Chapter Iii. the Root

I. OSMOSIS AND THE ACTION OF THE CELL

MATERIAL.—For experiments in osmosis provide fresh and boiled slices of red beet, a fresh egg, a piece of ox bladder or some parchment paper; glass tubing, thread, twine, elastic bands, salt and sugar solutions. A common medicine dropper with the small end cut off will answer instead of tubing for making an artificial cell; or an eggshell may be used, by blowing out the contents through a puncture in the small end, and carefully chipping away a portion of the shell at the big end, leaving the lining membrane intact. The different liquids can be put into the shell and the exposed membrane placed in contact with the liquid in the glass, by fitting over the latter a piece of cardboard with a hole in the center large enough for the exposed surface to protrude sufficiently to touch the water.

=55. Object of the experiments.=—In order to understand clearly the action of roots in absorbing nutrients from the soil, it will be necessary to learn something about the movement of liquids through the cells, upon which the physiological processes of the plant depend. For this purpose make an artificial cell by tying a piece of ox bladder or parchment paper tightly over one end of a small glass tube, as shown in Fig. 71.

EXPERIMENT 39. HOW DOES ABSORPTION TAKE PLACE IN THE CELL?—(a) Put some salt water in a wineglass, partly fill the tube of the artificial cell with fresh water, and mark on the outside of both vessels the height at which the contained liquid stands. Set the tube in the glass of salt water and wait for results, having first tested carefully to make sure that there are no leaks in the membrane. After half an hour, notice whether there is any increase of water in the glass, as indicated by the mark. If so, where did it come from? Is there any loss of water in the tube? What has become of it? How did it get out? Taste it to see if any of the salt water has got in. Which is the heavier, salt water, or fresh? (If you do not know, weigh an equal quantity of each.) In which direction did the principal flow take place; from the heavier to the lighter, or from the lighter to the heavier liquid?

(b) Put a sugar or salt solution in the tube, and clear, fresh water in the glass, marking the height in each as before. Does the liquid rise or fall in the tube? Does any of it escape into the water of the glass, and if so, is it more or less than before? Which now contains the denser fluid, the tube or the glass? What principle governs the course of the liquid? Try the same experiment with (c), the same liquid in both vessels, and notice whether there is a greater flow in one direction than the other, as indicated by a comparison with the marks on the outside. (d) Put in the tube some of the white of a raw egg, insert in a glass of pure water, and note the effect. (e) Reverse, with water in the tube and white of egg in the glass. Does the water rise in the tube as before? Test the contents for proteins; has any of the albumin passed through the membrane into the tube?

EXPERIMENT 40. TO TEST THE BEHAVIOR OF LIVING AND DEAD CELLS.—Slice a fresh piece of red beet into a vessel of water and of a boiled one into another vessel of the same liquid at the same temperature. What difference do you notice? Can you think of any reason why the boiled one gives up its juices and the other one does not?

=56. Osmosis.=—The passage of liquids or of solids in solution through membranes is known as osmosis. Our experiments have shown that the principles governing the osmotic movement are: (1) the passage of water from the thinner liquid toward the denser takes place more rapidly than in the opposite direction; (2) the rapidity of the transfer depends on the difference in density; (3) crystallizable substances in solution, like sugar and salt, osmose readily; (4) albuminous or gelatinous substances, such as the white of an egg, osmose so slowly that the cell wall may be regarded as practically impermeable to them.

=57. Osmosis a form of diffusion.=—Osmosis is related to diffusion as a part to the whole. In other words, it is a name given to the process when it takes place through a membrane, whether solid, as the outer wall of the cell, or semi-fluid, as the inner wall of living protoplasm. Diffusion may therefore take place without osmosis, that is, in the absence of a membrane, as, for example, when we sweeten our tea or coffee by allowing sugar to diffuse through it. Many membranes offer little resistance to the osmotic movement of crystallizable substances. Such membranes are said to be permeable. Membranes which are not permeable to the dissolved solids, are called semi-permeable, since they allow the diffusion of water but not of the substances in solution. Living protoplasm is of this class. It is only very slightly permeable to many substances toward which, when dead, it acts as a permeable membrane.

=58. Absorption in living and dead cells.=—There is one great difference between the action of the artificial cell used in the foregoing experiments and that of the cells of which a living body is built up. The living cell always has at least two membranes. One of these, the cell wall, is readily permeable, while the other, the protoplasm, is semi-permeable—that is, substances in solution usually diffuse more or less slowly, while water diffuses rapidly. Hence in the living cell the protoplasm exercises a power of absorption independent of the cell wall, sometimes rejecting substances admitted by the latter, sometimes retaining others to which it is permeable, as shown in Exp. 40. In the boiled beet the protoplasm had been killed and the red coloring matter passed through it unhindered, while in the living one it was held back by the protoplasmic lining, which is thus seen to control the absorptive properties of the cell.

