Similar to Thismia brunonis Griff., but differing in bearing a maximum of only three flowers (with a single flower at anthesis), and having a dark red perianth tube with the filiform appendages on the outer tepals remaining upright and forming a loose mitre over the annulus, longer staminal filaments with two distinct teeth at connective apex, and rounded stigmas.

China: 22°25'N, 114°11'E, Tai Po Kau Nature Reserve, New Territories, Hong Kong, 19 May 2014, S.S. Mar 1 (holotype: HK, in spirit).

Small achlorophyllous holomycotrophic herbs. Roots clustered, ± horizontal, vermiform, fleshy, 1.2–1.3 mm in diameter, cream-coloured. Stem cream-coloured, unbranched, erect, ca. 1.7 cm tall, 1.8–2 mm in diameter, glabrous, terete, with ca. 12 longitudinal ridges. Leaves white, appressed, clasping stem, narrowly triangular with acute apex, scale-like, 3–5.5 mm long, 1.7–2 mm wide; basal leaves smallest, upper leaves (equivalent to floral bracts) largest. Flowers in clusters of up to 3, developing sequentially with only one anthetic. Perianth actinomorphic, of 6 fused tepals, forming a perianth tube with free apical lobes. Perianth tube pinkish-white, membranous, urceolate, ca. 6.7 mm long, ca. 6.1 mm in diameter, with 12 dark red vertical ribs, abaxial surface distinctly verrucose; apex of perianth tube fused to form a dark red, rounded-hexagonal annulus, ca. 1.4 mm wide (top, externally), ca. 2.3 mm wide (base, externally) and ca. 1 mm (internal aperture); dark red, inverted V-shaped structures (putative nectaries) at apex of adaxial surface of perianth tube, opposite apertures between staminal filaments. Outer tepal lobes triangular, ca. 1.8 mm long, ca. 1.5 mm wide at base; inner tepal lobes spathulate, concave adaxially, ca. 3.3 mm long, ca. 1.7 mm wide at widest point, bearing a dark red filiform appendage on the abaxial surface, ca. 4 mm long, ca. 0.5 mm in diameter (towards base), 0.4 mm in diameter (towards apex); the three filiform appendages remain upright and cross each other, forming a persistent mitre. Stamens 6, pendent from the inner margin of perianth annulus, ca. 2.9 mm long, ca. 1.1 mm wide at widest point; filaments free, ca. 1 mm long; stamens laterally connate, forming an anther tube; individual stamens with two thecae (adaxial, dehiscing towards inner surface of perianth tube), ca. 0.7 mm long; apical connective of stamens ca. 1 mm long, with two distinct teeth, adorned with trichomes, ca. 0.5 mm long. Ovary inferior, obconical, ca. 2.7 mm long, ca. 4 mm wide towards apex; style ca. 0.6 mm long, ca. 0.6 mm in diameter, with three bilobed, rounded stigmas; stigmatic head ca. 1 mm long, ca. 1 mm in diameter. Fruit a capsule ca. 4 mm long, ca. 4.8 mm wide, cup-shaped, carnose, pale orange-brown, dehiscing apically; fruiting peduncle ca. 2.5 mm diameter. Seeds numerous, yellow-brown.

Flowering was observed between May and September. Flower development extends over several weeks (Fig. 1C–H). Based on our field observations, mature flowers are functional for up to three weeks, with up to three flowers developing successively in each individual (Fig. 3A). The perianth tube abscises after fertilization (Fig. 1I, J), with a clear abscission zone (ab in Fig. 2A); the epidermis on the upper surface of the carpel subsequently disintegrates and the stigma is shed, exposing the seeds (Fig. 3A, D). Fruiting was observed between June and October.

