Submerged monoecious glabrous aquatic herbs of alkaline or brackish water, sometimes in sea-water. Rhizome thin, matted, perennial, rooted in the substrate. Stem slender, erect, branching freely, monopodial below, sympodial above. Leaves alternate or opposite; blade simple, linear or setaceous, with a single midvein; sheath wide, distally open. Inflorescence usually a 2-flowered terminal spike with the flowers inserted at unequal heights and on opposite sides of the rachis, when young concealed in the sheath of the uppermost vegetative leaf, and just exserted above the water at anthesis; peduncle short or long, sometimes spirally coiled. Flowers bisexual, actinomorphic, protandrous, very small and inconspicuous. Perianth absent. Stamens 2, opposite each other; filaments very short and broad; anthers 2-thecous, extrorse, the 2 relatively large pollen-sacs divergent and well separated; pollen-grains luniform and of a unique type, buoyant. Ovaries 2-8, but usually 4, free, superior, sessile at first, becoming long-stipitate in fruit; ovule solitary, pendulous, campylotropous, crassinucellar; stigma sessile, peltate, ventroapical. Fruits ovoid-pyriform, often asymmetric, beaked drupes. Seeds hard, without endosperm.
Aquatic herbs of saline marshes. Leaves opposite or alternate, linear or setaceous, sheathing at the base. Flowers hermaphrodite, small, in terminal spikes at first enclosed by the sheathing base of the leaves, at length much elongated; bracts and perianth absent. Stamens 2; filaments short, broad; anthers extrorse, loculi reniform and separated by the connective. Carpels 4 or more, free, stigmas peltate or umbonate. Ovule solitary, pendulous. Fruits long-stipitate with spirally twisted stalks, indehiscent. Seeds pendulous, without endosperm.
Annual or rarely perennial submersed glabrous herbs growing in fresh or brackish waters. Roots fibrous, few, non-septate, from lower nodes of stems. Stems slender, branched or unbranched, terete, often dimorphic, the lower stems rhizomatous, the upper erect, leafy, the tips not modified into turions. Leaves alternate to subopposite, divided into blade and stipular sheath encircling the stem; blade entire below, serrulate above, 1veined, thin, linear; infravaginal scales present; turions rare, with 1 internode, the leaves undifferentiated. Inflorescence a I-few-flowered capitate axillary or terminal spike, at first enclosed by 2 subopposite foliage leaves. Peduncle elongating at anthesis, elevating the inflorescence to or near the water surface, or remaining short, occasionally becoming twisted in fruit, then pulling the developing fruit below the water surface. Flowers perfect, hypogynous, actinomorphic; tepals absent. Stamens 2, sessile; anthers extrorse, with two bisporangiate thecae dehiscing by longitudinal slits; connective broad. Carpels (2)4(-16), distinct, each forming a pistil with a sessile, peltate, lobed stigma and a single lateral, more or less pendulous, campylotropous, bitegmic, and crassinucellar ovule. Fruits long-stipitate or sessile drupes, the outer soft layers decaying, the inner sclerotic layers persistent and opening via a small lid at germination. Seeds exalbuminous, embryo with enlarged hypocotyl.
Subcosmopolitan, the single genus variously regarded as a single variable species or as comprising about ten species.
The fruits, after being released, float on the water for a period of time varying from less than a week to several months. They then sink and become covered by the substrate. According to Graves 1908, while covered by the substrate, the exocarp and mesocarp decay, leaving the endocarp intact. The seed coat remains permanently attached to the endocarp.
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