G. E. Briggs, F. Kidd, and C. West 213 



process only when the intensity is weak, and that normally the factor 

 which limits the assimilatory process is the rate at which the carbon 

 dioxide can reach the chloroplast — the seat of photosynthesis (2). A point 

 to be remembered, however, is that various environmental factors, such 

 as light intensity, temperature, humidity of the atmosphere, etc., may 

 affect the assimilation, not directly by affecting the assimilatory process 

 itself, but indirectly by affecting the degree of stomatal opening and 

 consequently the amount of carbon dioxide which can reach the chloro- 

 plast. We have calculated the correlation coefficients between Unit 

 Leaf Rate and the following — 



(1) Duration of light above certain intensities plus the duration 



of weaker lights diminished by factors as stated below. 



(2) Total light as calculated by Kreusler. 



(3) Hours of sunshine. 



(4) Duration of light of all intensities. 



(5) Average maximum temperature. 



(6) Rainfall. 



Correlation coefficients between Unit Leaf Rate and various environmental 



factors 1 . 



r L = correlation coefficient between Unit Leaf Rate and the duration of light, including 



light down to -05 sunlight; 

 r L = correlation coefficient between Unit Leaf Rate and the duration of light, including 



light down to -1 sunlight plus half the duration of light of -05 sunlight; 

 Tt = correlation coefficient between Unit Leaf Rate and the duration of light, including 



light down to -2 sunlight plus half the duration of light of -1 sunlight plus a charter 



the duration of light of -05 sunlight; 

 r L = correlation coefficient between Unit Leaf Rate and the duration of light, including 



light down to -3 sunlight plus two-thirds the duration of light of -2 sunlight, a third 



that of -1 intensity and a sixth that of -05. And so on for r L , etc.; 

 rr = correlation coefficient between Unit Leaf Rate and total light; 

 r L = correlation coefficient between Unit Leaf Rate and the hours of sunshine; 

 r T = correlation coefficient between Unit Leaf Rate and average maximum temperature ; 

 /jo = correlation coefficient between Unit Leaf Rate and rainfall; 

 r R = correlation coefficient between Unit Leaf Rate and rainfall of the previous week. 



1 The fact that the method of calculation used does not give the exact value for the 

 Unit Leaf Rate during the first few weeks, as already pointed out (see footnote, p. 203), 

 does not materially affect the significance of the correlation coefficients. When the Unit 

 Leaf Rates are calculated on the exponential basis the values for r L and r T , for example, 

 become -76 and -34 respectively instead of -77 and -30. 



