Smut, or BUNT (sometimes also called Dust-brand), is the popular name of certain small fungi which infest flowering land-plants, especially the grasses. This name is derived from the appearance of the spores, which are nearly black and very numerous. At the present time the group is called the Ustilagineæ. Some of them live in the intercellular spaces only, others penetrate the walls of the cells, especially of the parenchyma (simple) cells, and live within the living matter of the plant. Some species attack a part only of their host—e.g. Entyloma forms pustules on the leaves of certain species of Ranunculus, others spread throughout the tissues, forming spores in special places. Thus the mycelium of Tilletia caries, which causes the disease specially known as Bunt, may spread through the whole of a wheat-plant, but the spores are found only in the ovary. U. Carbo fructifies in the inflorescences of various grasses, and causes the disease known as Smut. The part of the plant where the fructifica- tion is being formed is generally enlarged, and becomes filled with the black resting spores. This swelling is especially noticeable in maize-plants attacked by U. Maidis. The mycelial hyphæ are not very densely spread within the tissues of a host, but the hyphæ that will bear spores branch repeatedly, and thus form a mass of compact tissue within that part of the host selected for the fructification, this compact mass taking the form of that part of the host, or covering with a flat layer a part of the surface, or eating a cavity out of the tissues of the plant and taking the form of that cavity. Generally the spore-bearing hyphæ become transformed into spores, so that nothing but spores remain, but some species form definite envelopes for the spores—e.g. Doassansia. The germination of the spores occurs when they have been well saturated with water. Typically, a germ tube is emitted which is called the promycelium. This, in most cases, gives forth from the far end a number (4 to 10) of smaller tubes, called sporidia. These sporidia, either before or after their separation from the promycelium, conjugate in pairs. From these conjugated cells there may arise a tube which takes all the protoplasm of the two cells; this tube, called an incipient mycelium, may enter into the tissues of a host and develop a true mycelium. Sometimes in a species which usually develops in this way any or all of these steps may be omitted—e.g. the spore may give rise to an incipient mycelium direct, or the sporidia though formed may not conjugate, but produce incipient mycelia direct. Species also occur in which these peculiarities are the general rule. In some species the sporidia, or what correspond to them, are occasionally formed direct from the hyphæ within the plant, the formation of a true resting spore being omitted; the parts so formed project from the host and are called gonidia.
See FUNGI; also De Bary's Comparative Morphology of Fungi, Mycetozoa, and Bacteria; or Goebel's Outlines of Classification and Morphology.