58 I-'OKK>T 



law. By this method it is . onceived that with every increase of 



9 I-', from 40 degrees to the optimum there is a doubling 



of tlie rate of growth and of activity of other physiological 



& L MaeDnugal (37) has applied this principle in de- 

 visiiiLT a method of integral ing temperatures, whereby the area 

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

 degrees may he 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, 0) herein 

 presented for .Montana the duration and intensity of the growing 

 tempi-ratlin's is shown. The spaces between the heaviest vertical 

 lines represent months. The same distance between hori/ontal 

 lines represents lo degrees of temperature. The tallest of these 

 verti.-al lines represents therefore 72 degrees as the mean for 

 the month of -Inly at P.illiim.s. The lowest here given is 61 de- 



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 irrowth. in the latter 21. The others may be seen to vary 

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

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

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

 Th- hi'_rhcr temperatures are sustained longer and the curve 

 accordingly has a flat or broadly rounded top. These features 

 fiL-nre largely in the sum total of the productive temperatures 

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

 plants usually 'jnmn under irrigation in the several places. 



The curve^ of total heat required for all species must nat- 

 urally fall within the limits of the curve thus determined for 

 cadi locality, when established upon the same basis. It is also 

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



he curve f,,r the h.calily the higher the latitude or altitude. 

 If lh- exponential interpretat ion of temperatures in relation to 

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

 peratm-e afl much substance of the plant body may be formed as 

 in a |oML r '-r j,,. r jod at lo\vep tempcrat ure. It must be borne in 

 mind. hn\\e\-er. that the brevity of the far northern summer 

 'Mipanied by the ylmrlrmiiLT of the ve^-i tat i ve period 

 and the production of seed in much less time than is required 

 b\ th ipeciefl in warmer climates. Wheats which require 



