130 FIRST FORMS OF VEGETATION. 
they follow boulders or erratic blocks of these 
rocks into localities that are widely different 
from their native habitats. Lichens peculiar to 
the mountain summits may thus be found on 
lowland plains and even at the sea level. One of 
the most remarkable examples of the connexion 
between lichens and the lithological character 
of their basis of support, may be seen in the 
development of species of lichens, sub-alpine and 
arctic in character, and totally different from the 
surrounding lichen-flora, upon the huge boulders 
spread over the great North German Plain, which 
came originally from Norway and Sweden. 
Certain trees attract certain lichens, which rarely 
desert them: for other trees. Many species are 
found most abundantly on pine-trees ; others on 
oak and ash, and others on the beech and birch; 
their growth on these trees being determined by 
the facilities which the bark affords for attach- 
ment or nourishment. Some lichens are found on 
rocks on the sea-shore, and do not flourish inland 
beyond the reach of the salt spray wafted by the 
winds. Of these the most remarkable are the 
Parmelia aguila, and the Ramalina scopulorum. 
The former is easily distinguished by its tawny- 
brown, sun-burnt colour, and the very numerous 
and much divided narrow segments of the thallus. 
It covers rocks on almost all our sea-shores, 
