XBROPHYTES 341 



The region of dunes may gradually pass landward into 

 sandy stretches or fields, covered with tufted grasses, 

 shrubs, and low trees. 



204. Plains. Under this head are included great areas 

 in the interior of continents, where dry air and wind prevail. 

 The plains of the United States extend from about the one- 

 hundredth meridian westward to the foot-hills of the Rocky 

 Mountains. Similar great areas are represented by the 

 steppes of Siberia, and in the interior of all continents. 

 On the plains of the United States the characteristic plant 

 forms are bunch-grasses, that is, grasses which grow in tufts 

 and do not form turf; and the low grayish shrubs called 

 sage-brush (Fig. 310). 



205. Cactus deserts. In passing southward en the plains 

 of the United States, the conditions are observed to become 

 drier, until the cactus deserts are reached (Fig. 311). This 

 region begins in western Texas, New Mexico, Arizona, and 

 Southern California, and stretches far southward into 

 Mexico. This vast arid region has developed a peculiar 

 flora, which contains our most highly specialized drought 

 plants.. The numerous forms of cactus are the most char- 

 acteristic, and associated with them are the yuccas and 

 agaves. * Not only are the adaptations for checking tran- 

 spiration and for retaining water of the most extreme 

 kind, but also there is developed a remarkable armature of 

 spines. 



206. Subtropical deserts. In these areas drought con- 

 ditions reach the greatest extreme in the combination of 

 great heat and scanty water-supply. It is evident that such 

 a combination is almost too difficult for plants to endure. 

 That the very scanty vegetation is due to lack of water, 

 and not to lack of proper materials in the soil, is shown by 

 the fact that where water does occur oases are developed, 

 in which luxuriant vegetation is found. The desert which 

 stretches from Egypt across Arabia may be regarded as a 



