LICHENOLOGY OF ICELAND 193 



of sometimes one, sometimes another species; for instance, as 

 Jonsson has shown, there often occurs a fairly pure vegetation of 

 Nardns on slopes (Lier); also a rather pure Agrostis (vulyaris)- vege- 

 tation, and Nardus-Agrostis-slopes (Lier). 



Consequently, it is not possible to give a more detailed division 

 according to associations, but from a lichen-ecological standpoint 

 this is of no consequence, because the different species of grass 

 differ only slightly as competitors with lichens, and can therefore 

 very well be treated collectively. 



On the other hand, w r e have good knowledge of the substrata 

 which support the grass, which is usually divided into associations 

 according to the substrata at least partially. 



Thoroddsen discusses this exhaustively and instructively (vol. 

 I, pp. 33536), stating that 



Grass-slopes (Grses-li) occur on sloping ground with loose 

 soil and a level surface (not knolly) at the foot of mountains, both 

 when the mountain is tuff and when it is basalt. 



Knolly grassland (Grses-Mo) is extremely knolly, clayey 

 ground, intermixed with humus. 



A third type ("dry uncultivated grassland" loc. cit. p. 337) is 

 without knolls and has a sandy, gravelly or pebbly substratum and 

 an open plant-covering. 



Home- field (Tun) is the cultivated, manured grassland around 

 the farm-buildings. 



The conditions afforded the lichens in the grass-vegetation are 

 chiefly characterized by the fact that the plant-carpet is quite low, 

 being only a few centimetre high; besides this the shoots, and 

 especially the leaves, frequently stand more or less erect, so that 

 abundant light usually falls between them. The amount of light is 

 very favourable to lichens even in the most luxuriant carpet; on 

 the other hand, the vertical direction of growth of the grass is a 

 very grave hindrance to the crustaceous earth-lichens, which cannot 

 of course force their way athwart this. On the other hand, as re- 

 gards the fruticose and the erect foliaceous lichens this hindrance 

 is of no great importance. Consequently, it will be easily under- 

 stood, that crustaceous lichens can occur in abundance only in 

 places, where the grass-carpet is open, so that they can grow di- 

 rectly on the surface of the ground, or here and there, where the 

 grass is closely cropped (by grazing sheep, etc.), they can grow 

 across the tufts. 



The Botany of Iceland. Vol. II. 13 



