13] 



VEGETATIONAL TYPES OF POLAR LANDS 



385 



philous Willows or other ground-shrubs and many hygrophilous or 

 ubiquitous forbs. Typical among these last are Viviparous Knot- 

 weed [Polygonum vwiparum) and the bright-flowered Yellow Marsh 

 Saxifrage {Saxifraga hirculus agg.). The fairly luxuriant cryptogamic 

 layer is largely composed of Mosses, and helps to consolidate the 

 whole. An example is seen in Fig. no. Often these marshy areas 

 are beset with small hummocks commonly about 25 cm, high, and 

 introducing drier conditions on their tops, which may then support 

 heathv plants and Lichens. Such hummocky tracts are known as 



Fig. 1 10. — Marshy tundra near the south shore of Hudson Strait, dominated by 

 Cotton-grasses, Sedges, and Grasses, with dwarf Willows creeping among the 



subdominant Mosses. 



' hillock tundra '. In other instances tundras, especially of the 

 damper types, are liable to be much interrupted by various of the 

 geodynamic influences prevalent in cold regions — such as, par- 

 ticularly, solifluction and * patterned soil ' (polygon — see p. 381) 

 formation of various kinds. 



The drier tundras of raised areas or well-drained surface material 

 in low-arctic regions tend to be much poorer and thinner than the 

 damper types. Typically they are composed of an extremely 

 various array of more or less xerophilous Sedges (such as the Rock 

 Sedge, Carex rupestris), Willows (particularly the Arctic Willow, Salix 

 arctica s.l.), Grasses (such as Alpine Holy-grass, Hierochloe alpina). 



