152 EVOLUTION OF PLANTS 



phyte is exceedingly rapid, in marked contrast to the 

 long-lived gametophyte of the homosporous ferns. The 

 ungerminated dried spores of Marsilia vestita (Fig. 39, 

 D), for example, a common species of the western 

 United States, on being placed in water will complete 

 their whole development within less than twenty-four 

 hours, the sexual organs being matured and fertilization 

 effected within that time. 



In the Equisetinese, heterospory, as already noted, 

 is known only in a few fossil forms, and in these there 

 is much less difference in the size of the two sorts of 

 spores than is the case in the heterosporous ferns. 



The club-mosses, as we have seen, show very marked 

 heterospory in the genus Selaginella, which includes the 

 majority of the existing species, mostly tropical in their 

 distribution. In Selaginella, as in Isoetes, the formation 

 of the female gametophyte is preceded by a repeated 

 division of the nucleus of the macrospores, and closely 

 resembles the endosperm formation of the flowering 

 plants. The male gametophyte is reduced to a single 

 vegetative cell as in Isoetes, but the number of sperm- 

 cells is much greater, and the spermatozoids are biciliate 

 as in Lycopodium or the mosses, and not multiciliate 

 like those of the other Pteridophytes. 



In Selaginella the germination of the spores begins 

 while they are still included in the sporangium, whose 

 wall-cells remain active, the inner layer of cells acting as 

 nourishing cells for the developing spores with the con- 

 tained gametophyte. The latter derives its sustenance, 

 not from reserve matter within the spore, but directly from 

 the sporophyte. In this respect Selaginella approaches 

 the condition found in the flowering plants, where the 



