Myriophyllum L.

Annual or perennial herbs, usually glabrous, monoecious, dioecious, or fls ☿; stems rooting at nodes, prostrate to erect. Lvs usually whorled, sometimes opposite or alternate, usually dimorphic, the submerged lvs pectinate and finely divided, the aerial lvs ± entire, rarely all lvs of either form. Hydathodes often present near lf axils. Infl. a simple or rarely branched, terminal spike; fls usually solitary, in the axils of aerial lvs, occasionally in the axils of upper submerged lvs, each with 2 bracteoles; ♂ fls usually above ♀ within same infl. in monoecious spp. ♂ fls (2)–4-merous; sepals present; petals not clawed, ± hooded; stamens (1)–4 or 8; filaments short; anthers linear; pistil rudiments present or 0. ♀ fls (2)–4-merous; sepals and petals very reduced or 0; ovary (2)–4-celled; styles as many as cells of ovary; stigmas capitate, fimbriate; staminodes rudimentary. Fr. of 2–4, 1-seeded mericarps; mericarps usually smooth and rounded on dorsal face, lacking a differentiated pericarp, sessile or shortly pedunculate.

[From: Webb et al. (1988) Flora of New Zealand. Volume 4.]