The deciduous forest formation of America presents in Indiana and Missouri associations in which oak and hickory are the chief features, whilst in the region of the great lakes it is represented by an association of birch and maple. In central Europe the same formation is represented by the very mixed forests of beech, hornbeam oak, etc., of the Danube region, by the Mediterranean chestnut forests, by the oak forests of Austria and other types. In Britain consociations are the rule, each characterized by a single dominant tree and associated with particular soil conditions. Thus the ashwoods are chiefly found on limestone soils, where the con ditions are moist ; beechwoods on dry soils, particularly those derived from chalk; birchwoods constitute the uppermost zone of woodland in mountain districts and also occur as a temporary phase on felled areas ; alderwoods occupy the wettest soils ; oak woods of Quercus Robur are a feature of the moister and more fertile clays and loams, and oakwoods of Q. sessiliflora of the less fertile, acid, and usually drier non-calcareous soils, especially on the valley slopes of the west. Within an English oakwood some areas are covered with a carpet of Mercurialis perennis and Scilla nutans, others with Anemone nemorosa or Ficaria verna, furnish ing examples of societies within the consociation. Such illustra tions serve to indicate the use of these synecological terms; but for an account of the plant communities of the world the reader is referred to the works cited on p. 18.
Every plant community has something of the attributes of an organic unit, in that it possesses a more or less definite structure and has a life history in which juvenile, adult, and senile phases can be recognized. The structural organization is well illustrated
in the temperate deciduous woodland, where three definite strata are usually present, respectively consisting of trees, shrubs and herbs, whilst beneath the lowest tier or ground flora there is often a carpet of mosses and the surface soil is rich in fungi and bac teria. Although less conspicuous, these flowerless plants are just as important a part of the community and have as definite a floristic composition. Beneath the surface the root systems of the higher plants exhibit a stratification comparable to that of the aerial organs, and the differing depths to which the roots of dif ferent species penetrate, as also the different periods at which they make their maximum demands on the food supply in the soil solution, are features which tend to reduce the competition between species and render possible that dense intermingling which results in the succession of seasonal changes. It must, however, be realized that the architecture of any community necessarily connotes different conditions in its various parts so that the environment occupied for example by the ground flora of a tropical or temperate forest, or by the lowest stratum of a fen is markedly different, as to humidity and illumination, from that of the tree layer in the one or the tall reeds in the other.
The different associations in a plant formation show an analo gous structure of ten built up by very diverse species, and no bet ter example of this could be furnished than by the desert vegeta tion of Texas and Mexico, characterized by succulent Cactaceae, and that of Africa, characterized by succulent Euphorbiaceae. Some of the members of these totally unrelated groups are so similar when not in flower as to be most readily distinguished by wounding, since the euphorbias possess milky juice which the cacti do not. Each association is distinguished by the presence and relative abundance of certain species and the absence of others. Those which are almost invariably present in a community are termed constants, but their diagnostic value is often low owing to their constancy in more than one association. Examples are furnished by Mercurialis perennis in the beechwoods of Britain and Viola sylvestris in the oakwoods. Of greater significance are the characteristic species which occur more or less exclusively in a particular community, as Hordeum sylvaticum and Monotropa hypopitys in the beechwoods of Britain or Psamma arenaria on European sand-dunes. Such characteristic species may be at the same time constants but are of ten of local occurrence. Many of the subordinate or accessory species are found in a variety of communities but have their characteristic frequency in each.