CHAPTER IX 

 THE EVOLUTION OF PLANTS 



97. The Problem Stated. — If we knew the entire 

 history of development of the plant world, we could ar- 

 range all plants now living, and that have lived, so as to 

 show their genetic relation to each other. The prob- 

 lem is illustrated on a small scale by various related culti- 

 vated plants, all known to be derived from a common 

 wild ancestor. Cabbage and its relatives are a case in 

 point. The botanical relatives of the cabbage include 

 such forms as kohlrabi, brussels-sprouts, collards, kale, 

 broccoli, and cauhflower (Fig. 44). All of these garden 

 vegetables are beheved to have been derived from the 

 common wild clifiE-cabbage {Brassica oleracea) of Europe 

 and Asia, by selecting mutations in various directions, 

 e.g., excessive development of the stem in kohlrabi, of 

 the terminal bud in cabbages, of the lateral buds in brus- 

 sel's sprouts, of the flower buds in cauhflower. Or, to 

 refer to de Vries's studies in experimental evolution, where 

 the course of descent was actually observed, we may 

 arrange the forms of Lamarck's evening-primrose so as to 

 show their known derivation. 



The general problem, therefore, is to estabUsh the 

 genetic relationship of all known plants, living and fossil. 

 Since the Angiosperms stand at the top of the series, the 

 problem resolves itself largely into ascertaining the 

 phytogeny, or hne of ancestry, of that group. 



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