PAULSON : NOTES ON THE ECOLOGY OF LICHENS. 281
development. In the environs of London, it is less abundant
in woods upon heavy London Clay than in those on a lighter soil.
It has been suggested that this is owing to a difference of light-
intensity ; for the woods of the lighter soils have a considerable
number of birches mixed with the other trees. Parmelia phy-
sodes does not require the extra light, as do P. sulcata and P.
caperata ; for, when foliaceous lichens are found on the northern
(darker) aspect of a tree, it is P. physodes that is most abundant.
As far as I can judge at present, it appears that the nature
of the soil influences the growth of lichens upon the tree-trunk.
The lichen is there as the result of the sum of the factors that
go to produce the wood in which it grows abundantly.
It is when we compare the Epping Forest heathlands (i.e.,
bare heath and oak-birch-heath associations) with similar asso-
ciations around London that we begin to realise how com-
paratively rich the small amount of heathland is. There is also
evidence that the number of species is gradually increasing.
The lichen flora around High Beach and away to the north
by Jack's Hill and Epping is not what it is solely because the
ground is higher and the atmosphere purer, but because of the
nature of the soil.
In the table on the following page, I have compared the
lichen flora of the heath on the drained gravel of Jack's Hill
with that of the dry heath at Oxshott, Surrey, on Bagshot
sand, and with a dry heath near Sevenoaks, Kent, on Lower
Greensand. Again, I have taken the wet heath of Woodredden
Hill and a portion of Oxshott Heath liable to flood owing to
the presence of "pan." The flora is practically the same in
the three localities for dry heath, and in the two others where
the heath is periodically very wet.
The lichens of the Forest heathlands have shown clearly
during the past year that growth and fertility vary from year to
year as with the higher plants—that there are good seasons and
bad seasons. Lowland lichens as a group should no longer be
regarded as plants living on from year to year in a semi-dormant
state and growing at such an excessively, slow rate as to appear
practically the same size at the end of a lengthened period as
they were at the beginning. Some are to be found upon a certain
spot during a succession of three or four seasons, and then
they suddenly disappear, as Peltigera spurea, described by