28 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



plains it may total 30 to 43 per cent (Shantz, 1911 : 35). This run-off 

 greatly decreases the amount of water which would enter the soil and 

 become available for plant growth. 



The fertile, dark-colored soil at the prairie station is of the type 

 commonly called loess, but is more or less confounded with glacial 

 drift. The soil at the upland prairie station consists of Marshall silt- 

 loam ; at the low-prairie station the soil is of alluvial origin and is desig- 

 nated as Wabash silt-loam. At both stations the soil is very deep. 

 The water-holding capacity of the surface foot at the two stations is 

 60 and 70 per cent. The wilting coefficients are 13.5 and 16.8 per cent, 

 respectively. At the mixed-prairie station the soil consists of a light- 

 colored loam intermixed with some sand. It is so very compact that 

 it is spaded with extreme difficulty and in digging trenches for root 

 examinations a large pick was kept in constant use. At a depth of 6 

 to 10 feet it is underlaid with sand. The water-holding capacity of the 

 surface foot is 45 per cent, while the wilting coefficient is 8.2 per cent. 



Water-content data have been obtained at the mixed-prairie station 

 for two successive growing-seasons (1918 and 1919), while at Lincoln 

 they extend, although somewhat interruptedly, over a longer period of 

 years. They show that both true-prairie and mixed-prairie plants 

 grow under semiarid climatic conditions in which the supply of water 

 is the chief limiting factor of plant growth. During certain portions 

 of the growing-season extremely xerophytic conditions prevail. It 

 has been shown that the water-content of the soil is reduced to the wilt- 

 ing coefficient of Briggs and Shantz (1912) to a depth of 4 to 5 feet in 

 both habitats, at least during certain years. This occurred during 

 1918, when the daily evaporating power of the air was very high, 38 

 c. c. and 49.5 c. c. in true prairie and mixed prairie respectively. The 

 fact that the plants, including many species whose roots do not extend 

 beyond a depth of 4 to 5 feet, did not wilt 1 has led the writer to con- 

 clude that grassland vegetation can extract water from the soil beyond 

 the point indicated by the wilting coefficient. Brown (1912), working 

 in Arizona, found that plants wilted with a greater amount of water in 

 the soil when the rate of evaporation was high than when it was low. 

 Caldwell (1913), and Shive and Livingston (1914) have shown that the 

 permanent wilting of plants occurred with varying amounts of moisture 

 in the soil, depending upon the evaporating power of the air. The 

 work of Alway, McDole, and Trumbull (1919) offers convincing evi- 

 dence that plants exhaust water to a greater degree than is indicated 

 by the wilting coefficient of Briggs and Shantz . They have shown from 

 soil-moisture data obtained in the mixed prairies near McCook and 



1 An exception to this statement occurred during the summer of 1919 at the mixed-prairie 

 station at Colorado Springs. Here, during certain periods when hot winds were blowing and 

 the evaporating power of the air exceeded 45 or 50 c. c. per day, certain broad-leaved herbs 

 partially wilted during the day, but always recovered at night. 



