THE ROCKY MOUNTAIN REGION 493 



The vegetation of the prairies consists primarily of a con- 

 siderable number of vigorous sod-forming grasses intermixed 

 with many other seed plants. Notable among these are several 

 species of the pea family, many golden-rods and asters, and 

 some larger Composites, such as sunflowers and rosinweeds 

 (Silphium). Especially striking is the display in late summer 

 and autumn of many showy Composites, such as the blazing 

 star (Liatris), the cone flower (Rudbeckia), and the tickseed 

 (Coreopsis). 



The vegetation of the high treeless plains is, in the eastern 

 portion (Plate IX, lower figure), characterized mainly by the close 

 mats of the short, xerophytic buffalo grasses and grama grasses 

 of a grayish-green color. Among these grasses are scattered 

 prickly pear cactuses (Opuntia), milkweeds (Asclepias), and 

 thistles. After the drying up of the grasses in early July, there 

 is sometimes hardly any living vegetation left above ground 

 except that of the cactuses. 



Toward the Rocky Mountains, as the soil becomes more alka- 

 line, various species of wormwood or sagebrush, and members 

 of the pigweed family (Chenopodiacece) become predominant. 

 The universal sagebrush (Artemisia tridentata) plainly shows 

 its xerophytic character by its deep-reaching roots, its reduced 

 leaf area, and its strongly hairy surface. 



470. The Rocky Mountain region. The Rocky Mountain 

 region includes a very great- variety of plant formations, from 

 the heavily wooded mountain slopes and valleys to high sterile 

 plains which are almost deserts. Cone-bearing evergreen trees, 

 especially the true spruces, the "Douglas spruce" (Pseudotsuga), 

 and several pines, are very characteristic of the forests (Plate 

 XII). Great numbers of alpine species of herbs and shrubs are 

 found on the mountains at and above the timber line. In the 

 " alkali " regions, where the soil is too full of mineral salts to 

 permit ordinary plants to grow, many kinds of halophytes, such 

 as the salty sage (A triplex), the greasewood (Sarcobatus), Sali- 

 cornia, and Suceda, occur. 



