LIGHT AND VEGETATION 109 



the corresponding energy per gram-molecule is very near to 

 one half and one third of the energy of the reaction of photosyn- 

 thesis; at 4,400 A, 2 photons would be necessary, at 6,600 A, 3 

 photons, giving, in both cases, a very sUght excess of energy. 



Without stressing what is only a coincidence, and approxi- 

 mate at that, we must remember that the participation of 

 several photons is always necessary for the assimilation of a 

 molecule of COg; that is an inescapable energy requirement, 

 whatever hypothesis may be formulated on the process of 

 assimilation. Now, we shall see how difficult it is to imagine 

 any probable process by which the energy of several photons 

 can be concentrated on a single molecule of CO 2 or of HgO. 



Briefly, the general theoretical considerations that we 

 have outhned enable us to deduce that at least 4 quanta of 

 red Ught, but perhaps less for light of shorter wave-length, 

 are required to supply the energy necessary for the assimi- 

 lation of an atom of carbon. 



Each active quantum contributes its energy in a particularly 

 effective way, for the molecule which absorbs it instan- 

 taneously acquires, for itself alone, the whole of this energy, 

 which has the effect of putting it in an activated state favour- 

 able to the chemical reaction. 



Such is the true photochemical action. Before or after, 

 the molecule in action, which is probably a molecule of 

 chlorophyll, doubtless unites with other molecules, of carbon 

 dioxide for example, but these are chemical and not photo- 

 chemical phenomena. These chemical phenomena which 

 precede or follow the photochemical action are designated 

 by the general name of the "dark reaction" (because they are 

 accomplished without the direct intervention of light) or the 

 Blackman reaction. Their speed is dependent on all the factors 

 which control the speed of chemical reactions : temperature, con- 

 centration of the bodies which participate in the reaction, etc. 



Measurement of the Maximum Real Efficiency 



We may define the efficiency as the ratio between the 

 chemical energy of the products obtained and the luminous 



