14 
NEIL E. STEVENS 
and duration aspects of the temperature factor it is improbable that 
any fundamental or general principle regarding the influence of tem- 
perature in a plant is derived from the relations thus brought out. 
They suggest as more satisfactory for measuring temperature effect- 
iveness a method of calculating temperature efficiencies based on the 
well-known chemical principle of van't Hoff and Arrhenius, that 
within limits the velocity of most chemical reactions doubles or some- 
what more than doubles for each rise in temperature of io° C. On 
this basis the Livingstons (5, p. 366) have prepared a table of approxi- 
mate efficiency indices for temperatures in whole degrees from 40° F. 
to 99° F., assuming the efficiency to be unity at 40° and to double with 
each rise in temperature of 18°, and have prepared maps of the United 
States comparing temperature summations with the temperature 
efficiencies calculated according to their tables. The results of the 
two methods show a rather close general agreement but there are 
numerous discrepancies in detail. 
For purposes of comparison both methods have been used in the 
present work. In all calculations the mean for each day was deter- 
mined by the formula: Mean = J (maximum -f minimum). The 
calculations have been made in the Fahrenheit scale, not because this 
scale is as convenient as the Centigrade but because all Weather 
Bureau data are so published. 
Direct Temperature Summations 
The direct temperature summations have been calculated for all 
the stations where complete data are available.^ Forty-five degrees 
F. has been regarded as the zero point, since it is undoubtedly slightly 
below the temperature at which Endothia parasitica is able to grow (11). 
The amount added each day is then one half the sum of the maximum 
plus the minimum temperature as given in the monthly reports of 
climatological data issued by the Weather Bureau. The sum of these 
amounts for the 365 days for which the growth of the canker was 
measured is the temperature summation for the year. Table VI gives 
these summations for the various localities and the percentage of each 
when the summation at Charlottesville for the year ending April 23, 
1915, is considered 100 percent. 
With the single exception of Wilmington there is a fairly regular 
^ The writer is indebted to Mr. Anthony Merryman for much assistance in 
calculating weather data. 
