PHYLOGENY 



which in turn appear to have given rise to the angiosperms. The more 

 primitive subclass of angiosperms, the dicots, has given rise to the more 

 speciaHzed monocots. 



Some general trends of evolution in the Plant Kingdom may be noted. 

 Among the most primitive plants, increasing complexity of the nuclear 

 apparatus, culminating with the typical cell structure as found in all plants 

 above the blue-green algae, is a major trend. Among the algae, develop- 

 ment of the colonial habit and multicellularitv is one major trend. A 

 second is the evolution of sexual reproduction, together with the alterna- 

 tion of generations. Finally, a transition from aquatic to terrestrial habitats 

 occurred, but little specific information is available regarding the steps 

 in this process. This required the development of vascular and strengthen- 

 ing tissues, the development of which is one of the most characteristic 

 features of the land plants. These tissues are, however, found only in the 

 sporophyte, and so the sporophytic generation has increased in size and 

 functional importance relative to the gametophytic generation until fi- 

 nally, in the angiosperms, the gametophytic generation is recognizable as 

 distinct from the sporophyte only by comparison to the lower plants. This 

 trend is illustrated in Figure 44. In a general way, there has been a tend- 

 ency to increase in size. This is obviously so among the algae, where the 

 most primitive species are all unicellular and the more advanced ones may 

 be (but need not be) multicellular. While the most primitive land plants 

 were smaller than large algae, all of the classes of tracheophytes have 

 produced species much larger than the largest of the algae. Nonetheless, 

 it is far from axiomatic that evolution is necessarily accompanied by pro- 





1 



AU3AE 

 (ULOTHRIXI 



MOSSES 



ALGAE FERNS GYMNOSPERMS ANGIOSPERMS 



(ULVA) 



Figure 44. Diagham Showing thi: C;ha»ual Increase in Size and Importance of 

 THE Sporophyte, and the Corresponding Decrease in Size and Importance of 

 THE Gametophyte. ( From Fuller and Tippo, "College Botany," 2nd Ed., Henr\- Holt 

 & Co., Inc., 1954. ) 



126 



