Myriophyllum robustum Hook.F.

Aquatic or semi-aquatic herb, rooted from lower nodes; erect stems robust, to 40 cm. tall and 5 mm. in diam., sparsely branched. Lvs verticillate in whorls of c. 5, c. 3 cm. × 6-8 mm., often crowded, all pinnate to pinnatifid, segs longer and finer in lowest lvs. Fls solitary in the axils of upper lvs, sessile, hermaphrodite and protandrous; bracteoles very short, pinnati-or palmati-sect; sepals small, triangular, minutely serrate; petals pale, c. 2.5 mm. long, falling early; stamens 8, anthers 2 mm. long; ovary c. 1 mm. long, styles 4, divergent, stigmas plumose; fr. pale when ripe, often broader than long, c. 2 × 3 mm., mericarps often developing unevenly; surface smooth and rounded or occ. with a few sharp tubercles towards base.