298 ANNUAL REPORT SMITHSONIAN" INSTITUTION, 1918. 



is peculiar, since paleobotany is the botany of all geological time, 

 while botany is that of but one geological period — namely, the 

 present. It is now quite impossible to obtain a broad foundation 

 for phylogenetic, morphological, or distributional studies of existing 

 plants without a consideration of their extinct ancestors. In the 

 interpretation of the far distant past paleobotany and its sister 

 science paleozoology contribute fundamental data to geology, not 

 only in furnishing the " medals of creation," wdiich constitute the 

 best basis for a geological chronology yet discovered; not only as 

 comprising the subject matter of the biology of the past, w T hich is a 

 legitimate part of earth history, but in the elucidation of the climate 

 and other physical conditions of past times, subjects wdiich may be 

 embraced under the terms of Paleoclimatology and Paleoecology. 

 Paleogeography, or the Physical Geography of past times, also 

 derives some of its most important facts from the study of the 

 character and distribution of fossil organisms. 



TYPES OF VEGETATION. 



The salient features of the various phyla into which the vegetable 

 kingdom is now divided will be passed in review, after which the 

 evolution of floras from the beginning of life on the globe down 

 through the Pleistocene will be briefly sketched. 



PHYLUM TIIALLOPHYTA. 



As a convenience and because of their lack of importance as fossils 

 the old term Thallophyta will be used for the great mass of thallus 

 plants, formerly thought to constitute a single subkingdom but now 

 known to represent several phyla. The thallophytes embrace plants 

 of the simplest type, but they exhibit nevertheless a very wide range 

 in the structure and degree of differentiation of the vegetative body 

 and in their methods of reproduction. The plant body may be a 

 single minute and often motile cell multiplying by fission, or the 

 thallus may be an aggregate of cells with a well-marked physiological 

 division of labor, and differentiated into parts analogous to the 

 roots, stems, and leaves of the higher plants, and with a correspond- 

 ing histological complexity. The best known thallus plants [to the 

 nonbotanist] are bacteria, such fungi as molds and toadstools, and 

 seaweeds. With but few exceptions, marine plants are thallophytes, 

 although immense numbers are also found in fresh water and in 

 various terrestrial habitats. In general, the larger and more complex 

 are marine and some of these are among the most gigantic of plants. 



The thallophytes are primitive types and their existence unques- 

 tionably antedates the geological record, although they are now 

 known in considerable variety from pre-Cambrian. They furnish 

 little that is of paleobotanical interest, since the} 7 are either so slightly 



