216 Quantitative Analysis of Plant Growth 



for the respiration of plants of mean age we have done the best we could 

 under the circumstances. The correlation coefficients of real assimilation 

 with various environmental factors are given below. 



Correlation coefficients of real assimilation with various environmental 



factors. 



(For years 1875-8.) With weekly mean temperature r = -78 



(For years 1876-8.) With weekly mean tern perature r=:-82 



(For years 1876-8.) With weekly hours of sunshine r = -60 

 (For year 1877, "nach Auswahl. ") 



'/., 



= 17 



r Ls = -28 r i6 = .26 r T = -56 



The partial correlation coefficients for the years 1876-8 are as follows: 

 With weekly mean temperature r=-76 

 With weekly hours of sunshine r = -38 



It will be seen that in the case of the five selected weeks for the 

 earlier portion of the life-cycle for the four years the correlation of real 

 assimilation with weekly mean temperature and with hours of sunshine 

 is of the same order as that between Unit Leaf Hate and these two en- 

 vironmental factors, that for temperature being the greater. In the case 

 of the larger portion of the life-cycle of the selected plants for the year 

 1877 it will be seen that the correlation with light, no matter how 

 measured, is insignificant, whereas the correlation with maximum tempe- 

 rature is considerably greater than it was in the case of the Unit Leaf 

 Rate. 



If the allowances made for respiration approach accuracy the indica- 

 tion is that the real assimilation of the plant is not governed by light 3 . 

 Taking into account the whole of the evidence afforded by the correla- 

 tion coefficients it would seem that the main factor governing real 

 assimilation is temperature. It must be pointed out that the averages 

 of the daily maximum or mean temperatures are not an accurate measure 

 of the average temperature for the days or for the days and nights of 

 the week respectively. The significant correlation of Unit Leaf Kate 



1 If it is found that the apparent assimilation of the leaves is more closely correlated 

 with light than is the real assimilation, then the indications are that light exerts its con- 

 trolling influence on assimilation under natural conditions, not by acting directly upon the 

 photosynthetic process itself, but indirectly via the diffusion stage (stomatal opening, etc.). 

 The data for deciding this fundamental question are not available in the case of maize^ 

 but the writers hope to be able to decide tins question in the case of Hdianthus for which 

 they have collected experimental data. 



