The Encyclopedia Americana (1920)/Plant Geography
PLANT GEOGRAPHY.— The relation of the plant covering to the surface of the earth is the basis of that division of botany which is called plant geography. This does not confine itself to the geographical distribution of plants, as was formerly the case, but comprises all the out-of-door relations of plants to each other, and to their environment. The subject falls somewhat naturally into several divisions with respect to the point of view. Floristic botany deals with the geographical distribution of species, and with we character of the plant population of different regions and countries. Ecology concerns itself especially with the relation existing between the plant and its environment, and with the grouping of species in particular areas called formations, such as prairie, forest, etc. Experimental ecology is merely a phase of the latter, in which changes in the form and behavior of plants are brought about by changing the physical conditions of the environment. It is of great importance because of its bearing upon die origin of species.
Environment and Plant.— The essential points of inquiry in plant geography are the environment, or home of the plant, usually called habitat by botanists, and the plant, either as an individual, or as a member of the vegetation. The habitat is to be regarded as the cause, the plant as the effect. This is true of the present relation between any habitat and the plants which grow in it only to a certain degree, for probably no plant of the present day owes its entire structure to one habitat. Each habitat, however, does have a modifying influence upon the plants in it. This influence will be great where the physical conditions are extreme, and it will be slight where they are more moderate. In either case this modification will leave its distinctive stamp upon the plant in such a way that one may readily tell whether it grew in pond, meadow, forest or desert. Of the many factors which make up a habitat not all are of the same importance. In some situations the water of the soil will be most important, in others light will be the controlling factor, while in still other places wind will have the most striking effect. Habitats show great differences also in the amount of water, light, wind and other factors. If one will know the causes of plant structure and distribution, it is necessary to look into the habitat with great care, and to determine the relative importance and the amount of each factor.
The factors of a habitat which are most intimately connected with the form of the plant are those that have to do with the water-supply, and with the food-making activities of the leaf. These are water and light. They have a direct influence upon the plant form, while all others affect the structure indirectly, as a result of their action upon the water of the soil, or the air, or upon light. Every habitat comprises the following factors: water-content, humidity, light, temperature, soil, wind, precipitation, physiography, dead vegetation and animals. Of these, water-content, soil, soil temperature, and physiography belong to the soil, and are in consequence called edaphic. Humidity (air moisture), light, air temperature, wind, and precipitation pertain to the air, and are termed atmospheric or climatic. Dead vegetation and animals are biological or biotic factors. Living vegetation has a marked effect upon its habitat, but this is to be regarded more as a reaction than as a cause.
Water as a Factor.— The simplest plants grow in water, and are in every way dependent upon it. Terrestrial plants — for example, practically all flowering plants and ferns — have adapted themselves to two media, air and water, and their dependence upon water is not so marked. The active root-hairs are still really aquatic, and must always be in contact with an adequate supply of water. The stems and leaves are aerial, but their behavior and form are largely determined by the water in the air, that is, the humidity, The water-supply is used by the root-hairs, while water-loss is the result of evaporation from the surface of the leaves. The excess of supply over loss will determine the form of the plant; it is evident that plants cannot grow where the loss exceeds the supply. The balance between these is so nice that plants grow well only where it is maintained. The most luxuriant vegetation is the forest, where both supply and loss of water are great. An excess of supply over loss is almost as certain to produce stunting and dwarfing, as seen in the plants of ponds and marshes, as is an excessive water-loss, which is the condition typical of deserts and high mountain peaks. The total amount of water present in the soil will vary with the rainfall, and with the behavior of surface and underground streams. Much of the rainfall runs off the surface, while a part of it sinks below the roots and is carried off by underground drainage. What is left remains in contact with the soil-particles as a thin film, and it is this which is absorbed by the root-hairs. The pull exerted by the absorptive power of the hair upon the water-film is greater than the attraction of the soil-particle, and the latter loses its water. If this continues, however, the ratio lessens, and in soils that are drying out the particles hold the water-films with increasing tenacity. In consequence, plants will wilt and die in soils that still contain water. Loose soils, such at sand and gravel, will give up all but 0.5 to 1 per cent of the water-content, while compact days retain as high as 12 to 30 per cent. An excess of salts in the soil, or a lack of air usually produces a similar effect. They decrease the absorptive power of the root-hairs, and lead to the production in marshes and bogs of plants showing the effects of an insufficient water-supply.
