NO. l6 CARBON DIOXIDE ASSIMILATION HOOVER ET AL. 3 



the neck of the flask, N, which holds the nutrient solution, is some- 

 what greater than the diameter of the inner tube of the growth chamber. 

 The two are connected by a rubber annulus, S. The plants are sup- 

 ported by inserting them through small holes in a paraffined cork 

 stopper which fits into the lower end of the growth chamber. The 

 cork, in addition to supporting the plants, separates the flask of 

 solution containing the roots from the space occupied by the tops of 

 the plants. The roots are aerated by passing air through a tube sealed 

 in the side of the flask and extending to the bottom, the air escaping 

 through a side tube near the toj) of the flask. Air for the plant stems 

 and leaves is admitted through a glass tube entering the growth 

 chamber just above the stopper. 



The supply of air containing any desired concentration of carbon 

 dioxide is derived from a 50-gallon galvanized iron tank, R. Air 

 in this tank under a pressure of 75' pounds per square inch is sufficient 

 for the plants for a period of 14 to 16 hours. An electric heater is 

 placed on the bottom of the tank to insure rapid mixing of the air 

 and carbon dioxide. 



The rate of flow of the air is regulated by means of the flowmeter, 

 M, reducing valve, V, and capillary tube. The capillary tube is 

 inserted in the air line between the reducing valve and the flowmeter 

 to produce a back pressure, as the reducing valve does not regulate 

 well unless the back pressure is 8 pounds per square inch or more. 

 This precaution insures that a sufficient quantity of air is delivered to 

 the flowmeter for all pressures in the tank between 10 and 75 pounds 

 per square inch. The flowmeter used is designed to maintain a constant 

 I)ressure difl:'erence between the intake and the discharge, for small 

 changes of pressure on the discharge side. To prevent leakage of car- 

 bon dioxide the entire air system is made of glass, with the exception 

 of the recirculating air pump, F, and three short rubber connections. 

 The rubber connections are coated with paraffin. To test for a possible 

 leakage of carbon dioxide the entire apparatus was assembled, without 

 plants, and the concentration of carbon dioxide in the air measured 

 before and after passing through the apparatus. No change in the 

 concentration was detected. 



A two-way stopcock is placed in the air line at i, one branch leading 

 directly to the carbon dioxide measuring apparatus for determining 

 the original concentration of carbon dioxide, and the other leading 

 to the growth chamber. During a determination of the original con- 

 centration, air is supplied to the plants through stopcock 2. 



The flowmeter is adjusted to give 600 cc of air per minute. Under 

 extreme conditions the plants absorbed as much as one half the avail- 



