12 Introduction 



The association has a very definite and usually a much 

 more restricted flora than the formation, largely owing in 

 the case of closed associations to the special conditions 

 determined by the dominants. Transitions between dif- 

 ferent associations of the same formation are constantly 

 met withj and in these a mixture of both vegetation and 

 flora occurs, a fact which serves to knit the various 

 associations together and to emphasise the unity of the 

 formation to which they belong. 



The Plant-society 



Locally within an association there occur more or less 

 definite aggregations of characteristic species or of small 

 groups of species, and these, which appear as features 

 within the association, may be recognised as smaller 

 vegetation-units or 'plant-societies. Sometimes their 

 occurrence may be due to local variations of the habitat, 

 at other times to accident and the gregarious habit 

 originating from a general scattering of seed in one 

 place or from the social growth of a rhizomic plant \ 

 While a plant-formation is always made up of associations, 

 an association is not necessarily or even generally made 

 up of societies, which are essentially local discontinuous 

 phenomena. 



Thus of the three orders of vegetation-unit or plant- 

 community which we recognise, the largest, the formation, 

 is absolutely determined by habitat. Transitions between 



abbreviation of Quercetum Querci Rob%f,ris. A qualifying adjective is 

 sometimes tentatively added. '\* 



^ It is a question whether it would not be better to separate these 

 two causes of the formation of societies within an association, and to 

 restrict the term society to the aggregations due to the latter alone. In 

 this -way we should obtain a more logically coherent conception. But 

 the more detailed analysis of vegetation has scarcely progressed far 

 enough at present to justify a finer classification of plant-communities. 



