| Author||C. den Hartog|
|Journal||Flora Malesiana - Series 1, Spermatophyta|
|Abstract||Annual or perennial freshwater and marine plants. Leaves submerged, rarely floating or partly emerged, radical or arranged along a stem, spirally whorled, distichous, or in pairs, linear, lanceolate, elliptic, ovate or orbicular, sessile or petioled; petioles mostly sheathing; nerves more or less parallel, straight or curved, connected by perpendicular or ascending cross-veins. Stipules sometimes present. Squamulae intravaginales often present. Flowers actinomorphous, rarely faintly zygomorphous (Vallisneria), unisexual, and then sometimes with rudiments of the other sex, or rarely bisexual, 1-~ enclosed between 2, more or less connate, rarely free segments (spathe). Spathe sessile or peduncled, often ribbed or winged, tip mostly bifid. Perianth segments free, 3 or 6, in the later case differentiated in petals and sepals; sepals often green, mostly valvate; petals mostly coloured, imbricate. Stamens 2—~, in 1 or more whorls, the inner ones sometimes staminodial (Hydrocharis), the outer ones often doubled (Stratiotes, Ottelia); anthers basifixed, 2—4-celled, dorsally or latrorsely lengthwise dehiscent; filaments more or less slender, sometimes absent. Ovary inferior, linear, lanceolate or ovate, consisting of 2-15 connate carpels, 1-celled, apex often narrowed into a long, filiform beak; parietal placentas sometimes protruding to the centre of the gynaecium, but never connate, sometimes split into 2 lamellae; styles 2-15, often more or less split into 2 crests. Ovules ~, anatropous, with 2 integuments. Fruits linear, lanceolate or ovate, opening by decay of the pericarp, rarely stellately dehiscent ( Thalassia). Seeds ~, fusiform, elliptic, ovate or globose; testa glabrous or densely set with spines or warts; embryo straight, with a very inconspicuous plumule at the base of a lateral groove, and a thick radicle; the marine genera and Stratiotes possess, however, a large, well-developed plumule; albumen 0.|
Distribution. About 15 genera with c. 100 spp., widely distributed in the tropical and subtropical zones with a few species in the temperate zones. Among the freshwater genera only Vallisneria and Ottelia occur both in the palaeotropics and in the neotropics. Most other freshwater genera are confined to the Old World [ Hydrilla, Hydrocharis, Blyxa, Lagarosiphon (Africa), Nechamandra (S. Asia), Stratiotes (Europe), Maidenia (Australia)], only 2 are restricted to America (Elodea and Limnobium). The marine genera are commonly dispersed along the coasts of the Indian and Pacific Ocean going East as far as Hawaii & Tahiti, but do not reach the American Pacific coast; 2 of them occur also in the West Indies, but further they are absent from the coasts of the Atlantic Ocean! See further under ecology.
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