430 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 Salvinza 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 
a ‘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 
Fic, 176.—GuRMination or MzGa- and its germination can be 
SPORE oF Salvinia. ei bass 
pra, prothallium; a young sporo- observed, it is found that 
Pare at teanaaaciacarare more of the gametophyte 
prothallium is protruding. 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 Isoétes, in which 
the prothallium is developed inside the spore, which only 
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