FIELD CROPS. 833 



" The potometers were 3 ft. deep and 16 in. in diameter. Each contained the 

 equivalent of 260 lbs. moisture-free silt loam soil. A 3-in. layer of gravel (E) 

 served to prevent surface evaporation. Rain was excluded by means of a 

 galvanized iron lid (D) having a 4-in. opening for the plant. This opening 

 was covered with oil cloth, closely fastened about the cornstallj. Buried in the 

 soil and connected at the top with a covered 6-qt. can was a spiral coil (B) 

 made of f-in. brass tubing 15 ft. in length and perforated every 8 in. All water 

 was added through the small can (C) and distributed uniformly throughout 

 the soil by means of the perforated coil. . . . 



" Five different degrees of soil saturation were maintained throughout the 

 growing season. This made 5 sets of 4 plants each, varying from 35 to 100 per 

 cent saturated soil. Saturation was regarded as the amount of water retained 

 by the soil after drainage ceased from water poured on the soil surface. It 

 required 100 lbs. of water to saturate the 260 lbs. moisture-free soil in each 

 potometer. The 5 degrees saturation maintained were 100 per cent, 80 per 

 cent, 60 per cent, 45 per cent, and 35 per cent. . . . 



" The daily transpiration was determined by loss in weight. The soil satura- 

 tions were kept fairly constant by restoring, each evening, the exact quantity of 

 water transpired during the day. When the loss was likely to be large, water 

 was added twice each day. 



" In connection with these transpiration determinations accurate records were 

 obtained in the immediate vicinity by means of standard self-recording in- 

 struments of those climatic factors which were likely to affect the rate of 

 water loss. 



" The daily evaporation rate from a free-water surface was obtained by 

 averaging the losses in weight from six 1-gal. glazed stone jars in which the 

 water level was daily restored to 1 in. below the top of the jar. These jars 

 were placed at different altitudes, ranging uniformly from the ground up to 10 

 ft. above the ground. By taking the average evaporation from free-water sur- 

 faces distributed in this manner, data were obtained which may be compared 

 with the transpiration from corn plants, the leaves of which vary in their 

 height above ground. 



" In one experiment the evaporation rate was obtained by averaging the 

 water loss from 5 Livingston porous clay cup evaporimeters. 



" Comparative data were also secured as to the wind velocity in a cornfield, 

 at altitudes of 4 ft. and 10 ft. above ground." 



It was found that " the optimum soil saturation for growth was from 60 to 80 

 per cent. The percentage of ear increased as saturation decreased down to 45 

 per cent, though the largest actual weight of ear was produced at 60 per cent 

 and the greatest total dry weight at SO per cent. 



" Least water per gram dry weight was used in 45 to 60 per cent saturation. 

 The percentage of leaf area to dry weight was least under the most nearly 

 optimum conditions. 



" When the hourly fluctuation in evaporation of free water was compared 

 with the transpiration of a corn plant, they were found to fluctuate in almost 

 perfect accord, the transpiration tending neither to lag uor to accelerate. This 

 indicates that ' transpiration ' is essentially ' evaporation.' 



" Transpiration records for daily or weekly periods gave similar results up 

 until the latter part of the season, when plants began to ripen. Then evapora- 

 tion exceeded transpiration. 



" The transpiration for the 12 hours of day was about 13 times greater than 

 for the 12 hours of night." 



Correlation studies of corn, E. G. Montgomery (Nebraska Sta. Rpt. 1910, 

 pp. 108-159, figs. //). — The object of this investigation was to secure data on (1) 



