290 BRITISH PLANTS 



II. Sub-Alpine and Lowland Rocks. 



Below the alpine region exposed rocks and cliffs are 

 frequent, especially in limestone districts. Where the 

 rock is protected from the wind and supplied with plenty 

 of water, shrubs and low trees" are common, in some cases 

 forming thickets or woods, as in the scar-woods of the 

 Pennines. But in exposed situations on vertical cliffs 

 only xerophytic herbs become established. The general 

 habit of the plants is similar to those of alpine rocks, for 

 the environment is much the same. The cold, however, 

 is not so severe, and the plants can grow in drier situations. 

 Owing to this often extreme dryness, succulents — e.g., 

 Sedwm — are more abundant, but rosette-plants are still 

 common. Competition is not severe, and many annuals 

 are found, among < them several autumn-annuals which 

 flower early in spring, before the hot sun has parched the 

 soil — e.g., Draba verna, Hutchinsia pefrcea, Myosotis 

 collina (see p. 107). 



On limestone rocks the following plants may occur, in 

 addition to the three annuals mentioned : 



Rosette-Plants : Arabis hirswta, A . stricta, Draba muralis, 

 D. incana, Thlaspi alpestre, Saxifraga tridactylites, Hiera- 

 cium Pilosella. 



Succulents : Sedum TelepMum, S. album, S. acre, S. 

 reflexum, S. rupestre, 8. anglicum. 



Other Plants : Thalictrum minus var. calcareum, Hdi- 

 anthemum Chamcecistus, Dianihus ccesius (Cheddar-pink, 

 found only in the Cheddar Gorge), Arenaria serpyllifolia, 

 A. verna, Geranium sanguineum, 0. lucidum, Lactuca 

 muralis, Parietaria officinalis, Festuca ovina, Asplenium 

 Adiantum-nigrum, A. Trichomanes, A. Ruta-muraria, 

 Cystopteris fragilis. 



III. Walls. 



The vegetation of a wall is in many of its features 

 similar to that of ordinary rocks. The substratum is 

 dry, and, as a rule, poor in nutritive material. The 

 extent and variety of its flora will depend on the material 

 of which the wall is composed, whether held together by 

 mud or by mortar, and on its age. A brick wall is first 

 tenanted by minute algae, lichens, and mosses, which 

 assist in the disintegration of the mortar, and so prepare 



