^Nov. 24, 1923 
Variations in Strains of Rhizopus nigricans 
365 
At the end of three days, notes were taken of the percentage of infec¬ 
tion and the approximate amount of decay. When the potatoes were 
almost decayed, usually at the end of five days, they were removed 
ifrom the incubator and final notes taken of the percentage of infection 
and the total amount of decay. An isolation was made from each 
infected potato and the organism identified. 
The results of these inoculations need not be given in detail. They 
show, however, that all of the organisms are parasitic, the percentage 
of infection varying from 80 to 100 per cent. Uninoculated controls 
did not become infected. Nearly all the infected potatoes were com¬ 
pletely decayed at the end of five days when the experiment was termi¬ 
nated. The results of the isolations showed that R . nigricans was 
obtained from every one of the infected potatoes. 
These investigations show that Rhizopus nigricans from a variety of 
hosts widely separated geographically is parasitic on sweet potatoes. 
Although there is some difference in the percentage of infection, this 
difference is probably no greater than would be expected if the same 
organism was employed in different sets of experiments. The con¬ 
clusion drawn is that there is no material difference in the parasitism 
of any of these strains. A repetition of these experiments gave similar 
results. 
DELATION OF TEMPERATURE TO GERMINATION OF SPORES AND SUBSE¬ 
QUENT GROWTH 
It has been shown above that all the strains of R. nigricans are para¬ 
sitic. It has further been shown that although certain so-called species 
{triticiy nodosus , oryzae , delemar) of Rhizopus are very similar morpho¬ 
logically, they are dissimilar physiologically (8). In view of these facts, it 
was proposed to make an investigation to determine whether or not 
different isolations of R. nigricans behave differently physiologically. 
The hanging drop method used for studying spore germination was 
selected from among others tried, as being the one likely to give the most 
reliable data. Sweet potato decoction was employed as a substrate. 
Care was taken that each hanging drop contained an approximately 
-equal number of spores. Seven incubators were employed, the tem¬ 
peratures of which are shown in Table II. One hanging drop of each 
organism was placed in each of the incubators and examined frequently 
for spore germination. By using all the organisms at one time in each 
of the incubators, variations due to fluctuations in temperature were 
largely obviated. These tests were duplicated and the average figures 
obtained are given in Table II. 
When it was desirable to examine the spores, the hanging drops were 
removed from the incubator and placed under a microscope near by, 
-every effort being made to examine them as quickly as possible. The 
slide was not out of the incubator usually more than one minute. Spore 
germination was considered to have taken place when the length of the 
germ tube of the first germinating spores was equal to the diameter of 
the spore. 
The same incubators were employed in determining the minimum, 
optimum, and maximum temperatures for growth. The same general 
methods were employed also with such modifications as were necessary 
to meet the requirements of the experiments. All the strains were 
