124 THE PHYCOMYCETES 



the Rhizopus type some of the paired nuclei fuse, the super- 

 numerary nuclei degenerate before dormancy begins, and reduc- 

 tional division is delayed until germination of the zygospores. In 

 the Phycomyces type the paired nuclei remain associated until 

 germination of the zygospore begins, ^^-hereupon some of them 

 fuse, but the unfused ones do not degenerate. As the germ 

 sporangium forms, some of the diploid nuclei undergo reduction, 

 but others enter the sporangiospores in the unreduced condition. 

 In the Sporodinia type nuclear fusion does not take place at all. 



Various aspects of the numerous later studies on sexuality 

 among Mucorales by Blakeslee and other students of this group 

 are summarized by Blakeslee (1920) and Satina and Blakeslee 

 (1929). 



Classificatiox. Among the older taxonomic works that of 

 Lendner (1908) is the most satisfactory. Sporangial characters 

 furnish the most usable bases for readily distinoruishinor families 

 and genera. To distinguish species, however, is difficult. 



KEY TO FAMILIES 



2 



1. Sporangia globular to pyriform, sometimes accompanied or re- 

 placed by sporangioles 

 2. Sporangium having a columella; zygospore not enveloped by 

 interwoven hyphae 3 



3. Sporangia always present, but sporangioles lacking 4 



4. Sporangial wall not cutinized Mucoraceae 



4. Sporangial wall cutinized Pilobolaceae 

 3. Sporangioles always present, but sporangia may be lacking 5 



5. Sporangioles and sporangia both present on same spo- 

 rangiophore Thamnidiaceae 



5. Sporangioles and sporangia, when present, on different 

 sporangiophores 6 



6. Sporangia absent; sporangioles on sub terminal branches 



Chaetocladiaceae 

 6. Sporangia present or absent; sporangioles on capitate 

 tips Choanephoraceae 



2. Sporangium lacking a columella; zygospores enveloped by 

 interwoven hyphae Mortierellaceae 



1. Sporangia narrowly elongate; spores arranged in a lineal series 



Piptocephalidaceae (Cephalidaceae) 



Remarks on important, better-known species. In spite of 

 the wide use of Rhizopus nigricans to demonstrate heterothallism, 

 no one appears to have succeeded in germinating its zygospores. 



