216 BOTANICAL GAZETTE [ocToBER 
even with careful inspection. Parthenospores have been obtained 
between certain strong races of different species in other genera 
with larger zygospores when no doubt of their true nature was 
likely to occur after a careful examination. Figures of partheno- 
spores formed on homothallic species, at stimulus of contact with 
a sexually vigorous race of a heterothallic species, are given in an 
earlier publication (9, pl. J). The possible presence of partheno- 
spores must not be overlooked in judging reports (19) of true 
hybridization between different species in the mucors. 
So far as the reactions between different species of Cunning- 
hamella have been tested, they argue for the sexual dimorphism 
of this genus. 
Discussion 
The data in the present paper refer only to the mucor genus 
Cunninghamella. A preliminary summary has already been given 
of tests with other genera (10), and it is hoped to publish a detailed 
account of these tests at a later date. The data so far accumulated 
show no behavior inconsistent with the idea of a strict sexual 
dimorphism. The work, especially with Cunninghamella, indicates 
that sex intergrades must be extremely rare if ever present in these 
forms, despite the fact that they would be expected on a priori 
grounds and the fact that other observers have thought they had 
found them. 
In the species of Cunninghamella there is apparent a graded 
series so far as the strength of sexual activity is concerned, ranging 
from a reaction with grade A between sexually strong races to 
grade O between sexually weak races. Races which have shown 
no reactions in any contrast tested are provisionally listed as 
“neutral.” The term neutral is obviously relative, and not meant 
to indicate absolute absence of sex. The number of races listed 
as neutral for a given collection tends to decrease as more testers 
are used in contrasts. Thus it is evident from table VII A that 
if strain no. 217 had not been used as a tester, strains nos. 719, 727, 
and 725 would have been listed as neutrals rather than as minus 
strains, since they would have shown no reaction against any of 
_ the plus or minus testers used. Neutrals seem to form the low 
