LIFE HISTORIES OF FUNGI 
265 
ago, when it was discovered that there are, in reality, two 
unlike strains of Rhizopus-mycelium, and therefore two 
kinds of spores. The two kinds of mycelia are designated 
FIG. 191. Petri-dish culture showing dark line of zygospores between 
the (+) and ( ) strains of a dioecious species of mold, Choanephora (Me) 
and white lines of "imperfect hybridization" between the strains of this 
species and the opposite strains of Mucor V (Mv). Note the total absence 
of zygospore formation between two (+) or two ( ) strains. (After 
Blakeslee.) 
as (+) and ( ). When the intermingling hyphae are of 
like strains, either all (+) or all ( ), conjugation fails 
completely, but when they are from unlike strains, zygo- 
spores form in great abundance. This fact is strikingly 
