GROWING SEASON TEMPERATURES 



237 



tion to plants Is, therefore, qualified. Fully recognizing the numerous 

 limitations inherent in the data at present available, they have, never- 

 theless, tentatively assigned "efficiency indices" to the various degrees of 

 temperature, reproduced in part in Table 1, and applied them to the 

 temperature data at various points in the United States. 



In a subsequent paper Livingston proposes a different system, based 

 on Lehenbauer's studies of root growth in maize. ^°^ 



This system differs from the others in that it is based on observed rates of 

 growth and in taking cognizance of a decreased rate of growth with temperatures 

 above the optimum. A comparison of the values obtained with the three 

 systems is given in Table 1. Livingston evidently regards this work only as a 

 step toward further study, since he states: "... these indices are to be re- 

 garded as only a first approximation and . . . much more physiological study 

 will be required before they may be taken as generally applicable. In the first 

 place, they are based upon tests of only a single plant species, maize, and there are 



Table 1. — A Comparison of Temperature Index Values, Starting with 40°F. 

 AS Unit, According to Three Systems 



probably other plants ... for which they are not even approximately true. 

 ... no doubt other phases of growth in the same plant may exhibit other 

 relations between temperature and the rate of shoot elongation. Third, these 

 indices refer to rates of shoot elongation, and there are many other processes 

 involved in plant growth, which may require other indices for their proper inter- 

 pretation in terms of temperature efficiency. Fourth, they apply strictly only 

 under the moisture, Hght and chemical conditions that prevailed in Lehenbauer's 

 experiments . . . Fifth, and finally, plants in nature are never subject to 

 any temperature maintained for any considerable period of time. . . ." 



Influence of Latitude on Heat Requirements. — Phenological data 

 on any single fruit plant gathered over a wide area are rather scarce at 

 present and those available are not altogether satisfactory. However, in 

 combination with temperature data compiled by the Weather Bureau 



