Carbon Assimilation. 
27 
in his paper “ Ein Beitrag zur Kenntniss der Ernahrungsthatigkeit 
der Blatter ” published in 1884. Sachs’ method is well-known. 
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 takes 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 weight 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 already referred to. 
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. 
Experiment. 
Increase in Dry 
Wt. per sq. deci¬ 
metre per hour 
mg. (observed). 
CO, absorbed per 
sq. decimetre per 
hour, ccs. 
Carbohydrate formed 
per sq. dec. per 
hour, mg. (calculated). 
1 
9-83 
1-41 
1-76 
2 
7-14 
1-43 
1-79 
3 
2-60 
2-35 
2-94 
4 
7-22 
2-33 
2-92 
Mean 
6-69 
2-35 
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— 
