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INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



the cushions formed by some species may be so large as to resemble 

 ' recumbent elephants ' (C. A. W. Sandeman in lift.). 



At the very highest altitudes, for example above 6,000 metres 

 (19,685 feet). Lichens are the chief or only macroscopic plants to 

 persist, their growth being usually poor. These very high altitudes 

 also introduce the so-called ' cold deserts ' of temperate and even 

 tropical regions which, surrounded by tundra and other high-alpine 

 zones, are developed in the Rocky Mountains of North America 

 and the Andes of South America, in the Himalayas (O. Polunin 

 voce), and to some extent in the Norwegian Alps as well as on 





Fig. 138. — Alpine puna-like formation near mountain summit in Colombia. 

 Farther south in South America the cushion-plants are often much larger. (Phot, 



R. E. Schultes.) 



mountain ranges elsewhere. They are often snow- or ice-bound, 

 as are of course the ice-sheets of the polar regions, of which the 

 most extensive are the Greenland ice-cap in the northern hemisphere 

 and the Antarctic ice-cap in the southern hemisphere. Even around 

 21,000 feet there may be occasional flowering plants persisting in 

 the Himalayas, although crustaceous Lichens are more frequent and 

 go higher (O. Polunin roce, and cf. above). 



Whereas in some cases particular zones of vegetation may encircle 

 a mountain at a fairly uniform level and preserving a fairly uniform 

 breadth, frequently they are tilted, being often higher on the 



