Flora of New Zealand Series

water buttercup

Aquatic perennial, rarely terrestrial; roots all fibrous. Rhizome creeping. Stems slender to stout, prostrate or ascending, up to 1-(2) m long. Lvs divided into many filiform segments, glabrous but for a few short hairs at segment tips, (1)-3-6-(8) cm long; segments narrowly linear in terrestrial plants; petiole (2)-5-30 mm long. Fls solitary, 8-15 mm diam. Pedicels terete, glabrous, (2)-3-6-(8) cm long. Sepals 5, bluish near tip, reflexed, 2.5-4 mm long. Petals 5, white with yellow base, 6-8 × c. 4 mm long; nectary single, 0.5-1 mm from petal base, lunate. Receptacle usually with a few sparse hairs at base, elongating slightly at fruiting. Achenes 30-60, glabrous, ovoid, slightly flattened, transversely rugose; body 1.5-2 mm long; beak straight, c. 0.5 mm long.

N.: throughout except Auckland and Taranaki; S.: throughout except Westland.

Temperate N. Hemisphere, Australia 1906

Streams, lakes and ponds, sometimes on sandy to muddy banks and dried-up stream beds.

Terrestrial forms are short-stemmed and tufted in habit. Their lvs have slightly wider segments, but the lamina is never as broad as in the emergent lvs of some other spp. of subgen. Batrachium. The short hairs at the segment tips distinguish this sp. from other N.Z. aquatic plants of similar habit. This sp. has been referred to in N.Z. as R. aquatilis and R. fluitans.

The N.Z. specimens of R. trichophyllus differ from European and Australian forms of the sp. in having glabrous achenes, elongating receptacles with only a basal collar of hairs, and glabrous stipules. Garnock-Jones (1981) treated them as an unidentified sp. However some E. Asian and N. American populations of R. trichophyllus are similarly almost glabrous.