510 ECOLOGICAL 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 
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. 
Fic. 460.—A xerophytic society, Plant Succession 
consisting of Lichens growimg on a : 
bare rock. After Bailey. 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 
Tia. 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 Lilies, Pondweeds, Eelgrass, 
and other representatives of pond societies are replaced by 
Rushes, Sedges, Sagittaraes, Cat-tails, Reeds, True Flags, and 
