
1905.] On the Physiological Processes of Green Leaves. 77 
1 square centimetre of leaf-leamina per minute by c, then the energy used up 
in photosynthesis, expressed in calories per square centimetre of lamina 
per minute will be 5°O2c. 
The total internal work due to water vaporization and assimilation together, 
per square centimetre of leaf-lamina per minute, will therefore be represented 
in calories by 
592°6 Q+5:02c = W+w, 
W being the generalized expression for the work effected in water-vaporiza- 
tion, and ~ that due to the photosynthetic process. 
Now W+w, owing to the complex interactions of a variety of conditions, 
may be equal to Ra, the total solar radiation absorbed by the leaf, or it may 
be less or greater than this value. 
If Ra=W-+w it is evident that when the steady thermal condition is 
attained, the leaf will have the same temperature as its surroundings. 
If, on the other hand, Ra exceeds (W +), then the excess of solar radiant 
energy will raise the temperature of the leaf above that of its surroundings, 
and the steady thermal condition will only be attained when there is a 
sufficiently high temperature gradient for the excess of energy thus received, 
to be dissipated by re-radiation and convective cooling. The thermal static 
condition will in this case be represented by 
Ra = (W+w) +7, 
7, being a measure in calories of the sum of the losses due to radiation and 
convective cooling, whilst at the same time it is the only portion of R which 
can produce a rise of temperature in the leaf. It will be observed that whilst 
Ra and (W +w) are values obtained by direct experiment, 7 is a difference 
value. 
The actual rise-of temperature of the leaf above its surroundings can be 
determined from 7 if we know the thermal emissivity of the leaf surface, which 
is ascertainable. Ife is taken to represent this emissivity in air in calories 
per square centimetre of leaf swrface* per minute, for a difference of 1° 
between the leaf and its surroundings and, as before, 6=the temperature of 
the surroundings, and @” that of the leaf, then, when the thermal static state 
is attained, the temperature difference 06n—0=7'/2e. 
We have still to consider the case when Ra, the absorbed solar radiation, 
* Care must be taken to differentiate between the “area of leaf-lamina ” and the “ surface-area 
of a leaf.” In the above equations R, W, w, and 7 are in terms of the former, whilst e is in 
terms of “surface-area”’ and has, therefore, to be multiplied by two when used for determinations 
of the temperature of the lamina. 
