Southeastern Washington and Adjacent Idaho. 111 
form in itself an intensely interesting and very practical field of 
investigation. 
CONCLUSION 
In the foregoing pages it has been shown that in this region 
lying in southeastern Washington and adjacent Idaho, three plant 
formations occur. In the western part of the region, that of least 
elevation and consequently of lowest precipitation, the desert 
scrub formation occurs. As the altitude becomes greater and the 
climatic conditions more favorable eastward, the desert scrub 
gives way to the grass land of the prairie-plains formation. In 
its early development this grass land is of a very open bunch-grass 
type, with much of the ground being either destitute of vegeta- 
tion or carpeted with interstitial plants only in early spring. As 
conditions become less xerophytic at higher altitudes the ground 
becomes rather completely occupied by grasses, some of which 
are sod-formers, and by non-grassy herbaceous vegetation which 
shows a great increase both in the number of species and in the 
number of individuals per unit area. The best developed prairies 
reveal a potential shrub community which, while held in check 
usually by competition with prairie plants, develops in the eastern 
edge of the grass lands and especially upon the non-basaltic soils 
into thickets of chaparral. Under cover of the chaparral, the 
yellow pine, a pioneer community in the development of the 
Pacific Coast forests, often becomes established. The pine may 
be replaced by Douglas fir and tamarack, and these in turn by 
mixed communities dominated by tamarack and white fir, or by 
white fir and cedar. Finally, in the most mesophytic areas the 
cedar community reveals the climax stage in the development of 
the Pacific Coast forest formation. 
It has been shown that the desert scrub formation is charac- 
terized by soils with lower water content and higher temperature, 
and with higher evaporating power of the air than any of the 
other plant communities. Likewise, there has also been found to 
exist in the several plant communities of the xerosere, from the 
bunch-grass community of the prairies to the climax cedar com- 
munity of the forest, a direct relation between the water content 
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