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.
(Rupestral plants are essentially plants that grow on shallow soils, or in the clefts and crevices of rocks. A large number are found in hilly regions, chiefly in the west of England, or in Scotland and Ireland. This section therefore is to some extent similar to Section VII, but in the latter case the soil is usually less shallow. Many of these plants are alpine arctic types; a large number grow on the borders of rills or wet rocks. The flora of a wall is artificial, but many plants have adopted this habit, being native on rocks. Sandy and gravelly places are more akin to walls and rocks than fields or hills and dry places, and some plants are therefore included here that might be placed in either of the latter.)