356 EXPERIMENTS WITH PLANTS 



a southern region develop from fifteen to twenty days 

 earlier than those which are accustomed to the warmer 

 regipn. Early varieties of flowers, etc., are constantly 

 imported from northern points. Temperature, more 

 than anything else, determines what kinds of plants 

 grow in a given region 1 . The tropics, the temperate 

 and the arctic zones have each their characteristic 

 vegetation. Plants which flourish in the tropics may 

 be cultivated in the temperate zone (e. g., Date Palm, 

 Rubber Plant, etc.), but remain stunted and refuse to 

 bear fruit. The study of the various zones to be found 

 in a country is very important in determining what 

 part is best adapted for various crops. 2 



Wherever there are high mountains there are 

 opportunities to study these zones to great advantage, 

 since as we ascend the mountain we pass through 

 successive zones. It will then be seen how much 

 depends on exposure; i. e., the southern face of the 

 mountain being so much warmer than the northern, 

 the zones will run higher, etc. The same may be seen 

 in even a small hill or elevation. For this reason 



J An interesting illustration of this is seen in the forests of the United 

 States and Canada. As we go northward from Florida we find, considering only 

 good and well-watered soils, that the hardwood forest gradually diminishes, 

 both in the number of trees and the number of kinds represented, until we come 

 to the great pineries of the northern United States and Canada. Still further 

 north these are succeeded by Birch, Willow and Alder: these gradually dim- 

 inish northward until we reach the treeless wastes. 



2 The " Crop Zones " of the United States are now being determined by the 

 United States Biological Survey. 



