ROOT SYSTEMS OF TRUE-PRAIRIE SPECIES. 



31 



An examination of table 2 shows that water in excess of the hygro- 

 scopic coefficient was present, with occasional exceptions in the surface 

 soil, at all depths at both stations. With the exception of a single 

 determination, this excess of water was much greater at the true- 

 prairie station at all depths below 1 foot, while in the surface soil it 

 was often much greater than at the mixed-prairie station. In this 

 comparison it should be kept in mind that the water-content data 

 were obtained from an upland station and are much lower throughout 

 than in lowland-prairie sites (cf. table 3). It is interesting to note 

 that at the mixed-prairie station the soil moisture at all depths fell 

 below the wilting coefficient, while even at Lincoln this condition ob- 

 tained to a depth of 3 feet. 



A study of water relations of plants must take into consideration 

 not only the available supply but the rapidity with which this is ex- 

 pended. The evaporating power of the air is the environmental factor 

 chiefly controlling the latter. During the growing-season of 1919, 

 synchronous readings of the evaporation from Livingston's non- 

 absorbing, white, cylindrical, porous-cup atmometers were made at the 

 true-prairie and mixed-prairie stations. These were operated in the 

 usual manner in duplicate, the evaporating surface extending from 2 to 

 5 inches above the soil surface. The readings are reduced to those of 

 the standard cup. These data are shown in figure 7, where the read- 



er 



45 

 40 

 35 

 30 

 Z5 

 20 

 \5 



IO 



Fig. 7. — Graphs showing the average daily evaporation rates 

 in the mixed prairie (upper line) and true prairie 

 (lower line) during 1919. 



ings from the atmometers on low and high prairie are averaged so as to 

 represent a mean between the two prairie types. The much higher 

 evaporation rate at the mixed-prairie station, where in fact it is often 

 more than twice that in the true prairie, is clearly evident, and is an 

 exceedingly important environmental difference in the two grassland 

 habitats. That higher evaporating power of the air in the mixed 

 prairie is the rule rather than the exception is further indicated in 



