under the microscope. In the case of some stems the presence of punctated woody tissue (Fig. 1) has proved of great service as regards fossil Botany; this structure, along with the absence of large pitted ducts, serving to distinguish Conifers. The presence of scalariform vessels indicates a plant belonging to the vascular Cryptogams, of which the fern is the best known example. The cautions to be observed in determining fossil plants are noticed by Dr. Hooker in the Memoirs of the Geological Survey of Great Britain (vol. ii. p. 387). At the present day, the same fern may have different forms of fronds, which, unless they were found united, might be reckoned distinct genera; and remarkable examples are seen in Niphobolus rupestris and Lindsæa cordata. Moreover, we find the same form of frond belonging to several different genera, which can only be distinguished by the fructification; and as this is rarely seen in fossil ferns, it is often impossible to come to a decided conclusion in regard to them. A leaf of Stangeria paradoxa was considered by an eminent botanist as a barren fern frond, but it ultimately proved to be the leaf of a Cycad. The leaf of Cupania filicifolia, a Dicotyledon, might easily be mistaken for that of a fern; it resembles much the frond of a fossil fern called Coniopteris. The diverse leaves of Sterculia diversifolia, if seen separately, might easily be referred to different plants. In the same fern we meet also with different kinds of venation in the fronds. Similar remarks may be made in regard to other plants. Harvey has pointed out many difficulties in regard to sea-weeds.
As regards the materials for a fossil flora, the following remarks of Hugh Miller deserve attention:—