Plants of Different Regions. 



345 



Fig. 187. 



Zilla spinosa, slightly reduced. From the Sa- 

 hara desert. After Prantl. 



growth, the rings having an average breadth of 0.15 

 millimeter. 



The polar regions 

 possess no families of 

 plants peculiar to them- 

 selves, the plants which 

 appear there being 

 dwarfed and xerophytic 

 representatives of fami- 

 lies which dominate the 

 north temperate zone. 



217. Vegetation of 

 Desert Regions. — The 

 deserts occupy a consid- 

 erable area of the earth. 

 In north Africa the Sa- 

 hara desert alone nearly 

 equals the United States in size, and its area is more than 



doubled by the deserts of Ara- 

 bia and of south and central 

 Asia. To this must be added a 

 large tract between the Sierra 

 Nevada and Rocky Moun- 

 tains, a narrow strip east of 

 the Rocky Mountains, a large 

 portion of central and south- 

 western Australia, and a nar- 

 row strip along the western 

 border and in the south cen- 

 tral part of South America. 

 The total area of these desert 

 regions equals approximately 

 the area of the North Ameri- 



FlG. 188. 



Alhagi maurorum. From the Sahara, 

 After TaUBERT. 



