58 LIGHT, VEGETATION AND CHLOROPHYLL 



the production should therefore rise to 4 tons of carbohydrates. 

 Now, one acre of lucerne can provide a crop of about 

 4-8 tons of dry matter. We can see therefore that the preceding 

 calculation leads to a correct estimate of the proportion of 

 the energy used for photosynthesis. 



As regards the part of the incident energy used for the 

 evaporation of water by the leaf, we can make a similar 

 attempt to check it by calculating the total quantity of water 

 thus evaporated per acre and comparing it with the annual 

 rainfall. As we have indicated, about half of the power of 

 solar radiation would be used for evaporation. Now, 600 kW. 

 is equal to 140,000 calories per second and can vaporize 

 240 grammes of water per second. In five million seconds, 

 1,200 tons of water are thus evaporated. The annual rainfall 

 in France averages 29-5 inches, corresponding to 3,000 tons 

 of water per acre. In six months our crop of lucerne would 

 therefore have taken nearly half for the needs of transpiration. 

 The rest is evaporated directly from the soil or soaks away. 

 There, again, our calculation leads to a reasonable conclusion 

 which confirms the estimates on the utihzation of radiation 

 by plants. 



Returning to the efficiency of photosynthesis, even if we 

 assume, as we have done, that a hundredth part of the 

 luminous energy received by the leaf is stored in the plant in 

 the form of the chemical energy of the products of synthesis, 

 we cannot conclude that one-hundredth of the energy received 

 on a cultivated field will be effectively used. In fact, especially 

 when the plants are young, a part of the light, passing between 

 the leaves, strikes the ground directly. As the earth is more 

 often bare in winter and vegetation is produced only during 

 part of the year, the overall efficiency, that is to say, the 

 fraction of the annual total solar energy which is effectively 

 recovered in the crop, is much less than one-hundredth. 



It must also be remembered that, in order to five, the 

 plant uses part of the substances that it has created; their 

 waste products in the form of carbon dioxide are given off 

 in respiration. 



