Carbon Assimilation. 
29 
the leaf calculated. These differences are summarised in the 
following table. 
Table XXXVIII. 
Difference in Dry Weight per Unit Area of Opposite Sides of 
Leaves due to Differences in Symmetry [Brown and Escombe). 
Species. Diffen 
Catalpa bignonoides ... 
11 11 • • • • • • • • • 
11 11 • • • • • • 
11 11 • • • • • • • • • 
11 11 • • • • • * • • • 
Catalpa purpurea 
Catalpa Bungei 
11 11 ••• ••• * • • 
Tropoeolum majus 
Polygonum Weyrichii 
Similar differences were found by Thoday for some other 
species and he concludes with Brown and Escombe that this source 
of error is inherent in the method. He points out that the error 
arising from this cause may be reduced by using parts of leaves 
free from big veins instead of whole half leaves. Thus with 
Paulownia imperialis the average percentage difference of four 
pairs of measurements was 1-4% when the veins were avoided and 
the average percentage difference of two pairs of measurements was 
5-95% when the veins were included. 
3. The third source of error suggested by Brown and Escombe 
is that due to change in area of the leaf during insolation. These 
investigators measured the area of leaves of Catalpa bignonioides 
before and after insolation and found resulting alterations in area 
from an increase of 0T4% to a decrease of 3T2%. According to 
Thoday, leaves of Helianthns annuus often diminish in area by more 
than 5 % between early morning and midday if the meteorological 
conditions are such as to favour rapid transpiration of water. 
From such data Brown and Escombe calculate the order of 
magnitude of the error likely to arise in Sachs’ half leaf method. 
It would thus be quite probable for the error in determination of 
the dry weight of a half leaf to equal 2%. In such a case Brown 
and Morris show that with a leaf having a dry weight of 0-5 gm. 
per sq. decimetre asssimilating 0-002 gm. carbohydrate per sq. 
nee in Dry Weight, 
Pfcr Cent. 
3- 9 
4- 3 
2-3 
5- 7 
0-7 
2-3 
1- 3 
2 - 2 
0-3 
M 
