LICHENS OF HATFIELD FOREST 169
bank at the edge of a place where gravel had long previously been
removed; C. rangiformis was collected from ant-hills dominated by
Hieracium pilosella.
The present list, although almost certainly not exhaustive, probably
includes at any rate most of the commoner and more conspicuous
elements now constituting the lichen flora. In its composition this flora
is probably typical of the woodlands of rural Essex generally. The
scarcity or absence of certain species (e.g. Ramalina spp.) coupled with
the abundance of others, particularly Lecanora conizaeoides, is usually
regarded as being indicative of a certain measure of atmospheric pol-
lution, a condition which has probably increased steadily in eastern
and central England over the last hundred years or so. In this con-
nection, it is noteworthy that Holmes stated in his remarks on his list
of 1890 that the following six species were the commonest on trees in
Hatfield Forest: Pertusaria amara, P. globulifera, 'Lecidea canescens'
(= Diploicia canescens), Ramalina fraxinea, R. farinacea and Calicium
hyperellum. He did however say that the frondose species were small
and badly developed. Only two of these six species appear to be plenti-
ful in the forest to-day; the other four, which I did not find at all. would
appear to have diminished to a state of unimportance even if they have
not disappeared altogether.
This annotated list, which I do not pretend constitutes anything
approaching a survey, is sufficient to indicate that the lichen flora of
Hatfield Forest is still quite rich in species, but that it has probably
changed considerably during the last sixty or seventy years. As Holmes
said in 1890, the area would repay more careful study!
Woodham Walter Common
A Further Study in Woodland Ecology
BY W. R. MASEFIELD
INTRODUCTION
IN A previous paper (Masefield, 1957) it has been shown that the dis-
tribution of the trees on Woodham Walter Common can be related to
the types of soil and topographical features of the area. In that work, no
differentiation was made between the several species of the Quercetum. It
was suggested to the author that a finer distinction would reveal a cor-
relation between the separate species of oak and their environment. This
investigation is the result of the suggestion, and it has been undertaken
with the very able assistance, once again, of Mr. G. Clements of
Oxford.