504 



RELATION TO ENVIRONMENT. 



Borea! Region. 



Austral Region 



Lines of stress due to heat. 



Arctic or Arctic- 

 Alpine Zone. 

 Hudsonian Zone. 

 Canadian Zone. 



f Transition Zone 



Upper Austral Zone. . . . 

 Lower Austral Zone. . . 



Lines of stress due to 

 rainfall or drought. 



Alleghanian Area. 

 Arid Transition Area. 

 Pacific Coast Transi- 

 tion Area. 



Carolinian Area. 

 Upper Sonoran Area. 



Austroriparian Area. 

 Lower Sonoran Area. 



( Humid Tropical 



Tropical Region \ . , _ . 



[ And Tropical. 



971. The great lines of stress. There are here shown two 

 great systems of parallel lines of stress which influence and limit 

 plant migration. One of these systems of stress is traceable to 

 the factor heat, and the stress lines are in general transconti- 

 nental (east and west). In the eastern and western portions of 

 the continent, however, these temperature stress lines are de- 

 flected far to the south because of the mountain ranges and 

 become parallel with continental lines. So mountain ranges 

 constitute lines of stress, but the great factor influencing plants 

 is heat, since plants in attempting to cross mountain chains 

 meet with the extreme cold of mountain-tops. The other sys- 

 tem of stress lines is due to difference in rainfall; the interior of 

 the continent having a slight amount of rainfall as compared 

 with the abundant rainfall nearer the coast and along the moun- 

 tain ranges (except in the southwest), these lines run across the 

 temperature stress lines. Large bodies of water also constitute 

 lines of stress. 



972. Lesser lines of stress. Lesser lines of stress are formed 

 by streams and small bodies of water, as well as by the lesser 

 physiographic variations of the earth's surface. For example, 

 between ponds, marshes, swamps, etc., and the surrounding 

 higher elevations; between rocky areas, sandy areas, and soil 



