Mid-vein impressed, margin revolute, pistillate inflorescence peduncle dichotomously branched. Typus seriei: Elatostema xingyiense L.D. Duan & D.H. Yin.

The staminate capitula of the species is long peduncelate, as such this new series is closely related to ser. Sublinearia W.T. Wang (Wang 1980). It also has similarity to ser. Melanocarpa W.T. Wang with its achene fawn, ovoid, longitudinally 4(-5)-ribbed and tuberculate (Wang 2013). However, the new series differs from these two series with regards to the following features: leaf mid-vein impressed, the margin revolute (compared to the mid-vein flat and margin flat in ser. Melanocarpa and ser. Sublinearia); pistillate inflorescence peduncle dichotomously branched (compared to the peduncle not dichotomously branched in ser. Melanocarpa and ser. Sublinearia) (Table 1).

China Guizhou: Xingyi City, Maling River Canyon Scenic Area, adarces and walls in the valley floor of middle mountains, 25°09'58.00"N, 104°57'20.07"E, 1110 m alt., 8 February 2019, Lin-Dong Duan & Zhen Lu, 6118 (holotype: HUSY!, isotype HNNU!, PE!, HUSY!).

This new species is closely related and similar to Elatostema melanocarpum (Wang 2013), Elatostema sublineare (Wang 1980), Elatostema obscurinerve (Wang 1980), Elatostema langicuspe (Wang 2013) and Elatostema youyangense (Wang 1984). This new species is visibly distinguished by a combination of characters: leaf mid-vein impressed, leaf margin revolute (mid-vein flat, margin flat in the other five species); pistillate inflorescence peduncle dichotomously branched (not branched in the other five species) (Table 1).

Herbs perennial. Young stems ca. 16–30 cm tall, glabrous, purple, simple, with 3–4 leaves. Leaves sub-sessile, glabrous; blades thin-papery, adaxially green, abaxially purple, obliquely long elliptic, lanceolate-elliptic or ovate-elliptic, 4.0–12.5 cm long, 1.2–3.7 cm wide; apex caudate-acuminate (acumens entire); base sub-orbicular to broad-cuneate at broad side and cuneate at narrow side; margins below mid-leaf entire, above mid-leaf crenate, revolute; venation pinnate, with 3–5 pairs of lateral nerves; cystoliths conspicuous, dense, bacilliform, 0.08–0.16 mm long; stipules subulate, 1.0–1.5 mm long. Mature stems ca. 25–45 cm tall, glabrous, simple or sometimes branched, with female inflorescences near apex with 3–5 leaves. Leaves sub-sessile or shortly petiole, 0.2–4.0 mm long, glabrous; blades papery, obliquely long elliptic, obliquely elliptic to obovate-elliptic, 15–16 cm long, 2.5–6.0 cm wide, apex caudate-acuminate, base broadly cuneate at broad side and cuneate at narrow side; margin below mid-leaf entire, above mid-leaf crenate, margin notably revolute; both surfaces glabrous; venation pinnate, with 4–7 pairs of lateral nerves, adaxially mid-vein impressed, lateral vein impressed near mid-vein, abaxially mid-vein and lateral vein notably ridged, cystoliths conspicuous the same as caulicles. Monoecious, male inflorescence axillary on young stems, female inflorescence axillary on mature stems. Staminate capitula singly axillary, peduncles round, glabrous, 4–20 mm long, apex dichotomously branched, branches 0.5–2.0 mm long, nearly glabrous, capitulum above each secondary peduncle, 3.6–4.8 mm long, 3.6–4.0 mm wide, receptacle cochleariform to oblong, ca. 2–3 mm long, ca. 1–2 mm wide and receptacle 1(–2)-lobed when oblong, glabrous, unilateral bract 3–5 (3 when receptacle is cochleariform, 5 when receptacle is oblong), oval to narrow triangular, 2.0–2.5 mm long, 0.6–2.3 mm wide; apical bract abaxial surface longitudinally 1(–3)-ribbed, ca. 1–2 mm long, abaxial surface nearly glabrous; lateral bract longitudinally 1-ribbed, ca. 1 mm long, abaxial surface nearly glabrous or puberulent, bracteoles few, membranous, semi-hyaline, white, lanceolate; abaxially puberulent with cystolith, margin ciliate, ca. 3 mm long. Staminate flowers peduncles glabrous, 3 mm long; tepals 5, oval, 3 mm long, base connate, glabrous, apex corniculate on 2–3 tepals; stamens 5. Pistillate capitula singly axillary, papilionaceous to quadrangular, 2–8 mm long, 2.0–6.5 mm wide; peduncles 1.0–1.5 mm long, flowers numerous, receptacle papilionaceous or elliptic, bipartite, margin indehiscent or lobed, 1.5–6.5 mm long, 1.5–5.4 mm wide, bracts numerous, linear-lanceolate, black, abaxially puberulent, margin ciliate; bracteoles numerous, linear-lanceolate, black, 1.0–2.6 mm long, abaxially puberulent, margin ciliate. Pistillate flowers sessile, tepals absent, ovary ovoid, stigma penicillate. Achenes brownish, ovoid, ca. 0.35–0.40 mm long, longitudinally 4(–5)-ribbed, tuberculate.

Figure 1. 

Elatostema xingyiense L.D. Duan & D.H.Yin A habit B male flower C male inflorescence in fresh specimen D male inflorescence E male inflorescence and secondary peduncle F female inflorescence in fresh specimen G female inflorescence H blades in fresh specimen I bract longitudinally 3-ribbed. Photos: Lin-Dong Duan and Dan–Hong Yin.

During our field trips, plants were observed in full bloom and without fruits on 10 February 2019, then flowers and fruits on 9 April 2019. The flowering in February to April, fruiting in March to May can be expected.

The new species grows on limestone in the valley floor of middle mountains, Maling River Canyon Scenic Area, Xingyi City, Guizhou Province, south-western China.

Elatostema xingyiense is only known from one locality in Maling River Canyon Scenic Area, Xingyi City, Guizhou Province, south-western China.

The new species was named after its type locality, Xingyi City, Guizhou Province, China.

兴义楼梯草(Xīng yì lóu tī cǎo) is Chinese Pinyin for Elatostema xingyiense, the first two characters are the place name of Xingyi City, the last three characters are the Chinese name for Elatostema.

Elatostema xingyiense is only known from one collection with about 1000 individuals in Maling River Canyon Scenic Area, Xingyi City, Guizhou Province, south-western China (ca. 74 km²). This species is under threat because of its fragmented habitat and there is tourism in the type location, Maling River Canyon Scenic Area. It is only in one small area of less than 100 km² and has threats from anthropogenic factors. We suggest that E. xingyiense should be considered as “Endangered” (EN) according to the IUCN Red List Categories and Criteria (IUCN 2019).