13] VEGETATIONAL TYPES OF POLAR LANDS 405 



Bryophytes and open-soil herbs such as the Mountain Sorrel 

 {Oxyria digyna). 



Many of the smaller snow-patches, of course, disappear well before 

 the end of summer, especially in the South, and consequently show 

 only the outermost zones. On the other hand around the centre 

 of the deeper drifts, which usually form in ravines, depressions, or 

 behind banks or ridges, the snow may melt only towards the very 

 end of summer or in a cool season not at all. Here most herbs are 

 unable to persist and even cryptogams are little in evidence, though 

 some tufts or limited mats of Bryophytes and investments of Algae 

 are usually to be found, together with the diminutive grass Phippsia 

 algida agg. (Frigid Phippsia). Towards such centres various plants, 

 including attractive Saxifrages and Buttercups {Ranunculus spp.), 

 are often to be found flow ering at the very end of summer, sometimes 

 being caught still in bud by the frosts and snow of a new winter. 

 In the Far North the zones are apt to be reduced in number but 

 extended in area, the fell-fields and barrens often representing inner 

 zones where the modest snow^-covering melts late under the pre- 

 vailingly cool conditions. Here, as on mountains farther south, 

 many of the snow-patches are perennial or even eternal, the chief 

 growths near their centres being of Bryophytes and Algae in the 

 run-oflF below. 



Seral Types 



Whereas the apparent arctic counterparts of many southern seral 

 types have already been dealt with, being often seemingly incapable 

 of leading to permanent higher vegetation or best regarded as sub- 

 climax (in view either of persistent disturbance or of the extreme 

 slowness of vegetable build-up), other instances of seral stages remain 

 to be mentioned. Outstanding are the marshy and boggy ones of 

 hydroseres (the fully aquatic communities comprising the early 

 stages of which will be dealt with in Chapter XV), various ones of 

 lithoseres, and the ' flower-slopes ' that probably belong to mesoseres. 



The hydrosere of arctic lakes and tarns usually has as its first 

 aerial stage a ' reed-swamp ' of aquatic Sedges (particularly the 

 Water Sedge, Carex aquatilis agg.) and/or Cotton-grasses (particularly 

 the Tall Cotton-grass, Eriophorum angustifoUum agg.), though some- 

 times Common Mare's-tail {Hippuris vulgaris s.l.), or such coarse 

 Grasses as the Tawny Arctophila {Arctophila fuha agg.), may largely 

 or wholly take their place. Any of these plants may form luxuriant 



