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., 
Sedum—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 petrea, Myosotis 
collina (see p. 107). 
On limestone rocks the following plants may occur, in 
addition to the three annuals mentioned : 
Rosette-Plants : Arabis hirsuta, A. stricta, Draba muralis, 
D. incana, Thlaspi alpestre, Saxifraga tridactylites, Hiera- 
cium Pilosella. 
Succulents : Sedum Telephium, S. album, S. acre, 8S. 
reflecum, S. rupestre, S. anglicum. 
Other Plants: Thalictrum minus var. calcareum, Heli- 
anthemum Chamecistus, Dianthus cesius (Cheddar-pink, 
found only in the Cheddar Gorge), Arenaria serpyllifolia, 
A. verna, Geranium sanguineum, G. lucidum, Lactuca 
muralis, Parietaria officinalis, Festuca ovina, Asplenium 
Adiantum-nigrum, A. Trichomanes, A. Ruta-muraria, 
Cystopteris fragilis. 
Ill. 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 alge, lichens, and mosses, which 
assist in the disintegration of the mortar, and so prepare 
