278 THE ESSEX NATURALIST.
lichens, e.g. Verrucaria nigrescens, V. maculiformis, Placodium
tegularis and P. lobulatum, which in many parts of the country
are restricted to calcareous rocks, were observed on silicious
rocks on the Arran coast. From this the authors conclude that
these exposed spray-washed maritime rocks afford the requisite
Xerophytic conditions which in other districts are only furnished
by limestone, and that the chemical composition of the sub-
stratum does not come into question in these cases.
It has elsewhere been demonstrated, "Lichens of Epping
Forest" (18), that substratum is not always the main determining
factor in the appearance of particular species. They do not always
grow upon the bark of the oak (for instance) mainly because
it is oak, for the oaks in a Quercus pedunculata wood on London
clay and those of a Quercus sessiliflora wood on a better-drained
lighter soil, at a short distance from the former, exhibit a marked
contrast relative to the percentage of trunk covered by lichens,
and also in the number and luxuriance of the species of the latter.
Edaphic conditions of the soil appear to exercise considerable
influence, favourable or unfavourable, on the lichen growth on
oak trunks of such woods.
S. West published a series of observations in 1914 on the Crypto-
gamic flora of tree trunks growing in exposed mountainous situa-
tions in the British Isles. He found that the moss Stereodon
cupressiformis var. filiformis was by far the most abundant
epiphyte, and that the lichen Parmelia saxatilis came next.
In the general summary of his observations he does not compare
trunk with trunk, i.e. oak with ash, or ash with beech, but
gives results as to percentage of surface of trunk covered.
On a low-lying sea-coast, bordered by shingle banks and
sand-dunes, powerful factors inimical to lichen development are
frequently active ; they are (1) the mobility of the sand, which
effectively prevents any form of vegetation from establishing
itself ; and (2) the scouring force of the sand blast. Vegetation
cannot withstand the constant attrition caused by blown sand.
Surfaces of pebbles exposed to it are rapidly denuded of all sub-
stances that have accumulated upon them during a period when
the sand blast was not active.
There is a marked difference between the surfaces of pebbles
exposed to the sand blast and those that are more or less sheltered.
The former are bare, while the latter have crustose forms of