Fungi, fun'jl, singular Fungus, a class of colorless plants, including mushrooms, puff balls, rusts, smuts, mildews, and yeast. Being without the green of ordinary plants, they cannot feed on true soil, however rich, but grow in plant and animal tissue, living or dead. They play a part in turning animal and plant matter, called organic matter, back into soil, and are useful. Were it not for fungi and bacteria, which see, we should have no decay, or way of turning plants and animals back into soil again. On the other hand, fungi do much damage. The decay which attacks a bruised apple, plum, orange, or other fruit is a fungus. Mildew is a fungus. Potato scab and the stain that attacks rose leaves are fungi. There are, in all, probably not less than 250,000 distinct fungi, many of them so small as to be seen only under the microscope. See BACTERIUM; MOLD; MILDEW; TRUFFLE; LICHEN; YEAST; SMUT; TOADSTOOL; MUSHROOM; ERGOT.