STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



89 



of a bicycle wheel which is suspended from the ceiUng of the room. 

 The wheel has excellent ball-bearings and is fastened, so that there 

 is no play and very little friction. As the weight descends it 

 lifts the cylinder out of the water, which causes the air-stream to 

 be drawn through the entire apparatus. It is, however, necessary 

 to make an adjustment for the increasing weight of the cylinder, as a 

 larger portion of this comes out of the water. This is attained by 

 increasing the pull on the other end of the cable through a supple- 

 mentary weight placed on the wheel. When 

 the cylinder is completely down this weight is 

 at the top of the wheel, exerting no pull. As 

 the cylinder is drawn out of the water, the 

 wheel turns and the pull of the supplemen- 

 tary weight increases as the horizontal dis- 

 tance between it and the center of the wheel 

 increases. Thus the increasing weight of the 

 cylinder is adjusted; and while the two phe- 

 nomena do not mathematically balance each 

 other, the adjustment can be made sufficiently 

 accurate so that no differences in the rate of 

 the air-stream are noticeable. 



The length of time for each observation 

 was usually two hours. The amount of car- 

 bon dioxid fixed by a single leaf in this length 

 of time is very small. It was therefore essen- , 

 tial that the accuracy of the method of de- ^ 

 termining carbon dioxid be correspondingly 

 high. For these experiments the same princi- 

 ple of carbon-dioxid determination was em- 

 ployed as in the respiration experiments. On 

 account of the shorter periods and smaller 

 amounts of carbon dioxid to be absorbed, a 

 smaller range in the specific resistance curve 

 of barium-hydroxide solution was used. 

 Greater accuracy was attained by using an 

 electrolytic cell of higher resistance. The 

 original barium-hydroxide solution was 0.1001 

 normal, and in the absorption of the carbon 

 dioxid this was reduced to about 0.086 nor- 

 mal. For each determination 68.12 c. c. of 

 solution was introduced by means of a pipette 



Fig. 23. — Electrolytic cell for determining the conductivity 

 of the barium hj'dioxide solutions used to absorb the CO2 in the 

 photosynthesis experiments. The solution is protected from the 

 air by a soda-lime tube. 



