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.
The plant formations hitherto considered have been either aquatic or terrestrial, and in the latter case made up of plants that grow upon soil derived from underlying rocks, often of considerable thickness.
The rupestral or rock plants differ in growing upon the rock itself or in clefts. In general they are called lithophytes. One condition affecting such lithophytes is the slope of the surface, which when steep has no loose soil or detritus, but when little inclined is covered with the latter. The composition of the rock is important, determining the distribution of plants that prefer lime or not, and so on. The hardness, cleavage, specific heat, and waterbearing capacity or porosity are also important factors. Owing to the irregularity of the surface, lithophilous plants are of variable character as regards their adaptation to dry or moist conditions. The formations are thus not generally continuous, but like a patchwork quilt, occurring in small societies or florulas.