78 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
is Jess than W +, representing the sum of the internal work of the leaf. The 
equation for static thermal equilibrium then becomes 
Ra+r = W-+w. 
The excess amount of energy requisite for producing the internal work under 
these conditions must be drawn from the surroundings of the leaf, that is to 
say, the temperature of the leaf must fall below that of its surroundings when 
thermal equilibrium is reached. The thermometric difference 0—@, will 
again be expressed by 7/2e, as in the preceding case. 
The influence of the heat evolved by the respiratory process has still to be 
considered. The true measure of the photosynthetic work effected by suitable 
radiation is, strictly speaking, not given by the amount of atmospheric carbon 
dioxide absorbed by the leaf, but by this amount plus the carbon dioxide which 
would have been evolved by respiration if photosynthesis were in abeyance. 
But this correction, although one to be taken into account under certain 
circumstances, does not affect the above thermal equations, since the heat of 
respiration is opposite in sign to the heat of reformation of the carbohydrate, 
and these values, representing a concurrent gain and loss of energy by the 
leaf, must exactly balance each other if the two carbohydrates are identical ; 
and if they are not identical the difference will be so extremely small that it 
may be safely neglected. 
In discussing the thermal relations of a leaf to its surroundings we have so 
far, for the sake of simplicity, imagined an ideal set of conditions under which 
all the determining factors, both internal and external, remain constant for a 
sufficient time to allow of the attainment of static thermal equilibrium. In 
practice this ideal condition is never attainable. In the first place the 
incidence of solar radiation, even under the most fair-weather conditions, is 
subject to rapid oscillations of considerable magnitude, as can readily be seen 
by watching the constant movement of the pen of the self-recording instrument 
or by reference to the final “graph” which is integrated by the attached 
planimeter. 
Every variation of this kind alters the value of Ra, the actual amount of 
energy absorbed by the leaf. Even if these changes are unaccompanied by 
any alteration in the degree of opening of the stomata, and the external air 
conditions remain constant, the amount of water vaporization must still be 
indirectly influenced, since the value of 7+ which represents the difference 
between the amount of the utilizable incident energy and the internal work 
performed, must also vary, and as this determines the temperature of the leaf, 
there will be corresponding changes in the “ diffusion-potential ” between the 
inner saturated air of the leaf and the surrounding atmosphere. 
