Bacteria are greatly affected by electricity ; they increase in numbers at a very marked ratio when stimulated. The process of fermentation by yeast is also greatly accelerated by the application of minute direct currents or by a single tiny spark from a frictional machine.
The range of currents acting favorably on growth is limited and may be represented as rang ing from .005 to .55 milliamperes. Direct currents are not so stimulating in all cases as alternating currents, but static electricity stimulates very appreciably. In the many thousand seeds which have been used there is no evidence that electricity awakens life in dormant seeds. It always acts as a decided accelerator to germination and growth, but the germinating capacity is in no way affected.
Monahan has shown that charging air with static electricity constitutes an important stimulus to seeds and plants. Germination and growth in such instances are greatly accelerated. He employed a potential ranging from 50 to 175 volts, with most excellent results. A too high potential or a too strong current prevents growth, and if the current is increased sufficiently it is easy to kill plants.
The maximum or death current is determined by the nature of the plant, as well as the conditions under which the plant is stimulated. On the other hand, too weak currents do not produce perceptible reactions. The optimum or best current the writer found to be about .22 milliamperes.
The connecting of copper and zinc electrodes placed in soil constitutes a very effective method, as well as one of the cheapest ways, of stimulating crops by electricity. Strips of copper and zinc one foot wide and four to six feet long connected with wires furnish a bat tery when placed in soil, which under certain conditions will generate an optimum current. The amount of cur rent which these will produce de pends, of course, on the size of the metal plates em ployed, together with the nature of the soil and other factors. A soil lack ing in organic mat ter and plant-food will give less cur rent than a richer soil. Plates six inches by three feet in some soils would give a current ranging from .02 to 1 milliampere when placed four feet apart, whereas, if these same plates were put in some of the highly manured Boston market-gar den soils, they would generate ten to twenty times as much current in a tolerably dry soil, when placed farther apart. The amount of resistance in well manured market-garden soils is extremely small, and it has been estimated that if a large house were provided with copper and zinc plates located at either end and these were connected with wires, a current could be generated sufficient to run a small incandescent lamp.
General observations.
The extensive use of electricity in a commercial way has introduced factors which have a bearing on vegetation. The numerous high tension wires used for street lighting purposes frequently come into contact with beautiful shade trees and cause 17.1r- Such injury, however, is mainly of a local nature,—that is, trees are injured or burned only at the point of contact of the wires with a tree, and it can be positively stated that there are no authentic cases of alternating current wires killing large trees. The circumstances, however,
might be different in the case of direct cur rent lighting wires, providing sufficientgrounding occurred ; nevertheless, so- called direct current trolley wires have been known to kill large trees where certain condi tions prevail. (Fig. 50.) There is also some evi dence in support of the prevailing opinion that a certain leakage or grounding from a trol ley system through a tree may cause its death in time without any material burning tak ing place. In such cases the tissues are over - stimulated, as it were, resulting in the possible disintegration of the protoplasm of the cells.
There is much evi dence in support of the idea that electric ity plays an important role in nature. The air and earth are constantly charged with it, and vegetation, being in contact with both, is un doubtedly affected. Grandean and others main tain that when plants are surrounded with wire netting they develop less in a given space of time than plants grown under similar conditions as regards light and other factors in a free atmos phere. The interpretation of this phenomenon is that wire screens modify the atmospheric potential to the detriment of the plant. Grandeau secured similar results by growing plants under chestnut trees, and he concluded that trees modify to a large extent the atmosphoric potential in their immediate neighborhood. Electrical experiments made for three years at the Massachusetts Agri cultural College Experiment Station show that the electrical potential at corresponding heights in the free atmosphere and in an elm tree are identical during the season when no foliage is present. When, however, the foliage develops, the potential drops materially in the air surrounding the tree and remains in this condition until the leaves fall, at which time the potential becomes identical again. This is apparently a case of the foliage of a tree absorbing atmospheric electricity or screen ing it in the some way as does a glass structure. It may be interesting to note in this connection that there is no atmospheric electricity in greenhouses, but the effect of its absence on plants is not easily discernible, since there are too many other factors in greenhouses which modify the configuration of plants. The electrical potential records secured by the writer and his assistants under conifers, such as the Norway spruce, proved the potential to be similar most of the time to that of the earth and not of the air, as secured under deciduous trees, like the elm. Lemstrom was of the opinion that the numerous small pointed leaves common to conifers serve as points of discharge or accumulators of electricity. This theory has some foundation, since the apices of leaves of trees have been known to discharge electricity, and the electric potential of the air and earth may be more or less equalized by vegetation.