CHAPTER IV

PLANT SOCIETIES

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.

In a large newly cleared area, plants usually _first establish themselves in dense colonies_. Note the great patches of nettles, jewel-weeds, smart-weeds, clot-burs, fire-weeds in recently cleared but neglected swales, also the fire-weeds in recently burned areas, the rank weeds in the neglected garden, and the ragweeds and May-weeds along the recently worked highway. The competition amongst themselves and with their neighbors finally breaks up the colonies, and _a mixed and intermingled flora is generally the result_.

[Illustration: FIG. 9.--A MID-REGION SOCIETY.]

In many parts of the world the _general tendency of neglected areas is to run into forest_. All plants rush for the cleared area. Here and there bushes gain a foothold. Young trees come up; in time these shade the bushes and gain the mastery. Sometimes the area grows to poplars or birches, and people wonder why the original forest trees do not return; but these forest trees may be growing unobserved here and there in the tangle, and in the slow processes of time the poplars perish--for they are short-lived--and the original forest may be replaced. Whether one kind of forest or another returns will depend partly on the kinds that are most seedful in that vicinity and which, therefore, have sown themselves most profusely. Much depends, also, on the kind of undergrowth that first springs up, for some young trees can endure more or less shade than others.

[Illustration: FIG. 10.--OVERGROWTH AND UNDERGROWTH IN THREE SERIES,--trees, bushes, grass.]

Some plants _associate_. They grow together. This is possible largely because they diverge or differ in character. Plants associate in two ways: _by growing side by side_; _by growing above or beneath_. In sparsely populated societies, plants may grow alongside each other. In most cases, however, there is _overgrowth_ and _undergrowth_: one kind grows beneath another. Plants that have become adapted to shade are usually undergrowths. In a cat-tail swamp, grasses and other narrow-leaved plants grow in the bottom, but they are usually unseen by the casual observer. Note the undergrowth in woods or under trees (Fig. 10). Observe that in pine and spruce forests there is almost no undergrowth, partly because there is very little light.

On the same area the societies may _differ at different times of the year_. There are spring, summer, and fall societies. The knoll which is cool with grass and strawberries in June may be aglow with goldenrod in September. If the bank is examined in May, look for the young plants that are to cover it in July and October; if in September, find the dead stalks of the flora of May. What succeeds the skunk cabbage, hepaticas, trilliums, phlox, violets, buttercups of spring? What precedes the wild sunflowers, ragweed, asters, and goldenrod of fall?

=The Landscape.=--To a large extent the _color of the landscape_ is determined by the character of the plant societies. Evergreen societies remain green, but the shade of green varies from season to season; it is bright and soft in spring, becomes dull in midsummer and fall, and assumes a dull yellow-green or a black-green in winter. Deciduous societies vary remarkably in color--from the dull browns and grays of winter to the brown greens and olive-greens of spring, the staid greens of summer, and the brilliant colors of autumn.

The _autumn colors_ are due to intermingled shades of green, yellow, and red. The coloration varies with the kind of plant, the special location, and the season. Even in the same species or kind, individual plants differ in color; and this individuality usually distinguishes the plant year by year. That is, an oak which is maroon red this autumn is likely to exhibit that range of color every year. The autumn color is associated with the natural maturity and death of the leaf, but it is most brilliant in long and open falls--largely because the foliage ripens more gradually and persists longer in such seasons. It is probable that the autumn tints are of no utility to the plant. _Autumn colors are not caused by frost._ Because of the long, dry falls and the great variety of plants, the autumnal color of the American landscape is phenomenal.

=Ecology.=--The study of the relationships of plants and animals to each other and to seasons and environments is known as =ecology= (still written _œcology_ in the dictionaries). It considers the habits, habitats, and modes of life of living things--the places in which they grow, how they migrate or are disseminated, means of collecting food, their times and seasons of flowering, producing young, and the like.

SUGGESTIONS.--One of the best of all subjects for school instruction in botany is the study of plant societies. It adds definiteness and zest to excursions. =7.= Let each excursion be confined to one or two societies. Visit one day a swamp, another day a forest, another a pasture or meadow, another a roadside, another a weedy field, another a cliff or ravine. Visit shores whenever possible. Each pupil should be assigned a bit of ground--say 10 or 20 ft. square--for special study. He should make a list showing (1) how many kinds of plants it contains, (2) the relative abundance of each. The lists secured in different regions should be compared. It does not matter greatly if the pupil does not know all the plants. He may count the kinds without knowing the names. It is a good plan for the pupil to make a dried specimen of each kind for reference. The pupil should endeavor to discover why the plants grow as they do. Note what kinds of plants grow next each other; and which are undergrowth and which overgrowth; and which are erect and which wide-spreading. _Challenge every plant society._