The Leaf and Its Functions

The vegetative organs of the plant naturally fall into two groups: The root-system, situated in the soil and concerned primarily with the absorption therefrom of water and certain nutrient materials; and the stem and leaves (together called the shoot), which unfold in the air and are concerned primarily with the manufacture of food, the raw materials for which they derive from both air and soil. Of the two members of the shootsystem the leaf is the primary and more important one, the stem serving merely to expose the leaves to light and air and to provide a means of communication between them and the rootsystem. It is logical, therefore, for us to follow our study of the root with a study of the leaf.

The Structure of The Leaf

Before we can understand clearly the functions which the leaf performs, we shall need to observe with some care its rather complicated structure.

External Structure

Externally, the typical leaf consists of a broad, flat, and thin portion, the blade, which is the seat of its important activities. This is green is color and provided with a system of ribs or veins of stouter texture than the rest of the tissue. The blade may sometimes be attached directly to the stem, but is usually supported by a leaf-stalk or petiole, which holds it out in a place favorable for the performance of its function and serves as a highway for transportation of water and food between blade and stem. At the base may be even, or it may be lobed or sometimes actually divided into separate portions, the leaflets, in which state the leaf is said to be compound. The margin is sometimes quite smooth, but is more commonly broken into teeth of various sizes. The vein-system, is either parallel where the veins run side by side with no conspicuous branches; or netted, where they divide and anastomose repeatedly. The petiole and stipules vary greatly in their development

Internal Structure

Internally, the structure of the leaf is highly differentiated. A transverse section cut at right angles to the surface of the blade displays three important tissues: The epidermus, or protective covering; the mesophyll, constituting the major portion of the leaf substance, and the veins, each of which is a separate fibro-vascular bundle and represents a final branch of the vascular system which runs through root and stem.

The epidermis covers the entire leaf surface and is usually but one cell-layer in thickness. Its cells are generally thin-walled and are filled with a transparent cell-sap. Spread over the outside wall is a thin, waxy, water-proofing layer, the cuticle, extending from cell to cell and forming a continuous skin which covers the epidermis. It is usually much thicker on the upper than on the lower surface of the leaf. The epidermis is not an unbroken layer but is provided with minute openings, the stomata (singular, stoma), through which an exchange of gases between the tissues of the leaf and the outside air may take place. These are much more numerous in the lower epidermis than in the upper, and, indeed, are often absent from the latter altogether. Each stoma is a slit-like pore formed by the pulling apart of two modified epidermal cells, the gaurd-cells, which are unlike other cells of the epidermis in containing chlorophyll. These guard-cells are so constructed that when plump and turgid with water they tend to pull apart, thus enlarging the opening. On becoming limp and partially collapsed, however, they spring together again and close it. The degree of stomatal opening is thus continually flucuating as the water supply of the guard-cells rises and falls in response to changing internal or external conditions.

The mesophyll consists of tissue which is characteristically thin-walled, soft, and green. The cytoplasm within its cells contains very small, roundish bodies, denser than the rest of the living substance, and green in color. These are the chloroplasts which contain within them the green pigment chlorophyll, to which the characteristic color of foliage is due. The mesophyll is not homogeneous tissue but in typical leaves is divided into two main regions. That part lying next to the upper side of the leaf is composed of cells which are elongated at right angles to the leaf surface, packed rather closely together, and provided with a great abundance of chloroplasts. This region is known as the palisade layer and here is carried on most activity the process of food-manufacture or photosynthesis. The lower region of the mesophyll consists of a mass of cells which are so very irregular in shape that large air-spaces occur between them, and a very loose, sponge-like tissue, the spongy layer, is produced. These air-spaces communicate directly with the outside air through the stomata. Chloroplasts are present in the spongy layer, but not abundantly. Through the exposure to the air of a large area of cell-surface, opportunity is provided in this portion of the mesophyll for those has-exchanges which are continually taking place between the leaf and the atmosphere, such as the absorption and excretion of both carbon dioxide and oxygen, in the processes of the photosynthesis and respiration, and the evaporation of water in the process of tranpiration.

Running through the blade are the fibro-vascular bundles or veins, the channels by which the leaf tissues are kept in communication with the rest of the plant. The main veins are stout, often projecting somewhat below the lower surface of the blade. These break up into smaller and smaller veins, and finally into minute veinlets which consist of only a few cells. Each vein is surrounded by a bundle-sheath of heavy-walled cells, to which most of its rigidity is due. Within this are two tissues: The wood, consisting largely (as elsewhere in the plant body) of elongated; water-conducting cells, the tracheids and ducts, which distribute water and dissolved substances brought up through the stem from the root; and the bast, consisting of especially modified, the sieve-tubes, which collect from the mesophyll the food manufactured there and convey it to the bast of the stem, along which it is transported to other parts of the plant.

