Elementary Botany

I. Factors Influencing Vegetation Types.

900. All plants are subject to the influence of environment from the time the seed begins to germinate until the seed is formed again, or until the plant ceases to live. A suitable amount of warmth and moisture is necessary that the seed may germinate. Moisture may be present, but if it is too cold, germination will not take place. So in all the processes of life there are several conditions of the environment, or the “outside” of plants, which must be favorable for successful growth and reproduction. Not only is this true, but the surroundings of plants to a large extent determine the kind of plants which can grow in particular localities. It is also evident that the reaction of environment on plants has in a large measure caused them to take on certain forms and structures which fit them better to exist under local conditions. In other cases where plants have varied by mutation (p. 338) some of the new forms may be more suited to the conditions of environment than others and they are more apt to survive. These conditions of environment acting on the plant are factors which have an important determining influence on the existence, habitat, habit, and form of the plant. These factors are sometimes spoken of as ecological factors, and the study of plants in this relation is sometimes spoken of as ecology,[48] which means a study of plants in their home or a study of the household relations of plants. These factors are of three sorts: 1st, physical factors; 2d, climatic factors; 3d, biotic factors.

901. Physical factors.—Some of these factors are water, light, heat, wind, chemical or physical condition of the soil, etc. Water is a very important factor for all plants. Even those growing on land contain a large percentage of water, which we have seen is rapidly lost by transpiration, and unless water is available for root absorption the plant soon suffers, and aquatic plants are injured very quickly by drying when taken from the water. Excess of soil water is injurious to some plants. Light is important in photosynthesis, in determining direction of growth as well as in determining the formation of suitable leaves in most plants, and has an influence in the structure of the leaf according as the light may be strong, weak, etc. Heat has great influence on plant growth and on the distribution of plants. The growth period for most vegetation begins at 6° C. (= 43° F.), or in the tropics at 10°-12° C., but a much higher temperature is usually necessary for reproduction. Some arctic algæ, however, fruit at 1.8° C. The upper limit favorable for plants in general is 45°-50° C., while the optimum temperature is below this. Very high temperatures are injurious, and fatal to most plants, but some algæ grow in hot springs where the temperature reaches 80°-90° C. Some desert plants are able to endure a temperature of 70° C., while some flowering plants of other regions are killed at 45° C. Some plants are specifically susceptible to cold, but most plants which are injured by freezing suffer because the freezing is a drying process of the protoplasm (see p. 374). Wind may serve useful purposes in pollination and in aeration, but severe winds injure plants by causing too rapid transpiration, by felling trees, by breaking plant parts, by deforming trees and shrubs, and by mechanical injuries from “sand-blast.” Ground covers protect plants in several ways. Snow during the winter checks radiation of heat from the ground so that it does not freeze to so great a depth, and this is very important for many trees and shrubs. It also prevents alternate freezing and thawing of the ground, which “heaves” some plants from the soil. Leaves and other plant remains mulch the soil and check evaporation of water. The influence of the chemical condition of the soil is very marked in alkaline areas where the concentration of salt in the soil permits a very limited range of species. So the physical and mechanical conditions of the soil influence plants because the moisture content of the ground is so closely dependent on its physical condition. Rocky and gravelly soil, other things being equal, is dry. Clay is more retentive of moisture than sand, and moisture also varies according to the per cent of humus mixed with it, the humus increasing the percentage of moisture retained.

902. Climatic factors.—These factors are operative over very wide areas. There are two climatic factors: rainfall or atmospheric moisture, and temperature. A very low annual rainfall in warm or tropical countries causes a desert; an abundance of rain permits the growth of forests; extreme cold prevents the growth of forests and gives us the low vegetation of arctic and alpine regions.

903. Biotic factors.—These are animals which act favorably in pollination, seed distribution, or unfavorably in destroying or injuring plants, and man himself is one of the great agencies in checking the growth of some plants while favoring the growth of others. Plants also react on themselves in a multitude of ways for good or evil. Some are parasites on others; some in symbiosis (see p. 85) aid in providing food; shade plants are protected by those which overtop them; mushrooms and other fungi disintegrate dead plants to make humus and finally plant food; certain bacteria by nitrification prepare nitrates for the higher plants (see p. 83).

