CHLOROPHYLL AND ENERGY 189 



If the exposure is longer, the number of substances which 

 are carriers of C^^ rapidly increases and saccharose appears. 

 At first the fructose possesses twice as many carriers as the 

 glucose. At the end of five minutes, in barley leaves, 95 per 

 cent of the carbon fixed has already reached the stage of 

 saccharose and other osides, while in green algae half has been 

 distributed in glucosides and half in albumins. The end product 

 of photosynthesis, glucosides, is reached and all the inter- 

 mediate stages are passed with disconcerting rapidity. 



If the luminous intensity is lowered, this speed greatly 

 diminishes and 4,000 lux for thirty seconds causes only tri- 

 carbonate substances to appear — phosphopyruvic acid and 

 especially phosphoglyceric acid. At low temperatures, the 

 result will be particularly phospho-2-glycerate (phosphoric 

 acid fixed on the central atom of carbon), which is less stable 

 at ordinary temperature and more slowly formed than 

 phospho-3-glycerate. If the luminous intensity is reduced still 

 more and falls to 500 or 600 lux, malic acid becomes abundant. 



These experiments throw a new light on the path followed 

 by photosynthesis. It must not be thought, however, that all 

 the intermediate products are known or even that the first 

 have been identified with certainty. It may be that phospho- 

 glycerate is the first compound formed by the carbon dioxide 

 fixed, but there is still some doubt, for the chemical cycles in 

 biology are so rapid that if one stage is passed slightly more 

 slowly than the others it will very probably eclipse them and 

 make them imperceptible. Moreoever, a substance may seem 

 to be abundant because in the cycle it appears in a "side- 

 track" or as a storage product which is more or less stable. 



It is therefore possible that substances other than those 

 found "labelled" are produced in the course of photosynthesis 

 and that some of those "labelled" do not enter into it directly. 



It cannot be doubted, however, that the first three sub- 

 stances found are important and occupy among the inter- 

 mediate substances a central place from which the diversion is 

 made towards the principal syntheses, while sooner or later 

 phosphoric acid is detached from their molecules. 



