2§0 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, Hypmim 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 v/oods 
of the chalk area of the tv'o counties just mentioned. This 
group is represented to some extent on the pebbles among the 
beeches of Epping Forest, where Lecidea cnistulata 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 w r ell covered 
with lichens on the sunny side of trees on the south of the w'ood. 
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, wirere the bark, is deeply furrowed) have very 
few r 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 w'oodland 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, w r hen other points are taken into consideration, that 
bark is not ahvays a factor of the greatest importance in deter¬ 
mining the presence of common lichens in a given association. 
Pavmelia 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 