=59. Plasmolysis.=—Cells can be killed or injured in other ways than by heat; for example, by cold, by poisons, by starvation, and by overfeeding through the use of too much fertilizer or too rich a one. In this last case, the soil water becomes impregnated with soluble matter from the manure, which may render it denser than the sap in the roots. When this happens, it will cause the osmotic flow to set outward and thus deplete the cell of its water; whence we have the paradox that a cell, or even a whole plant, may be starved by overfeeding. This action of osmosis in withdrawing the contents from a cell is termed plasmolysis, and you can easily understand how very important a knowledge of the principles governing it is to the farmer in determining the application of fertilizers to his crops.

Dead cells, although powerless to carry on the life processes of a plant, have nevertheless important uses in serving the purposes of mechanical support and also to some extent in assisting in the work of absorption, though their function here is a purely mechanical one.

=60. Selective absorption.=—Different plants through their roots absorb different substances from the soil water, or the same substance in varying degrees. Hence, one kind of crop will exhaust the soil of certain minerals while leaving other kinds intact, or very little diminished; and vice versa, another kind will take up abundantly what its predecessor has rejected. In this sense, plants are said to exercise a selective power in the absorption of nutrients. The expression must not be understood, however, as implying any kind of volitional discrimination. It is merely a short and convenient way of saying that the cells of different plants possess different degrees of permeability to certain substances, some being more permeable to one thing, some to another. But beyond this rejection of untransmissible substances there is no active power of discrimination, any substance that can pass through the cell wall and its protoplasmic lining being taken in, whether useful, unnecessary, or even harmful. These may, however, be got rid of by excretion, as the superfluous water taken in with dissolved minerals is exhaled from the leaves; or if incapable of passing out by osmosis, rendered harmless and retained in the form of the curious “crystalloids” found in various parts of plants. But while the kind of selection exercised by vegetable cells implies no power of choice, as a matter of fact those substances most used by the plant in carrying on its life processes are absorbed in much greater quantities than others, being transferred to parts where growth or other changes in the plant tissues are going on, and there used up in the work of nutrition, or excreted in part as waste products. In either case their passage from cell to cell will give rise to a continuous osmotic current in that direction, and the absorption of new matter will go on in proportion to the amounts used up.

=61. Definition.=—Tissue is a word used to denote any animal or vegetable substance having a uniform structure organized to perform a particular office or function. Thus, for instance, we have bony tissue and muscular tissue in animals; that is, tissue made of bone substance and muscle substance and doing the work of bone and muscle respectively. Likewise in plants, we have strengthening tissue made up of hard, thick-walled cells, serving mainly for purposes of mechanical support, and vascular tissue, made up of conducting vessels for conveying sap—and so on, for every separate function.

Practical Questions

1. Why do raspberries and strawberries have a flabby, wilted look if sugar has been put on them too long before they are served? (7, 56.)

2. Where has the juice gone? What caused it to go out of the berries? (56, 59.)

3. Is a knowledge of the principles governing osmosis of any practical use to the housekeeper?

4. Why cannot roots absorb water as freely in winter as in summer? (Suggestion: which is the heavier, cold or warm water?)

5. Why does fertilizing too heavily sometimes injure a crop? (59.)

6. Do you see any apparent contradiction between the action of plasmolysis and the selective power of protoplasm? Can you reconcile it?

7. If a piece of beet that has been frozen is placed in water it will behave just as the slice of boiled beet did in Exp. 40; explain. (58, 59.)

II. MINERAL NUTRIMENTS ABSORBED BY PLANTS

MATERIAL.—A dozen or two each of different kinds of seeds and grains. A small portion from a growing shoot of a woody and a herbaceous land plant, and of some kind of succulent water or marsh plant, such as arrow grass (Sagittaria), water plantain, etc.

APPLIANCES.—A pair of scales; a lamp, stove, or other means of burning away the perishable parts of the specimens to be studied.

EXPERIMENT 41.—DO THE TISSUES OF PLANTS CONTAIN MINERAL MATTER?—Take about a dozen each of grains and seeds of different kinds, weigh each kind separately, and then dry them at a high temperature, but not high enough to scorch or burn them. After they have become perfectly dry, weigh them again. What proportion of the different seeds was water, as indicated by their loss of weight in drying?

Burn all the solid part that remains, and then weigh the ash. What proportion of each kind of seed was of incombustible material? What proportion of the solid material was destroyed by combustion?

EXPERIMENT 42.—DO THEY CONTAIN DIFFERENT KINDS AND QUANTITIES OF MINERALS?—Test in the same way the fresh, active parts of any kind of ordinary land plant (sunflower, hollyhock, pea vines, etc.), and of some kind of succulent water or marsh plant (Sagittaria, water lily, fern). Do you notice any difference in the amount of water given off and of solid matter left behind? In the character of the ashes left? Have you observed in general any difference between the ashes of different woods; as, for instance, hickory, pine, oak? Compare with the residue left in Exp. 21; would you judge that the residual substances are of the same composition?

