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Genus Acianthus Family Orchidaceae

Synonyms: Nemacianthus M.A.Clem. & D.L.Jones APNI*

Description: Terrestrial herb, sympodial; roots filamentous; tuberoids paired and also produced at the tips of stolons.

Leaves well-developed, held horizontally well above the soil surface, 1 per shoot, base sheathing, entire, upper surface green and lower surface purple; not petiolate.

Inflorescence racemose, 1–many-flowered, terminal. Flowers resupinate. Dorsal sepal free, similar to, or broader and/or longer than lateral sepals. Lateral sepals free. Lateral petals free, shorter and usually narrower than dorsal sepal. Labellum free, attached at base of column; lamina not lobed, with entire margins, with 2 basal gland-like calli and a longitudinal, central, channelled plate extending from the base to the apical half. Column lacking free filament and style; column wings fused to column, vestigial or absent. Column foot absent. Anther erect, parallel to the axis of the column, persistent. Pollinia 4, hard and waxy. Stigma entire. Rostellum ventral.

Distribution and occurrence: World: 9 species, Australia, New Zealand. Australia: 8 species, endemic, Qld, N.S.W., Vic., Tas., S.A.

We have chosen to recognise most, but not all of the genera segregated from Acianthus by Jones et al. (2002, Orchadian 13: 437-468) and Jones & Clements (2004, Orchadian 14: 331). Acianthus, as circumscribed by Jones (1993, Flora of New South Wales vol. 4), was shown by Kores et al. (2001, American Journal of Botany 88: 1903-1914) and Clements et al. (2002, Lindleyana 17: 135-171) to be polyphyletic and thus requiring nomenclatural change at generic level. Jones et al. (2002, Orchadian 13: 437-468) segregated Acianthopsis and Nemacianthus from Acianthus and Jones & Clements (2004, Orchadian 14: 331) segregated Acianthella from Acianthus in order to ensure that only monophyletic genera were recognized in this group. However, the segregation of the monotypic Nemacianthus (Acianthus caudatus) from Acianthus was unnecessary since, according to the analysis of Clements et al. (2002), Acianthus was a strongly supported monophyletic group with the inclusion of A. caudatus. Segregating Acianthus caudatus into a new genus was thus an unnecessary change of name. (P.H. Weston March 2007)

Text by D. L. Jones, key updated by Matt A.M. Renner (4 Feb 2021)
Taxon concept: P.H. Weston

 Key to the species 
1Sepals with thread-like filiform points more than 20 mm longAcianthus caudatus
Sepals with thread-like filiform points less than 3 mm long, or absent2
2Inflorescence shorter than or equal to the leaf length, labellum ovate, plane with a prominant central raised band flaring into a low, indistinct, trulliform callus, labellum margins not recurved, floral segments predominantly green except for labellum which is green with red-brown pigments, filiform sepal appendages absentAcianthus saxatilis
Inflorescence longer than the leaf length, labellum various, usually triangular oblong with a distinct central grove and recurved margins; floral segments pale or with reddish-purple pigments, filiform sepal appendages present
                       Back to 1
3Petals reflexed against the ovary, dorsal sepal not hooded closely over the column4
Petals spreading, dorsal sepal hooded closely over the column
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4Flowers 8–12 mm long, dull green to dull pink; labellum 4–4.5 mm long, 2–3 mm wideAcianthus pusillus
Flowers 12–16 mm long, pinkish purple to dark purple; labellum 5–6 mm long, 3.5–4 mm wide
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Acianthus exsertus
5Flowers 5–7 mm longAcianthus exiguus
Flowers more than 8 mm long
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6Labellum callus narrow-cordate, labellum margins strongly inrolled causing the labellum to appear lanceolate when viewed from the front (flowers Feb.–Apr.)Acianthus apprimus
Labellum callus cordate to broad-cordate, labellum margins inrolled, but labellum retaining an oblong-triangular outline when viewed from the front (flowers May–Aug.)
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7Flowers 5–7 mm longAcianthus exiguus
Flowers 8–14 mm long
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8Flowers 8–11 mm long, dorsal sepal 6–8 mm long, 4–5 mm wide, labellum callus cordateAcianthus collinus
Flowers 10–14 mm long, dorsal sepal 9–12 mm long, 5–6 mm wide, labellum callus more or less cordate
                       Back to 7
Acianthus fornicatus

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