196 



PHYSIOLOGY OF NUTRITION 



air, which enters the plant chamber {a, Fig. 91) after having been freed of carbon 

 dioxide through the action of soda lime. From the plant chamber the air 

 passes into the lower end of the Pettenkoffer tube, forming small bubbles which 

 ascend slowly through the baryta water. The air, again freed of carbon dioxide, 

 passes out to the aspirator from the upper end of the tube. Since the aspirator 

 would usually produce a more rapid air stream than can be passed through the 

 Pettenkoffer tube, a pressure regulator (b, Fig. 91) is introduced, which also 

 prevents too great rarification of the air in the. plant chambei;. -The. carbon 

 dioxide produced by the plants is precipitated in the tube as barium carbonate. 

 After a suitable time the air stream is turned into a second Pettenkoffer tube 

 and the solution is removed from the first and titrated [with standard oxalic 

 acid solution and phenolphthalein as indicator]. Thus the amount of unprecipi- 

 tated barium hydroxide that remains is determined, and a simple calculation 

 gives the weight of the carbon dioxide produced by the plants during the given 

 period. The temperature of the plant chamber is maintained constant by 

 immersing it in a large vessel of water which is warmed as necessary. 



Fig. 91. — Respiration apparatus. (After Pettenkoffer.) 



The amount of oxygen absorbed by a plant may be measured by means 

 of the apparatus of Wolkoff and Mayer (see note 3, p. 1 90), which consists essen- 

 tially of a large inverted U-tube with one arm broad and the other narrow and 

 graduated for volume readings. In the broad arm of this tube are placed the 

 seedlings, etc., to be studied, and also a small, open vessel of potassium hydroxide 

 solution, and the larger opening is tightly closed with a glass stopper. The 

 other, narrow arm of the tube is closed by dipping into mercury below. The 

 carbon dioxide produced by the plant is absorbed by the potassium hydroxide 

 solution and the volume of the oxygen absorbed is measured by the rise of the 

 mercury meniscus in the narrow, graduated arm. 



For the simultaneous determination of the oxygen absorbed and the carbon 

 dioxide given off, the apparatus of Bonnier and Mangin may be employed (Fig. 

 92). The bell-jar. A, serves as plant chamber, into which air passes tlfrough 

 the tube a, having first been freed of carbon dioxide by bubbhng through potas- 

 sium hydroxide solution in the wash-bottle, F. A vessel of water in the chamber 

 keeps the atmosphere moist. The chamber is first filled with air that has been 

 freed from carbon dioxide, suction being applied through tube b, by means of 

 an aspirator. Then the* two cocks, r and 2, are closed. From time to time a 



