16 



from the growth of maize under controlled conditions, with 

 different maintained temperatures may possibly express a 

 general relation between plant growth and .temperature and 

 may thus be applicable to plants growing under other conditions. 

 The graph of these physiological indices, as rela'^ed to tem- 

 perature is of course the graph of mr.ize seedling growth as so 

 related, and it exhibits the same direction of slope between 

 low temperature and the optimum graphs of temperature efficien- 

 cies as iorived by the other t7/o methods, but for this portion 

 of the temperature range the slope of thp graph of physiologi- 

 cal indices is generally .:;teeper than that of the graph of 

 remainder indices, the latter graph having a much steeper 

 slope than that of the exponential indices. This is shown 

 by Livingston and also by Stevens, in the papers cited above. 

 Since they are derived from the actual growth rates :;f a plant, 

 the physiological temperature indices appear to have a more 

 rational basis trian either the remainder or the exponential 

 indices. For this reason, and for others that will appear 

 below, the physiological indices are used in this study for 

 expressing the tempera■^nre as it aff'^cts the growth of the 

 plants. Two other series of temperature values sire presented 

 in the talles of this paper but neither has been found to be 

 as satisfactory for exioressir.g this climatic condition as a:'e . 

 the physiological indices. ":hese two other temperature values 

 are fl) the average daily mean temperature for each period, 

 in decrees Fahrenheit and (2) the remainder summation index 

 for eahh period. In the case of temperature, as in the case 

 of light and evaporation, the value given for each period 



