In the long course of time in which plants have been accommodating themselves to the varying conditions in which they are obliged to grow, _they have become adapted to every different environment_. Certain plants, therefore, may live together or near each other, all enjoying the same general conditions and surroundings. These aggregations of plants that are adapted to similar general conditions are known as =plant societies=. Moisture and temperature are the leading factors in determining plant societies. The great geographical societies or aggregations of the plant world may conveniently be associated chiefly with the moisture supply, as: _wet-region societies_, comprising aquatic and bog vegetation (Fig. 8); _arid-region societies_, comprising desert and most sand-region vegetation; _mid-region societies_, comprising the mixed vegetation in intermediate regions (Fig. 9), this being the commonest type. Much of the characteristic scenery of any place is due to its plant societies. Arid-region plants usually have small and hard leaves, apparently preventing too rapid loss of water. Usually, also, they are characterized by stiff growth, hairy covering, spines, or a much-contracted plant body, and often by large underground parts for the storage of water. Plant societies may also be distinguished with reference to latitude and temperature. There are _tropical societies_, _temperate-region societies_, _boreal_ or _cold-region societies_. With reference to altitude, societies might be classified as _lowland_ (which are chiefly wet-region), _intermediate_ (chiefly mid-region), _subalpine_ or _mid-mountain_ (which are chiefly boreal), _alpine_ or _high-mountain_. The above classifications have reference chiefly to great geographical floras or societies. But there are _societies within societies_. There are small societies coming within the experience of every person who has ever seen plants growing in natural conditions. There are roadside, fence-row, lawn, thicket, pasture, dune, woods, cliff, barn-yard societies. _Every different place has its characteristic vegetation._ Note the smaller societies in Figs. 8 and 9. In the former is a water-lily society and a cat-tail society. In the latter there are grass and bush and woods societies. [Illustration: FIG. 8.—A WET-REGION SOCIETY.] =Some Details of Plant Societies.=—Societies may be composed of _scattered and intermingled plants_, or of dense _clumps_ or _groups of plants_. Dense clumps or groups are usually made up of one kind of plant, and they are then called =colonies=. Colonies of most plants are transient: after a short time other plants gain a foothold amongst them, and an intermingled society is the outcome. Marked exceptions to this are grass colonies and forest colonies, in which one kind of plant may hold its own for years and centuries. [Illustration: FIG. 9.—A MID-REGION SOCIETY.]