A Fungus is a plant of very low organization consisting of one or more cells multiplying its kind by cell division and by spores. It contains no green substance (chlorophyll), and grows either as a saprophyte upon non-living organic matter or derives its food directly from another living organism, and is then a true parasite.

Fungi are very common, and range in size from the large hard-shell Fungus upon logs and the puff ball and toadstool in the rich earth to the delicate moulds that infest bread and other foods, and the still more microscopic forms that produce fermentation, as yeast in dough and other species employed in making beer. Some of the toadstools are very richly tinted with red, yellow, brown and even blue, and a few are deadly poisonous, as the "death cup" and the "fly agaric,” which grow upon the decaying organic matter in shaded places. Others are wholesome, and are grown as articles of food, the leading species of which is the mushroom, Agaricus campestris. More highly prized still is the truffle, which is produced under ground and hunted by hogs, which find them by their odor, and even muzzled hogs are trained to unearth them.

One of the parasitic species, ergot, infests the heads of rye, changing the grains into much enlarged horny purplish masses called "spurred rye” because of the resemblance of the fungous grains to a cock's spur. This is extremely poisonous, and when eaten by live stock with the hay or grass has been known to destroy whole herds. This Fungus furnishes one of the most valuable in the whole long list of drugs. Many other fungi grow in the heads of grain, the most conspicuous being the corn smut, which changes the whole ear possibly into a large mass of dark slime when wet, and brown dust when dry. This dust is the myriads of spores which the Fungus produces to secure its reproduction. In a similar manner other smuts destroy the oats, wheat and very many kinds of grasses and other plants.

The rusts are similar Fungi which thrive upon the juices of plants and produce patches of orange or yellow upon leaf or stem, the discolored portion being usually swollen and the skin more or less broken. There is another group of Fungi known as the mildews, and these usually produce a fine whitish coating to the diseased part, due to the fine stalks that come from the surface of the plant and bear the spores.

Fungi love warmth and moisture for their greatest activity, and therefore they are more in evidence in midsummer when wet weather prevails than at other times. The spores are so small and light that they float in the air, and it is only when substances like canned goods have these spores excluded by first killing those present and preventing access of others, that they will keep unattacked, that is, sweet and edible. Substances can be easily inoculated by introducing the germs, as mold into potatoes, rust into a tree, or yeast into dough.

The number of kinds of Fungi is high among the thousands, and new species are being found each month, but they are so small that only specialists can understand the microscopic differences that separate one kind from another. Many Fungi have certain forms which are assumed in the cycle of life, and in this they resemble insects with their larval, pupal and imago stages. This polymorphic nature has made the study of the Fungi very puzzling. While a few of these plants are poisonous, and many destructive to life, the greatest majority are scavengers, reducing the waste products to simpler and harmless forms. We could not get on well without this minute and humble race of plants.

For further discussion, see Diseases.