224 CHAP. XXV. PHOTOSYNTHETIC STORAGE OF SOLAR ENERGY 



the heat of combustion of the product which are not fully 

 justified. They calculated the dry weight produced by 

 multiplying the weight of carbon dioxide absorbed by the 

 carbohydrate-factor 0-64. This factor is, however, not a 

 definite constant, but varies from o- 60 to 0-74 in different 

 plants. The carbohydrate-factor would depend on the 

 proportions of different carbohydrates formed, which will 

 affect the calculation, the heats of combustion of starch 

 and glucose being 41,000 and 37,600 gm. cal. respectively. 

 Brown and Escombe adopted the lower value in their 

 calculation of the energy stored in photosynthesis. 



Turning next to the exact determination of the energy 

 absorbed for the internal work of assimilation, the difficulties 

 encountered are even more serious. For a large proportion 

 of the incident energy is lost in various ways ; (a) in trans- 

 mission through and (b) reflection from the surface of the 

 leaf ; (c) by re-radiation of heat from the leaf ; (d) by 

 convection-currents in the surrounding air ; and finally 

 (e) a portion of the energy absorbed is utilised for the main- 

 tenance of transpiration by the leaf. Of the total energy 

 of the incident radiation only a small proportion is used in 

 assimilation. 



The various factors in the problem may be indicated as 

 follows : 



I = The incident energy of light. 



E A = Energy absorbed in photosynthesis. 



T = Radiation transmitted through the leaf. 



r = Radiation reflected from the surface of the leaf. 



e = Loss by emission, which includes loss by re-radiation 



and convection by air-currents. 

 t r = Loss of energy in transpiration. 



E A , or energy absorbed in photosynthesis, is then 



= 1 — T— r— e — t r . 



Brown and Escombe measured the energy of the incident 

 radiation, I, and that of the radiation transmitted, T, through 



