I9I9] 



OSTERIIOUT— PHOTOSYNTHESIS AND RESPIRATION 



61 



The bubbling must be continued until the color becomes constant. 

 When this is achieved we know that equilibrium has been estab- 

 lished between the CO2 in the liquid and that in the chamber. 



In order to determine when the color of the indicator has 

 become constant it is compared with a series of buffer solutions 

 having the same concentration of in- 

 dicator and contained in Pyrex tubes 

 of the same size, as described in a 

 previous article. 



The plant is now exposed to sun- 

 light.' After exposure the gas is 

 again bubbled through the liquid. 

 If the plant has taken CO2 from the 

 air it will be evidenced by the change 

 in the color of the indicator, which 

 will show a greater degree of alka- 

 linity than before. From the amount 

 of change in alkalinity the change in 

 tension of CO2 can be calculated; or 

 the indicator may be cahbrated.^ 



For quahtative results the calcula- 

 tion or calibration is not necessary. 

 In the opinion of the writer, leaves 

 of land plants are not suited to 

 quantitative investigations on photo- 

 synthesis, since on exposure to sun- 

 light their temperature (and conse- 

 quently the rate of photosynthesis) 

 fluctuates greatly (as much as 10° C. 

 in half an hour). This difficulty may 

 be obviated by using suitable aquatics. 



For class demonstration it is not necessary to have a tube 

 projecting through the lower stopper. The apparatus may be 



" The rise of temperature which occurs in sunlight tends to force gas out through 

 the joints, which should therefore be made tight or sealed with water. 



* For methods see Henderson, L. J., apd Cohn, E. J., Proc. Nat. Acad. Sci. 

 2:618. 1916; McClendon, J. F., Gault, C. C, and Mulholl.a.nd, S., Publ. no. 

 251, Carnegie Inst. 1917. p. 21. 



Fig. I. — .\pparatus for meas- 

 urement of photosynthesis and 

 respiration: p>lants are placed in 

 the chamber; by means of bulb B 

 gas in chamber is bubbled through 

 indicator contained in tube .4 ; in- 

 take of bulb is at C so that no air 

 enters from outside; valve (not 

 shown in figure) prevents air from 

 passing out through C; changes 

 in color of indicator show changes 

 in tension of CO2. 



