Cardamine unicaulis Heenan

Perennial herb, single rosette or with short lateral branches, stem and branches 0.9–2.5 mm diam. Leaves up to 85 mm long, simple or pinnatisect; lamina 1.8–25.0 × 1.3–14.0 mm, light green to green, membranous, glabrous to sparsely or moderately hairy on adaxial surface, margin and petiole, glabrous on abaxial surface. Terminal pinna 1.8–14.5 × 1.3–14.3 mm, simple, reniform, orbicular-rhomboid to broadly elliptic, apex obtuse with an inconspicuous hydathode, base truncate to cordate. Lateral pinnae absent or 1–4, 1.8–12.0 × 0.8–5.5 mm, orbicular, orbicular-rhomboid, to broadly elliptic, base often oblique, petiolule 0.2–1.7 mm long; petiole up to 60 mm long; hairs 0.2–0.4 mm long, spreading. Cauline leaves, usually simple, with fewer and narrower leaflets than rosette leaves, becoming smaller in all parts; upper leaves up to 10.0 × 1.6 mm, increasingly linear. Inflorescence with flowers solitary or corymbose, corymbs 2–6-flowered; peduncle up to 70 mm long, 0.4–0.6 mm diam. at base, spreading to ascending, glabrous. Pedicels 1.5–14.0 mm long, 0.2–0.4 mm diam., glabrous. Sepals 1.1–1.6 × 0.5–0.8 mm, elliptic-oblong to narrowly elliptic-oblong, saccate, green or red-brown, glabrous or sparsely hairy distally, margin white and membranous, apex obtuse, base truncate. Petals 3.5–4.7 × 1.7–2.5 mm, white, limb obovate; apex obtuse; base cuneate, tapering to claw up to c. 1.5 mm long. Stamens 6; median filaments 4, 1.0–2.0 mm long; lateral filaments 2, 0.8–1.7 mm long; anthers 0.4–0.5 mm long, cream to pale yellow, when dehiscent held at a similar height to or slightly below the stigma. Ovary 1.1–1.7 mm long, 0.15–0.25 mm diam., ± terete, green, glabrous; ovules 8–11; style 0.2–0.3 mm long, ± terete; stigma 0.15–0.25 mm diam. Siliques 8.0–13.5 × 0.9–1.2 mm, glabrous, style sessile or up to 1.0 mm long; valves green to green-brown at maturity; straw-coloured when dehiscent, replum 0.4–0.45 mm wide. Seeds 0.9–1.1 mm long, 0.6–0.8 mm wide, 0.25–0.45 mm thick, yellow-brown to henna; wing absent.

Cardamine unicaulis occurs in low-elevation sites, with the majority of collections being from coastal areas, but it does occur inland. It grows in a variety of habitats, including lake and pond margins, river banks, the base of rock outcrops, among coastal turf, sand and rocks, and on limestone rocks. It seems to prefer partially shaded sites such as occur with an open forest canopy or among rock outcrops, and also moist areas such as muddy or damp ground.

Flowering September–March; Fruiting November–April.

Specimens from the Southland coast, Foveaux Strait, and Stewart Island have broader siliques and cauline leaves than the other collections.