The Root and Its Functions

The portion of the plant most intimately related to the soil is the root. This organ has two major functions - to anchor the plant firmly and to abosrb water and certain important nutrient materials from the soil. Beyond this, the root often serves as a storage reservoir for food, and may perform various other functions.

External Structure

The most common type of root is a rather slender and profusely branched structure, penetrating the soil in all directions and forming a fibrous root-system. Its advantages for anchorage and absorption are obvious. Somewhat less common are types which possess a single main root or tap-root, penetrating deeply into the soil and much stouter than the lateral roots which arise from it. Tap-roots lend themselves readily to storage purposes and frequently become large and fleshy. The root-systems of many plants are intermediate between these two main types. Others may sometimes depart radically from the normal forms in response to certain special and unusual functions which they have assumed.

The Absorbing Region

Absorption of water and nutrient material is carried on only by the younger portions of the root, near its tip. The very tip itself is covered with a sheathing root-cap of cells, which protects the delicate underlying tissues as the root pushes its way through the soil. Back of this is a short region of growth, the only place where elongation of the root occurs. Behind this, in turn, is the absorbing region, a somewhat longer zone the surface of which is covered with thousands of exceedingly delicate filaments, the root-hairs. Each hair is an elongated projection growing out from one of the surface cells of the root, its sap-cavity and lining of cytoplasm being continuous with those of the root-cell of which it forms a part. The root-hair may reach a length of several millimeters and force its way into the minute crevices of the soil, thus coming into most intimate contact with the soil particles. Through the enormous surface which these root-hairs expose to the soil, absorption of water and mineral salts takes place. Root-hairs are generally short-lived, dying away as the corky bark begins to appear. A root-hair zone of fairly constant length thus follows behind the growing root-tip, new hairs appearing in its younger portion to replace the oldest ones, which are continually dying away.

The Plant Cell

The root-hair (including its basal portion) is a plant cell. Since a knowledge of the structure and functions of cells is obviously essential if we are to understand how the root-hair is constructed and does its work, or, indeed, how any other part of the plant is put together and functions, it will be nevessary at this point, before we discuss the physiology of the root, to describe briefly some of the most important characteristics of cells in general and of plant cells in particular.

We have already spoken of that remarkable living material which is called protoplasm, the seat of all the various activities which are maintained in animals and plants, and the only portion of their bodies which is truly alive. Physically, protoplasm is a thin, jelly-like, colloidal substance, but its minute structure is not clearly known. Chemically, it is a mixture of very complex proteins and is thus composed chiefly of carbon, oxygen, hydrogen, and nitrogen. Protoplasm is the “physical basis” of all life and the most extraordinary material known to man.

The protoplasm of the plant body is not a directly continuous mass but is broken up into minute parts, the cells or protoplasts, each of which is a distincy and more or less independent unit, possessing a definite structure and carrying on within itself a variety of physiological processes. Around the cell the protoplasm secretes a dead cell-wall composed of the characteristic plant substance cellulose. This is firm in texture but easily penetrated by water. The protoplasm of the cell has two distinct portions - the nucleus, a dense, somewhat spherical body which appears to be the directive center for the cells activities; and the cytoplasm, thinner and more watery in texture, which lines the inner surface of the wall in a tenuous layer and is bounded, without and within, by a very delicate membrane. Embedded in the cytoplasm frequently appear small, somewhat denser bodies, the plastids. These perform special functions, such as carrying on the manufacture of food, building up starch-grains or producing colors. In many cases it has been shown that the cytoplasm is not passive immobile, but that within it a slow, streaming movement often takes place. In mature cells, the entire central portion of the cell is occupied by a sap-cavity or vacuole, filled with water in which various substances are dissolved, and surrounded externally by the layer of cytoplasm. A typical plant cell may thus perhaps be likened roughly to an inflated football or basketball, the firm leathery covering corresponding to the cell-wall; the thin inner bladder of rubber, tightly pressed against it, to the layer of cytoplasm; and the air-space to the sap-cavity. A comparison to an automobile tire, with its stout shoe or casing, its delicate inner tube, and its central air-cavity, might also be made.

