140 



Carbon Assimilation. 



Table XLl. 



The Absorption of Radiant Energy by the Leaf in the Presence 



and in the Absence of Carbon Dioxide (Puriewitsch). 



*The values marked with an asterisk were obtained by means of a Rubens 

 thermopile, the remainder by means of the Bolometer. 



Brown and Escombe suppose that the total radiant energy 

 falling on the leaf is used in the following ways ; 



(1) in assimilation, 



(2) in transpiration, 



(3) by transmission through the leaf, 



(4) by thermal emission (if the leaf temperature is higher than 

 that of its surroundings, as it usually is, this is positive, but if lower 

 the thermal emission is negative, that is, the leaf gains energy from 

 its surroundings). 



We have indicated earlier in this chapter that Brown and 

 Escombe estimated the assimilation by calculating the increase in 

 dry weight from the intake of carbon dioxide and the assumed heat 

 of combustion of the products. Thus, in this first determination, 

 two assumptions are made, the accuracy of which is not 

 confirmed by measurement. One of these is the assumption that 

 1 gram of carbon dioxide absorbed is equivalent to 0'64 gram of dry 

 matter ; the other that the heat of combustion of the products is 

 3-76 X 10' gram-calories. 



The transpiration was determined by weight, and the energy 

 used in transpiration calculated from the heat of vaporisation of 

 water at the particular temperature. 



The energy transmitted was calculated from the coefficient of 

 absorption of the leaf, which was found in the following manner. 

 A day of bright sunshine was selected and the intensity of radiation 



