ECOLOGY 143 
of the surface is covered by a sod. Stipas and various 
Andropogonee, especially Andropogon, are often dominant 
species. Over much of the Great Plains, a single species, 
Bulbilis dactyloides (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 
