LICHENOLOGY OF ICELAND 193 
of sometimes one, sometimes another species; for instance, as 
Jønsson has shown, there often occurs a fairly pure vegetation of 
Nardus on slopes (Lier); also a rather pure Agrostis (vulgaris)-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, we 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. 335—36), stating that 
Grass-slopes (Gres-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 (Græs-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 
