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1903] FLORA OF SOUTHERN CALIFORNIA 267 
boundaries than those of a biological province. Plants such as 
these are without phytogeographical value in the study of more 
limited areas. For an opposite reason plants which are very 
localized are likewise without value. Only those whose limits 
are neither too widely extended nor too restricted are serviceable 
to the phytogeographer in determining the biological subdivi- 
sions of a region. For this purpose trees and shrubs are more 
useful than humbler plants. Not only are they more readily 
observed, but their greater duration requires a closer adaptation 
to climatic conditions, while their stature and their depth of root 
render them less immediately dependent on conditions of pure 
locality, such as surface moisture or shelter. 
PHYSIOGNOMIC CHARACTERISTICS OF THE FLORA. 
The most striking feature of the southern Californian flora, 
taken as a whole, is the prevalence of shrubs. The Nevadan is, 
indeed, largely a forested region; but its open growth is inter- 
spersed with vast tracts of chaparral, and altogether fails to pro- 
duce an effect comparable to the vaster and denser forests of 
moister climes. Except in the mountains, trees are seldom 
numerous, and when present form park-like groves rather than 
true forests. Each region has, too, its meadows, never of large 
extent, and except in the mountains mostly confined to soils 
somewhat alkaline. 
But throughout the whole territory, shrubs form the common 
plant-covering of plain and hillside. In the higher mountains 
impenetrable thickets of Castanea sempervirens and Ceanothus cor- 
dulatus extend for miles. Lower on the Cismontane slope other 
species of Ceanothus, intermixed with Arctostaphylos, Rhamnus, 
Ribes, and many other shrubs, cover expanses as wide. To these 
Succeed dense chaparrals of Adenostoma and scrub-oak. 
But it is in the deserts that this characteristic is especially 
developed. Large areas are thickset with opuntias, or with a 
great variety of other shrubs, daleas, lyciums, ephedras, tetrady- 
Mias, and many others, whose rigid and thorny growth renders 
passage painful or impossible. Indeed, in this region, and toa 
considerable extent in the Cismontane as well, for half the year 
