322 PHOTOSYNTHESIS 



W + w, the sum of the internal work of the leaf, by 0.1762 — 0.1260 = 

 0.0502, which represents the value of r in calories in the equation 



"This value for r of 0.0502 calory per square centimeter per minute 



represents the only part of the solar radiation ivhich can have had any 



heating effect on the leaf. From the value of r we can determine the 



mean temperature-difference between the exposed leaf and its surroundings 



r 

 0n — 0, if we know the thermal emissivity of the leaf, for ©n — = — . 



"The thermal emissivity of a leaf of this nature, for 'still air' condi- 

 tions, is approximately 0.015 calorie per square centimeter of leaf surface 

 per minute for a temperature excess of 1°, and the emissivity increases 

 by 0.00017 calorie per square centimeter per minute for an increased air 

 speed of 1 meter per minute. Hence, since the average velocity of the 

 wind in this case was 25.7 kilometers per hour, or 428 meters per minute, 

 the corrected 'emissivity' becomes 0.0150 + 0.00017 X 428 = 0.0577 cal- 

 orie per square centimeter of surface per minute for a temperature excess 

 of 1°. Hence the temperature excess of the leaf-lamina above its surround- 

 ings will be 



^ ^ r 0.0502 ^ o , ^ ^ 



®»-e = 2; = aTT5-4 = °-^^^- 



"Since the average temperature of the air during the experiment was 

 16.°9 C, that of the leaf was about 17.°3 C. 



"We are now in a position to state with a fair approach to accuracy the 

 manner in which the leaf has disposed of the energy incident upon it, 

 and to obtain some idea of the 'economic coefficient' of the leaf under 

 these average conditions. If we denote R, the total energy incident on unit- 

 area of the leaf in unit time, by 100. then the disposal of this radiation 

 will be accounted for in the following manner : 



(w) Energy used in photosynthesis 0.66 



(W) " " " transpiration 48.39 



(W -t- w) Total energy expended in internal work 49.05 



R — Ra. Solar radiant energy transmitted by leaf 31.40 



(r) Energy lost by thermal transmission 19.55 



100.00" 



From the results of Brown and Escombe it is apparent that a very 

 considerable portion of the energy absorbed by the leaf is used in trans- 

 piration. That the rate of transpiration varies greatly with changes in 

 the external conditions is, of course, a familiar fact. The changes 

 brought about by variations of the transpiration rate in the distribution 

 of the radiant energy absorbed by the leaf and probably also in the photo- 

 synthetic rate are too complicated to predict. 



