ARCTIC VEGETATION. 



583 



or prairie formation which is a transition belt to the true alpine 

 formation above and which is comparable to a desert formation 

 because of the austere conditions which so limit plant growth. 

 While there is a general re- 

 semblance between alpine and 

 arctic vegetation, there is an 

 essential difference in structure 

 even of individuals in the same 

 species. As a matter of his- 

 tory many alpine species are 

 arctic species which were left 

 behind as the continental ice- 

 sheet retreated northward at 

 the close of the glacial peroid. 

 Bonnier has found that aerial 

 parts of Salix polaris and Sax- 

 ifraga oppositifolia are more 

 weakly developed in polar than 

 in alpine individuals, while polar 

 plants in general have thicker 

 leaves than alpine plants, but 

 the leaves of polar plants show 

 less differentiation in structure 

 and larger intercellular spaces. 

 So the present form and struc- 

 ture of alpine plants are not 

 due to these historical causes, 

 since alpine conditions are 

 sufficient in themselves to in- 

 duce the peculiar habit and in addition to induce changes in 

 structure since the glacial period. These peculiarities of alpine 

 plants are, therefore, due chiefly to physiological causes and not 

 merely to heredity. 



1073. Factors of alpine climate influencing plants. These 

 factors are: ist, decrease in precipitation; 2d, decrease in heat 

 (the temperature being very low) ; 3d, rarity of the atmosphere, 



Fig. 517. 



Compact, radiate cushion type of 

 vegetation of North Polar region, 

 Draba alpina. (After Kjellman.) 



