6 Introduction 



though a strict determination of the various cases is lack- 

 ing. But these phenomena, interesting as they are, do 

 not count in the determination of the fundamental units 

 of vegetation, the plant-formations, and we may fairly 

 say that, on the whole, the plant-formations of the British 

 Isles are mainly determined by edaphic factors^ i.e. by 

 soil. 



Many plant-formations are easy to determine, because 



they are limited by quite obvious habitats 



Determina- ^q which very definite and characteristic 



tion of plant- , , . , . n oi i n 



formations. plant-communities correspond, buch are tor 



instance the two chief "maritime" forma- 

 tions, those of the sand dunes and of the salt marshes. 



The moor-formations are also well characterised, if we 

 take the word "moor" in its current German sense to 

 apply to types of vegetation developed on deep peat'. 

 Two principal formations with essentially diilerent plant- 

 communities may be distinguished, that on peat relatively 

 rich in mineral salts and neutral or alkaline in reaction — 

 " fen " as it is called in this country (Niedermoor of Weber, 

 Flachmoor of various authors) — contrasting strongly with 

 the formation on peat poor in mineral salts and acid in 

 reaction — the moor proper (Hochmoor of German writers) . 

 Whether we should distinguish a third formation — the 

 "transition-moor" (Uebergangsmoor) — is open to question. 



The vegetation of chalk and limestone, of the non- 

 calcareous clays and loams, of the sands relatively rich 

 in plant-foods, and of the sands and gravels relatively 

 poor in plant-foods and developing an acid humus, fall 

 into different plant-formations. Bach of these types of 

 soil bears vegetation with characteristic features and 

 characteristic species of plants, though many of the 

 species are common to several. There are however certain 



' This use is not, however, in accord with the current use of the 

 common English word "moor." For a discussion of the subject see 

 Chapter IX. 