Cells are normally very small objects, averaging about .01 mm in diameter, and varying widely in shape and character according to the function which each performs, whether this be support, absorption, conduction, storage, protection, food-manufacture, growth, or reproduction. The plant body is composed of a multitude of cells, bound firmly together by cementing substances to form an entire, coherent organism. As we consider the various tissues and organs in detail, we shall have occasion to describe the particular characteristics which their cells display.

Internal Structure of Roots

The epidermis of the root, like that of all other plant organs, is a single layer of cells in thickness. These cells are normally protective in function, but in the root-hair zone many of them produce on their outer surface a characteristic projection, the root-hair itself.

The cortex lies just under the epidermis and is of varying thickness. In the young root, its cells serve to transmit water and dissolved substances from root-hair to fibro-vascular cylinder, and, in the older roots, to store food. Most of the fleshy portion of storage-roots consists of enlarged cortex. The innermost layer of cortical cells is often especially modified and is then known as the endodermis.

The fibro-vascular cylinder occupies the core of the root, furnishing mechanical strength and serving as a highway for conduction. As in other organs of the plant, it is composed of two main tissues, the wood or xylem and the bast or phloem. The wood, which forms the central axis of this cylinder, is usually star-shaped in cross section and is composed for the most part of thick-walled and much elongated dead cells, the walls of which have become woody. It provides rigidity for the root and conducts upward the water and dissolved substances which enter from the soil. Between the points of the star are patches of bast, formed of thin-walled cells which transport manufactured food upward and downward.

In older roots the fibro-vascular cylinder, particularly as to its woody portion, increases greatly in thickness through the activity of a growing zone or cambium, just as does the stem; and a corky bark is usually developed on the outside.

Diffusion and Osmosis

The most important function of the root is to absorb from the soil the water and mineral substances needed for the plants life and growth. This involved the physical processes of diffusion and osmosis, a consideration of which is necessary before we can understand clearly this primary activity of the root.

Diffusion

Diffusion may be defined as the tendency of any substance, when it occurs as a gas or in solution, to become evenly distibuted throughout the whole space available to it by moving from points of greater to points of lesser concentration. Its operation is familiar in the diffusion of odors, for the minute particles given off by any strongly scented substance will move outward rapidly, even in perfectly still air, and will soon become equally distinquishable in all directions from their point of origin. Two gases liberated within a closed space soon diffuse throughout its whole extent and become thoroughly mixed. In the same way, a crystal of salt dissolved in a vessel of water will in time have its molecules dispersed so uniformly, even though the liquid is free from moving currents, that samples taken from any part of the contents of the vessel will be salt solutions of exactly the same strength. This constant tendency toward diffusion is explained by the fact that in gases and liquids the molecules are in very active movement, continually striking against one another and rebounding. There are obviously fewest collisions, and thus most frequent opportunity for unobstructed movement, in those directions where there are fewest molecules, and in such directions a dispersal of the molecules therefore necessarily takes place until they are present everywhere in uniform abundance. The principle of diffusion is operative in so many of the physiological processes of plants that it must be thoroughly grasped if these processes are to be understood.

When two liquids are separated by a membrane through which they can pass, diffusion between them will still take place. Such diffusion through a membrane is called osmosis, and it tends to continue (if the permeability of the membrane allows) until the composition of the liquids on both sides of the membrane is the same. If a solution of salt in water, for example, is present on one side and pure water on the other, the salt will tend to diffuse through the membrane until its concentration is the same throughout. It is important to note that the concentration (the amount of substance dissolved per unit of volume) rather than the total amount of the substance or the bulk of the solution, is the factor which determines the direction and rate of diffusion. It is by diffusion through the cytoplasmic membranes of the root-hair that mineral salts in the soil solution enter the plant. This inward movement of any given salt will continue so long as its concentration is greater in the soil water than in the sap of the root-hair.

