CHAPTER III. 

 THE SYNTHESIS OF CARBOHYDRATES. 



THE term carbon assimilation, although unfortunate from 

 some points of view, is employed to designate all those 

 activities, in part physical, in part chemical, which play a 

 r61e in the anabolism of carbon dioxide by green tissues. 

 The conspicuous facts of the process are that active chloren- 

 chyma on exposure to light forms, by means of its chlorophyll, 

 carbohydrate from the initial substances carbon dioxide and 

 water; oxygen, in volume roughly equivalent to the volume 

 of carbon dioxide consumed, is evolved during the process.* 

 Carbohydrate is the obvious and chief end product, but protein 

 also may be so formed, and such diverse materials as fat, 

 tannin and various organic acids have been considered, prob- 

 ably on insufficient evidence, to be of direct photosynthetic 

 origin. The earlier phases in these synthetic processes are 

 photochemical, a mutation of radiant into chemical energy, 

 and it is during this phase that the oxygen, a waste product, 

 is evolved. The presence of oxygen in the air-space system 

 of the active chlorenchyma may thus be considerably greater 

 than in normal air, and since this gas is continually excreted 

 during the process, it is not surprising to find that the quantity 

 of oxygen in the surrounding atmosphere is immaterial to 

 the process and that it may be decreased to 2 per cent or 

 increased to 50 per cent without adverse effect : f but since 



* Bonnier and Mangin (" Ann. Sci. Nat. Bot.," 1886, 3, i) found by various 

 experimental methods that the ratio O 2 /CO 2 was always greater than unity 

 for ordinary plants; the lilac gave the smallest value 1*05, and the holly the 

 largest 1-24. A similar range was found by Aubert (" Rev. gn. Bot.," 1892, 

 4, 203) to obtain in ordinary plants, but succulents, which have a peculiar 

 metabolism, gave generally a higher value ; as high as 7-59 in the instance of 

 Opuntia tomentosa. Maquenne and Demoussy (" Compt. rend.," 1913, 156, 

 506) conclude from a large number of observations that the assimilatory 

 quotient approximates to unity. 



t Friedel: " U.S. Dept. Agric.," 1901, Bull. 28. 



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