LEAVES, in the ordinary sense of the word, are the structures on which devolves the duty of nourishing the plant. They invariably arise as lateral protuberances from the growing-points or terminal vegetative cones of the shoots, that is, from a part of the plant which is still in an embryonic condition. In cases where a leaf seems to arise from an older part of a plant, as from the trunk of a tree, close inspection shows that it is really developed from a shoot perhaps not readily visible. Its growth is first at the apex, but this soon ceases, and is followed by continuous enlargement throughout the tissues, by which the upper part or blade of the leaf is soon distinguished from the basal part, and the stalk or petiole (where present) is subsequently formed between them. The development may result in a variety of structures, some of which are far different from typical foliage-leaves, yet are strictly homologous; such are scale-leaves, bracts and the parts of a blossom (floral leaves). The higher the rank of the plant in the scale of development the more these diversities are manifested; and the observations here to be made apply mainly to the phanerogams from the ferns (q.v.) upward.
Leaves collect from the atmosphere the great essential of plant-food, carbon, and conduct the processes of its assimilation, or, in other words, apply it by chemical conversion to the vitality and growth of the plant. In order to understand how they perform this function it will be necessary to investigate their structure and properties. Each leaf is composed of three parts, an outside layer on each surface of compact, flattened, and usually colorless cells, forming a skin or epidermis; an inner part (mesophyll) consisting of irregular cellular tissue and intercellular spaces. These cells of the mesophyll contain minute bodies (chloroplasts) of green coloring matter called chlorophyll, which also abounds in the bark of the stems of herbs and all other green parts of plants, and is the working element in their composition. Through the spongy mesophyll extends the network of veins which form the skeleton of the leaf, and are at once its support and its channels of communication with the other parts of the plant; these form the third part of the leaf. One other important feature must be mentioned — the breathing-pores, or stomata. These are excessively minute openings in the epidermis, which occur wherever chlorophyll lies underneath, but are most numerous on the under or earthward side of the leaves, where, on the average, about 60,000 may be counted per square inch of surface, although in some leaves they are six or eight times as numerous. Each of these pores lies between the “guard-cells” which form an automatic valve, opening or closing the pore, by their swelling or shrinking, according to varying conditions and the requirements of plant-health, especially in respect to evaporation. The chlorophyll grains (chloroplasts) also change their positions in the cells so as to take all possible advantages of a weak illumination, or to guard against a bad effect from excessive light.
Of the 10 essential elements of plant-food nine are drawn from the soil by means of the roots, but the tenth, which is the most important and the largest in amount of all, is obtained by all green plants solely from the carbonic acid of the atmosphere and is taken up by the green leaves alone; also a little of the oxygen required. The air enters the stomata, is seized, as it were, by the chlorophyll, and within it is so decomposed (in a manner not yet explained) that the carbon is chemically extracted and is transformed into plant-food and plant-substance, that is, is assimilated; and botanists restrict their use of the term “assimilation” to this physiological absorption of carbon alone. In order to be able to do this work, however, the leaves require the aid of sunlight, without which the chlorophyll becomes inactive, and in total darkness a green plant will speedily die of starvation, however rich may be the soil in which it is rooted. The “rest” of plants at night is thus accounted for; and also the greater rapidity of growth in northern plants where in summer they enjoy more hours of sunlight each day than southern plants get.
But the service of leaves in the nutrition of the plant does not cease here. They perform a most important function in the transpiration of water. Plants must always draw from the soil a quantity of water far in excess of their needs, or of their capacity to hold, in order to get a sufficient supply of the mineral food dissolved in it, but in exceedingly small quantities; and after that sustenance has been extracted the extra useless surplus of water must be got rid of. This is accomplished through the stomata of the leaves, out of which water is always passing in gaseous evaporation or sometimes even in globules. A secondary but most important accompaniment of this is the suction thus formed, by which the constant up-flow from the root-ends is maintained.
A third essential office of leaves is as the lungs of the plant, which must breathe in essentially the same manner and for the same purposes as does an animal; that is, they must take up oxygen and give off carbonic acid. This independent process (the converse of the simultaneous assimilation) is carried on steadily by all plants, night and day; but in those having leaves it is mainly performed by these organs, because they spread the greatest surface.
In addition to these foremost and general services, leaves are adapted in particular cases, almost as numerous as the plant species, to such special purposes as a depository of food for the young plant in the cotyledons or seed-leaves; as bulb-scales in plants like the hyacinth and lily, where part of the nourishment in the foliage of one year is stored up in the scales or subterranean thickened leaves, for the early growth and flowering of the next year; as bud-scales, forming the protective coverings of buds, as tendrils, pitchers, fly-traps, etc.
These complicated requirements and duties, under varied conditions and circumstances, have produced the extraordinary modifications of form and texture which leaves present, and which must now be briefly considered.
