POLYGONACEAE, in botany, a family of dicotyledons, containing 4o genera with about 750-800 species, chiefly in the north temperate zone, and represented in Great Britain by three genera, Polygonum, Rumex (dock, q.v.) and Oxyria. They are mostly herbs characterized by the union of the stipules into a sheath or ocrea, which protects the younger leaves in the bud stage. Some are climbers, as, for instance, the British Polygonum Con volvulus (black bindweed). In Muehlenbeckia platyclada, a na tive of the Solomon islands, the stem and branches are flattened, forming ribbon-like cladodes jointed at the nodes. The leaves are alternate, simple and generally entire ; the edges are rolled back in the bud. They are generally smooth, bitt sometimes, especially in mountain species, woolly. The small regular, generally her maphrodite flowers are borne in large numbers in compound inflo rescences, the branches of which are cymose. The parts of the flower are whorled (cyclic) or acyclic. The former arrangement may be derived from a regular trimerous flower with two whorls of perianth leaves, two staminal whorls and a three-sided ovary— this type of flower occurs in the Californian genus Pterostegia. The flower of rhubarb (Rheum) is derived from this by doubling in the outer staminal whorl and that of the dock (Rumex) by doubling in the outer staminal whorl and suppression of the inner whorl. Dimerous whorled flowers occur in Oxyria (mountain sorrel), another arctic and alpine genus, the flowers of which otherwise resemble those of Rumex. In the acyclic flowers a pentamerous perianth is followed by five to eight stamens as in Polygonum. The perianth leaves are generally uniform and green, white or red in colour. They are free or more or less united, and persist till the fruit is ripe, often playing a part in its distribu tion, and affording useful characters for distinguishing genera or species. Thus in the docks the three inner leaves enlarge and envelop the fruit as three membranous wings one or more of which bear on the back large fleshy warts. The number of the carpels is indicated by the three-sided (in dimerous flowers two sided) ovary, and the number of the styles; the ovary is uni locular and contains a single erect ovule springing from the top of the floral axis. The fruit is a dry one-seeded nut, two-sided in bicarpellary flowers, as in Oxyria. The straight or curved embryo
is embedded in a mealy endosperm. The flowers are wind-polli nated, as in the docks (Rumex), where they are pendulous on long slender stalks and have large hairy stigmas ; or insect-pollinated, as in Polygonum or rhubarb (Rheum), where the stigmas are capitate and honey is secreted by glands near the base of the stamens. Insect-pollinated flowers are rendered conspicuous chiefly by their aggregation in large numbers, as for instance in bistort (Polygonum Bistorta), where the perianth is red and the flowers are crowded in a spike. In buckwheat (q.v., P. Fagopyrum) the numerous flowers have a white or red perianth and are perfumed; they are dimorphic, i.e., there are two forms of flowers, one with long styles and short stamens, the other with short styles and long stamens. In other cases self-pollination is the rule, as in knot grass (P. aviculare), where the very small, solitary odourless flowers are very rarely visited by insects and pollinate themselves by the incurving of the three inner stamens on to the styles.
Polygonaceae is mainly a north temperate order. A few genera are tropical, e.g., Coccoloba, which has 125 species restricted to tropical and sub-tropical America. Polygonum has a very wide distribution spreading from the limits of vegetation in the north ern hemisphere to the mountains of tropical Africa and South Africa, through the highlands of tropical Asia to Australia, and in America as far south as Chile. Most of the genera have, however, a limited distribution. In the British Isles, Polygonum has 14 species; Rumex ( 2 species) includes the various species of dock (q.v.) and sorrel (R. Acetosa) ; and Oxyria digyna, an alpine plant (mountain sorrel), takes its generic name (GrAin, sharp) from the acidity of its leaves. Rheum (rhubarb, q.v.) is central Asiatic.