350 



GAMETOPHYTE OF SELAGINELLA 



sitic upon the sporophyte (see page 281). After the spores are 

 shed they complete the growth of the gametophyte by forming 

 the archegonia and filHng the space within the spores with a soUd 

 mass of cells. This growth ruptures the spore walls at the apical 

 regions, thus exposing the archegonia to the male gametes (Fig. 

 243, 3). The archegonia are rudimentary structures consisting 



Fig. 243. Gametophyte and young sporophyte of Selaginella: i, section 

 of a microspore that has nearly completed its germination — p, a single cell 

 that is possibly a remnant of the large gametophyte or prothallium of pre- 

 vious groups of ferns; aw, antheridium consisting of a layer of wall cells 

 which enclose the gamete mother cells. lA, a male gamete. 2, section 

 of a megaspore, showing the stage of germination that is usually attained 

 in the sporangium. 3, mature female gametophyte that has ruptured the 

 wall of the megaspore, thus exposing the archegonia, ar, one of which has 

 developed a young sporophyte or embryo with root, r; stem bearing two 

 leaves, st, foot, /, and the suspensor, s. 4, young sporophyte with root, 

 stem and leaves emerging from the gametophyte. 



of but two neck cells, and a single canal cell leads to the female 

 gamete. 



It is evident that these variations are of great advantage in 

 ensuring the perpetuation of the species. Especially is the 

 nourishment of the young gametophyte by the highly organized 

 sporophyte a distinct gain. In previous types the formation of 

 the gametophyte was dependent upon favorable conditions, such 

 as moisture, light, temperature, etc., and it should be said that 



