768 TEXTBOOK OF BOTANY 



At this lower surface calcium carbonate and other less soluble salts ac- 

 cumulate and form a hard layer. The water in the moist layer is lost by 

 evaporation from soil and plants each growing season, and there is 

 rarely a surplus from one year to the next. The deeper soil is perma- 

 nently dry. During a period of wet years taller grasses appear on the 

 short grass sod, and during dry years the short grasses form an inter- 

 rupted cover. The buffalo grass is most abundant from Colorado south- 

 ward, but the grama grass continues far to the north. Toward the 

 mountains sagebrush (Artemisia) intermingles with the grasses and in 

 Wyoming and Montana it dominates in scattered areas. 



This is grazing land and farming is perilous. During a succession of wet 

 years, however, profitable crops of hard wheat have been harvested; but 

 in succeeding dry years there have been crop failures, accompanied by 

 wind erosion, dust storms, and poverty. 



The desert o;rasslands extend still farther west and south from Texas 

 to Arizona and far into Mexico. These grasslands are much like the short 

 grass of the high plains, but the plants are farther apart and areas occu- 

 pied by grasses alone become smaller and more scattered. More often 

 the grasses form the ground cover under scattered desert shrubs such as 

 mesquite, creosote bush, vuccas, cacti, and scrub oaks. The rainy period 

 occurs in summer and evaporation is excessive. The growth period there- 

 fore is short and does not extend much beyond the rainv season. Curly 

 mesquite grass, black and crowfoot grama, tobosa, and "winter-fat" 

 grasses are characteristic, together with other species common to the 

 high plateaus. 



Desert shrubland (Figs. 395-396). The evergreen creosote bush char- 

 acterizes the southern part, and sagebrush the northern part, of the vast 

 arid plateaus from Idaho to central Mexico and from eastern California 

 to western Colorado. Here precipitation is alwavs insufficient for long 

 periods of growth, and the perennials that form the conspicuous vegeta- 

 tion are either those that can dry out without injurv between rainy 

 periods, or those that accumulate water in thickened stems or roots. In 

 southern Arizona and California there are two rainy periods, one in win- 

 ter which is followed by a profusion of flowering winter annuals, the 

 other in summer which is also accompanied by an abundance of summer 

 annuals. Of course in the desert there are seepage areas and rather con- 

 stant underground water in many valle\'S, where water covers the river 

 bottoms onlv after torrential rains. In such areas trees mav grow, and 

 some of the characteristic shrubs may become tree-like in size. 



