CHAPTER II. 



THE SYNTHESIS OF CARBOHYDRATES. 



The term carbon assimilation, although unfortunate from 

 some points of view, is employed to designate all those ac- 

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

 in the anabolism of carbon dioxide by green tissues. The 

 conspicuous facts of the process are that active chlorenchyma 

 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 quan- 

 tity of oxygen in the surrounding atmosphere is immaterial 

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



* Bonnier and Mangin (" Ann. Sci. Nat. Bot.," 1886, 3, 1) found by 

 various experimental methods that the ratio 2 /C0 2 was always greater 

 than unity for ordinary plants ; the lilac gave the smallest value, 1-05, and 

 tin- holly the largest, 1-24. A similar range was found by Aubert (" Rev. 

 gen. 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 Opuntia tomentosa. Maquenne and Demoussy (" Compt. rend.," 

 IQ I3. '56> 506) conclude from a large number of observations that the 

 assimilatory quotient approximates to unity. 



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