Ruppia

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, ventro- apical. Fruits ovoid-pyriform, often asymmetric, beaked drupes. Seeds hard, without endosperm.