Polygonum (Gr. , many, and , knee, from the numerous joints), a large genus of annual or perennial herbs, rarely under-shrubs, giving its name to a somewhat important family, the polygonacece, which includes among others the rhubarb, buckwheat, the docks, and sorrel. In most of the genera the stipules, above the swollen joints of the stem, are united to form a sheath, a character quite con-, spicuous in most species of polygonum, by which they may be recognized at sight. The flowers are apetalous, with a five-parted, mostly petal-like calyx; stamens four to nine; pistil with a single-celled ovary, with two or three styles, and forming in fruit a lenticular or a three-angled akene. There are about 25 species east of the Mississippi, most of which are natives; but few are showy, and some are common weeds. Recently glowing accounts have been published of a remarkable tanning plant discovered in Nebraska; this proves to be the water persicaria, P. amphibium, common all over the country and in Europe, and like several other species containing some tannin, though not a very large amount.
P. orientale is a tall species from India, sometimes seen in old gardens under the names of prince's feather and ragged-sailor; it is sparingly naturalized in waste grounds. The lady's-thumb (P. persicaria) has its peach-like leaves usually marked with a blackish spot near the middle; it is an introduced plant and a very common weed in cultivated grounds, as is P. Pennsylvanicum, a native species which closely resembles it, but has hairy branches. Several of the species have a highly acrid juice, which is capable of producing inflammation and even blistering when applied to the skin, whence they are known as smartweeds and water pepper; P. hydropiper is the most common of these, a decoction of which thickened with meal is very popular with western physicians as a local stimulant. Knot grass (P. avicu-lare), also called goose grass and door weed, is a small species and one of the commonest weeds about dwellings. Black bindweed (P. convolvulus) is a small twining vine introduced from Europe, and here as well as there often troublesome among grain crops. There are several native climbing species, the most conspicuous of which, P. dumetorum, is known as climbing buckwheat, and is abundant in moist places.
Bistort (P. bistorta), so called because the root is sometimes bent twice, is a European species, occasionally seen in old gardens, where it is cultivated for its rather pretty pink flowers and for its astringent roots, which were formerly used in domestic medicine; the roots contain an abundance of starch, and, after the astringency has been extracted by steeping in water, are in times of scarcity used as food in Siberia and Eussia. Polygonum cuspidatum is a hardy perennial species from Japan, introduced into our gardens a few years ago; it is 3 or 4 ft. high, and an effective plant, though difficult to eradicate when once established.