270 Plant Life and Evolution 



adorn the woods, azalea, and rhododendron, syringa, 

 honeysuckle, crab-apples and hawthorn, dogwood 

 and redbud; and especially in the southern woods, 

 beautiful creepers, grape-vines, clematis, wistaria, 

 bitter-sweet, yellow jasmines, trumpet-creepers, and 

 passion-flowers, and others, remind one of the lianas 

 of the tropics. Only in the extreme South do we 

 encounter palms, perhaps the most striking tree types 

 of the tropics. 



Passing inland from the Atlantic coast there is 

 a marked diminution of the rainfall, accompanied 

 by a corresponding falling off in the forest flora. 

 In Western Michigan, Indiana, and Illinois the for- 

 est assumes a more open character, and shows much 

 less variety in the trees. Oaks predominate, and 

 these " oak openings " are very characteristic of the 

 territory abutting on the prairie region lying to 

 the west, and small prairies already appear in spots 

 between the forested areas. These patches of for- 

 est finally disappear entirely, and the great plains 

 extending from the Mississippi to the Rocky 

 Mountains are quite treeless, except along the 

 streams, where a belt of cottonwoods or willows 

 often marks the course of some shallow muddy 

 river. 



The Great Plains. The great plains constitute 

 the second phytogeographical region. A level or 

 slightly rolling plain, with meager rainfall and great 

 extremes of heat and cold, and with fierce winds 

 sweeping it, the conditions are not favorable for 



