212 Quantitative Analysis of Plant Growth 



(3) Correlation of Unit Leaf Rate of plants harvested "nach AuswahV 

 with environmental factors. 



The most complete record of the environmental factors presented 

 by Kreusler for one year is that for 1877, the year in which the selected 

 mean plants were harvested. For this year Kreusler not only gives the 

 usual records, temperature, rainfall, sunshine, and total light, but in 

 a separate paper (12) he gives a very detailed record of the variations in 

 light intensity for each day. The duration of light intensity of the 

 following fractions of full sunlight, -05, -1, -2, -3 ... 1, is recorded, the 

 duration for each intensity being measured to the nearest five minutes. 

 From these figures we have been able to determine the duration in 

 minutes of all the light above any given intensity for individual days, 

 and hence for individual weeks at the end of which each harvest was 

 taken. These values are presented in Table VI. Kreusler himself deter- 

 mined the "total light-effect" ("Gesammtlichteffekt") by multiplying 

 each light intensity by its duration and summing the results as "Beson- 

 nungsstunden." If one attempts to correlate the "total light-effect" with 

 dry-weight increase per unit leaf-area, there is the underlying assump- 

 tion that this increase is proportional at any moment to the light 

 intensity, that is, is limited by the light intensity throughout. Our 

 knowledge of assimilation would lead us to believe that light limits the 

 rate of assimilation in nature by direct action on the photosynthetic 



Table VI. — Weekly records of light intensity for the year 1877. 



Duration Duration Duration Duration Duration Duration 



in mins. in inins. in mins. in mins. in mins. in mins. 



of light of light of light of light of light of light 



1/20 of 1/10 of 2/10 of 3/10 of 4/10 of 5/10 of 



sunlight gunlight sunlight sunlight sunlight sunlight 



* For example, in the case of light of 3/10 full sunlight, 2/3 duration of light of 2/10, 1/3 duration 

 of light of 1/10, and 1/6 duration of light of 1/20 sunlight are added. 



