Other Moldlike Fungi.--Mildews are near relatives of the molds found in our homes. They may attack leather, cloth, etc., in a damp house. Other allied forms may do damage to living plants. Some of these live upon the lilac, rose, or willow. These fungi do not penetrate the host plant to any depth, for they obtain their food from the outer layer of cells in the leaf of their host and cover the leaves with the whitish threads of the mycelium. Hence they may be killed by means of applications of some fungus-killing fluid, as Bordeaux mixture.[19] Among the useful plants preyed upon by mildews are the plum, cherry, and peach trees. (The diseases known as black knot and peach curl are thus caused.) Another important member of this group is the tiny parasite found on rye and other grains, which gives us the drug ergot. Footnote 19: See Goff and Mayne, _First Principles of Agriculture_, page 59, for formula of Bordeaux mixture. Among other parasitic fungi are rusts and smuts. Wheat rust is probably the most destructive parasitic fungus. Indirectly this parasite is of considerable importance to the citizen of a great city because of its effect upon the price of wheat. YEASTS IN THEIR RELATION TO MAN Fermentation.--It is of common knowledge to country boys or girls that the juice of fresh apples, grapes, and some other fruits, if allowed to stand exposed to the air for a short time will _ferment_. That is, the sweet juice will begin to taste sour and to have a peculiar odor, which we recognize as that of alcohol. The fermenting juice appears to be full of bubbles which rise to the surface. If we collect enough of these bubbles of gas to make a test, we find it to be carbon dioxide. Evidently something changed some part of the apple or grape, the sugar, (C{6}H{12}O{6}), into alcohol, 2(C{2}H{6}O), and carbon dioxide, 2(CO{2}). This chemical process is known as _fermentation_. [Illustration: Apparatus to show effect of fermentation. _N_, molasses, water and yeast plants; _C_, bubbles of carbon dioxide.] Yeast causes Fermentation.--Let us now take a compressed yeast cake, shake up a small portion of it in a solution of molasses and water, and fill a fermentation tube with the mixture. Leave the tube in a warm place overnight. In the morning a gas will be found to have been collected in the closed end of the tube (see Figure on page 138). The taste and odor of the liquid shows alcohol to be present, and the gas, if tested, is proven carbon dioxide. Evidently yeast causes fermentation. What are Yeasts?--If now part of the liquid from the fermentation tube which contains the settlings be drawn off, a drop placed on a slide and a little weak iodine added and the mixture examined under the compound microscope, two kinds of structures will be found (see Figure below), starch grains which are stained deep blue, and other smaller ovoid structures of a brownish yellow color. The latter are yeast plants.