34 
Journal of Agricultural Research 
Vol XXIV. No I 
Experiments were next conducted to determine whether the influence 
of temperature on the germination of the spores is reflected in the rate 
of mycelial growth. Irish potato agar contained in 5 Petri dishes was 
inoculated with a loop of a spore suspension from cultures of triiici and 
delemar grown at 20.5°, 27.5^, 31.5°, and 36.5° C. The plates were held 
at 34® to 35^. At the end of 24 and 48 hours the growth of tritici had 
reached a diameter of 50.6, 50.8, 48, and 45.8 mm. and 84, 92, 82, and 
79.5 mm., respectively. The growth of delemar measured at the end 
of 24 and 48 hours 46, 42, 40.5, and 34.6 mm. and 79.5, 81.4, 77, and 
78.8 mm., respectively. In general tritici showed a slight decline in 
the rate of growth when the spores were grown at the two highest tem¬ 
peratures. Delemar showed a more marked difference at the end of the 
first 24 hours, there being a gradual decrease from 46 to 34.6 mm.; but 
this was largely overcome during the next 24 hours, as shown by the 
fact that there was less than i mm. difference between the two extreme 
temperatures. The temperature at which the spores are produced 
undoubtedly influences their rate of germination and the early period 
of the growth of the mycelium developed therefrom. The evidence 
seems to point to the fact that different species react differently in this 
respect, some being much more sensitive to small changes in temperature 
than others. 
CULTURE media 
Tests were also made to determine to what extent, if any, different 
media would affect the rate of germination of nigricans spores. The 
media used were sweet potato decoction, beef bouillon, distilled water, 
string bean agar, Irish potato agar, and a synthetic agar. The media 
were placed on cover slips, spores from a single culture were sifted on, 
and the slips were then inverted over glass rings on slides in the usual 
manner and incubated at 26® C. The time necessary for germination in 
each case was as follows: Sweet potato decoction, 2 hours; beef bouillon, 
2 hours and 25 minutes; distilled water, hours; string bean agar, 2 
hoinrs and 10 minutes; Irish potato agar, 2 hours and 20 minutes; and 
sjmthetic agar, 3 hours. Sweet potato decoction proved the best solu¬ 
tion tried, and string bean the best agar. This experiment was dupli¬ 
cated on different days, using spores from a different culture with very 
similar results. It seems clear that Rhizopus spores require something 
more than water for good germination, since in distilled water a much 
smaller percentage of the spores germinated, and the germ tubes pro¬ 
duced were considerably more slender than those supplied with nutrients. 
Tap water was found by comparative tests to be a less favorable medium 
for the germination of these spores than distilled water. 
In order to test the effect of the substrate upon the rate of mycelial 
growth, plates were prepared in the usual manner, using string bean and 
Irish potato agars. The plates were inoculated with a platinum loop of 
the same suspension of nigricans spores in sterile distilled water. Five 
plates of each agar were used at each temperature. Measurements of 
the diameters of the mycelial disks were made in the usual way, and an 
average taken of the growth on each medium at the different tempera¬ 
tures. The results are given in Table II. 
