28 Ingvar Jorgensen and Walter Stiles. 
1. Possible changes after assimilation in the power of retention 
of water by the colloids 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 
