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An organ (sometimes a single cell) the main function of which is to build up one or more specific chemical compounds, called secretions, that are passed to the outside of the gland. Most glands in animals are of the exocrine type. Exocrine glands discharge their secretions through a tube or duct on to a surface, either the outer (epidermal) surface of the body (e.g. sweat glands in mammals), or the inner surface of the gut, as in the case of digestive glands. Endocrine glands secrete hormones directly into the blood.

Plants, too, have glands. These may be superficial, discharging secretions externally, e.g., gland hair (lavender), nectary, or hydathode; or embedded in tissue, occuring as isolated cells containing the secretion or as a layer of cells surrounding an intracellular space (a secretory cavity or canal) into which a secretion is discharged, e.g. the resin canal of a pine.

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