The Vegetation of North America 323 



and consequently the most extensive forest is dominated by the 

 longleaf and other pines. The pine barrens are comparatively 

 open woods on the dry or moist sandy plains ; on better and on 

 moist soils the forest is more dense and has an undergrowth of 

 small oaks and other trees, some of which are evergreen. Among 

 the undergrowth is the low-growing palmetto, which suggests 

 an approach to subtropical conditions. 



Along the eastern coasts white cedar swamps occur as far north 

 as southern Maine. The cypress and tupelo swamps are common 

 along the lower courses of the rivers from Chesapeake Bay south- 

 ward, and extend as far inland as southern Illinois. 



One of the remarkable plant associations of the South is that 

 of the canebrakes, our only native representatives of the bamboos, 

 which are so abundant in Asia. The canebrakes were formerly 

 extensively developed on the low hills bordering both sides of the 

 Mississippi flood plain, and in central Alabama as undergrowth 

 on the oak-covered black-soil areas. 



Just as the deciduous forest trees like the maple occur in the 

 best habitats in many places as far north as Nova Scotia, so dense 

 growths of oaks, beech, hickories, and magnolias occupy the most 

 mesophytic habitats, called '* hammocks," as far south as central 

 Florida. As we go southward from the northern evergreen forest, • 

 the rate of evaporation gradually increases, and an increasing 

 amount of rainfall becomes necessary to permit the growth of 

 forests. 



The tropical evergreen forest formation. The southern third 

 of the peninsula of Florida, the West Indies, the lowlands of 

 Mexico, and the eastern slopes of Central America are occupied 

 by tropical forests. Where the rainfall is more than 50 inches, 

 these forests attain magnificent proportions and great density. 

 Where the rainfall is less, one finds tropical scrub and desert. 

 As the trade winds of the tropics blow from the east, the greatest 

 rainfall occurs on eastern slopes, and there the conditions for 

 forest growth are at their best. On western slopes the rainfall 



