442 PTERIDOPHYTES (FERN PLANTS) 
diums the spores are alike as to size, although in some cases they 
differ in the kinds of gametophytes produced. Other Pterido- 
phytes differentiated spore-bearing and vegetative tissues, but 
the Selaginellas have differentiated spores both in size and func- 
tion. The larger spores, which are many times larger than the 
smaller ones, produce only female, while the smaller ones produce 
only male gametophytes. The two kinds of spores are borne 
in separate sporangia which also differ in size. The prefixes, 
macro, meaning little, and mega or macro, meaning large, are used 
to designate these spores and also the sporangia and sporophylls 
which bear them. Thus we speak of microspores and megaspores, 
microsporangia and megasporangia, and microsporophylls and mega- 
sporophylls (B and C, Fig. 394). 
This habit of producing two kinds of spores in regard to size is 
called heterospory (meaning different spores), while the habit of 
producing spores alike in size is called homospory (meaning same 
spores). The introduction of heterospory by Selaginella is a 
significant feature because all Seed Plants are heterosporous. In 
Seed Plants the pollen grains are microspores and within the 
ovules occur the megaspores. 
Gametophytes. — The second notable feature which Selagi- 
nella presents is that the gametophytes are so much reduced that 
they develop within the spores, where food and protection are 
provided. Thus in Selaginella there are no green independent 
gametophytes as we have been used to in other Pteridophytes 
and in Bryophytes, but the gametophyte now lives on the sporo- 
phyte just as the sporophyte of the Bryophytes lives on the 
gametophyte. This, also, is a feature that is characteristic of 
Seed Plants. 
The male gametophyte is extremely simple, consisting of one 
vegetative cell and a simple antheridium containing only a few 
sperms, each of which nas two slender cilia (C, Fig. 395). In 
developing, the male gametophyte breaks the spore wall, so that 
a crack is produced through which the sperms escape. 
The megaspores germinate and form the female gametophytes 
while still in the sporangium, and this is a third feature that is 
characteristic of Seed Plants. The female gametophyte is much 
larger than the male gametophyte (A and B, Fig. 395). Its much 
larger size is permitted by the greater size of the megaspore and 
is also necessary because the female gametophyte must support 
