80 



THE PHYCOMYCETES 



anisos^ametes (unequal in size) are liberated and fuse in the water 

 to form a motile, biciliate zygote. The zygote soon comes to rest 

 and develops into a thallus bearing sporangia. On germination 

 these sporangia produce motile spores that develop, in turn, into 



Fig. 23. Life cycle of Allomyces arbiiscula. A. Germinating resistant 

 sporangium with outer wall split. B. Zoospores emerging from germinat- 

 ing resistant sporangium. C. Germination of one of these zoospores. D. 

 Young plant of gamete-producing phase. E. Hypha bearing male and 

 female e^ametangia. F. Emergence of male and female gametes. G. Copu- 

 lation of gametes. H. Motile zygote. /. Germination of zygote. /. Young 

 plant of sporangium-producing phase. K. Mature hypha bearing both 

 thin-walled and thick-walled zoosporangia. L. Emergence of zoospores 

 from zoosporangium, M. Germinating zoospores. R. Resistant sporan- 

 gium. Z. Thin-walled zoosporangium. (Courtesy of Ralph Emerson.) 



thalli, each of which bears male and female gametangia, thus con- 

 tinuing the alternation. Some \\orkers regard this sequence of 

 sporogenic and gametogenic phases as an "alternation of sporo- 

 phyte and gametophyte generations," w^hereas others interpret 



