58 FOREST DISTEIBUTIOX 



law. By this method it is conceived that with erers- increase of 

 18 degrees F. from 40 degrees to the optimum there is a doubling 

 of the rate of growth and of acti\'ity of other physiological 

 processes. MacDougal (37) has applied this principle in de- 

 vising a method of integrating temperatures, whereby the area 

 below the line of a thennograph record and above the line of 40 

 degrees may be made to represent hour-degrees in relation to 

 plant growth, when applied to the rate of growth within fixed 

 limits of temperature. In the diagrams (Figures 8, 9) herein 

 presented for ^lontana the duration and intensity of the growing 

 temperatures is sho^\'n. The spaces between the heaviest vertical 

 lines represent months. The same distance between horizontal 

 lines represents 10 degrees of temperature. The tallest of these 

 vertical lines represents therefore 72 degrees as the mean for 

 the month of July at Billings. The lowest here given is 61 de- 

 grees which is the mean for the same month at Ovando. In the 

 former case there are as a rule 32 degrees of temperature favor- 

 able for growth, in the latter 21. The others may be seen to vary 

 from 22 to 28 degrees. The duration, however, varies, as indi- 

 cated by the character of the curve which in some cases ascends 

 directly to the maximum and falls off again quickly, in others 

 the higher temperatures are sustained longer and the curve 

 accordingly has a flat or broadly rounded top. These features 

 figure largely in the sum total of the productive temperatures 

 of the locality, and stand in relation also to the variety of crop 

 plants usually grown under irrigation in the several places. 



The curves of total heat recjuired for all species must nat- 

 urally fall within the limits of the curve thus determined for 

 each locality, wlien established upon the same basis. It is also 

 evident that the curve for the species must more nearly approxi- 

 mate the curve for the locality the higher the latitude or altitude. 

 If the exponential interpretation of temperatures in relation to 

 plant life is applied, it means that in one month of higher tem- 

 perature as much substance of the plant body may be formed as 

 in a longer period at lower temperature. It must be borne in 

 mind, however, that the brevity of the far northern summer 

 seasons is accompanied by the shortening of the vegetative period 

 and the production of seed in much less time than i.s required 

 bj' the same species in warmer climates. Wheats which require 



