Apr., 1922] STEVENS — ROTS OF EARLY STRAWBERRIES 207 
of central Florida are more sandy than those of southern California. The 
surface of a sandy soil of course dries quickly. Humidity conditions thus 
seem to be at least as favorable for the development of Botrytis during these 
months in southern California as in central Florida. If anything, they are 
somewhat more favorable. In fact, as Bennett has recently shown (2), 
the region about Tampa has, for so southern a station, a remarkably low 
relative humidity. 
Temperature Relations 
The difference in the temperature of the two regions presents a very 
interesting correlation with the difference in the temperature relations of 
the fungi under consideration, and seems to account in large part for the 
difference in the relative abundance of these fungi on strawberry fruits. 
The exact determination of the minimum and optimum temperatures 
for the growth of an organism is very difficult. Both minimum and 
optimum temperatures of fungi have been shown to vary with the sub- 
stratum used and with the duration of the test. It is unlikely, also, that 
the behavior of a fungus under constant temperatures in the laboratory 
exactly corresponds to its behavior under changing climatic temperatures. 
However, its average temperature reactions under maintained conditions 
furnish our only available basis of comparison Avith the behavior of a fungus 
under natural conditions. 
Earlier studies (9) of the writer agree with those of Ames (i) and Han- 
zawa (8), and with the recent results of Brooks and Cooley (4), in placing 
the minimum for active growth of Rhizopus nigricans at about 7.5° to 8° C. 
(45.5 to 46.4 F.) and the optimum between 30^ and 35° C. (86° and 95° F.). 
In contrast (3), Botrytis cinerea will grow somewhat at 0° C. (32° F.) and 
freely at 2° C. (35.6° F.). Its growth is most abundant at about 25° C. 
(77° F.) and falls off rapidly at 30° C. (86° F.). Whether these tempera- 
tures represent absolute minima and optima for the growth of the two fungi 
is of little present interest. They may fairly be taken to indicate that the 
temperatures favorable for the growth of Rhizopus nigricans are markedly 
higher than those for Botrytis cinerea. 
As yet no thoroughly satisfactory way of expressing the value for plant 
growth of any given series of climatic temperatures has been devised. 
Where thermograph records are available, however, the monthly mean 
hourly temperatures certainly give some indication of the temperature 
conditions to which an organism is subjected in a given locality. Figures i 
to 4 show the mean hourly temperatures for the first four months of the 
year at two stations in southern California and two in central Florida. 
The curve for Tampa is based on mean temperatures for the years 1910 
to 1 91 9 computed by W. J. Bennett, and that for Los Angeles on mean 
temperatures for the years 1919 and 1920 furnished by H. B. Hersey. The 
curves for Brooksville, Florida, and Pomona, California, are based on data 
