VII. ROOTS AND THEIR WORK

Problem XIV. A study of roots. (Laboratory Manual, Prob. XIV)

(a) Factors influencing direction of growth.

(b) Structure.

(c) How they absorb soil water.

The development of a bean seedling has shown us that the root invariably grows first. One of the most important functions of the root to a young seed plant is that of a holdfast, an anchor to fasten it in

A root system, showing primary and second roots.

the place where it is to develop. In this chapter we shall find many other uses of the root to the plant, the taking in of water and the mineral and organic matter dissolved therein, the storage of food, climbing, etc. All other functions than the first one stated arise after the young plant has begun to develop.

Root System.

If you dig up a young bean seedling and carefully wash off the roots, you will see that a long root is developed as a continuation of the hypocotyl. This root is called the primary root. Other smaller roots which grow

from the primary root are called secondary, or tertiary, depending on their relation to the first root developed.

Downward Growth of Root. Influence of Gravity.-Most of the roots examined take a more or less downward direction. We are all familiar with the fact that the force we call gravity influences life upon this earth to a great degree. Does gravity act on the growing root? This question may be answered by a simple experiment.

Plant mustard or radish seeds in a pocket garden, place it on one edge and allow the seeds to germinate until the root has grown to a length of about half an inch. Then turn it at right angles to the first position and allow it to

remain for one day undisturbed. The roots now will be found to have turned in response to the change in position, that part of the root near the growing point being the most sensitive to the change. This experiment seems to indicate that the roots are influenced to grow downward by the force we call gravity.1

The reaction of the plant (or any living thing) to this force is called geotropism. Roots are stimulated by gravity to grow downward; hence they are said to be positively geotropic.

tion of the arrows to see if the roots of the radish respond to gravity.

Experiments to determine Influence of Moisture on a Growing Root. - The objection might well be interposed that possibly the roots in the pocket garden grew downward after water. That moisture has an influence on the growing root is easily proved.

Plant bird seed and the seed of mustard or radish in the underside of a sponge, which should be kept wet, and may be suspended by a string under a bell jar in the schoolroom window. Note whether the roots leave the sponge to grow downward, or if the moisture in the sponge is sufficient to counterbalance the force of gravity.

1 The Pocket Garden. A very convenient form of pocket germinator may be made in a few minutes in the following manner: Obtain two cleaned four by five negatives (window glass will do); place one flat on the table and place on the glass half a dozen pieces of colored blotting paper cut to a size a little less than the glass. Now cut four thin strips of wood so as to fit on the glass just outside of the paper. Next moisten the blotter, place on it some well-soaked radish or mustard seeds or grains of barley, and cover it with the other glass. The whole box thus made should be bound together with bicycle tape. Seeds will germinate in this box, and with care may live for two weeks or more.

Another experiment is the following: Divide the interior of a shallow wooden box into two parts by an incomplete partition. Partly fill the box with sawdust and place the opening in the partition so that it is below the surface of the sawdust. Plant peas and beans in the sawdust on one side of the partition, water very slightly, but keep the other side of the box well soaked. After two weeks, take up some of the seedlings and note the effect on the roots.

Water a Factor which determines the Course taken by Roots. Water, as well as the force of gravity, has much to do with the direction

taken by roots. Water is always found below the surface of the ground, but sometimes at a great depth. In order to obtain a supply of water, the roots of plants frequently spread out for very great distances. Most trees, and all grasses, have a greater area of surface exposed by the roots than by the branches. The mesquite bush, a low-growing tree of the American and Mexican deserts, often sends roots downwards for a distance of forty feet after water. The roots of alfalfa, a cloverlike plant used for hay in the Western states,

often penetrate the soil after water for a distance of ten to twenty feet below the surface of the ground.

Structure of a Taproot. To understand fully the structure of the root, it will be necessary for us to examine some large, fleshy root (a taproot), so that we may get a little first-hand evidence as to its internal structure. If you cut open a parsnip or carrot so as to make a cross section of the root, you find two distinct areas outer portion, the cortex, and an inner part, the wood. If you cut another parsnip in lengthwise section, these structures show still

an

more plainly. An additional fact is seen; namely, that all the smaller roots leaving the main or primary root have a core of wood which bores its way out through

the cortex wherever the small rootlets are given off.

Fine Structure of a Root. - If we could now examine a much smaller and more delicate root in thin longitudinal section under

the compound microscope, we should find the entire root to

be made up of cells, the walls A cross section through a taproot (a

parsnip): C, cortex; W, wood. Notice in the right-hand specimen, which has been dipped in iodine, that the core of wood continues out into the rootlets which leave the main root. Where is most starchy food stored in a parsnip?

of which are uniformly rather thin. (Cross sections and longitudinal sections of tradescantia roots are excellent for demonstration of these structures.) Over the lower end of the root is found a collection of cells, most of which are dead, loosely arranged so as to form a cap over the growing tip. This is evidently an adaptation which protects the young and actively growing cells just under the root cap. In the body of the root the central cylinder can easily be distinguished from the surrounding cortex. The cells of the former have somewhat thicker walls.

g

In a longitudinal section a series of tubelike structures may be found within the central cylinder. These structures are cells which have grown The end of a growing root, tipped together at the small end, the long and protected by the root cap; axis of the cells running the length g, the growing point. (Consider- of the main root. In their developably magnified.) ment the cells mentioned have grown together in such a manner as to lose their small ends, and now form continuous hollow tubes with rather strong walls. Other cells have come to develop greatly thickened walls; these cells give mechanical support to the tubelike cells. Collections of such tubes and supporting woody cells together make up what is known as fibrovascular bundles.

Root Hairs.

Careful examination of the root of one of the seedlings of mustard, radish, or barley grown in the pocket germinator

d

a

shows a covering of tiny fuzzy structures. These structures are very minute, at most 3 to 4 mm. in length. They vary in length according to their position on the b root, the most and the longest root hairs being found near at the point marked R. H. in the Figure. These structures are outgrowths of the outer layer of the root (the epidermis), and are of very great importance to the living plant.

Cross section of a young taproot: a, a, root hairs; b, epidermis; c, cortex; d, fibrovascular cylinder or wood.

Structure of a Root Hair. A single root hair examined under a compound microscope 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. 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. and cell membrane are alive; all the rest of the root hair is dead

Young embryo of corn, showing root hairs (R. H). and growing stem (P.).