1913] LIVINGSTON—TEMPERATURE COEFFICIENTS 363 
Since the individual data from which was derived Day’s chart 
of the average length of the frostless season in the United States 
have not been published as such, the average lengths of the frost- 
less season for the various stations in the United States with which 
we have been concerned have been taken from the 106 climatic 
summaries by sections,’5 published by the Weather Bureau. 
Not only is it necessary to fix upon the length of time during 
which temperature integration is to be made, it is also requisite, for 
each station, to establish the month and day with which the 
summation is to begin, and likewise that with which it is to termi- 
nate. These dates must of course be chosen with reference to the 
beginning and ending of the period of active growth. The first is 
here taken as the date next following the average date of the last 
frost in spring, and the second as the average date of the first frost 
in autumn. If the dates for the beginning and end of the assumed 
growth period were determined from daily mean temperatures (as 
in case this period were taken as beginning with the first day in the 
year having a daily mean of 40° F. or above, and ending with the 
last day having this temperature), then they should bear a definite 
relation to the real first and last dates of the active period. Such 
methods have been frequently used in phenology. By our method, 
the beginning and end of the average period of active growth are 
determined from frost data rather than from mean daily tempera- 
tures. The two methods would probably prove equally — 
factory, with no great discrepancy in most cases. ~~ go” 
The geographical distribution of plant association. al’ of 
species involves the effectiveness of climatic features throughout 
many years, and a knowledge of these features for any single year 
is of but little value in the present connection. This consideration 
involves still another sort of climatic integration that has not 
yet been mentioned here; namely, the averaging of the effective 
conditions for long periods of time. This operation has been 
implied, however, in the preceding treatment of the average length 
"Day, P.-C. _ data of the United States, etc. U.S. Dept. Agric., Weather 
Bureau, Bull. V. 
*s Summary i is climatological data of the United States, by sections, U.S. 
Dept. Agric., Weather Bureau. No date. These pamphlets appear to have been 
prepared about 1909-1910. The data generally extend through 1908 or 1909. 
