34 THE VEGETATION OF THE UNITED STATES. 



occurs in very open stand with a height of a few inches or at other 

 times grows so thickly as nearly to cover the ground, reaching a height 

 of 4 feet or more. Together with the sagebrush are to be found several 

 other microphyllous shrubs of similar size and distribution (Sarco- 

 batus vermiculatus, Grayia spinosa, Coleogyne ramosissima, Kunzia 

 tridentata, Tetradymia glahrata, etc.). The Great Basin desert is dis- 

 tinctively a region of microphyllous shrubs, in which succulent plants 

 are rare and confined to the highest mountain-slopes. The simplicity 

 of the vegetation and the uniformity in the character of the several 

 shrubs which play a secondary role in it are features very unlike the 

 other desert areas about to be described. " 



Texas Succulent Desert. — The central valley of the Rio Grande and 

 the valley of the Pecos River form a desert area in which extensive 

 tracts are dominated by evergreen shrubs in open stand, while other 

 areas are chiefly occupied by deciduous shrubbery {Acacia, Flour ensia, 

 Brayodendron) . Large areas are covered by the low leaf -succulent 

 lechuguilla {Agave lechuguilla) , but not to the exclusion of the shrub- 

 bery. Other areas are dominated more conspicuously by the sotol 

 {Dasylirion texanum), a plant with perennial foliage and with a store 

 of water and reserve materials in its stout stem. Succulents are 

 abundantly represented in this desert, but chiefly by small species 

 which do not play an important part in the physiognomy of the vege- 

 tation. Perennial bunch-grasses are common in certain portions of 

 the Texas Desert, and the number of plants perennial by underground 

 succulent or semisucculent parts is larger than in the Arizona Desert. 

 This is distinctively a region of leaf-succulents and semisucculents as 

 contrasted with the Arizona region of stem-succulents. 



Arizona Succulent Desert.— This area comprises the southern por- 

 tion of Arizona drained by the Bill Williams and Gila Rivers and 

 Ijdng below 4,000 feet. In this desert the vegetation is largely made 

 up of microphyllous shrubs, but there is everywhere a rich commingling 

 of other types of non-succulent plants and of several types of succu- 

 lents. The vegetation is open and low, but of irregular height. The 

 sclerophyllous shrubs comprise the evergreen creosote-bush {Covillea 

 tridentata), deciduous acacias {Acacia paucispina, A. constricta), and 

 bitter bush {Flourensia cernua), the drought-deciduous ocotillo {Fou- 

 quieria splendens), several small-leaved or leafless trees and shrubs with 

 green bark and stems {Parkinsonia, Canotia, Holacantha, Koeberlinia, 

 Ephedra), as well as the columnar giant cactus {Carnegiea gigantea) 

 and numerous species of flat-jointed and round-jointed cacti {Opuntia). 

 This desert is by no means poor in perennial grasses, and the seasonal 

 rains are followed by the appearance of carpets of annual grasses and 

 other herbaceous plants. The leafy succulents are rare in this desert, 

 except at its upper edges around the higher mountains, and the plants 

 with subterranean water-storing organs are very infrequent. 