II. Vegetation Types and Structures.

904. Responsive type of vegetation.—In studying vegetation in relation to environment we are more concerned with the form of the plants which fits them to exist under the local conditions than we are with the classification of plants according to natural relationships. Plants may have the same vegetation type, grow side by side, and still belong to very different floristic types. For example, the cactus, yucca, three-leaved sumac, the sage-brush, etc., have all the same general vegetation type and thrive in desert regions. The red oaks, the elms, many goldenrods, trillium, etc., have the same general vegetation type, but represent very different floristic types. The latter plants grow in regions with abundant rainfall throughout the year, where the growing season is not very short and temperature conditions are moderate. Some goldenrods grow in very sandy soil which dries out quickly. These have fleshy or succulent leaves for storing water, and while they are of the same floristic type as goldenrods growing in other places, the vegetation type is very different. The types of vegetation which fit plants for growing in special regions or under special conditions, they have taken on in response to the influence of the conditions of their environment. While we find all gradations between the different types of vegetation, looking at the vegetation in a broad way, several types are recognized which were proposed by Warming as follows:

905. Mesophytes.—These are represented by land plants under temperate or moderate climatic and soil conditions. The normal land vegetation of our temperate region is composed of mesophytes, that is, the plants have mesophytic structures during the growing season. The deciduous forests or thickets of trees and shrubs with their undergrowth, the meadows, pastures, prairies, weeds, etc., are examples. In those portions of the tropics where rainfall is great the vegetation is mesophytic the year around.

906. Xerophytes.—These are plants which are provided with structures which enable them to live under severe conditions of dryness, where the air and soil are very dry, as in deserts or semideserts, or where the soil is very dry or not retentive of moisture, as in very sandy soil which is above ground water, or in rocky areas. Since the plants cannot obtain much water from the soil they must be provided with structures which will enable them to retain the small amount they can absorb from the soil and give it off slowly. Otherwise they would dry out by evaporation and die. Some of the structures which enable xerophytic plants to withstand the conditions of dry climate and soil are lessened leaf surface, increase in thickness of leaf, increase in thickness of cuticle, deeply sunken stomates, compact growth, also succulent leaves and stems, and in some cases loss of the leaf. Evergreens of the north temperate and the arctic regions are xerophytes.

907. Hydrophytes.—These are plants which grow in fresh water or in very damp situations. The leaves of aerial hydrophytes are very thin, have a thin cuticle, and lose water easily, so that if the air becomes quite dry they are in danger of drying up even though the roots may be supplied with an abundance of water. The aquatic plants which are entirely submerged have often thin leaves, or very finely divided or slender leaves, since these are less liable to be torn by currents of water. The stems are slender and especially lack strengthening tissue, since the water buoys them up. Removed from the water they droop of their own weight, and soon dry up. The stems and leaves have large intercellular spaces filled with air which aids in aeration and in the diffusion of gases. Some use the term hygrophytes.

908. Halophytes.—These are salt-loving plants. They grow in salt water, or in salt marshes where the water is brackish, or in soil which contains a high per cent of certain salts, for example the alkaline soils of the West, especially in the so-called “Bad Lands” of Dakota and Nebraska, and in alkaline soils of the Southwest and California. These plants are able to withstand a stronger concentration of salts in the water than other plants. They are also found in soil about salt springs.

909. Tropophytes.[49]—Tropophytes are plants which can live as mesophytes during the growing season, and then turn to a xerophytic habit in the resting season. Deciduous trees and shrubs, and perennial herbs of our temperate regions, are in this sense tropophytes, while many are at the same time mesophytes if they exist in the portions of the temperate region where rainfall is abundant. In the spring and summer they have broad and comparatively thin leaves, transpiration goes on rapidly, but there is an abundance of moisture in the soil, so that root absorption quickly replaces the loss and the plant does not suffer. In the autumn the trees shed their leaves, and in this condition with the bare twigs they are able to stand the drying effect of the cold and winds of the winter because transpiration is now at a minimum, while root absorption is also at a minimum because of the cold condition of the soil. Perennial herbs like trillium, dentaria, the goldenrods, etc., turn to xerophytic habit by the death of their aerial shoots, while the thick underground shoot which is also protected by its subterranean habit carries the plant through the winter.

