122 LIGHT, VEGETATION AND CHLOROPHYLL 



of magnitude of those used in the experiments on efficiency, 

 i.e., 1,000 times lower than sunlight, it is found to be fifty 

 minutes. 



Now, however low the illumination may be, photosynthesis 

 begins immediately. We are forced to admit that the 4 quanta 

 whose combined action on a single molecule of CO2 is 

 indispensable have not been absorbed by the same molecule 

 of chlorophyll and that their energy has been propagated, 

 directed — how, it is not known — from the point of absorption 

 to the molecule of CO 2 about to be reduced. 



These difficulties still remain, even if we assume the 

 formation of intermediate compounds each of which requires 

 for its creation only a part of the total energy suppUed. If 

 the first quantum gives a first compound A, the second 

 quantum must then encounter A to form B, and so on. More- 

 over, from the beginning, an encounter, direct or indirect, is 

 necessary between CO 2 and the activated molecule of chloro- 

 phyll, and all these phenomena are unimaginable without the 

 transmission of energy. 



Again, it must be remarked that the necessity for a 

 cumulative action of 4 quanta ought, in conformity with 

 what is known in physics of actions of that kind, to result in a 

 very low efficiency at low illuminations and a very rapid 

 increase of efficiency with increasing illumination, according 

 to a 4th power progression. This means that if the illumination 

 were doubled, the rate of photosynthesis ought to be 16 

 times greater. Nothing of the kind is observed ; the variation 

 observed is, within certain limits, a simple proportionaUty, 

 as would be expected if a single quantum were sufficient to 

 accomphsh the reaction. 



In fact, the number of centres excited the first time is 

 proportional to the illumination; the number of centres 

 excited the second time is proportional, not only to the 

 exciting illumination, but also to the number of acceptors 

 already excited. It is therefore proportional to the product 

 of these two quantities and so to the square of the illumi- 

 nation. Similarly, the number of acceptors excited three times 



