32 Naturalized Plants. [ZOE 
This smaller portion contains the farms, the towns and almost 
all the population, and in its limits are found also most, of the 
naturalized plants. The other region consists of deserts or 
rough mountains, the former largely preponderating. Its popula- 
tion is sparse, and there is comparatively little travel and few roads. 
The opportunities for the introduction and dissemination of foreign © 
plants are consequently slight, and as there is almost no agriculture 
the weeds of cultivation are seldom seen. The climate of the 
desert is an additional safeguard to the integrity of its vegetation. 
Few exotic plants are capable of adapting themselves to the aridity 
of airand soil which there prevail. Hence, the desert flora is but 
slightly contaminated by intruding species, and these are easily re- 
cognized, since they cling closely about houses or camping places. 
A few early introduced plants, such as the Erodiums, have a some- | 
what wider range, but in no degree comparable to their abundance 
in the inhabited districts. So little is this region receptive of new 
comers that the prevalence there of any plant away from habita- 
tions and routes of travel is almost conclusive evidence of its in- 
digenuous character. With our present imperfect knowledge it is 
‘not possible to give the number of introduced species which extend 
to the desert region, but they are probably not more than twenty. | 
Numerically they constitute a very small proportion of the whole — 
vegetation, and in the mountain districts it js hardly greater, 
Very different is the case in the region of fertility. Here intro-_ 
duced plants constitute a very considerable element of the vegetive 
life. These have not, indeed, supplanted the natives to such an 
extent as is unhappily the case in the Atlantic States, where the 
European visitor finds the agrestial and ruderal vegetation largely _ 
made up of the weeds of his own fields and waysides.’ Our meadows — 
_are yellow with native buttercups, azure with Brodizeas, or whitened © 
with Yerba Mansa. By the highway the sunflower and the tarweed | 
maintain their ground against all intruders. On the hills and | 
plains the foreign Erodiums, bur clover and wild oat are ever present 
and often cover mile on mile of land, but they do not exclude the 
natives of the soil, and in the spring there are burnished acres of — 
Eschscholtzia and glowing leagues of Beeria. Even in cultivated © 
fields there are weeds of native origin quite able to compete with 
their foreign rivals. The black mustard alone succeeds in excluding 
all other plants from the soil it occupies, ae 