910. While these different vegetation types are generally dominant in certain climatic regions or under certain soil conditions, they are not the exclusive vegetation types of the regions. For example, in desert or semidesert regions the dominant vegetation type is made up of xerophytes. But there is a mesophytic flora even in deserts, which appears during the rainy season where temperature conditions are favorable for growth. This is sometimes spoken of as the rainy-season flora. The plants are annuals and by formation of seed can tide over the dry season. So in the region where mesophytes grow there are xerophytes, examples being the evergreens like the pines, spruces, rhododendrons; or succulent plants like the stonecrop, the purslane, etc. Then among hydrophytes the semiaquatics are really xerophytes. The roots are in water, and absorption is slow because there are no root hairs, or but few, and the aerial parts of the plant are xerophytic.

III. Plant Formations.

911. The term plant formation is applied to associations of plants of the same kind, though there is a great difference in the use of the word by different writers which leads to some confusion.[50] It is sometimes applied to an association of individuals of a species, or of several species occupying a rather definite area of ground where the soil conditions are not greatly different (individual formation); by others it is applied to the plants of a definite physiographic area, as a swamp, moor, strand, or beach, bank, rock hill, clay hill, ravine, bluff, etc. (principal formation); and in a broad sense it is applied to the plants of climatic regions, of those in bodies of water, etc. (general formations). Space here is too limited to discuss all these kinds of formations, but the nature of the general formations will be pointed out. The general formations may be grouped into four divisions:

• 1st. Climatic formations.
• 2d. Edaphic formations.
• 3d. Aquatic formations.
• 4th. Culture formations.

912. Climatic formations.—Climatic influences extend over wide regions, so that climate controls the general type of vegetation of a region. In the sense of control there are two climatic factors, temperature and moisture, especially soil moisture. Temperature exerts a controlling influence over the vegetation type only where the total heat during the period of growth and reproduction is very low. This occurs in polar lands and at high elevations where the climate is alpine. In the temperate and tropical regions of the globe moisture, not heat, controls the general vegetation type. These vegetation types in general are coincident with rainfall distribution, and Schimper recognizes here three types, which with the arctic-alpine type would make four climatic formations as follows:

1st. The woodland formation.—This formation is characterized by trees and shrubs, and it is what is called a close formation. By this it is meant that so far as the climate is concerned the conditions are favorable for the development of trees and shrubs in such abundance that they become the dominant vegetation type of the region and grow close together. Other plants, as herbs, grasses, etc., occur, but they grow as subordinate elements of the general vegetation type, and as undergrowth. The land portion of the globe, therefore, outside of arctic and alpine regions, where the annual precipitation is 40 to 60 or more inches, is the area for woodland formation. In some places, the eastern part of England, for example, the annual precipitation is 25 to 30 inches, but the cool temperature permits a forest growth. It is true there are places where forests do not grow,—where man cuts them down, for example. But if cultivated lands in this region were allowed to go to waste, they would in time grow up to forest again. So there are swamps where the soil is too wet for trees, or sandy or rocky areas where there is not a sufficient amount of soil or water to support forest trees. But here it is the soil conditions, not climatic conditions, which prevent the development of the forest. But we know that swamps are being filled in and the ground gradually becoming higher and drier, and that soil is slowly accumulating in rocky areas, so that in time if left to natural forces these places would become forested. So this area of heavy annual rainfall is a potential forest area. These areas are determined by warm currents of moisture-laden air from the ocean moving over cooler land areas where the moisture is precipitated. In general these areas are along the coasts of great continents and on mountains. Therefore the interior of a continent is apt to be dry because most of the moisture has been precipitated before it reaches the interior. Deserts or steppes are therefore usually near the interior of continents. Some exceptions to this general rule are found: central South America, which is a region of exceptional rainfall because the moisture-laden winds here come from the warmest part of the ocean; the desert region west of the Andes mountains, where the winds are not favorable; southern California, where the winds come chiefly from a cooler portion of the Pacific ocean and move over an area of high temperature, etc.

