PALAEOBOTANY or VEGETABLE PALAEON TOLOGY, a branch of the science of botany, is concerned with the study of plants which lived in past ages, by means of their remains found as fossils in the rocks. Fossil plants may be re garded as the documents on which the history of the plant-world is founded. A surprising amount is known about the form of f os sil plants from rocks as ancient as those of the Ordovician (see GEOLOGY), and, according to the latest estimates of geological time, about 500,000,000 years have since elapsed. During that time the constitution of the flora of the earth has been changing continuously. Starting with the fossils found in the older sedi mentary rocks and reviewing those of successively later f orma tions, it becomes apparent that each species, genus, or larger sys tematic grouping has three phases in its history : its first appear ance and development, its period of maximum abundance, and its dying out and final extinction. As one would expect, the fossils found in the more recently formed rocks are most like the plants living at the present day. While on the one hand the evolution of some families of plants appears to be an exceedingly slow pro cess, for even as far back as the Ordovician there are found algae very similar in form to some of the living ones, there are on the other hand examples illustrating comparatively rapid evolution. Although the Angiosperms (plants with flowers) appear late in geological time they have already become the dominant plants of to-day. A reference to the diagram below will show that there is evidence that the more highly specialised groups such as the Cycadales, Bennettitales and Angiosperms have been evolved later than the comparatively simple types represented by the Algae and Bryophyta and that simplicity in structure is in general an index of primitiveness. This agrees in a general way with the theory that the more specialised and complex plants have been evolved from simpler forerunners. It must be remembered, however, that al though Lycopods, Cycads and Ginkgoales are represented in the present day flora the chances are that an investigator at a future period would find no trace of them among the fossil debris of the present age since they form such a small part of the vegetation. There is no reason to believe that the present day flora of the world has attained a permanent constitution ; on the contrary, it is probably changing as rapidly, if not more rapidly, than at any part of its developmental history.
forms, incrustations and petrifactions. The incrustations are the commonest type found and have been formed by plant-fragments becoming embedded in the mud or silt of a lake or estuary. As the sediment was changed in the course of ages into shale or sandstone rocks the plants became compressed by the vertical stress of the overlying sediment and are finally found, when the rock is split open, in the form of a thin layer of black carbonaceous matter which gives silhouettes of the plants. In many fossils this carbonaceous matter is similar to coal (q.v.) in composition and retains to a varying extent the outward form of the plant. (Plate I., fig. 7) while the internal structure has mostly dis appeared. It is possible to remove such a fossil from the rock and then the film may be translucent and some structure visible (Plate I., fig. 6). It may happen that in an incrustation the original cuticle of the plant and sometimes the spores are pre served and may be isolated by chemical treatment. The petrifac tion is the most valuable type of fossil for investigation of the internal structure but is comparatively rare. In the petrifaction the entire plant fragment has been converted into solid rock, mainly by a gradual substitution of mineral matter for the water which forms such a large proportion of most plant tissues, before the plant was subjected to any considerable pressure by the sand and mud subsequently deposited over it. The mineral substances reach the plant by a process of diffusion from the water contained in the sediment in which the plant is embedded. The most perfectly preserved plants are those petrified in silica. The Middle Devonian chert found in Aberdeenshire contains embedded in its siliceous matrix fossil-plants in which every cell-wall is so ' clearly defined (Plate I., fig. 2) that the knowledge of their internal structure is practically complete. The beds of silica of Permian age at Autun in France have also furnished some very perfectly preserved plants. Silicified wood is common in many geological formations and a considerable number of silicified Mesozoic plants is known.
Concretionary masses known as "coal-balls" or "bullions" are found embedded in the coal of certain seams in Lancashire and Yorkshire, sometimes in such abundance that further working of the seam is unprofitable. These coal-balls consist largely of cal cium and magnesium carbonates and contain well preserved plant fragments ; they represent part of the vegetable matter of the seam which became petrified before the rest became compressed to form the coal. Similar concretions are found in the Belgian, Dutch, Westphalian and Silesian coal-fields. It is from the study of the plants in the coal-balls from Lancashire and Yorkshire that Binney, Williamson and recently Scott, have been able to reveal so much about the structure of f ossil-plants of the Carbon iferous period. The coal-ball or other petrified mass may be cut into thin slices which are ground so thin and translucent that the cellular structure of the contained plants is made visible when viewed by transmitted light.