38 Ingvar Jorgensen and Walter Stiles. 
We may quote the results of atypical experiment of Puriewitsch. 
Two leaves of Acer platanoides were used. The intensity of 
radiation incident on the leaf was measured every 10 minutes 
during the experiment which lasted 6 hours, and the total energy 
calculated. 
The increase in dry weight was obtained by the half-leaf 
method. 
The following results were obtained :— 
Before Insolation. 
Area of half leaf ... ... 316-6 sq. cm. 
Dry weight of half leaf ... T2494 gm. 
Dry weight per sq. cm. ... 0-0039 gm. 
Heat of combustion of 1 gm. 
dry weight ... ... 4300-21 gm.-cal. 
Heat of combustion per sq. 
cm. ... ... ... 16-770 „ 
After Insolation. 
... 316-8 sq. cm. 
1-3952 gm. 
... 0-0044 gm. 
... 4313-46 gm.-cal. 
18-978 „ 
Increase of heat of combustion after insolation per sq. cm.— 
2-208 gm.-cal. 
Total energy incident on leaf per sq. cm. 361-03 gm.-cal. 
Quantity of radiant energy used in assimilation 0-6%. 
As we have stated before, conclusions in regard to the relation 
between the intensity of the radiant energy and that part of it used 
in assimilation, cannot be drawn until our information in regard to 
the various factors is considerably enlarged. It can, however, be 
concluded that with high light intensities only a small part of the 
incident radiant energy is utilised for assimilation. The lowest 
value, for instance, obtained by Puriewitsch was 0-6% for an average 
light intensity of 1-003 gm.-calories per minute. But it will clearly 
be seen from the table below that other factors besides light intensity 
are operating. 
Brown and Escombe’s figures exhibit similar variations in the 
percentage of sun energy utilised in assimilation, but on the whole 
they are lower. This difference is easily accounted for by the 
different method employed in measuring the assimilation. Brown 
and Escombe, as we have shown, used the intake of carbon dioxide 
in conjunction with a carbohydrate factor and an estimated value 
of the heat of combustion in order to obtain a measure of the 
energy used in assimilation. Their method is likely to yield more 
uniform though perhaps not more accurate results than the half 
leaf method employed by Puriewitsch, who did not attempt to 
correct any of the sources of error of the method, which it might 
