
1905.| On the Physiological Processes of Green Leaves. 73 
remains in abeyance; and this temperature-excess will increase pari passu 
with any increase in the respiratory process. Since, however, any rise of 
temperature in the leaf, no matter how small, will increase the partial pressure 
of the water-vapour in the interspaces of the leaf, a diffusion potential will 
be produced, and water-vapour will flow from the leaf into the surrounding 
air, hence even this small theoretical maximum will never be reached. 
The effect of transpiration ou the leaf temperature can be best studied in a 
general way by supposing the leaf to be under the same conditions as above, 
but surrounded by air which is not fully saturated with aqueous vapour for 
the temperature 0; and here again, for the purpose of argument, we will 
assume that at the commencement there is no difference of temperature 
between the leaf and its suroundings. 
These conditions are manifestly unstable as a consequence of the excess of 
the partial pressure of the saturated air of the leaf-interspaces over the 
partial pressure of the water-vapour in the unsaturated air of the enclosure. 
Owing to the “ diffusion-potential” thus set up water-vapour will diffuse 
through the stomata if these are in any degree open, and the temperature of 
the leaf will fall. This will continue until the temperature-gradient between 
the leaf and its surroundings is steep enough to allow energy to flow into the 
leaf from without at a rate equal to that of the energy expended in the work 
of vaporization. A thermal static state will then be re-established which will 
remain constant as long as the conditions remain unaltered, the leaf assuming 
a temperature 6@,, which will be J/ess than @, the temperature of the 
surroundings. 
Neglecting for the moment the very slight disturbance due to the 
exothermic respiratory process, as soon as the above thermal equilibrium 
has been attained the amount of water, Q, lost by unit-area of the leaf surface 
im unit-time, is a measure of the energy flowing into the leaf from its surround- 
ings, and if we know the temperature difference between the leaf and its 
surroundings, 2.¢., the temperature gradient 0—9, we can determine the rate 
of interchange of energy between the leaf and its surroundings in absolute 
units for a temperature difference of 1° C., that is to say, the coefficient of 
thermal emissivity. 
This method of determining the emissivity to which reference has already 
been made, is elaborated in an accompanying paper.* When once the 
constant of emissivity has been determined the value of the temperature 
difference 6—@, is calculable, providing we know Q, the amount of water 
transpired by the leaf for unit-area and unit-time. 
* erat and Wilson, infra, p. 122. 
