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, i.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 



