Nitrogen in a Usable Form necessary for Growth of Plants. We learned that humus is made up of decayed plant and animal bodies. A chemical element needed by the plant to make protoplasm is nitrogen. 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 which oxidize the compounds, first into nitrites and then nitrates.1 Nitrogen Nitrates Nitrogen 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 that ground 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 out nitrogen from the atmosphere and fix it so itrates t that it can be used by the plant; that is, they form 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, it is evident that any new supply of usable nitrogen must come by means of these nitrogen-fixing bacteria. Tubercles containing the nitrogenfixing bacteria. 1 It has recently been discovered that under some conditions these bacteria are preyed upon by tiny one-celled animals living in the soil and are so reduced in num 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. This is known as rotation of crops. The annual yield of the average farm may be greatly increased by this means. Soil Exhaustion may be Prevented. Besides the rotation of crops, other methods are used by the farmer to prevent the exhaustion of raw food material 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 fertilizers, material rich in plant food. Manure is most frequently used, but many artificial fertilizers, most of which contain nitrogen, are used, because they can be more easily transported and sold. Such are ground bone, guano (bird manure), nitrate of potash, and many others. These contain as well 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. Forms of Roots and their Relation to the Life of the Plant. Roots assume various forms. The form or position of the root is usually dependent on the needs of the plant, the roots acting to help it succeed in certain localities. Food Storage in Roots and its Economic Importance. The use to the plant of the food stored in the taproot may be understood if we take up the life history of the parsnip. Such a plant produces no seed until near the end of the second year of its existence, its growth the first summer forming the root we use as food. After forming seeds the plant dies. bers that they cannot do their work effectively. If then the soil is heated artificially or treated with antiseptics so as to kill the protozoa, the bacteria which escape multiply so rapidly as to make the land much richer than before. The food stored in its root enables it to get an early start in the spring, so as to be better able to produce seeds when the time comes. Such plants live only under rather cool climatic conditions. Examples of other roots which store food are carrot, radish, yam, sweet potato, etc. 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, 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. Water Roots... In the duckweed, a plant living in water, the roots are short and contain few root hairs. The water supply is so great that few root hairs have been called forth. The water hyacinth is another example of slight development of roots. The plant is buoyed up by the water and does not need strong roots to hold it firm. Adventitious Roots. Roots are often developed in unusual places. plants by striking root at any point on the reclining stem where it touches the ground. This fact is made use of by practical gardeners in the layering of plants. Examine the Indian corn for another they serve as props for the tall stem. notice how early these roots develop. they arise on the stem. kind of adventitious roots. Here In the young seedlings of corn, Air Roots. In tropical forests, where the air is always warm and moist, some plants have come to live above the soil on the trunks of trees, or in other places where they can get a favorable foothold. Such plants are called epiphytes, or air plants. The tropical orchid seen in our greenhouses is an example. Examine the roots of such a plant. Notice how thick they are. They are usually provided with a spongy tissue around the outside which has the function of absorbing water. Parasitic Roots. A few plants live on other living plants, and develop by the aid of nourishment taken at their expense. Such a plant is called a parasite. The plant or animal on which the parasite lives is called the host. The mistletoe is an example of a parasitic plant. An examination of its roots shows that they have bored their way into the stem of the host. These roots not only penetrate the bark, but push toward the center of the tree, taking nourishment from the cells there. The dodder is another seed-bearing plant which has this habit. Dodder produces from seed, but is unable to live alone after it has passed the seedling stage, and will die if it cannot find a suitable host. It is found on many common weeds, as jewelweed and goldenrod. Many of the lower plants live as parasites, among them being mildew, rusts, and smuts found on roses, grain, and corn. REFERENCE BOOKS ELEMENTARY Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Company. Andrews, Botany all the Year Round, Chap. II. American Book Company. Goff and Mayne, First Principles of Agriculture. American Book Company. ADVANCED Coulter, Barnes, and Cowles, A Textbook of Botany, Part II. American Book Com pany. Detmer-Moor, Practical Plant Physiology. The Macmillan Company. Goodale, Physiological Botany. American Book Company. Gray, Structural Botany, pages 27-39, 56-64. American Book Company. Green, Vegetable Physiology, Chaps. V, VI. J. and A. Churchill. Farmers' Bulletin 86, U.S. Department of Agriculture. Kerner-Oliver, Natural History of Plants. Henry Holt and Company. MacDougal, Plant Physiology. Longmans, Green, and Company. HUNT. ES. BIO.-7 |