144 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



growth of many species of tall, sod-forming grasses and numerous 

 societies of tall herbs. This association constitutes the portion 

 of the climax grassland with greatest rainfall and merges into the 

 subclimax prairie eastward. In central Kansas and Nebraska it gives 

 way to mixed prairie. Short-grass plains (Bulbilis-Bouteloua associa- 

 tion) are covered with a sparser growth of a few sod-forming short- 

 grasses and fewer herbaceous societies. This association constitutes 

 that portion of the formation with the smallest precipitation, greatest 

 run-off, and highest rate of evaporation. Its typical expression may be 

 seen in western Kansas and eastern Colorado. With an increase in 

 rainfall or with more favorable edaphic conditions it gives way to mixed 

 prairie. Mixed prairie (Stipa-Bouleloua association) differs from true 

 prairie in the practically universal presence of one or more of the short- 

 grasses or sedges as a lower layer under the taller prairie species. Here 

 tall-grasses are intimately mixed with the shorter ones. The pre- 

 cipitation over this association is intermediate between that of short- 

 grass plains and true prairie. Mixed prairie forms a distinct belt of 

 vegetation in central Nebraska and Kansas between true prairie and 

 short-grass plains; it recurs along the mountain front in Colorado, and 

 often as alternes governed by edaphic conditions, especially in short- 

 grass plains (Clements, 1920). 



The root systems of 43 species of plants have been examined in the 

 true prairie of eastern Nebraska; 17 of these were grasses. All of the 

 dicotyledonous plants were found to extend well beyond a depth of 2 

 feet, while the roots of 6 grasses were confined to the surface 2 feet of soil. 

 Four of the grasses and 5 other species were found to penetrate well 

 below the second foot of soil, but seldom deeper than 5 feet. The re- 

 maining species, including 7 grasses, 4 of which are dominants, have 

 roots which reach depths greater than 5 feet — in fact, most of them 

 penetrate to distances of 7 to 9 feet and a few to a maximum depth of 

 13 to 20 feet. Thus, 14 per cent of the typical prairie species examined 

 were shallow-rooted, 21 per cent had roots of intermediate depth, while 

 65 per cent were very deeply rooted. The roots of prairie species are 

 grouped into about three more or less definite absorbing layers, many of 

 the deeper-rooted species having few or no absorbing roots in the first few 

 feet of soil. The layering of the roots reduces competition, and, since 

 available moisture is present throughout the subsoil, permits the growth 

 of a larger number of species. However, no relation between layering 

 and seasonal activity is apparent. The period of most active growth and 

 flower production of plants rooted at various levels occurs synchronously, 

 and, like the successive development of aerial parts of different species 

 in time and space above ground (which results in seasonal aspects), 

 probably equalizes the demands upon the habitat, distributing them 

 throughout the growing-season. True-prairie species as a community 

 emphasize depth of penetration and widely spreading, deep laterals. 



