
1905.| On the Physiological Processes of Green Leaves. 75 
was taken as the difference between the solar radiant energy falling on the 
leaf in full sunshine and the amount transmitted, and takes no account of any 
possible reflection of radiation from the surface of the leaf. With perpendicular 
incidence the reflected radiation must be very small in amount, but it is well 
to bear in mind that, strictly speaking, the value of the coefficient of absorption 
employed includes this reflected portion. Although the absorptivity varies in 
the leaves of different plants there is a remarkable constancy in the results 
obtained at different times with the leaves of plants of the same species. 
The loss of water by the leaves due to transpiration was determined by 
weighing the leaf and its attached water-tube at suitable intervals. 
In those cases where it was desired to submit two leaves to solar radiation 
of definite relative intensity, the leaf to be partially shaded was placed under 
the radial arms of a revolving sector which could be adjusted for any required 
degree of “exposure.” In this way a leaf could be submitted to a definite 
fraction of the full solar radiation falling on another leat placed alongside 
without the risk of introducing any error from selective absorption of any 
special rays, which it is difficult to avoid when ordinary screens are used. 
The area of the leaves was ascertained by the planimeter method described in 
an earlier part of the paper, and the amount of energy used up in the photo- 
synthetic process was deduced from the rate of assimilation as determined by 
the absorption of carbon dioxide from the air, a method which has been fully 
described in Part IT. 
Before discussing the results of some typical experiments it will be well to 
consider in a general way the thermal relations of a leaf to its surroundings 
when it is exposed to sunlight under free air conditions, just as we have 
considered these relations in the case of a leaf under the still air conditions of 
an enclosed space cut off from direct solar radiation. 
In the first place let us suppose a healthy green leaf, well supplied with 
water, to be exposed to sunlight under perfectly constant conditions as 
regards intensity of solar radiation, and the temperature, humidity and degree 
of movement in the surrounding air; and further that the apertures of the 
stomata undergo nc variation in dimensions. Under these ideal conditions, 
just as in the case previously considered, a state of thermal equilibrium 
will be speedily established between the leaf and its surroundings, when 
the loss and gain of energy by the leaf in a given interval of time will just 
balance. 
Let R represent the total solar radiation expressed in water-gramme-units 
(calories) falling on a square centimetre of the leaf-lamina in one minute, and 
let a represent the “absorptive coefficient” of the leaf for this radiation. 