=62. Essential constituents.=—The composition of the ash of any particular plant will depend upon two things: the absorbent capacity of the plant itself and the nature of the substances contained in the soil in which it grows. But chemical analysis has shown that however the ashes may vary, they always contain some proportion of the following substances: potassium (potash), calcium (lime), magnesium, phosphorus, and (in green plants) iron. These elements occur in all plants, and if any one of them is absent, growth becomes abnormal if not impossible.

The part of the dried substances that was burned away after expelling the water consists, in all plants, mainly of carbon, hydrogen, oxygen, nitrogen, and sulphur, in varying proportions. These five rank first in importance among the essential elements of vegetable life, and without them the plant cell itself, the physiological unit of vegetable structure, could not exist. They compose the greater part of the substance of every plant, carbon alone usually forming about one half the dry weight. Other substances may be present in varying proportions, but the two groups named above are found in all plants without exception, and so we may conclude that (with the possible addition of chlorine) they form the indispensable elements of plant food. Carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus compose the structure of which the plant is built. The other four ingredients do not enter into the substance as component parts, but aid in the chemical processes by which the life functions of the plant are carried on, and are none the less essential elements of its food. Figure 74 shows the difference between a plant grown in a solution where all the food elements are present, and others in which some of them are lacking.

=63. How plants obtain their food material.=—Plants obtain their supply of the various mineral salts from solutions in the soil water which they absorb through their roots. With a few doubtful exceptions, they cannot assimilate their food unless it is in a liquid or gaseous form. Of the gases, carbon dioxide, oxygen, and hydrogen can be freely absorbed from the air, or from water with various substances in solution, but most plants are so constituted that they cannot absorb free nitrogen from the air; they can take it only in the form of compounds from nitrates dissolved in the soil, and hence the importance of ammonia and other nitrogenous compounds in artificial fertilizers. Some of the pea family, however, bear on their roots little tubers formed by minute organisms called bacteria, which have the power of extracting nitrogen directly from the free air mingled with the soil; and hence the soil in which these tuber-bearing legumes decay is enriched with nitrogen in a form ready for use.

Practical Questions

1. Could any normal plant grow in a soil from which nitrogen was lacking? Potash? Lime? Phosphorus? (62.)

2. Could it live in an atmosphere devoid of oxygen? Nitrogen? Carbon dioxide? (62.)

3. Why are cow peas or other legumes planted on worn-out soil to renew it? (63.)

4. Is the same kind of fertilizer equally good for all kinds of soil? For all kinds of plants? (60, 62.)

5. Why does too much watering interfere with the nourishment of plants? (Exps. 26, 27.)

6. Are ashes fit for fertilizers after being leached for lye? (62.)

7. Why will plants die, or make very slow growth, in pots, unless the soil is renewed occasionally? (60, 62.)

III. STRUCTURE OF THE ROOT

MATERIAL.—Taproot of a young woody plant not over one or two years old; apple and cherry shoots make good specimens. For showing root hairs, seedlings of radish, turnip, or oat are good, also roots of wandering Jew grown in water; for the rootcap, corn, sunflower, squash.

=64. Gross anatomy of the root.=—Cut a cross section of any woody taproot, about halfway between the tip and the ground level, examine it with a lens, and sketch. Label the dark outer covering, epidermis, the soft layer just within that, cortex, the hard, woody axis that you find in the center, vascular cylinder, and the fine silvery lines that radiate from the center to the cortex, medullary rays (in a very young root these will not appear). Cut a section through a root that has stood in coloring fluid for about three hours and note the parts colored by the fluid. What portion of the root, would you judge from this, acts as a conductor of the water absorbed from the ground?

Make a longitudinal section passing through the central portion of the root and extending an inch or two into the lower part of the stem. Do you find any sharp line of division between them? Notice the hard, woody axis that runs through the center. This is the vascular cylinder and contains the conducting vessels, the cut ends of which were shown in cross section in Fig. 76.

=65. Distinctions between root and stem.=—Pull off a branch from the stem and one from the root; which comes off the more easily? Examine the points of attachment of the two and see why this is so. This mode of branching from the central axis instead of from the external layers, as in the stem, is one marked distinction between the structure of the two organs. In stems, moreover, branches occur normally above the points of leaf insertion at the nodes (46), while in the root they tend to arrange themselves in straight vertical rows. The shoots and cions that often originate from them are not normal root branches, but outgrowths from irregular or adventitious buds, that may occur on any part of a plant. The root is not divided into nodes like the stem, and never bears leaves.

=66. The active part of the root.=—It is only the newest and most delicate parts of the root that produce hairs and are engaged in the active work of absorption, the older parts acting mainly as carriers. Hence, old roots lose much of their characteristic structure and take on more and more of the office of the stem, until there is practically no difference between them. On the sides of gullies, where the earth has been washed from around the trees, we often see the upper portion of the root covered with a thick bark and fulfilling every office of a true stem.