Osmotic Movement of Solvents

This phenomenon of osmosis that is complicated, however, by the fact that the dissolving liquid or solvent (water in ithis case) as well as the dissolved substance will pass through the membrane, and by the remarkable circumstance that where such movement occurs, it is always more rapid in one direction that in the other. Experiment has shown that if two solutions of different densities (or a solution and purse water) are separated by a membrane, a movement of water take place from the less concentrated to the more concentrated solution, and tends to continue till both are of the same density; and that the rate of this movement is proportional to the difference in concentration. The more concentrated solution will therefore tend to expand through this access of water, and, if it is confined within a closed space, a pressure, often of considerable magnitude, will develop.

As to why such a movement of water occurs no complete agreement of opinion yet exists, for the process of osmotic interchange involves some of the less clearly understood phenomena of physical chemistry. We may assume that the dissolved substance has an affinity or attractive power for water, and that this attraction increases with the concentration of the substance; or that the molecular movement of water molecules, so that where there is little material in solution the water molecules strike the membrane and pass through it oftener than they can where much material is in solution; or we may regard the whole phenomenon as really a manifestation of the fundamental principle of diffusion, since the tendency is for the solutions on both sides of the membrane of to become equal in concentration, although this is here brought about a movement of the dissolving liquid as well as of the dissolved substance itself. None of these explanations is entirely satisfactory, but they may perhaps help to picture the process more clearly to our minds. The essential fact remains that water will pass through a membrane toward the denser solution, explain it as we may; and upon this fact depends the power of the plant to withdraw water from the soil.

Permeability of the Membrane

Thus far, we have assumed that both thedissolved substance and water may pass with perfect freedom through the membrane, or that the membrane is permeable to them. All osmotic membranes are readily permeable to water, but we find that they differ markedly in the ease with which dissolved substances of various sorts can diffuse through them. One membrane may be perfectly permeable to a given substance; another may allow it to pass slowly and with difficulty, and another may exclude it altogether. Nor does even the same membrane display an equal degree of permeability to all substances, for some may pass through it easily, others with difficulty, and others not at all. To what these differences in permeability are due we do not know, but they are presumably caused by the relations between the structure of the membrane and the size and character of the molucules of the dissolved substance.

A membrane which allows water to diffue through it but does not allow a given dissolved substance to do so is called semi-permeable, and it is a highly important biological fact that all membranes in living cells seem to belong to this class. The membrane of a root-hair cell, for example, allows water to pass readily but is impermeable to such substances as sugar, which are dissolved in the sap solution. The cell is thus able not only to retain these valuable materials within itself, unwasted by outward diffusion, but to use them as a permanent means of drawing in osmotically a supply of water from the soil, since their presence within the root-hairs normally maintains the sap of these cells at a higher concentration than the adjacent soil solution. This same membrane, however, is permeable to most of the mineral salts present in the soil, which are thus able to diffuse readily into the root-hair.

Other Principles of Osmotic Action

Before we attempt to apply the principles of osmosis to the living plant, however, we should fix clearly in mind certain facts with regard to osmotic phenomena in general about which confusion frequently arises. First, substances which are not soluable or which for any reason are not in solution cannot diffuse and have no osmotic effect whatever. Sugar, for instance, is soluble and is osmotically active, but the moment it is converted into stach, which is an insoluble substance, it loses its osmotic effect entirely. Second, the osmotic strength of a solution, and consequently its power to attact water, is determined not by the chemical nature of the dissolved substances but by the total concentration of material, of whatever kind, which is in solution. A solution of sugar, one of salt, one of a mixture of the two, or one containing half a dozen substances, may all have exactly the same osmotic concentration. Third, the diffusion of water through a membrane and the diffusion of salts through the same membrane occur quite independently of one another. Water will move through a membrane from a solution of lesser to a solution of greater total concentration, but a dissolved substance, following the general law of diffusion, will pass from a point where that substance is abundant to one where it is scarce, always providing that the membrane is permeable to it. Given the proper conditions, it is quite possible for a dissolved substance to pass through a memberane osmotically with no movement of water taking place at all, or for water to move without a movement of dissolved subtances in the other. Fourth, if there is more than one substance in solution, each will tend to diffuse quite independently of all others. Differences in the concentration of each substance, considered by itself, are what determine the rate and direction of diffusion of that substance.

