A Civic Biology, Presented in Problems. George W. Hunter
Reader in Botany, pages 24–49. Ginn and Company.
Sharpe, A Laboratory Manual in Biology, pages 55–65. American Book Company.
advanced
Bailey, The Evolution of our Native Fruits. The Macmillan Company.
Bailey, Plant Breeding. The Macmillan Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Vol. I. American Book Company.
De Candolle, Origin of Cultivated Plants. D. Appleton and Company.
Duggar, Plant Physiology. The Macmillan Company.
Farmers' Bulletins, Nos. 78, 86, 225, 344. U. S. Department of Agriculture.
Hodge, Nature Study and Life, Chaps. X, XX. Ginn and Company.
Kerner (translated by Oliver), Natural History of Plants. Henry Holt and Company, 4 vols. Vol. II, Part 2.
Sargent, Corn Plants. Houghton, Mifflin, and Company.
VI. THE ORGANS OF NUTRITION IN PLANTS—THE SOIL AND ITS RELATION TO THE ROOTS
Problem.—What a plant takes from the soil and how it gets it.
(a) What determines the direction of growth of roots?
(b) How is the root built?
(c) How does a root absorb water?
(d) What is in the soil that a root might take out?
(e) Why is nitrogen necessary, and how is it obtained?
Laboratory Suggestions
Demonstration.—Roots of bean or pea.
Demonstration or home experiment.—Response of root to gravity and to water. What part of root is most responsive?
Laboratory work.—Root hairs, radish or corn, position on root, gross structure only. Drawing.
Demonstration.—Root hair under compound microscope.
Demonstration.—Apparatus illustrating osmosis.
Demonstration or a home experiment.—Organic matter present in soil.
Demonstration.—Root tubercles of legume.
Demonstration.—Nutrients present in some roots.
Uses of the Root.—If one of the seedlings of the bean spoken of in the last chapter is allowed to grow in sawdust and is given light, air, and water, sooner or later it will die. Soil is part of its natural environment, and the roots which come in contact with the soil are very important. It is the purpose of this chapter to find out just how the young plant is fitted to get what it needs from this part of its environment; namely, the soil.
The development of a bean seedling has shown us that the root 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 the place where it is to develop. It has many other uses, as the taking in of water with the mineral and organic matter dissolved therein, the storage of food, climbing, etc. All functions other than the first one stated arise after the young plant has begun to develop.
A root system, showing primary and secondary roots.
Root System.—If you dig up a young bean seedling and carefully wash the dirt from 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.
Revolve this figure in the direction of the arrows to see if the roots of the radish respond to gravity.
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 of gravity.
Experiments to determine the Influence of Moisture on a Growing Root.—The objection might well be interposed that possibly the roots in the pocket garden[8] grew downward after water. That moisture has an influence on the growing root is easily proved.
Plant bird seed, mustard or radish seed 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.
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. Most trees, and all grasses, have a greater area of surface exposed by the roots than by the branches. 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.
Cross section of a young taproot; a, a, root hairs; b, outer layer of bark; c, inner layer of bark; d, wood or central cylinder.
Fine Structure of a Root.[9]—When we examine a delicate root in thin longitudinal section under the compound microscope, we find the entire root to be made up of cells, the walls of which are uniformly rather thin. 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 a central cylinder can easily be distinguished from the surrounding cells. In a longitudinal section a series of tubelike structures may be found within the central cylinder. These structures are cells which have grown together at the small end, the long axis of the cells running the length of the main root. In their development 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 are known as fibrovascular bundles.
Young embryo of corn, showing root hairs (R. H.) and growing stem (P.).
Root Hairs.—Careful examination of the root of one of the seedlings of mustard, radish, or barley