=67. Minute structure of the root.=—(a) Mount in water and place under the microscope a portion of the root of an oat or radish seedling containing a number of hairs. In studying the thin, transparent roots of very young seedlings a section will not be necessary. Observe whether the hairs originate from the epidermis or from the interior. Are they true roots, or mere outgrowths from the cells of the epidermis? Do they consist of a single cell or a number of cells each? Notice what very thin cell walls the hairs have; is there any advantage in this? The interior, transparent portion of the hair contains the sap, and the protoplasm forms a thin lining on the inner surface of the wall; why not the sap next the wall and the protoplasm in the interior? (58, 60.)

(b) Next examine a portion of the body of the root and try to make out the parts as shown in Fig. 79, and compare them with your observations in 64. The light line running through the middle is the central cylinder, up which the water passes, as was shown by the colored liquid in 64. Outside this is a darker portion (a, Fig. 79), corresponding to the cortex (rr, Fig. 77). Besides other uses, the cortex serves to prevent the loss of water as it passes up to the stem, and also, in fleshy roots like the carrot and turnip, for the storage of nourishment. Its innermost row of cells is thickened into the sheath, or endodermis (e), which serves as an additional protection to the conducting tissues. The extreme outer layer, from the cells of which the root hairs are developed, is, as already stated, the epidermis, and in the older and more exposed parts of perennial roots is displaced by the bark, which becomes indistinguishable from that of the stem. (66.)

(c) Look at the tip of the root for a loose structure (c) fitting over it like a thimble. This is the rootcap. Do you see any loose cells that seem to have broken away from it? These are old cells that have been pushed to the front by the formation of new growth back of them, and, being of no further use, are rubbed off by friction as the root bores its way through the soil. Draw a longitudinal section of the root as it appears under the microscope, labeling all the parts. If they cannot be made out distinctly in the specimen examined, use sections of young corn or bean roots, which are larger and show the parts more distinctly.

(d) Place under the microscope a thin cross section through the hairy portion of a primary root of a bean or pea seedling, and try to make out the parts noted above and shown in cross section in Fig. 80. Make a sketch of what you see, labeling all the parts you can recognize. Show in your drawing the differences in the size and shape of the cells composing the different tissues. Notice in the central cylinder (Fig. 80) several groups of what look in the section like little round pits, or holes, sp. These are the cut ends of large-sized tubes or ducts that convey the water absorbed by the roots to the stem. Each set of these tubes, together with a number of smaller ones belonging to the same group, constitutes a fibrovascular bundle—a very important element in the structure of all roots and stems, as these bundles make up the conducting system of the plant body.

IV. THE WORK OF ROOTS

MATERIAL.—Germinating seedlings of radish, bean, corn, etc.; a potted plant of calla, fuchsia, tropæolum, touch-me-not (Impatiens), or corn; a plant that has been growing for some time in a porous earthen jar.

APPLIANCES.—Glass tumblers; coloring fluid; wax; some coarse netting; dark wrapping paper, or a long cardboard box; a sheet of oiled paper; some half-inch glass tubing; a few inches of rubber tubing; an ounce of mercury; some blue litmus paper; a flower pot full of earth; a few handfuls of sand, clay, and vegetable mold; a pair of scales; a half dozen straight lamp chimneys, or long-necked bottles from which the bottoms have been removed as directed in Exp. 53.

EXPERIMENT 43. USE OF THE EPIDERMIS.—Cut away the lower end of a taproot; seal the cut surface with wax so as to make it perfectly water-tight, and insert it in red ink for at least half the remaining length, taking care that there is no break in the epidermis. Cut an inch or two from the tip of the lower piece, or if material is abundant, from another root of the same kind, and without sealing the cut surface, insert it in red ink, beside the other. At the end of three or four hours, examine longitudinal sections of both pieces. Has the liquid been absorbed equally by both? If not, in which has it been absorbed the more freely? What conclusion would you draw from this, as to the passage of liquids through the epidermis?

From this experiment we see that the epidermis, besides protecting the more delicate parts within from mechanical injury by hard substances contained in the soil, serves by its comparative imperviousness to prevent evaporation, or the escape of the sap by osmosis as it flows from the root hairs up to the stem and leaves.

EXPERIMENT 44. TO SHOW THAT ROOTS ABSORB MOISTURE.—Fill two pots with damp earth, put a healthy plant in one, and set them side by side in the shade. After a few days examine by digging into the soil with a fork and see in which pot it is drier. Where has the moisture gone? How did it get out?

EXPERIMENT 45. TO SHOW THAT ROOTS SHUN THE LIGHT.—Cover the top of a glass of water with thin netting, and lay on it sprouting mustard or other convenient seed. Allow the roots to pass through the netting into the water, noting the position of root and stem. Envelop the sides of the glass in heavy wrapping paper, admitting a little ray of light through a slit in one side, and after a few days again observe the relative position of the two organs. How is each affected by the light?

EXPERIMENT 46. TO FIND OUT WHETHER ROOTS NEED AIR.—Remove a plant from a porous earthenware pot in which it has been growing for some time; the roots will be found spread out in contact with the walls of the pot instead of embedded in the soil at the center. Why is this?

