Page:An illustrated flora of the Pacific States (vol. 1).djvu/14

 The aim of the present work is to furnish an authentic reference book that will be of of greatest service not only to the trained botanist but to everyone interested in the native plant life of the Pacific States. It is patterned after the classical work of Britton and Brown,—"Illustrated Flora of the Northern United States and Canada."—and is the second comprehensive illustrated flora published in this country. Every species of fern, flower, tree, and shrub known to grow wild in the Pacific States is illustrated and described.

The three Pacific States—Washington, Oregon, and California—form the western boundary of the United States extending along the shore of the Pacific from 49° to 32° 35′ north latitude, a distance of 1,300 miles. They have an average width of about 250 miles, and an area of 324,123 square miles.

The outstanding physical features are the two great mountain systems, the Cascade-Sierra Nevada and the Coast Ranges, which in general parallel the coast. Of these the former is much the more prominent, and exerts the greater influence on climatic conditions, forming a remarkably distinct divide between the Pacific slope on the west and the interior or Great Basin region on the east. The Pacific slope has an oceanic climate with mild, rainy winters and cool (especially near the coast), dry summers, while east of the divide the climate is continental, with cold winters and hot summers.

Rainfall is greatest in the Northwest, diminishing southward and toward the interior. From northwestern California to the Puget Sound region the average annual rainfall is 75 to 100 inches; in the San Francisco Bay region 20 to 25 inches, and at San Diego 10 inches. East of the Cascade-Sierra Nevada Divide it is much reduced, producing semi-arid conditions in eastern Washington and Oregon, and typical desert conditions in southeastern California, where the annual rainfall is only 2 to 5 inches.

The great coniferous forests of the Northwest and the mountains of California are the most prominent floral feature. Nowhere are there more extensive forests, more majestic trees, or greater variety of cone-bearing species. In the foothills and coastal valleys of California chaparral and oaks replace the conifers, the open park-like groves of the latter contributing much to the charm of the region. Finally, forming a striking contrast to the forests, are the bunch-grass plains of eastern Washington and Oregon, the sage-brush tracts of southeastern Oregon and adjacent California, and the weird cacti and yuccas of the southern California deserts.

The usual manual method of noting geographical distribution by states is inadequate in the Pacific region, where, owing to the diversity of topography and climate, plant distribution is unusually complex. As an aid therefore in designating distribution, Merriam's system of Life Zones has been used. These zones as represented in the Pacific States are as follows: