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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
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 long2
2Petals reflexed against the ovary, dorsal sepal not hooded closely over the column3
Petals spreading, dorsal sepal hooded closely over the column
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3Flowers 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
                       Back to 2
Acianthus exsertus
4Flowers 5–7 mm long5
Flowers more than 8 mm long
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5Labellum flatAcianthus exiguus
Labellum margins recurved
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Acianthus sp. A sensu Harden (1993)
6Labellum callus narrow-cordate (flowers Feb.–Apr.)Acianthus apprimus
Labellum callus cordate to broad-cordate (flowers May–Aug.)
                       Back to 4
7Flowers 8–11 mm long, dorsal sepal 6–8 mm long, 4–5 mm wide, labellum callus cordate (inland ranges)Acianthus collinus
Flowers 10–14 mm long, dorsal sepal 9–12 mm long, 5–6 mm wide, labellum callus more or less cordate (coastal and near-coastal ranges)
                       Back to 6
Acianthus fornicatus

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