Plant The vegetation of the earth's surface is not at all uniform, but con sists of a large number of different areas, de termined by climate and soil. The most exten sive of these are known as formations, illus trated by the deciduous forest of the Mississippi Basin, the prairie-plains grassland, and the sage brush desert of the Great Basin. Each forma tion is the product of a particular climate and hence represents the highest type of vegetation possible under it. For this reason, each forma tion is often called a climax or climax forma tion. The entire plant covering of the globe is made up of such climax formations. These are far from continuous or uniform, however, owing to the interruptions due to bodies of water, outcrops of rock, fire, cultivation and other disturbances. In such areas are to be found pioneer communities which develop through a series of stages until the final stage or climax for that particular climate is reached. Thus, each formation consists of two kinds of communities, developmental or successional ones which disappear in turn until the final stage is reached, and climax ones which persist for long periods and over vast areas as long as the climate remains essentially the same. The climax divisions of a formation are known as associations, consociations and societies, and are well illustrated by the grassland formation which covers the prairies and plains. The tall grasses of the prairie constitute one association made up of consociations of Stipa, Agropyrum and Koeleria, while the short grasses form a plains association consisting of the Bouteloga and Bulbulis consociations primarily. The socie ties are due to the presence of characteristic per ennial herbs such as Amorpha, Psoralea, Aster, Solidago, etc., which dominate more or less dis tinct areas within the association.
Areas or The vegetation of the earth's surface is not at all uniform,' but consists of a multitude of different areas, each corresponding to a habitat. These areas are called formations and each is composed of an association or groups of plants determined by the physical factors of its habitat. A pond will be occupied by a community composed of water plants ; a forest formation will consist of me sophytes, and desert plant formations will be found in dry, sandy regions. Even within each formation it will be found that the plants are not uniformly distributed; some will occur in masses, while others are scattered singly, and one species will be met again and again, while another will be found but once. Furthermore, communities are not fixed groups of plants. One species will find that the conditions of life be come more and more difficult; and will gradu ally disappear. Other species will prosper and increase rapidly in number, this very prosperity often producing the conditions unfavorable to another. The seeds of species from other places will be brought in by the wind, by birds or by animals, and will find a new home, or, after struggling for a while, the plants will disappear. Frequently, new plants come in to such a degree that they finally replace the original species en tirely and the community is replaced by a new one.
The development of a formation may be readily followed where rocks are disintegrating, or where an original vegetation has been re moved by fire. In the first case, the pioneer plants are small crust-like lichens, which de compose the surface of the rock, and by their decay prepare a thin soil for the larger leaf like forms, which sooner or later appear. With these usually enter the rock mosses and the two by their activity and ultimate decay finally form a soil sufficient for some of the grasses and other herbs which are able to withstand extreme dryness. Meanwhile, the action of rain and
frost has produced rifts in the rocks, which are first filled with mosses, and then by a soil deep enough to support larger plants. The ultimate result of the activity of these various factors is the breaking down of the rock into soil. In the case of the harder rocks, this will be a coarse sand or gravel; with the softer ones, it is a fine sand or marl At this stage, leaf-like lichens and mosses play some part in binding the soil particles together, but they soon dis appear before the grasses, which in their turn yield in a few years to other herbs. These are sooner or later displaced by bushes and shrubs and the latter make way for the trees which mark the close of the process. Such a primary succession takes place very slowly and may of ten extend over a century or more. When a forest is burned, the revegetation is much more rapid, as the soil is already prepared and the succession is termed secondary. Tiny mosses and fungi first appear and in a year or two at the most are replaced by low herbs. These disappear before the invasion of grasses and afireweed,o and these are replaced by fast growing trees, such as the birch and the aspen. Such trees are usually shortlived and are dis placed after a decade or so by pines, spruces or firs, which in many cases are at last conquered by the hardwoods. It is significant of the plants of each stage of such successions that they bring about conditions in their action upon the habi tat which finally cause their own disappearance. Each stage represents a community, but the change from one stage to the next takes place so slowly that it is not at all uncommon to find associated with the plants typical of one stage some survivors of the preceding community, as well as a few pioneers of the next stage.
Migration of Plants.— The movement of plants in vegetation is known as migration. In the case of the simple water-plants, the alga, the whole plant moves of its own accord, or is carried by some agent. The same holds for a few of the flowering plants of floating habit. All terrestrial plants are fixed, however, and migration must act regularly upon the spore or seed. In tumbleweeds, the whole plant is fre quently carried away by the wind, but it is no longer in a living condition. Spores are readily scattered by the wind on account of their light ness, but seeds and fruits have been especially modified for migration on account of their greater weight. Plants growing in or near the water often have fruits with corky or inflated envelopes, which serve to keep them afloat. The great majority of the modifications for securing migration, however, are concerned with wind and animals. In the former, the con trivances are uniformly for the purpose of light ening the fruit or seed, so that it may readily be carried by the wind. Fruits that are to be distributed by animals are provided with spines, hooks or glands for attachment, or are made attractive by a bright or edible envelope. Wind carried fruits are especially common; they are provided with wings, as in the maple; with hairs, as in the milkweed, and with parachute-like tufts, as in the dandelion. Man plays the most important part of all distributive agents, if vol untary as well as involuntary carriage be con sidered. He has carried cultivated plants and weeds all over the globe and to thousands of places where they could never have gone of themselves.