214 Quantitative Analysis of Plant Growth 



In calculating these correlation coefficients we have omitted the first 

 phase where the young leaves are most probably assimilating at a much 

 lower rate than the normal leaves, and also the phase marked by the 

 high peak at the end of the life-cycle. In calculating r L to r Lg the 

 underlying assumption is that light is not limiting until it is lower 

 than the lowest value before deductions are made. For example, in the 

 case of r L it is assumed that light is not limiting the Unit Leaf Rate 

 until the intensity is less than «3 sunlight and that at lower values the 

 Unit Leaf Rate is proportional to the light intensity. It will be seen that 

 the best correlation is obtained when we make the assumption that light 

 up to one-fifth full sunlight is limiting. 



III. Correlation of Real Assimilation with 

 Environmental Factors 1 . 



We have investigated the question as to how real assimilation per 

 unit leaf-area is correlated with environmental factors. Unit Leaf Rate 

 is the net result of gain due to real assimilation per unit leaf-area plus 

 salt- uptake less loss due to the respiration of the whole plant per unit 

 leaf-area. As the ratio of ash to total dry-weight is of the order of -06 

 and undergoes no marked change from week to week (9, li, 17), the Unit 

 Leaf Rate plus the respiration of the whole plant per unit leaf-area is a 

 fair comparative measure of the real assimilation. 



In order to arrive at values for the respiration of maize we determined 

 the respiration of plants about nine weeks old at 2-8°, 10° and 25° C. 

 respectively. The results are given in Table VII. 



Table VII. — Respiration of maize. 



Mgs. of CO.. per gm. 

 Temperature dry-weight per hour. 



2-8° C. 0-311 



10 0-557 



25 1-818 



From these results we have calculated the loss in dry-weight per 

 unit leaf- area per week on the basis of the average dry- weight and the 

 average leaf-area, and have thus been able to make an estimate of the 

 real assimilation (Tables VIII and IX) 2 . In making this estimate of loss 



1 Gregory (8) states a few relations between the average rate of assimilation of cucumber 

 plants and the average intensity of radiation, but the figures he gives are too few to es- 

 tablish any general result. 



2 The respiration is calculated for the average mean temperature for the week. Twenty- 

 four hours per day have been allowed. 



