Hydrocleys nymphoides (humb. & Bonpl. ex Willd.) Buchenau

Aquatic perennial herb, often forming dense mats. Stolons to several metres long, terete, rooting at nodes. Leaves floating or emergent; petiole sheathed at base, terete, to 60 cm long and 8 mm diam., with transverse septa; blade ovate to broadly ovate or elliptic to suborbicular, (2.5–) 3–13.5 cm long, (2–) 3–12 cm wide, shallowly cordate base, obtuse to rounded apex, mid to dark green and glossy above, paler below, glabrous, usually with spongy tissue on undersurface near midrib, with (1–) 2–4 (–6) parallel longitudinal veins on either side of the more prominent midvein and numerous transverse interconnecting veins. Bracts basally subtending petioles and peduncles, membranous, ovate to lanceolate, to 6.5 cm long. Juvenile leaves narrower than the adult leaves, ± narrowly elliptic. Inflorescences simple with flowers solitary in the leaf axils or in clusters of up to 6 flowers arising at stolon nodes; peduncles to 30 cm long, terete, septation similar to petioles. Flowers emergent, (4–) 5–8 cm across, short-lived; sepals 3, green, shiny, narrowly ovate to elliptic, subobtuse to obtuse, c. 15–20 (–25) mm long, longitudinally striate, with a membranous, translucent wing around margin, persistent even in fruit. Petals 3, broadly obovate–rounded triangular (± fan-shaped) to rounded-rhombic, c. 3–4 cm long, 3–5 (–5.5) cm wide, delicate, yellow, often a deeper yellow towards base, pale yellow or reddish resin/oil gland-dotted (seen as scattered minute translucent globules). Stamens c. 60 % as long as sepals, the outer sterile, the inner fertile; filaments linear, dorsiventrally flattened, c. 3–4 mm long, reddish purple to deep maroon; anthers 5–6 mm long, lobes yellowish, connective reddish purple to deep maroon; staminodes flattened, awl-shaped, same colour as filaments. Carpels 5–8 (usually 6), 11–13 mm long; stigma purple. Fruiting carpels (not observed in Australian populations) c. 15 mm long.

Grows in stationary or slow-flowing, shallow to deeper, freshwater streams, swamps and dams with high nutrient levels.

In Australia, Hydrocleys nymphoides (Water Poppy) has been cultivated as an attractive ornamental in ponds and other water features, however, it has occasionally escaped and become naturalised.