MANGROVE

Mangroves (Rhizophora apiculata) at low tide on the coast of Thailand. any of certain shrubs and trees, of the families Rhizophoraceae, Verbenaceae, Sonneratiaceae, and Arecaceae (Palmae), that grow in dense thickets or forests along tidal estuaries, in salt marshes, and on muddy coasts and characteristically have prop rootsi.e., exposed, supporting roots. The term mangrove also applies to thickets and forests of such plants. Respiratory or knee roots (pneumatophores) are characteristic of many species; they project above the mud and have small openings (lenticels) through which air enters, passing through the soft, spongy tissue to the roots beneath the mud. Mangrove flora along the Atlantic coast of tropical America and along the coast of the Gulf of Mexico to Florida consists chiefly of the common, or red, mangrove (Rhizophora mangle) of the family Rhizophoraceae and the black mangrove (Avicennia nitida, sometimes A. marina) of the family Verbenaceae. Mangrove formations in Southeast Asia include Sonneratia of the family Sonneratiaceae and the nipa palm (Nypa fruticans) of the family Arecaceae. The trunks and branches of the common mangrove are typical of the growth habit of all mangroves. They constantly produce adventitious roots, which, descending in arched fashion, strike at some distance from the parent stem and send up new trunks. While the fruit is still attached to the parent branch, the long embryonic root emerges from the seed and grows rapidly downward. When the seed falls, the young root is in the correct position to be driven into the mud; the plant being thus rooted, the shoot makes its appearance. The young root may grow to such a length that it becomes fixed in the mud before the fruit separates from the parent tree. The common mangrove grows to about 9 m (30 feet) tall. The leaves are 5 to 15 cm (2 to 6 inches) long, opposite, oval or elliptic, and smooth-edged; they are thick, have leathery surfaces, and are borne on short stems. The flowers are pale yellow. Black mangroves (Avicennia germinans). The black mangrove, usually of moderate height, sometimes grows 18 to 21 m tall. The leaves are 5 to 7.5 cm long, opposite, oblong or spear-shaped; the upper surface is green and glossy, the lower surface whitish or grayish. The white flowers are small, inconspicuous, and fragrant and are frequented by honeybees for their abundant nectar. The wood of some species is hard and durable. The astringent bark yields a water-soluble tanning substance. The fruit of the common mangrove is sweet and wholesome.