igig] VESTAL— PIIYTOGEOGRAPIIY OF COLORADO 180 



grassland in the debris-covered soil of the mesas and outwash-plains 

 of the mountain-front (short-grass mixed association). The foot- 

 hills mixed grassland, very similar to the mixed short-grass, is 

 typical over the coarse surface of the granitic foothills. 



Wheat-grass. — This taller but also shallow-rooted grass, Agro- 

 pyron Smithii, dominates areas of loose clay in the mountain-front 

 and plains. Its ecological character is not well understood. 



Stipa-Aristida association. — These tufted xerophytic grasses of 

 coarse soil occur frequently but not extensively, together or singly, 

 with other rather deep-rooted plains xerophytes. 



Bunch-grass. — Tufted perennial and deep-rooted grasses, 

 depending on continued moisture, such as Andropogon sco partus, 

 A. furcatus, Sorghastrum nutans, etc., are notable through most 

 of the prairie region, almost absent in dry plains, but abundant in 

 the rain belt of eastern Colorado; at the mountain-front and in 

 the foothills, scatteringly in the north, but very frequent at the 

 Platte-Arkansas divide and southward into New Mexico. 'In 

 the foothills there are grasses of similar habit which mingle with 

 the prairie bunch-grasses. 



Mesophytic grasslands. — Mesophytic herbaceous growths are 

 made up partly of prairie plants and partly of Rocky Mountain 

 forest plants. The latter element is very considerable in occasional 

 foothill ravines. Meadow growths of both foothills and mountain- 

 front, in moist soil, with showy flowering plants like Delphinium, 

 Cerastium, Castilleja, Orthocarpus, etc., are conspicuous in early 

 summer, but not very frequent. The mountain-front in many 

 places shows a mixed grassland much like that of eastern prairie, 

 which has been called western prairie-grass. It has plants of the 

 mixed short-grass, with components from bunch-grass and eastern 

 prairie or forest border, with some foothills mesophytes, and a few 

 plants characteristic of the mountain-front, like Stipa viridula. 



Primitive grasslands. — Early stages of grassland developing 

 in areas recently bared, or remaining for long in loose shifting 

 slopes, are frequently seen. Prostrate plants with heavy taproots 

 (rosette plants) are common. Gravel-slides in the foothills and 

 dry stony crests of mesas, buttes, and ridges in the plains and 

 mountain-front are the typical habitats. The Bouteloua hirsuta 



