FERNS 341 



the fern. When the sporophytes of the sensitive fern are forced 

 to grow into vegetative leaves, large numbers of young pro- 

 thallia are formed on the intermediate leaves in place of the 

 sporangia.* 



509. Comparative review of the ferns. f The ferns show a 

 striking advance in the evolution of the sporophyte over the 

 sporophyte of the liverworts and mosses. The principal features 

 in this progress or higher development are as follows : 



First. The sporophyte has become an independent plant and 

 can obtain its own food without the aid of the gametophyte, 

 while in the liverworts and mosses it is dependent on the game- 

 tophyte. 



Second. The sporophyte is much larger in size and differen- 

 tiated in form into roots, stem and leaves. 



Third. Its structure is more complex with highly developed 

 tissue systems for aeration, interchange of gases, and a well- 

 developed vascular system for the transport of water and food. 



Fourth. The sporophyte or second generation has become the 

 prominent stage, or generation, in the life cycle of the plant, 

 whereas the first generation, the gametophyte, is the larger and 

 more prominent stage in the liverworts and mosses. 



Fifth. Another evidence of the advance and increasing import- 

 ance of the sporophyte in the ferns is the decrease in size of -the 

 prothallium or gametophyte, which is much smaller than the 

 majority of the gametophytes of the liverworts and mosses. 



Sixth. The sporophyte of the ferns is a structure better 

 adapted to live on dry land and to obtain great size, thus enab- 

 ling it to compete successfully over all the lower plants because 

 it can rise above them to obtain the light relation. 



Seventh. The differentiation of the leaves in some species 

 bringing about a division of labor between vegetative leaves 

 and spore-bearing leaves (sporophylls) . 



510. Formula for life history of the f eras. $ The game- 

 tophyte is the prothallium or first generation, and bears the sexual 



* See Chapter XXVIII, College Botany, by the author, 

 f For reference. J For reference. 



