Page:Popular Science Monthly Volume 42.djvu/662

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THE POPULAR SCIENCE MONTHLY.

by placing particles of poison upon the surface of the food-plant. A large proportion of our injurious insects have such biting mouth-parts, but there is also an important class which have, instead of jaws, a pointed beak that they push into the cells of the plant and suck out the sap. Insects of this kind can not be destroyed by coating the food-plant with particles of poison, because the particles will not be taken into the alimentary system. Consequently one must use against them some insecticide which kills by contact. There are several such insecticides in common use, the most important perhaps being an emulsion of kerosene, soap, and water, called the kerosene emulsion. It is usually prepared by adding two parts of kerosene to one part of a solution made by dissolving half a pound of hard soap in a gallon of boiling water, and churning the mixture through a force pump until the whole forms a creamy mass, which will thicken into a jelly-like substance on cooling. The emulsion thus made is diluted before using with nine parts of cold water, and is then sprayed directly upon the offending insects, killing them by simple contact. Among the more important pests against which this insecticide is used, I may mention the aphides or plant lice, the chinch-bug, the various cabbage worms, the lice of domestic animals, etc.

These examples will perhaps suffice to illustrate how valuable an adjunct the spraying machine has become in preventing the injuries of the hordes of destructive insects that overrun our farms, orchards, and gardens. Turning now to the other class of noxious organisms—the parasitic fungi—we shall find that it plays an equally important rôle in their subjection.

As the first illustration under this heading we will take the downy mildew or brown rot of the grape, a disease which for many years has troubled the vineyardists of the Eastern half ofPSM V42 D662 Section of leaf showing mycelium of fungus.jpgFig. 5.-Section of Leaf showing Mycelium of Fungus. Magnified. (After Farlow.) the United States, and has proved especially destructive in the great fruit belt of northern Ohio, along the southern shore of Lake Erie. It has often destroyed nearly the entire crop, and several times has threatened to ruin the vineyard industry over a wide area. Fortunately however, this disaster has been averted by the timely introduction of the spraying machine. The brown rot of grapes is a diseased condition of the fruit caused by the presence of a minute parasitic plant—a fungus—that develops by absorbing the tissues of its host. It attacks not only the fruit but also the leaves and young shoots, on which it often appears as a whitish, mildew-like covering, which has given