4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8/ 



able carl)on dioxide. It is necessary, therefore, to recirculate the air 

 in order to reduce the change in concentration of carbon dioxide along 

 the plants. The air is recirculated by pump F, the direction of cir- 

 culation, as indicated l)y the arrows, being down the double-walled 

 tube, C, through the coil in I, and up through the growth chamber. 

 Air enters the circulating system at a and is discharged at b for 

 analysis. Pump F delivers about 21 liters of air per minute. Thus 

 the effective cpiantity of air passing over the plants is increased by 

 a factor of 35. If we assume the air entering at a contains .04 per 

 cent carbon dioxide and the air discharged at b .02 per cent, the differ- 

 ence between the two or .02 per cent represents the amount absorbed 

 by the plants. If the air is not recirculated the change in concentra- 

 tion of carbon dioxide along the plants amounts to 50 per cent. Since 

 the effective flow has been increased by a factor of 35, the actual 

 change in carbon dioxide concentration along the plants is 

 .02-^35, or .00057 P^r cent of the total gas supply. Then if .02 per 

 cent represents the concentration of the discharged air, .02057 per 

 cent represents the concentration of carbon dioxide in the air entering 

 the growth chamber. This amounts to a change in concentration along 

 the plants of 2.77 per cent of the initial concentration. This repre- 

 sents about the maximum change in the course of the experiment. 



Temperature control.- — Water is recirculated through the space be- 

 tween the walls of tube G by a small propeller in the inner tube of P, 

 through the cylinder, I, and the small bottle containing the thermostat, 

 Ti. A continuous stream of water flows past the electric heater, Hi, 

 and through the space between the walls of P. A resistance in series 

 with the heater is adjusted to give about the desired temperature and 

 the final regulation obtained by the thermostat, Ti, which changes 

 the resistance in series with the heater. There is some lag in this 

 method of controlling the temperature, but thermometers at the two 

 ends of the growth chamber showed differences of only one half of 

 a degree or less. 



Humidity. — The tube, C, is maintained at a temperature of about 

 3° C. below the tem|)erature of the growth chamber by a continuous 

 flow of water past the heater, H2, the tube, C, and past the thermostat, 

 To. In order to hold the humidity constant it is necessary to have the 

 humidity of the air supply high enough so that the addition of moisture 

 due to transpiration will produce saturation at the temperature of 

 C ; any excess will be condensed on the walls of tube C. Due to the 

 rapid recirculation of air we have air saturated at the temperature 

 of C raised to the temperature of the growth chamber in I. From 

 these data the relative humidity may be calculated. This calculated 



