PTERIDOPHYTES 333 
In this case two gametophytes are involved, one pro- 
ducing a sperm, the other an egg, which fuse and form the 
oospore, which in germination produces the sporophyte, 
which produces two kinds of asexual spores (megaspores 
and microspores), which in germination produce the two 
gametophytes again. 
One additional fact connected with heterospory should 
be mentioned, and that is the great reduction of the gam- 
etophyte. In the homosporous ferns the spore develops 
a small but free and independent prothallium which pro- 
duces both sex organs. When in heterosporous plants this 
work of producing sex organs is divided between two gam- 
etophytes they become very much reduced in size and lose 
their freedom and independence. They are so small that 
they do not escape entirely, if at all, from the embrace of 
the spores which produce them, and are mainly dependent 
for their nourishment upon the food stored up in the spores. 
