152 PLANT STRUCTURES 



porous, or sometimes isosporous, both terms meaning 

 "spores similar." The corresponding noun form is homos- 

 pory or isospory. Bryophytes and most Pteridophytes are 

 homosporous, while some Pteridophytes and all Spermato- 

 phytes are heterosporous. 



It is convenient to distinguish by suitable names the 

 two kinds of asexual spores produced by the sporangia of 

 heterosporous plants (Fig. 139). 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 (Pig. 139). 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 

 functioning megaspore. 



The differentiation goes still further. If the sporangia 

 are born upon sporophylls, the sporophylls themselves may 

 differentiate, some bearing only megasporangia, and others 

 only microsporangia, the former being called megasporo- 

 phylls, the latter microsporophylls. In such a case the 

 sequence is as follows : megasporophylls produce megaspo- 

 rangia, which produce megaspores, which in germination 

 produce the female gametophytes (prothallia with archego- 

 nia) ; while the microsporophylls produce microsporangia, 

 which produce microspores, which in germination produce 

 male gametophytes (prothallia with antheridia). 



A formula may indicate the life history of a heteros- 

 porous plant. The formula of homosporous plants with 



