VEGETATION OF THE PRAIRIE. 559 



cristata, Eatonia obtusata, Panicum scribnerianum ; 2d, the 

 " bunch "-grasses, like buffalo-grass (Bulbilis dactyloides), beard- 

 grasses or broom-sedge (Andropogon furcatus and scoparius), 

 grama-grass (Bouteloua oligostachya), etc. These two types are 

 more or less intermixed, but the "sod-formers" ( = " prairie-grass 

 formation") are especially characteristic of the lower prairie 

 areas, while the bunch-grasses ( = " buffalo-grass formation") are 

 especially characteristic grasses of the arid region or steppes 

 farther west and southwest. They make a close formation, but 

 in the drier regions tend more and more to an open formation 

 (see paragraph 1049). 



III. The Plains Formations. 



1046. The Great Plains of the United States. The Great 

 Plains, as the arid region of the United States is sometimes 

 called, extend from about the looth meridian to the eastern foot- 

 hills of the Rocky Mountains, and they form a transition area 

 from the prairie or meadow region to the desert area between the 

 Sierra Nevada and western slope of the Rockies, the dryness of 

 the area increasing in intensity toward the southwest. The 

 prevailing grasses in this arid area are the bunch-grasses (buffalo- 

 grass, beard-grass, Indian grass, etc.), which in the typical prairie 

 usually grow in close formation, but here, while often forming 

 meadow-like expanses, grow in more open formation. The other 

 prevailing characteristic plants are the sage-brush (Artemisia 

 tridentata), low, compact, tufted shrubs, grayish in color. The 

 prickly-pear cactus (Opuntia) is also characteristic of the area, 

 but is subordinate and less common, though it is the dominant 

 plant over extensive areas. Reaching the Lower Sonoran area 

 of the Great Plains there is a gradual change of the flora as the 

 climate becomes hotter and drier in western Texas and New 

 Mexico, where a semi-desert area (see paragraph 10540) exists 

 and extends into the high table-lands of Mexico, characterized 

 by giant cacti, forming the so-called cactus deserts. Thus there 

 are cactus societies (or cactus formations). Opuntia is common, 



