

GENERAL TOPOGRAPHY OF CALIFORNIA. 19 



fortunate because they form the most conspicuous vegetative features 

 of the landscape. Over certain large areas of the United States, on the 

 other hand, such as prairies and certain sorts of arid lands, none but 

 herbaceous vegetation exists, and in such cases we must have recourse 

 to perennials. It is not to be inferred, however, that annuals are en- 

 tirely erratic in their distribution. Ou the contrary, most of the species 

 are found to fall conformably into some one of the primary floras, though 

 often overstepping the true zonal limits. 



DISTRIBUTION OF PLANTS IN SOUTHEASTERN CALIFORNIA. 



CLASSIFICATION OF FLORAS. 



A full discussion of the characteristics and limits of the life zones in 

 the region examined by the expedition is given by Dr. C. Hart Mer- 

 riam in the general report. The object of the present outline is to 

 present a few of the more strictly botanical aspects of distribution in 

 this region, in order to render intelligible the zonal references employed 

 in other parts of the report. 



So far as plant lite is concerned, the most important topographic fea- 

 ture of California, and in fact of the whole region of North America west 

 of the Rocky Mountains, is the great Cordilleran system of mountains. 

 This, composed of the Cascade Mountains, Sierra Nevada, San Bernar- 

 dino Mountains, and San Jacinto Mountains, with their connecting 

 ridges, extends as the backbone of the western country from British Co- 

 lumbia to Lower California. From the Pacific Ocean, blowing eastward, 

 come winds which, under the proper conditions, would bring rain and 

 fertility to all the region west of the Rocky Mountains; but the cold 

 summits of the Cordilleras precipitate all the water that is brought to 

 them by the west winds, so that none is left for the ultramontane 

 basin. The interior of the State of California is occupied by an im- 

 mense valley, made up of the valleys of the Sacramento and San 

 Joaquin rivers, and bounded on the west, except at the bay of San 

 Francisco, by a smaller mountain system, the Coast Range, separating 

 the valley from the true coastal lowlands. At the southern end of this 

 great interior valley the Coast Range sweeps eastward to meet the 

 Sierra Nevada. 



We have, therefore, as the most important physical characteristic 

 of our region, a series of parallel lowlands separated by intervening 

 highlands. The essential climatic feature of these lowlands is their 

 successive increase in aridity from west to east. 



To the effect of various degrees of aridity upon the flora of California 

 must be added another set of influences dependent, not primarily upon 

 any striking topographic feature of the State, but upon variations in 

 latitude. The cold of northern California is in marked contrast with 

 the mild climate of the southern part of the State. On the basis of 