Air Humidity.— The humidity of the air exerts a direct control upon the amount of water evaporated from the leaves. It is evident that the water-loss of the plant will be slight where the amount of moisture in the air is great, and that the evaporation will be great where the air is dry. This effect of humidity is so marked that plants which grow in moist climates often have structures designed to increase water-loss, while those living in desert-like places regularly protect themselves by thickening their epidermis, and decreasing the amount of surface exposed. The relative humidity of lowlands and sea-coasts, especially in the tropics, is above 80 per cent; in deserts and upon high mountains it is rarely more than 30 per cent, and often falls below 15 per cent.
Influence of Light.— The amount of light present in a habitat influences directly the food-making activities of the plant. The green coloring matter of plants, the chlorophyll, is formed readily and abundantly only in the light, and the combination of crude materials, water, carbon dioxide, and salts into foods available for the protoplasm can occur only in the presence of this pigment. Light thus bears a peculiar relation to the nutrition and growth of plants, and in a large degree determines their form and size. Sunlight produces vigorous, stocky stems, and thick leaves, as a rule, while plants grown in the shade have tall slender stems, and broad thin leaves. Plants that occur underground, in caves, or grow within organisms, do not develop chlorophyll, and without exception belong to the flowerless forms known as fungi. The intensity of the light varies throughout the day and year; it ts greater in the tropics than at the poles, and on the tops of high peaks than at their bases. In forests and thickets the light is often very diffuse, varying from .01 to .003, summer sunlight being 1.
Temperature.— This is directly concerned with the nutrition and growth of plants. Heat is necessary for the germination of seeds, and for the sprouting of bulbs and tubers. It must be present in a considerable degree for the food-making activities of plants, and upon it in a large part depends the size of individuals, and the luxuriance of vegetation. The growing period is the season during which temperatures favorable to plant growth prevail; the length of this period determines in great measure the native vegetation of a country, and the cultivated plants which can be grown there. Indirectly, temperature exerts a pronounced effect upon the form of plants, by decreasing the moisture of the air, and thus increasing the water-loss. In tropical and subtropical deserts this indirect action of heat is a predominant factor. Soil temperatures are of much less importance, though they have much to do with germination and the activity of underground parts.
Wind Effects.— Wind influences plant life both directly and indirectly. Its mechanical action is marked in regions where forceful and constant winds prevail, notably seacoasts and high mountain peaks. Shrubs and trees become bent or prostrate, and their branches are developed almost wholly on the leeward side. Over great open stretches where strong winds prevail, for example prairies and steppes, the plant forms are largely grasses and grass-like plants, which are not easily torn or whipped by the wind. As is well-known, winds play an extremely important part in carrying the pollen of trees and grasses, and in scattering seeds and spores. Like heat, wind decreases the humidity of the air, and correspondingly increases the evaporation from leaf-surfaces. It does this by removing the more or less saturated air in contact with the plant, replacing it with air containing less moisture. This action is characteristic of the dry southwest winds in the corn-belt, which rapidly carry away the moisture of the leaves, causing the latter to curl, thus decreasing the surface and affording some measure of protection. The stunted forest vegetation of arctic and alpine timber-lines is largely due to the drying action of almost constant winds. The direction, force, and duration of the wind must all be taken into account in the study of vegetation.
