REGIONAL DISTRIBUTION 503 



after careful endeavors to introduce the plant into all the 

 kinds of environment in which it might have a fair chance of 

 success. The student will find it most instructive to watch 

 for instances of the occurrence of roadside weeds, such as 

 knotgrass, wild peppergrass, Indian chickweed, and dog fennel, 

 in deep, rich ground, or to plant these and similar tramp plants 

 in good soil and observe how they succeed. 



455. Plant geography of the United States. Briefly stated, 

 the four great vegetation areas of the continuous territory of 

 the United States 1 may be designated as follows : 



(1) The eastern and central forest region, occupying the east- 

 ern and central portions of the country. It extends westward 

 to an irregular boundary line, lying mostly to the east of the 

 hundredth meridian. The easternmost portions of this bound- 

 ary run considerably east of the Mississippi River, while the 

 westernmost extend at least 500 miles west of the river. 



(2) The Plains region, stretching westward from the forest 

 region to the Rocky Mountain plateau. 



(3) The Rocky Mountain region, including the Rocky Moun- 

 tains, the Sierra Nevada, and the plateaus between them. 



(4) The Pacific slope, extending from the Cascade Range 

 and the Sierra Nevada to the Pacific coast. 



It must be understood that the same species of plant may 

 occur in several or even in all of these regions. 



Compare the temperature requirements of the white pine 

 and bald cypress (as far as shown by the maps on page 504). 



Do the ranges of the two anywhere overlap ? If so, where ? 



Consult Fig. 380 and compare the moisture requirements 

 of the two trees (as far as shown by the maps). 



Which species avoids the Appalachian highlands? What 

 two reasons may be given for this ? 



In using the maps shown in Figs. 374 and 375 it should 

 be kept in mind that they are not drawn to exactly the 

 same scale. 



1 That is, excluding Alaska, Porto Rico, the Hawaiian Islands, and the 

 Philippines. 



