246 PHOTOSYNTHESIS 



a modified form of the Haldane-Barcroft blood gas analysis apparatus 

 was employed. The method has the great advantage that determinations 

 of the amount of photosynthesis can be made in relatively short periods 

 of time and thus the course of the photosynthetic activity followed with 

 considerable exactness. 



A fundamental assumption in this method is that the photosynthetic 

 quotient equals unity, that the volume of carbon dioxide absorbed equals 

 that of oxygen emitted. This implies that when a plant is enclosed in 

 an atmosphere of air containing carbon dioxide there will be no change 

 in pressure when the plant is illuminated. A change in pressure does 

 occur if an aquatic plant is employed, and the volume of water is not 

 small as compared to that of the gas volume. Under these conditions 

 the difference in the absorption coefficient of oxygen and of carbon 

 dioxide is noticeable. Also a change in gas pressure in the illuminated 

 vessel containing the plant is produced when the carbon dioxide used by 

 the plant is taken from a compound dissolved in water. 



Both of these principles have been applied by Warburg. In one 

 the dififerences in the absorption coefficient of carbon dioxide and oxygen 

 for water at known temperatures and pressures are taken as the basis of 

 calculation. In the other the carbon dioxide is derived from mixtures 

 of sodium carbonate and sodium bicarbonate, the COj-content of which 

 is known for certain temperatures. When the amount of carbon dioxide 

 used is small in comparison to the total amount of salt the effect of the 

 change produced in concentration of the latter is very small and the 

 carbonate mixtures play the role of buffers. The change in gas pressure 

 is very nearly entirely due to alteration in the partial pressure of the 

 oxygen, the amount of change in pressure due to alteration in the partial 

 pressure of the carbon dioxide being relatively very small. 



One form of apparatus which was used by Warburg is shown in 

 Figure 15. The entire apparatus was immersed in a thermostat and the 

 changes in pressure read on the manometer. The latter was filled with 

 a solution made by Brodie for this purpose with which 10,000 mm. = 

 760 mm. of mercury. The algae were introduced into the containers in 

 the form of a suspension in water. As neither the weight nor the 

 surface of cells could be determined, it was not possible to obtain abso- 

 lute values of photosynthetic activity by this method. Warburg has also 

 developed several modifications of the apparatus shown in Figure 15 for 

 work with suspensions of unicellular algae. 



3. The Absorption of Carbon Dioxide 



For the determination of the rate of photosynthesis of land plants the 

 method which has found most general application is the one based upon 

 the estimation of the amount of carbon dioxide withdrawn by the leaves 

 (or usually a single leaf) from a current of air containing carbon dioxide. 

 The carbon dioxide-content of the air stream is first determined. The 



