38 The Botanical Survey of Nebraska 



An examination of these data shows that plants at Peru grow 

 under a much more favorable condition of soil moisture. 



It has already been pointed out that below a depth of 12 inches 

 the soil of the linden forest always had water available for plant 

 growth. That at the Salt Creek station had no available moisture 

 at depths of 1-4 feet on April 2 and again at 2 feet on July 30 

 and August 6 respectively. Such differences in the distribution 

 of soil moisture coupled with corresponding differences in evap- 

 oration readily account for the variations in the structure of the 

 vegetation. 



EVAPORATION 



The evaporating power of the air at the different stations was 

 measured from June i to September 22, 191 7. The atmometers 

 were placed in duplicate at the various stations and at a height of 

 about 17 cm. above the soil surface. The data are shown in the 

 form of graphs in figure 14. 



The marked parallelism among the graphs is no more striking 

 than the differences in the average daily evaporation, the latter 

 always being least in the forest and highest in the prairie com- 

 munity. 



Wind movement exerts a profound effect in accelerating trans- 

 piration and evaporation. During the interval from June 29 to 

 July 28, 3,900 miles of wind, as measured by a standard anemom- 

 eter, passed over the prairie vegetation at a height of one-half 

 meter; while only about 6 per cent, as much (250 miles) was 

 recorded by an anemometer similarly placed in the scrub. Again 

 from July 28 to September 22, 4,077 miles of wind were recorded 

 by the instrument in the prairie and only 666 miles (16 per cent.) 

 in the thicket. 



A comparison of the average daily evaporating power of the air 

 obtained from the readings of the atmometers at Peru, which 

 were not fitted with the rain correcting device, and similar in- 

 struments at Lincoln operated beside the rain correctors, gives 

 the following comparison : in the prairie at Lincoln the average 

 daily evaporation from June 4 to August 27 was 19.5 c.c. At 

 Peru it was about 2 c.c. less, i.e., 17.6 c.c. This reduction in the 



44 



