
1905.| On the Physiological Processes of Green Leaves, 91 
and finally a second observation of the unobstructed radiation. The effect 
produced by the leaf was referred to the mean of the first and third obser- 
vations with full radiation, and the result was expressed in the form of a 
coefficient of absorption (a) for the leaf-iamina, the full radiation falling on the 
leaf being taken as unity. 
The only sensible error to which this method is hable is that due to 
reflection of radiation from the leaf-surface, an error which would tend to 
unduly increase the estimated coefficient of absorption. With the perpen- 
dicular incidence of sunlight such as was generally employed in these experi- 
ments the amount of reflection must, however, have been very small. 
The estimation of the coefficient of absorption cannot be perceptibly 
influenced by re-radiation from the leaf to the platinum-spirals, since, in the 
first place, during the very short time the experiment lasts, the rise of 
temperature of the lamina will be very small, especially as transpiration is 
going on; and, secondly, the glass cover of the radiometer is very opaque to 
the obscure radiation which the leaf would emit. 
With the same leaf and at different times of the day the method gives 
very concordant results, as will be seen from the following obtained with 
a leaf of Helianthus annuus. In these experiments the actual values ot 
the unobstructed solar radiation (R) varied from 0:591 to 0°636 calorie 
per square centimetre per minute. 
Coefficient of absorption 
for sunlight. 
Heolvanthus ammniiis (1), .0..0..0+e002-s 0°687 
, Sa) ero 0-689 
a EN (2) 0 ee aoe eee 0-684 
Although the coefficient of absorption is fairly constant for mature and 
healthy leaves of the same species of plant it shows considerable variation in 
the leaves of different species, as will be seen from the following Table III, 
in which are given the mean values for a number of different plants. The 
values of the coefficients of transmission (1.e., 1—a) ‘are also given in this 
instance. 
A series of observations was now undertaken in order determine how far 
individual leaves of the same plant differ in their coefficient of absorption for 
solar energy, and in order to bring out any possible correlation which may 
exist between this property and the age of the leaf, the experiments were 
carried out on leaves taken from the plant in the serial order of thevr 
development. 
