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A sundew with a leaf bent around a fly trapped by mucilage.

Mucilage is a gooey polar glycoprotein; an exopolysaccharide; a polymer produced by most plants and some microorganisms.

It occurs in various parts of nearly all classes of plant, usually in relatively small percentages, and is frequently associated with other substances, such as tannins and alkaloids.

Mucilage in plants is thought to aid in water storage and seed germination, and to act as a membrane thickener and food reserve. Among the richest sources are cacti (and other succulents), and flax seeds.

Mucilage has a unique purpose in some carnivorous plants. The plant genera Drosera (Sundews), Pinguicula, and others have leaves studded with mucilage-secreting glands, and use a "flypaper trap" to capture insects.

Exopolysaccharides are the most stabilising factor for microaggregates and are widely distributed in soils. Therefore exopolysaccharide-producing "soil algae" play a vital role in the ecology of the world's soils. The substance covers the outside of, for example, unicellular or filamentous green algae and cyanobacteria. Amongst the green algae especially, the group Volvocales are known to produce exopolysaccharides in a certain part of their life cycle.

Mucilage is edible, but tastes rather bland. It is used in medicine for its demulcent properties. Traditionally marshmallows were made from the extract of the mucilaginous root of the marshmallow plant and due to the demulcent nature of the extract, worked as a cough suppressant. Some carnivorous plants with mucilage are used for the traditional production of a dairy product in Sweden, called filmjölk.

The following plants are known to contain far greater concentrations of mucilage than is typically found in most plants:

• Aloe vera
• Cactus
• Chondrus crispus (Irish moss)
• Drosera (sundews)
• Fenugreek
• Flax seeds
• Kelp
• Liquorice root
• Marshmallow
• Mullein
• Okra
• Parthenium
• Pinguicula (butterwort)
• Ulmus rubra bark (slippery elm)

Mucilage is also a term for an adhesive composed of a solution of a sticky vegetable product or vegetable gum in water, used primarily to seal paper (e.g., postage stamps and envelope flaps).

• Mucilage McGraw-Hill Encyclopedia of Science and Technology, 5th edition
• Mucilage Columbia Encyclopedia, Sixth Edition (2007)