The petiole, usually circular in cross section, has within it a cylinder or half-cylinder of fibro-vascular bundles which are continuous with the main veins of the blade above and which enter directly into the vascular cylinder of the stem below.

Photosynthesis

The primary activity of green leaves is the manufacture of food from certain simple inorganic materials - carbon dioxide and water - by energy derived from light. This process of photosynthesis is fundamental in organic nature, for it is of the utmost significance to animals and man, because it constitutes the sole ultimate source of food in the world. Food for living things. In the green parts of plants, and nowhere else among organisms, is active or kinetic energy - in this case the energy of light - converted into a latent or potential form, readily available to living organisms for use in maintaining their vital activities; and, moreover, in green plants alone are produced those fundamental organic materials out of which plant and animal bodies are constructed. All the complex metabolic changes which later take place in the organic world are simply elaborations or simplifications of the primary products of photosynthesis. A more detailed account of the various types of foods and their uses, and of the energy-relations of the plant, will be given in our chapter on Metabolism.

Materials

The materials combined by the plant in this process are but two - water and carbon dioxide. Water is absorbed from the soil by the roots, passes upward through the stem, the petiole, and the veins of the leaf, and thence enters the mesophyll cells. None is obtained by the leaf directly from the atmosphere. It should be remembered that only a relatively small portion of the water taken in by the plant is used in food-manufacture; for much the larger part soon leaves the plant again, passing out of the leaf into the air by transpiration. The carbon dioxide used in photosynthesis is derived entirely from the air. Here it is always present, but in such small quantities that it constitues only about three parts in ten thousand of the atmosphere or three hundreths of 1%. Experiments have shown that a higher concentration would be advantageous to plant growth, since up to a certain point the rate of photosynthesis rises if the proportion of carbon dioxide in the air is artificially increased. It is through this comparatively rare gas alone that the plant derives its supply of carbon, that element so vitally necessary to all living organisms. No other carbon compounds, not even the abundant supplies present in the complex organic materials of humus, can apparently be drawn upon by ordinary green plants. Carbon, oxygen and hydrogen, together with the seven essential elements derived from the soil, constitute the necessary chemical basis for plant life.

Mechanism

The mechanism or apparatus by which water and carbon dioxide are combined is remarkable green pigment chlorophyll. This is present only in the chloroplasts, portions of the cytoplasm slightly denser than the rest. Chloroplasts may be few and large in certain lower plants but in the higher ones are almost always small, numerous, and more or less spherical in shape. They are most abundant in the palisade layer of the leaf. As to chlorophyll itself we know comparatively little except that it is a complex protein and contains magnesium. Iron is essential for its production but apparently does not enter into the construction of the substance itself. The presence of light is also necessary for the full development of chlorophyll, as is shown by the pale color of leaves which have grown in darkness. We are even more ignorant as to the manner in which chlorophyll operates in bringing about the union of carbon dioxide and water, nor have we yet succeeded in imitating this process in the laboratory. We know, however, that chlorophyll does not contribute material to the product formed and that it is not used up itself in the process, and we may therefore infer that it acts somewhat as does a catalyzer.

Associated with chlorophyll is usually another pigment or group of pigments, yellow in color instead of green, to which the general terms xanthophyll or carotin are ginven. These are not concerned with photosynthesis and their function is poorly understood. To them are due most of the yellow colors which occur in plants. Chlorophyll is a very unstable compound and tends to break down quickly when extracted from the leaf or when the leaf loses its vitality, but the yellow pigments are much more resistant and often survive long after chlorophyll has disintegrated.

Energy

Energy is necessarily expended in the process of breaking up the molecules of water and cabon dioxide and recombining their atoms into a new compound. We know that this energy is derived not from heat, as in so many cases, but entirely from light, which thus plays an essential part in the physiology of plants. According to the most widely accepted theory, light is due to minute and enormously rapid vibrations, the length of the vibration - its wave-length - determining the color of the light produced. Sunlight, or any white light, is composed of vibrations of a great variety of different wave-lengths, but when such light is passed through a prism these become sorted out into a many-colored spectrum. The visible spectrum runs from red rays, the wave-length of which is approximately 750 millionths of a millimeter, to the violet ones, where it is approximately 400. These visible radiations are by no means the only ones which occur, however. Rays of longer wave-length than red - the infra-red rays - pass gradually into heat-waves, and those shorter than violet - the ultra-violet rays - are active chemically. When falling upon different objects, light behaves differently. All of it may be absorbed by the object and converted into heat or some other form of energy, the object then appearing black; or all may be reflected, the object then appearing white; or certain wave-lengths may be absorbed and certain others reflected, the object in such a case displaying to oiur eyes the color of the light which it reflects. We know, for example, that a green substance like chlorophyll absorbs in general those wave-lengths which are not green and reflects those which are green or greenish-yellow. We can determine more accurately, however, the kind of light which is absorbed by a substance, if we break up into a spectrum the light which has passed through that substance. Such a spectrum displays perfectly dark regions, or absorption bands, in those portions which correspond to the particular kinds or wave-lengths of light which the substance has absorbed. The absorption spectrum of chlorophyll shows a narrow, sharp, black band in the orange-red and a wider, less definite one in the blue, thus indicating that it is chiefly these two kinds of light which chlorophyll absorbs, and suggesting that the red and blue rays in sunlight, and no others, furnish the energy used in the process of photosynthesis. Chlorophyll possesses the remarkable power of utilizing energy from this source in the manufacture of food, an ability that is unique in the organic world.

