PART Y. LIFE UPON THE EARTH 



CHAPTER XXIII 

 THE PLANT COVERING OF THE EARTH 



245. Abundance of plants. In the preceding chapters there 

 has been frequent reference to living things, and some of them, 

 such as bacteria, have been studied. We must now give more 

 attention to living things, for they are a very important part of 

 our environment. Almost everywhere the surface of the land 

 is covered with plants, if they have not been removed by man ; 

 and even where this has been done, man has usually introduced 

 other plants to take the place of those he has removed. In 

 countries with abundant rainfall they hide the earth so com- 

 pletely that the landscape is almost wholly green. Even in 

 the desert there are few large areas without sufficient plants 

 to give tone to the landscape (fig. 111). 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. In the waters of the ocean, 

 plants are found in great abundance. The only land areas of 

 considerable 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 algse and commonly known as " red snow." 



246. Variety among plant formations. The plant covering 

 of the earth is not everywhere the same. Instead, there is in- 

 finite variety, and at first sight there seems to be little system 

 or order. The plants, however, do not grow altogether at 



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