366 
Journal of Agricultural Research 
Vol. XXVI, No. 8 
used at one time in each of the seven incubators, in order to eliminate 
variations due to fluctuations in the temperatures of the incubators. 
The organisms were grown on Irish potato agar in Petri dishes. A small 
drop of a spore suspension was placed in the center of the dish by means 
of a platinum loop. At the end of twenty-four hours and as often there¬ 
after as desirable the diameter of the growth was measured. The plate 
was usually entirely covered at the end of forty-eight hours at the opti¬ 
mum temperature. The optimum temperature was considered to be 
the one at which the entire plate was covered in the fewest number of 
hours; the maximum, that at which spore germination and growth 
would just take place. It was more difficult to establish a minimum 
temperature, since the time element played such an important part. 
It was found that at some of the lower temperatures germination took 
place after about 30 days. Usually this was about as long a time as 
was allowed for germination. However, it is not unlikely that in a 
longer time germination might have taken place at a lower temperature. 
Table II shows in detail the results of the germination tests. Perhaps 
in no case has the minimum been established. The spores of five organ¬ 
isms germinate at i.8° C. in from five to seventeen days. It is not 
unlikely if sufficient time had been allowed the spores of other strains 
would have germinated at the same or even at a lower temperature. 
Nevertheless, they had not done so by the end of thirty days. Three 
hours at a temperature of 31 0 C. was found to be the average time re¬ 
quired to germinate the spores. Therefore, according to the criteria 
established by the writers, a temperature of 31 0 C. is the optimum for 
germination of the spores of the different strains of R. nigricans. At tem¬ 
peratures both above and below 31 0 C. the time required to germinate 
the spores is increased. At a temperature of 32.5 0 C. it is increased 60 
minutes and at 33.5 0 C. 1 hour and 37 minutes. No germination took 
place at 35.5 0 C., a temperature only four degrees higher than the opti¬ 
mum, which shows that only a slight rise in temperature above the opti¬ 
mum is very injurious to the spores. At temperatures below the optimum 
the time required to germinate the spores increases much more gradually. 
If a curve was constructed to show the average time required to germi¬ 
nate the spores at the different temperatures, it would be found that the 
ascent from the lowest temperatures to the optimum is quite gradual. 
Above the optimum temperature the curve would fall quite abruptly to 
zero at 35.5 0 C. This is the type of curve usually obtained from the 
results of experiments of this kind. The authors (<?) have obtained such 
curves for several different species of this genus. 
It is of interest to note some of the variations between the different 
strains at the same temperature. For example, at 31 0 C., which is the 
optimum for all of the organisms with the exception of 4652, there is a 
variation of from 2 hours and 41 minutes to 4 hours and 37 minutes; the 
spores of the majority of the strains, however, germinated in less than 3 
hours. One strain, 4652, requires special consideration. This organism 
has been used by the writers in a large number of experiments of various 
kinds. It was used by them (<8) when a comparative study was made 
of the minimum, optimum, and maximum temperatures for spore germi¬ 
nation and mycelial growth of the different species of Rhizopus. It has 
also been used for the study of pectinase secretion (/), hydrogen-ion con¬ 
centrations, etc. It was found to be an especially low temperature form 
and to secrete a small amount of pectinase. It will be noted from an 