Diffusion and Osmosis in the Plant Cell

It is upon the principles of diffusion and osmosis that the plant depends, not only for the absorption of water and mineral substances from the soil, but for most of the circulation of materials which goes on within the plant body. We have already outlined briefly the structure of a typical plant cell and may now consider the osmotic interchanges which go on therein.

The cell wall in plants is ordinarily composed of cellulose. Like most organic materials, cellulose has the capacity of absorbing water vigorously by imbibition and will therefore swell considerably if placed, when dry, in contact with water. This expansive ability of the cell wall is some value in certain of the plants activities, as in the germination of the seed, but the wall of an ordinary living cell is moist and has imbibed water to the limit of its capacity. Water, and practically all substances in solution, pass through this cellulose wall with great readiness, and since it thus offers practically no resistance to diffusion, its osmotic effect is slight.

We have noted that, in the mature plant cell, the cytoplasm is dispersed in a thin layer closely pressed against the inner surface of the cell-wall, and that it completely surround a large central vacuole or sap-cavity, filled with water in which various substances, sugar usually prominent among them, are dissolved. On its outer surface next the wall, and on its inner surface next the sap-cavity, the cytoplasm is bounded by a delicate membrane, so that we find here fulfilled all conditions necessary for osmotic activity - one solution, in sap-cavity, separated by a membrane or membranes from another solution, which may be the soil water (in the case of a root-hair) or the sap-solution of an adjacent cell.

These cytoplasmic membranes, unlike the cell-wall, offer resistance to the diffusion of certain things and are thus highly important in cell physiology. We find that they are characteristically semi-permeable, preventing the passage of such substances as sugar, which are dissolved in the sap-cavity; and we have already noted that the cell is thus able to retain these valuable materials within intself and to use them as a means for bringing in osmotically a continous supply of water from the soil or from adjacent cells. To the essential mineral salts and to many other dissolved substances, however, these membranes are generally permeable, though in varying defrees, and the cell is therefore readily able to absorb a supply of such substances from any adjacent solution. It has been found by experiment that the degree of permeability of the cell membranes is not a fixed and constant one but is subject to chnage from moment to moment in response to changes in the environment or in the protoplasm itself. A cell which at one time admits a given substance very readily may at another allow it to enter but slowly, or may exclude it altogether. Many of the phsiological activities of the cell are probably regulated by changed in the permeability of its membranes.

The rapidity with which a substance passes through a membrane is due not only to these differences in permeability but to differences in the concentration of the solutoins on the two sides of the membrane. Where this difference is great (other things being equal) osmotic diffusion will be more rapid than where it is slight. Therefore if the concentration of a dissolved subtance within a cell is reduced, either by its diffusion into an adjacent cell or its conversion into an insoluble form (as must occur when it enters into the construction of a complex organic molecule in the protoplasm) the rate at which a new supply enters the cell from without is at once correspondingly increased; but the moment its concentration becomes equal, within and without the cell, the movement of this particular substances ceases, even though others are passing rapidly through the membranes.

The Absorption of Water and Salts

This activity of the cell as an osmotic system evidently controls its most important functions. Let us first consider the role played by osmosis in that process which the immediate subject of this chapter, the absorption of water and nutrient materials from the soil. Each root-hair, as we have seen, is merely a projection from one of the epidermal cells of the root. The cytoplasm and sap-cavity of the cell extend into the root-hair, the whole of which is thus lined by a thin cytoplasmic layer, with its membranes. The root-hair penetrates the soil and comes intimately in contact with the soil-particles, to the surface of each of which a thin water-film normally adheres. In this water are dissolved a great variety of substances, but the total concentration of the soil solution is normally less than that in the sap-cavity of the root-hair, where sugar and other materials are dissolved. In obedience to the law of osmosis, therefore, water will pass from the soil solution through the cytoplasmic membranes of the root-hair and into its sap-cavity. This flow of water will continue so long as there is a difference in total density between soil-solution and sap-solution. Of course if the soil becomes dry and the film around each particle grows so thin that the surface attraction of the particle equals the osmotic attraction of the root-hair solution, the flow will necessarily cease; and if this condition occurs throughout the whole soil mass, the plant will suffer from drought.

