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 Equisetine, 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 Isoétes, 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 Isoétes, 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 
