86 



PLANT PHYSIOLOGY 



of the apparatus. Previous to entering the apparatus, the cur- 

 rent of air must be dried. Frequently, a titratable solution of 

 barium hydroxide is used as an absorber of CO 2, with Petten- 

 koffer's tubes used as a convenient equipment (Fig. 18), through 

 which an air current passes in the form of fine bubbles that 

 discharge into the barium hydroxide their carbon dioxide. The 

 quantity of the latter is determined by the difference between 

 the initial and final titration figures of the solution. 



The intensity of respiration may also be determined by 

 measuring the amount of oxygen absorbed. To observe and 

 measure the absorption of oxygen by the respirating organism, 

 a small cup with caustic alkali is placed in the container holding 



Fig. 18. — Pettenkoffer's apparatus for quantitative determination of carbon 

 dioxide eliminated in respiration. The germinating seeds are placed in U-shaped 

 tubes connected with cylinders containing soda lime and absorbing CO2 of the air. 

 The air current proceeds from right to left {after Palladin). 



the respiring organs of the plant to absorb carbon dioxide 

 liberated in the process of respiration. With such an equipment, 

 the visible decrease of the total volume of gas in the container 

 will correspond to the amount of oxygen absorbed during respira- 

 tion. To measure the decrease in volume, the container is 

 provided with a graduated lateral tube of small diameter forming 

 a manometer. This apparatus is called a ''respirometer" 

 (Fig. 19). Not so long ago, this apparatus was assumed to be 

 very crude and was used only for demonstrations to students 

 of the phenomenon of absorption of oxygen. Recently, it has 

 been considerably improved by Warburg, so that now it attains 

 a high degree of precision and sensitivity. With an apparatus 

 of this type Warburg performed important investigations on 

 respiration, which will be further described in Art. 24. 



Thers is considerable variation in the intensity of respiration 

 in different plants and in different parts of the sam< plant. 