Salts and other substances in the soil-solution diffuse through the root-hair membranes quite independently of the passage of water, and the rate at which they enter depends upon the factors which we have above considered. Any substance which is in greater concentration in the cell-sap than the soil-water, and to which the membranes are permeable, will of course, diffuse outwardly into the soil; but except for carbon dioxide, which is given off in considerable quantities as a product of respiration in the root, there seems to be comparatively little loss of material from the plant in this manner.

As water is taken into the sap-cavity of the root-hair, the solution there becomes less dense; and the first cell of the cortex is consequently able in turn to withdraw water osmotically from the root-hair cell. The second row of cortical cells may now withdraw water from the first, and this process will continue until the water reaches the central cylinder. The water-ducts here, however, are nothing but dead shells, their living cytoplasm having disappeared as soon as the thick cell walls were completed. They are filled with water, and it is hard to understand why water should move into them from the cells of the cortex rather than in the reverse direction. In the innermost layer of cortical cells, a considerable pressure is probably developed by osmosis, and water may simply be squeezed through the cytoplasm and into these ducts. We know, at any rate, that water is forced up through the ducts under a good deal of pressure. This root-pressure may be measured by a gauge attached to an opening in the stem. As to what causes water to rise to great heights in the trunks of trees we shall speak later; but root-pressure is apparently only one of the factors involved.

Other Osmotic Phenomena in the Plant

Not only the absorption of water from the soil, but the whole process of circulation within the plant body, as well, is primarily an osmotic one; for the salts taken in by the root-hairs, and any dissolved subtances in other cells throughout the plant, move from cell to cell by diffusion through the cytoplasmic membrane.

Still another contribution of osmosis to the plants activities lies in maintaining the turgidity of the tissues. It is evident that if a cell has a strong sap-solution and is thus able to absorb water vigorously, it will become plump and fully expanded and will press tightly against its neighbors. If all the cells become turgid in this way the whole plant will tend to be erect and rigid, like an inflated balloon. In the case of parts which do not possess a firm skeleton, such as the leaf blades, floral organs, or other comparatively soft structures, this turgidity is necessary to maintain their form, firmness, and proper functioning. Conversely, if a cell is exposed to a solution of greater concentration than its cell-sap, water will be withdrawn from it, it will collapse, and its cytoplasm will be pulled away from the walls. Such a condition of plasmolysis, if extreme or long-continued, will result in the death of the cell; and, if extensive, in the death of the plant.

Osmosis also plays an essential part in growth, for at any growing region we find a point where the cells are multiplying in number but are still small, and another point behind this where each expands rapidly to its final size. This expansion, with the consequent stretching of the cell-walls and growth of the tissues, is due to the rapid absorption of water by the young and delicate cells, the sap of which is rich in dissolved sugar. The force exerted by any growing region is thus primarily due to osmotic pressure.

Other Functions of the Root

We have briefly discussed the root as an organ of anchorage and of storage, and in more detail as an organ of absorption. It has less frequently certain other functions which should be mentioned here. Roots may arise from almost any part of the stem and sometimes from the leaves. In many climbing plants they are produced abundantly on these aerial organs and serve to hold the plant firmly to its support. In corn, stout roots arise from the stem at a little distance above the ground and pass into the soil, thus acting as props or guy-ropes for the tall plant. In epiphytes, the roots are sent out directly into the air and possess a characteristic spongy envelope which absorbs and holds rain-water and dew. In parasitic plants the roots are converted into short, sucking organs, penetrating the host-plant and withdrawing therefrom the food upon which the parasite lives.

The root and the leaf are the two most important vegetative organs of the plant, and it is therefore the leaf which we shall next discuss.