194 



FIXATION OF CARBON DIOXIDE 



CHAP. 8 



production which partially saturates the buffers during the short time between prehmi- 

 nary evacuation and the admission of carbon dioxide. This may explain the smaller 

 CO2 uptake after the admission of this gas, but not the increased quantity of "irrevers- 

 ible" carbon dioxide found in living leaves. Furthermore, this explanation imphes that 

 respiration builds up a large internal pressure of carbon dioxide before the latter escapes 

 into the atmosphere — which is improbable (compare Vol. II, Chapter 33). 



Despite these difficulties of quantitative interpretation, which show 

 the desirabihty of continued experimentation, Smith's general conclusion 

 that leaves contain two main carbon dioxide-absorbing factors — solid 

 carbonates, and a water soluble buffer — appears plausible. 



The behavior of the aqueous fraction in particular can be quantita- 

 tively accounted for by the action of a phosphate buffer, as shown by 

 measurements of the CO2 uptake by this fraction under varying partial 

 pressures of carbon dioxide {cf. Table 8. XII and Fig. 19). 



Table 8.XII 

 Carbon Dioxide Absorption by Water Extract of Sunflower Leaves 



A comparison of pH values in table 8. XII with those in table 8. II 

 confirms the presence of buffers in the sap. The second column in table 

 8.XII shows that if the bicarbonate concentration is calculated from the 

 carbon dioxide absorption, by assuming that all absorption in excess of 

 solubility is due to bicarbonate formation, the result is equal to that 

 derived from acidity. This is taken by Smith as a proof that carbamina- 

 tion plays no part in the carbon dioxide absorption by the water-soluble 

 leaf fraction. In figure 19, the experimental absorption values are com- 

 pared with the calculated absorption by the phosphate buffer alone; the 

 comparison shows that the phosphate can account for most, but not quite 

 all, the bicarbonate formation in the extract. The remaining discrep- 

 ancy indicates the presence of some minor buffering components. 



The assumption that the carbon dioxide absorbing capacity of the 

 insoluble leaf fraction is caused by the presence of alkaline earth car- 

 bonates is supported not only by the solubiHty of the absorbing agent in 

 carbon dioxide-saturated water, but also by the analysis of the ash. 

 It shows the presence of 6.7 X 10"" gram atom of calcium and 1.8 X IQ-" 

 gram atom of magnesium in 10 grams of fresh leaves. In the form of 



