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vi The Arctic-Alpine Zone occupies the high ahitudes of the mountains alcove timber-line. On Mt. Rainier,. Washington, timber-hne is about 7,500 feet altitude; on Mt. Shasta, northern California, about 9,500 feet, while on Mt. San Gorgonio, southern California, which is 11,425 feet in elevation, trees extend to the summit, although a few Arctic-Alpine plants are present. The zone is characterized by the absence of trees, and the dwarf (often matted) habit of the shrubs and perennial herbs. Among the characteristic species are: Festuca alpina, Cares hrczveri, Salix nivalis, Oxyria didyna, Ranunculns eschscholtcii, Draba brewcri, Sineloz^'skia calycina, Saxifraga toluiiei. Liitkea pectinafa, and Dryas octopctala. The Hudsonian and Canadian Zones include the subalpine forests, and are not always readily separable, especially in the Sierra Nevada where the former is poorly developed. The Hudsonian is the uppermost timber zone extending from timber-line to 2,000 or 3,000 feet below. Characteristic trees are: Finns albicaulis, Finns balfouriana, Abies lasiocarpa, Chaiuaeeyparis nootkatensis, and Tsnga inertensiana. The Canadian Zone comprises the lower portion of the subalpine forests. In the Cascades of Washington it is usually between 3,000 and 5,000 feet altitude, and in the central Sierra Nevada between 6,500 and 9.000 feet. Among the char- acteristic trees are: Finns nionticola, Finns contorta nuurrayana, Finns jeffreyi. Abies aniabilis, Abies magnifiea, and Fopnlns tretnitloides. A narrow coastal strip extending from southern Alaska to northern California, in which the dominating tree is Ficca sitchensis, is to be considered as belonging to the Canadian Zone. The presence of this boreal flora is undoubtedly due to the cold, foggy summer months. The Transition Zone is represented in the Pacific States by two clearly-defined areas, the Humid Transition Area and the Arid Transition Area. The Humid Transition includes the redwood belt of northern California and the great conifer- ous forest belt of western Oregon and Washington, where the dominating tree is Fsendotsnga mucronata. The Arid Transition is the yellow pine belt {Finns ponderosa) of western North America. In Washington and northern Oregon it is restricted to the eastern side of the Cascade Divide, but in southern Oregon and California it extends to the inner Coast Ranges. The most characteristic tree is Finns ponderosa, but other typically Arid Transition trees are: Libocedrns decnr- rens, Finns lauibertiana, Abies concolor, and Qnerens kelloggii. The bunch-grass plains of eastern Washington and eastern Oregon also belong to this zone. The Upper Sonoran Zone comprises the oak and chaparral belts of the Cali- fornia foothills and valleys, and the sage-brush and pifion belts east of the Cascade- Sierra Nevada Divide. In the California Upper Sonoran the most important char- acteristic trees and shrubs are: Finns sabiniana, Qnerens lobata, Qnerens doitglasii, Qnerens dnmosa, Aeseulns californiea, Adenostoma faseicnlata, Ceanothus cnneatns, and Ccanothns divaricatus. In eastern Oregon and Washington the most conspicuous plant is the sage-brush, Artemisia tridentata. Other characteristic lignescent plants are: Eriogonnm m-icrotheca, Grayia spinosa, Knnzia tridentata, and Chrysothamnns nanscosus. On the desert slopes of the California mountains the Upper Sonoran represents the pihon-juniper belt, of which the two most con- spicuous plants are Finns nionopliylla, and Sabina californiea. The Lozcer Sonoran Zone is the typical desert flora of the Mojave and Colo- rado Deserts, but it also includes the San Joaquin ^alley and the inner coastal valleys of southern California. Among the characteristic trees and shrubs are: Yucca brevifolia, Yncca mohavensis, Fopnlus frenionti, Salix gooddi)igii. Atriplex lentiforuiis, Olneya tesota, Fonqnieria splendens, and various species of cylindrical opuntias.

Boreal Element. The Boreal Zones, — Arctic, Hudsonian, and Canadian, — have a flora essentially boreal in origin. With scarcely an exception the genera are widely distributed throughout the subarctic and cool temperate regions of the northern hemisphere. Many of the species are spread over most of northern North