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=g>o— S— 0— G=8>o— S, etc. 



In the case of heterosporous plants (some Pterido- 

 phytes and all Spermatophytes) it would be modified as 

 follows : 



q=8>o— S=8=g=8>o— S:=&rfcr.g>o— S, etc. 



