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, G5 to GOO meters (200 to 1,9(K) feet). 

 Humid Transition, to 1.200 meters (0 to 3.8(X) feet). 

 Arid Transition, 500 to 1,3(X) meters (1,000 to 4,200 feet). 

 Canadian, 400 to 1,500 meters (1,400 to 5,000 feet). 

 Iludsonian, 1.500 to 2.100 meters (5,000 to 7.000 feet). 

 Arctic, 1,800 to 3,200 meters (G.OOO 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 aljout 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 3G0 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 conspicnons plant of this zone is the 

 sagebrush (Artemisia tridentata) (PI. 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. viscidiflonis) , hop sage {Grayia spinosa), antelope brush 

 {Knnzia tridentata^ locally known as hlach sage), and, in alkaline 

 situations, greasewood (Sarcohatus verwAculatus). 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 30 

 species are confined in their distribution to the Columbian Basin of 

 Washington, Oregon, and Idaho, several of them being quite rare 

 and local. 