Thismia hongkongensis is only known from the type locality in Hong Kong. The habitat is lowland secondary forest (west-facing slope, ca. 240 m above sea level); the site is not close to a stream and is therefore likely to dry out during periods of low precipitation. Cooccurring species include Adiantum flabellulatum L. (Adiantaceae), Ardisia quinquegona Blume (Myrsinaceae), Breynia fruticosa (L.) Hook.f. (Euphorbiaceae), Burmannia wallichii (Miers) Hook.f. (Burmanniaceae), Clematis meyeniana Walp. (Ranunculaceae), Cratoxylum cochinchinense (Lour.) Blume (Clusiaceae), Desmos chinensis Lour. (Annonaceae), Diplospora dubia (Lindl.) Masam. (Rubiaceae), Garcinia oblongifolia Champ. ex Benth. (Clusiaceae), Lindsaea orbiculata (Lam.) Mett. ex Kuhn (Lindsaeaceae), Lophatherum gracile Brogn. (Poaceae), Lygodium japonicum (Thunb.) Sw. (Lygodiaceae), Machilus chekiangensis S.K.Lee (Lauraceae), Psychotria asiatica L. (Rubiaceae), Psychotria serpens L. (Rubiaceae), Rourea microphylla (Hook. & Arn.) Planch. (Connaraceae), Sarcandra glabra (Thunb.) Nakai (Chloranthaceae), and Sciaphila ramosa Fukuy. & T.Suzuki (Triuridaceae).

The specific epithet reflects the geographical origin of the species in Hong Kong.

(paratypes). China. Hong Kong: 22°25'N, 114°11'E, Tai Po Kau Nature Reserve, New Territories, 29 May 2014, S.S. Mar 2 (HK, dissected flower in spirit); idem, 2 October 2014, S.S. Mar 3 (HK, immature fruit in spirit).

Thismia hongkongensis is most similar to Thismia brunonis Griff. (1844, 1845; as ‘Thismia brunoniana’ in the latter), the type species in the genus. Thismia brunonis is only known from a single collection from Tenasserim in Myanmar, dating from October 1834. According to Jonker’s (1938) supraspecific classification of the genus, both species would be included in sect. Thismia (‘Euthismia’) subsect. Brunonithismia Jonker as they have unequal and free tepal lobes. Comparison of the two species reveals several significant differences, however, including flower number per inflorescence. The protologue of Thismia brunonis includes reference to flowers clustered in a terminal raceme (“Flores pauci, in racemum brevem terminalem dispositi”: Griffith 1845: 341) with the accompanying illustration in the same publication (Fig. 1 in Pl. XXXIX) showing inflorescences with four and six flowers; Jonker (1938) subsequently described the species as bearing 3–8 flowers per raceme. In contrast, Thismia hongkongensis invariably has fewer flowers, with a maximum of three per inflorescence, reaching anthesis consecutively. The perianth tube of Thismia brunonis is pale yellow with red ribs (Griffith 1845), whilst it is pink with red ribs in Thismia hongkongensis (Figs 1G–J, 2A, B). The filiform appendages on the outer tepals of Thismia brunonis appear to be spreading at maturity (Griffith 1845: Pl. XXXIX), whereas those of Thismia hongkongensis invariably remain upright and cross each other to form a loose mitre over the annulus (Figs 1G–J, 2A, B, 4A). Thismia brunonis also differs from Thismia hongkongensis as it has a much shorter filament: although a measurement was not included in Griffith’s descriptions of Thismia brunonis, the accompanying plate (Griffith 1845: Fig. 7 in Pl. XXXIX) indicates that it is considerably shorter than the rest of the stamen. The shape of the stigma lobes also differs: it is acute in Thismia brunonis (Griffith 1845: Fig. 9 in Pl. XXXIX; Jonker 1938), but rounded in Thismia hongkongensis (Fig. 4D).

Thismia hongkongensis is strikingly different from its congener in Hong Kong. Thismia tentaculata has a white perianth tube with a bright yellow annulus, and three divergent orange-red filiform appendages on the inner tepals (Ho et al. 2009).

CR D (IUCN 2001). Only one population is known, consisting of ca. 10 individuals, covering an area of approximately 4–5 m². The population is located within the Tai Po Kau Nature Reserve, but is close to a major hiking path and the population is therefore susceptible to human disturbance and vegetation clearance.