26 PLANT RESPIRATION 



then directed into a container with clear baryta water. The 

 separation of barium carbonate, if necessary with subsequent 

 analysis of the precipitate, serves to identify the carbon dioxide. 

 Still, only quantitative determinations of plant respiration have 

 real physiological significance. These quantitative determina- 

 tions either measure the amount of the carbon dioxide liberated 

 (determination of the respiratory intensity) or give the value of 



CO 



~ (determination of the respiratory quotient) . 

 O2 



For the determination of the carbon dioxide formed by 

 plants, various types of apparatus are used. The research 

 material is enclosed in a container with an entry and an exit 

 tube {Zu- und Ahleitungsrohr) and left for a time in a stream of 

 air free from CO2. The carbon dioxide which is formed is 

 intercepted in a special absorption apparatus. 



In order to free completely the incoming air of atmospheric 

 CO2, the current of air is first passed through a drying tower 

 filled wdth soda lime.^ The containers for the research mate- 

 rial are of various forms and sizes according to the nature of 

 the object of study. One must make sure that the container 

 is nearly filled with plants, for the use of large ones and small 

 quantities of plants will introduce experimental errors. For 

 considerable amounts of research material, glass bell jars are 

 used. The upper openings of the jars are closed with close 

 fitting rubber stoppers, each with an entry and an exit tube. 

 The former must extend nearer to the bottom. The bell jars 

 are sealed to ground glass plates by means of vacuum stop cock 

 grease (of course the lower rim of the bell jar must be evenly 

 ground or an air-tight seal is impossible) . 



For experiments with smaller quantities of plants, U-tubes 

 or thick-walled flasks may be used to advantage." For his 

 studies of the respiration of molds, Puriewitsch- arranged 



1 Soda lime must be renewed frequently and contains so much water that the heating of a 

 small quantity in a test tube frees some water vapor which condenses on the walls of the 

 upper part of the tube. The use of too dry soda lime prevents a complete absorption of CO:. 



■'■ Puriewitsch, K. Ber. d. bot. Ges. i6: 290. 1898; Jahrb. f. wiss. Bot. 35: 573- iQoo. 



" The study of the respiration of bacteria is a special problem. While it may 

 be followed as regards its COo-production by means of certain indicator methods, 

 the complete study requires such apparatus and technique as that described by 

 Novy, Roehm and Soule (Jour. Infect. Diseases. 36: 109-167. 1925). — Ed. 



