3i8 REPRODUCTION OF SELAGINELLA 



spores. The larger cell of the microspore now forms a central 

 mass of gamete mother cells which are surrounded by a single 

 layer of wall cells (Fig. 243, 7). The male gametes are of the 

 same character as noted in the Lye op odium and are set free by 

 the disintegration of the wall cells. The megaspores begin to 

 germinate even before they have reached their full size. The 

 nucleus of a megaspore divides repeatedly, forming numerous 

 nuclei which become arranged about the walls of the young spore. 

 Subsequently walls are formed about the nuclei at the apex of 

 the spore (Fig. 243, 8) and by further division a mass of cells 

 results, in the outer part of which the first divisions of the arche- 

 gonia arise. At this stage of development, or, earlier, the mega- 

 sporangium opens by a vertical cleft permitting the discharge of 

 the spores. It must be borne in mind that this origin of the 

 gametophyte is of a radically different nature from any case pre- 

 viously noted. Heretofore the gametophyte always had an inde- 

 pendent existence and the sporophyte for varying periods of time 

 was a parasite upon it. In Selaginella you note the beginning 

 of a reversal of this relation, for the sporangia remain green ; 

 and continue to nourish the spores during their germination ; in 

 other words, during the early stages in the development of the 

 gametophyte. We see that the gametophyte is becoming para- 

 sitic upon the sporophyte (see page 252). After the spores are 

 shed they complete the growth of the gametophyte by forming 

 the archegonia and filling the space within the spores with a solid 

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

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

 243, 8). The archegonia are rudimentary structures consisting 

 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 nourish- 

 ment of the young gametophyte by the highly organized sporo- 

 phyte 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 only 

 in rare cases do the spores fall in such favorable situations and 



