36 THE VEGETATION OF THE UNITED STATES. 



and the Desert regions to the west. It is essentially a very open stand 

 of perennial grasses, together with the herbaceous annuals or peren- 

 nials of the Desert and an extremely scattered stand of succulent or 

 semisucculent plants. In the Llano Estacado and in southern New 

 Mexico and Arizona the latter group comprises sotol (Dasylirion) and 

 bear-grass (Nolina), while farther north in Texas and New Mexico the 

 commonest succulent or semisucculent forms are a yucca {Yucca 

 glauca) and a round-jointed cactus {Opuntia arhorescens) . In northern 

 Arizona and northwestern New Mexico there are low, shrubby sages 

 (Artemisia), Mormon tea (Ephedra), and other scattered bushes, and 

 small cacti (Opuntia hystricina, 0. whipplei). Throughout the Transi- 

 tion region the grasses are omnipresent, sometimes forming nearly 

 as dense a carpet as they do in the Grassland itself. There is a con- 

 siderable variety in the grass flora, but the commonest forms are 

 species of Bouteloua, Hilaria, Bulbilis, and Aristida. 



Grassland. — The Grassland region extends from central Texas to the 

 Canadian boundary, merging on the east into the transition region 

 which separates it from the Deciduous Forest, and on the west either 

 merging into the Desert-Grassland Transition or else terminating at 

 the eastern base of the Rocky Mountains. Smaller detached areas 

 of Grassland also surround the northernmost salients of the desert. 



Throughout the Grassland region the vegetation is dominated by a 

 more or less continuous cover of perennial grasses — in some locaUties 

 by a dense sod, in others by an open sod, and in still others by an open 

 stand of bunch-grasses. The types of grasses which form the grassland 

 are varied, both in the region as a whole and in any small portion of it. 

 A score of grass species form the great bulk of the vegetation, several 

 of them being of very widespread occurrence throughout the region, 

 as Bouteloua oligostachya, Bulbilis dactyloides, Koeleria cristata, and 

 species of Andropogon, while others are confined to different portions 

 of the area or to particular soils, as the species of Hilaria in Texas, 

 the species of Sporoholus and Stipa in Kansas and Nebraska, and the 

 species of Agropyron in the northwestern part of the area. In addition 

 to the score of commonest species, there are a hundred or more that are 

 either frequent over large areas or common over smaller portions of the 

 area. From a floristic standpoint the Grassland presents two grada- 

 tions, one encountered in going from the eastern edge toward the 

 Rocky Mountains, the other encountered in going from south to north 

 through its entire length of over 1,200 miles. From a vegetational 

 standpoint, however, this is all a region of great uniformity. Its prin- 

 cipal variations are in the relative density or openness of the grassy 

 cover, in the character of the areas in which grasses are sparse or absent, 

 and in the frequence of plants other than grasses. It may be said, 

 in general, that the carpet of grasses is most evenly closed along the 

 eastern edge of the area and in the central portion. In central Texas 



