968 



of the alpine division, and to defer any remarks on Ihe less typical 

 vegetation on rocky flåts at lower altitudes. 



The cliff vegetation is a concept meant to cover a number 

 of dilTerent formations, which extend as a continuous series from 

 the exposed vertical cliff-w^all with its covering of lichens, to the 

 sheltered »Gjov« (ravine), with its terraces so closely clothed with 

 a luxuriant vegetation that the grassy sward overhangs the edge 

 and reaches down to the next terrace. The coast-cliff formation, 

 which is a special type of the cliff vegetation , has already been 

 described (pp. 931—934). 



The conditions of life of the cliff vegetation^ are characteristic 

 in that the piants must be capable of enduring wide ranges of 

 temperature, and of resisting periodical drought. The variations of 

 temperature are caused by the configuration of the clitTs, insolation 

 being greatest, when the sun's rays strike at an angle approaching 

 90'^ on a cliff-wall with a southerly exposure; cliffs facing north- 

 wards can only receive diffuse light. As a result, the clifF vegeta- 

 tion varies, according to aspect, much more than the grass-slope. 

 The important effects of exposure on vegetation have been poinled 

 out by nearly every author who deals with the plant-associations 

 of northern countries (Warming 1888, Hartz 1895, Rosen- 

 vinge 1897, Kjellman, Gunnar Andersson 1900, Kihlman, 

 Vestergren 1902, Hesselman 1905, etc). Special reference to 

 the Færoese conditions has been made by C. Jensen (1897, p. 

 165, 192, etc.) and in my own papers (Ostenfeld 1901, p. 27, 33, 

 35, etc). 



Great variations in the water-supply are also closely connected 

 with the insolation. The solar heat falling directly on south slopes 

 will rapidly reduce the water-content of the soil, especially on 

 projecting parts of the cliffs where a continuous fresh supply of 

 telluric water is frequently impossible, and only rain and fog are 

 available. The piants growing on such piaces must therefore be 

 able to endure periodic drought. On slopes with a north expo- 

 sure, there will always be, in a country like the Færoes, a rather 

 considerable degree of moisture in the soil. Apart from exposure 

 the amount of water available in each separate crevice or terrace 

 will vary considerably according to the amount of loose soil, the 

 nature of the water-supply, and other local circumstances. 



Whereas phanerogams and mosses have formed the integral 

 1 Cfr. E. Warming 1895, pp. 182—185, and Alb. Nilsson 1899, p. 93. 



