1905.| On the Physiological Processes of Green Leaves. 93 
Table V.—Coefficient of Absorption of the Radiant Energy of Sunlight for 
Leaves of Polygonum Sachalinense taken from the Plant in Serial Order. 

Coefficient of 
absorption. 
Heat i (youngest) ..........06.060 0°687 
Erase. 5A ed oepiginn sok ves 0681 
Pe a. does vatinc te eb basa 0-696 
Sa Caner, A ne nen 0°702 
(c) The Selective Absorption of Radiant Energy by the Leaf. 
In considering the general thermostatic and thermodynamic problems 
arising out of the reception and utilization of radiant energy by the leaf, we 
have so far only been concerned with the gross amount of energy received, 
irrespective of its particular grade. As long as we confine ourselves to the 
consideration of single leaves the principles which have hitherto guided us are 
sufficient to determine the thermal relations of a leaf to its environment 
without reference to the particular quality of the rays transmitted or absorbed 
by the leaf-lamina. On the other hand, any attempt to determine the thermal 
and other effects induced by sunlight which has already been filtered through 
one or more leaves must necessarily take into account the property of 
selective absorption possessed by the lamina, a property which is mainly due to 
the colouring matter of the leaf. 
If the leaf-lamina absorbed equal proportions of the various undulations 
incident upon it in the form of solar radiation, that is to say, if it possessed 
no power of selective absorption, we should expect the transmitted portion of 
the radiation to diminish in geometric proportion as the number of similar 
leaves was increased in arithmetic proportion. Thus, taking the incident 
radiation as unity, and assuming that we have a leaf with a coefficient of 
absorption of 0°687, and therefore a coefficient of transmission of 0°313, if 
selective absorption does not come into play we ought to obtain a transmission 
through two superimposed and similar leaves of 0°313? = 0:098, with three such 
leaves of 0°313? = 0:030, and with 7 leaves a transmission of 0°313”. 
In a green leaf, however, we have to deal with a screen which 1s eminently 
selective in its absorbing power, a fact which is clearly brought out by the 
results given in the following table, which show the increased transmission by 
a second and third leaf of the heat rays which have passed through the first. 
The experiments were made with the Callendar’s radiometer by the successive 
superposition of the leaves. 