2d. Grassland formation.—Grasses form the dominant vegetation type where the annual rainfall is approximately 15 to 25 inches. In true grasslands the formation is a close one since there is still a sufficient amount of moisture to provide for all the plants which can stand on the ground. Yet there is not enough moisture to permit the growth of forest as the dominant type without aid and protection by man. The so-called prairie regions are examples. Trees and shrubs do occur, but they cannot compete successfully with the grasses because the climatic conditions are favorable for the latter and unfavorable for the former. On the border line between forest and prairie the line of division is not a clear-cut one because conditions grade from one to the other. The two formations are somewhat mixed, like the outposts of contending armies, arms of the forest or prairie extending out here and there. In the United States the prairies extend from Illinois to about the 100th meridian, and beyond this to the foothills of the Rockies and southwest to the Sonora Nevada desert the region is drier, the rainfall varying from 10 to 20 inches. This is the area of the Great Plains, and while grasses of the bunch type are dominant, they make a more or less open formation because the moisture is not sufficient to supply all the plants which could be crowded on the ground, each individual tuft needing an area of ground surrounding it on which it can draw for moisture. Such a formation is an open one, and in this respect is similar to desert formations.

3d. Desert formations.—These occur where the annual rainfall is still lower, 10 to 4 inches or even less, 2 to 3 inches, while in one place in Chili it is as low as ½ inch. In the great Sahara desert it is about 8 inches, while in the Sonora Nevada desert in the southwestern United States it is 4 to 8 inches. Here the formation is an open one. In the forest and prairie formations the plants compete with each other for occupancy of the ground, since climatic conditions are favorable, so that the struggle against climate is not severe. But in the desert plants do not compete with each other; since the climate is so austere, the struggle is against the climate. Hence plants stand at some distance from each other because the roots need the moisture from the ground for some distance around them. There is not enough moisture for all the plants that begin, and those which get the start take the moisture away from the intervening ones, which then die. Since the struggle is against the adverse conditions of climate and not a competition between plants to occupy the ground, no one floristic type dominates as in the case of the grasses and forests of the grassland and woodland formations, but grassland and woodland types grow together. So we find grasses, trees, and shrubs growing without competition in the desert. The dominant vegetation type is xerophytic.

4th. Arctic-alpine formation. This formation extends from the limit of tree growth to the region of perpetual ice and snow. The forest here comes in competition with climate, with the severe cold of the long winter night, so that tree growth is limited, and on the border line with the woodland formation the trees are stunted, bent to one side by the heavy snows, or the tops are killed by the cold wind. The arctic zone of plant growth is sometimes spoken of as the “cold waste,” since conditions here are somewhat similar to those in the desert, the extreme cold exercising a drying effect on vegetation, and the vegetation type then is largely xerophytic.

913. Edaphic[51] formations.—Edaphic formations may occur in any of the climatic-formation areas. They are controlled by the condition of soil or ground. The condition of the soil is unfavorable for the growth of the general vegetation type of that region, or is more favorable for another vegetation type, so that soil conditions overcome the climatic conditions. These areas include swamps, moors, the strand or beach, rocky areas, etc., as well as oases in the desert, warm oases in the arctic zone, river bottoms in the prairie and plains region, alkaline areas, etc. The edaphic formations may be close or open according to the nature of the soil. The edaphic formations then are infiltrated in the climatic formations, the different vegetation types fitting together like pieces of mosaic, which can be seen in some places from a mountain top, or if one could take a bird’s-eye view of the landscape or from a balloon.

914. Aquatic formations.—These are made up of water plants and are of two general kinds: fresh-water plant formations in ponds, lakes, streams; and salt-water plant formations in the ocean and inland salt seas.

