Halophila Thouars
Annual or perennial marine herbs, monoecious or dioecious. Roots adventitious, branchless, 1 or a few at each rhizome node. Rhizomes creeping, monopodial, thin, with roots and a stem bearing at each node; scales 2, persistent, the upper one embracing a stem, the lower one clasping the rhizome. Stems erect, alternate on the rhizomes and distichously arranged. Leave in pair or in imbricated pseudo-whorl, petiolate, with or without sheath at the base of the petiole. Leaf blades linear, elliptic, lanceolate, oval or obovate, with or without scatterd hairs; leaf apex acute or rounded; leaf margin entire or spinulose; trinerve, 1 middle and 2 intramarginal, with or without crossing veinlets between nerves. Flowers unisexual, solitary. Spathes sessile, elliptic to obovate, one embracing the other, keeled or not; the apex retuse, rounded or acute; keel smooth or slightly serrulate; margin entire of sometimes irregular undulate. Staminate flowers pedicellate; tepals 3, imbricate; stamens 3, alternating with tepals; anthers sessile and extrorse, longitudinal dehiscing. Pistallate flowers sessile; tepals 3 or none, minute; ovary ellipsoid or ovoid, unilocular, long beaked at the apex; style terete; stigmata terete, 2-3. Fruits ovoid or globose; pericarp membranaceous. Seeds numerous, globose; cotyledon coiled.
14 species widely distributed in tropical, subtropical and warm-temperate seas. 3 species in Taiwan, Penghu Archipelagos(the Pescadores Islands), and the Pratas Islands.