XEROPILYTES 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 on 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 
