LIFE HISTORY OF A FERN 



175 



organism similar to that with which we started — a full- 

 grown fern-plant, capable of producing spores, which can 

 develop into prothallia again, with antheridia and arche- 

 gonia, producing sperms and eggs, and so on. Thus we 

 see that the steps in the life history of a fern constitute 

 a life-cycle. At whatever point or with whatever struc- 

 ture we start, if we follow the course of development we 

 are brought back again to the same point, or the same 

 kind of structure with which we began. 



Fig. 132. — Prothallia of a fern, i, Before the sporophyte had appeared; 

 2-5, with sporophytes attached; I, cotyledon or first leaf of the sporophyte; 

 V, circinate vernation of a leaf; s, mass of soil adhering to the rhizoids and 

 roots. 



160. Simpler Ferns. — In addition to the leptosporan- 

 giate ferns, which have served as a basis for the general- 

 ized description given above, there is another group, 

 having a more primitive type of organization. Repre- 

 sentatives of this group include the "moonworts" (species 

 of Botrychium, Fig. 133), and the "adder's tongue" 

 (Ppkioglossum vulgatum, Fig. 134). The species of 