EXPERIMENT 47. TO SHOW THAT ROOTS SEEK WATER.—Stretch some coarse netting covered with moist batting over the top of an empty tumbler. Lay on it some seedlings, as in Exp. 45, allowing the roots to pass through the meshes of the netting. Keep the batting moist, but take care not to let any of the water run into the vessel. Observe the position of the roots at intervals, for twelve to twenty-four hours, then fill the glass with water to within 10 millimeters (a half inch, nearly) or less of the netting, let the batting dry, and after eight or ten hours again observe the position of the roots. What would you infer from this experiment as to the affinity of roots for water?

EXPERIMENT 48. WHAT BECOMES OF THE WATER ABSORBED BY ROOTS.—Cover a calla lily, young cornstalk, sunflower, tropæolum, or other succulent herb with a cap of oiled paper to prevent evaporation from the leaves, set the pot containing it in a pan of tepid water, and keep the temperature unchanged. After a few hours look for water drops on the leaves. Where did this water come from? How did it get up into the leaves?

EXPERIMENT 49. TO SHOW THE FORCE OF ROOT PRESSURE.—Cut off the stem of the plant 6 or 8 centimeters (3 or 4 inches) from the base. Slip over the part remaining in the soil a bit of rubber tubing of about the same diameter as the stem, and tie tightly just below the cut. Pour in a little water to keep the stem moist, and slip in above, a short piece of tightly fitting glass tubing. Watch the tube for several days and note the rise of water in it. The same phenomenon may be observed in the “bleeding” of rapidly growing, absorbent young shoots, such as grape, sunflower, gourd, tobacco, etc., if cut off near the ground in spring when the earth is warm and moist. By means of an arrangement like that shown in Fig. 81, the force of the pressure exerted can be measured by the displacement of the mercury. This flow cannot be due to the giving off of moisture by the leaves, since they have been removed. Their action, when present, by causing a deficiency of moisture in certain places may influence the direction and rapidity of the current, but does not furnish the motive power, which evidently comes, in part at least, from the roots, and is the expression of their absorbent activity.

EXPERIMENT 50. TO SHOW THAT ROOTS CAUSE THE OCCURRENCE OF ACIDS.—Lay a piece of blue litmus paper on a board or on a piece of glass slightly tilted at one end to secure drainage. Cover the surface with an inch of moist sand and plant in it a number of healthy seedlings. Acids have the property of changing blue litmus to red; hence, if you find any red stains on the paper where the roots have penetrated, what are you to conclude?

Carbon dioxide has a slight acid reaction and is caused to form in varying quantities by all roots. Probably other substances, and these not a few, are actually excreted.

EXPERIMENT 51. CAN THE ABSORBENT POWER OF ROOTS BE INTERFERED WITH?—Place the roots of a number of seedlings with well-developed hairs in a weak solution of saltpeter—10 grams (about ⅓ of an ounce) to a pint of water, and others in a stronger solution—say 30 grams, or 1 ounce, to a pint. Try the same experiment with weak and strong solutions of any conveniently obtainable liquid fertilizer. After 45 minutes or an hour examine the roots under a lens and note the change that has taken place. What has gone out of them? What caused the loss of the contained sap?

EXPERIMENT 52. TO TEST THE WEIGHT OF SOILS.—Thoroughly dry and powder a pint each of sand and clay, measure accurately, and balance against each other in a pair of scales. Which weighs more, bulk for bulk, a “light” soil, or a “heavy” one? (77.)

EXPERIMENT 53. TO TEST THE CAPACITY OF SOILS FOR ABSORBING AND RETAINING MOISTURE.—Arrange, as shown in Fig. 82, a number of long-necked bottles from which the bottom has been removed. This can be done by making a small indentation with a file at the point desired and leading the break round the circumference with the end of a glowing wire or a red-hot poker. The crack will follow the heated object with sufficient regularity to answer the purpose. Tie a piece of thin cloth over the mouth of each bottle and invert with the necks extending an inch or two into empty tumblers placed beneath. Fill all to the same height with soils of different kinds—sand, clay, gravel, loam, vegetable mold, etc.—and pour over each the same quantity of water from above. Watch the rate at which the liquid filters through into the tumblers. Which loses its moisture soonest? Which retains it longest?

Next leave the soils in the bottles dry, fill the tumblers up to the necks of the bottles, and watch the rate at which the water rises in the different ones. The power of soils to absorb moisture is called capillarity. Which of your samples shows the highest capillarity? Which the lowest? Do you observe any relation between the capillarity of a soil and its power of retention?

=68. Roots as holdfasts.=—One use of ordinary roots is to serve as props and stays for anchoring plants to the soil. Tall herbs and shrubs, and vegetation generally that is exposed to much stress of weather, are apt to have large, strong roots. Even plants of the same species will develop systems of very different strength according as they grow in sheltered or exposed places.

