GRASSES — HITCHCOCK 307 



Plains is the 100th meridian. The annual rainfall for the Plains 

 varies from about 28 inches on the east to 16 inches on the west. The 

 grasses of the Great Plains are known as " short grasses " to distin- 

 guish them from the " tall grasses " of the prairies. The dominant 

 grass over much of this region is buffalo grass {Buchloe dactyloides) ^ 

 mentioned on page 303. In Texas Buffalo grass is partially or wholly 

 replaced by curly mesquite {Hilaria helangeri) . Blue grama {Bou- 

 teloua gracilis) and black grama {B. hirsuta)^ both bunch grasses, 

 are also abundant throughout the Great Plains and are excellent 

 forage grasses. 



The Great Plains, which extend beyond the boundaries of the 

 United States, far northward into Canada, and southward to merge 

 with the Mexican Plateau, form one of the great grass regions of 

 the world. Other important comparable grasslands are to be found 

 in the llanos (Spanish for plains) of northern South America, the 

 pampas (another Spanish word for plains) of southern South Amer- 

 ica, the steppes of European and Asiatic Russia, and the high plains 

 of Africa extending from Kenya to Rhodesia. 



In the Great Basin, between the Rocky Mountains and the Sierra 

 Nevada, there are large areas with scant rainfall (mostly less than 16 

 inches annually). These areas are semiarid; or, if the rainfall is 

 very low and the temperature high, they are classed as deserts. The 

 culmination of aridity is found in the Colorado Desert of the lower 

 Colorado River Valley, and the Mohave Desert (including Death 

 Valley) lying to the north. In this desert region the rainfall is 

 mostly below 8 inches. 



The grasses of the Great Basin are mostly in widely scattered 

 bunches of highly drought-resistant species of the genera Bouteloua^ 

 Aristida, Sitanion, and Stipa. 



ALKALI GRASSES 



There are many areas, especially in the drier regions of the United 

 States, where the soil is so strongly impregnated with soluble salts 

 (alkali) that, even though moist, vegetation is more or less inhibited. 

 Only plants resistant to alkali can thrive under such conditions. 

 As the soluble salts of the soil increase, the number of species dimin- 

 ishes until a point is reached where no vegetation can exist. These 

 salt or alkali (often soda) deserts are found around salt lakes (such 

 as Great Salt Lake) or depressions that are shallow lakes after 

 rains. 



The characteristic grasses of such regions not so strongly alkaline 

 or saline as to inhibit vegetation are species of saltgrass {Distichlis) , 

 alkali-grass {Puceinellia), and alkali sacaton {Sporoholus airoides). 



111666—35 21 