Soil Action.— The soil acts directly upon the behavior and form of plants by reason of its influence upon water-content and temperature. In plant geography, all inorganic strata upon which plants grow are termed soils. The extreme types are rock and water, between which are found all manner of gradations from gravel to mud. The weathering of rocks produces two kinds of soils quite different in their behavior with respect to water. The one is loose and composed of large particles — for example, sands and gravels; the other is composed of fine, compact particles, clays and loams. Sands and gravels absorb nearly all of the rain falling upon them but much of the water taken up passes through and is carried away along some impervious stratum. What remains as water-content is readily absorbed by the root-hairs or is lost by evaporation from the soil. On account of the large air-spaces between the grains, the water in the lower layers is raised with difficulty by capillary action. Plants withdraw from loose soils nearly all of their water-content. Compact soils, especially clay do not absorb rain rapidly, and much of the latter is carried away by surface drainage. Water once absorbed is held tenaciously, and the loss by underground drainage is slight. The pores of the soil are fine, and capillarity plays an important part in raising water from the lower levels. The particles attract the water-films strongly, and in consequence clay yields its water to plant or air very slowly. Ordinary plants will wilt in clay soils which still contain as high as 12 to 30 per cent of water. Rock is all but absolutely impervious to water. Mosses and lichens alone can grow upon it, in consequence of their power to decompose the surface and their ability to withstand drying out. The amount of soluble material in the soil has a direct effect upon the growth of plants. Nearly all ordinary soils contain an adequate supply of soluble minerals; a few, however, are deficient in these, and are unable to support more than a scanty vegetation. Other soils contain in excess soluble salts and acids which are harmful. The sparse, desert-like vegetation of salt basins and alkali wastes is due to the large quantities of sodium chloride, sodium carbonate, and other salts present. In swamps and bogs the decay of plant and animal remains uses up the available oxygen and thus hinders the absorptive powers of the roots. The kind of material in the soil and the fineness and compactness of the grains determine its behavior with reference to the absorption and radiation of heat. The color of the soil, the amount of water present and other conditions have also to do with this matter. Rock is warmed most readily in the sunshine, and at night most readily parts with its heat. Water is at the other extreme; it warms up slowly and, conversely, yields its heat reluctantly.
Utilization of Moisture.— It is a well-established fact that plants in general are unable to absorb water-vapor from the air. The moisture must be condensed into water, and then must regularly find its way into the soil before it can be used by the plant. A comparatively small number of plants, lichens, mosses, and tree-dwelling orchids absorb rain or dew directly through their leaf or thallus surfaces, but with the great majority of plants the water can only be taken in from the soil or substratum. The moisture of the air is condensed or precipitated in various forms — rain, dew, snow, sleet, hail, frost and fog. With the exception of the last all of these contribute sooner or later to the water-content of the soil, the important difference being that the solid forms usually melt gradually and are in consequence absorbed more completely. The water which falls upon the surface of the soil is partly absorbed, and partly carried away by drainage. The latter is known as “run-off”; its amount will depend upon the compactness of the soil and the steepness of the slope. The absorbed water passes into the lower layers in part, where some of it is drained off as gravitation water, and some is retained as capillary water to be raised by pillary action into the upper layers of the soil. That which remains in the soil about the roots forms thin films about the soil-grains, and is known as the water-content. This alone can be used by the root-hairs. Part of it, however, is lost by direct evaporation from the soil. Rain has little or no mechanical action upon plants, except perhaps in the tropics, where it falls in torrents. In the form of “run-off,” however, it acts powerfully upon the surface of hills and mountains, and plays in consequence an extremely important part in the development of vegetation. Sleet and hail are very destructive in the breaking of twigs and branches and the cutting and tearing of leaves, but are of little importance because of their relative infrequence. Snow, on the other hand, has had a great deal to do with the forms of trees, particularly the pines, spruces, and firs, in northern and mountain regions. It is a poor conductor, and for this reason affords much protection to plant parts covered by it. Unlike rain, it is often unequally distributed by the wind, and therefore produces important local differences in the water-content of the soil.
Physiography.— The surface features of a region — its physiography — affect directly several of the physical factors of habitats. Altitude not only influences the rainfall, but it also increases water-loss by reason of the reduced air-pressure and the decreased humidity. The sunlight may be stronger, as the rays pass through fewer air-layers, and are absorbed in a less degree. The degree of slope is especially important, as it determines very largely the ratio between “run-off” and absorbed water. In mountain regions particularly, it modifies the angle at which the sun's rays strike the surface, and increases the amount of heat and light received. The exposure of the surface, that is, the direction in which it lies, affects the amount of heat and light, the intensity of the wind, and the snowfall. Furthermore, the character of the surface itself, whether level or uneven, will influence all of these factors in a less degree. Physiographic changes, such as elevation and subsidence, the erosion of river-valleys, and the upbuilding of swamps, deltas, etc., have a profound effect upon the distribution of plants, and the development of vegetation.
