FLOWERS OF THE ROCKS AND WALLS 



197 



&c.. play a considerable part in ihe accuniu- 

 lalion of talus, as does rain-wash against verli- 

 cal cli(Ts, whilst in the past glacial action has 

 produced a considerable scree or detritus at 

 higher elevations. 



Rock debris is formed and is colonized by 

 lichens and mosses, which provide a layer of 

 humus and pulveri/.ed grit or sand, &c., lor 

 higher plants. Indeed, a large part of it is 

 colonized by Rhacomilriiini, lorniing an open 

 association with bare patches of rock between, 

 which lower down becomes closed and includes 

 higher plants, and still lower is succeeded by 

 heath. 



It is exposed to wind, to intense heat, and 

 drought, owing to the thin soil and steepness 

 of the slopes, and is subjected, also, to insola- 

 tion and the prolonged influence of a snow 

 covering. The intensity of the struggle for 

 existence under such conditions is shown by 

 the paucitj- of species. 



Habitats of Plants of Walls and Rocks.— 

 Rock pUuits are aflected by a variety of 

 factors, which in dilVerent areas and circum- 

 stances help to render the habitat diverse. 

 The vegetation of the arctic-alpine rocks is 

 typically of a dwarf habit, owing to altitude 

 and exposure and the poverty or thinness ef 

 the soil layer. It varies in respect of the 

 derivation of the latter. The mountain -top 

 detritus forms layers on the surface. Else- 

 where, in corries, &c., there are crevices with 

 deeper soil and wetter conditions. At lower 

 altitudes the rocks and soils vary in com- 

 position. 



There are siliceous soils with a characteristic 

 vegetation which is found in some cases on 

 bare rocks of this type or in crevices. Sand- 

 stones and sands have an equally characteristic 

 flora. 



On limestone the soil is thin, and the flora 

 m.iy be regarded as petrophilous to a great 

 extent, limestone pavements in particular 

 illustrating this point. The soil of the chalk 

 is markedly deficient, and many chalk plants 

 are petrophytes. .Along the coasts on cliffs 

 and rocks a halophytic rupestral flora is found. 

 Sandy fields and gravelly commons are special 

 types of sandstone or loose sand formations, but 

 of newer date. Walls have a particular flora of 

 their own, and many plants are confined to 

 them and to old ruins. The plants found 

 upon rocks are also common to mural habitats, 

 owing to the occurrence in both cases of dry 

 conditions and a thin soil layer. 



Habits of Plants on Rocks and Walls.— 

 The outstanding feature of plants growing in 

 these habitats is the adaptation (generally 

 speaking) to dry conditions. Practically all of 

 them are xerophytes. .Mural [)lants are in 



other habitats largely mesophilous, but on 

 walls develop xerophytic adaptations. 



Upon the summitsof lofty mountains 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 acaiilis, Arenaiia seJuiih's, Sa.xifra^^a 

 ItyjiHoidcs. Others form mats, as Azalea pro- 

 cumbens, PolentiUa Sibbaldi, Rmpetrum ni- 

 grum, Saxifraga unihrosa, S. oppositifolia, Lu- 

 sulaspicala, Carex rigida, Vaccinium mvrlillus, 

 Sa/ix herhncea. The rosette habit is exhibited 

 by JiatiiinLuhis acris, &c. 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 Ornie's Head the Cotone- 

 aster is similarly dwarfed, owing to the ex- 

 posure to wind. Hairs, &c. , 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. 



The Height of Rock and Wall Vegetation. — 

 .\ltitude is largely responsible for the dwarf 

 habit of alpine plants. When these are found 

 at lower altitudes, or in moist situations, they 

 are taller. The plants that grow on rocks are 

 in general not very tall, nor are the crevice 

 plants in exposed situations, but in lowland 

 districts in wooded areas Orpine may be 

 luxuriant and grow to a height of 2 ft. The 

 habit of the plants is intimately connected with 

 their height; and mat, cushion, or rosette 

 plants are consequently more or less dwarf. 

 Thus Vernal Whitlow Grass is rarely more 

 than 5 in. in height even in flower; Cheddar 

 Pink, 8 in.; Field Mouse-ear and Sandwort 

 Spurrey, both trailers, are 6 in. and 2 in. respec- 

 tively; Bird's-foot is 2 in.; Horse-shoe Vetch, 

 6 in.; Rue-leaved Saxifrage 6 in.; Meadow 

 Saxifrage and Navelwort, growing in more 

 sheltered situations, reach a foot or more in 

 height. 



Dry conditions limit not only the height 

 but also the extent of tlii' branching or size 

 of leaves. 



The Flowering of Rock and Wall Plants. — 

 The principal factors that regulate fl(}wering 

 are, in the case of terrestrial plants, excessive 

 rainfall, increased altitude, exposure to wind, 

 insolation, or interrupted sunlight. The dura- 

 tion of the plant also influences the flowering, 

 and to some extent size enters into these 



