A Civic Biology Part 7

To a little starch in half a cup of water we add a very little (1 gram) of diastase and put the vessel containing the mixture in a warm place, where the temperature will remain nearly constant at about 98 Fahrenheit. On testing part of the contents at the end of half an hour, and the remainder the next morning, for starch and for grape sugar, we find from the morning test that the starch has been almost completely changed to grape sugar.

Starch and warm water alone under similar conditions will not react to the test for grape sugar.

Digestion has the Same Purpose in Plants and Animals.--In our own bodies we know that solid foods taken into the mouth are broken up by the teeth and moistened by saliva. If we could follow that food, we would find that eventually it became part of the blood. It was made soluble by digestion, and in a liquid form was able to reach the blood. Once a part of the body, the food is used either to release energy or to build up the body.

Summary.--We have seen:

1. That seeds, in order to grow, must possess a food supply either in or around their bodies.

2. That this food supply must be oxidized before energy is released.

3. That in cases where the food is not stored at the point where it is to be oxidized the food must be digested so that it may be transported from one part to another in the same plant.

The life processes of plants and animals, so far, may be considered as alike; they both feed, breathe (oxidize their food), do work, and grow.

REFERENCE BOOKS

ELEMENTARY

Hunter, _Laboratory Problems in Civic Biology_. American Book Company.

Andrews, _A Practical Course in Botany_, pages 1-21.

American Book Company.

Atkinson, _First Studies of Plant Life_, Chap. x.x.x. Ginn and Company.

Bailey, _Botany_, Chaps. XX, x.x.x. The Macmillan Company.

Beal, _Seed Dispersal_. Ginn and Company.

Bergen and Davis, _Principles of Botany_, Chaps. XX, x.x.x.

Ginn and Company.

Coulter, _Plant Life and Plant Uses_. American Book Company.

Dana, _Plants and their Children_. American Book Company.

Mayne and Hatch, _High School Agriculture_. American Book Company.

Lubbock, _Flowers, Fruits, and Leaves_. The Macmillan Company.

Newell, _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?_ _(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 ill.u.s.trating 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.

Footnote 8: _The Pocket Garden._--A very convenient form of pocket germinator may be made as follows. Obtain two cleaned four by five negatives (window gla.s.s will do); place one flat on the table and place on this half a dozen pieces of colored blotting paper cut to a size a little less than the gla.s.s. Now cut four thin strips of wood to fit on the gla.s.s just outside of the paper. Next moisten the blotter, place on it some well-soaked radish, mustard seeds or barley grains, and cover with the other gla.s.s. 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.

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 gra.s.ses, 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.

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_.

Footnote 9: Sections of tradescantia roots are excellent for demonstration of these structures.

[Ill.u.s.tration: 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.]