374 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



woody plants, the Mosses seem to control matters as long as they 

 remain fully active. However, with draining or natural drying out 

 of the surface, the succession usually proceeds to the local forest 

 or other climax — such as highmoors or drier heathlands in exposed 

 situations. Lowland- or meadow-moors are usually hydroseral 

 communities occurring on circumneutral peat accumulations result- 

 ing from the filling up of lakes and preceding the timbered stages 

 of succession. Here the upper layers above the level of ground- 

 water are often acidic in reaction, and either support heathland 

 communities dominated by Heaths in drier parts, or bog-like ones 

 dominated by Cotton-grasses or Moor-grass [Molinia coerulea) in 

 damper parts. Where acid conditions are developed by such humus 

 accumulation above basic rocks, ' flushes ' from springs or superficial 

 drainage may continually bring down fresh supplies of basic salts 

 and lead to ' spring flush ' communities very locally. Especially in 

 regions of marked topographical and geological variation is it common 

 to find whole series of moorland, dry heathland, pastured grassland, 

 and scrub or woodland communities existing side by side within a 

 relatively small area. 



Besides the more extensive hinterland communities already men- 

 tioned, those of sea and other cliflFs, and of river and other banks, 

 may be distinctive, though usually they are very limited in area and 

 too variable for detailed consideration here. Notable, however, is 

 the occurrence of cushion and other alpine plants along the dry 

 margins or beds of watercourses in the lowlands of mountainous 

 temperate countries on both sides of the Equator. Presumably their 

 disseminules are washed down from the uplands and their growth 

 is favoured by the ' open ' conditions and general lack of competition 

 by ranker lowland types, which would prevent their ecesis or rapidly 

 succeed them in most other habitats. 



Finally there are the various weed and allied communities that 

 follow anthropic disturbance ; for such activities as cultivation or 

 forest-cutting and their aftermaths are so widespread that in tem- 

 perate regions there is relatively little truly natural vegetation left. 

 Indeed of the more favourable areas capable of supporting climax 

 forest there can scarcely remain any that are wholly undisturbed 

 by Man or his domestic animals, though there are many which we 

 think of (and study) as practically natural. Even crops may be 

 considered to comprise communities of a sort, however artificial and 

 temporary they may be. Representative crops and some weeds 

 were dealt with earlier in this work ; especiallv do weeds form many 