915. Culture formations.—Culture formations are largely controlled by man, who destroys the climatic or edaphic formation and by cultivation protects cultivated types, or by allowing land to go to “waste” permits the growth of weeds, though weeds are often abundant in the culture areas. In general the culture formations may be grouped into two subdivisions: 1st, the vegetation of cultivated places; and 2d, the vegetation of waste places, as abandoned fields, roadsides, etc.

IV. Plant Societies.

916. Plant societies are somewhat definite associations of the vegetation of an area marked by physiographic conditions. A single plant society is nearly if not altogether identical with a “principal formation,” but is a more popular expression, and besides includes all the plants growing on the area, while in the use of the term “principal formation” we have reference mainly to the dominant plants and the most conspicuous subordinate species.

917. Complex character of plant societies.—In their broadest analysis all plant societies are complex. Every plant society has one or several dominant species, the individuals of which, because of their number and size, give it its peculiar character. The society may be so nearly pure that it appears to consist of the individuals of a single species. But even in those cases there are small and conspicuous plants of other species which occupy spaces between the dominant ones. Usually there are several or more kinds in the same society. The larger individuals come into competition for first place in regard to ground and light, the smaller ones come into competition for the intervening spaces for shade, and so on down in the scale of size and shade tolerance. Then climbing plants (lianas) and epiphytes (lichens, algæ, mosses, ferns, tree orchids, etc.) gain access to light and support by growing on other larger and stouter members of the society.

Parasites (dodder, mistletoes, rusts, smuts, mildews, bacteria, etc.) are present, either actually or potentially, in all societies, and in their methods of obtaining food sap the life and health of their hosts. Then come the scavenger members, whose work it is to clean house, as it were, the great army of saprophytic fungi (molds, mushrooms, etc.), and bacteria ready to lay hold on dead and dying leaves, branches, trunks, roots, etc., disintegrate them, and reduce them to humus, where other fungi change them into a form in which the larger members of the plant society can utilize them as plant food and thus continue the cycle of matter through life, death, decay, and into life again. Mycorhiza (see Chapter IX) or other forms of mutualistic symbiosis occur which make atmospheric nitrogen available for food, or shorten the path from humus to available food, or the humus plants feed on the humus directly. Nor should we leave out of account the myriads of nitrate and nitrite bacteria (see Chapter IX) which make certain substances in the soil available to the higher members of the society. Most plant societies are also benefited or profoundly influenced in other ways by animals, as the flower-visiting insects, birds which feed on injurious insects, the worms which mellow up the soil and cover dead organic matter so that it may more thoroughly decay. In short, every plant society is a great cosmos like the universe itself of which it is a part, where multitudinous forms, processes, influences, evolutions, degenerations, and regenerations are at work.

918. Forest Societies.[52]—Each different climatic belt or region has its characteristic forest. For example, the forests of the Hudsonian zone in North America are different from those of the Canadian zone, and these in turn different from those in the transition zone (mainly in northern United States). The forests of the Rocky mountains and of the Pacific coast differ from those of the Alleghanian, Carolinian (mainly middle United States) or Austroriparian (southern United States) areas. Finally, tropical forests are strikingly different from those of other regions. Similar variations occur in the forests of other regions of the globe. The character of these forests depends largely on climatic factors. The character of the forest varies, however, even in the same climatic area, dependent on soil conditions, or success in seeding and ground-gaining of the different species in competition, etc.

919. General structure of the forest.—Structurally the forest possesses three subdivisions: the floor, the canopy, and the interior. The floor is the surface soil, which holds the rootage of the trees, with its covering of leaf-mold and carpet of leaves, mosses, or other low, more or less compact vegetation. The canopy is formed by the spreading foliage of the tree crowns, which, in a forest of an even and regular stand, meet and form a continuous mass of foliage through which some light filters down into the interior. Where the stand is irregular, i.e., the trees of different heights, the canopy is said to be “compound” or “storied.” Where it is uneven, there are open places in the canopy which admit more light, in which case the undergrowth may be different. The interior of the forest lies between the canopy and the floor. It provides for aeration of the floor and interior occupants, and also room for the boles or tree trunks (called by foresters the wood mass of the forest) which support the canopy and provide the channels for communication and food exchange between the floor and canopy. The canopy manufactures the carbohydrate food and assimilates the mineral and proteid substances absorbed by the roots in the soil; and also gets rid of the surplus water needed for conveying food materials from the floor to the place where they are elaborated. It is the seat where energy is created for work, and also the place for seed production.