Vegetable, Animal and Human Factors.— Dead vegetation increases the water-content of a habitat by checking the movement of the “run-off,” and thus increasing the absorption, and by protecting the surface of the soil from excessive evaporation. It equalizes the soil temperature by hindering the warming action of the sun's rays, and the cooling effect of radiation. It also diminishes the force of the wind, and, finally, by decay, returns to the soil much of the nutrient material taken from it. Living vegetation has the same effect, but is different in that it constantly draws water and nourishment from the soil, and often reduces the amount of light present. The activities of animals and man are extremely diverse. Earthworms and burrowing mammals enrich the soil by working it over repeatedly. Grazing animals have more or less effect upon grasslands. Insects are fundamentally important in fertilization, and doubtless often act decisively in the struggle for existence by destroying some plants and not others. Man is a biological factor of the first importance, even if we leave out of consideration all the changes that he has brought about in plants and vegetation in consequence of cultivation. He changes habitats fundamentally by the removal of forests, by fires, by the construction of railroads and canals, by drainage, by irrigation, etc.
Classification of Habitats.— Habitats are usually grouped with respect to the two direct factors, water and light. They are first classed as wet, moist and dry and the moist habitats arc further divided into sun and shade. Wet habitats comprise all bodies of water, oceans, lakes, ponds, springs, streams, swamps, marshes, river-banks, seashores, tanks, etc. Dry habitats are principally deserts, sandhills, prairies, gravel-slides, strands, dunes, bad lands, cliffs, rocks, heaths, humus-marshes, moors, alpine and polar barrens. Sunny moist lands are meadows, pastures, grain-fields and waste places. Shady moist habitats are forests, groves, woodlands and thickets.
Effects of Habitat.— These, as regards the individual, are either evident or demonstrable, as in the case of the habitat form seen in bog-plants, shade-plants, etc., or they are obscure and remote and can in consequence no longer be traced. The latter is true of vegetation forms — trees, shrubs, bulb-plants, etc. Three well-defined groups of habitat forms are recognized, based upon the water-content of habitats. These are water-plants (hydrophytes), moist-land or middle plants (mesophytes), and desert plants (xerophytes). Upon the basis of light differences, mesophytes are further divided into sun-plants (heliophytes), shade-plants (sciophytes) and darkness-plants (scotophytes). Water plants owe their peculiar stamp to the fact that the water-supply is always greatly in excess of the water-loss. The roots are superficial in position, owing to the abundance of water at or on the surface of the soil. Root-surfaces are slightly developed and root-hairs often lacking, because the amount of water renders absorption easy. The surplus of water is a disadvantage, however, as it reduces the amount of air in the soil and hence cuts off the supply of oxygen necessary for the activity of the roots. This lack of aeration is compensated by the development of large air-passages leading down from the leaves through the stem and roots. Stems and leaves are almost invariably smooth and without any sort of protective covering. Breathing-pores are usually abundant and the necessity that the plant should lose a large amount of water has led to the development of water-pores and papillæ. This structure is typical of amphibious plants, that is, those that grow in the mud or in shallow water. Floating plants are usually much the same, with the exception that the breathing pores become useless and disappear on the under surface of the leaf, which is in contact with the water. Certain plants, such as the duckweed, have become greatly reduced in consequence of the floating habitat, and consist merely of a tiny, leaf-like disc, with a few rootlets. Submerged plants grow entirely beneath the water and are not subject to water-loss. As a result their leaves and stems are greatly reduced. The leaves are thin and divided into narrow segments; in structure they are almost uniform. The characteristic air-passages of the other water forms are lacking, as all the air must be dissolved in water.
Desert-plants are in most respects the exact opposites of water-plants. Not only is the water-supply scanty, but all the factors which increase water-loss are present in a large degree. Such plants must use all their power of adaptation to absorb and store all the water they can and to lose just as little by evaporation as possible. The roots of desert-plants are for the most part deep-seated and branch for the most part only in the deeper, moist soil. They are covered elsewhere with a corky layer to prevent the loss of water where the root passes through the dry upper layers. In the moist soil, root-hairs are produced in large numbers. In many cases the upper portion of the root consists of tissue especially adapted to the storage of water. The stems of xerophytes are short and stout. The leaves are thick and much reduced in size; and in extreme forms they are entirely lacking. Both leaves and stems are covered with a thick coating of hairs, or wax, or the epidermis is greatly thickened, all for the purpose of protection against water-loss. The breathing-pores are generally confined to the underside of the leaf and are often sunken far below the surface for still greater protection. In the cactus the leaves are reduced to mere scales and the stem often contracted into a cylinder or ball, thus decreasing the exposed surface to the minimum. Succulent plants, such as the live-for-ever and ice-plant have, on the other hand, been modified so that the leaves serve for the storage of water. Lichens and mosses which grow on rocks are capable of withstanding extreme dryness, a faculty seemingly inherent in their protoplasm, as they are without ordinary protective contrivances.
