136 



Carbon Assimilation. 



the leaf calculated. These differences are summarised in the 

 following table. 



Table XXXVIII. 



Di^erence in Dry Weight per Unit A rea of Opposite Sides of 

 Leaves due to Differences in Symmetry [Brown and Escoiube). 



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 

 Panlownia 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 hignonioides 

 before and after insolation and found resulting alterations in area 

 from an increase of 0-14% to a decrease of 3'12%. According to 

 Thoday, leaves of Helianthns animus 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. 



