Hibbertia acerosa (R.Br. ex DC.) Benth., Fl. Austral. 1: 24 (1863).

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Pleurandra acerosa R.Br. ex DC., Syst. Nat. [Candolle] 1: 422. Type citation: "Hab. in Novae-Hollandiae ora australi loco Lucky Bay dicto [likely an error for Kings George Sound], R. Brown. (v.s. sp.)." (syn: G 201255; probable syn: NY 428751).

Pleurandra cognata Steud., Pl. Preiss. [J.G.C.Lehmann] 1(2): 265 (1845). Type citation: "In fissuris rupium ad latus orientale montis Melville, ditionis Plantagenet, 5. Oct. 1840. Herb. Preiss. No. 2156" (syn: LD 1087556 image!, L2385642!, MEL 666814!, MEL 666813!, S 08-20063 image!).

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Map 4. Distribution of Hibbertia acerosa in southwestern Western Australia.

Low, often spreading, openly-branched to compact shrubs (sometimes cushion-like), 0.1–0.3(–0.5) m high; young branchlets sparsely to moderately and minutely pubescent with white, multi-armed hairs, soon glabrescent, with obscure tufts of short hairs in the leaf axils. Leaves spreading, scattered, linear, (5–)10–15(–20) mm long, 0.6–1 mm wide, the margins revolute and tightly abutting the midrib (which is usually level with or sunken relative to the margins) or to each other, the abaxial surface obscured; adaxial surface smooth to tuberculate, glabrescent but usually with sparse, hooked hairs until mature, sometimes also with sparse, minute, antrorse hairs from the tubercles when young (rarely almost glabrous from a young age), with short, multi-armed hairs at the very base; abaxial midrib smooth including along its flanks; abaxial lamina glabrous (surface almost or completely hidden within the lacuna formed between the margins and the midrib and usually requiring dissection to examine); petiole abaxially glabrous; apex tapering and pungent-pointed. Flowers pedicellate, from upper leaf nodes or on lateral shoots, the pedicels 8–28 mm long, glabrous or with scattered minute stellate hairs; bract 1, subtending the flower at the apex of the pedicel, linear to narrowly triangular, herbaceous, 2–4 mm long, abaxially glabrous to sparsely and minutely hairy, adaxially and on the margins usually minutely pubescent. Sepals ovate-acute, 3–4.5 mm long, herbaceous, with scattered hooked hairs usually along most of the length, mixed with minute stellate hairs; midribs prominent especially at the base; outer and inner sepals similar but the inner slightly broader, scarious-margined and with fewer hooked hairs. Petals 5, yellow, broadly obovate, 6.5–7.5 mm long, deeply emarginate. Stamens 10(12), all on one side of the gynoecium and curving over it like a hand of bananas; filaments 0.4–0.6 mm long, fused at the base into a robust claw; anthers rectangular, 1.4–2 mm long, dehiscing by introrse, longitudinal slits. Staminodes 2 either side of the stamens. Carpels 2; ovaries globular, densely pubescent; styles inserted excentrically on the carpel apex, parallel and curved beneath the stamens, 1.2–1.8 mm long. Ovules 2 per carpel. Fruiting carpels somewhat elongated, usually 2-seeded and slightly constricted between the seeds, which are dark, glossy red-brown and 1.5–2 mm diam; aril covering 1/3–3/4 of the seed, membranous, sparsely to moderately appressed-pubescent.

Selected specimens examined (all PERTH): Beenup (3041018), Beverley (5549809), Boddington (3106411), Bowelling, (4742052), Byford (5307864), Cape Riche (8598584), Cranbrook (4598113), Dryandra State Forest (1444646), Kalamunda (2863243), Kojonup, (5888409), Mount Clarence (7762631), Mundaring (3040801), Rocky Gully (8994242), Serpentine (6740049), Stirling Range National Park (2863219), 2863251), Stokes Inlet (2863200), Wungong (7187483), York (3041042)

For full specimen details, see the following batch search of the ALA for the above set of specimens: https://biocache.ala.org.au/occurrence/search?q=qid%3A1695269540236&qualityProfile=ALA&disableQualityFilter=scientific-name#tab_mapView

Map 4. Distribution of Hibbertia acerosa in southwestern Western Australia.

Diagnostic features. Hibbertia acerosa is readily distinguishable from the species segregated from it by the abaxial leaf midrib being entirely smooth, including along its flanks, the abaxial leaf lamina being glabrous, and the sepals being usually fairly evenly covered in sparse to mid-dense hooked hairs.

