CLIMATE AND PLANT GEOWTH. 37 



sum of the positive or effective temperatures, that is, the sum of the 

 means above 40° F., as originally proposed by Merriam,^ and (3) the 

 sum of the daily mean temperatures. For comparison with plant 

 growth in this study, the sum of the temperature efficiencies for the 

 growth periods has been used instead of the average temperature 

 efficiency. This was done for the reason that the plant measurements 

 represent total growth for the respective periods. 



Physiological temperature coefficients are based upon data obtained 

 by Lehenbauer in the study of the elongation of the shoots of maize 

 sprouts when exposed to practically constant temperature for 12-hour 

 periods. These 12-hour exposures were made degree by degree at 

 temperatures ranging from the minimum at which growth takes 

 place, through the optimum, and on to the maximum temperature at 

 which growth is possible. Varymg increments of elongation 

 naturally took place according to the temperature to wdiich the 

 sprouts were exposed; and these growth rates were platted against 

 the temperature used, giving a curve showing the relation between 

 temperature and the rate of growth of the plant. The lengths of the 

 ordinates of this growth curve furnish a series of numbers which rep- 

 resent the efficiency of the various temperatures in promoting the 

 growth of maize. The application of the physiological temperature 

 coefficient to any plant other than the one used by Lehenbauer is 

 based on the assumption that the general relation of growth and 

 temperature is the same as for the maize. Whether or not the physio- 

 logical temperature indices obtained under controlled conditions will 

 apply to field plants where the temperatures fluctuate widely can not 

 be stated. It may be presumed for the present, however, that they 

 will more closely account for physiological responses of field plants 

 than will direct temperature summations. 



Since these indices are ba^ed on physical and chemical processes 

 taking place within the plant, temperatures at which no appreciable 

 activities take place are at once eliminated; at the same time the 

 efficiency of the temperature up and down the thermometer scale 

 receives the proper weight. 



In applying this method of temperature summation the daily 

 mean temperatures were first obtained from the hourly corrected 

 thermometer readings for the period during which the plants in the 

 type stations were grown. The corresponding physiological indices 

 were then substituted for the daily means and these indices summed 

 for the period in question. 



By positive or effective temperatures is meant the number of de- 

 grees of temperature above the minimum at which growth can take 



1 Merriam, C. Hart. Life zones and crop zones in the United States. U. S. Department 

 of Agriculture Bull. 10 : 55-73. 1898. 