The intensity as well as the character of the light affects the rate at which photosynthesis proceeds. The process begins at illuminations of very low intensity, reaches its maximum at about that of bright diffuse daylight, and decreases again in light which is so bright as to injure protoplasm. Photosynthesis may be readily accomplished in artificial light of the proper intensity and wave-lengths.

Given a supply of the necessary raw materials, a sufficient temperature, the presence of chlorophyll, and light of proper character and intensity, photosynthesis may go on anywhere in the plant. Although these conditions are preeminently fulfilled in the mesophyll of the leaves, they may also be present to a lesser extent in petioles, stipules, calyx-lobes, and other organs, thus insuring a utilization of these regions to produce a small supplementary food supply.

Products

Let us now turn from a consideration of the necessary conditions for photosynthesis to a study of its products. The details of the process whereby carbon dioxide unites with water are not yet known, but the formation of formaldehyde (CH2O) id perhaps one the the preliminary steps. The first product which can be recognized, however, and a substance which is therefore of unique interest, is glucose or grape sugar, C6H12O6, formed according to the following equation:

6CO2 + 6H2O = C6H12O6 + 6O2

Glucose is present in the sap of practically all plant cells. It is the fundamental carbohydrate and the basis for all other foods; and from it are ultimately derived, through the action of enzymes and by various additions and chemical modifications, all the organic compounds of plants and animals.

The presence of a large amount of sugar in a chlorophyll-bearing cell results in a stoppage of its manufacture there, and is disadvantageous for other reasons. We find, accordingly, that before photosynthesis has long continued, the resulting sugar becomes converted into another type of carbohydrate, starch (C6H10O5)n (n stands for the unknown number of smaller molecules which are united into one of the large and complex ones of such a substance of starch). Starch is complex and insoluble, occuring in minute but definite bodies or grains, the size, shape and markings of which are characteristic and constant in any plant species. The starch molecule is very large - just how large we do not know - and is derived through the combination of many glucose molecules, with the liberation of a molecule of water from each, thus:

nC6H12O6 - nH2O = (C6H10O5)n

Neither sugar nor starch are accumulated in very great quantities in the leaf-blade, for most of the products of photosynthesis are removed shortly after their production to those regions of the plant where they are to be used or stored.

By-Product

In the recombination of the atoms of carbon dioxide and water out of which glucose is produced there is evidently a surplus of oxygen, and we find this oxygen given of as a by-product of photosynthesis, passing forth into the air continually from green plants during daylight. This is of little significance to the plant itself but is often important to other organisms.

Photosynthesis is, therefore, c constructive process by which the food of the plant is manufactured from very simple inorganic materials, through the agency of the characteristic substance chlorophyll, and by energy derived from light. The significance of photosynthesis lies in the fact that it is the only process among living things whereby organic compounds are built up from simple inorganic substances, with the resultant storage of energy. All other chemical changes in plants and animals are concerned either with the transformation of one type of organic material into another or with the breaking down of complex organic compounds into simpler ones. Photosynthesis alone is fundamentally constructive, and the activity of green plants thus underlies that of all other organisms.

Transpiration

The lower portion of the mesophyll, or the spongy layer, is not concerned primarily with photosynthesis but with the interchange of gases between plant and atmosphere. Notable among these interchanges is the evaporation of water from the tissues and its passage into the air, a process which we know as transpiration.

The Importance of Water

The water-relations of a plant are of the utmost importance to it and profoundly influence its structure and activities. We have seen that water constitutes the major portion (75-90%) of plant tissues in general, and a very much larger share of protoplasm itself. An sbundance of water keeps the cells plump by maintaining the turgidity of the tissues, enables the soft parts of the plant to preserve their firmness and to function successfully. Water is one of the raw materials entering into the process of photosynthesis. It is the solvent of the mineral nutrients, which can enter the plant and move about within it only when in solution, and in watery solutions all the important physiological processes of the plant take place. The maintenance of an abundant supply of water in its tissues is therefore essential for the life and growth of the plant.

