266 



STRUCTURE AND LIFE HISTORIES 



shown in Fig. 191, which illustrates the result of growing 

 mycelia from unlike strains side by side. The zone where 

 they come into contact is sharply denned by the line of 

 black zygospores, resulting from conjugation. 



267. Distinction of Sexes. When first discovered, the 

 two unlike strains were designated as (+) and ( ), 

 because it was not possible to decide, with certainty, 

 which was male and which female. Externally both 



FIG. 192. Diagram of life-cycle of a dioecious mold. 



strains looked very much alike, except that one appeared 

 to be vegetatively more vigorous than the other. Recent 

 experiments seem to indicate that the vegetatively more 

 vigorous (+) strain is female, while the less vigorous 

 ( ) strain is male. Not all molds are dioecious , like 

 Rhizobus. In some species the mycelium from a single 

 spore produces conjugating branches of both (+) and 



