440 



PTERIDOPHYTES (FERN PLANTS) 



The strobilus, therefore, arose as a result of differentiating the 

 leaves in function and aggregating the sporophylls. Differing in 

 function, sporophylls and vegetative leaves would come to differ 

 in form. One can see considerable advantage in this to the plant. 

 It permits a large amount of leaf tissue to be devoted entirely to 

 the manufacture of food, while the sporophylls, since they are 

 not depended upon for food, can be much crowded, and as a result 

 many spores can be produced on a small region. In scattering 

 the spores there is also an advantage in having the sporophylls 

 at the top of the stem. 



Gametophyte. — When the spores fall to the ground and 

 germinate, they develop fleshy gametophytes consisting usually 

 of a tuberous subterranean portion from which small, aerial, 

 green lobes arise on which the sex organs are produced. Within 



Fig. 393. — The sporophyte of a Selaginella. After J. M. Coulter. 



the tissues of the gametophyte there lives a filamentous Fungus, 

 and thus it is seen that the gametophyte resembles the gameto- 

 phyte of Botrychium in a number of ways. 



The fertilized egg begins to develop immediately after fertiliza- 

 tion, and the young sporophyte is soon formed and the life cycle 

 thus completed. 



Selaginella. — The Selaginellas, called Little Club Mosses, com- 

 prise 300 or more species. Although chiefly tropical, some forms 

 are found in all parts of 'the world. They are decorative plants 

 and most all greenhouses grow them. 



