FERNS 371 



threads are very much contracted and wrinkled, and can be 

 easily distinguished from the normal form of the fern. When 

 the spore-bearing leaves of the sensitive fern are forced to grow 

 into vegetative leaves (paragraph 522), large numbers of young 

 prothallia are formed on the intermediate leaves in place of 

 the sporangia.* 



524. Comparative review of the ferns. 1 " 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 gametophyte. 



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 im- 

 portance 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 enabling 

 it to compete successfully over lower plants because it can rise 

 above them to obtain the light. 



Seventh. The differentiation of the leaves in some species 

 brings about a division of labor between vegetative leaves and 

 spore-bearing leaves (sporophylls). 



525. Formula for life history of the ferns. 1 The game- 



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

 f For reference. J For reference. 



