NATIVE WOODY PLANTS OF THE UNITED STATES. 21 
in between the lines expressing 1 and 10 will be places where the 
average may be 2, 3, 4, or any number between 1 and 10. 
With this explanation, the climatic characters of each region may 
now be considered very briefly, bearing in mind that the values given 
are averages, not absolute figures. Study of the two maps (figs. 
2 and 3, in the pocket) may assist in a better understanding of the 
climate and soil in any given plant growth region. In figure 2 the 
climatic regions of Thornthwaite (in color) are superimposed on 
Mulford’s plant-growth regions (hachured). In figure 3 the impor- 
tant soil regions (in color) are superimposed upon Mulford’s plant- 
growth regions (hachured). 
CLIMATES OF GROWTH REGIONS 
REGION 1, NORTH PACIFIC COAST 
Climate wet, microthermal, with adequate precipitation at all sea- 
sons in all portions, except for two areas with a summer deficiency, 
one extending for 100 miles south of the Oregon-California line, the 
other extending the same distance north from San Francisco Bay. 
Between these two areas is a mesothermal area of some 150 miles in 
north-south extent. Fogs are frequent and winter rainfall is heavy. 
The lowest temperatures are 22° to 24° F. in the north, to about 
32° in the south. 
The average annual growing season is from 210 days to less than 
90 days in the higher northern portions and more than 240 days in 
the extreme south. The average annual number of days with snow 
cover ranges from 1 to 10 over most of the region, increasing to as 
many as 60 in the central portion. 
REGION 2, WILLAMETTE VALLEY-PUGET SOUND 
A humid, microtherma! climate throughout, with adequate precipi- 
tation at all seasons in the north, but with a summer deficiency 
south of the Columbia River. Summers warmer and drier than 
those of region 1. Commonest low temperatures range from 10° 
to 20° F. 
The average growing season is 210 days in the south and 150 days 
in the north. Average annual number of days with snow cover 
ranges from 1 to 10. 
REGION 3, CENTRAL CALIFORNIA VALLEYS 
A subhumid, mesothermal climate throughout except for the semi- 
arid to arid southwestern third. Precipitation is deficient in the 
summer through the northern two-thirds but deficient at all seasons 
in the semiarid to arid third. Winters mild; lowest temperature 22° 
to 24° F., with slightly higher temperatures on the hillsides. 
Average annual frost-free season more than 240 days. Number 
of days with snow cover less than 1 annually. 
REGION 4, CASCADE-SIERRA NEVADA 
A mountainous region where conditions vary with elevation and 
slope, the eastern side being drier than the western side. South- 
ward the eastern side is decidedly arid; northward less so. The 
