THE ENERGY RELATIONS IN PHOTOSYNTHESIS 319 



, 0.8X0.78 --„, 

 in temperature of the leaf expressed by ^ ^ ^-g — J^- 4 per minute. 



Such an increase in temperature of a leaf on illumination by direct sun- 

 light would soon be fatal. A means of dissipating the absorbed energy 

 is offered by: 1. the vaporization of water and 2. photosynthesis, both 

 endothermic processes. Under circumstances the dissipation of the ab- 

 sorbed energy by vaporization of water and by photosynthesis is less than 

 the amount of solar energy absorbed. As a consequence the temperature 

 of the leaf will rise above that of its surroundings and a steady thermal 

 state will be attained only when, with a sufficiently high temperature gradi- 

 ent, the excess of energy received by the leaf is dissipated by re-radia- 

 tion and convective cooling. There is also the other possible condition, 

 that through photosynthesis and vaporization of water the leaf is cooled 

 more rapidly than energy is supplied by the incident radiation. Under 

 these circumstances the temperature of the leaf will fall below that of 

 its surroundings. In order to make determinations of the energy re- 

 lations of the leaf during photosynthesis it therefore becomes essential 

 to have some knowledge of the interchange of heat between the leaf and 

 its surroundings, that is. what Brown and Escombe termed the thermal 

 emissivity of the leaf surface. 



The relation of thermal emission to the other modes of energy change 

 in the photosynthetically active leaf was deduced by Brown and Escombe 

 about as follows : 

 R = Incident solar radiation in calories per square centimeter per minute. 



a = Coefficient of absorption of the leaf. 



= Heat loss per square centimeter of leaf lamina per minute. 

 7 = Photosynthesis, i.e., cc. of carbon dioxide absorbed by 1 square 



centimeter of leaf lamina per minute. 

 \V = Generalized expression of work effective in vaporization of water. 

 zu = That due to the photosynthetic process. 



r= Loss or gain of heat by the leaf through radiation, convective cool- 

 ing or absorption of heat from the air surrounding the leaf. 



It is important to note that R, W, zv and r are calculated in terms of 

 the "area of leaf lamina," while the thermal emissivity is calculated in 

 terms of the "surface area of the leaf." 



The means of obtaining the value of the total radiant energy falling 

 on the leaf and the amount absorbed by the leaf have already been men- 

 tioned. A measure of the heat dissipated by the vaporization of water 

 was obtained bv a determination of the water lost through transpiration 

 per unit area and time. The latent heat of vaporization of 1 gram of 

 water at 20° was taken as 592.6 calories, so the heat dissipated by vaporiza- 

 tion of water i>er unit area and time will be 592.6 Q. 



The energy used in photosynthesis was determined from the volume 

 of carbon dioxide absorbed in unit time and area on the assumption that 

 a hexose is the product formed and that the heat of combustion, with a 



