A naturally occurring, high-molecular-weight (ranging 200,000 and up), organic plant product of unknown detailed structure. The term is loosely used, often interchangably with the term gum. Chemically, mucilage is closely allied to gums and pectins but differs in certain physical properties. Although gums swell in water to form sticky, colloidal dispersions and pectins gelatinize in water, mucilages form slippery, aqueous colloidal dispersions which are optically active and can be hydrolyzed and fermented. Mucilages are not pathological products but are formed in normal plant growth within the plant by mucilage-secreting hairs, sacs, and canals, but they are not found on the surface as exudates as a result of bacterial or fungal action after mechanical injury, as are gums. Mucilages occur in nearly all classes of plants in various parts of the plant, usually in relatively small percentages, and are not infrequently associated with other substances, such as tannins. The most common sources are the root, bark, and seed, but they are also found in the flower, leaf, and cell wall. Any biological functions within the plant are unknown, but they may be considered to aid in water storage, decrease diffusion in aquatic plants, aid in seed dispersal and germination, and act as a membrane thickener and food reserve. Mucilages are commonly identified by physical properties, most recently by infrared spectroscopy.