190 THE BIOLOGY OF FLOWERING PLANTS 



According to Farmer the specific conductivity of the wood 

 of trees and shrubs with sclerophyllous foliage (both of arid 

 and of temperate regions) is small ; for Quercus Ilex, the 

 holly oak, the figure is 32, and for the holly it is 9. 



§ 28. Moorland Xerophytes 



Such plants occur in the most diverse stations. The 

 rock-roses grow in dry exposed gravels and chalks. Loise- 

 leuria is an alpine in this country, and arctic in distribution 

 elsewhere ; it is sometimes subject to strong insolation, but 

 its xerophytic character must be partly related to its ever- 

 green habit. The heaths are typically a South African 

 genus ; some species occur in dry stations in the Mediter- 

 ranean region ; in central and northern Europe they are 

 plants of the heaths and moors. Calluna, in Britain, is 

 characteristic of the dry moors on thin peat, really a type of 

 heath, where in summer extreme drought may prevail. 

 But it grows also in the wet soil of true moor on deep peat, 

 and is there accompanied by Erica Tetralix, by Myrica Gale, 

 and by species of sedge, cotton-grass, and xerophytic grasses. 

 In fact, the vegetation of our wettest moors is markedly 

 xerophytic, and this is the case in the moors of temperate 

 climates in general. The problem of reconcihng the 

 xerophytic character of the vegetation with the wet soil 

 conditions has always exercised the ingenuity of botanists. 

 Though we cannot yet be said to have a full understanding 

 of the subject, we now know that a whole series of factors 

 is involved. 



The earliest clear explanation was that offered by 

 Schimper, who distinguished soils bearing a xerophytic 

 type of vegetation into those which were physically and those 

 which were physiologically dry. The latter include moorland 

 soils, saline soils, and cold soils. By physiological dryness 

 he meant simply the introduction of some factor which 

 made the absorption of water diflicult ahhough t"here was 

 plenty available ; in the case of cold soils the low tempera- 

 ture, in saUne soils the high osmotic pressure of the solution. 