Phenology. Flowers from late August to December (and sporadically thereafter), with a peak in late October.

Distribution & habitat. Occurs in south-western Western Australia from the vicinity of Julimar south to Cookernup along the edge of the Darling Scarp, with scattered occurrences in the western Avon Wheatbelt bioregion between York and Bowelling, and along the south coast hinterland between Lake Unicup and inland of Cape Riche (Map 4), mostly in jarrah-marri forests and woodlands on sandy to clayey soils. A single collection (A.E. Orchard 1195) is from well east of the main range, near Stokes Inlet west of Esperance, and is within the range of H. callida. Similar disjunctions along the south coast of Western Australia are not uncommon, and there is no reason to doubt the specimen or its location.

Conservation status. Common and widespread and not considered to be at risk.

Notes. Hibbertia acerosa was named (as Pleurandra acerosa R.Br. ex. DC.) by De Candolle in 1817, based on material collected by Robert Brown at Bay 1 (Lucky Bay) in 1802. The combination into Hibbertia was effected by Bentham (1863) in Flora Australiensis. At the same time, Bentham placed two prior names, Pleurandra juniperina Turcz. and P. cognata Steud., into synonymy under H. acerosa and erected a variety, var. ulicifolia Benth. (later elevated to species rank as H. ulicifolia (Benth.) J.R.Wheeler). The new combination H. juniperina based on P. juniperina is made in this paper; it is clearly distinct from H. acerosa.

There are three Brown specimens labelled Pleurandra acerosa on JSTOR Global Plants and catalogued in Mabberley & Moore (2022). A sheet at NY ex BM (NY 428751) bears a label 'Nova Hollandia Ora Meridionalis, King George's Sound' and has two specimens, both small, much-branched plants with short leaves. A sheet at G-DC bears a label with 'Pleurandra acerosa, Lucky Bay or. Austr.' in Brown's hand, annotated 'm. Rob. Brown 1816' in a different (unknown) hand. The single small specimen on the sheet is a close match to the NY specimen. The annotation indicates that it was sent to De Candolle by Brown in 1816 (possibly as a result of De Candolle's visit to Brown in London in January of that year; Mabberley 1985).

A third sheet, at BM, bears a blue Bennett slip with the number 4886 and an affixed label with 'Curtelloides acerosa/Bay 1 [Lucky Bay] S. Coast' in Brown's hand. A determinavit slip by David Mabberley and David Moore dated 15 July 2011 indicates that this is an isotype of Pleurandra acerosa. The specimen, however, is clearly H. ulicifolia, which is common at Lucky Bay; Hibbertia acerosa, by contrast, does not occur there. The NY and G-DC sheets closely match the protologue of P. acerosa in all respects, particularly in being small and much-branched with glabrous stems (protologue: '[s]uffrutex humillimus ramosissimus; rami glabri').

Taking the locations at face value, only the G-DC specimen matches the location (Lucky Bay) given in the protologue. However, the fact that it does not occur there and the close match with the NY (King George Sound) specimen makes it likely that the G-DC label is in error, making both specimens syntypes. Hibbertia acerosa is common around Albany (King George Sound, from where the type of P. cognata was also collected by Baxter). It is clear that the Brown 4886 sheet at BM is not a type; it is the first known collection of H. ulicifolia.

Three other sheets, of Brown 4885 (K000687507, BM000571219 and E00791174), are also of H. acerosa. The BM sheet bears a slip in Brown's hand with 'Curtelloides recurva/K G III|^rd^| Sound' and a BM Type sticker. These specimens are not regarded as type material.

Pleurandra cognata was described by Steudel (1845) based on Preiss 2156, collected 'in broad clefts of the cliffs on the east side of Mount Melville, District Plantagenet' ('[i]n fissuris rupium ad latus orientale montis Melville, ditionis Plantagenet') on 5 October 1840. There are two Mounts Melville, one in what is now Albany and one at Cape Riche, further east along the Western Australian south coast and beyond the bounds of Plantagenet County (which was designated in 1829). Preiss collected at both localities, but was in Albany on 5 October 1840, not collecting in the Cape Riche area until 17–26 November of that year (AVH 2022). Hibbertia acerosa sens. str. is common in the Albany area, including at Mount Melville and Mount Clarence.