Apium nodiflorum (L.) Lag.

Procumbent to ascending, aquatic or amphibious perennial, rooting at lower nodes.  Stems hollow, grooved, striate, up to 180 cm long.  Basal lvs 1-pinnate, petiolate;  leaflets in 2–8 pairs, ovate to lanceolate, serrate, sometimes lobed, particularly the terminal leaflet, 1–8–(12) cm long, sessile;  stem lvs similar to basal, but uppermost often reduced to 3–5 leaflets.  Umbel 2–4 cm diam., compound, subsessile or shortly pedunculate, lf-opposed;  rays 4–14; bracts 0–2, linear to lanceolate;  bracteoles 4–8, ovate-triangular.  Fls numerous, white, 1–2 mm diam.  Fr. ovoid, dark brown, 1.5–2.5 mm long;  ribs light brown, slender or slightly thickened.  

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

Flowering: Nov.–Feb.