THE PRAIRIE-PLAINS GRASSLAND FORMATION. 15 



Winner in South Dakota, and Long Pine and McCook in Nebraska. Along the 

 west it occurs from near Calgary, Alberta, southward to Lewiston and Billings, 

 Montana, Douglas and Laramie, Wyoming, and Colorado Springs and Trini- 

 dad, Colorado. Beyond the eastern limit, Bouteloua and Bulbilis merely per- 

 sist as alternes in xerophytic situations in the midst of the prairie." (Cf. 

 Pound and Clements, 1900; Shantz, 1906, 1911, 1917; Pool, 1914; Weaver, 

 1919; and Clements, 1920.) 



In concluding this summary statement of the grassland associations, 

 it may be well to point out that while the major factor is water-content, 

 precipitation and evaporation are so greatly modified by edaphic con- 

 ditions that two types of vegetation are often found in juxtaposition. 

 Thus at Limon, Colorado, native and crop plants in adjacent fields 

 have been excavated, one lot from typical short-grass land, the other 

 in sandy soil in typical mixed prairie. Even in the midst of the prairie 

 region, at Lincoln, Nebraska, an epitome of decreasing rainfall and con- 

 sequent mixed prairie and short-grass plains vegetation may be found 

 on a single hillside (Weaver, 1919). The base is clothed with tall 

 prairie grasses and herbs, while the gravelly crest, which is of glacial 

 origin, is covered with a nearly pure growth of Bouteloua gracilis and 

 B. hirsuta. On the upper slopes the short-grass layer is overtopped by 

 Koderia cristata, Stipa spartea, and Andropogon scoparius. Here, as 

 everywhere throughout the formation, root relations are important 

 and root competition plays a decisive role. 



In "Ecological Relations of Roots" it has been pointed out that 

 every plant association has a rather definite community root habit. 

 The peculiar set of edaphic conditions of an association, especially its 

 water-content, leaves its impress upon root distribution. Roots of 

 true prairie show distinct differences in habit from those of the plains, 

 and these in turn are unlike those of sandhills. In fact, the same 

 species when grown in the different associations usually takes on the 

 root habit best adapted to meet its needs in that particular environ- 

 ment. Following the description of the root habit of the individuals 

 of each association there is a summary statement of the root habit of 

 the plants as a group. Finally, the correlations between root habit 

 and factors of the environment are pointed out. 