Moist-land plants, or mesophytes, stand as intermediate between the two preceding groups. The water-supply, though not excessive, is usually sufficient, and the humidity of the air is great enough to preclude the danger of excessive water-loss. In consequence, mesophytes have well-developed, more or less branched, root-systems, which are usually intermediate in position, but many of them are deep-seated. The development of surface is moderate, as well as that of the protective cork. Stems are for the most part tall and vigorous and much-branched. The leaves are large and mostly entire, or at least rarely finely dissected. Hairs are common, but seldom compacted into a dense covering. The epidermis is not greatly thickened and while the breathings pores are often more abundant on the lower side, they are present in large numbers on both surfaces. The leaves of mesophytes are characterized regularly by compact rows of oblong cells placed at right angles to the surface, which are called palisade-tissue, and by loose irregular cells with large inter-cellular spaces, the sponge tissue. The former is usually in the upper, the latter in the lower half of the leaf. The palisade is differentiated in response to the action of strong light, while sponge-tissue results from the need of the rapid diffusion of the carbon dioxide and oxygen absorbed from the air.
Sun-plants and shade-plants are especially different in their leaves; this is to be expected, as the leaf is the organ most dependent upon light. The root-system will be more superficial in shade-plants, as the moisture is nearer the surface of the soil in shaded than in sunny places. The stem will be more slender, taller and often more branched, since it is necessary to place the leaves in the position to receive the most of the diffuse light. The leaves are broad, thin and entire, increasing the exposed surface, while in sun-plants they are thick and more or less divided. In typical shade-leaves, the palisade-tissue is reduced to a single row, or is altogether absent. The cells are not crowded closely and their longest axis often coincides with the epidermis of the leaf. Sun-leaves have one or more rows of typical palisade on the upper side and are frequently palisaded on the lower side also. The epidermis develops more wax and hairs in the sun; the breathing-pores are more numerous on the lower than upon the upper surface, while there is little difference in the shade.
Origin and Distribution of Forms.— Vegetation-forms doubtless originated in response to physical conditions, but this relation is hardly evident to-day. We can only see in trees, shrubs, herbs, etc., an expression of the success which different plants have obtained in the struggle for existence. It is also evident that the vegetation form of a plant has much to do with its persistence and hence with its importance in vegetation. The main groups of vegetation-forms are woody plants, herbs and thallus (flowerless) plants. The former are the largest, the most dominant, and the most persistent of all forms; the latter are tiny, subordinate and fleeting. The various woody forms are trees, shrubs, bushes and climbers; the first are the most important, the last the least so. Trees constitute the most permanent type of vegetation, the forest, to which shrubs, bushes and climbers also contribute. The relation between these forms is easily seen in the development of a forest, in which bushes precede and are followed by shrubs; these give way to the trees, the climbers coming in after the latter.
Herbs are specially distinguished from woody plants by their lack of woody stems and by the fact that their persistent parts are underground. They can never be very large, for their stems lack support, though they may persist for years. They are either perennial, blooming each year, or annual-biennial, blooming at the end of the first or second year, and then dying. Various forms of herbs are distinguished with reference to the position of the stem and leaves. In some, like the dandelion, the leaves are grouped in rosettes in response to light and heat. In others, such as the everlasting, the plants are set close together for somewhat similar reasons and perhaps for mutual protection also. Climbers, sweet peas, vetches, etc., develop partly for increased support, partly to secure more light. Grasses form sod because of their abundant rootstalks; it is this faculty which enables grasses to control the vegetation of meadows and prairies. Bunch-grasses are an exception, but they generally grow where the formation of a sod is impossible on account of unfavorable physical conditions. The position and abundance of herbs in a formation will depend also upon the character of the underground parts. Species with underground runners or rootstalks will be more abundant and more widely distributed than those with nearly stationary bulbs or tubers. The vegetation forms of thallus plants play a very subordinate rôle in vegetation. Mosses, liverworts and lichens are regularly present in tree and herb formations, but their small size and transient nature make them of little importance. They are significant of the early stages of vegetation on rocks, new soils, etc., but they soon disappear before the grasses and other herbs. Fungi are entirely dependent upon their host plant or stratum and are relatively insignificant, except where they are necessary to the nutrition of the host, as in the case of certain trees.
