DISTRIBUTION OF VEGETATION IN UNITED STATES. 37 



and in the extreme north much of the Grassland is relatively open, 

 particularly in the sandhills of Nebraska, where Andropogon scoparius 

 is predominant, and in the portions of the region lying in Washington 

 and Oregon, where Agropyron spicatum and Poa sandhergii are pre- 

 dominant. In the highly eroded "bad lands," such as occur in southern 

 South Dakota, low, shrubby perennials are predominant, as the rabbit- 

 brush {Chrysothamnus graveolens), the white ''sage" {Eurotia lanata), 

 and greasewood {Sarcobatus vermiculatus) . Throughout the portion 

 of the Grassland which lies nearest the Rocky Mountains there is a 

 complicated patchwork of vegetation, in which closed grassland, open 

 grassland, and open stands of low bushes, with or without grasses, are 

 found to alternate in habitats of different character. The shrubby 

 perennials found in such areas are chiefly those which have just been 

 mentioned, together with species of true sage {Artemisia tridentata, 

 A. frigida). The Grassland is locally invaded by plants of the types 

 which are dominant in all of the surrounding regions. In the south- 

 western portion of the area arborescent round-jointed cacti {Opuntia 

 arhorescens) are sporadic, and also yucca {Yucca glauca), which is 

 likewise common in the sandhills of Nebraska. Throughout the area 

 a low prickly pear {Opuntia missouriensis) is abundant on coarse soil, 

 particularly in the Bad Lands. 



In the vicinity of the mountains the Grassland is invaded by shrubs 

 {Cercocarpus, Quercus, Symphoricarpus) , and in some localities even by 

 coniferous trees {Pinus ponderosa), while the bottomlands of the 

 rivers are the westernmost localities for many eastern broad-leaved 

 deciduous trees. 



In every portion of the Grassland there are to be found very many 

 types of low annual or root-perennial plants other than grasses. Among 

 these certain composites are perhaps the most abundant, as Grindelia 

 squarrosa and Chrysopsis villosa, although there are many plants of 

 other types. The seasonal habits of the grasses and of these associated 

 non-gramineous plants are such as to give rise to one of the most 

 striking characteristics of the Grassland, namely, its different aspect 

 in different portions of the growing-season, and the difference in the 

 conspicuously predominant plants in different months. 



Grassland-Deciduous Forest Transition. — This is the broadest and 

 most extensive of the transition areas, but is so purely transitional 

 in its character that it does not merit recognition on any other basis. 

 Its eastern limit has been fixed along the line at which the Deciduous 

 Forest ceases to be an unbroken formation and begins to exhibit the 

 islands of grassland locally known as "oak openings." The western 

 limit has been placed where timber ceases to occur on the upland. 

 The transition area is seen, therefore, to increase in the amount of 

 timber found on going east and to increase in the extent of grassland 

 exhibited on going west. 



