38 
Journal of Agricultural Research 
Vol. XXIV, No. r 
concentration of the cell sap, thereby protecting the protoplasm of the 
cell against coagulation by the heat to account for the raising of the 
temperature maximum i° to 1.5° C. The high concentration of sugar 
in the substrate may also act to lower the water content of the cells of 
the fungus and thereby increase the concentration of the cell sap which 
would tend to raise its maximum and lower its minimum. However, 
the true explanation of this phenomenon must await the accumulation 
of more experimental data. 
DISCUSSION 
It is apparent from the foregoing curves and tables that the cardinal 
temperatures for spore germination, growth, and fruiting of the fungi 
studied vary somewhat. In general the spores will germinate at a 
temperature too low for mycelial growth. A higher temperature is 
required for fruiting than for growth. The same thing holds true for 
the^ optimum temperatures, although not to the same extent. The 
optimum for germination is always higher than for growth and fruiting, 
while in most cases the optimum for fruiting is about the same as that for 
Jfrowth. The slight differences shown in Table I may be due to the fact 
that these figures give the optimum temperatures tried rather than the 
true optimums. The latter probably does not differ from the former 
more than a plus or minus i or 2° C., since the temperature of the incu¬ 
bators usually differed only from 2 to 5°, the maximum differences being 
between the extreme temperatures. The optimum temperature for 
fruiting is often not so well defined as that for growth, and the latter less 
so than for spore germination. 
In each case there is a gradual gradation in the maximum from that 
at which the spores will germinate to that at which fruiting will take 
place, the maximum for growth being about midway between that for 
germination and that for fruiting. This fact may be explained in part 
at least in two ways; first, the spores germinate quickly, probably before 
the heat acts injuriously; and second, the spores are probably less sensi¬ 
tive to heat than are the germ tubes. Spores which before germination 
are uninjured by the heat may be readily killed after they germinate. 
At a slightly lower temperature growth becomes evident but ceases after 
a time, and the mycelium is killed before sporangia are formed. 
The time factor is of greater importance at the minimum than at any 
other temperature. At low temperatures, germination is greatly 
retarded, the growth being often so sparse that no measurable felt is 
produced within the time limit of these experiments. 
The genus Rhizopus includes some of the best known, most widely 
distributed and most destructive of the fungi. Nigricans is perhaps the 
most destructive, since it attacks a wide range of hosts under widely 
different conditions, being especially destructive to tomatoes, straw¬ 
berries, and sweet potatoes. However, this species is somewhat limited 
in its field of destruction by temperature. For example, in these experi¬ 
ments its spores were killed by a comparatively short exposure at a 
temperature of 34® to 35*^ C., and it made a very slow growth at 6.5^ 
(35 mm. in 30 days) and no appreciable growth at 1.5° after 30 days. 
Although some variation in its response to temperature on different 
substrates may be expected, there is little doubt that decay of fruits and 
vegetables by this organism can be prevented or retarded by proper 
cold storage. A temperature as high as 7° or thereabouts will prevent 
fruiting for several days and hence retard the spread of this fungus. It 
