It is morphologically close to Pet. hechiensis, but can be distinguished by its petioles, peduncles and pedicels densely curly rubiginous to ferruginous villous, bracts lanceolate to subulate and both surfaces densely rubiginous to dark brown pubescent, corolla lobes oblong to nearly rounded and margin entire, calyx lobes densely rubiginous to ferruginous pubescent.

Figure 1. 

Petrocodon rubiginosus Y.G.Wei & R.L.Zhang, sp. nov. A flowering plant in natural habitat (Jingxi, Guangxi, China) B flowering plant cultivated in GCCC C cyme D corolla in front view E corolla in lateral view and bud on cyme F indumentum of peduncle G flower in top view H flower in lateral view I flower in upward view J resolved flower for showing corolla, calyx lobes, pistil and pedicel K pistil and calyx lobes (adaxial & abaxial surfaces) L style and stigma. Photographs by authors.

CHINA. Guangxi Zhuangzu Autonomous Region: Jingxi city, Yuexu Town, Siming village, 22°56'N, 106°37'E, alt. 470 m, 12 November 2015, flowering, Wen Fang et al. WF151112-01 (holotype: IBK; isotype: IBK).

Perennial herb, stemless. Rhizome subterete, 3–8 cm long, 5–7 mm in diam. Leaves 8–16, all basal; leaf blades herbaceous, green to yellowish-green, zygomorphic to slightly unequal in each pair, broadly ovate to nearly rounded, 3.5–20 × 3.8–15 cm, apex obtuse to rounded, base cordate or obliquely cordate and bases on both sides usually overlapping, margin obviously crenate to undulate, pubescent on both surfaces, 5–6 nerves on each side, adaxially impressed, abaxially prominent, densely ferruginous and erect villous along abaxial main vein and lateral veins; petioles 15–25 cm or longer, densely ferruginous and erect villous. Cymes 4–8, 20–30 cm long, usually curved or squiggly,1–2-branched, 8–12-flowered; peduncle 15–20 cm long, ca. 2 mm in diam., densely curly rubiginous to ferruginous villous; bracts 2, opposite, lanceolate to subulate, 5–6 × 1–2 mm, entire, both sides densely rubiginous to dark brown pubescent; bracteoles 2, opposite, linear-lanceolate, ca. 5 ×0.8 mm, both sides densely rubiginous to dark brown pubescent; pedicel 4–7 mm long, 0.8–1 mm in diam., densely rubiginous to dark brown pubescent. Calyx 5-parted to the base, sepals lanceolate-linear to subulate, ca. 10 mm long, 0.6–0.7 mm at base, both sides densely rubiginous to dark brown pubescent. Corolla bilabiate, pale yellow, throat with two brightly yellow longitudinal stripes and dark yellow glands on stripe surface, ca. 3 cm long, outside white pubescent; tube slender, 2–2.2 cm long, 6.5–7 mm in diam. at middle, slightly constricted at ca. 6 mm above base of corolla base; adaxial lip 7–8 mm long, 2-partite nearly to base, lobes oblong to nearly rounded, apex acuminate, abaxial lip 2–2.5 cm long, 3-partite nearly to base, lobes nearly rounded, apex obtuse. Stamens 2, sparsely glandular-puberulent, adnate to 1.1–1.2 cm above the base of corolla tube; filaments pale yellow to white, 5–5.5 mm long; anthers 1.2–1.5 mm long, elliptical, 1.8–2 mm long, 1–1.5 mm in diam. Staminodes 3, glabrous, lateral ones 6–7 mm long, adnate to ca. 10 mm above the base of corolla tube, median ca. 1.5 mm long, adnate to ca. 7 mm above the base of corolla tube. Disc glabrous, ca. 1.2 mm high, margin entire. Pistil ca. 2 cm long; ovary ovoid, ca. 5 mm long, covered pale rubiginous or white pubescent, style ca. 1.5 cm long, densely pubescent; stigmas inapparent triangular, 2, each one semicircular, ca. 0.8 mm long. Capsule ovoid, valvular dehiscence, pubescent.

Figure 2. 

Petrocodon rubiginosus Y.G.Wei & R.L.Zhang, sp. nov. (1) and its congener, Pet. hechiensis (Y.G.Wei, Yan Liu & F.Wen) Y.G.Wei & Mich.Möller (2) A adaxial surfaces of leaf blades and petioles of two species B abaxial surfaces of leaf blades and petioles of two species C indumentum of petioles D indumentum of main nerves on adaxial surface E cymes and flowers F indumentum of peduncles G indumentum of bracts and pedicels H abaxial and adaxial surfaces of bracts I corolla in frontal views J corolla in top views K corolla in lateral views L indumentum of abaxial surfaces of calyx lobes M stigma. Photographs by authors.

Flowering occurs in November and fruiting from December to January of next year.

The specific epithet is derived from the conspicuous indumentum of petioles, peduncles and pedicels, which are covered with densely long ferruginous or rubiginous hairs.

Xìu Géěng Shí Shān Jù Tái (Chinese pronunciation); 锈梗石山苣苔 (Chinese name).

Petrocodon rubiginosus is hitherto only known from two close localities at elevational ranges from 450–500 m in Jingxi city, Guangxi, China, growing on shaded and moist rock surface with no more than 50 individuals at each of the localities. Two localities are about three kilometres apart. The species grows in subtropical broad-leaved evergreen monsoon forest with sufficient seasonal run-off water.

Population information of Petrocodon rubiginosus is still unclear, which makes it difficult to determine an assessment of the extinction risk faced by this new taxon. At present, two distribution points of the species are known and its estimated area of occupancy is less than 10 km². The main threat now comes from environmental damage caused by grazing and there is a risk of poaching in the future because its distribution is not far away from the villages. Furthermore, prolonged droughts and illegal logging in the area, including nearby potential habitat, should be considered as potential risks to the persistence of Pet. rubiginosus. Thus, following the IUCN Red List Categories and Criteria (IUCN 2017), it is assessed temporarily as endangered [EN B2ab (ii, iii)].

It is morphologically close to Petrocodon hechiensis because two congeners share some similarities, for example, they have look-alike leaf shape and size and a pale-yellow corolla (Wei et al. 2008, 2010). As a result, it is sometimes mistaken for Pet. hechiensis. However, one of the largest differences between Pet. rubiginosus and Pet. hechiensis is the indumentum of petioles, peduncles and pedicels. There are absolutely no glandular-hairs on petioles, peduncles and pedicels of Pet. rubiginosus, but in Pet. hechiensis, all are covered by densely short and sticky glandular-hairs and pubescent-hairs (Figure 3). The other major differences between the species are outlined in Table 1.

Figure 3. 

The difference of indumentum between Petrocodon rubiginosus Y.G.Wei & R.L.Zhang, sp. nov. (A) and Pet. hechiensis (Y.G.Wei, Yan Liu & F.Wen) Y.G.Wei & Mich.Möller (B): densely curly rubiginous to ferruginous villous on surface of petiole (A–1), pedicel (A–2) and calyx lobes (C–1) and densely short glandular-pubescent and pubescent on surface of petiole (B–1), pedicel (B–2) and calyx lobes (C–2).