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VEGETABLE PHYSIOLOGY 



be examined by studying a series of forms. The earliest 

 indication of it which we can find is exhibited by the 

 Hydropteridea, of which Salvinia is a characteristic type. 

 Salvinia is a heterosporous form, each microspore of which 

 gives rise to a very rudimentary prothallium bearing only 

 one antheridium with four antherozoids (fig. 175). The 

 megaspore, like the microspore, is liberated from the 

 sporangium and on germination it produces a prothallium, 

 part of which remains in the spore and part protrudes from 

 it (fig. 176). The inclusion of the part of the gameto- 

 phyte within the spore is the first step towards the 



formation of the seed. The 

 young sporophyte arises upon 

 this prothallium upon the 

 exposed portion, originating 

 as in other cases from the 

 zygote produced in the arche- 

 gonium in consequence of 

 fertilisation. 



A more advanced stage is 

 seen in Selaginella, which also 

 is a member of the Pterido- 

 phyta, though not a fern. 

 The heterospory is just as 

 pronounced as in Salvinia. 

 When the megospore is set 

 free from the sporangium 

 and its germination can be 

 observed, it is found that 

 more of the gametophyte 

 remains inside the spore 

 (fig. 177). The process of germination begins while the 

 spore is still in the sporangium, and by the time the spore 

 opens the prothallium has reached a fair degree of develop- 

 ment. 



A still further advance is shown by Isoetes, in which 

 the prothallium is developed inside the spore, which only 



FIG. 176. GERMINATION OF MEGA- 

 SPOKE OF Salvinia. 



pro, prothallium. ; a, young sporo- 

 phyte. The thick wall of the spore 

 has been ruptured and part of the 

 prothallium is protruding. 



