210 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
from original thermograph records corrected by daily maximum and 
minimum thermometer readings for two seasons (Brooksville, 191 9 and 
1921; Pomona, 1919 and 1920). While the curves are not strictly com- 
parable, they nevertheless furnish a fairly reliable basis of comparison, 
since including data for a longer period at the three latter stations would 
alter the shape of the curves but little. The data for the field stations, 
Pomona and Brooksville, of course approximate the conditions under which 
strawberry plants grow more closely than those for the city stations. 
The range of mean hourly temperatures in the field in central Florida, 
January to April, as shown by the curves for Brooksville, is from about 
47° F. to 80° F., temperatures known to be favorable for the growth of 
Rhizopus nigricans. In the region about Pomona, California, on the other 
hand, there is a period each day during which the temperature is less than 
47° F., the minimum temperature at which R. nigricans grows under most 
conditions. In January there are about twelve hours out of the twenty-four 
during which the temperature is below 45° F., in February there are nearly 
ten, in March nine, and even in April there are five hours each day below 
this temperature. During this period, more than one third of the day, 
even in March, Rhizopus nigricans is able to make little if any growth. 
The mean temperature, however, does not go below 35° F., at which tem- 
perature Botrytis cinerea grows readily. There is thus throughout the 
winter a daily period, averaging one half the day in January, during which 
B. cinerea grows without competition from Rhizopus. 
The writer believes that in this fact is found one of the chief causes of 
the great prevalence of Botrytis on strawberries and perhaps on other hosts 
in California during the winter. Under the temperature conditions which 
prevail in Florida, the rapid growth and abundant spore production of 
R. nigricans enable it to compete successfully with the numerous fungi 
which are known to infect strawberry fruits. Indeed, it may well be that 
the temperature of strawberries in Florida is frequently so high as to hinder 
the grow^th of Botrytis. Previous studies (11, p. 179) have shown that the 
temperature of strawberry fruits in the sun is often ten or more degrees C. 
above that of the air. Under mean shade temperatures of 72° to 80° F., 
strawberries in the sun may be expected to reach temperatures of 90° 
to 100° F., which are well above those most favorable for the growth of 
Botrytis. This may indeed be an important factor, hitherto overlooked, 
in the favorable influence of rainy weather on the growth of Botrytis. For 
not only are rainy days generally cooler than clear ones, but on such days 
the berries remain at or near the temperature of the air, and thus at tem- 
peratures more favorable for the growth of Botrytis than the much higher 
temperatures reached on clear days. 
In California, on the other hand, the ability of Botrytis cinerea to grow 
at the lower temperatures gives it an enormous advantage over Rhizopus 
nigricans and over fungi with similar temperatuie requirements. In the 
