36 
Journal of Agricultural Research 
Vol. XXIV. No. I 
in Table III. In the cases where no growth is recorded the spores usu¬ 
ally had germinated and formed short germ tubes but never formed a 
spot which spread appreciably beyond the area covered by the drop of 
spore suspension. Exposure for 48 hours to a temperature at which no 
grcfwth took place was sufficient to kill the fungus, as was shown by the 
fact that it failed to make a further growth when held for 24 hours at a 
temperature suitable for its development. 
Table III .—Comparison of the growth made by Rhizopus nigricans at C. on different 
agars with and without dextrose added 
Agar used. 
Not modified. 
With addition of dextrose. 
I per 
cent. 
5 per 
cent. 
10 per 
cent. 
15 per 
cent. 
20 per 
cent. 
Irish potato.... 
Beef. 
No growth.... 
.do. 
Na growth. 
.do. 
Mm. 
7 
No growth. 
.do. 
Mm. 
49 
30 
23 
Mm. 
59 
52 
Mm. 
57 
54 
Carrot.. 
.do. 
.do. 
f^trifig he.ari. 
.do. 
Com rriftal 
.do. 
-Sweet potato.... 
.do. 
Synthetic. 
‘t! . 
^ Contained 20 per cent desctrose. 
. . The table shows that the strain of nigricans used (4652) did not grow 
at 30° C. on any of the unmodified agars tried except synthetic agar, 
which contained 20 per cent dextrose as originally prepared. Upon 
the addition of 10 per cent or more of dextrose to IrivSh potato, beef, 
or carrot agar growtJi did take place at this tempera tine. The addition 
of more than 15 per cent of sugar seemed to have little effect. 
After it was determined that the maximum temperature for growth 
was raised by the addition of dextrose to the medium, experiments 
were conducted to learn the extent of this change. A loop of the same 
spore suspension was placed in the center of each of 20 Petri dishes, 
one-half of which contained unmodified Irish potato agar and the 
other half some of the same agar to which 20 per cent dextrose was 
added. The plates were then held at 31° C. Observations after 48 
hours showed that the spores had not germinated on the unmodified 
agar while an average growth about 5 mm. in diameter had been made 
on the agar with 20 per cent dextrose added. A slight additional 
growth was made after 48 hours longer on the agar with sugar added. 
The results of these experiments seem to justify the conclusion that 
the maximum temperature for the growth of this organism on Irish 
potato agar is raised from to 1.5° C. by the addition of 20 per cent 
dextrose to the medium. 
That the presence of dextrose in the medium influences the minimum 
temperature was demonstrated by placing a loop of spore suspension 
in the center 6f each of 20 plates of Irish potato agar to one-half of 
which 20 per cent dextrose was added. The plates were then placed 
at a temperature of from 0.8"^ to 2° C. The spores had not germinated 
at the end of 17 days on the unmodified agar. On the other hand, germ 
