14 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



The leaves of the grasses are seldom over 2 to 8 inches in height, while 

 the flowering stalks do not usually exceed 0.5 to 2 feet. While Boute- 

 loua gracilis or Bulbilis dactyloides, or the two intermixed, constitute 

 the mass of the cover, Muhlenbergia gracillima and Bouteloua hirsuta 

 (with Hilaria jamesii southward, Clements, 1920:141) frequently 

 form consociations. Compared with the prairies, herbaceous societies 

 are not only much fewer in number, but of much less importance in the 

 vegetational structure, nor are the individuals so well developed. 

 Typical of these are Artemisia frigida, Liatris punctata, Senecio aureus, 

 Gutierrezia sarothrce, Psoralea tenuiflora, Opuntia polyacantha, Helian- 

 thus pumilus, Grindelia squarrosa, and Aragallus lambertii. (Cf. Pound 

 and Clements, 1900; Shantz, 1911; Pool, 1914; Weaver, 1919; and 

 Clements, 1920.) 



THE MIXED PRAIRIE. 



The mixed prairie (Stipa-Bouteloua association) occupies a region 

 of greater precipitation or more favorable edaphic conditions than the 

 short-grass plains. As pointed out by Clements (1920: 137), the most 

 significant difference from true prairie is the practically universal 

 presence of one or more of the short-grasses or sedges as a lower layer 

 under the taller prairie species, while the distinctive feature of the as- 

 sociation is the intimate mixing of the tall-grasses with the shorter 

 ones. Such dominants as Bouteloua gracilis, Bulbilis dactyloides, 

 Carex stenophylla, and C. filifolia form a layer beneath Stipa comata, 

 Agropyrum glaucum, Kceleria cristata, and other true prairie species 

 (plates 3, 7, and 8). Competition between the two types of vegetation 

 seems nicely adjusted. While the short-grasses have a disadvanta- 

 geous light-relation because of their height, this is counterbalanced by 

 their more favorable position as regards evaporating power of the air 

 and especially their greater ability to successfully withstand grazing. 

 Eastward, under more favorable growth conditions, the short-grass 

 layer disappears and the mixed prairie merges into true prairie; while 

 in drier areas, especially southward and westward, the gradual dis- 

 appearance of the taller grasses reveals the changed climatic or edaphic 

 conditions and gives the plant cover the impress of the plains short- 

 grass association. Thus Clements (1920:135) states as follows re- 

 garding the mixed-prairie association: 



"It is composed of the dominants of both prairie and plains, but it is essen- 

 tially prairie in its tall-grasses, numerous societies, and successional relations. 

 Mixed prairies occur from central North and South Dakota, central Nebraska, 

 and northwestern Kansas, throughout Montana and Wyoming to the Rocky 

 Mountains, and southward in Colorado along the foothills of the Front Range. 

 They extend well north into Saskatchewan and Alberta, and are assumed to 

 have covered much of northern New Mexico before the period of intensive 

 overgrazing. On the east, the association is found in more or less typical form 

 at Medicine Hat in Saskatchewan, Minot and Mandan in North Dakota, 



