490 PLANT GEOGRAPHY 



irregular line, most of which is east of the hundredth meridian. 

 In some places this forest boundary lies considerably east of 

 the Mississippi Eiver, while in others it extends from the river 

 five hundred miles or more to the westward. 



The plains region. This stretches westward from the region 

 above-named to the Eocky Mountain plateau. 



The Rocky Mountain region. This includes the Eocky Moun- 

 tains, the Sierra Nevada, and the various plateaus between 

 them. 



The Pacific slope. This extends from the Cascade Eange and 

 the Sierra Nevada to the sea. 



468. The forest region. The forest region is mainly remark- 

 able for its great variety of hardwood trees, of which it contains 

 a larger number of useful species than any equal area of the 

 earth with a temperate climate. Perhaps the most important of 

 these are the oaks ; but other genera, such as the hickory, the 

 tulip tree, and the sassafras, are more characteristically Ameri- 

 can. In the northeasterly portion there are extensive forests of 

 the cone-bearing evergreens, such as pines, spruces, hemlocks, 

 and cedars ; the other trees which accompany these are mostly 

 deciduous hardwood species. In the southerly portion the for- 

 ests are partly of coniferous evergreens (Fig. 392) and partly of 

 deciduous mesophytes, such as hickories, beeches, oaks, elms, 

 hackberries, magnolias, and sycamores. There is also a consider- 

 able admixture of such hydrophytes as the water hickory, the 

 sweet bay (Magnolia], the anise tree (Illicium), the custard apple 

 (Anona), the red bay (Persea), the loblolly bay (Gordonia), and 

 the sour gum (Nyssa), due to the mild, moist climate. 



This region was never completely forest-covered. Areas of 

 prairie, so-called "openings" in the hardwood forests (Fig. 393), 

 extensive marshes, and some heaths have for ages been treeless, 

 or nearly so. Generally, in the older states, the land most desir- 

 able for cultivation has been tilled so long that it is difficult to 

 find portions in anything like their primitive condition. It is 

 only in broken country like that of the mountainous regions 



