LIGHT AND VEGETATION 57 



used? It is to these questions that we shall give an approximate 

 answer. 



Taking as a basis the estimates made some years ago by 

 Brown and Escombe, who appear to have studied the question 

 fairly thoroughly, we may assume that in the conditions of 

 illumination generally prevailing in agriculture the 70 per 

 cent of Ught absorbed is distributed in the following way: 



Energy used for photosynthesis .... 1 per cent 



Energy used for evaporation . . . . 49 „ „ 



Energy transformed into heat and re-emitted by 



radiation 20 , 



>> 



Photosynthesis transforms the hght into chemical energy, 

 which is stored in the chemical products made by the plant 

 with water and carbon dioxide as the principal raw materials. 

 These products — wood, cellulose, starch, sugars, fats, etc. — 

 are capable of restoring later the energy that they contain. 

 This happens during their combustion, i.e., their destruction 

 and oxidation, which re-creates the water and carbon dioxide 

 from which they were originally formed. They constitute, 

 therefore, a store of solar energy. 



In particular experimental conditions it is possible, as we 

 shall see in the chapter on photosynthesis, to transform a 

 much greater proportion of the Ught into chemical energy. 

 In agriculture, it is this hundredth part of the luminous 

 energy which constitutes the value of the crop. If we succeed 

 in doubling the proportion of hght used by photosynthesis, 

 we shall double the profit. 



We can verify the amount of tliis proportion by com- 

 paring the weight of dry matter of the crops from one acre 

 with the total solar energy received by the same area during 

 the period of their growth. Let us assume that during the 

 summer the average power supplied by the sun is 1,200 kW. 

 per acre; 1 per cent, i.e., 12 kW., would be used for photo- 

 synthesis. This power corresponds ^to the synthesis of 

 0-8 grammes of carbohydrates per second; for a period of 

 six months at eight hours a day, totalUng five miUion seconds, 



