This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Rock plants are affected by a variety of factors, which in different areas and circumstances help to render the habitat diverse. The vegetation of the arctic-alpine rocks is typically of a dwarf habit, owing to altitude and exposure and the poverty or thinness of the soil layer. It varies in respect of the derivation of the latter. The mountain-top detritus forms layers on the surface. Elsewhere, in corries, etc., there are crevices with deeper soil and wetter conditions. At lower altitudes the rocks and soils vary in composition.
There are siliceous soils with a characteristic vegetation which is found in some cases on bare rocks of this type or in crevices. Sandstones and sands have an equally characteristic flora.
On limestone the soil is thin, and the flora may be regarded as petrophilous to a great extent, limestone pavements in particular illustrating this point. The soil of the chalk is markedly deficient, and many chalk plants are petrophytes. Along the coasts on cliffs and rocks a halophytic rupestral flora is found. Sandy fields and gravelly commons are special types of sandstone or loose sand formations, but of newer date. Walls have a particular flora of their own, and many plants are confined to them and to old ruins. The plants found upon rocks are also common to mural habitats, owing to the occurrence in both cases of dry conditions and a thin soil layer.