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.
Chomophytes are plants that can colonize rocks where detritus has accumulated, whether on the surface or in crevices. In this country chomophytes are developed upon the detritus formed at high altitudes, in Scotland and elsewhere, at altitudes of 2000-3000 ft. or more, and in ravines or corries at 3000 ft. or more, where arctic-alpine vegetation occurs. The plants are generally the highland species of Watson. The mountain-top detritus is largely of Glacial origin, and the plants are northern, perhaps Preglacial. The rocks vary in different districts in composition. Owing to the frequent displacement of rock the surface is continually being recolonized. It is in the corries that the chomophytes occur, such as many arctic Saxifrages, Saussurea, Gentiana nivalis, Erigeron alpinum, Veronica saxatilis, Arenaria rubella, Draba rupestris, Carex atrata, etc. Others not arctic-alpine occur, as Globe Flower, Hairy Bitter Cress, Wood Sorrel, Northern Bedstraw, etc.
Chasmophytes are crevice plants in fine sand, etc, which accumulates, with particular conditions depending on water content. The true chomophyte vegetation is thus almost entirely upland, though lowland plants occur at high elevations amongst the others. Some of these have already been mentioned in discussing the flora of hills and dry places, including Silky Mountain Vetch, Oak-leaved Mountain Avens, Wild Thyme. There are certain other types that at lower elevations may be considered as chomophytes or chasmophytes of the more lowland rocks, as Biting Stonecrop and Mouse-ear Hawkweed (chomophytes), Navelwort, Orpine, Golden Rod, Wall Lettuce, and Ivy-leaved Toad-flax (chasmophytes).