272 Plant Life and Evolution 



however, and the beautiful glacial parks of the 

 Rocky Mountains, adorned with luxuriant mead- 

 ows and fine forests, present a great contrast to the 

 barren deserts of Arizona and Nevada. Except in 

 a few places these deserts support a scanty, but ex- 

 tremely characteristic flora, which has adapted itself 

 to the rather strenuous conditions of desert life. 

 Especially interesting are the desert forms of the 

 Southwest. Here the cacti, Yucca, century-plants, 

 and many other striking desert types are especially 

 well developed, and in some of the canyons opening 

 into the hot sandy waste of the Colorado desert in 

 Southern California, are groves of lofty palms that 

 might have been transported bodily from the 

 tropics. 



The Pacific Slope. The Pacific coast forms al- 

 most a distinct botanical region of its own. Shut 

 off from the country to the east by the great barrier 

 of the Cascades and the Sierra Nevada, it shows 

 many peculiarities in its flora, these being particu- 

 larly marked in California, where the isolation is 

 practically complete. This great mountain barrier 

 exercises a profound influence on the climate of the 

 Pacific coast, which is dominated by winds blowing 

 from the Pacific, the temperature of which varies 

 but little, so that it acts as a great thermostat. 

 Instead of the sudden changes characteristic of 

 the continental climate of most of the United 

 States, the Pacific coast has a climate which is re- 

 markably uniform. The difference between the mean 



