CHAP. L MOSS-TUNDRA OR MOSS-HEATH 207 



warm, and thaws rapidly after the snow has melted, so that in such 

 places there is no ground-ice. On the contrary, where the covering of 

 snow is thin or wanting, the low temperature descends deep into the soil, 

 and there is formed ground-ice which cannot be melted in the deeper 

 layers during the cool summer. Ground-ice melts with the greatest 

 difficulty in peat-soil, and consequently persists during the summer 

 at a depth of only a few centimetres. 



The vegetation of the peat-hillocks is definitely xerophilous. Species 

 of Sphagnum occur scantily ; whereas species of Polytrichum abound 

 in the form of tall mosses, whose erect stems are closely crowded and 

 produce a low, soft carpet. Even when the soil on which they grow 

 is very wet with melted snow below the surface, it may be superficially 

 dried by the summer sun and consequently become hard. While in winter, 

 when great dryness of air prevails over the northern tundra, it is to wind 

 that plants owe their desiccation. The Polytricha can hold water in 

 their dense, matted tufts, but despite this they show xerophilous 

 structure, as some species have peculiar leaves, which when dry can close 

 their margins over the assimilatory tissue. In addition to this genus, 

 Dicranum elongatum, D. tenuinerve, and other species of Dicranum, 

 form similar dense, firm tufts ; and intermingled in this air-excluding 

 carpet of mosses that produce raw humus are species of Hylocomium, 

 Hypnum, Rhacomitrium, Jungermannia and other Bryophyta, lichens, 

 dwarf -shrubs, including Empetrum, Betula nana, and Vaccinium Myrtillus, 

 also herbs belonging to the same species as on ' fell-field '. 



Mosses are enabled to colonize the inhospitable region in virtue, not 

 only of their capacity of drying up and reviving after a fresh supply of 

 moisture, but also of their hardy nature, and of their powers of assimi- 

 lating, apparently at very low temperatures, more readily than can 

 Spermophyta. 



The puddles have no ground-ice. The soil is not so cold as that of 

 the peat-hillocks, and in the dry season water accumulates in these 

 places. Here a formation belonging to the temperate forest-domain, 

 namely Sphagnum-moor, can invade a portion of the tundra. These 

 Sphagnum-moors do not extend far beyond the hmit of forest ; east 

 of the Timan Mountains and in Asiatic tundras they are merely sporadic, 

 while far north they are completely absent. 



The production of peat in the tundra varies. Pohle ^ in northern 

 Russia saw layers of peat more than six metres in thickness. According 

 to Kihlman,'^ in the Kola Peninsula there is not so much a production 

 of moss-peat as one of raw humus, which is permeated with hving parts 

 of plants. As a whole, the modern production of peat in the tundra does 

 not seem to be great ; on the contrary, the tendency seems often to be 

 retrograde,^ as has been mentioned already. The soil must necessarily 

 abound in humous acids, and this character must materially contribute 

 to the xeromorphy of the constituent plants. 



Moss-heath apparently occurs only in the Northern Hemisphere, and 

 especially in Siberia and Lapland. Heuglin describes it on the Straits 

 of Jugor ; it is also known in North America and Greenland. In the 



' Pohle, 1903. ' Kihlman, 1890, 



* Cajandcr, is)05. 



