THE GREAT GROUPS OF PTERIDOPHYTES 



167 



ar 



of water the antheridium wall breaks down, as also do the 

 walls of the mother cells, and the small biciliate sperms 

 are set free. 



The much larger megaspores germinate and become 

 filled with a mass of numerous nutritive cells, representing 

 the ordinary cells of a prothallium (Fig. 141). The spore 

 wall is broken by this growing prothallium, a part of which 

 thus protrudes and becomes exposed, although the main 

 part of it is still invested by the old megaspore wall. In 

 this exposed portion 

 of the female gameto- 

 phyte the archegonia 

 appear, and thus be- 

 come accessible to the 

 sperms. In the case 

 of Isoetes (see 90) 

 the reduction of the 

 female gametophyte is 

 even greater, as it does 

 not project from the 

 megaspore wall at all, 

 and the archegonia 

 are made accessible 

 through cracks in the 

 wall immediately over 

 them. 



The embryo of Se- 

 laginella is also impor- 

 tant to consider. Be- 

 ginning its development in the venter of the archegonium, 

 it first lies upon the exposed margin of the prothallium, 

 while the mass of nutritive cells lie deep within the mega- 

 spore (Fig. 141, emb^ emb^). It first develops an elongated 

 cell, or row of cells, which thrusts the embryo cell deeper 

 among the nutritive cells. This cell or row of cells, formed 

 by the embryo to place the real embryo cell in better rela- 



spni 



FIG. 141. Female gametophyte of a Selaginella: 

 spm, wall of megaspore ; pr, gametophyte ; 

 ar, an archegonium ; emb^ and m6 a , em- 

 bryo sporophytes ; et, suspensors ; the gam- 

 etophyte has developed a few rhizoids. 

 After PFEPFEB. 



