316 



ELEMENTS OF GENERAL SCIENCE 



are few large areas without sufficient plants to give tone to 

 the landscape (fig. 151). The " bare" rocks of either dry or 

 moist countries are usually partly covered with the plants 

 called lichens, and there are plants on the mountains close 

 to perpetual snow. Plants are found in great abundance in 

 the waters of the ocean. The only land areas of considerable 



FIG. 151. A desert landscape, Southern Arizona 



The plants are not sufficiently numerous to entirely cover the soil. Photograph 

 by Dr. D. T. MacDougal 



extent which appear to be almost wholly devoid of plants 

 are the interior of Greenland and the region about the south 

 pole, and these are covered with ice. Even here the surface 

 of the snow may be colored by a small red plant belonging 

 to the algae and commonly known as " red snow." 



320. Variety among plant formations. The plant covering 

 of the earth varies greatly in different regions. Each kind 

 of plant needs for its growth certain conditions that may be 



