142 ADAPTATIONS. OECOLOGICAL CLASSIFICATION SECT, in 



It may be asked why arrange the various types of vegetation in the classes 

 named on p. 136 ? Why not use each growth-form as a foundation upon which 

 to build a special class ? The following classes could then be distinguished : that 

 of forest-formations, of bush-formations, of shrub-formations, of dwarf-shrub 

 formations, of perennial-herb formations, of moss-formations, and of alga-forma- 

 tions. 1 Within each such class one would further be able to distinguish hygrophilous, 

 mesophilous, and xerpphilous formations, and to define them. From a morpho- 

 logical standpoint this would possess a certain interest, but from a phytogeo- 

 graphical one it must be dismissed, because it would involve the separation of 

 formations that are oecologically closely allied. It is nature of locality that must 

 be represented by formations ; and naturally allied localities may include different 

 collections of growth-forms, yet they must be grouped in the same class. 



According to the isolation or combination of the growth-forms in 

 a formation we have to distinguish simple formations and compound 

 formations. 



Simple formations : Formations consisting solely of one type of 



rowth-form are few. An example is the phyto-plankton formation, 

 he species composing this microphytic free-floating flora belong to 

 different families or even different systematic classes, but they may all 

 be grouped together as belonging to one growth-form adapted to the 

 free-floating mode of existence in water ; plankton is a purely edaphic 

 oecological formation. 



Compound formations : Usually many growth-forms, and often, 

 to some extent, different formations, are combined to form a single 

 whole. For instance, as will be explained hereafter, the reed-formation 

 is one dominated by divers monocotylous herbs, which are social and 

 perennial in habit and varied in stature ; but on the ground, also in 

 the water between and beneath the reeds, there sometimes flourish 

 other growth-forms comprising what may be termed subordinate commu- 

 nities : thus there may be communities composed of Schizophyceae, 

 plankton, pleuston, and limnaea, 2 which are more or less influenced in 

 their composition by the dominant community. Again, in forest the 

 different lower storeys are constituted of growth-forms which, for the 

 most part, are able in themselves to give rise to distinct formations 

 (bush, grassland, moss-formation, and others), but in the two cases 

 the species occurring would usually be different. For the shade of forest 

 or of tall vegetation affects not only the conditions of illumination, but 

 also the humidity and temperature of air and soil. As an example 

 of a species capable by itself of giving rise to one independent formation, 

 also of occurring as a subordinate member in another, we may cite 

 Calluna vulgaris. This species as a dominant plant forms a widely 

 distributed community belonging to the type of dwarf-shrub heath ; 

 but it can occur as low vegetation in thin pine-forest ; to what extent 

 the latter differs from the open ling-heath has not been adequately 

 investigated. 



A formation consisting of several storeys may therefore be composed 

 of both xerophytes and mesophytes : there are forests for instance, 

 sclerophyllous forest in which markedly xerophytic species form the 

 uppermost storey, but mesophytic species the lower storeys. 



1 This approximates to the method adopted by Kerner, 1891, p. 821, who 

 distinguished nine kinds of societies. 



2 See Chap. XLIX. 



