THE XATURE OF PHOTOSYNTHESIS 131 



however, exceedingly small. Brown and Escombe have calculated that 

 the heat of respiration of a Hcliunthus ainuius leaf is 0.000582 calory per 

 square centimeter of leaf lamina per minute when the leaf is respiring 

 0.7 cc. CO. per sq. decimeter per hour. Brown and Wilson ^^° have de- 

 termined the thermal emissivity of such a leaf per square centimeter for 

 still air and for a temperature excess in the leaf of 1" C. as being 0.015 

 callory per sq. cm. per minute. This amount must be doubled for the 

 two sides of the leaf so that the rate of cooling becomes 0.030 calory 



0.000582 

 per sq. cm. per minute for 1" excess. Thus, - =0.019 will 



represent the maximal excess temperature which the leaf will attain above 

 its surroundings under still-air conditions when there is no transpiration, 

 and respiration remains constant at 0.7 cc. CO2 per sq. decimeter per 

 hour. 



Such conditions are, of course, purely theoretical. Any rise of tem- 

 perature within the leaf will increase the partial pressure of the water- 

 vapor in the intercellular spaces of the leaf : this will diffuse from the 

 leaf into the surrounding atmosphere and even this small theoretical ex- 

 cess temperature of 0.019"" will not be reached. Moreover, in experi- 

 mental work we can rarely deal with still-air conditions nor with fully 

 saturated atmosphere. Also, the loss of heat due to transpiration is nor- 

 mallv of a much greater magnitude than the energy liberated in respiration. 



If the slight exothermic disturbance due to respiration is neglected, 

 Brown and Escombe consider that: "the amount of water, Q, lost by 

 unit-area of leaf surface in unit-time is a measure of the energy flowing 

 into the leaf from its surroundings, and if we know the tem^^erature 

 difference between the leaf and its surroundings, i.e. the temperature 

 gradient 6 — ©n we can determine the rate of interchange of energy be- 

 tween the leaf and its surroundings in absolute units for a temperature 

 difference of P C, that is to say, the coefficient of thermal emissivity." 

 This method has been applied to leaves in still and moving air by Brown 

 and Wilson. 



Brown and Escombe regard that the following data are required to 

 determine the thermal relations of a leaf to its surroundings when it is 

 exposed to direct solar radiation : 



1. The total amount of radiant energy incident on the leaf per unit 



time and area. 



2. The amount of this energy absorbed by the leaf. 



3. A measure of the internal work due to (a) evaporation of water, 

 (b) photosynthesis. 



4. The influence of air currents on the thermal emissivity of the leaf. 



The first. 1. can be determined by direct measurement of the intensity 

 of solar radiation ; 2 is the absorption coefficient of the leaf. A measure 



""Brown and Wilson, Proc. Roy. Soc, B 76, 122 (1905). 



