254 DISTRIBUTION OF PLANT COMMUNITIES 



the top but the highest mountains are covered by perpetual snow 

 and so are destitute of vegetation while some others have only 

 lichens and a few xeric mosses at their summits. 



None of the mountains in the eastern part of the United States are 

 high enough to support typical alpine vegetation but in the west 

 there are many mountains that extend beyond the tree line and 

 there alpine communities are encountered. 



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