CARBON DIOXIDE CONSUMPTION 853 



consumption of carbon dioxide can be followed acidimetrically, c. g., by- 

 means of an appropriate color indicator (Osterhout and Haas 1918; Oster- 

 hout 1918), or a glass electrode (Blinks and Skow 1938). 



Two sensitive, purely physical methods have been used for the deter- 

 mination of carbon dioxide in the gas phase. They were based on spec- 

 trophotometry and thermal conductivity, respectively. McAlister (1937) 

 built an apparatus (fig. 25.5) in which the carbon dioxide content of cir- 

 culating gas was recorded continuously by means of an infrared-sensitive 

 spectrophotometer, using the strong carbon dioxide absorption band at 

 4.2 to 4.3 M- The apparatus was later altered to register changes in carbon 

 dioxide concentration (uistead of concentration itself) ; cf. McAlister and 

 Myers (1940). 



A simplified method of estimation of small quantities of carbon dioxide 

 in the air by infrared absorption, without the use of a monochromator 

 (after eliminating water vapor as the only other infrared-absorbing com- 

 ponent), was described by Dingle and Pryce (1940). Scarth, Loewy and 

 Shaw (1948) improved this apparatus, increasing considerably its sensitiv- 

 ity, and adapting it for the determination of both carbon dioxide and water. 

 They used it to follow the course of photosynthesis and of transpiration of 



leaves. 



Harder and Aufdemgarten (1938) and Aufdemgarten (1939) described 

 an automatic carbon dioxide recorder, in which the rate of cooling of an 

 electrically heated wire was used to determine the content of the ambient 

 gas in carbon dioxide. (Changes in the concentration of carbon dioxide 

 affect the heat conductivity more strongly than equivalent changes in oxy- 

 gen concentration.) Some registration curves obtained with this instru- 

 ment are reproduced in figs. 33. 10A,B and 1 1 A,B. An improved apparatus 

 ^\■as described by van der Veen (1949). 



5. Measurements of Carbohydrate Production and Energy Conversion 



Two methods — analytical determination of the carbohydrate produc- 

 tion, and determination of the heat of combustion of the synthesized or- 

 ganic material — were much used in the earlier work on photosynthesis. 

 They are, however, hardly suitable for exact kinetic investigations. As de- 

 scribed in chapter 3 (Vol. I, page 35) the amount of analytically determin- 

 able carbohydrates found in the plant after a prolonged period of photo- 

 synthesis often is considerably smaller than was expected from the rate of 

 consumption of carbon dioxide — a result attributable to rapid secondary 

 transformations of the primary product, and possibly also to direct photo- 

 synthetic production of compounds other than carbohydrates. 



The combustion of organic material is an appropriate method for the 



