THE CLUB-MOSSES 165 



The germination of the megaspores, resulting 

 in the formation of a prothallus, takes place 

 while they are still inside the sporangium (see 

 fig. 18). As a rule the large spores are then 

 shed; the prothallus develops further and bursts 

 through the spore-wall; it does not, however, 

 become green. In the meantime the micro- 

 spores have likewise been shed, and have each 

 developed a small group of cells, which is prac- 

 tically nothing but an antheridium, and from 

 this a few spermatozoids, each with two cilia, 

 are set free. If water is present, they swim to 

 the archegonia of any female prothallus which 

 is close by, attracted by a secretion of malic 

 acid as in the Ferns. Fertilisation is accom- 

 plished and the result is an embryo, which re- 

 mains enclosed for some time in the megaspore, 

 embedded in the prothallus. 



It has already been mentioned that in some 

 species fertilisation takes place inside the spor- 

 angium, and an embryo is formed, which may even 

 grow into a young plant, without the megaspore 

 ever being shed at all. This is an extreme case 

 and not altogether normal, though interesting as 

 a parallel to what happens in the regular course of 

 development of the higher Seed-plants. In Sel- 

 aginella, however, there is no seed, for the sporan- 

 gium remains unaltered. 



The heterospory of Selaginella is thus extremely 



