PAULSON : NOTES ON THE ECOLOGY OF LICHENS. 279
was discussed in the Report already referred to. An appreciable
increase in the volume of smoke drifting; over the Forest for a
considerable period of time would undoubtedly arrest the return
to a more vigorous growth of the lichens which has been evident
during the past twenty-five years.
As to the terricolous lichens, they are similar to those of the.
oak wood, though not so numerous or luxuriant in growth.
Where clearings have been made and by the sides of rides and
paths, there is a distinct increase in the number of species.
Graphidiaceae is very poorly represented in the oak woods
of the northern environs of London. Whenever I have noted
members of this Family, they have been on the bark of young
oaks of from 15 to 25 years growth and mostly in Quercus
sessiliflora associations. They develop before the bark has become
deeply furrowed, and that portion which they occupy remains
smooth and polished, resembling the bark of the birch. Many
of these lichens are hypophloeodal :—that is growth starts under
the surface. This appears to prevent the bark from becoming
furrowed, and, if this be the case, must mean that growth
commences some years before apothecia are formed. The lichens
upon the young oaks gradually give place to members of the
Parmeliaceae, as Parmelia physodes Ach., P. sulcata Tayl., P.
Borreri Turn. From the fact that some lichens are transitory,
it becomes necessary, when dealing with plant-associations, not
only to note those present, but also to record the approximate
age of the tree on which they are growing. The trunks of oaks,
Quercus Robur, on the low ground in the southern portion of
the Forest, are singularly bare of any epiphytes, except one
alga, Pleurococcus. This state we find elsewhere quite charac-
teristic of Quercus Robur woods on heavy London clay.
From the above notes, we conclude that the lichen flora of
an oak-hornbeam association, differing considerably from the
Quercus Robur association, tends to strengthen the view that
such a wood represents a definite sub-type.
We do not find that the beech woods to the north of London
have an abundant lichen flora, not even some of those on the
chalk in south-east Bucks, the smooth bark of the trees being
frequently covered with a growth of alga. The trees are often
crowded, and the light intensity is low, as at High Beach.
In Monk Wood, there is much greater space between the trees.