Fibrovascular Bundles. Collections of such tubes and supporting woody cells together make up what is known as fibro

vascular bundles.

Structure of a Root Hair. The cells of the cortex are almost uniform in character. The outmost layer, however, differs from the rest of the cortex. This layer is called the epidermis. It is the prolongations of the cells of the epidermis that form the structures we have already seen and know as root hairs.

Let us now take out one of the small radish seedlings from the pocket garden, mount it in water, and examine it under the low power of the microscope. A single root hair will be found to be a long round structure, almost colorless in appearance. The wall, which is very flexible and thin, is made up of cellulose, a substance somewhat like wood in chemical composition, through which fluids may easily pass. If we had a very high power of the microscope focused upon this cellulose wall, we should be able to find under it another structure, far more delicate than the cell wall. This is called the cell membrane. Clinging close to the cell membrane is the protoplasm of the cell, which in the root hair is found close to the membrane. The interior of the root hair is more or less filled with a fluid called cell sap. Forming a part of the living protoplasm of the root hair, sometimes in the hairlike prolongation and sometimes in that part of the cell which forms the epidermis, is found a nucleus. The protoplasm, nucleus, and cell membrane are alive; all the rest of the root hair is dead material, formed by the activity of the living substance of the cell. The root hair is a living plant cell with a wall

C.M.C.W.

C.S.

Diagram of a root hair: C.M., cell membrane; C.S., cell sap; C.W., cell wall; P, protoplasm; N, nucleus; S, soil particles.

so delicate that water and mineral substances from the soil can pass through it into the interior of the root.

Place the
Examine

HOW THE ROOT ABSORBS WATER. This process can best be understood by means of the following experiment:1 Crack the shell of a fresh egg at one end and pick it, bit by bit, from the delicate membrane that lies underneath it until about one square inch of the membrane is exposed. Now break a small hole in the opposite end of the egg just large enough to admit a small glass tube. After putting the tube in place, cement it in with sealing wax or paraffin. egg with the large end in a glass of water. it after a few hours, and the contents of the egg will be found to have risen in the glass tube to a considerable distance. The membrane through which the water has passed has no holes in it. It allows the passage of certain fluids through it, and is hence called a permeable membrane. In the experiment just performed, a little of the contents of the egg passes into the glass, as can be proved by the proteid test applied to the contents of the glass. On the other hand, a considerable amount of water from the glass has passed into the egg through the membrane.

Osmosis. The process by which two fluids, separated by a membrane, pass through the membrane and mingle with each other is called osmosis. In this process the greater flow is always toward the more dense medium. The method by which the root hairs take up soil water is exactly the same process as we see in the egg. It is by osmosis. The white of the egg is the best possible substitute for living matter; it has, indeed, almost the same chemical formula as protoplasm. The animal membrane separating the egg from the water is much like the delicate membrane which separates the protoplasm of the root hair from the water in the soil surrounding it. The fluid in the root hair is more dense than the soil water; hence the greater flow is toward the interior of the root hair.

Passage of Soil Water within the Root. We have already seen that in an exchange of fluids by osmosis the greater flow is always toward the denser fluid. Thus it is that the root hairs

1 This experiment, although not illustrating osmotic action in the strict sense, appeals to the pupil as does no other.

A potato osmometer. The lower end of the potato was cut off and the remainder peeled for about one third of its length. A hole was bored to within three fourths of an inch of the cut end; a small hole was bored at the side of the potato. In the latter was inserted a small L-shaped tube, the lower end being vaselined to make it air tight. Sugar was then placed in the hole at the top and a cork inserted; water was poured into the dish below. Within two hours the water had risen in the tube as shown in the right-hand figure.

take in more fluid than they give up. The cell sap, which partly fills the interior of the root hair, is a fluid of greater density than the water outside in the soil. When the root hairs become filled with water, the density of the cell sap is lessened, and the cells of the epidermis are thus in a position to pass along their supply of water to the cells next to them and nearer to the center of the root. These cells, in turn, become less dense than their inside neighbors, and so the transfer of water goes on until the water at last reaches the central cylinder. Here (as we shall see later) it is passed over to the tubes of the fibrovascular bundles and started up the stem. The pressure created by this process of osmosis is sufficient to send water up the stem to a distance, in some plants, of twenty-five to thirty feet. Cases are on record of water having been raised in the birch a distance of eighty-five feet. How water gets to the summits of tall trees is a problem which we shall discuss in a later chapter.

Physiological Importance of Osmosis. It is not an exaggeration to say that osmosis is a process not only of great importance

to a plant, but to an animal as well. Foods are digested in the food tube of an animal; that is, they are changed into a soluble form so that they may pass through the walls of the food tube and become part of the blood. Without the process of osmosis we should be unable to use the food we eat.

Composition of Soil. If we examine a mass of ordinary loam carefully, we find that it is composed of a number of particles of varying size and weight. Between these particles, if the soil is not caked and hard packed, we can find tiny spaces. In welltilled soil these spaces are constantly being formed and enlarged. They allow air and water to penetrate the soil. If we examine soil under the microscope, we find considerable water clinging to the soil particles and forming a delicate film around each particle. In this manner most of the water is held by the soil.

L

Experiment to illustrate the kind of soil which best retains water: A, gravel; B, sand; C, barren soil; D, rich soil; E, leaf mold; F, dry leaves.

KIND OF SOIL FAVORABLE TO EVAPORATION. The picture shows an easily constructed apparatus to show which kind of soil can retain most water. Fill each of the vessels with a given weight (say 100 grams each) of gravel, sand, barren soil, rich loam, leaf mold, and 25 grams of dry pulverized leaves, then pour equal amounts of water (100 c.c.) on each. Measure all that runs through. The water that has been retained constitutes the water supply that plants could draw on from such soil.

HOW WATER IS HELD IN SOIL. - To understand what comes in with the soil water, it will be necessary to find out a little more about soil. Scientists who have made the subject of the composition of the earth a study, tell us

that once upon a time at least a part of the earth was molten. Later, it cooled into solid rock Soil-making began when the ice and frost, working with the heat, chipped off pieces of rock. These pieces in time became ground into fragments by action of ice, glaciers, or the atmosphere. This process is called weathering. Weathering is largely a process of oxidation. A glance at crumbling stone will convince you of this, because of the oxide of iron (rust) disclosed. So by slow degrees this earth became covered with a coating of what we call inorganic soil. Later, generation after generation of tiny plants and animals which lived in the soil died, and their remains formed the first organic materials of the soil. You are all familiar

dark soil simply contains more dead plant and animal life, which forms the portion called humus.

A simple experiment may be performed to show the amount of vegetable and mineral matter in different soils.

Amount of Organic Matter in Soil. Gather

about a pound of leaf mold from a forest, a like amount of the rich loam taken from beneath the leaf mold, and the same amount of soil taken from a barren roadside or field. Dry them carefully and then weigh equal amounts (say 100 grams) of each kind of soil. Place them on pieces of tin and heat them redhot over a coal fire or in

This picture shows how the forests help to cover the

inorganic soil with an organic coating.