rue, common name for various members of the family Rutaceae, a large group of plants distributed throughout temperate and tropical regions and most abundant in S Africa and Australia. Most species are woody shrubs or small trees; many are evergreen and bear spines. The family is characterized by the presence of glands producing an essential oil, and the foliage, fruits, and flowers are noticeably aromatic and fragrant. The aromatic principle is widely utilized for flavorings, perfume oils, and medicines. Chief in importance are the citrus fruits, source of numerous extracted oils but best known as a major tropical-fruit industry, rivaled only by the banana and, to a lesser extent, the pineapple. Also of value medicinally are angostura bark and the rues (both now more commonly used for flavoring) and the poisonous jaborandi. Leaves of the latter (Pilocarpus spp. Brazil) are the source of pilocarpine, used to treat glaucoma. Several species of the Rutaceae yield lumber used for cabinetwork, e.g., the orange and the species called satinwood. The prickly ash, native to North America, is used in domestic brews and is often planted as a fragrant garden ornamental, as are the citrus trees and the varieties of dittany or fraxinella (Dictamnus alba), Old World woody perennials with a strong, lemonlike aroma. The name rue is properly restricted to the shrubby herbs of the genus Ruta, ranging from the Mediterranean to E Siberia. The common rue of history and literature is R. graveolans, which has greenish-yellow flowers and blue-green leaves sometimes variegated, with a very strong odor and a bitter taste. The leaves are now sometimes used in flavorings, beverages, and herb vinegars and in the preparation of cosmetics and perfumes. In medieval times rue was much used as a drug; its use as a condiment was thought to prevent poisons from affecting the system. Rue was strewn about law courts in parts of Great Britain as a preventive against diseases carried by criminals. It was sometimes associated with witches but also symbolized grace, repentance, and memory. Shakespeare in Richard II refers to it as the "sour herb of grace." The family Rutaceae is classified in the division Magnoliophyta, class Magnoliopsida, order Sapindales.

meadow rue, any plant of the genus Thalictrum of the family Ranunculaceae (buttercup family). Most are tall perennials (up to 7 ft/2.1 m high) bearing summer flowers with showy, pendent tassels of long stamens, greenish sepals, and no petals. Meadow rues are found in moist, open places throughout northern temperate regions; in the United States they are especially abundant in the Northeast. A few species are cultivated for ornament. The European herb called rue is an unrelated plant. Meadow rue is classified in the division Magnoliophyta, class Magnoliopsida, order Ranunculales, family Ranunculaceae.

De la Rue, Warren, 1815-89, British scientist and inventor. Especially noted as an astronomer, he was a pioneer in celestial photography. He adapted the wet-plate process to lunar photography and invented (1858) for Kew Observatory a photoheliograph, the first device to give good solar pictures. His photographs of a solar eclipse in 1860 demonstrated that prominences observed at the sun's edge are of solar origin. De la Rue is known also for his research in chemistry, solar physics, and electrical discharge through gases. His inventions include an envelope-folding machine (1851).