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POLYGONACEAE

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Originally appearing in Volume V22, Page 27 of the 1911 Encyclopedia Britannica.
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POLYGONACEAE , in See also:

botany, a natural See also:order of See also:Dicotyledons, containing 30 genera with about 700 See also:species, chiefly in the See also:north temperate See also:zone, and represented in See also:Great See also:Britain by three genera, Polygonum, Rumex (See also:Dock, q.v.) and Oxyria. They are mostly herbs characterized by the See also:union of the stipules into a sheath or ocrea, which protects the younger leaves in the bud See also:stage (fig. 1). Some are climbers, as, for instance, the See also:British Polygonum Convolvulus (See also:black bindweed). In Muehlenbeckia platyclada, a native of the See also:Solomon Islands, the See also:stem and branches are flattened, forming ribbon-like cladodes jointed at the nodes. The leaves are alter-FIG. 1.—See also:Leaf of Polygonum, nate, See also:simple and generally entire; with See also:part of stem (g, ocrea). the edges are rolled back in the bud. They are generally smooth, but sometimes, especially in See also:mountain species, woolly. The small See also:regular, generally hermaphrodite See also:flowers are See also:borne in large See also:numbers in See also:compound inflorescences, the branches of which are cymose. The parts of the See also: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—such a flower occurs in the Californian genus Pterostegia (fig. 2).

The flower of See also:

rhubarb (Rheum) is derived from this by doubling in the See also:outer staminal whorl (fig. 3), and that of the dock (Rumex) by doubling in the outer staminal whorl and suppression of the inner (fig. 4). In Koenigia, a tiny See also:annual less than an See also:inch high, native in the See also:arctic and sub-arctic regions and the Himalayas, there is one perianth and one staminal whorl only. Dimerous whorled flowers occur in Oxyria (mountain See also:sorrel), another arctic and alpine genus, the flowers of which resemble those of Rumex but are dimerous (fig. 5). In the acyclic flowers a 5-merous perianth is followed ov, Pericarp formed from ovary See also:wall. s, See also:Seed. e, Endosperm. pl, Embryo with radicle pointing upwards and cotyledons downwards. by 5 to 8 stamens as in Polygonum (fig. 6).

The perianth leaves are generally See also:

uniform and See also:green, See also:white or red in See also:colour. They are See also:free or more or less See also:united, and persist till the See also:fruit is ripe, often playing a part in its See also:distribution, and affording useful characters for distinguishing genera or species. Thus in the docks the three inner leaves enlarge and envelope the fruit as three membranous wings one or more of which See also:bear on the back large fleshy warts. Less often, as in the See also:South See also:American genus Triplaris, the three outer perianth leaves See also:form the See also:agent of distribution, developing into See also:long See also:flat membranous wings, the whole mechanism suggesting a shuttlecock. 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 unilocular and contains a single erect ovule springing from the See also:top of the floral See also:axis (fig. 7). The fruit is a dry one-seeded See also:nut, two- Rumex obtusifolius, See also:Common Dock. 1. Upper part of plant, showing the flowers. 2. Leaf from See also:base of the stem. 3.

Fruit enlarged. 4. Fruit of Rumex Acetosa (sorrel) (enlarged). sided in bicarpellary flowers, as in Oxyria. The straight or curved embryo is embedded in a mealy endosperm. The flowers are See also:

wind-pollinated, as in the docks (Rumex), where they are pendulous on long slender stalks and have large hairy stigmas; or See also:insect-pollinated, as in Polygonum or rhubarb (Rheum), where the stigmas are capitate and See also:honey is secreted by glands near the base of the stamens. Insect-pollinated flowers are rendered conspicuous chiefly by their See also: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 See also: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 See also:short stamens, the other with short styles and long stamens. In other cases self-See also:pollination is the See also:rule, as in See also:knot-grass (P. aviculare), where the very small, solitary odourless flowers are very rarely visited by See also: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 See also:

America. Polygonum has a very wide distribution spreading from the limits of vegetation in the See also:northern hemisphere to the mountains of tropical See also:Africa and South Africa, through the See also:highlands of tropical See also:Asia to See also:Australia, and in America as far south as See also:Chile. Most of the genera have, however, a limited distribution. Of the three which are native in the- British Isles, Polygonum hao 12 species; Rumex (fig. 8) (II species) includes the various species of dock .(q.v.) and sorrel (R. Acetosa); and Oxyria digyna, an alpine 1 plant (mountain sorrel), takes its generic name (Gr. 411s, See also:sharp) from, the acidity of its leaves. Rheum (Rhubarb, q.v.) is central See also:Asiatic.

End of Article: POLYGONACEAE

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