=69. Root pull.=—Roots are not mere passive holdfasts, but exert an active downward pull upon the stem. Notice the rooting end of a strawberry or raspberry shoot and observe how the stem appears to be drawn into the ground at the rooting point. In the leaf rosettes of herbs growing flat on the ground or in the crevices of walls and pavements, the strong depression observable at the center is due to root pull. (Fig. 84.)

=70. Storage of food.=—Another office of roots is to store up food for the use of the plant. This is done chiefly in the tissues of fleshy roots and tubers, and gives to them their great economic value. Next to grains and cereals, roots probably furnish a larger portion of food to the human race than any other crop. In addition to this they are also the source of valuable drugs, condiments, and dyes.

=71. Absorption and conveyance of sap.=—But the most important function of roots is that of absorption. By their action the soil water and the minerals contained in it are drawn up into the plant body and made available for conversion by the leaves into organic foods, as will be explained in another chapter. From the nature of their function, most roots have naturally a strong affinity for water, and its presence or absence has a marked influence on their direction of growth, being often sufficient to overcome that of geotropism (Exp. 47). There are many trees and shrubs, notably willow, sweet bay, red birch, and the like, that grow best on the banks of streams and ponds, where their roots can have direct access to water. Excess of moisture, however, is injurious to most land plants by preventing the roots from getting sufficient air for respiration.

=72. The conditions of absorption.=—The sap in the root cells is normally denser than the water in the soil, so there is a continuous flow from the latter to the former. But if, for any reason, the density of the liquids should be reversed, the flow would set in the opposite direction, and if continued long enough, the strength of the plant would be literally “sapped” by the exhaustion of its tissues, so that it would die. What is this process of cell exhaustion called?

=73. The use of acid secretions to the root.=—It was shown in Exp. 50 that carbon dioxide and probably other substances occur in the immediate vicinity of roots. Carbon dioxide is an active agent in dissolving the various mineral matters contained in the soil, and as these last can be absorbed only in a liquid or a gaseous state (63), the advantage to the root as an absorbent organ, of being able to secrete such active solvents, is obvious.

=74. Relation of roots to the soil.=—In order to perform their work of absorption, roots must have access to a suitable soil. To produce the best results a soil must contain (1) all the essential mineral constituents (62); (2) moisture for dissolving these materials; and (3) air enough to supply the oxygen which is necessary to the life processes of all green plants.

=75. Composition of soils.=—Sand, clay, and humus, or vegetable mold, with the various substances dissolved in them, constitute the basis of cultivated soils. A mixture of sand, clay, and humus is called loam. When the proportion of humus is very large and well decomposed, the mixture is called muck. Pure sand contains but little nourishing matter and is too porous to retain water well. Pure clay is too compact to be easily permeable to either air or water. Most soils are composed of a mixture of the two with vegetable mold in varying proportions, giving a sandy loam, or a clay loam, as the case may be.

=76. Tillage.=—The advantages of tillage are: (a) that by breaking up the hard lumps it renders the soil more permeable to air and water and more easily penetrable by the roots in their search for food; (b) the covering of loose, friable earth left by the plow and the harrow acts as a mulch, and by shading the soil below, prevents too rapid a loss of water by evaporation. Where the essential food ingredients are present, good tillage counts for more in making a crop than the original quality of the soil.

=77. Light and heavy soils.=—These terms are used by farmers not in relation to the weight of soils, but in reference to the ease or difficulty with which they are worked. Light soils contain a preponderance of sand; heavy ones, of clay.

Practical Questions

1. Will plants grow better in an earthen pot or a wooden box than in a vessel of glass or metal? Why? (Exp. 46.)

2. Which absorb more from the soil, plants with light roots and abundant foliage, or those with heavy roots and scant foliage? (Suggestion: roots absorb from the soil; leaves, mainly from the air.)

3. Why are willows so generally selected for planting along the borders of streams in order to protect the banks from washing? (71.)

4. Why are the conducting tissues of roots at the center instead of near the surface as in stems? (67, b.)

5. Why does corn never grow well in swampy ground? (74; Exp. 46.)

6. Why are fleshy roots so much larger in cultivated plants than in wild ones of the same species? (74, 76.)

7. When the use of a particular kind of fertilizer causes the leaves of the plants to which it has been applied to turn brown, so that the farmer says they have been “burned” by it, to what cause is the trouble due? (59, 72.)

8. Why do farmers speak of turnips and other root crops as “heavy feeders”? (70, 71.)

9. Which is more exhausting to the soil, a crop of beets, or one of oats? Onions, or green peas? (See 2, suggestion.)

10. Why will inserting the end of a wilted twig in warm water sometimes cause it to revive? (Exps. 48, 49.)

V. DIFFERENT FORMS OF ROOTS

MATERIAL.—Examples of taproots: bean, pea, cotton, maple seedlings, or any kind of very young woody root. Fibrous: any kind of grass or grain. Fleshy: parsnip, turnip, carrot, dahlia, sweet potato. Water: duckweed, pondweed, or a cutting of wandering Jew grown in water. Parasitic: mistletoe, dodder, beech drops. Aërial and adventitious: the aërial roots of old scuppernong vines, climbing roots of ivy and trumpet vine, prop roots from the lower nodes of cornstalks and sugar cane.

