50 PLANT RESPIRATION 



(which is very accurate) and the electrical conductivitj- method were found to 

 be less satisfactory than a form of the indicator method which uses colored glass 

 slips for standards and bromcresol purple for the indicator. 



One of the most satisfactory and adaptable forms of the indicator methods is 

 that originally described by Osterhout (J. Gen. Physiol, i: 17-22. 1918-19) 

 for measurement of the rate of COj-production (or consumption) and used in 

 different forms by his students and associates. The diagram shown in Fig. 10 

 is that of a useful form of this system, showing in some detail the practical modi- 

 fications (but not the correct proportions by volume) of the several elements of 

 the system. The total volume should be no larger than is necessary for conven- 

 ience in manipulation of the parts and for adjustment of the amount of living 

 material (or chemical mixtures) required to effect a rapid change in the color of the 

 indicator. 



Fig. 10. — The Osterhout apparatus. 



The diagram shows two chambers, P and R, one of which may be eliminated 

 as indicated by the dotted lines. The second one, R, may be used to guard 

 against a possible buffer efTect of a reagent which maj' be emptied into the plant 

 chamber by way of the flexible tube C without stopping the current of air. This 

 current is caused to flow as indicated by the arrows by the regular compression 

 and expansion of the rubber bulb B, and by the action of the two Bunsen valves, 

 Vi and V2. The CO2 is thereby passed from the chamber P to the indicator 

 tube I unless the stopcock Si is closed and S2 and S3 are open, in which case the 

 CO2 is removed by passing over solid NaOH in the U-tube. Under the latter 

 conditions CO2 is removed from the indicator solution (preferably phenol red). 



The relative rate of CO2 release in (or withdrawal from) the system is the recip- 

 rocal of the time required for the color of indicator solution to change from 

 an exact match with the color of one standard to that of another. The indicator 



