MUCORALES 



105 



sporangium bisporale (Thaxter 1914) exhibits this condition as a general rule. 

 The sporangia remain very small and retain only 1 or 2 spores. The spore 

 wall is delicate, the sporangial wall thick and sculptured. The same process 

 of reduction as in the unrelated Thamnidium and Chaetocladium has occurred 

 here and has led to the formation of 1- or 2-spored sporangioles, biologically 

 functioning as conidia. 



Both sexual and asexual reproduction are known in most Mucoraceae. 

 The type of reproduction in the homothallic forms depends mainly on con- 

 ditions of nutrition ; the heterothallic forms require the presence of both sexes. 

 Mycelia of one sex may be cultivated alone indefinitely without the appear- 

 ance of normal sexual reproduction, which appears promptly whenever the 

 opposite sex is brought into the vicinity. 



Fig. 8. — Mortierella niveovelutina. a, g, hyphal anastomosis ; h, spoiangiospore ; c, 

 sporangia ; d, stylospores or aerial conidia ; e, chlamydospores ; /, sporangia attached to sporan- 

 giophores. 



When the environmental conditions are favorable, the mutual approach 

 of two sexually mature (in heterothallic forms also dynamically opposite) 

 hyphae results in the formation of outgrowths toward each other. Each out- 

 growth is cut off from the hypha close behind the tip by a septum laid down 

 from the wall inwards. The tip cell is the gametangium, the hypha is the 

 suspensor. As the homothallic forms are bisexual, apparently there occurs 

 in their hyphae at sexual reproduction, a spatial sepai-ation of + and - energids. 

 In some species, the copulating branches arise from ordinary hyphae, in others 

 they are developed on special branches, the zygophores (Fig. 9). 



The two separating walls between the gametangia are gradually dis- 

 solved from the middle toward the edge, and the zygote becomes a hypnospore 



