406 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



beds where the bottom is soft and not more than about 40 cm. deep. 

 They generally project some 20-40 cm. above the water in the south 

 but usually less in the Far North, where Mare's-tail is normally 

 absent. Commonly such beds are accompanied by aquatic Mosses 

 and, of coiu-sc, ninnerous small Algae. Behind there stretches 

 typically a marshy sedge-meadow with the same ' grassy ' dominants 

 and, in addition, Arctagrostis and lowly Willows. This in turn 

 merges into damp tundra. Fig. 131 shows such a sequence in 

 the low- Arctic, though it should be noted that in exposed situations, 

 especially farther north, wave action may prevent reed-swamp 

 formation, a definite ' hard line ' then delimiting terrestrial from 



Fig. 131. — Luxuriant lakeside marsh dominated by Water Sedge (Corex aquatilis 



agg.) and Tall Cotton-grass {Eriophoium aiigiistifoliitin agg.), which both extend 



out into the water. Southampton Island, Hudson Bay. 



aquatic communities. On the other hand there mav be an ecotone 

 of Willow scrub that, at least on its lower side, probably represents 

 a later stage in the hydrosere. 



Boggy areas, typically dominated by Bog-mosses (Sphagnum spp.) 

 and rather strongly acidic in reaction, are chiefly developed in the 

 southern portions of the Arctic. Baked-apple {Rubiis chamaemorus) 

 is a characteristic inhabitant of them. Often they are well developed 

 around pools in peaty tracts and are colonized by heathy plants, 

 with little doubt ultimately turning into heathlands. In the Far 

 North, however, many tarnside areas remain to this day uncolonized 

 by higher plants — as in the right background of Fig. 132, although 

 the foreground is vegetated by a fine bed of Scheuchzer's Cotton- 

 grass {Eriophorum scheuchzeri). 



