China. Guangxi: Daxin county, Shuolong town, Detian village, 22°52'N, 106°43'E, elevation 650–750 m, 30 March 2018 (fl.), Zhao-Jie Wen & Guang-Fu Mou 18033001 (holotype: CSFI [CSFI069613]; isotypes: CSFI, IBK).

The new species is similar to Spiradiclis cordata and S. tubiflora, but it differs from the former mainly by the cuneate leaf base (vs. cordate) and much longer corolla tubes (1.8–2.2 cm long vs. ca. 5 mm long), and from the latter by its subrosulate habit (vs. procumbent to creeping), longer and wider leaf blades (7.0–10.5 × 2.0–3.5 cm vs. 0.5–2.5 × 0.4–1.5 cm), more secondary veins (7–10 pairs vs. 3–5 pairs) and longer corolla tubes (18–22 mm vs. 14–16 mm).

Perennial herb, up to 15 cm tall; stems densely pubescent, erect or ascending, lower part rooting at nodes. Leaves subrosulate; petiole 0.8–1.7 cm long; leaf blade drying papery, obovate-lanceolate or oblong, 7.0–10.5 × 2.0–3.5 cm, obtuse or acute at apex, cuneate at base, adaxially dark green, puberulent, abaxially light green, pubescent, densely pubescent along veins at lower surface; secondary veins in 7–10 pairs; stipules persistent, pubescent, narrowly triangular, 3.2–5.7 mm long, or 2–5-lobed, lobes linear-triangular, upper part filiform. Inflorescence cymose, 2–7-flowered; peduncle 9–14 cm long, pubescent; bracts linear-triangular, 4–6 mm long, subglabrous; pedicels 0.5–5 mm long, pubescent. Flowers distylous. Calyx puberulent; hypanthium portion obconic, ca. 2 mm long; lobes triangular, 1.2–2.9 mm long, acute at apex. Corolla white to pink, tubular-funnel shaped, puberulent outside; tube 1.8–2.2 cm long, sharply enlarged at the 1/3 lower part of the corolla tube, 4.5–6.8 mm in diam. at throat, 1.8–2.5 mm in diam. near base; lobes triangular-ovate, 4.5–5.6 × 3.5–4.3 mm long. Stamens 5; anthers linear. Stigma bilobed; ovary 2-celled. Long-styled flowers: corolla tube inside with densely pubescence near base and densely puberulent above anther and on to lobes; anthers inserted near base of corolla tube, 1.9–2.3 mm long; style 1.2–1.4 cm long, puberulent; stigma inserted at between middle and throat of corolla tube, 2-lobed, lobes elliptic, 1.8–2.2 mm long. Short-styled flowers: corolla tube inside densely puberulent; anthers inserted often at or a little above middle of corolla tube, 2.6–3.3 mm long; style 1.5–3.4 mm long, glabrous; stigma near base of corolla tube, lobes ovate-triangular, 1.3–1.6 mm long. Capsules subglobose, 2.5–3.2 mm in diam., valves 4 when matured. Seeds many, angular.

. Flowering March to April, fruiting from May to July.

The specific epithet refers to the type locality, where a famous attraction, Detian Waterfall, is situated.

德天螺序草 (de-tian-luo-xu-cao in Mandarin).

Until now, only two populations of the new species have been found. They are both known from limestone hills of southern Guangxi. Plants of the new species prefer to grow at humid places at elevation range of 500‒800 m, under evergreen broad-leaved forests with tree species of Fagaceae, Lauraceae, Tiliaceae, Theaceae, Myrsinaceae, Magnoliaceae and Sapindaceae.

Figure 1. 

Spiradiclis detianensis A habit B stipule C inflorescence, side view D opened long-style flower E opened short-style flower F capsule, side view G capsule under matured, face view. Scale bar: 1 cm (A, C, D, E); 2 mm (B, F, G). Drawn from the holotype by X.Y. Zeng.

Figure 2. 

Spiradiclis detianensis A habit B inflorescence C leaves D, E stipule F, G short- and long-styled flower, front view H opened long- and short-styled flower, showing style and stamens I ovary, side view J infructescence, side view. S. tubiflora K habit L inflorescence. Scale bars: 1 cm (B, C, H, J, L); 2 mm (D, E, F, G, I). Photos by Z.J. Wen and L. Wu.

. During a series of field investigations at the China-Vietnam border over the past ten years (2009–2019), only three populations of Spiradiclis detianensis have been observed. One site with nearly 59 matured individuals is distributed in Detian Waterfall (type locality), while the other two sites with more than 250 and 114 individuals are in the Longzhou county and Ningming county, respectively. The habitats of the three sites are in good condition and have been rarely influenced by humans. Considering the above, the species can be assigned a status of ‘Least concern’ [LC] following the guidelines of IUCN (2019).

Our unpublished molecular data indicates that Spiradiclis detianensis shows the closest genetic relationship with S. cordata. Both species prefer to grow on limestone hills from southern Guangxi, China, and have similar habit, such as short stems, subrosulate leaves, long peduncles, heterostylous flowers, funnel-shaped corollas and subglobose capsules. However, the former can be easily distinguished from the latter mainly by its leaf blade basally cuneate (vs. basally cordate) and corolla tubes 1.8–2.2 cm long (vs. ca. 5 mm long) (more detailed comparisons are listed in Table 1).

Spiradiclis detianensis is very similar to S. spathulata in morphology, since both have subrosetted habit, obovate-oblanceolate leaf blades, long peduncles and subglobose capsules, but it differs mainly by its corolla tubular-funnel shaped (vs. slenderly salver shaped), 4.5–6.8 (vs. 1.5–2) mm in diam. at the middle and densely pubescence inside corolla near base and densely puberulent above anther and onto lobes (vs. glabrous except a ring of long hairs at the middle) (more detailed comparisons are listed in Table 1).

Spiradiclis detianensis is a distinct species in the genus Spiradiclis due to the corolla tubes 18–22 mm long and having the shape of a reversed wine bottle, with an abruptly narrowed lower third, 4.5–6.8 mm in diam. at throat, 1.8–2.5 mm in diam. near base. Until now, only one other known species, S. tubiflora L.Wu, B.M.Wang & B.Pan (Wu et al. 2019b), has a similar corolla shape (see Fig. 2 K & L). However, S. detianensis differs from S. tubiflora principally by its rosulate habit (vs. procumbent to creeping), leaf blades longer than 7 cm and wider than 2 cm (vs. shorter than 2.5 cm and narrower than 1.5 cm), secondary veins 7–10 pairs (vs. 3–5 pairs) and longer corolla tubes (vs. 18–22 mm vs. 14–16 mm) (more detailed comparisons are listed in Table 1).

(paratypes). China. Guangxi: Longzhou county, Nonggang National Nature Reserve, 2 April 2019 (fl.), Zheng-Quan Nong nzq0004 (CSFI); Niming county, Tingliang Town, Lixin village, 28 July 2011 (fr.), Yu-Song Huang 9422 (IBK).