THE CHEMISTRY OF PHOTOSYNTHESIS 311 



for by the solubility of this gas in the water of the tissues.^*" Willstatter 

 and Stoll observed a decided absorption of carbon dioxide by leaves that 

 had been dried and powdered and had subsequently been allowed to 

 absorb water again. Their method of determining the amount of carbon 

 dioxide absorbed by the leaf material consisted in passing a stream of 

 air containing 5 to 10 per cent of carbon dioxide over the leaf material 

 at 5° ; on raising the temperature to 30° more carbon dioxide was given 

 off than could be accounted for by the difference in solubility between 

 the two temperatures and after making proper allowance for the post 

 mortal respiration. Two species of leaves were used, Helianthus annnus 

 and the nettle, Urtica dioica. The absorption was determined at differ- 

 ent partial pressures of carbon dioxide, 1 to 19 volume per cent. The 

 amount of absorption of carbon dioxide which is attributed to some 

 constituent in the leaf is obtained by subtracting the quantity dissolved 

 in the amount of water present (at the particular temperature and pres- 

 sure) from the total absorption observed by the moist leaf material. 

 They thus find that the leaf absorbs considerably more carbon dioxide 

 than can be accounted for by the solubility of this gas in the water pres- 

 ent. The difference between the amounts absorbed by water and the leaf 

 substance becomes greater with smaller partial pressure of carbon dioxide. 

 Thus, with the sunflower, in 5 per cent carbon dioxide, the absorption 

 of the leaf material is four times that of the solubility in water, while 

 with 1 per cent carbon dioxide it is six times the solubility, and it 

 is calculated that with 0.1 per cent carbon dioxide the absorption of the 

 leaf substance would be twelve times that of the solubility of carbon 

 dioxide in water. In view of the low partial pressure of carbon dioxide 

 in the atmosphere, there can be little doubt but that this high absorp- 

 tion capacity of the leaf material for carbon dioxide would result in 

 making more of it available for photosynthesis. 



Willstatter and Stoll did not determine the nature of the substance 

 or substances which are responsible for the absorption of carbon dioxide, 

 although they show that this is not related to the pigments of the leaf. 

 They did not regard it as improbable that the absorption of carbon dioxide 

 by the leaf material was due to inorganic carbonates, but favored the 

 idea that the absorption was due to organic compounds. They thus came 

 in accord with the conceptions which had previously been developed by 

 Sies^fried. The latter considered that it was not carbon dioxide which 

 was reduced in the photosynthetic process, but that this gas united with 

 components of the protoplasm on the basis of the carbamino reaction. 

 Siegfried ^*' showed that carbon dioxide combines with amino acids, sugars, 

 alcohols and hydroxyacids. Thus the carbamino reaction with neutral 

 amino acids or proteins and bicarbonates may be represented as follows : 



""Bohm, Ann. Chem.. 185, 248 (1876). Carey, Physiological Researches, 2, 407 

 (1923). Willstatter and Stoll, "Unters. li. Kohlensaureass.," 174 (1918). 



'"Siegfried, Zeit. physiol. Chem., 44, 85 (1905); 46, 401 (1905); 54, 423, 437 

 (1908) ; 59, 376 (1909). 



