THE NATURE OE PHOTOSYNTHESIS 83 



components changes. However, if the quantity of carhon dioxide taken 

 up by the plant is small in comparison to the total amount of the 

 carbonate salts present, the changes of concentration are negligible and 

 the carbonate mixtures play the role of buffers. Thus, for example, 

 taking Warburg's figures for 25° for the carbonate mixture No. 9 



CNaHCOa = 0.085 moles per liter. 



Ceo = 91 X 10-*' " " " 

 K^ " = 5.3 X 10^ " " " 



If from 10 cc. of this mixture 0.2 cc. COo are removed, there will 

 be taken 0.9 X lO"' moles CO, per liter. Thereby the same amount of 

 NezCOs molecules will be formed and twice the number of NaHCOa 

 molecules will disappear. So that from equation (2) 



r (0.085-1.8X10-^)^ -g-^xio-B 



^°^- (0.015 + 0.9 X 10-=*) X 5.3 X 10^ 



i.e. Cec\, decreases from 91 X lO"' to 82 X 10"' or about 10 per cent. 



Nathanson ^^ has tried to show that it is not the accumulation of OH 

 ions but only the reduction of free CO2 which is responsible for the de- 

 crease in the rate of photosynthesis in carbonate mixtures as these be- 

 come more alkaline. This is based upon the fact that certain aquatic 

 plants which are capable of storing small quantities of carbon dioxide 

 can utilize this for photosynthesis in relatively strongly alkaline solu- 

 tions. In view of the complexity of such a system the evidence is not 

 altogether convincing, however. 



Large bodies of water and particularly the sea are solutions of car- 

 bonates which must be viewed in the light of the equilibrium between 

 atmospheric carbon dioxide and carbonates. The water, being in con- 

 stant contact with the atmosphere, an equilibrium has been established be- 

 tween solution and gas so that the carbon dioxide pressure in the air is 

 about equal to that in the water. The conditions under which plants 

 carry on photosynthesis in the sea and in the atmosphere are therefore 

 about the same as far as the carbon dioxide supply is concerned. Although 

 the total carbon dioxide-content of sea- water, i.e. both free and com- 

 bined CO2, is about 50 times greater than that of the atmosphere, the 

 larger portion of this is not available to the plant. 



It appears that no other gas can take the place of carbon dioxide in the 

 photosynthetic process. The effect of carbon monoxide in low concentra- 

 tions is like other indifferent gases, e.g. nitrogen, and plants do not 

 survive when confined in an atmosphere containing carbon monoxide 

 but no carbon dioxide.^* Higher concentrations of carbon monoxide are 



"Nathanson, Stoffwechsel der Piianzen, p. 166. (Leipzig. 1910.) 

 " Boussingault, Agronomic, 4, 300 (1868). Stutzer, Ber. chem. Ges., 9, 1570 

 (1876). 



