122 



OUTLINE OF PLANT GEOGRAPHY 



The arid climate of the Great Basin is due largely to its almost 

 complete encirclement by lofty mountains. The western Cordil- 

 lera forms an effective barrier against the moist Pacific winds, 

 and the southern portion is open to the hot winds from the Mexican 

 plateau. Except for the Columbia and its tributaries, which 

 traverse the northern parts of the basin, there are no streams of 

 importance within the area, and these are often lost in the desert 



sands, or as in the case of the 

 Jordan, flow into saline lakes. 

 Parts of the region, like that 

 adjacent to the Great Salt 

 Lake, are deserts of the most 

 pronounced type, quite desti- 

 tute of vegetation; or the arid, 

 often alkaline soils support 

 only a thin growth of such 

 plants as sage-brush (Arte- 

 misia), and grease- wood (Sar- 

 cobatus) and similar shrubs 

 capable of enduring long 

 periods of drought. 



This region is traversed by 

 the Union Pacific railway, and 

 between Cheyenne and Reno 

 one has an opportunity of see- 

 ing the most characteristic por- 

 tions of this Great American 

 desert. 



From an elevation of over 

 8,000 feet at the continental 

 divide there is a descent of about 4,000 feet into Utah and Nevada. 

 The bunch-grass prairie of Wyoming passes gradually into a dry 

 steppe with thinly scattered bunch-grasses interspersed with sage- 

 brush and other xerophytic growths. The rocky hillsides support 

 a few scrubby junipers and firs, and various low shrubs and herba- 

 ceous plants in the sheltered hollows among the rocks; but these 

 mostly disappear in the lower parts of the basin. Where there 

 are streams, the banks are lined with willows and cotton-woods, as 

 well as a number of deciduous shrubs, currants, elder, roses, 



Fig. 31. — Sub-alpine meadow, Glacier 

 National Park. 



