268 BOTANICAL GAZETTE [october 



upper end was set a small tin cup such as may be obtained at any 

 hardware store, and over this a screen lid was placed for use when 

 working in a high wind. For ventilation above the flame, a ring of 

 small holes was cut about 2 . 5 cm. below the level of the inserted 

 cup. This lamp was found to be readily controlled, burns alcohol 

 or kerosene equally well, can be used successfully in a strong wind, 

 and burns continuously for 48 hours or more without refilling or 

 adjustment. 



Relative humidity was calculated hourly by means of the sling 

 psychrometer, such as is in use by the United States Weather 

 Bureau, and the wet bulb depression was referred to a standard 

 table as given by Marvin (12) to obtain the relative humidity 

 values. 



Soil temperatures were recorded for depths of 2 dm. and 4 dm. 

 by means of a centigrade thermometer mounted on a cylindrical 

 piece of hickory which was well adapted for inserting into the 

 sand. Atmospheric temperature records were taken and recorded 

 hourly during each day of experimentation. 



Soil samples were collected at the different stations on the days 

 when these were worked and the moisture computed on the basis 

 of dry weight. From these results growth water was calculated 

 by the equation GW = SW— WC, in which SW = total soil water 

 and WC = the wilting coefficient. Samples were taken from 

 depths of 2 and 3 dm. and dried for 6 days at a temperature of 

 ioo° C. 



The wilting coefficient of the soil was computed by the centri- 

 fuge method of Briggs and McLane (4). The moisture equiva- 

 lent was obtained directly and the wilting coefficient derived from 



this by the equation of Briggs and Shaxtz (3), : — — =WC. 



The evaporating power of the air was recorded by means of 

 the porous clay cup of Livingston (7). Hourly readings were 

 readily secured by mounting the atmometer on a graduated 

 burette tube which was refilled when the water column fell to a 

 point about 25 cm. below the level of the cup. 



Field work was conducted under diverse weather conditions 

 to discovei to what extent the varying of such factors as relative 



