120 LIGHT, VEGETATION AND CHLOROPHYLL 



At the same time as the last reaction, Rabinovitch supposes 

 that the following is accomplished, the simultaneity rendering 

 the whole process easier: 



4HB+4A=4HA+4B 



The photosynthetic action is terminated by: 

 4[HC02]=3C02+H20+[CH20] 



The resultant of these five reactions is simply: 



4H20+4[C02]=[CH20]+02+3H20+3C02 



Rabinovitch says, without being able to state precisely 

 which, that one of these partial reactions is caused by light. 

 As the total energy necessary for the whole of these trans- 

 formations is contributed by fight, it would seem that the true 

 photochemical reaction ought to be by far the most endo- 

 thermic and thus to be clearly distinguished from the others. 

 On this point at least his theory appears therefore rather 

 defective. We can only confess our ignorance, as, of course, 

 he does himself. 



The identification of some of the intermediate products 

 would be of the greatest interest. An experimental method is 

 available, since a radioactive isotope of carbon can be 

 separated and is easy to distinguish from ordinary carbon, even 

 when it is present only in minute traces. If a plant is supplied 

 for a short period of time with CO 2 containing some atoms of 

 radioactive carbon, among the multiple organic compounds 

 of the tissue of the leaf those which contain this special 

 carbon can be recognized as the products which have just 

 been created by photosynthesis. This distinction between the 

 compounds just formed and those which were already in 

 existence was scarcely possible before a label could be fixed 

 to atoms of carbon by making them radioactive without 

 modifiying their chemical properties. 



The use of isotopes opens a new, and certainly promising, 

 line of research, for it has already given results conducive to 

 a better understanding of plant photosynthesis and encouraged 

 scientists to hope to extend their knowledge of one of the 

 most essential processes for life. 



