4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 94 



25° to 28° C. as it came from the pipe line. A further reduction in 

 temperature was brought about by placing a white cloth reflecting 

 surface back of the plants and by operating a movable " half-shade." 



A battery of these cylinders is shown in plate i. They were placed 

 on a small platform about 6 feet above ground and in front of a small 

 frame building that faced south. The flow of water was adjusted by 

 the valves at the top. The waste pipe is shown below. On cloudy 

 days, and at night, the " half-shade " was raised by means of a rope 

 and pulleys. 



In the space beneath the platform were located the air and carbon- 

 dioxide flow gauges, the mixing flasks, and the gas tanks. These are 

 illustrated in plate 2. Commercial carbon dioxide of high purity, 

 supplied in heavy steel cylinders, was passed under 15 pounds pressure 

 into a cushion tank and then through a flow gauge into the mixing 

 flask for the proper dilution with air. The air was supplied from the 

 high-pressure compressed-air line from the United States National 

 Museum. It was reduced to 15 pounds pressure and passed into a 

 cushion tank and then into the mixing flask. The proper mixture of air 

 and carbon dioxide was then passed into the cellulose acetate cylinders. 

 The concentration of carbon dioxide in these growth cylinders was 

 checked from time to time by analyses. 



Several preliminary experiments were run during the summer of 

 1933, but the wheat grew so poorly that no definite conclusions could 

 be made other than that the plants receiving the liigher concentrations 

 of carbon dioxide grew better than those receiving the lower con- 

 centrations. Because of the fact that the plants were too closely con- 

 fined in the cylinders, where the temperature was abnormally high, 

 and because of the necessity for using a shade and water filter, it was 

 decided to repeat the experiment the following summer after modify- 

 ing the conditions so as to make them a little less artificial. 



On April 14. 1934, Marquis wheat was planted in the six 8-inch 

 pots used the previous summer and in three plots of soil 2x2 feet 

 laid off in the yard of the Astrophysical Observatory. The conical 

 tops of the cylinders were removed to minimize the rise in tempera- 

 ture of the air surrounding the plants. Neither the water screen nor 

 the " half-shade " was used. At the corners of two of the 2 x 2-foot 

 plots were placed slotted posts into which sheets of glass 24 x 30 inches 

 in size could be fitted. A plot with glass walls either 30 or 60 inches 

 high could be built up as circumstances warranted. To minimize the 

 removal of carbon dioxide from within these glass-walled plots by 

 air currents, there was laid across the top a frame over which two 

 layers of fly netting were stretched. 