=78. Basis of distinction.=—Roots vary in form and external structure according to their origin, function, and surroundings. In reference to the first, they are classed as primary or secondary; in regard to the second, as dry or fleshy; while as to surroundings, they may be adapted to either the soil, water, air, or the parasitic habit. Soil roots are the normal form. According to their mode of growth they are either fibrous or axial.

=79. Taproots.=—These are the common form of the axial type. Compare the root of any young hardwood cion a year or two old with one of a mature stalk of corn or other grain, and with the roots of seedlings of the same species. Notice the difference in their mode of growth. In the first kind a single stout prolongation called a taproot proceeds from the lower end of the hypocotyl and continues the axis of growth straight downward, unless turned aside by some external influence. A taproot may be either simple, as in the turnip, radish, and dandelion, or branched, as in most shrubs and trees. In the latter case the main axis is called the primary root, and the branches are secondary ones.

=80. Fibrous and fascicled roots.=—Where the main axis fails to develop, as in the corn and grasses generally, a number of independent branches take its place, forming what are known as fibrous roots. Both fibrous and taproots may be either hard or fleshy. The turnip and carrot are examples of fleshy taproots, the dahlia and rhubarb of fascicled roots. The function of both is the storage of nourishment. The sweet potato is an example of a tuberous root.

=81. Practical importance of this distinction.=—The difference between axial and fibrous roots has important bearings in agriculture. The first kind, which are characteristic of most dicotyls, strike deep and draw their nourishment from the lower strata of the soil, while the fibrous and fascicled, or radial kinds, as we may call them for want of a better name, spread out near the surface and are more dependent on external conditions.

=82. Roots that grow above ground.=—The kinds of roots that have just been considered are all subterranean, and bring the plant into relation with the earth, whether for the purpose of absorbing nourishment, or of mechanical support, or, as in the majority of cases, for both. Many plants, however, do not get their mineral nutrients directly from the soil, and these give rise to various forms suited to other conditions of alimentation.

=83. Adventitious roots.=—This name applies to any kinds of roots that occur on stems, or in other unusual positions. They may be considered as intermediate between the two classes named in 81; for while their starting point is above ground, they generally end by fixing themselves in the soil, where they often function as normal roots. Familiar examples are the roots that put out from the lower nodes of corn and sugar cane stalks, and serve both to supply additional moisture and to anchor the plant more firmly to the soil. Most plants will develop adventitious roots if covered with earth, or even if merely kept in contact with the ground. The gardener takes advantage of this capacity when he propagates by cuttings and layers.

=84. Water roots.=—These are generally white and threadlike and more tender and succulent than ordinary soil roots, because they have less work to do. Floating and immersed plants, such as bladderwort and hornwort (Ceratophyllum) have no need of absorbent roots, since the greater part of their surface is in contact with water and can absorb directly what is needed.

Land plants will often develop water roots and thrive for a time if the liquid holds in solution a sufficient quantity of air and mineral nutrients. Place a cutting of wandering Jew in a glass of clear water, and in from four to six days it will develop beautiful water roots in which both hairs and cap are clearly visible to the naked eye.

=85. Haustoria=, from a Latin word meaning to drain, or exhaust, is a name given to the roots of parasitic plants, or such as live by attaching themselves to some other living organism, from which they draw their nourishment ready made. Their roots are adapted to penetrating the substance of the host, as their victim is called, and absorbing the sap from it. Dodder and mistletoe are the best-known examples of plant parasites, though the latter is only partially parasitic, as it merely takes up the sap from the host and manufactures its own food by means of its green leaves.

=86. Saprophytes.=—Akin to parasites are saprophytes, which live on dead and decaying vegetable matter. They are only partially parasitic and do not bear the haustoria of true parasites. Many of them, of which the Indian pipe (Monotropa) and coral root are familiar examples, obtain their nourishment in part, at least, by association with certain saprophytic fungi, which enmesh their roots in a growth of threadlike fibers that take the place of root hairs and absorb organic food from the rich humus in which these plants grow. Such growths are called mycorrhiza, meaning “fungal roots.” Similar associations are formed by some of the higher plants also. The rootlets of the common beech and of certain of the pine family, for instance, are often enveloped in a network of fungus fibers, and in this case root hairs are developed very poorly, or not at all. Besides greatly increasing the absorbent surface by their ramification through the soil, the mycorrhizal threads may possibly benefit the plant in other ways also, as, for instance, by bringing about chemical changes that might aid in the work of nutrition.

=87. Epiphytes, or air plants.=—In the proper meaning of the word these are not parasitic, but use their host merely as a mechanical support to bring them into better light relations. The name, however, is loosely applied to all plants that find a lodgment on the trunks and branches of trees, whether parasites or true epiphytes that draw no nourishment from the host. Not infrequently the latter is killed by them through suffocation, overweighting, or the constriction of the stems by close clinging twiners.

