Pogostemon petelotii Doan ex Gang Yao, Y. F. Deng & X. J. Ge, sp. nov.

Diagnosis: —The species is similar to Pogostemon cruciatus (Benth.) Kuntze in habit, but differs from the latter in its stems and leaves being whitish adpressed tomentose, leaf margins not revolute and dentate apically, midrib conspicuously elevated, and lateral veins in 3–5 pairs.

Type: — VIETNAM. Thonh Hoa: Bim-son, anciennes rizieres, December 1923, P. A. Pételot 1300 (holotype P! [sheet no. P00344448]; isotypes P! [sheets nos. P00344449, P00344450]).

Annual herb, to 40 cm tall. Stem terete, unbranched or rarely branched, straight, erect, striate on lower portion, adpressed whitish tomentose with long hairs. Leaves 4–6-verticellate, sessile; blades elliptic-linear, 1–3 cm × 3–7 mm, apex obtuse, margin not revolute, sometimes dentate apically, base rounded or broadly cuneate, tomentose on both surfaces, hairs unicellular, midrib elevated abaxially, lateral veins 3–5 on each side of midrib. Inflorescence terminal, 5–7 cm × 38 mm, densely flowered, not interrupted, tomentose; bracts linear, villose, longer than the calyx, long-hairy; calyx campanulate, 1.5–1.8 mm in length, 4-angled and more or less quadrangular, villose outside, lobes 5, subequal; corolla pinkish, very shortly exserted from the calyx tube, lobes oblong, subequal, pubescent outside, corolla tube about 2 mm in length; stamens 4, about 4 mm in length, inserted at the mid-part of the corolla tube; filament twice as long as the corolla, joined to the middle of the corolla tube, hairy; anther stalks tapered at the base which is pubescent, bearded at the mid part and more than twice as long as the corolla; anther 1-locular, cell apex dehiscent. Ovary glabrous; stigma shortly bifid at apex. Nutlets not seen.

Distribution and habitat: — Pogostemon petelotii is found in Thailand and Vietnam. It grows in grassland in open places at elevations of 1000–1300 m.

Etymology: —The specific epithet “ petelotii ” was originally proposed by Doan (1936) for the species to be in honor of French botanist P. A. Pételot (1885–after 1940), who first collected the species in Vietnam.

Discussion: —According to the classification of Bhatti & Ingrouille (1997), Pogostemon petelotii belongs to P. subgen. Dysophylla. Obviously, it is closely related to P. cruciatus, but differs from the latter by having the stems and leaves whitish adpressed tomentose (Fig. 1 C), leaf margins rarely revolute and sometimes dentate apically, midrib elevated conspicuously (Fig. 3) and lateral veins in 3–5 pairs (Fig. 1 C). Pogostemon cruciatus has the stems and leaves brown spreadingly hirsute (Fig. 1 D), leaf margin usually revolute and quite entire, midrib inconspicuously elevated (Fig. 1 D) and lateral veins invisible (Fig. 1 D). P. cruciatus is distributed in India, Nepal, Myanmar, China, Thailand, Vietnam, Laos and Cambodia.

Trichomes are widely used in taxomomic studies in the family Lamiaceae (Cantino 1990, Gairola et al. 2009, Xiang et al. 2010, Hu et al. 2012). In Pogostemon, most species have multi-cellular, simple hairs except for two species that have unicellular hairs (Bhatti & Ingrouille 1997). We observed the trichome morphology of leaves of P. petelotii and P.cruciatus under scanning electron microscopy and light microscopy. The results showed that P. petelotii has unicellular hairs (Fig. 3 F) more densely covered on the leaf surfaces (Fig 3 B, D), and P.cruciatus has 2–3-cellular hairs (Fig. 3 E) more sparsely covered on the leaf surfaces (Fig. 3 A, C).