KEPBODUCTION 



489 



be illustrated by studying a series of forms. The earliest 

 indication of it, which we can find, is exhibited by the 

 Hydropteridece, 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 was 

 probably the first step to- 

 wards the formation of the 

 seed. The young sporophyte 

 arises upon the exposed por- 

 tion of this prothallium, orig- 

 inating as in other cases from 

 the zygote produced in the 

 archegonium after fertili- 

 sation. 



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 pro- 

 nounced as in Salvinia. When 

 the megaspore is set free from 

 the sporangium and its germi- 

 nation can be observed, it is 

 found that more of the game 

 tophyte 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 development. 



A still further advance is shown by Isoetes, in which 

 the prothallium is developed inside the spore, which only 



FEG. 176. GERMINATION OF MEGA 

 SPORE OF Salvinia. 



pro, prothalUum ; a, young sporo- 

 phyte. The thick wall of the spore 

 has been ruptured and part of the 

 prothallium is protruding. 



