510 



E('OLOOICAL CLASSIFICATION OF PLANTS 



In Asia, Africa, and North America, there is much land that 

 is xerophytic. Much of the Southwestern part of the United 

 States is xerophytic. One of the important problems in Agri- 

 culture is to bring xerophytic 



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areas into cultivation. This 

 may be done by making these 

 areas mesophytic through irri- 

 gation or by securing crop 

 plants through selection or 

 breeding that are drought re- 

 sistant, that is, able to grow- 

 under xerophytic conditions. 



Fig. 460. — A xerophytic society, 

 consisting of Lichens growing on a 

 bare rock. After Bailey. 



Plant Succession 



One society of plants com- 

 monly prepares the way for 

 another. For example, the Lichens and Mosses, growing on bare 

 rocks, disintegrate the rocks and form soil in which other plants 

 can get a start {Fig. 460). Ponds and lakes are gradually, filled up 



Fig. 461 — A desert xerophytic society consisting chiefly of Sage Brush 

 and Yuccas. After R. G. Kirby. 



through the growth of pond societies until they are transformed 

 into swamps, in which the Pond Lihes, Pondweeds, Eelgrass, 

 and other representatives of pond societies are replaced by 

 Rushes, Sedges, Sagittaraes, Cat-tails, Reeds, True Flags, and 



