13] VEGETATIONAL TYPES OF POLAR LANDS 409 



both rock and sand, a dense mat of the ' silvery ' Moss Rhacomitrium 

 lanuginosuni may cover substantial areas and apparently persist for 

 many years, though in the end it is usually colonized by Lichens 

 and Grasses, etc., to form what is sometimes termed ' Rhacomitrium 

 heath '. 



The mesosere is represented by relatively short-lived communities 

 in such favourable situations as alluvial deltas and the beds of 

 receding tarns, and apparently by longer-lived types on the earthy 

 or gravelly ' flower-slopes ' that form such a pleasing feature on 

 steep south-facing inclines particularly in low-arctic lands. Fig. 

 133 shows an example in which Fleabanes {Erigeron spp.) and 

 Saxifrages are prominent, although a large variety of other forbs 

 occur — typically much-mixed and in such profusion that the usual 

 dominants are excluded. The writer has even encountered very 

 limited communities of this type near 78° N. in Spitsbergen and 

 at high altitudes in southern Greenland, growing under an unusually 

 favourable combination of conditions of shelter, aspect, water, 

 aeration, and soil — cf. Fig. 134. 



High Altitudes 



In general, temperatures get lower and lower as we ascend moun- 

 tains, and higher and higher as we go farther and farther south at 

 a particular altitude (such as sea-level) in the northern hemisphere. 

 Precipitation, windiness, fogginess, and the intensity of radiation 

 also tend to increase with altitude on mountains, though precipitation 

 falls off at the higher altitudes and other conditions often interfere 

 with radiation. Consequently, particular zones of vegetation in 

 general get higher and higher in the mountains as we travel towards 

 the Equator, though something like arctic conditions and attendant 

 vegetation may still be found at very high altitudes in tropical lands. 

 But although a general similarity to high-latitude lands prevails in 

 high mountains even near the Equator, the light and other climatic 

 regimes are by no means identical, while the floras are not necessarily 

 even comparable. Nevertheless the very general (but often only 

 superficial) similarity between polar regions and high altitudes else- 

 where, which to some degree may extend to the vegetation, makes it 

 desirable to treat high-altitude plant communities here, though 

 very briefly. 



While in high-arctic lands there is the usual general tendency 

 towards limitation of flora and depauperation of vegetation as 



