Carbon A ssimilatioil. 
Actual determinations of the heat of combustion of the 
material produced.in assimilation have been made by Krasheninikov 
(1901) and by Puriewitsch (1914). They measured the increase in 
dry weight per unit area per hour by Sachs’ dry weight method 
and also the increase in the heat of combustion per unit area per 
hour. The increase in heat of combustion per unit increase in dry 
weight gives the heat of combustion per gram of the products of 
assimilation. Krasheninikov obtained an average value for this of 
4-4 X 10 3 gram-calories. From Puriewitsch’s data we have calculated 
the values for different species set out in the following table. It 
will be observed that the values agree well with the number 
obtained by Krasheninikov, but not with the value assumed by 
Brown and Escombe (3'76 X 10 3 ). 
Table XL. 
Heats of Combustion of the Products of Assimilation. 
Species. 
Increase in 
Dry Wt. per 
sq. metre 
per hr., grm. 
Increase in Heat 
of Combustion 
per sq. cm. 
per hr. 
Heat of 
Combustion of 
Product of Ass. 
in gm.-cal. 
per gram. 
Acer platinoides ... 
1-2 
0-526 
4-4 x10 3 
Polygonum sacchalinense 
2-7 
1-41 
5-2 x10 3 
J • 5 > 
2-0 
0-903 
4-5 xlO 3 
D. The Quantitative Measurement of the Radiant Energy 
Incident on the Leaf and the Utilisation 
of this Energy. 
A full discussion of the methods used and principles involved 
in the measurement of radiant energy would be out of place here. 
The instruments generally employed are of four kinds, the 
thermopile, the bolometer, the radiometer and radiomicrometer. 
For a description of these instruments the reader is referred to 
physical text books, and for a more complete discussion on the 
relative merits of the various methods, to Kayser’s Spectroscopie > 
Baly’s Spectroscopy, and Coblentz’s “ Instruments and Methods 
used in Radiometry” (1908). Generally speaking, the radiant 
energy is absorbed and transformed to heat in the measuring 
instrument, and thus a measure of the total energy obtained, but 
if by a suitable method a spectrum of the source of light is produced, 
the same method can be used for measuring the distribution of 
energy in the different parts of the spectrum. 
