Apr. 7, 1923 
Temperature Relations of Eleven Species of Rhizopus 35 
TABi.e II .—Comparative rate of growth of Rhizopus nigricans on string bean and 
Irish potato agars at different temperatures 
Temperature. 
24 hours. 
48 hours. 
String bean 
agar. 
Irish potato 
agar. 
String bean 
agar. 
Irish potato 
agar. 
°C, 
Mm. 
Mm. 
Mm. 
Mm. 
33. 
0 
0 
0 
0 
25-7 . 
41. 4 
23. 8 
92 
46. 6 
23-3. 
40. 4 
22 
92 
42. 2 
19-4 . 
22. 4 
13.8 
73-3 
39 
12. 9. 
7-4 
6. 2 
48 
24 
II -9 . 
0 
0 
9 
7.8 
10. 7. 
0 
1 
0 
1 
0 
1 
0 
The data show that for the most part nigricans grows nearly twice as 
fast on string bean agar as on Irish potato agar. The plates of bean 
agar at 25.7° C. after 48 hours were completely covered, and it was evi¬ 
dent that the diameter of the growth would have been somewhat greater 
had it not come in contact with the edge of the dish. The plates at 
23.3® were just covered. The difference in the amount of growth on 
the two agars at 11.9° was very slight but yet apparent. The cardinal 
temperatures on the two media appear from the result of these experi¬ 
ments to be about the same. However, to determine this point with 
certainty, much closer temperatures than those used by the writers would 
have to be employed. 
DEXTROSE 
While studying the effect of different culture media upon growth it 
was observed that nigricans grew at 30° C. on a synthetic agar but 
failed to do so on the other media used. 
It was thought that perhaps the differences in the acidity of the media 
might account for this shifting of the maximum temperature. Hence, 
an experiment was prepared in which Irish potato and beef agars made 
up to the same H-ion concentration as the S5mthetic agar, were used, 
together with synthetic agar as a control. The fungus, however, failed 
to grow on either of the modified agars at 30° C., but did grow on the 
synthetic agar. This experiment demonstrated that the H-ion concen¬ 
tration of ffie media was not the cause of the shifting of the maximum 
temperature of this fungus. 
The synthetic agar differed from the other agars in sugar content as 
well as acidity, and for this reason tests were made to determine the 
effect of the addition of different quantities of dextrose to the media. 
Irish potato and beef agars were prepared to contain roughly i, 5, 10, 
15, and 20 per cent and carrot agar i, 5, and 10 per cent dextrose. Ten 
plates were prepared in the usual way with these as well as with unmodi¬ 
fied string beans, com meal, and sweet potato agars. Plates of synthetic, 
Irish potato, beef, and carrot agars were prepared in the same manner 
and held as controls. All the plates were held at 30° C. for 48 hours, 
when the diameter of the growth was measured. This experiment was 
repeated several times. The average figures for all tests are presented 
