THE MAIN LINES OF PLANT EVOLUTION 



gressive size increase. The most successful members of many groups are 

 characterized by small size. The club mosses and horsetails, for example, 

 were once represented by great trees, but only the small members of the 

 group have survived to the present. Among the angiosperms, the trees 

 seem to have been primitive, and from these the shrubs and grasses have 

 been derived. This has been accompanied by the development of the 

 annual habit, presumably as an adaptation to prevent extinction by win- 

 ter-killing, for a dormant seed may easily survive severe weather which 

 would kill a mature plant. Thus development of small size, rapid growth 

 and maturity, and the annual habit comprise an adaptation to arctic and 

 subarctic conditions, and such plants appear to be on the increase in 

 temperate lands as well. 



REFERENCES 



Andrews, H. N., 1947. "Ancient Plants and the World They Lived In," Comstock 

 Publishing Co., New York, N.Y. A readable summary of paleobotany. 



AxELROD, D. I., 1959. "Evolution of the Psilophyte Paleoflora," Evolution, 13, 264-275. 

 Evidence is adduced that the higher plants arose much earlier than generally sup- 

 posed, and from algal ancestors. 



Bold, H. C, 1957. "Morphology of Plants," Harper Brothers, New York, N.Y. A pene- 

 trating treatment, going far beyond the minimum presented in this chapter. 



Fuller, H. J., and O. Tippo, 1954. "College Botany," 2nd Ed., Henry Holt & Co.. Inc., 

 New York, N. Y. An excellent text, and the source of the classification used in this 

 chapter ( Lignier, Kidston, and Lang ) . 



Thomas, H. H., 1936. "Paleobotany and the Origin of the Angiosperms," Botan. Rev., 

 2, 397-418. Documentation for the statement that the origin of the angiosperms is 

 still "an abominable mystery." 



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