15] 



VEGETATIONAL TYPES OF FRESH WATERS 



501 



the centre of the bog or margin of the lake. Beyond this zone of 

 often gnarled dwarfs there normally extends one of lowly Heaths, 

 such as species of Vaccinium (including Blueberries, Bilberries, 

 Whortleberries, etc.), Chamaedaphne (Leather-leaf), Ox_ycoca<5 (Cran- 

 berries), Andromeda (Marsh Andromeda), and Ledum (Labrador- 

 tea). The flora is liable to be limited by, among other conditions, 

 poor aeration not far down. 



The surface is typically of slight mounds or hummocks set in a 

 network of depressions occupied by small shallow puddles. These 

 last are particularly prevalent towards the centre of the bog or 

 persisting lake, where they often coalesce to form larger bog-pools. 

 The vascular plants of these very wet depressions are chiefly hygro- 



Sphagnetum 

 .x^.i"^'^ Highmoor peat ^ '"^^^^f I 



j=^ j_-_^ natmoor _£eat_^.__ ^_i_-jj.^ - 

 Lake sediments" 



Fig. 168. — Diagram of cross-section through a ' highmoor ' bog that has arisen 

 from a small lake, the contours being exaggerated for clarity. (After Ruttner.) 



phytic Sedges [Carex spp.) and Cotton-grasses {Eriophortim spp.), 

 but the zones of recent extension may be of almost pure Mosses. 

 Quaking bogs are those in which such vegetation, owing to extension 

 over waters, in part at least floats like a raft, while hanging bogs 

 are those developed on moist slopes. Fig. 168 shows in diagram- 

 matic form how a lake may become filled by sediments and ultimately 

 by Bog-mosses, etc., to form an extensive bog (Sphagnetum). 



The bog-waters, including those of puddles, pools, and embedded 

 lakes, tend to be extremely poor in dissolved salts, strongly acidic 

 in reaction, and possessed of a high content of humic materials 

 which often impart a brownish coloration. Such waters are char- 

 acterized, from the boreal regions to the mountains of the tropics, 

 by a special microflora of species apparently unable to withstand an 

 alkaline medium. This microflora is particularly rich in Desmids 

 but also contains certain Cyanophyceae such as Chroococcus turgidus. 

 Some Diatoms, Peridinians, and other Green Algae also occur, but 

 usually in limited variety compared with the predominant Desmids. 



