110 LIGHT, VEGETATION AND CHLOROPHYLL 



energy expended in their formation. But it is necessary also 

 to define the manner of calculating the energy expended. 

 If by that is understood the total energy of the radiation 

 which strikes a plant, the efficiency in good conditions of 

 cultivation is less than 1 per cent. 



It is better to consider only the radiation which is really 

 usable, i.e., that which is neither reflected nor transmitted 

 but is absorbed by the plant, and then, of the radiation 

 absorbed, to reckon only that which is absorbed by chloro- 

 phyll, and not by the parts devoid of chlorophyll or by the 

 carotenoids. The efficiency calculated in this way is the real 



efl^iciency. 



An attempt has been made to determine the maximum 

 real efficiency by providing conditions such that no factor 

 other than light intervenes to slow down photosynthesis. 

 The experiments are made in illumination which is low enough 

 to ensure that the factor which limits photosynthesis is the 

 quantity of light available. Thus this quantity of light is used 

 to the best advantage and the real efficiency must be 

 maximum. 



The measurements made by Wurmser and Warburg (1923) 

 on chlorella with light of diff'erent wave-lengths in the visible 

 region gave the following results showing that the real 

 efficiency may exceed 50 per cent. According to Warburg, 

 this energy efficiency is: 



In the red (A =6,600 A) 59% 



„ „ yellow (A=5,780 A) 53-5% 



, green (A=5,460 A) 44% 



blue (A =4,360 A) 34% 





These results have been criticized, but before discussing 

 them and comparing them with others, let us examine their 

 meaning. 



For the yellow, for example, the energy efficiency is 

 53-5 per cent. This means that for the initiation of the 

 reaction of a gram-molecule of CO 2 and the Hberation of a 

 gram-molecule of oxygen, with the formation of a molecule of 



