ECOLOGY 143 



of the surface is covered by a sod. Stipas and various 

 Andropogonese, especially Andropogon, are often dominant 

 species. Over much of the Great Plains, a single species, 

 Bulbilis dadyloides (buffalo-grass) or this combined with 

 Bouteloua gracilis (grama -grass) gives a characteristic 

 aspect to the vegetation. Those grasses often called 

 "short grasses" may occupy vast areas almost to the 

 exclusion of other species of plants. Farther south, the 

 Bulbilis is replaced by Hilaria cenchroides (curly mes- 

 quite). This portion of the Great Plains is known locally 

 as the ''short-grass country" because the uniform com- 

 pact curly growth is only a few inches high. The regions 

 described above are known as semi-arid regions. 



180. Sandy soil. — Plants characteristic of sandy soil 

 are sometimes called psammophytes. The best illustration 

 of this kind of xerophytes may be observed upon sand- 

 dunes. These are found along sandy seacoasts of temper- 

 ate regions, the sandy shores of lakes, along the banks of 

 rivers, especially in arid regions, and in dry interior regions 

 far removed from bodies of water. Such areas are found 

 in the United States along the Atlantic seaboard, especi- 

 ally on Cape Cod, along the Great Lakes, especially the 

 eastern and southern shore of Lake Michigan, and along 

 certain large rivers, such as the Columbia east of the 

 Cascades, and the Arkansas in western Kansas. Large 

 areas of sand hills are found in interior regions such as 

 central Nebraska. 



181. Sand-dunes may be so far removed from water 

 or in such rapid motion that no vegetation can be sup- 

 ported. Dunes near the sea, though completely dried 

 out at the surface may be moist beneath on account of 

 the drawing of water from below by capillary attraction. 

 Many grasses of sand-dunes produce a well-developed 



