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.
In the case of petrophytes or plants that grow upon rocks, or rock debris, or in crevices, the composition of the rock is of importance in determining distribution. But since the highest rocks in this country are as a whole the oldest rocks, and are mainly siliceous or sandy, the factor that plays the greatest part is apparently altitude. There are, in fact, no alpine chalk plants, and limestone plants do not range as high as 2000 ft. Hence the arctic-alpine vegetation more or less monopolizes the most elevated siliceous or sandstone (or volcanic) rocks.
In the more lowland areas the modified petrophytic vegetation is dependent more especially upon soil or rock characters. Thus there are plants that grow on siliceous rocks, sandstones, limestone, chalk, along the coast, where the conditions of soil or rock are modified by halophytic conditions, or on the shallow soils of sandy or gravelly areas.
The characteristic limestone types here described are Rue-leaved Saxifrage, Horseshoe Vetch, Cheddar Pink, Golden Rod (in part), Mouse-ear Hawkweed. A few need clay, as Meadow Saxifrage, whilst some humus is needed by Yellow Fumitory and Houseleek, and gravel is required by Rampion and Field Speedwell. The rest, when not rupestral or crevice plants, are typically found upon sandy or siliceous soils.
The wall plants are found upon rock or brick, or on sandy, loamy, or gravelly soil.