920. Longevity of the forest.—The forest is capable of self-perpetuation, and, except in case of unusual disaster or the action of man, it should live indefinitely. As the old trees die they are gradually replaced by younger ones. So while trees may come and trees may go, the forest goes on forever.

921. Autumn colors.—One of the striking effects produced by the deciduous forests is that of the autumn coloring of the leaves. It is more pronounced in the forests of the United States than in corresponding life zones in the eastern hemisphere because of the greater number of species. With the disintegration of the chlorophyll bodies, other colors, which in some cases were masked by the green, appear. In other cases decomposition products result in the formation of other colors, as red, scarlet, yellow, brown, purple, maroon, etc., in different species. These coloring substances to some extent are believed to protect the nitrogenous substances in the leaf from injury. The colors absorb the sun’s rays, which otherwise might destroy these nitrogenous substances before they have passed back through the petiole of the leaf into the stem, where they may be stored for food. The gorgeous display of color, then, which the leaves of many trees and shrubs put on is one of the many useful adaptations of the plants.

922. Importance of the forest in the disposal of rainfall.—The importance of the forest in disposing of the rainfall is very great. The great accumulation of humus on the forest floor holds back the water both by absorption and by checking its flow, so that it does not immediately flow quickly off the slopes into the drainage system of the valley. It percolates into the soil. Much of it is held in the humus and soil. What is not retained thus filters slowly through the soil and is doled out more gradually into the valley streams and mountain tributaries, so that the flood period is extended, and its injury lessened or entirely prevented, because the body of water moving at any one time is not dangerously high. The winter snow is shaded and in the spring melts slowly, and the spring freshets are thus lessened. The action of the leaves and humus in retarding the flow of the water prevents the washing away of the soil; the roots of trees bind the soil also and assist in holding it.

923. Absence of forest encourages serious floods.—The great floods of the Mississippi and its tributaries are due to the rapidity with which heavy rainfall flows from the rolling prairies of the west, and from the deforested areas west of the Alleghany system. The serious floods in recent years in some of the South Atlantic States are in part due to the increasing area of deforestation in the Blue Ridge and southern Alleghany system.

924. The prairie and plains societies.—These are to be found in the grassland formation. In the prairies “meadows” are formed in the lower ground near river courses where there is greater moisture in soil. The grasses here are principally “sod-formers” which have creeping underground stems which mat together, forming a dense sod. On the higher and drier ground the “bunch” grasses, like buffalo-grass, beard-grass, or broom-sedge, etc., are dominant, and in the drier regions as one approaches desert conditions the vegetation gradually takes on more the character of the desert, so that in the plains sage-brush, the prickly-pear cactus, etc., occur. Besides the dominant vegetation of the society there are subordinate species, and the societies are especially marked by a spring and autumn flora of conspicuous flowering plants which are mixed with the grasses.

925. Desert societies.—These are composed of plants which possess a form or structure which enables them to exist in a very dry climate where the air is very dry and the soil contains but little moisture. The true desert plants are perennial. The growth and flowering period occurs during the rainy season, or those portions of the rainy season when the temperature is favorable, and they rest during the very dry season and cold. Characteristic desert plants are the cacti with thick succulent green stems or massive trunks, the leaves being absent or reduced to mere spines which no longer function in photosynthesis; yuccas with thick, narrow and long leaves with a firm and thick cuticle; small shrubs or herbs with compact rounded habit and small thick gray leaves. All of these structures conserve moisture. The mesquite tree is one of the common trees in portions of the Sonora Nevada desert. Besides the true desert plants, desert societies have a rainy-season flora consisting of annuals, which can germinate, vegetate, flower, and seed during the period of rain and before the ground moisture has largely disappeared, and these pass the resting period in seed.

