280 PAULSON : NOTES ON THE ECOLOGY OF LICHENS.
There the lichens are not abundant on the tree trunks, but the
exposed thick horizontal roots are sometimes covered with
common species, mixed with the moss, Hypnum cupressiforme
var. filiforme. The lichens of the Forest beech woods compare
very favourably with those of the beech woods of Hertfordshire
and south-east Buckinghamshire in the matter of number of
species and the percentage of trunk covered. There is a special
group of saxicolous lichens on the exposed flints in the woods
of the chalk area of the two counties just mentioned. This
group is represented to some extent on the pebbles among the
beeches of Epping Forest, where Lecidea crustulata Korb and
Rhizocarpon confervoides D.C. occur.
I am unable for the present to suggest the reason for the
beeches on some parts of the North Downs being so well covered
with lichens on the sunny side of trees on the south of the wood.
If comparing the beeches of the Forest with those of the Downs
only, one would be inclined to suggest that the more humid
atmosphere of the Forest area is suitable for the develop-
ment, of algae on beech trunks ; but one has to remember that
the beeches of S.E. Bucks in similar situations (that is. the
outer trees on the south side of the woods) are not so well covered
with lichens as those of the woods on the North Downs in the
neighbourhood of Dorking.
Owing to the constant peeling of the bark, birches (except
towards the base, where the bark, is deeply furrowed) have very
few lichens upon them ; but Lecanora varia, which develops
rapidly, does obtain a hold, even upon very young branches.
The great majority of birches within the Forest are of no great
age. They are mostly on those parts where, through indis-
criminate felling or through fires, the woodland is degenerating
into heath.
Although I have mentioned a number of lichens on the smooth
bark of hornbeams and the absence of such on the bark of birches,
I conclude, when other points are taken into consideration, that
bark is not always a factor of the greatest importance in deter-
mining the presence of common lichens in a given association.
Parmelia physodes, perhaps the commonest of foliaceous lichens
within our area, is met with on the bark of most trees and is a
member of a number of associations : yet there are conditions
that at first appear remote which are unfavourable to its healthy