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 1 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 
sessiliftora 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 
Ouercus 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. 
