370 GENERAL BIOLOGY 



I. Local distribution of green plants. 



The distribution of plants over the larger regions of the 

 earth is determined chiefly by physical and climatic condi- 

 tions. We can see the effect of temperature by passing 

 from the stunted and scanty vegetation of polar regions to 



FIG. 215. Engelmann's Spruce from a sheltered valley (altitude 

 7600 ft.). Photo by D. M. Andrews. 



the luxuriant forests of the tropics; of winds (figs. 215 

 and 216) and altitude and drouth, by crossing mountain 

 and desert. We can seethe effect of water and sunshine by 

 crossing a narrow ravine, from its moist and shaded north 



