LIGHT AND VEGETATION 121 



Unanswered Questions 



Experimental results fully confirm the photochemical 

 character of photosynthesis — activation and accumulation 

 of energy by the action of light, and chemical transformations, 

 capable of being produced in darkness, resulting in the 

 assimilation of carbon from the air. Nevertheless, several 

 very simple but very important problems remain and at 

 present their solution appears to the physicist to be difficult 

 to foresee. 



It is assumed, from experiments on the maximum 

 efficiency of photosynthesis, that 4 quanta of Hght are 

 necessary, and may be sufficient, for the formation of a formal 

 group, combined or not. This statement alone gives rise to 

 several difficulties. 



The 4 quanta are absorbed in the tissue, doubtless by 

 molecules of chlorophyll, on a molecular scale and at 

 different points, separated from one another. It must of 

 necessity be assumed that these 4 quanta of energy are 

 capable of moving and of converging towards the exact spot 

 where there is a molecule of CO2 or of HgO; this transmission 

 of energy must be made without loss and without degradation. 

 Now, there is no phenomenon of this kind in any of the 

 conditions physically known in the photochemistry of gases 

 and vapours ; resonance radiation is an example of a transfer 

 of energy without degradation, but it occurs at random in 

 any direction and not towards a precise objective. 



There must be in the molecular associations of the chloro- 

 plasts an organization which is well adapted to this transfer 

 and which does not exist in a gas. The energy is propagated in 

 the form of electrons raised to high energy levels. These 

 energy levels will have to be determined, as well as the laws 

 which govern their occupation and the mobiUty of the electrons 

 excited. This work has not even been started in the case with 

 which we are concerned. 



There is another difficulty related to the preceding one. 

 If the time necessary for 4 quanta to attack the same molecule 

 of chlorophyll is calculated, for an illumination of the order 



