332 PLANT STUniES 



to gumotopliytes of different sexes, we have the condition 

 called lielcroKjinrii (" spores different"), and such jjlants are 

 called lielerasjxirous (Fig. 307). In contrast with hetero- 

 sporous plants, those in which the asexual spores appear 

 alike are called homosjJoroiif!, or sometimes isoxporous, both 

 terms meaning " spores similar." The corresponding noun 

 form is linmospory or isospory. Bryophytes and most Pteri- 

 dophytes are homosporous, while some Pteridopihytes and 

 all Spermatophytes are heterosporous. 



It is convenient to distinguish by suitable names the 

 two kinds of asexual spores produced by the sporangia of 

 iK'terosporons plants (Fig. 307). The large ones are called 

 mcf/d.y/orex, or by some writers macrosporcs, both terms 

 meaning " largo spores " ; the small ones are called micro- 

 spart's, or " small spores." It should be remembered that 

 megaspores always produce female gametopliytes, and mi- 

 crospores male gametopliytes. 



This differentiation does not end with the spores, but 

 soon involves the sporangia (Fig. 307). Some sporangia 

 produce only megaspores, and are called nirfjasjinrdiKjlit ; 

 othei-s produce only microspores, and are called /iiirriis/io- 

 riiiKjiii. It is important to note that while microsporangia 

 usually produce numerous micros^iores, the mcgasporaugia 

 pi-oduce much fewer megaspores, the tendency being to 

 diminish tlie number and increase the size, until finally 

 there are megaspiirangia which produce but a single large 

 megaspore. 



A formula may indicate the life history of a hetero- 

 sjiorouK ]ilant. Tlie formula, nf homosporous ]ilants with 

 alternation of generations (Bryophytes and nmst Pterido- 

 phytes) was given as follows (§ l'.)7) : 



Gjii;><i— S—o— (!—';> o— 8—0— (i:::;;>()^S, etc. 



In tlu^ case of heterosp(u-ous jilauts (some Pterido- 

 lihytcs and all Spermatophytes) it would be modified as 

 follows : 



Sz:S> o-S=S=S=S> o-S=:C=t8> o-S, etc. 



