TEN YEARS' PROGRESS IN LICHENOLOGY. 279
lichens upon them. Rhizocarpon confervoides is frequently the
most abundant lichen on pebbles by the sea-shore.
Among the more stable sand-dunes, and in situations between
the parallel sand-ridges, there is often an abundant lichen-flora
as can be well seen at Studland, south of Poole Harbour. Al-
though growth may be luxuriant, the number of species is small,
the dominant species being often Cetraria aculeata var. acanthella,
or some species of Cladonia. The Cetraria is well adapted to
growth in a sand-dune area, for it is more luxuriant and more
frequently fertile in such a situation than on the sand and gravel
of the upland heath.
R. C. McLean (15) recognizes two plant formations, (1) the
Dune, and (2) the Shingle. From these arise five associations,
three belonging to the Dune formation as Bare Sand, Grey
Dune and Derelict Dune, and two to the Shingle formation,
distinguished as High Shingle and Low Shingle. The Shingle
is not that on the seaward side of the main bank, which for very
obvious reasons is totally bare, but shingle on the landward side,
which forms promontories projecting into the sea-marshes, and
is washed only occasionally by the waves of a high-tide.
McLean's investigation of lichens among shingle throws
additional light upon the comparatively rapid growth of certain
species, a feature that had been described in "Lichens of Epping
Forest" (2). He concludes that the growth of certain lichens
is more rapid than has been generally assumed to be the case,
the reasons for such conclusion being fully explained in the
paper referred to.
In "Lichens of S. Lancashire" (5b), the authors express
the opinion that lichens among sand-dunes are largely dependent
on higher plants for the creation of edaphic conditions requisite
for their development and existence. As a rule, however, on
rock surfaces lichens are the pioneers, and are followed by mosses,
grasses, etc.
Breckland, or the Breck country, a district covering an
area of about 400 square miles, is situated partly in south-west
Norfolk and partly in north-west Suffolk. It comprises for the
most part a series of sandy heaths and commons of low elevation.
The ecology of the vegetation of this area is described by E.
Pickworth Farrow (17). His attention had been drawn to certain
areas, varying in size, that were quite bare of vegetation, but