Australian Journal of
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Hydrocotyle demissa A.R.Bean sp. nov.
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Type: Queensland. Warrego district: Tank near headwaters of Paroo River, “Varna”, 7 November 2016, J.L. Silcock JLS2190 & R.J. Fairfax (holo: BRI).

Hydrocotyle sp. (Lake Broadwater K.A.Williams AQ230829) in Bean (2021).

Hydrocotyle sp. Lake Broadwater (K.A.Williams AQ230829) Qld Herbarium, Australian Plant Census

Hydrocotyle sp. 1 (Byrock), Australian Plant Census, https://id.biodiversity.org.au/instance/apni/764793
Briggs, J.D. & Leigh, J.H. (1988), Rare or Threatened Australian Plants: 19, 195.
Hydrocotyle sp. 1 (Byrock; Hj.Eichler 22868), Australian Plant Census, https://id.biodiversity.org.au/name/apni/242793
Briggs, J.D. & Leigh, J.H. (1996), Rare or Threatened Australian Plants: 21, 213.

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Figure 2. Hydrocotyle demissa. a. leaf, upper surface b. fruits. (a from Moore 934; b from Silcock JLS2190 & Fairfax).

Prostrate stoloniferous herb; stems filiform, glabrous. Stipules orbicular or deltate, 0.5–0.6 x 0.5–0.8 mm, white; margin entire or minutely erose. Petioles of fully developed leaves terete, 4–25 mm long, inserted at base of lamina; petioles glabrous. Lamina reniform to orbicular-cordate, radius 1.5–4.0 mm, with 5 major palmate veins; margin with 3–5 deep lobes, each lobe entire to 3-toothed; radius at major sinuses 45–60% of lamina radius; upper surface glabrous, without reddish punctate glands; lower surface glabrous. Inflorescence umbellate, 3–8-flowered; all flowers bisexual; peduncles absent or up to 1 mm long, much shorter than the adjacent petiole, glabrous. Involucral bracts 3–6, linear, c. 0.2 mm long; pedicels c. 0.1 mm long; petals white, c. 0.15 mm long, anthers c. 0.1 mm long. Schizocarps symmetrical, compressed; mericarps 2, ± lenticular, 0.6–0.7 mm long, 0.5–0.55 mm wide, green, glabrous; dorsal ribs narrowly winged; mericarps with a central white or green mound surrounded by an annulus of raised tissue and bisected by the lateral rib. Fruiting styles c. 0.1 mm long, style base rudimentary. Fig. 2.

Specimens examined: Queensland. Warrego district: Twin Sisters spring, Idalia National Park, 15 Jul 2015, R.J. Fensham RJF6517 (BRI); Harlot spring, Idalia National Park, 6 Jul 2009, R.J. Fensham 5957 (BRI). Mitchell district: Milo station, Powell Ck, NNW of Adavale, 4 Aug 2009, P.I. Forster PIF35400 & M.B. Thomas (BRI); Hell Hole Gorge National Park, NW of Adavale, above Hell Hole waterhole on Powell Ck, 7 Aug 2009, P.I. Forster PIF35663 & M.B. Thomas (BRI). Maranoa district: Toulby section, Culgoa Floodplain National Park, 16 Dec 2008, R. Moore 934 (BRI). Darling Downs district: “Lakeview”, Lake Broadwater, via Dalby, 1 Oct 1988, K.A. Williams s.n. (BRI, AQ230829). New South Wales. North western plains: Mulgowan Station, at end of track 12 km SSW of homestead, c. 65 km SSW of Bourke, 27 Sep 1981, H. Eichler 22840 (BRI, CANB, NSW).

Figure 2. Hydrocotyle demissa. a. leaf, upper surface b. fruits. (a from Moore 934; b from Silcock JLS2190 & Fairfax).

Distribution & habitat. Sporadically distributed from near Blackall in Queensland to near Bourke in New South Wales, and eastwards to Dalby (Fig. 1). It grows on the edges of lagoons and dams, along creeks, and in wetlands associated with springs.

Etymology. From the Latin demissus, meaning low, weak, dwarf. This is in reference to the small size and prostrate habit of the species.

Notes. Hydrocotyle demissa is completely glabrous with very small leaves (radius 1.5–4 mm), very short fruiting styles (c. 0.1 mm long), and the mericarp morphology is distinctive. It is perhaps close to H. dipleura A.R.Bean, but H. demissa differs by the peduncles 0–1 mm long (3–17 mm long for H. dipleura); the stipules entire or minutely erose (with laciniate margins for H. dipleura); mericarp with one lateral rib (2 lateral ribs for H. dipleura); and the petals c. 0.15 mm long (c. 0.4 mm long for H. dipleura).{p}{p}