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VEGETABLE PHYSIOLOGY 



microspore of which gives rise to a very rudimentary pro- 

 thallium bearing only one antheridium with four anthero- 

 zoids (fig. 177). The megaspore, like the microspore, is 

 liberated from the sporangium and on germination it pro- 

 duces a prothallium, part of which remains in the spore 

 and part protrudes from it (fig. 178). The inclusion of 

 part of the gametophyte 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 archegonium in conse- 

 quence 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 megaspore is set 

 free from the sporangium 

 and its germination can be 

 observed, it is found that 

 more of the gametophyte re- 

 mains inside the spore (fig. 

 179). The process of germi- 

 nation begins while the spore 

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

 the prothallium has reached a fair degree of development. 

 A still further advance is shown by Isoetes, in which 

 the prothallium is developed inside the spore, which only 

 opens a little at the apex when the archegonia are mature, 

 in order that fertilisation may be possible. 



When we pass to the Phanerogams two further 

 advances may be seen. The spore never escapes from the 

 sporangium, and the prothallium does not emerge even in 



FIG. 



178. GERMINATION OF MEGA- 



SPOBE OF Salvinia. 

 pro, prothallium ; a, young sporo- 

 phyte. The thick wall of the spore 

 has been ruptured and part of the 

 prothallium is protruding. 



