122 



THE FHYCOMYCETES 



Absidia caeritlea, Blakeslea trispora, Choanephora ciiciirbitannn^ 

 Miicor miicedo, Fhy corny ces nitejis, and Rhizopiis nigricans are 

 heterothallic. Under certain environmental conditions unmated 

 gametangia become thick-walled and may be designated azygo- 

 spores, or chlamydospores. 



Cytological features associated with fertilization have been 

 studied in several species of Alucorales. Each gamete is multi- 



FiG. 38. Zygotes of several Mucorales. A. Absidia glaiica. (Adapted from 



Lendner.) B. ZygorhyiicJms heterogavms. (Adapted from Blakeslee.) 



C. Piptocephalis jreseniana. (Adapted from Brefeld.) 



nucleate in Sporodinia grandis, and multiple fusions of paired + 

 and — nuclei occur soon after the protoplasts intermingle. Un- 

 mated nuclei degenerate and disappear. In Phycomvces all ex- 

 cept 2 pairs of nuclei degenerate. These 4 nuclei fuse after sev- 

 eral months [Burgeff (1915)]. Keene (1919) determined, how- 

 ever, that 6 to 8 pairs of nuclei, rather than 2 pairs, fused. De- 

 layed nuclear fusion has been reported in Zygorhy7icbiis dan ge- 

 ar di and Rbizopiis sexualis ICallen (1940)]. 



Germination of zygospores. After a rest period zygospores 

 may be induced to germinate by the formation of a short germ 

 tube that is terminated by a germ sporangium. Blakeslee (1906) 

 observed with reference to + and — characters that there are 



