REGIONAL DISTRIBUTION 



507 



and rosin weeds 1 are among our largest herbaceous prairie 

 plants, reaching a height of ten or more feet and forming a 

 striliiing feature in most prairie landscapes in late summer 

 and autumn. 



In their eastern portion the treeless areas of the high plains 

 are largely covered by close mats of short xerophytic grasses 

 known as buffalo grass ^ and 

 grama grass.^ Some prickly- 

 pear cacti,^ milkweeds,^ and 

 thistles^ are also found. In 

 early July the grasses dry 

 up, and then hardly any 

 vegetation remains alive 

 aboveground except the 

 succulent cacti. 



The western portion of 

 the high treeless plauis is 

 the beginning of the foot- 

 hill region, of high, rather 

 barren table-lands, extend- 

 ing from Montana to New 

 Mexico inclusive, with an 

 altitude in many portions 

 of 5000 feet above sea level. 

 Among the most character- 

 istic woody plants of the 

 region are several wormwoods, especially the sagebrush,'^ and 

 (in " alkali " soils) the greasewood.^ The sagebrush (Fig. 

 378) is highly xerophytic, with deep roots, small leaf area, 

 and hairy surface. 



458. The Rocky Mountain region. The Rocky Mountain 

 region includes the most diverse formations, ranging from 

 coniferous forest to alpine meadow. Characteristic conifers are 

 several true spruces, the Douglas spruce,^ and a considerable 



1 SilpMum. 2 Bulbilis. ' Bouteloua. * Opwrdia. ^ Asclepias. 

 8 Cirsium. ' Artemisia trideniata. ^ Sarcobaius vermiculatus. ^ Pseudotsuga, 



Fig. 377. One-sided growth of trees 

 near timber line due to severe winds 

 from one direction. Rocky Mountains 



