14 INTRODUCTION 



radiation used effectively for photosynthesis? To take the 

 best examples, the chemical energy accumulated in the 

 products of the earth (we should say of the sun) represents 

 about 1 per cent of the solar energy which has reached the 

 cultivated area in the form of visible light, ultra-violet or 

 infra-red. This is already a strikingly good result if we compare 

 it with the results obtained at the few experimental stations 

 which have been set up to study the utiUzation of solar energy 

 and where the efficiencies scarcely attain this figure. 



Are we right, however, to think that it is theoretically 

 possible to obtain an enormous increase in vegetative growth 

 by a better utilization of hght? Some progress is certainly 

 conceivable, but a few simple considerations show that it is 

 limited. 



Chlorophyll seems to be the necessary intermediary between 

 hght and photosynthesis, but chlorophyll absorbs only the 

 visible hght, while about 50 per cent of the solar energy 

 consists of invisible infra-red radiations. The width of the 

 absorption band of chlorophyll means that it can absorb 

 only a third of the solar energy. Then again chlorophyll can 

 do nothing by itself, it plays its part only inside a living 

 vegetable organism, which, Uke all matter, absorbs, this time 

 without any benefit for photosynthesis, a part of the hght 

 which could have been absorbed by chlorophyll. Hence there 

 is not quite a third, but perhaps only a quarter, of the solar 

 energy available to chlorophyU. 



Finally, the plant, hke every living thing, must breathe, 

 which means that it bums a part of its substance by com- 

 bination with the oxygen in the air. This process, which 

 continues day and night throughout its life, consumes a part 

 of the products accumulated during the day by photo- 

 synthesis and the crop is consequently deprived of them. 



Therefore the agricultural efficiency of photosynthesis 

 could never exceed 20 per cent in the use of sunlight. We are 

 still very far short of this, since the best efficiencies obtained 

 are twenty-five times lower; there is still room for enormous 

 improvement. 



