76 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
Then Re will represent the radiant energy absorbed per square centimetre of 
leaf-lamina per minute. 
It is worth while at this point to see how the temperature of the leaf would 
be affected on the assumption that none of this absorbed energy were dissi- 
pated. If we denote the mass of a square centimetre of the leaf-lamina by m, 
and its specific heat by s, the rise of poised ete of the leaf-lamina per 
minute will be represented by Ra/ms. 
Taking kh, the solar radiation, at the comparatively low value of 0°8 calorie 
per square centimetre per minute, a, “the coefficient of absorption” of the 
leaf as 0°78, m the weight of a square centimetre of leaf as 0:020 gramme, 
and s its specific heat as 0°879; then the rise of the temperature of the leaf 
under the conditions postulated would be eae wae = 35°4 C. per minute, 
a result which would be speedily fatal to the leaf if there were no means 
of dissipating the absorbed radiation. 
The dissipation of energy necessary to keep within safe limits the tempera- 
ture of a leaf exposed even to moderate solar radiation is provided for by the 
internal work performed, and by the loss of heat due to radiation and air- 
convection. The internal work attended with any sensible absorption of 
energy 1s (1) vaporization of water, and (2) photosynthesis of carbohydrates 
from atmospheric carbon dioxide. 
We have a measure of (1) in the water lost by transpiration, and denoting 
this loss per square centimetre of leaf-lamina per minute by Q, the heat 
dissipated by vaporization for the same units of leaf-area and time will be 
992°6 Q calories; 592°6 being the latent heat of vaporization of 1 gramme of 
water In water-gramme-units.* 
The energy used up in photosynthesis can be determined from the mass or 
volume of carbon dioxide absorbed by the leaf under the conditions of the 
experiment. 
The heat of combustion, and therefore with a reversed sign, the heat of 
formation of the carbohydrate synthesized, must closely approximate 3760 
calories per gramme, hence it can readily be shown that the assimilation of 
1 cubic centimetre of carbon dioxide, measured at normal temperature and 
pressure, corresponds to 0001336 gramme of a hexose, and to the absorption 
of 0001336 x 3760=5:02 water-gramme-units of energy. If, therefore, we 
denote the volume in cubic centimetres of carbon dioxide assimilated by 
* This is the latent heat of vaporization of water at 20°C. The value varies slightly with the 
temperature, according to the formula 606°5—0°695 #¢°, being the temperature in degrees 
Centigrade. 
