212 



ENVIRONMENTAL CONDITIONS. 



nomena as a whole can not be maintained beyond the hmits set bj^ the 

 minimum and optimum temperatures for growth. However, these 

 physiological constants are not the same for different plant-forms, and 

 they may also be assumed to vary with other conditions within and 

 without the plant. It therefore seemed desirable, in an attempt to 

 apply this law of temperature coefficients to the climatological delimit- 

 ation of geographical areas, to choose such limiting temperatures as 

 should give promise of merely approximating the physiological limits 

 for a large number of plant-forms. In this we follow Livingston and 

 Livingston, who give a more complete discussion of this whole question 

 than is required here. 



The authors just mentioned calculated their indices of temperature 

 efficiency on the basis of the supposition that general plant activity 

 occurs at unity rate when the daily mean temperature is 40° F., and 

 that this rate is doubled with each rise of 18° F. in the daily mean. 

 Thus, with a daily mean temperature of 58° F. the rate becomes 2.0, 

 with a mean of 76° F. it becomes 4.0, etc. It thus becomes clear that 

 the relation here assumed between any index of temperature efficiency 

 and the corresponding index of temperature itself is an exponential 



one, expressed by the equation 7 = 2~i8", where / is the index of 

 efficiency and t is the corresponding index of temperature. From this 

 equation, by substituting the various values of t, Livingston and 

 Livingston prepared a table of the efficiency index values corresponding 

 to the temperature indices from 40° to 99° F. This table is reproduced 

 as our table 4. Of course, constants other than 2, 18, and 40 might be 

 employed, thus giving other values to the indices of efficiency; these 

 indices, as has been stated, are based on the supposition that plant 



Table 4. — Exponential indices of temperature efficiency for plant groivth, based on a coeffi- 

 cient of 2.0 for each rise in temperature of 18° above 40° F., for each temperature from 

 41° to 100° F. 



^The temperature efficiency index is here assumed to be doubled for each rise of 10° C. (18° F.) 

 above 40° F., at which temperature the index of efficiency is taken to be unity. 



