IM 



Carbon Assimilation. 



The dry weight of unit area of one half of a leaf measured at the 



beginning of an experiment is compared with the dry weight per 



unit area of the other half of the leaf after its exposure to the 



required conditions. The difference of the two values is regarded 



as the weight of products which have accumulated in unit area of 



the leaf during the experiment. As Sachs found a greater increase 



in dry weight in detached leaves than in leaves still attached to the 



plant, he assumed that in the latter case translocation of the 



products away from the leaf taUes place concurrently with 



assimilation. To obtain the true value for assimilation in attached 



leaves, Sachs therefore added the loss in dry weight of leaves 



during the night to the increase in dry weigiit of the same area 



during the same time during the day. 



Brown and Escombe pointed out that Sachs obtained much 



higher values for assimilation by his half leaf method than they 



obtained by direct determination of the carbon dioxide absorbed. 



They therefore carried out a series of experiments in which the 



assimilation of the same leaves were measured by both methods. 



The following table gives the results they obtained for Catalpa 



bignonioides. The results in the last column are obtained by the 



use of the carbohydrate factor 0'64, to which reference has already 



been made. 



Table XXXVII. 



Comparison of the Values obtained for Assimilation of Leaves of 



Catalpa bignonioides by the Half-Leaf Method and by 



measuring the Intake of Carbon dioxide. 



It will be observed that the divergence between the results 

 obtained by the two methods is much larger than can be accounted 

 for by experimental error or error in the estimation of the carbo- 

 hydrate factor. Brown and Escombe attribute this divergence to 

 three sources of serious error to which the half leaf method is 

 liable. These are — 



1. Possible changes after assimilation in the power of retention 



