QuaiiUtative Estimation of Carbon Assimilation. 135 



of water by the colloiils of the cell contents when these are dried 

 at 100°C. 



2. Differences due to lack of symmetry between the two halves 

 of the leaf in regard to venation and thickness. 



3. Alterations of area of the leaf as a result of insolation. 

 Thus if a leaf suffered shrinkage during insolation so that its area 

 as measured afterwards was less than at the beginning of the 

 experiment, the dry weight of unit area would be correspondingly 

 increased. Consequently the values found for the increase in dry 

 weight of unit area would be larger than the true values as, of course, 

 the initial dry weight is measured before insolation on an unshrunken 

 half leaf. 



1. The possible error due to changes of composition during 

 insolation which might produce a different water retaining capacity 

 was investigated by Thoday (1909), who measured both the dry 

 weight and carbon content of the experimental and control half 

 leaves and so calculated both the gain in dry weight and of carbon 

 per unit area. Thoday concludes that the correspondence between 

 the increase in dry weight and the starch equivalent of the gain in 

 carbon is sufficiently close to make it clear that fixation of water 

 cannot play an appreciable part in determining the dry weight 

 increase. However, as the starch equivalent of the gain in carbon 

 found in Thoday's experiments varied from 20% less to 40% (and in 

 one extreme case 90%) more than the actual increase in dry weight 

 of the same leaf, it is not clear why Thoday should come to 

 this conclusion from his results. We hesitate therefore to accept 

 Thoday's own opinion that his results indicate that the dry weight 

 method is " not vitiated by any large indeterminable errors such as 

 would arise if varying quantities of water were retained by the 

 colloids of the leaf after drying it at 100°C." The numbers show 

 indeed that changes in composition of the leaf during assimilation 

 will not account for the whole of the discrepancy between the two 

 methods as observed by Brown and Escombe, but they tell us 

 nothing as to whether such change is negligible or not. 



2. Brown and Escombe made a number of determinations of 

 the degree of symmetry of the two halves of various leaves by 

 measuring the two halves separately with a planimeter and then 

 drying them to a constant weight. The dry weight per square 

 decimetre of the two halves was calculated and the percentage 

 difference between the dry weight per unit area of the two sides of 