926. Arctic-alpine societies.—The most striking of the arctic plant societies are the “polar tundra,” extensive mats of vegetation largely made up of mosses, lichens, etc., only partially decayed because of the great cold of the subsoil, and perhaps also because of humus acid in the partially decayed vegetation. These tundras are brightened by numerous flowering plants which are characterized by short stems, a rosette of leaves near the ground, and by large bright-colored flowers. Heaths, saxifrages, and dwarf willow abound. Alpine plant societies are similar to the arctic, although some of the conditions are more severe than in the arctic region. This is principally due to the fact that during the summer while the plants are growing they are subject to a high temperature during the day and a very low temperature at night, whereas during the summer in arctic regions while the plants are growing there is continuous warmth for growth and continuous light for photosynthesis. Five types of alpine plants are recognized by some. 1st. Elfin tree. This type has short, gnarled, often horizontal stems, as seen in pines, birches, and other trees growing in alpine heights. 2d. The alpine shrubs. In the highest alpine belts they are dwarfed and creeping, richly branched and spreading close to the ground, while at lower belts they are more like lowland shrubs. 3d. The cushion type. The branching is very profuse and the branches are short and touch each other on all sides, forming compact masses (examples saxifrages, androsace, mosses, etc.). 4th. Rosette plants. These are perennial, short stems and very strong roots, and play an important part in the alpine meadows. 5th. Alpine grasses. These usually have much shorter leaves than grasses of the lowlands and consequently form a low sward.

927. Edaphic plant societies.—These are equivalent to edaphic plant formations, and the vegetation is of course controlled by the peculiar conditions of the soil. There are a number of different kinds of edaphic plant societies determined by the character of the physiographic areas. 1st. Sphagnum moors. These are formed in shallow basins originally with more or less water. The growth of the sphagnum moss along with other vegetation and its partial decay in the water builds up ground rapidly so that in course of time the pond may be completely filled in. This filling in proceeds from the shore toward the center, and in the early stages of course there would be a pond in the center. The partial decay of vegetation creates an excess of humus acid which retards absorption by the roots. The conditions are such, then, as require aerial structures for retarding the loss of water, and plants growing in such moors are usually xerophytes. Some of the plants are identical with those growing in the arctic tundra. 2d. Sand[53] strand of beach. The quantity of sand with very little or no admixture of humus or plant food makes it difficult for plants to obtain a sufficient amount of water even where rainfall is abundant. The same may be said of the sand dunes farther back from the shore. The plants of these areas are then usually xerophytes. Some of the plants accustomed to growing in such localities are American sea-rocket, seaside spurge, bugseed, sea-blite, sea-purslane, the sandcherry, dwarf willow, marram-grass, certain species of beard-grass, etc. 3d. Rocky shores or areas. Here lichens and mosses first grow, later to be followed by herbs, grasses, shrubs, and trees, as decayed plant remains accumulate in the rock crevices. 4th. Shores of ponds, or swamp moors. Here the vegetation often takes on a zonal arrangement if the ground gradually slopes to the shore and out into the pond. In Fig. 493 is shown zonal distribution of plants. The different kinds of plants are drawn into these zones by the varying amount of ground water in the soil, or the varying depth of the water on the margin of the pond as one proceeds from the land towards the deeper water. On the border lines or tension lines between the different zones the plants are struggling to occupy here ground which is suitable for each adjacent individual formation. Other edaphic societies are those of marl ponds, alkaline areas, oases in deserts, warm oases in arctic lands, the forested areas along river bottoms in prairie or plains regions, etc.

928. Aquatic plant societies.—In general we might distinguish three kinds, 1st. Fresh-water plant societies, with floating algæ like spirogyra, œdogonium, etc., the floating duck-meats, riccias; the plants of the lily type with roots and stems attached to the bottom and leaves floating on the surface, like the water-lily and certain pondweeds, and finally the completely submerged ones like certain pondweeds, the bassweed (Chara), etc. 2d. Marine plant societies, which are made up mostly of the red and brown algæ or “seaweeds,” though some green algæ and flowering plants also occur. 3d. The salt marshes where the water is brackish and there is usually a luxuriant growth of marsh-grasses.