90 Vegetation of Sands and Sandstones 



the highest type of plant-community these soils have 

 developed, though it is also possible that they have 

 been completely disforested. But in many cases there is 

 a well-marked natural woodland-type characteristic of 

 coarse sandy soils. Such woodland generally alternates 

 with heathland or dry grassland, which often occupies 

 its edges and open places within the wood and bears 

 the same relation to the wood that neutral pasture bears 

 to damp oakwood. In other words much heathland 

 as well as dry heathy grassland is clearly degenerate 

 woodland of a certain type, while other heathland is 

 possibly primitive. It is frequently difficult or impossible 

 to decide whether the heathland is primitive or has been 

 derived from woodland. 



The occupation of a sandy soil by the heath-com- 

 munity is connected with an important 

 drv neat modification of the habitat by the accumu- 



lation of a comparatively dry acid peaty 

 humus (Trockentorf) in or on the surface soil. The 

 formation of this characteristic type of humus excludes 

 many species of plants from the flora, and at the same 

 time introduces a well-defined vegetation of its own. 

 These facts make it necessary to recognise the existence 

 of a new plant-formation, the heath-formation, allied to 

 the moor-formation but characteristic of a drier climate 

 or of drier edaphic conditions. Where woodland degene- 

 rates into heath we must therefore recognise a case of 

 succession in which one formation is replaced by another. 

 The researches of WoUny, P. B. Miiller, Ramann, 

 Graebner and others have made it probable that the 

 fundamental cause of the degeneration of woodland into 

 heath in north-west G-ermany (and it is natural to extend 

 their conclusions to south-east England, which has much 

 the same climate, and often repeats the same edapliic 

 features) is the slow washing out or leaching of the sandy 

 soil. 



