108 LIGHT, VEGETATION AND CHLOROPHYLL 



The liberated atomic hydrogen H acts on a molecule of 

 chlorine : 



H+CI2 — ^HCl+Cl (3) 



The atom of chlorine can repeat reaction (2) which 

 Uberates H, and so on; we could thus explain, by these 

 chain reactions, the combination of an infinite quantity of 

 gas under the impulse of a single photon. 



But each of these partial reactions is exothermic. In photo- 

 synthesis by plants, when the reactions form an endothermic 

 process, according to the principle of the conservation of 

 energy, only those quantities of matter will combine which 

 have, through the absorption of light, sufficient energy at 

 their disposal. 



Now, a gram-molecule of CO 2 and a molecule of HgO 

 require, in order to be converted into carbohydrates, an 

 energy estimated at 112,000 calories. 



If all the molecules of a gram-molecule absorbed a photon 

 of radiation of wave-length A, the energy absorbed per gram- 

 molecule would be (in calories so that it may be compared 

 with the chemical energy): 



X (A) Nh-^- (calories) 



Energy per gram-molecule 



4,000 71,200 



5,000 56,900 



6,000 47,400 



7,000 40,700 



8,000 35,600 



In red monochromatic illumination for which A= 7,500 A 

 photosynthesis is accomphshed; to obtain the 112,000 calories 

 required, the concentration of the energy of 4 photons on 

 one molecule of CO2 or of HgO is necessary; 3 would not be 

 sufficient unless the kinetic energy of thermal agitation were 

 used, as sometimes happens. At the wave-length of 4,000 A, 

 2 photons would suffice. 



It is rather curious to note that the two maxima of efficacy 

 of photosynthesis as a function of wave-length correspond 

 very closely to the radiations of 4,400 A and 6,600 A and that 



