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. 
257. 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 
Rhizopus. In some species the mycelium from a single 
spore produces conjugating branches of both (+) and 