Plant Formations.— The vegetation of the earth's surface is not at all uniform, but consists of a large number of different areas, determined by climate and soil. The most extensive of these are known as formations, illustrated by the deciduous forest of the Mississippi Basin, the prairie-plains grassland, and the sagebrush desert of the Great Basin. Each formation is the product of a particular climate and hence represents the highest type of vegetation possible under it. For this reason, each formation is often called a climax or climax formation. 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 at 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 Bouteloua and Bulbulis consociations primarily. The societies are due to the presence of characteristic perennial herbs such as Amorpha, Psoralea, Aster, Solidago, etc, which dominate more or less distinct areas within the association.
Areas or Formations.— 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 mesophytes, 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 become more and more difficult; and will gradually 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 entirely 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 removed by fire. In the first case, the pioneer plants are small crust-like lichens, which decompose 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 disappear 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 often 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 “fireweed,” and these are replaced by fast-growing trees, such as the birch and the aspen. Such trees are usually shortlived and are displaced 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 habitat 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 algæ, 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 frequently 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 lightness, 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 contrivances are uniformly for the purpose of lightening 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 voluntary as well as involuntary carriage be considered. He has carried cultivated plants and weeds all over the globe and to thousands of places where they could never have gone of themselves.
The movement of the seed or fruit of a species into a new formation or country is often determined by natural barriers. Winds bear seeds for long distances, but they are powerless to carry them across oceans, or over high mountain ridges. Similarly, a desert region is a barrier to seeds brought from a moist climate, and a cold climate prevents the naturalization of species coming from a warm country. The chance that seeds will germinate and grow is greatest when they are carried into a habitat similar to the original one and it is least when they are left in habitats very different from it. It is unquestionable that seeds have often been carried into many places where they were unable to secure a foothold. This fact explains why many species are found only in certain countries, or localities, and why it is that each formation retains a more or less distinctive impress.
North American Vegetation.— The vegetation of the North American continent owes its general features to the gradual decrease of heat to the northward, and the more or less constant decrease in the rainfall in passing from the coasts to the interior. The greatest development of forests is found in the warm coast-regions of the southeast, and of the Pacific. The poorest vegetation is found in the north, and on high mountains, where the temperatures are low, and in the interior where the rainfall is slight. The character and distribution of vegetation are chiefly determined by heat and water. As a result, the vegetative covering falls into zones corresponding in a general way to zones of temperature. If the distribution of moisture were uniform over the continent, the series of zones would be as follows: (1) the zone of evergreen tropical and subtropical trees; (2) the zone of deciduous trees; (3) the zone of cone-bearing trees; (4) the zone of grasses and other herbs; (5) the zone of mosses and lichens; (6) the zone of ice and snow. The rainfall decreases regularly from the coast inland, while a high mountain range makes an abrupt change in the amount. The Appalachian, Rocky Mountain and Sierra Nevada ranges act as barriers to the passage of moisture-laden winds, and turn into grass land or desert, regions that are sufficiently warm to be forested. The Appalachian barrier is too low to be very effective, and the forests yield to prairies only slowly and far inland. The Sierra Nevada and the Rocky Mountains are almost complete barriers, and they enclose a parched desert. The height of these ranges causes an abundant condensation on their slopes, and in consequence they are more or less heavily wooded. On account of the altitude, the temperature is tow, and the forests are merely southerly extensions of the great boreal zone of pines and spruces. A general survey of the North American continent would show it to be wooded on the eastern, western, and southern coasts. In the north, there is a zone of grass and moss-covered barrens. In the interior there is a region of plain and prairie, stretching unbroken from Athabasca to Texas, and between the two great Cordilleran ranges from Washington to Central America lies a great desert region, broken repeatedly by intersecting lines of mountains. Running southward from the great northern forest mass of the continent are the three mountain systems. In the low Appalachian system, the arctic vegetation of the north is found on a few alpine peaks alone, but in the higher Rocky Mountains and the Sierra Nevada this long southward extension of dwarf herbaceous vegetation is almost continuous. All carry the northern pines and spruces far south, but in the lower range, these disappear in Virginia, while on the higher ranges they persist almost to the Mexican boundary. North America, is thus seen to be covered with belts of vegetation running east and west, which are completely interrupted in the interior by high mountain ranges, which, together with the Appalachians, also serve to carry the northern forests southward in three long tongues.