292 PRACTICAL COURSE IN BOTANY 



330. Boundaries of the zones. — While the broad conti- 

 nental zones of vegetation follow, in a general way, the 

 climatic zones outlined above, they are not sharply defined, 

 but run into each other and overlap in various degrees, so 

 that a map depicting the range of vegetation in any wide 

 area would show a marked deviation from those of latitude. 

 Various other geographical factors, such as mountain ranges 

 and bodies of water, influence the direction and character of 

 the prevailing winds and rains, and through them the mois- 

 ture and temperature, to so great an extent that they become 

 the controlling factors over wide areas. In countries border- 

 ing on the sea, the coast line always marks a belt of its own, 

 and on the sides of a mountain range, all the climatic zones 

 from the equator to the pole may be repeated during an 

 ascent of a few miles. 



In our own country, where the mountain chains and coast 

 lines run approximately north and south, the great conti- 

 nental zones have been superseded, for all practical purposes, 

 by four regional divisions running almost at right angles to 

 them. These are, disregarding minor subdivisions : — 



(1) The Forest region, occupying the eastern and south 

 central portion of the Union. In classifying this territory 

 as forest, it is not meant to imply that it is now, or ever 

 was, one unbroken jungle, like parts of central Africa, but 

 that it combines the conditions most favorable to a vigorous 

 and varied forest growth. 



(2) The Plains region, extending from the very irregular 

 western boundary of the forest region to the Rocky Moun- 

 tains. 



(3) The Rocky Mountain region, including the Rockies 

 and the Sierra Nevadas with the desert area between them. 



(4) The Pacific Slope, a narrow strip between the Sierras 

 and the Pacific Ocean. 



The boundaries of these regions, like those of the great 

 continental zones, overlap in various ways, the plants of one 

 region often appearing in another, like an arm of the sea 



