FLOWERS OF THE ROCKS AND WALLS 



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SECTION XII 



FLOWERS OF THE ROCKS AND WALLS 



Lithophytes. The plant formations hitherto 

 considered have been either aquatic or terres- 

 trial, and in the latter case made up of plants 

 that grow upon soil derived from underlying 

 rocks, often of considerable thickness. 



The rupestral or rock plants differ in grow- 

 ing upon the rock itself or in clefts. In general 

 they are called lithophytes. One condition 

 affecting such lithophytes is the slope of the 

 surface, which when steep has no loose soil or 

 detritus, but when little inclined is covered 

 with the latter. The composition of the rock 

 is important, determining the distribution of 

 plants that prefer lime or not, and so on. The 

 hardness, cleavage, specific heat, and water- 

 bearing capacity or porosity are also important 

 factors. Owing to the irregularity of the sur- 

 face, lithophilous plants are of variable char- 

 acter as regards their adaptation to dry or 

 moist conditions. The formations are thus 

 not generally continuous, but like a patch- 

 work quilt, occurring in small societies or 

 florulas. ~ 



The plants included in this section, which 

 embraces artificial habitats such as walls, 

 sandy and gravelly fields, &c., are also found 

 upon soils derived from rock surfaces in some 

 cases. 



Chomophytes and Chasmophytes. Litho- 

 phytes (or petrophytes) form the vegetation, 

 including cryptogams, on the surface of rocks 

 or stones, which can colonize steeply inclined 

 or bare rock. 



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 else- 

 where, 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 fre- 

 quent displacement of rock the surface is con- 

 tinually 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, &c. 

 Others not arctic-alpine occur, as Globe Flower, 

 VOL. v. 



Hairy Bitter Cress, Wood Sorrel, Northern 

 Bedstraw, &c. 



Chasmophytes are crevice plants in fine sand, 

 &c., which accumulates, with particular con- 

 ditions 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, Or- 

 pine, Golden Rod, Wall Lettuce, and Ivy- 

 leaved Toad-flax (chasmophytes). 



Walls. Walls are the artificial counterparts 

 of rock habitats, and are dispersed uniformly 

 over the country in the lowlands, as well as 

 at higher altitudes. In the neighbourhood of 

 villages and towns they are composed of a 

 variety of materials. Where natural exposures 

 of hard rocks do not occur, walls are largely 

 composed of brick. The upper surface forms 

 a habitat for a number of plants, where some 

 detritus has accumulated, and here one may 

 find Rue-leaved Saxifrage ; but as it is a lime- 

 loving plant to a great extent, it is more fre- 

 quent in the interstices of the mortar, where 

 Ivy-leaved Toadflax also grows. 



There were formerly a large number of mud 

 walls in differents parts of the country, a sur- 

 vival from the days when houses were similarly 

 constructed of mud; barns and outhouses are 

 frequently built of mud to-day, having in 

 many cases a top or roof of thatch. This 

 latter is a favourite habitat for Vernal Grass, 

 Rue-leaved Saxifrage, Thale Cress, Biting 

 Stonecrop, Houseleek, Field Speedwell, and 

 Flat-stalked Poa. 



Whilst walls vary in composition, the habitat 

 is resorted to by the plants that occur upon it, 

 not so much because of such differences, but 

 because it is uniformly a dry habitat. Upon 

 old walls, especially in less -frequented dis- 

 tricts, a large number of plants may be found. 

 Sometimes Furze establishes itself, whilst in 

 sheltered, moist spots Navelwort may grow in 

 the crevices, Shining Crane's Bill, Wall Rue, 

 and many ferns. 



Artificial Character of Walls, &c The 



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