1913] LIVINGSTON—TEM PERATURE COEFFICIENTS 355 
tion indices. So far as we know, this is the first chart of its kind to 
be prepared upon the basis of temperature summation, and the 
temperature zones of this writer have come into rather general use 
among American phytogeographers. 
A method of direct summation, similar to those employed by 
phenologists but dealing in a more refined way with the tempera- 
ture conditions of plant environments, has been described by 
MacDovuecat.’ This author summed the temperatures above the 
freezing-point of water for the period occupied by certain develop- 
mental phases of certain plants; but, instead of adopting as the 
terms of the summation the daily means or the means of daily 
maxima and minima, his integration was performed, with a pla- 
nimeter, upon automatically traced thermograph records. The 
resulting indices of environmental heat conditions are expressed 
by MacDoveat in terms of “hour-centigrade-degree” units. This 
method has never been employed in climatological or phyto- 
geographical studies, so far as we are aware. It seems to be the 
simplest and most promising of all the direct summation methods, 
but of course requires reliable thermograph tracings. Each year 
or season is to be treated separately, and the resulting annual 
indices may be averaged for a period of years, to give a normal 
index. 
Temperature efficiencies 
Although such temperature summations as those of the phenolo- 
gists and of MEerRrAM have seemed in many instances to furnish 
data consistent among themselves and constituting on an empirical 
basis an apparently reliable criterion for the measurement of the 
intensity and duration aspects of the temperature factor, yet it 
must be regarded as highly improbable that. any fundamental and 
general principle regarding the influence of temperature on plant 
life may be derived from the relations thus brought out. It seemed 
to us that the apparent value of temperature summations must rest 
upon some basic principle of physiology not indicated in the summa- 
tions themselves. 
7 egies D.T., The temperature of the soil. Jour. N.Y. Bot. Garden 
3:125-131. 190 
