36 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
Making due allowance for the overlapping of the various zones, 
the following approximations of their altitudinal limits in Washing- 
ton may be made: 
Upper Sonoran, 65 to G00 meters (200 to 1,900 feet). 
Humid Transition, 0 to 1.200 meters (0 to 3,800 feet). 
Arid Transition, 500 to 1,300 meters (1,600 to 4,200 feet). 
Canadian, 400 to 1,500 meters (1,400 to 5,000 feet). 
Hudsonian, 1,500 to 2,100 meters (5,000 to 7,000 feet). 
Arctic, 1,800 to 3,200 meters (6.000 to 10,500 feet). 
UPPER SONORAN LIFE AREA. 
This comprises the western or arid portion of the Upper Austral 
life zone. It occupies much of the Columbia and Great basins, and 
the lower portions of the Great Plains eastward to about the one 
hundredth meridian. It also extends southward into Mexico at 
increasing elevations along both sides of the Rocky Mountain system. 
In Washington the area is confined to that. portion east of the Cascade 
Mountains below a contour line approximating 360 meters (1,200 
feet), but on southerly slopes it may extend up to 510 meters (1,700 
feet), or even more. 
From an agricultural standpoint this zone is that in which the 
commercial culture of such crops as tomatoes, peaches, apricots, and 
watermelons is possible. 
In Washington the most conspicuous plant of this zone is the 
sagebrush (Artemisia tridentata) (Pl. VI). It marks quite sharply 
the limits of the Upper Sonoran zone, seldom extending into the zone 
above, as it commonly does farther southward. Other characteristic, 
if less abundant, shrubs are rabbit brush (Chrysothamnus nauseosus 
and C. viscidiforus), hop sage (Grayia spinosa), antelope brush 
(Kunzia tridentata, locally known as black sage), and, in alkaline 
situations, greasewood (Sarcobatus vermiculatus). In a few locali- 
ties the sagebrush is absent, but in such cases one or more of the other 
characteristic shrubs is sure to be present. 
Excepting such species as are confined to the moist ground along 
perennial streams, the great majority of the Upper Sonoran plants 
are either shrubs or thick-rooted perennial herbs or short-lived 
annuals. 
There are in Washington about 500 species of plants which occur 
in this life zone. Of this number 243 species occur in no other life 
zone—that is, are distinctive. Furthermore, of this last element 39 
species are confined in their distribution to the Columbian Basin of 
Washington, Oregon, and Idaho, several of them being quite rare 
and local. 
