A Civic Biology, Presented in Problems. George W. Hunter
leaves, then pour equal amounts of water (100 c.c.) on each and measure all that runs through, the water that has been retained will represent the water supply that plants could draw on from such soil.
The Root Hairs take more than Water out of the Soil.—If a root containing a fringe of root hairs is washed carefully, it will be found to have little particles of soil still clinging to it. Examined under the microscope, these particles of soil seem to be cemented to the sticky surface of the root hair. The soil contains, besides a number of chemical compounds of various mineral substances—lime, potash, iron, silica, and many others—a considerable amount of organic material. Acids of various kinds are present in the soil. These acids so act upon certain of the mineral substances that they become dissolved in the water which is absorbed by the root hairs. Root hairs also give off small amounts of acid. An interesting experiment may be shown (see Figure on page 80) to prove this. A solution of phenolphthalein loses its color when an acid is added to it. If a growing pea be placed in a tube containing some of this solution the latter will quickly change from a rose pink to a colorless solution.
Effect of root hairs on phenolphthalein solution. The change of color indicates the presence of acid.
A Plant needs Mineral Matter to Make Living Matter.—Living matter (protoplasm), besides containing the chemical elements carbon, hydrogen, oxygen, and nitrogen, contains a very minute proportion of various elements which make up the basis of certain minerals. These are calcium (lime), sulphur, iron, potassium, magnesium, phosphorus, sodium, and chlorine.
That plants will not grow well without certain of these mineral substances can be proved by the growth of seedlings in a so-called nutrient solution.[11] Such a solution contains all the mineral matter that a plant uses for food. If certain ingredients are left out of this solution, the plants placed in it will not live.
Nitrogen in a Usable Form necessary for Growth of Plants.—A chemical element needed by the plant to make protoplasm is nitrogen. The air can be proven by experiment to be made up of about four fifths nitrogen, but this element cannot be taken from either soil, water, or air in a pure state, but is usually obtained from the organic matter in the soil, where it exists with other substances in the form of nitrates. Ammonia and other organic compounds which contain nitrogen are changed by two groups of little plants called bacteria, first into nitrites and then nitrates.[12]
Diagram to show how the nitrogen-fixing bacteria prepare nitrogen for use by plants; t, tubercles.
Relation of Bacteria to Free Nitrogen.—It has been known since the time of the Romans that the growth of clover, peas, beans, and other legumes in soil causes it to become more favorable for growth of other plants. The reason for this has been discovered in late years. On the roots of the plants mentioned are found little swellings or nodules; in the nodules exist millions of bacteria, which take nitrogen from the atmosphere and fix it so that it can be used by the plant; that is, they assist in forming nitrates for the plants to use. Only these bacteria, of all the living plants, have the power to take the free nitrogen from the air and make it over into a form that can be used by the roots. As all the compounds of nitrogen are used over and over again, first by plants, then as food for animals, eventually returning to the soil again, or in part being turned into free nitrogen, it is evident that any new supply of usable nitrogen must come by means of these nitrogen-fixing bacteria.
Rotation of Crops.—The facts mentioned above are made use of by careful farmers who wish to make as much as possible from a given area of ground in a given time. Such plants as are hosts for the nitrogen-fixing bacteria are planted early in the season. Later these plants are plowed in and a second crop is planted. The latter grows quickly and luxuriantly because of the nitrates left in the soil by the bacteria which lived with the first crop. For this reason, clover is often grown on land in which it is proposed to plant corn, the nitrogen left in the soil thus giving nourishment to the young corn plants. In scientifically managed farms, different crops are planted in a given field on different years so that one crop may replace some of the elements taken from the soil by the previous crop. This is known as rotation of crops.[13] The annual yield of the average farm may thus be greatly increased.
Nitrogen in the soil is necessary for plants. Explain from this diagram how nitrogen is put into the soil by some plants and taken out by others.
Five of the elements necessary to the life of the plant which may be taken out of the soil by constant use are calcium, nitrogen, phosphorus, potassium, and sulphur. Several methods are used by the farmer to prevent the exhaustion of these and other raw food materials from the soil. One method known as fallowing is to allow the soil to remain idle until bacteria and oxidation have renewed the chemical materials used by the plants. This is an expensive method, if land is dear. The most common method of enriching soil is by means of fertilizing material rich in plant food. Manure is most frequently used, but many artificial fertilizers, most of which contain nitrogen in the form of some nitrate, are used, because they can be more easily transported and sold. Such are ground bone, guano (bird manure), nitrate of soda, and many others. These also contain other important raw food materials for plants, especially potash and phosphoric acid. Both of these substances are made soluble so as to be taken into the roots by the action of the carbon dioxide in the soil.
The Indirect Relation of this to the City Dweller.—All of us living in the city are aware of the importance of fresh vegetables, brought in from the neighboring market gardens. But we sometimes forget that our great staple crops, wheat and other cereals, potatoes, fruits of all kinds, our cotton crop, and all plants we make use of grow directly in proportion to the amount of raw food materials they take in through the roots. When we also remember that many industries within the cities, as mills, bakeries, and the like, as well as the earnings of our railways and steamship lines, are largely dependent on the abundance of the crops, we may recognize the importance of what we have read in this chapter.
Food Storage in Roots of Commercial Importance.—Some plants, as the parsnip, carrot, and radish, produce no seed until the second year, storing food in the roots the first year and using it to get an early start the following spring, so as to be better able to produce seeds when the time comes. This food storage in roots is of much practical value to mankind. Many of our commonest garden vegetables, as those mentioned above, and the beet, turnip, oyster plant, sweet potato and many others, are of value because of the food stored. The sugar beet has, in Europe especially, become the basis of a great industry.
[8] The Pocket Garden.—A very convenient form of pocket germinator may be made as follows. Obtain two cleaned four by five negatives (window glass 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 glass. Now cut four thin strips of wood to fit on the glass 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 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.
[9] Sections of tradescantia roots are excellent for demonstration of these structures.
[10] For an excellent elementary discussion of osmosis