CLIMATIC ZONES 31 



tain avens (Dry as octopetala) and the curious arctic Pedicularis 

 (P. Groenlandica) , common species of the arctic lowlands, occur 

 also as alpines in the Rocky Mountains and Sierra Nevada, while 

 the much lower White Mountains have a few plants, e. g., Dia- 

 pensia lapponica, natives of the far north. 



We may assume that the presence of these arctic species on the 

 mountains to the south is to be explained by the assumption that 

 they are remnants of the wide-spread arctic flora which at the 

 period of maximum glaciation occupied the lowlands of these 

 lower latitudes. With the retreat of the ice-sheet, and the increas- 

 ing warmth of the lowlands, some of these arctic species managed 

 to survive by ascending the mountains until conditions were 

 reached adapted to their needs. 



Strictly arctic conditions are found in Europe only in parts of 

 central and north Scandinavia, including parts of Lapland. 

 In north Siberia, the most extreme arctic conditions prevail, 

 and the lowest temperatures that have been recorded occur. 

 In spite of this excessive cold, to which they are exposed with- 

 out any snow-cover, a surprising number of species are able to 



survive. 



The north Siberian tundra region x has as its commonest 

 formation the " Moss-tundra," whose principal constituents 

 are mosses of the common genus Polytrichum. Among the mosses 

 are various grasses and sedges, among them " cotton-grass " (Erio- 

 phorum), common also in cold bogs much further south. Dryas, 

 and the heath-like Cassiope tetragona are characteristic of the 

 moss-tundra. 



Where there is water, true peat-bogs occur, peat mosses (Sphag- 

 num) replacing the Polytrichum of the dry moss-tundra. On 

 the slopes and precipices of the higher ground there is often 

 a fairly abundant development of mats of grasses interspersed 

 with bright flowers. Among the most notable, are species of 

 Oxytropia, with bright yellow pea-flowers, blue Polemonium, 

 various Saxifrages, and a showy avens (Geum). A bright blue 

 forget-me-not (Erytrichium), is also characteristic of the Siberian 



arctic flora. 



All of these plants grow in low compact tufts, with small leaves 

 exposing a minimum surface to evaporation. 



1 Drude, loc. cit., i>. 356. 



