AUTO-THERMAL INTEGRATION OF CLIMATIC COMPLEXES 1 87 
words, it is proposed to measure climate in terms of protoplasmic 
activity, a procedure that has become necessary in the experimental 
tests of the effect of climatic complexes upon plants now being carried 
out at the Desert and Coastal Laboratories and the attached planta- 
tions. If any rational analysis is to be made of the direct and in- 
heritable effects that have come under observation in this work it 
must be done upon the basis of determinations of the influence of 
the separate environic components, of which of course temperature 
is one of the most important. 
The author's work led him to a realization of the necessity for such 
a method in 1900 and the first attempts at anything definite were 
presented at the Denver meeting of the A. A. A. S. in 1901. The 
* method then proposed consisted simply in estimating the area of 
the thermographic diagram by the line of freezing point and by the 
temperature tracing from the beginning of a season until a plant had 
attained a certain stage of its development. The vertical component 
in such figures being degrees of temperature, and the horizontal ele- 
ment being elapsed time, the resulting amounts were designated as 
hour-degrees. Thus the silver maple {Acer saccharinum) was found 
to have been exposed to 3,466 hour-degree units from the winter 
solstice until the time of blooming on March 26, 1901. Draba verna 
reached a similar state in 974 hours afte^r an exposure of 1,644 units 
of exposure.^ 
This method was superior to all previous methods of summation of 
temperature-effects in that it gave full value to the time factor of exr- 
posure, which the older methods of cumulation, or of totalizing maxima 
or averages of daily temperatures did not. It was obviously a purely 
empirical procedure, as growth at all temperatures above the freezing 
was taken as uniform. The use of the freezing point as a physio- 
logical zero has much in its favor in phenological observations such as 
those upon Acer and Draba noted above, but a method of wider use- 
fulness would assume a zero above this point. 
As applied to the possibilities of growth it gave a value for the year 
of 78,836 hour-degree units to a meadow in the New York Botanical 
Garden and to the floor of the hemlock forest within four hundred 
yards of 68,596 units, a basis of comparison generally in harmony 
2 MacDougal, D. T. The temperature of the soil. Jour. N. Y. Bot. Garden 3: 
125. 1902, and Factors affecting the seafonal activities of plants. Plant World 10': 
218. 1907. 
