CONVERSION OF CARBON DIOXIDE INTO BICARBONATE 



191 



that the comparatively high pressure required for saturation argues 

 against this hypothesis and in favor of carbamate formation. The com- 

 parison of figure 18 with Smith's figure 19, which contains the calculated 

 absorption curve for a solution of primary phosphate, shows, however, 

 that the Willstatter-Stoll results can be accounted for almost entirely by 

 the phosphate buffer equilibrium. 



20 



16 



12 



E 



O 8 

 O 



20 



16 



.12 



E 



d^8 

 u 



3 6 9 12 15 18 



(A) COi,56 



3 6 9 12 15 18 



(8) COjV. 



Fig. 18.— Absorption of carbon dioxide at 5° C. A. Helianthus annuus (about 20 g. 

 fresh leaves); B. Urtica dioica (same quantity) (after Willstatter and Stoll 1918). 



Leaves 



Water in the leaves 



- - Leaves without water (calculated) 



According to Willstatter and Stoll, sunflower leaves absorb, under 

 150 mm. partial pressure, twice as much carbon dioxide as can be dissolved 

 in pure cell water and, under a partial pressure of 0.75 mm. (0.1% CO2 

 in the air), twelve times as much. The ratio between chemically bound 

 and physically dissolved carbon dioxide must become even larger at still 

 lower pressures. Thus, Schafer (1938) found that leaves of Vicia faha 

 (broad beans) may liberate, in vacuo, fifty times as much carbon dioxide 

 as could have been dissolved in the cell water under the partial pressure 

 of carbon dioxide in the air (0.23 mm.). 



Sj^stematic attempts to elucidate the nature of the carbon dioxide 

 absorbing agents in plants were first undertaken by Spoehr and coworkers. 

 Spoehr and McGee (1923, 1924) proved that dried or frozen leaves retain 

 the capacity for carbon dioxide absorption. They found that the absorb- 

 ing compounds can be extracted from the leaves by ether-saturated water, 

 and considered this at first as a proof of their proteinaceous nature. 

 Later, however, Spoehr and Newton (1925, 1926) found that the ab- 

 sorbing agent (reprecipitated by alcohol from the ether-water extract) 

 does not contain enough nitrogen to account for the carbon dioxide 

 absorption on stoichiometric basis. They therefore abandoned the 

 carbamate hypothesis and turned to the bicarbonate hypothesis. 



