190 



FIXATION OF CARBON DIOXIDE 



CHAP. 8 



dissolved carbon dioxide and one-half the carbonic acid of the bicar- 

 bonates) by pumping, then extracted the leaves with water, acidified 

 the extract and the insoluble residue, and measured the evolved carbon 

 dioxide. Table 8. IX contains some of the results obtained in this way. 



Table 8.IX 

 Carbonates in Plants (after Berthelot and Andre) 



The quantities of soluble carbonates (presumably, alkaU carbonates) 

 found by Berthelot and Andre appear too high when one considers the 

 comparatively low pH of the cell sap, and do not agree with the quantity 

 of carbon dioxide which the leaves absorb under an increased pressure of 

 carbon dioxide, and liberate in vacuo. Recent investigations of Smith 

 make it probable that divalent cations account for all the carbonate 

 anions found in the leaves. 



The presence of phosphates in the cell sap of green plants has been 

 demonstrated by Martin (1927). Their concentration is of the order of 

 10-- mole per liter. Primary phosphate absorbs carbon dioxide according 

 to the equation: 



(6.31) 



CO2 + H2O + HPO, 



:^ HCO3- + H2PO4- 



The presence in leaves of alkaline earth carbonates and primary 

 phosphates makes it necessary to consider these factors first in the 

 interpretation of the reversible carbon dioxide absorption by plants. 



The first determinations of the reversible carbon dioxide absorption 

 by leaves were carried out by Willstatter and Stoll (1918), with Urtica 

 dioica (nettle) and Helianthus annuus (sunflower). The absorption 

 isothermals are reproduced in figure 18. Half-saturation is reached in 

 Helianthus at about 40 mm., and in Urtica at a somewhat higher pressure. 

 The maximum absorbed quantities (after correcting for solubility) are, 

 in both cases, of the order of 1 ml. CO2 per 10 g. fresh leaves (5 X 10"^ 

 mole/1., or about 0.1% of the dry weight of the leaves). 



Willstatter and Stoll mentioned the carbonate-bicarbonate conversion 

 as a possible explanation of the carbon dioxide absorption, but thought 



