LIGHT AND VEGETATION 113 



optimum efficiency give varying results, one indicating the 

 necessity of 4 quanta, the other of 20 quanta, of visible light 

 for the combination of a molecule of HgO with a molecule of 

 CO2 and the Uberation of a molecule of Og. 



To conclude this discussion of the experimental results, 

 we may say provisionally that the elementary action can be 

 accomplished, in the most favourable conditions, by means 

 of a contribution of energy corresponding to the absorption 

 by chlorophyll of 4 photons. It remains to be seen in the 

 future whether the few experiments which contradict this 

 result are confirmed or not. 



This being admitted (rather arbitrarily), the most obvious 

 and important consequence is that the photosynthetic capacity 

 may be extraordinarily good ; in fact, according to estabhshed 

 therm ochemical data, the energy absorbed by the photo- 

 synthetic reaction corresponds, for the extreme red radiations, 

 to 4 quanta of light. In the other regions of the spectrum, the 

 efficiencies are of the order of 50 per cent. 



The above is an assertion which greatly hmits the processes 

 imaginable in considering the detailed reactions of assimi- 

 lation through chlorophyll; in the course of the partial 

 reactions which result finally in a photosynthetic elementary 

 reaction, the utilization of the luminous energy absorbed 

 must be nearly perfect, without any appreciable waste. 



The Dark Reaction 



We have seen at the beginning of this chapter that fight 

 produces an activation of the molecules capable of absorbing 

 it and have explained what this means. 



After the activation, the role played by light generally 

 comes to an end; the activated molecules are in a higher 

 energetic state and the course of events no longer depends on 

 the agent by which they have been brought to that state, but 

 only on its nature and on the medium in which the molecules 

 are placed. 



The cascade of reactions which follows the absorption of 

 fight, like those which have preceded it, therefore constitutes 



H 



