13] VEGETATIONAL TYPES OF POLAR LANDS 407 



Lithosere stages are abundant in the Arctic, where much of the 

 terrain is of more or less bare rock that has, in many cases, been 

 freed from glaciation only in relatively recent times. Nevertheless 

 there is no doubt that succession is proceeding, however slowly — 

 at all events in areas that are not too rigorously exposed or lastingly 

 snow- or ice-covered. Thus rock faces, whether of glacial boulders 

 or of detrital, cliif-face, or some other nature, are apt to be largely 

 invested with crustaceous and foliose Lichens, and to occupy con- 

 siderable areas. On the other hand, rock crevices or interstices 



Fig. 132. — Fine bed of Scheuchzer's Cotton-grass (En'ophonim scheiichzeri) beside 

 tarn in northern Spitsbergen, though the waterside behind (on right) is devoid 



of higher plants. 



often support higher life-forms, so that in time a moss-mat or mixed 

 herbaceous community develops, and, ultimately, heathy vegetation 

 in suitable situations. 



Screes, if not too active, may also be bound by hardy plants — 

 especially in low-arctic regions, where dark strips stabilized by 

 vegetation often extend down scree slopes. Also commonly stabilized 

 by vegetation are inland sandy areas, though the psammosere may 

 advance little beyond the pioneer stage of sand-binding Mosses (such 

 as Polytrichum spp.) and ground-shrubs (such as Crowberry and 

 Alpine Bearberry, Arctostaphylos alpina agg.). Consolidation of the 

 ground-shrubs produces already an advanced type of vegetation. On 



