The Pteridophytes 



491 



and eggs. The sperms of lycopods differ from those of other 

 pteridophytes in being very small and in having only two cilia. 



Fig. 304. Gametophy tes : A , Lyco podium complanatum, longitudinal section showing 

 antheridia, archegonia, and one embryo; B, Lycopodium annotinum, with three young 

 sporophytes; C, Equisetum Icevigatum, with four young sporophytes; D, Equisetum 

 debile; E, the fleshy prothallus of a fernwort, Ophioglossum vulgatum. {A, B, after 

 Bruchmann ; C, after Walker; D, after Kashyap; E, after Frank.) 



In Selaginella the spores are of two kinds, produced in two dif- . 

 ferent kinds of sporangia. The small spores (microspores) 

 are produced by hundreds in small sporangia (microsporangia) ; 

 and four large spores (megaspores) develop in each large sporan- 

 gium (megasporangium) . These spores have special interest 

 because from the microspores only antheridial gametophytes 

 develop, and from the megaspores only archegonial gametophytes 

 are formed (Fig. 306). 



Heterospory. The occurrence of two kinds of asexually 

 formed spores is known as heterospory, in contrast to homospory, 

 the formation of only one kind of spores. While we have de- 

 scribed heterospory only in the case of Selaginella, it should be 

 mentioned that heterospory occurs among the ferns as well as 



