Hydrilla Rich.

Submerged, freshwater, annual or perennial herbs. Roots simple, adventitious, without root-hairs, arising from nodes. Stems elongate, stoloniferous, or erect, terete, simple or branched from axils. Leaves sessile, opposite towards base of stem or branch, otherwise in whorls of 3–8(–12), linear to ovate, light to dark green, opaque or somewhat translucent, with 1 apical spine, margins hyaline, denticulate. Nodal scales 2 per leaf, narrowly triangular to narrowly lanceolate, fringed with orange-brown hairs. Flowers spathe solitary in leaf-axil, sessile, appendages at the apex, 1-flowered, shortly pedicellate, becoming detached and floating before anthesis; sepals 3, ovate, obtuse, reflexed at anthesis; petals 3, linear, obtuse, ± reflexed at anthesis; ovary of 3 carpels, 1-locular, placentation parietal. Fruit cylindric, smooth or with lateral spine-like appendages. Seeds up to 5, narrowly ellipsoid, smooth.

Worldwide: 13 species in tropical and subtropical Old World waters.

Burundi: 1 taxon.