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 outstanding feature of plants growing in these habitats is the adaptation (generally speaking) to dry conditions. Practically all of them are xerophytes. Mural plants are in other habitats largely mesophilous, but on walls develop xerophytic adaptations.
Upon the summitsof loftymountains the main feature of the vegetation is the dwarfed nature of all the plants. This is more or less what might be expected, since the vegetation merges into, or follows upon, the lichen and moss zone, which is dwarf in an extreme degree. Many of the plants have the cushion habit, e.g. Silene acaulis, Arenaria sedoides, Saxifraga hypnoides. Others form mats, as Azalea pro-cumbens, Potentilla Sibbaldi, Empetrum nigrum, Saxifraga umbrosa, S. oppositifolia, Lu-zula spicata, Carex rigida, Vaccinium myrtillus, Salix herbacea. The rosette habit is exhibited by Ranunculus acris, etc. Many plants are viviparous.
The trees that occur are mere bushes. The Juniper on Snowdon is only an inch or two in height, and on Great Orme's Head the Cotone-aster is similarly dwarfed, owing to the exposure to wind. Hairs, etc, are developed which act as screens from the wind, and help to retain moisture. The dwarf character may be assigned to the effect of intense sunlight. The mat, cushion, and rosette habits are largely due to wind exposure, and to the ready evaporation, and generally dry conditions.