37 



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 

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



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

 and 216) and altitude and drouth, by crossing mountain 

 and desert. We can see the effect of water and sunshine by 

 crossing a narrow ravine, from its moist and shaded north 



