20 BOTANY OF THE DEATH VALLEY EXPEDITION. 



temperature California is divided iuto belts extending theoretically east 

 and west, but in reality bent northward or southward in deep folds by 

 variations in the altitude of different sections of the State. 



The flora of California may be likened to a checker- board, the lines 

 between the squares representing isohygromctrie and isothermal lines, 

 the former running north and south, the latter following the course of 

 the mountain ranges. The separate blocks, as they actually exist in na- 

 ture, do not present the form of squares, but are quite misshapen and 

 distorted. The chaparral belt, for example, which represents one of 

 these blocks, lies along the western slope of the Sierra Nevada in a 

 narrow strip extending from north to south quite throughout the 

 region examined by the expedition. It is not the object of this part of 

 the reportto give the exact geographic and topographic limits of these 

 blocks, but to state their relative position and to name their principal 

 characteristic plants. 



We shall consider first the region east of the Cordilleran system 

 commonly known as the desert. In every portion of it containing 

 mountains of high altitude, there is a prominent feature of the vegeta- 

 tion which marks a well-defined and almost complete change in the 

 flora. This feature is the lower limit of limber. On all the higher 

 ranges there exists a covering of coniferous forest extending down to 

 the altitude of about 1,800 meters. Between the Sierra Nevada and 

 the great bend of the Colorado the timber-crested mountains that came 

 under my personal observation were the Inyo, Coso, Argus, Panamint, 

 Grapevine, Charleston, and Desert ranges. The timbered area of these 

 mountains, compared with the total area of the region in which they 

 lie, or even with the whole surface of the mountains themselves, is very 

 small. To this great treeless area we shall now turn our attention. 



This part of the whole region is not separated by geographic lines 

 into smaller divisions, characterized by different degrees of moisture 

 or heat; but the temperature of its various portions, and therefore the 

 humidity as well, changes with the altitude. Of two points with dif- 

 ferent altitudes, the higher has a lower temperature and greater hu- 

 midity than the other. It is in conformity with these relations that 

 the vegetation of the treeless desert area varies. A particular species 

 usually grows between two rather evenly defined limits of altitude. 

 If these limits fall upon the same mountain side, the area there covered 

 by the species is narrow; if a broad plain stretches out at an altitude 

 intermediate between the same limits, the plant grows over the whole 

 area. ' The customary belt of any species, therefore, can be obtained 

 by passing two imaginary parallel planes across the face of the country, 

 cutting its surface at the limiting altitudes of the species. All that 

 portion of the surface lying between the cutting lines constitutes the 

 customary belt of the species in question; that is, the area over which 

 the plant may be expected to grow. 



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