206 OXYLOPHYTES SECT, vi 



and denotes any open, forestless stretch of land, and therefore also 

 includes mountain-tops devoid of forest. In phytogeography the term 

 refers solely to treeless moor-like plant-communities occurring within the 

 Polar climate. The tundra arises from arctic ' fell-field ', when mosses 

 gain the upper hand over all other plants, and form a continuous soft 

 sward. Transitional stages between the tundra and ' fell-fields ' have 

 been described by Porsild 1 as occurring in Greenland. 



According to Middendorff 2 the tundra is always associated with wet 

 soil and moist air ; moreover, the exceeding prevalence of marshy soil 

 is characteristic of Polar regions. As the first cause of this we must 

 regard the very short duration of the snowless season coupled with low 

 temperature in summer and with frequent mist. The arctic summer 

 is like spring in temperate zones. In winter, however, great atmospheric 

 aridity prevails. Evaporation is small, the soil is wet. In Polar lands 

 there is, however, no warm season during which the soil can dry. Added 

 to these climatic factors are edaphic factors, and especially the ground- 

 ice which prevents the water from sinking. Ground-ice is not the chief 

 cause of the marshiness, as is shown by the fact that in the northern 

 part of the forest-domain of North Russia the larger part of the soil 

 is marshy, although there is no ground-ice. Here also, there is not 

 sufficient time during the short summer for the water to flow away. 

 On steeply inclined soil the flow of the water is facilitated, and the produc- 

 tion of marsh is consequently checked. And this explains why it is 

 that in mountainous country like Greenland tundra occupies only a small 

 area, whereas in northern Siberia it seizes upon nearly the whole of 

 the land. According to Porsild, 3 in Greenland moss-tundra occurs on 

 rocks in depressions of the terrain, on horizontal, not-drained terraces 

 of the basalt cliffs, on the flat headlands beneath the basalt cliffs, and 

 on the horizontal moist moraines. The soil is cold, especially when 

 ground-ice lies near the surface. 



Coldness of soil checks the absorption of water by plants, so that, 

 especially when the wind is strong, the plant cannot replace the water 

 lost by transpiration, and is in danger of desiccation. The mosses are 

 therefore xerophilous, and the Spermophyta are in various ways protected 

 against being dried up. 



The moss-tundra of the peninsulas of Kola and Kanin in northern 

 Russia has been described by Kihlmann 4 and Pohle. 5 In these districts 

 the tundra is differentiated into two components peat-hillocks and 

 puddles : 



The peat-hillocks are large cushions of moss which form isolated 

 hillocks or long ridges. They rise to a height of two or three metres 

 above the general level of the surrounding surface. Pohle is of opinion 

 that the peat-hillocks are quite normal clumps of moss, which, in the 

 course of centuries or possibly millennia, have arisen by gradual growth. 6 

 The peat-hillocks during winter are devoid of snow, whereas the surround- 

 ing depressions are filled with it. Consequently there arises a distinction 

 in the soil. Where the snow lies deep, the low temperature prevailing 

 during winter cannot penetrate deep into the soil, which remains relatively 



1 Porsild, 1902. * Middendorff, 1867. 3 Porsild, 1902. 



4 Kihlmann, 1890. ' Pohle, 1903. 



* See the remarks made upon regressive development by Cajander, 1905 b. 



