Many households are supplied with vinegar by preparing a weak solution of sugar or of molasses, and placing in this a fragment of a gelatinous substance known as vinegar plant. If left at a proper temperature (about 70°) for some days, the sugar will be found to have disappeared, and the liquid become a weak vinegar. The fragment of vinegar plant increases rapidly in size, and if the quantity of sugary liquid present be sufficient, it will be found to cover the surface completely, conforming itself to the shape of the vessel, and often half an inch thick or more. This floating mass has much the color and texture of a piece of glue that has been soaked in cold water previous to dissolving it, and may be separated into thin transverse layers; examined under a microscope, it is seen to consist of innumerable delicate branching threads, which interlace and form the mass. Under favorable circumstances, the surface is soon covered with the blue (sometimes yellow or green) mould (penicillium glavcum) so common on bread and other vegetable substances, showing that the vinegar plant is the mycelium or vegetative portion of a fungus of which the colored mould is the reproductive portion.

The penicillium does not of necessity produce its mycelium always in liquids, as wherever the mould is seen the mycelium will be found; but in favorable liquids and at a proper temperature the plant will increase its mycelium very rapidly and to an indefinite extent, with* out, under these conditions, any attempts to reproduce itself by spores. The "mother " of vinegar found in old vinegar barrels is a form of this mycelium, and it is now considered that the yeast plant, formerly regarded as an alga, is a form of the same, with differently shaped and detached cells. (See Fermentation, and Fungi.) Mycelia appear in various saline solutions and favor their decomposition, as in some pharmaceutical preparations; in the solutions of sulphate of copper in electrotyping establishments one appears often to an annoying extent; most, if not all, of these are supposed to be modified forms of the mycelium more highly developed in the vinegar plant.