FLOWERS OF THE ROCKS AND WALLS 



199 



rocks in this couiitiy are as a whole the oldest 

 rocks, and are mainly siliceous or sandy, the 

 factor that plays tlie greatest part is apparently 

 altitude. There are, in fact, no alpine chalk 

 plants, and limestone plants do not range as 

 high as 2000 ft. Hence the arctic-alpine vege- 

 tation more or less monopolizes the most 

 elevated siliceous or sandstone (or volcanic) 

 rocks. 



In the more lowland areas the modified 

 petrophytic vegetation is dependent more es- 

 pecially upon soil or rock characters. Thus 

 there are plants that grow on siliceous rocks, 

 sandstones, limestone, chalk, along the coast, 

 where the conditions of soil or rock are modi- 

 fied by halophylic conditions, or on the shallow 

 soils of sandy or gravelly areas. 



The characteristic limestone types here de- 

 scribed are Rue-leaved Saxifrage, Horseshoe 

 Vetch, Cheddar Pink, Golden Rod (in part), 

 Mouse-ear Hawkweed. .\ few need clay, as 

 Meadow Sa.xifrage, whilst some humus is 

 needed by Yellow Fumitory and Houseleek, 

 and gravel is required by Rampion and Field 

 Speedwell. The rest, when not rupestral or 

 crevice plants, are typically found upon sandy 

 or siliceous soils. 



The wall plants are found upon rock or 

 brick, or on sandy, loamy, or gravelly soil. 



Methods and Objects of Survey. — The pro- 

 gressive or retrogressive character of the 

 vegetation of mountain-tops, where the soil 

 is constantly changing its position, demands 

 a close study of the ground, so that the order 



of succession of the colonizing plants, whether 

 initially cryptogams or not, may be elucidated. 

 The open or closed nature of the formation 

 requires a similar method In either case, and 

 a survey of the surface in detail is essential. 

 The special adaptations of the plants, owing 

 to the particular conditions, such as wind, 

 slope, soil, depth, sun, rainfall, snow-line, in- 

 solation, Sec, are only to be determined by 

 such methods. 



The exact altitudes attained by each species, 

 and the aspect and extent of the formation, 

 association, or society, are other factors that 

 are of especial Interest. As a rule, the study 

 of highland vegetation is open only to the 

 favoured few who live In the vicinity of such 

 areas. But it Is suggested that an attempt 

 be made to study a typical area, by visiting 

 one of the mountainous areas in summer or 

 autumn. 



In the more generally accessible lowland 

 areas the relation of the vegetation upon bare 

 rocks to the more stable vegetation upon the 

 deeper soils derived from them may be studied 

 in the same way and with greater ease. 



The flora of a wall, as it is artificial, requires 

 rather different treatment; It need not be merely 

 a florlstic study or enumeration of species upon 

 particular walls, but may also be studied from 

 the wider ecological standpoint. The position 

 of mural plants upon the wall, their relation to 

 aspect, the extent of each society, the adapta- 

 tions to dry conditions, are some of the points 

 that should be considered. 



