30 PLANT RESPIRATION 



part the so-called Pettenkoffer tubes/ which are made in vari- 

 ous forms. In the laboratory of plant physiology at the 

 University of St. Petersburg the thick-walled tubes, bent 

 once at an angle of 130°, are used. The shorter arm is 10 cm. 

 long and the longer one is 120 cm. The diameter of the 

 tubes is 14 mm. The second bend with a spherical enlargement 

 first described by Pfeft'er- is unnecessary. If the tubes are 

 long enough no baryta water flows out of the tube with as 

 rapid a gas current as desired. The singly-bent tube is much 

 more easily cleaned. To clean the tubes there is needed a 

 thin wooden rod the end of which is wrapped in Joseph paper 

 or cotton. The fore end of the Pettenkoffer tube is connected 

 with the plant container by means of a i-hole rubber stopper 

 and entry tube. The end of the entry tube must be drawn out 

 in order that the gas may pass through the Pettenkoffer tube 

 in the form of the smallest possible bubbles. The Pettenkoffer 

 tubes are mounted on a wooden frame which consists of two 

 vertical strips of unequal height, fastened together. Both 

 strips are provided with grooves which are Hned with cloth. 

 The tubes are laid in these grooves and fastened with wooden 

 screw clamps. The rear end of each Pettenkoffer tube is con- 

 nected with one of the sidetubes of a wide collecting tube, 

 which itself communicates with an ordinary aspirator. Each 

 Pettenkoffer tube is provided with 100 cc. of baryta water. 

 First, the fore end of the Pettenkoffer tube is closed with the 

 rubber stopper and entry tube. The latter is furnished wdth a 

 rubber tube which remains hermetically closed by a screw 

 pinchcock. The tube is now filled with baryta water and closed 

 at once with a second rubber stopper with an exit tube. The 

 Pettenkoffer tube is now ready for use. 



The baryta water is all prepared at once in large quantities 

 in vessels of lo-i 5 hters capacity. Usually 7 g. of cryst. barium 

 hydroxide are dissolved in each liter of distilled water. For 

 determinations of exceptionally large amounts of CO. two or 

 three times this concentration of barium hydroxide is used. 



1 Pettenkoffer, M. Abdh. math.-physik. Klasse. Bayr. Akad. d. Wiss. (II) 9: 231. 1862. 

 [See also the several types of more recently devised apparatus described at the end of this 

 section.! 



^Pfeffer, W. Untersuch. aus d. bot. Inst. Tubin^jcn i : 636. 1881-1885. 



