The desert regions of North America lie in the arid regions of the West where annual precipitation is generally less than 250 mm. On the basis of higher plant floristic, geographic, geologic and climatic differences, four major desert regions are usually recognized: the Chihuahuan, Sonoran, Mohave and Great Basin Deserts (Jaeger, 1957;Shreve, 1951). The first three are frequently referred to as “hot deserts”, Because of their geographic position within a few degrees of 30°N, summers are generally hot with July temperatures frequently averaging over 30°C and winters are mild with the mean January temperature over 10°C. In contrast, the Great Basin Desert lies farther to the north (up to 45°N), where mean January temperatures are less than 0°C, in part because of the relatively high elevation (generally above 1000 m). As a consequence, winter precipitation frequently occurs as snow in the Great Basin Desert; whereas, snow is almost nonexistent to the south. The Great Basin Desert is underlain by sedimentary sandstones and limestones and is a desert primarily because of the rain shadow effect of the Sierra Nevada and Cascade Mountains, which lie immediately to the west. In the south the low lying Sonoran Desert (generally 0-1000 m) is underlain by a variety of volcanic rocks, such as basalts, rhyolites, dacites and granites. The relatively small Mohave Desert (generally 600-1300 m) occupies an intermediate position between the Sonoran and Great Basin Deserts and it lies within the rain shadow of the southern Sierra Nevada Mountains. The Mohave is underlain by a mixture of limestone and volcanic (chiefly granite) rocks. The Chihuahuan Desert, situated to the east of the Sonoran Desert, is a fairly high plain (1000-2000 m), principally lying between the Sierra Madre Oriental and Occidental Mountains of Mexico. Because it was previously an inland sea, the Chihuahuan Desert is largely underlain by limestone. Precipitation differences also occur among the southern deserts: winter precipitation dominates the Mohave; summer precipita tion, the Chihuahuan; and both summer and winter precipitation occur in the Sonoran. A few higher plants, such as Larrea tridentata and Atriplex confertifolia, have rather wide-spread distributions, but in general each desert is characterized by a suite of unique species, of which Carnegia gigantea and Ambrosia deltoidea are typical in the Sonoran; Ambrosia dumosa and Yucca brevifolia in the Mohave; Flourensia cernua and Prosopis juliflora var. torreyana in the Chihuahuan; and Areimesia tridentata in the Great Basin.