FLOWERS OF THE ROCKS, WALLS, STONY 

 AND GRAVELLY PLACES 



W'e have already alluded, under Section V'l on Mountains, Hills, 

 and Dry Places, to certain Lithophytes, chiefly surface plants, which 

 were grouped together primarily on a physical basis — their altitude. 

 In this section we include not only the surface plants (Exochomo- 

 phytes) but true Lithophytes. Rock plants may be called Petrophytes, 

 and include Lithophytes and Chomophytes. Here, also, as a sub- 

 division of the latter, are some crevice plants growing in vertical 

 fissures in rocks, or Chasmophytes. 



As the group is kindred in character to, but diverse in origin from, 

 the plants found on walls with the rupestral plants, a number of 

 purely mural plants are included, and a few found on sandy or gravelly 

 fields, of a local character, allied to those growing on rubble or shingle, 

 but not identical. .Several features characterize the existence of 



vegetation on rocks. 



The plants are lovers of dry soil, and may be called in general 

 rupestral or saxicolous. There are about i8o of these, and of that 

 number 120 are alpine, 60 lowland species. Those here alluded to 

 chiefly belong to the latter. About a dozen prefer wet rocks, and 

 twenty are distinctly calciphilous or lime-lovers, and so on. Much 

 depends on how far the rocks are exposed in relation to sunlight. 



The slope of the surface is of importance, as horizontal surfaces 

 accumulate detritus, whilst vertical rocks do not. Of paramount im- 

 portance is the composition of the rock chemically, or looked at in 

 a different way, to what geological formation it belongs. If it is 

 siliceous and hard, it nourishes a certain flora quite distinct from that 

 found on calcareous rocks. The porosity, again, is of great importance. 



The true Lithophytes are chiefly Cryptogams, such as Alg:e, 

 Lichens, and Mosses, which are all typically Xerophytes. Upon the 

 same rocks, where humus has accumulated, grow Biting Stone Crop 

 and Ling. Chasmophytes are those plants that grow with their roots 



131 



