164 BOTANICAL GAZETTE [SEPTEMBER 
is less readily recognized, however, when the invading growth is 
another race of the same species. Syncephalastrum and Cunning- 
hamella are forms which bear their spores externally and shake them 
off at the slightest touch or breath of air. When a dish containing 
a mature culture of these species is opened, the spores may be seen 
to rise in clouds. The spores of Rhizopus, although inclosed in a 
sporangium, are readily scattered into the air by the rupture of 
the brittle sporanguim wall, and in consequence Rhizopus also is 
a common source of infection. That the air may be filled with 
viable spores of both sexes of Cunninghamella has been shown by 
exposing Petri dish cultures for a short time and growing them 
at a temperature suitable for zygospore formation. In the 
laboratory where Cunninghamella had been recently grown, it was 
always possible to find growths of the same species in Petri dishes 
exposed in this way, and frequently both the sexual races were 
obtained, as shown by the production of their zygospores. . Experi- _ 
ence has shown, therefore, that in working with Cunninghamella one 
must observe greater precautions to avoid air-borne infection with 
other strains of the same species than is necessary when working 
with many other forms. 
Not only are cultures of Cunninghamella especially liable to 
infection with spores of the same species, but the spores of this 
species which gain access to a mature culture are able to germinate 
and grow on the aerial mycelium which they infect. With most 
mucor species, vigorous growth on the nutrient substratum is 
necessary before zygospore formation is possible. With Cunning- 
hamella, however, connection with the nutrient agar does not appear 
to be necessary in order to allow the mycelia to assist in zygospore 
formation, since if spores of one sex are planted on an aerial growth 
of the other sex, zygospores are likely to be produced. This 
peculiarity of Cunninghamella was responsible in the earlier cultures 
for the appearance of zygospores where they should not be found 
on the theory of sexual dimorphism, and led to a repetition of the 
first series of cultures and a revision of the technique. 
NUTRIENT MEDIA.—The method of growing and testing races 
of Cunninghamella for sexual reactions was the same as that adopted 
for other forms, with such slight modifications as the peculiarities of 
