1 84 TEXT-BOOK OF FUNGI 



evident and constant feature, in fact most of the generic 

 and specific characters are founded on it. It is interesting 

 to note that even in the conidiophore or superadded phase 

 the asexual reproductive bodies of the earlier forms pro- 

 duced motile zoospores, whereas in the more highly evolved 

 and later types the homologues of the bodies that produced 

 zoospores are transformed into what are termed conidia ; 

 that is, reproductive bodies distributed by wind, and which 

 on germination produce germ-tubes or hyphae directly; 

 hence during the accommodation of the Phycomycetes to 

 aerial conditions the motile zoospore as an asexual repro- 

 ductive organ is completely eliminated. 



In proportion as the conidiophore phase of the Phyco- 

 mycetes became differentiated, so the primitive sexual 

 phase diminished in importance, and in some forms has 

 entirely disappeared, the species being represented solely 

 by its well-developed and characteristic asexual conidio- 

 phore condition. This gradual elimination of the primitive 

 sexual stage is sometimes met with in different members of 

 the same genus, as in Phytophthora, where in some species 

 both sexual and asexual conditions are in existence, whereas 

 in other species the asexual or conidiophore stage alone 

 remains as in P. infestans. 



During the evolution of the conidial phase a second new 

 feature was added, the breaking up of the hyphae into cells 

 by the formation of transverse walls or septa, a character 

 which is continued henceforward throughout the entire 

 group of fungi. 



In the Phycomycetes there are two markedly different 

 modes of sexual reproduction. In the Oosporeae section 

 we have an antheridium and oogonium, exactly as in the 

 algal ancestors, whereas in the Zygosporeae there is often 



