332 PLANT STUDIES 
to gametophytes of different sexes, we have the condition 
called heterospory (" spores different "), and such plants are 
called heterosporous (Fig. 307). In contrast with hetero- 
sporous plants, those in which the asexual spores appear 
alike are called homosporous, or sometimes isosporous, both 
terms meaning " spores similar." The corresponding noun 
form is homospory or isospory. Bryophytes and most Pteri- 
dophytes are homosporous, while some Pteridophytes and 
all Spermatophytes are heterosporous. 
It is convenient to distinguish by suitable names the 
two kinds of asexual spores produced by the sporangia of 
heterosporous plants (Fig. 307). The large ones are called 
megaspores, or by some writers macrospores, both terms 
meaning " large spores " ; the small ones are called micro- 
spores, or " small spores." It should be remembered that 
megaspores always produce female gametophytes, and mi- 
crospores male gametophytes. 
This differentiation does not end with the spores, but 
soon involves the sporangia (Fig. 307). Some sporangia 
produce only megaspores, and are called megasporangia ; 
others produce only microspores, and are called microspo- 
rangia. It is important to note that while microsporangia 
usually produce numerous microspores, the megasporangia 
produce much fewer megaspores, the tendency being to 
diminish the number and increase the size, until finally 
there are megasporangia which produce but a single large 
megaspore. 
A formula may indicate the life history of a hetero- 
sporous plant. The formula of homosporous plants with 
alternation of generations (Bryophytes and most Pterido- 
phytes) was given as follows ( 197) : 
G=g>o S o G=ig>o S o G^>o S, etc. 
In the case of heterosporous plants (some Pterido- 
phytes and all Spermatophytes) it would be modified as 
follows : 
