Potamogetonaceae Bercht. & J.Presl

Aquatic herbs, perennial or rarely annual, glabrous, rooted in the substrate. Vegetative shoot system comprising horizontal shoots, mostly stoloniferous, non chlorophyllous (here called rhizomes) and vertical shoots, erect or sometimes ± floating, often densely branched, chlorophyllous (here called stems); tubers or turions sometimes present on the horizontal and/or the erect shoots. Leaves alternate or subopposite, rarely ternate, sessile or petiolate, simple, served by 3 traces, parallel-veined or with a single midvein; stipules present throughout or only on young shoots ( Groenlandia ), often forming a tubular sheath around the stem and young inflorescence, either free or adnate to the base of the blade; intravaginal scales 2– several, situated in leaf axils, mostly linear to subulate, rarely ovate, entire and acute; bladeentire, denticulate or serrate. Inflorescences pedunculate, with 2 opposite flowers ( >i>Groenlandia ) or more than 2 flowers in a dense or interrupted spike ( >i>Potamogeton ); peduncle rigid and mostly slightly elevated above the water level, but sometimes flexuous, the flowers resting then on the water surface, rarely submerged, but often withdrawing the fruits below the surface; bracts abortive or absent. Flowers& #2695; (hermaphrodite), small, regular, hypogynous, tetramerous. Perianth: a singlewhorl of 4 valvate shortly clawed tepals. Stamens 4, opposite the tepals and basally adnate to the claw; anthers sessile, bilocular, extrorse, opening by longitudinal slits. Gynoecium superior, of (1– 3) 4 (5– 8) carpels, sessile, free or shortly connate at the base, alternating with the stamens; styles usually short; stigma unicellular-papillate and strongly secretory or smooth and dry or weakly secretory; ovule solitary in the carpel and attached to its ventral margin, orthotropous at first, but becoming campylotropous at maturity, pendant and filling the locule, bitegmic, crassinucellate. Fruit apocarpous consisting of distinct sessile fruitlets, ± drupaceous, but opening with a dorsal lid, with a rather fleshy parenchymatous mesocarp and a sclerified endocarp, the latter multi-layered ( >i>Potamogeton )or 1-layered ( >i>Groenlandia ). Seed without endosperm; embryo unciform or spiral (i.e. coiled more than 1 complete turn), with a large hypocotyl and a single obliquely terminal cotyledon that encloses the plumule