DESERTS AND DESERT FLORA OF THE WEST 

 Transition and Upper S9noran zones of the Great 

 Basin. The shrub is readily recognized by its small, 

 bright green, wax-coated leaves and heavy medic- 

 inal odor. In rocky or gravelly situations, wherever 

 the soil is slightly alkaline, the desert holly(A/np/ea; 

 hymenelytra) may be found. This is an attractive 

 shrub with the leaves toothed as the true holly but 

 silvery white in color. 



The yuccas occupy a belt between the creosote 

 and the junipers of the mountains. The tree yucca, 

 Yucca (Cleistoyucca) arborescens, locally known as 

 the Joshua tree, is the most striking feature of the 

 desert. These weird and fantastic trees grow in 

 open forests that suggest great orchards as one looks 

 down upon them from the mountain slopes. Young 

 trees have tall erect simple trunks, densely clothed 

 with stiff sword-like leaves. (See PL XXI.) Older 

 trees are branched into rounded heads, and are often 

 thirty feet in height. A comprehensive account of the 

 distribution of the yuccas of the Mohave Desert was 

 given by Dr. G. H. Merriam as one of the results of 

 that important biological survey known as the 

 "Death Valley Expedition." 



About twenty-five to thirty species of cacti are 

 found in the Mohave Desert. Most of these belong 

 to the genus Opuntia, but there are also species of 

 Cereus, Echinocactus and Mamillaria. Opuntia 

 echinocarpa is the common arborescent cactus and 

 is widely distributed. Its branches are cylindrical 

 and densely clothed with light-colored spines, and 

 its flowers are greenish yellow. 'Opuntia basilaris 

 is another common species. This is a platopuntia 

 type, and is easily distinguished by the scantiness 

 or absence of spines. Its flowers are a deep purple 

 red. 



Flora of the Desert Mountains. Certain desert 

 ranges lying between the southern Sierra Nevada 

 and the Colorado River push up into cold Boreal 

 climates humid enough to support coniferous for- 

 ests. The Charleston Mountains of southern Ne- 

 vada, for example, rise to nearly 12,000 feet. On 

 their slopes are found all the extra-tropical life zones 

 of North America, from the Lower Sonqran at the 

 base to the Arctic-Alpine above timberline on the 

 highest peaks. Ascending one of these ranges, the 

 floral belts above the typical desert are encountered 

 in the following succession: 



The juniper-pinon belt, or Upper Sonoran life 

 zone, extends from about 6000 to 7500 feet. The 

 juniper (Juniperus utahensis) is principally con- 

 fined to the lower half of the belt, appearing 200 to 

 300 feet lower down than the pinon (Pinus mono- 



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