Any substance that swells in water to form a slimy solution. Mucilages are usually concerned with water retention; for example, the pentosan mucilages produced in the interior of succulent xerophytes serve to increase the water-holding capacity of the cells and hence reduce the transpiration rate. Many seeds have a mucilaginous coating that aids water uptake during germination.
Structurally, mucilages are very complex: linseed mucilage is a mixture of a complex polyuronide, proteinaceous matter, and cellulose. Mild hydrolysis of the polyuronide yields xylose and galactose residues and a more resistant fraction consisting of galacturonic acid and rhamnose residues.