494 PLANT GEOGRAPHY 



Most notable among the saline areas is the Great Basin, west 

 of the Great Salt Lake, a dreary region in general, destitute of 

 natural grass lands or trees, but with a scattered vegetation of 

 low gray or dull green shrubs and herbs. In the lower highly 

 alkaline valleys are found such halophytic species as those 

 above-named, while the drier valleys and foothills are somewhat 

 evenly covered with sagebrush. 



In the South, cactuses, palms, and tree yuccas abound. Wher- 

 ever the soil is gravelly throughout the southern arid region, up 

 to an elevation of five thousand feet or somewhat more, the 

 creosote bush (Larrea tridentata) is often as exclusive in its 

 occupancy of the ground as the sagebrush is in the central 

 and northern parts of the Great Basin. 



Here are some of the most notable arid regions of the United 

 States, such as the Mohave Desert, the Ralston Desert, and the 

 Colorado Desert of southern California. The intense dryness of 

 such areas may be understood from the fact that the average 

 rainfall of ten of these deserts is only five inches a year, and the 

 temperature in one of them (at Fort Yuma, Arizona) remains 

 for weeks as high as 118 during the day, with sometimes only 

 a little over one half inch of rain a year. 



471. The Pacific slope. The Pacific coast region offers far 

 less marked contrasts between the summer and winter temper- 

 ature than are found along the Atlantic coast. 



On the other hand, there is, in the southern portion of the 

 region, a sharply defined division of the year into a dry and a 

 rainy season. At San Diego the dry season begins with April 

 and lasts for seven months. The development of vegetation, 

 therefore, as in the northerly part of the plains region east of the 

 Eocky Mountains, is most rapid in spring and largely ceases 

 when the soil has become parched by the summer's heat. 



The flora of the Pacific slope is best known by its extraordi- 

 nary coniferous evergreen trees. In the moss-carpeted woods of 

 the northern portion (bounded on the south by the forty-first par- 

 allel) are found the Port Orford cedar (Cupressus Lawsoniana), 



