General description:- Submerged, perennial herbs of saline water.

Leaves:- Narrowly linear, sheathing at the base.

Flower:- Hermaphrodite, small, ebracteate, in 2-flowered, pedunculate spikes; peduncles slender, often accrescent in fruit. Perianth absent. Stamens 2; anthers sessile, opening by longitudinal slits. Carpels 4(-10), free, with 1 ovule, sessile in flower but usually becoming slender-stipitate in fruit.

Fruit:- Fruitlets indehiscent; seeds without endosperm.

RUPPIA

Leaves:- Alternate, or the involucral subopposite, minutely denticulate at the apex, otherwise entire; sheaths shortly auriculate.

Flowers:- Spikes subtended by 2 subopposite involucral leaves with more or less inflated sheaths; flowers arranged one above the other on opposite sides of the rhachis. Stamens with a small dorsal appendage at the base. Carpels with a sessile, peltate stigma.