148 EVOLUTION OF PLANTS 
be compared with those of existing species. Occasion- 
ally even the spore-bearing parts have been well pre- 
served, and it is evident that Lepidodendron and its 
allies were structurally much like the living genera 
Lycopodium and Selaginella. Especially does Lepido- 
dendron resemble the latter in the character of the 
spores, which are of two kinds, macrospores and micro- 
spores. The genus Lycopodium seems to be very old, 
fossils apparently very close to the living species occur- 
ring in the older rocks. These simpler forms have held 
their own in the struggle for existence, while the more 
highly specialized ones seem to have been crowded out 
by the still more specialized seed plants, some of which 
may be their direct descendants. 
HETEROSPORY 
In all of the principal groups of Pteridophytes, in 
passing from the simpler to the more specialized forms, 
a striking phenomenon manifests itself, z.e. ‘ hetero- 
spory,” or the development of two sorts of spores, produc- 
ing respectively male and female gametophytes. In the 
lower members of each series, the “ homosporous ” forms, 
the spores are all alike, and on germination produce 
a thallus of considerable size showing more or less 
resemblance to that of the lower liverworts, and in 
extreme cases living for several years. Upon this 
thallus the reproductive organs are borne, antheridia 
and archegonia usually growing upon the same plant, 
but sometimes upon separate ones. Where the gameto- 
phyte is unisexual, as in Equisetum and some ferns, 
the male plants are smaller, in extreme cases being 
