416 BOTANICAL GAZETTE [JUNE 
It may be said at the start that, as every laboratory mycologist knows, 
Rhizopus shares with Penicillium the distinction of being the worst fungus 
weed in laboratory cultures, and is practically certain to come as a con- 
tamination of substrata rich in carbohydrates unless they be sterilized and 
kept under sterile conditions. In the Harvard laboratory, as in most other 
botanical institutions, Rhizopus is obtained for class use by spontaneous 
infection of bread. Moreover, the sporangium wall is brittle and when 
ruptured the spores are discharged into the air. It is somewhat difficult, 
therefore, to obtain a pure transfer from a single sporangium without con- 
tamination with spores of other sporangia near by, and, before the discovery 
of heterothallism in this form, the writer more than once obtained zygo- 
spores from transfers which at the time were thought to have come from 
single sporangia. 
NAMYSLOWSKI’s conclusions are based for the most part on two series 
of cultures. In the first, 19 bread cultures were made from spores from 
one single sporangium, and zygospores were produced in every case. In 
the second series, 46 single spores were isolated by the separation method 
and used in the inoculation of cultures chiefly of bread. Of these 6 were 
destroyed by bacteria, 13 produced only a feeble growth of mycelium 
(probably also infected by bacteria), 13 only sporangia, and 14 zygospores. 
The failure to obtain zygospores in so large a number of the sowings is 
ascribed to improper moisture content of the air in the cultures and to other 
unfavorable conditions which are not further investigated. 
Two explanations are possible for these results: (1) that NAmy- 
SLOWSKI was in fact dealing with a homothallic form of Rhizopus and zygo- 
spores were the result of single spore sowings; (2) that infection from the 
air brought the (+) and (—) strains into contact and caused the production 
of zygospores. Mr. H. A. Epson, of the Vermont Experiment Station, has 
recently sent the writer zygospores of Rhizopus which he writes were 
secured synthetically by opposing two strains, one obtained from Vermont, 
the other from Virginia. A dozen strains were tested by him from as many 
different sources, but failed to produce zygospores alone in pure cultures. 
The writer himself has tested 60 different strains from localities in such 
widely separated parts of the world as North and South America, England, 
Germany, France, Honolulu, and the Philippine Islands, and has found 
none which alone in pure cultures will produce zygospores. A homothallic 
form of the ordinary species, therefore, would seem a priori rather improb- 
able, and to be proven only by careful cultures strictly under sterile 
conditions. 
NAMYSLOWSKI says nothing about sterilizing his culture media, and the 
— dp eed 
