MUCORALES 237 



these forms any given thallus bears sporangiospores which give 

 rise to thalH of the same nature as the parent thallus. He has 

 cultivated the opposite strains of Phycomyces nitens and Mucor 

 mucedo by means of sporangiospores for more than one hundred 

 asexual generations without any apparent change in their sexual 

 behavior. The two different strains of a heterothallic form can 

 sometimes be distinguished by a slight difference in the luxuri- 

 ance of their growth, although they are otherwise morphologically 

 indistinguishable. On account of this distinction the terms plus 

 (+) and minus ( — ) were suggested by Blakeslee for the two 

 strains. Although he believed the difference to be a sexual one, 

 he was unable to state which strain was male and which female. 



In a few species of the Mucorales the zygospores were found 

 to arise by the interaction of hyphae of a single mycehum derived 

 from the germination of a single spore. Blakeslee termed such 

 forms homothallic. To this group belong Sporodinia grandis, 

 Spinellusfusiger, Zygorhynchus spp., Dicranophora fulva, Absidia 

 spinosa, and a few species of Mucor, while the heterothalHc 

 group includes Rhizopus nigricans, Mucor mucedo, Phycomyces 

 nitens, Absidia caerulea and a very considerable number of others. 



Blakeslee made the further very interesting observation that 

 imperfect hybridization (the production of the early stages of 

 zygospores which never mature) will take place between opposite 

 strains of different heterothalHc species and between both 

 (+) and ( — ) strains of heterothalHc species on the one hand, 

 and homothallic species on the other. A homothalHc species 

 placed between the two strains of a heterothalHc species may, 

 therefore, give rise to two lines of imperfect zygospores. This 

 power of hybridization between different species of the Muco- 

 rales has made it possible to place in the proper category of 

 (_|_) or ( — ) the strains of such heterothallic forms as Mucor 

 mucedo in which there is no visible difference in the luxuriance 

 of the two strains. It also makes possible the determination 

 of the (+) or ( — ) character of the thalH in those heterothallic 

 species in which only one strain has been obtained for study. 



The results obtained by Blakeslee (1906 a) in the germination 

 of zygospores have proved of particular interest. In the Mucor- 

 ales the germ tube which arises from the zygospore usually 

 bears a single sporangium termed the germ sporangium, though it 

 may occasionally produce a branching mycelium. The examina- 

 tion of the spores in the germ sporangium with reference to their 



