CLIMATIC CONDITIONS OF THE UNITED STATES. 215 



This physiological method of deriving temperature efficiency indices 

 is of course empirical, but it is not more so than either the direct or the 

 remainder method, and it has what seems to be a better experimental 

 foundation than has either of these. Furthermore, Livingston's 

 physiological indices appear to be much more in accord with what is 

 actually observed in the case of growing plants than are the exponen- 

 tial indices of Livingston and Livingston, and it may be said that we 

 have at last obtained a method for estimating temperature efficiency 

 that is applicable throughout the entire range of possible temperatures, 

 the graph of these indices showing approximately the same form as 

 Lehenbauer's graph of actual growth-rates. Of course it is desirable 

 that this set of relations be eventually broadened so as to include other 

 plants than maize and other developmental phases than that of the 

 seedling, and when this is accomplished the efficiency index values for 

 general use will probably need to be altered; it would be surprising if 

 indices for seedling maize plants should prove to be representative for 

 plants in general. Enough data are now at hand, however, for a some- 

 what rational beginning, and we regard these physiological indices of 

 Livingston as the most promising of all the different kinds of indices 

 of temperature efficiency that have been proposed. 



We turn now to the employment of these four different kinds of 

 temperature efficiency indices in our study of the climatic conditions 

 of the United States. In the application of temperature efficiency 

 indices to climatological study we have followed phenological workers 

 in summing these indices throughout a certain period, the resulting 

 summation being supposed to represent, in the form of a single number, 

 the efficiency value of that period, and for the station in question, as 

 far as temperature is concerned. We have used Bigelow's normal 

 daily mean temperatures (Bulletin R of the United States Weather 

 Bureau) as our daily temperature indices, with which all our computa- 

 tions begin. For the time-period we have here again used the period 

 of the average frostless season. The procedure has been (1) to find the 

 efficiency index corresponding to the temperature index for each day of 

 the average frostless season, at each station, and (2) to add these daily 

 efficiency indices all together to give the seasonal efficiency index for 

 the station in question. This method allows the length of the average 

 frostless season, as well as the values of the daily efficiency indices 

 throughout that season, to take part in the control of the final value 

 which represents the seasonal temperature efficiencj^ for growth. 

 Thus, a long season with relatively low temperatures may give a higher 

 summation index than does a shorter season with higher daily mean 

 temperatures. The use of Bigelow's normal daily means of tempera- 

 ture as original data should give to these studies an exceedingly broad 

 and general character, and may be supposed to produce some approxi- 

 mation to the average conditions of temperature for many years. 



