5. Acronema tenerum (de Candolle) Edgeworth, Trans. Linn. Soc. London. 20: 51. 1846.
丝瓣芹 si ban qin
Helosciadium tenerum de Candolle, Prodr. 4: 105. 1830; Carum tenerum (de Candolle) Franchet; Pimpinella tenera (de Candolle) C. B. Clarke.
Plants 5–30 cm. Rhizome narrowly conic, or elongate-globose, ca. 5 mm across. Stem solitary, slender. Basal leaves petiolate, petioles 2–5 cm, sheaths narrowly lanceolate; blade triangular, 1.5–2.5 × 1.5–3 cm, 2–3-ternate-pinnate; ultimate segments cuneate-obovate or obovate, ca. 5 × 5 mm, apex often 3-toothed. Umbels 2–3 cm across; peduncles 2–5 cm, slender; bracts usually absent; rays 3–4, 1–2 cm, unequal; bracteoles 1–3, 2–3 mm; umbellules 6–14 mm across, 3–5-flowered; pedicels 2–7 mm, unequal. Calyx teeth obsolete. Petals purple-red, ovate, ca. 2 mm; apex linear, ca. 1 mm. Fruit broad-ovoid, ca. 1–1.5 × 1–1.5 mm; vittae minute, 2 in each furrow, 2 on commissure. Fl. Aug.
Damp shady crevices; 3400–3500 m. E Xizang, NW Yunnan [Bhutan, NE India, E Nepal, Sikkim, N Thailand].
Despite this species being the most widespread in the genus, fruiting material is still lacking for many areas (including China). This species is similar and closely related to Acronema hookeri, with which larger specimens of A. tenerum can be confused. The presence of bracteoles and shorter petal apices are useful in recognizing such specimens of A. tenerum.