34 



PLANT RESPIRATION 



analyzed by the usual methods which need not be fully described 

 here. For the general technique of manipulation of gases and 

 the description of long-established, accurate methods of gas 

 analysis, consult the complete manuals. ^ Only two pieces of 

 apparatus, which have proven to be particularly suitable for 

 plant physiology experiments and which are not described in 

 manuals of gas analysis, will be mentioned here.^ 



For exact work in biochemical gas analysis, the Polowzow- 

 Richter apparatus^ is particularly worthy of recommendation, 

 for it yields very accurate results and can be used for very small 

 quantities of gas. The most important part of the apparatus 

 (Fig. 6) is the measuring tube AA'A"A'". The part of the 

 tube which is furnished and calibrated with millimeter divisions 

 is contained in a glass cylinder B filled with water, the outer 

 end A'", in the cylindrical mercury bath C. The ends of the 

 glass pipettes D and E are also placed in the same bath. The 

 left end A of the measuring tube is connected with the glass 

 bulbs H by means of a thick-walled rubber tube P, which is 

 filled with purest mercury. By the aid of this pear-shaped 

 bulb and the glass stopcock a, the level of the mercury in the 

 measuring tube can be set at any height. The steel screw e 

 which is set in mercury in a side tube is used for delicate adjust- 

 ments of the level of mercury. The left side of the measuring 

 tube marked in millimeters is a thick-walled capillary tube of 

 about 2 mm. bore. From the line o on, the hollow of the tube 



1 Bunsen, R. Gasometric Melhoden, 1877; Hempel. Gasanalytische Methoden. 

 1913; Berthelot, M. Traite pratique de I'analyse des gas. 1906; The method of Doyere 

 (Ann. de chim. et de physique. (Ill) 28: 5- 1850) is very commendable. [Reliable man- 

 uals in English include: Haldane, J. S. Methods of Air Analysis. London. 1912; Hempe!, 

 W. Methods of Gas Analysis. Trans, by L. M. Dennis. London. 1892.] 



2 Barcroft's method, which is much used in animal physiology, together with its later 

 modifications (measurement of changes in pressure of tne gas in small experimental flasks 

 with the aid of sensitive manometers), has recently been used by some plant physiologists. 

 It will not be described here, however, since it is not very suitable for plant material and is 

 less reliable than the following gasometric method. The change in pressure of the oxygen 

 plays an important role in the Barcroft method. Various plants absorb considerable 

 amounts of oxygen not for combustion of the respiratory material but for other purposes 

 (formation of acids, amino acids, etc.). Likewise the marked variations in pH which occur 

 in the case of many plants and especially with microorganisms, might give rise to 

 considerable errors, since the solubility of CO: and other gases is thereby altered. Such 

 sudden fluctuations in pH are made impossible in the blood of animals and are not taken into 

 consideration by Barcroft's method which was designed especially for blood e.\periments. 



Hence in studies of plant respiration, only chemical methods of gas analysis are absolutely 

 reliable [for absolute measurements]. 



» Polowzow, V. Untersuch. Uber Pflanzcnatmung. 1901. Russian; Richter, A. Arb. 

 d. Naturf.-Gcs. in Petersburg. 33: 311- 1902. 



