578 RELATION TO ENVIRONMENT. 



their prostrate condition protect them from the drying effect of 

 the fierce winds in the very cold weather. As one passes be- 

 yond the line of tree growth the trees and shrubs become greatly 

 stunted and dwarfed. Growth in length of the stems takes 

 place proportionally slower than growth in diameter, and yet 

 even the diameter increases very slowly. For example, a dwarf 

 tree 83 mm (3^ in.) in diameter showed 544 annual rings! 



1066. The polar tundra. Tundra is the name given to a 

 large portion of the polar waste. The tundra are related in 

 their formation to heath or peat moors. Heaths, saxifrages> 

 dwarf willow, and other dwarf shrubs abound. The forma- 



Fig. 514- 

 Polar tundra with scattered flowers Alaska. (Copyright by E. H. Harriman.) 



tions are often open, and the vegetation varies over different 

 areas. In some places devoid of shrubs the tundra is covered 

 with a growth of mosses and lichens, often the reindeer-moss 

 (Cladonia rangiferina) being very abundant. So there are 

 'moss- tundra" (chiefly the moss Polytrichum) or "lichen- 



