96 



BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 



plant increases in size in accordance with the compound interest 

 law. 



This method has been employed by Shantz and the writer 

 in the examination of the daily transpiration of Sudan grass, 

 corn, sorghum, and alfalfa. 3 The results indicate that during 

 the early stages of the growth of these crops an approximately 

 linear relationship does exist between the logarithm of the 

 transpiration ratio and the time, as is shown in the case of 

 Sudan grass in figure 1. In the case of alfalfa, this relationship 

 held approximately up to the period of flowering. A comparison 

 of the computed transpiration ratio (solid line) of Sudan grass 

 with that actually observed (circles) is given in figure 2. 



TABLE 1 



Hate of Increase in the Transpiration Coefficient of Different Crops 

 in 1914, According to Briggs and Shantz 



CROP 



Corn, Northwestern Dent 



Corn, Algeria 



Sorghum, Minnesota Amber 



Sudan grass (in inclosure) 



Sudan grass (in open) 



Alfalfa, E23 (first crop in open). 

 Alfalfa, E23-20-52 (first crop). . . 

 Alfalfa, E23-20-52 (second crop) 



OBSERVATION PERIOD 



June 18- July 9 

 June 18-July 11 

 June 18-July 9 

 June 18-July 10 

 June 24- July 11 

 June 16- July 10 

 June 19- July 9 

 July 18-Aug. 5 



DATS 



REQUIRED 



FOR k TO 



DOUBLE IN 



VALUE 



11.6 

 6.8 

 7.3 

 4.6 

 3.7 

 9.1 

 8.1 

 8.1 



It is also possible to compute from the slope of the graphs 

 the value of the exponent a in equation (8) and so to determine 

 the daily rate of increase in the size of the plant. The results of 

 such calculations for a number of plants are given in table 1. 

 It will be seen that in the case of Sudan grass the plant system 

 was doubling in size every four days, and in the case of alfalfa 

 about every eight days. 



Therefore during the early stages of growth the size of some of 



3 Briggs, L. J., and Shantz, H. L. Daily transpiration during the normal 

 growth period, and its correlation with the weather. Journ. Agr. Research, 7: 155- 

 212. 1916. 