To this end the primary requisites are evidently the presence of a sufficient amount of available water in the soil and its abundant absorption therefrom by the roots. Of no less significance in the water-relations of the plant is the process by which this water evaporates from the plant tissues and passes into the air. Absorption must equal or exceed transpiration if the plant is to thrive, for should there be a deficiency in income or an excess of outgo, a shortage of water will result in the tissues, and the plant will suffer accordingly.

Only a very small fraction of the water which enters the root-hairs and passes upward to the leaves takes part in the manufacture of sugar. The remainder becomes distributed through the cells of the spongy layer and evaporates from their moistened walls, departing through the stomata as water vapor. A smaller amount may be evaporated directly from the surface of the epidermal cells. During the growing season a constant stream of water is thus passing through the plant body, entering at the root-hairs and leaving through the stomata. The total quantity of this water often amounts to several hundred times as much as the final dry weight of the plant itself.

The Rate of Transpiration

The rate of water-loss varies greatly according to the kinds of plant, the soil conditions, the season of the year, the time of day, and various environmental factors. As a general rule, we find that the rate tends to increase under conditions which favor increased evaporation, such as high temperature, bright light, rapid air movement and low humidity; and to decrease under environments of the opposite character.

Transpiration is by no means controlled entirelly by factors which influence evaporation alone. The rate of water-loss from a given leaf-surface and from an equal area of free water do not rise and fall exactly together, the transpiration from the living leaf sometimes being relatively higher and sometimes relatively lower. There must, therefore, be factors in the leaf itself (as opposed to those in the external environment) which tend to accelerate or to retard transpiration. The most important of thses is doubtless the opening and closing of the stomata, which we have already discussed. Changes in the concentration of the sap in the mesophyll cells also probably determine to some extent the rate at which water evaporates from their surfaces.

The actual amount of water transpired during the growing season may be large or small, depending on the size of the plant, its leaf-area, its transpiration-rate, and the moisture and fertility of the soil. Of most significance to the plant, however, is not the actual bulk of transpiration, but the efficiency with which the water is used. This is determined by comparing the weight of the total water transpired with the weight of the dry plant material ultimately produced, their quotient being known as water-requirement of the plant. Thus when we say that the water-requirement of corn under certain conditions is 400, we mean that for every gram of dry weight of corn plant produced at maturity, there have been transpired through its leaves 400 grams of water. Species vary markedly in their water-requirement, and so do plants of the same species when grown under different conditions.

The Significance of Transpiration

Excessive water-loss is an ever-present danger to land plants, and many structural modifications have been developed by various species, or may appear in particular individuals growing under dry conditions, which tend to reduce this loss. The question therefore arises as to whether transpiration is an unmixed evil, made necessary by the fact that the stomata must be open to allow the exchange of gases which take place in photosynthesis; or whether it is really a function of the leaf and performs a useful part in the plants economy. It was long thought that water must be taken in through the roots in large quantities to insure an abundant absorption of nutrient materials from the soil, but a fuller understanding of the phenomena of osmosis and root absorption shows the fallacy of this conclusion. It has also been contended that transpiration is useful in concentrating the very dilute solutions of nutrient salts take from the soil - “boiling them down”, so to speak. We have seen, however, that the factors which preclude such an explanation; and, indeed, experiment shows that the amount of salts absorbed is practically independent of the amount of water transpired. Transpiration from the leaves, however, is evidently what causes the transpiration stream, or continuous movement of water from root to leaf through the lifeless ducts in the wood of the stem. We shall consider this movement more fully when discussing the functions of the stem; but it is well to note here that by this stream the dissolved substances are transported bodily from the central cylinder of the root upward throughout the plant as far as ramifications of the dead conducting elements of the wood extend. This movement is probably far more rapid than would result through diffusion from cell to cell, and in tall plants, particularly, the transpiration stream probably performs a distinct service in distributing rapidly throughout the plant the supply of nutrient materials absorbed from the soil.

Transpiration is also of distinct usefulness in regulating the temperature of the leaf. The blade absorbs much more energy than it uses in photosynthesis, particularly in bright light; and the excess, as heat, would sometimes raise the temperature of the tissues dangerously, were it not absorbed in evaporating water from the mesophyll cells.

Transpiration is carried on primarily in the leaves, but may occur in any other organs which are exposed to the air. Excessive loss of water is often prevented in such regions by the development of cell layers with corky walls.