THE ENERGY RELATIONS IN PHOTOSYNTHESIS 321 



ably higher. Table 35 taken from the pubHcation of Brown and Wilson 

 illustrates the influence of air currents. 



TABLE 35 



Thermal Emissivity of Leaves of Liriodcndron tidipifera in Moving Air. 



(Brown and Wilson.) 



Thermal Emissivity 

 in Calories per Minute 

 Rate of Air Current, per sq. cm. for 1° C. 



Meters per Minute Temperature Excess 



Still air 0.0119 



36.2 0.0173 



71.4 0.0238 



108.0 0.0304 



139.0 0.0361 



In order to illustrate the manner in which Brown and Escombe cal- 

 culated their results, their own description of an experiment is here given : 



"In (1) the leaf was enclosed in a glazed case through which ordi- 

 nary air was passed, and the amount of carbon dioxide assimilated was 

 estimated from its determination in the entering and emergent air, the 

 results being corrected to the partial pressure of the carbon dioxide in the 

 outer atmosphere, which in this instance corresponded to 2.71 parts per 

 10,000 of air. 



"Leaf (2) for determining the rate of transpiration was freely ex- 

 posed to the air, which had an average temperature of 16. °9 C, and an 

 average velocity of 25.7 kilometers per hour. The partial pressure of 

 the water vapor in the air was 9.21 mm. of mercury as determined from 

 the average readings of the wet and dry bulb thermometers. The area 

 of each leaf was, of course, accurately determined. 



"The results were as follows : 



(R) Average total solar radiation falling on the insolated 



leaves per square centimeter per minute 0.2596 calories 



(a) Coefficient of absorption of the leaves 0.686 



The actual amount of solar energy intercepted was therefore Ra = 

 0.2596 X 0.686 = 0.1762 calory per square centimeter per minute. 



"The amount of carbon dioxide assimilated, reduced to free air con- 

 ditions, was 2.134 cc. per square decimeter per hour or 0.000355 cc. X 

 5.02 = 0.0017 calory per square centimeter per minute (w). 



"The water transpired from the leaf under free air conditions amounted 

 to 1.259 grams per square decimeter per hour, or 0.000209 gram per square 

 centimeter per minute. The amount of internal work of vaporization, W, 

 is therefore 0.000209 X 592.6 = 0.1243 calory per square centimeter per 

 minute. 



"The total amount of internal work W + w performed by the leaf 

 is therefore equivalent to 0.1243 + 0.0017 = 0.1260 calory per square 

 centimeter of leaf lamina per minute. 



"In this instance Ra, the solar radiation absorbed by the leaf, exceeds 



