THE TUNDRA 507 



The Arctic Tundras. The permanently frozen areas of 

 northern Europe, Asia and America are covered for the most part 

 by a growth of peat which rises in the form of rounded hills or 

 banks of approximately uniform height. Frozen solid in winter, 

 they are thawed out on the surface during the summer months. 

 Throughout northern Europe the old peat vegetation, which con- 

 sisted largely of Sphagnum, has perished, only on the borders of 

 the natural drainage channels are cotton grass and Sphagnum still 

 found growing. The causes of this widespread destruction of the 

 peat moss and of the consequent cessation of peat formation are 

 the rising of the permanently frozen ground-level beneath the moss, 

 and the overgrowing of the moss by lichens, especially Lccanora 

 tartarca, which covers both living and dead organisms with a uni- 

 form mantle. The peat protects the ice beneath it from melting, so 

 that in Lapland ice exists the year round under peat only. Melt- 

 ing of the peat bogs proceeds down to a depth of 30 to 40 cm. 

 With the increase in thickness of the peat, the ice is more protected 

 and its surface rises, the result being that the plants are less and less 

 supplied with moisture. This eventually leads to their destruction. 

 The tundra of Alaska covers the whole surface, except the faces of 

 steep cliffs, along the borders of Behring Sea and the Arctic Ocean. 

 It is typically "a swampy, moderately level country, covered with 

 mosses, lichens and a great number of small but exceedingly beauti- 

 ful flowering plants, together with a few ferns. The soil beneath 

 the luxuriant carpet of dense vegetation is a dark humus, and at a 

 depth exceeding about a foot is always frozen." (Russell-39 : 125.) 

 Lakelets and ponds abound in the level parts, and they occur even 

 on the hillsides, where, except for the spongy retaining vegetation, 

 no such accumulation would be possible. The dense vegetation ex- 

 tends up the mountainsides wherever conditions are favorable and 

 covers even steep crags. "On the steep slopes, as in the swamps, 

 the vegetation is always water-soaked, owing to the extreme hu- 

 midity of the climate in which it thrives." Mosses and lichens char- 

 acterize the flora with a notable absence of trees. "Cryptogamic 

 plants make more than nine-tenths of its mass. On their power to 

 grow above as they die and decay below depends the existence of 

 the tundra." Two species of Equisetum flourish with rank luxur- 

 iance over great areas along the Yukon. Excavations show "that 

 the fresh luxuriant vegetation at the surface changed by insensible 

 gradations to dead and decaying matter a few inches below and 

 finally became a black, peaty humus, retaining but few indications 

 of its vegetable origin." ( Russell-39 : /j-d.) The depth of the 

 humus layer at St. Michaels is about two feet. A mile east of the 



