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nental zones have been superseded, for all practical purposes, 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 