=88. Aërial roots= are such as have no connection at all with the soil or with any host plant, except as they may lodge upon the trunks and branches of trees for a support. In other than purely epiphytic plants, which get all their nourishment from the air, they are generally subsidiary to soil roots, like the long dangling cords that hang from some species of old grapevines; or they subserve other purposes altogether than absorbing nourishment, as the climbing roots of the trumpet vine and poison ivy. A very remarkable development of aërial roots takes place in the “strangling fig” of Mexico and Florida, which begins life as a small epiphyte, from seeds dropped by birds on the boughs or trunks of trees. When it gets well started, the young plant sends down enormous aërial roots, which find their way to the ground, and in time so completely envelop the host that it is literally strangled to death (Plate 3, p. 73). When this support is removed, the sheathing roots take its place and become to all intents and purposes the stem of the fig tree, which now leads an independent life.

=89. The root system.=—The entire mass of roots belonging to a plant, with all its ramifications and subdivisions, composes a root system. The extent of root expansion is in general about equal to that of the crown, thus bringing the new and active parts under the drip of the boughs where the moisture is most abundant. Some plants have root systems out of all seeming proportion to their size. A catalpa seedling six months old showed, by actual measurement, 250 feet of root growth, and it is estimated that the roots of a thrifty cornstalk, if laid end to end, would extend a mile. In the development of the root system, a great deal depends upon external conditions. In a poor, dry soil, the roots have to travel farther in search of a livelihood, and so a larger system has to be developed than in a more favorable location.

Practical Questions

1. Which is better to succeed a crop of turnips on the same land, hay or carrots? (81.)

2. Write out what you think would be a good rotation for four or five successive crops based on the forms of the roots.

3. Study the following rotations and give your opinion about them, on the same principle. Suggest any improvements that may occur to you, and give a reason for the change. Beets, barley, clover, wheat; cotton, oats, peas, corn; oats, melons, turnips; cotton, oats, corn and peas mixed, melons; cotton, hay, corn, peas.

4. Give three good reasons in favor of a rotation over a single-crop system. (24, 60, 62, 81.)

5. Which will require deeper tillage, a bed of carrots or one of strawberries? (81.)

6. Explain why some plants keep green and fresh when the surface of the soil is dry, while others wilt or die. (81, 89.)

7. Which will better withstand drought, a crop of alfalfa or one of Indian corn? Why? (81.)

8. Which will interfere less with the trees if planted in an orchard, beets or onions? (81.)

9. Ought a crop of hemp and tobacco to succeed each other on the same land? (81, 89.)

10. Why does a gardener manure a grass plot by scattering the fertilizer on the surface, while he digs around the roses and lilacs and deposits it under ground? (81.)

11. Do the adventitious roots of such climbers as ivy and trumpet vine draw any nourishment from the objects to which they cling? (83-88.)

12. How can you tell?

13. Do partial dependents of this kind injure trees by climbing upon them; and if so, how? (87, 88.)

14. What is the use of the aërial roots of the scuppernong grape? (88.)

15. Is the resurrection fern (Polypodium incanum), that grows on tree trunks in our Southern States, a parasite or an air plant? (87.)

16. On what plants in your neighborhood does mistletoe grow most abundantly? Dodder?

17. Is mistletoe injurious to the host? (85.)

18. Name some plants that are propagated mainly, or solely, by roots and cuttings.

19. Where do aërial roots get their nourishment? (88.)

20. Would they be of any use to a plant in a very cold or dry climate?

21. Where should manure be placed to benefit a tree or shrub with wide-spreading roots? (66, 89.)

22. Is it a wise practice to mulch a tree by raking up dead leaves and piling them around the base of the trunk, as is often done? Why, or why not? (66, 89.)

Field Work

(1) Examine the underground parts of hardy winter herbs in your neighborhood, also of any weeds or grasses that are particularly troublesome, and see if there is anything about the structure of these parts to account for their persistence. Note the difference between roots of the same species in low, moist places and in dry ones; between those of the same kind of plants in different soils; in sheltered and in exposed situations. Study the direction and position of the roots of trees and shrubs with reference to any stream or body of water in the neighborhood. (The elm, fig, mulberry, and willow are good subjects for such observations.) Notice also whether there is any relation between the underground parts and the leaf systems of plants in reference to drainage and transpiration.

(2) Observe the effect of root pull upon low herbs. Look along washes and gullies for roots doing the office of stems, and note any changes of structure consequent thereon. Study the relative length and strength of the root systems of different plants, with reference to their value as soil binders, or their hurtfulness in damaging the walls of cellars, wells, sewers, etc. Dig your trowel a few inches into the soil of any grove or copse you happen to visit, note the inextricable tangle of roots, and consider the fierce competition for living room in the vegetable world that it implies.

(3) Tests might be made of the different soils in the neighborhood of the schoolhouse by planting seeds of various kinds and noting the rate of germination; first, without fertilizers, then by adding the different elements in succession to see what is lacking. The field for study suggested by this subject is almost inexhaustible.