Forms and Arrangement of Leaves.—The typical and ordinary foliage leaf is a thin, flat structure composed of stalk (petiole) and blade (lamina) of symmetrical form, and growing in the plane of the horizon, so that one side (the dorsal) is presented upward to the sky and sunshine, and the other (ventral) is downward and in shadow; and these sides usually present appropriate differences in texture, the upper surface being usually more smooth and compact than the lower. A great variety of textures, from smooth, polished or waxy, to rough, downy or spiny, are distinguished by botanists and used in the description of plants; these variations of surface are largely defensive in their character. Some leaves nave no stalk and are said to be sessile, in which case the base of the leaf may partly clasp or completely surround the stem, or be otherwise modified; similarly the stalk takes many forms, sometimes with two lesser subsidiary leaves (stipules) at the base. The rigid woody centre of the stalk may continue straight on through the middle of the leaf to its apex, forming a midrib which throws out branches alternately on each side toward the margin of the blade, each again branching repeatedly and connecting with its neighbor, and so forming a network or skeleton of woody fibres which strengthen and support the leaf. These ribs arc called veins or nerves, and the whole is the “venation” of the leaf. Such a simple leaf (for example of the beech) is called reticulate or net-veined. In a large class of cases, however, the branches of the midrib do not spring at approximately equal intervals along its length, but all diverge from a point near its base, making a palmately veined arrangement, as in the maple. This reticulate veining is characteristic of dicotyledons. In another very distinct type of venation, characteristic of monocotyledons, there is no midrib, but the stalk divides at the base of the blade into many equal veins which extend in a more or less curving line through the length of the leaf, converging at the apex; such a leaf is said to be parallel-veined, as in grasses. Upon the plan of the skeleton depends mainly the form of the leaf, of which a great number of variations are named in botanical manuals and used in descriptions of species, depending mainly on the character and extent of the indentation or incisions.
The arrangement of leaves upon the plant is an important matter. That it follows certain regular plans is apparent in buds, which when cut across exhibit their young leaves packed together in one or another of certain definite ways; and their relative position on the stem of an herb or the twig of a tree follows as a result of the law of growth in that group. The theoretical perfection of arrangement, however, is often greatly disturbed by the interference of older leaves with the development of the younger, and by other causes affecting the unsymmetrical growth of the whole plant. The arrangement of leaves upon the stem, called phyllotaxis, is in most cases one of alternation, thus securing the uninterrupted exposure of the upper surface of the leaf to the sun. It is to obtain this exposure that plants struggle to become tall and bear their leaves most profusely at the summit; and that the branches of trees reach outward as far as possible; and the lower early leaves of many soon die off because shaded by the later, higher growth. The arrangement is carried out in two principal ways: the leaves are either alternate, one after another, only a single leaf arising from each node or joint of the stem; or opposite, when there is a pair of leaves on each joint of the stem; but sometimes the leaves are whorled or verticillate, there being three or more in a circle on one joint of the stem. The result of this arrangement in an alternate-leaved stem is to cause the leaves to follow one another up the stem in a spiral manner; while any two successive leaves on the same species will also be separated from each other by just and equal portion of the circumference of the stem. The same principle governs the parts of the flower in which the sepals of the calyx typically alternate with the petals of the corolla, the petals with the stamens and the stamens with the pistils, but it is often disguised in a very puzzling way, especially by the absence of one or more series of organs. See Flower.
Modifications of Leaves.— Leaves exist in other forms than the typical ones of foliage. Scales, such as those which envelop and protect buds in winter, and the seeds in cones, are leaves of simple structure which have no assimilative powers or functions; they most frequently originate from an enlarged leaf-base upon which a proper leaf never develops. Bracteal leaves, or bracts, are of similar character and grow beneath and about the flowers, of which, when they are colored, they often form the most conspicuous part; but frequently they are green and are connected with true leaves by intermediate forms. Both scales and bracts have been forced, under experimental conditions, to develop into true leaves.
The modified leaves which form the flowers of phanerogams are termed “floral leaves,” and, as has been said, typically succeed one another in whorls from below upward, as sepals, petals, stamens and carpels. The sepals are usually green and much like foliage-leaves; the sepals often retain a likeness, but the interior whorls usually bear no resemblance to leaves, yet occasionally, in ill-health, revert to a shape which betrays their origin and genetic history.
Periodicity of Leaf Growth.— Leaves are a temporary part of the plant, arising and disappearing at more or less regular intervals, usually once a year. This is especially noticeable in the higher plants, some of which (annuals) die in autumn completely, surviving as species only in their seeds; others die down to the roots in the fall and put forth entirely new stems as well as leaves the following spring; while others, as shrubs and tree, die only so far as their leaves are concerned, putting forth new foliage after the stated period of rest. This period is due to the arrival of annually recurring unfavorable conditions of temperature or moisture or both, when the activity of life in the plant is suspended and it ceases to feed or grow. In such a state leaves having no function are needless — in fact often harmful — and in many cases die and fall off in so sudden and conspicuous a manner that we say such trees are deciduous; while others, which we call evergreen, retain most of their leaves in a green condition until gradually replaced by new ones, so that the foliage seems to be perpetual. The brilliant colors of the dying leaves of many trees and herbs in autumn are due partly to chemical changes in the decaying chlorophyll and partly to the exposure of pigment cells previously concealed by the abundance of chorophyll and other features of vital activity. The leaf drops because it no longer receives nourishment from the stem or twig. The cells at its base close up, transmitting no more sap, and in so doing separate from those in the base of the leaf, which is thus cut off and thrown away.
Bibliography.— The morphology, genesis and functions of leaves have been studied most deeply by German students, as Haeckel, Fritz Müller, Göbel, Schwender, Marchlewski, Fisher and others. These and other authorities have been well summarized in the English translation by Porter of Strasburger's ‘Textbook of Botany’ (1903). For the forms and nomenclature of leaves, consult the botanical manuals and textbooks of Gray, Wood and other American authors. Consult also Atkinson, ‘Elementary Botany’ (New York 1898); Coulter, ‘Plant Structures’ (ib. 1900); id., ‘Elementary Studies in Botany’ (ib. 1913); Ganong, ‘The Living Plant’ (ib. 1913).