Impatiens L., the largest genus of Balsaminaceae, comprises about a thousand species distributed primarily in the Old World tropics and subtropics. Impatiens species occur from sea level to 4 000 m altitude, often in forest understory, roadside ditches, valleys, abandoned fields, along streams, and usually in mesic or wet conditions, but some species can tolerate drier habitats^[1]. In Vietnam, several taxonomic works on Impatiens had been done in the past^[2-10]. Tardieu described and recorded 20 species of Impatiens from Vietnam in 1944^[3]. Pham made short descriptions and illustrations for 35 Impatiens species in 2003^[4]. Recently, some new species and new records of Impatiens for the flora of Vietnam have been reported^[6-10], which increased the number to around 40 in Vietnam^[2-10].

During a field survey of plant diversity in Northeastern Vietnam in 2017, we collected several specimens of an interesting Impatiens species, which is different from all the previously recorded species from Vietnam. It has prostrate stems, pubescent branchlets, leaves and inflorescences, ovate to ovate- elliptic leaf blade with 6-7 lateral veins, 1-2 flori- ferous inflorescences, pink or purple flowers, ovate to broadly elliptic lateral sepals, and linear ovary with incurved-rostrate apex (Fig. 1). After careful comparison with species from neighboring countries, we came to the conclusion that all these morphological charac- teristics of this plant are fully consistent with I. napoensis Y. L. Chen^[11], a species previously only recorded from Guangxi, China^[12-13]. Thus, it represents a new record for the flora of Vietnam, and its morphological description, color photos and data on its distribution, phenology and ecology are provided herein.

(Fig. 1)

Fig. 1 Impatiens napoensis Y. L. Chen. A: A flowering branch; B: Leaves; C: Stem and radicant roots; D: Inflorescence; E: Flower (front view); F: Flower (lateral view); G: Flower (back view); H: Flower dissection (DP: Dorsal petal; LAS: Lateral sepal; LUP: Lateral united petal; UP: Upper petal; LP: Lower petal; LOS: Lower sepal; SP: Spur); I: Stamens. (Photographed by K. S. Ngyuen)

Impatiens napoensis Y. L. Chen, Acta Phytotax. Sin. 38(6): 557. f. 1. 2000; Fl. China 12: 84. 2007; S. X. Yu, Balsaminaceae of China: 101, 2012. Type: China, Guangxi, Napo, Defu Nature Reserve, alt. 1 300 m, in rocky valleys. 5 Nov.1998, H. N. Qin et al. 1939 (lectotype PE01879289!, designated here; isolectotype PE01879290!).

Terrestrial plants, 40-60 cm tall. Stem prostrate or procumbent, branched, sparsely short pilose, with adventitious roots at old nodes. Branchlets erect or ascending, dull white to brown pubescent, densely when young. Leaves simple, alternate; petiole (0.7) 1.0-1.5(2.0) cm long; leaf blade membranous, ovate to ovate-elliptic, (3.5)4.5-6.5(7.5) cm×(2.5)3.0-3.5 (4.0) cm, base broadly cuneate to subrounded, atte- nuate into petiole, apex acute to shortly acuminate, margin serrulate, pale green abaxially, green to dark green adaxially, covered by dull white pilose on both surfaces, with 2-3 pairs of stipitate glands at basal margin or petiole, mid-vein and 6-7 pairs of lateral veins prominent abaxially and depressed adaxially. Inflorescences arising from leaf axils of branchlets, 1-2 flowered. Peduncles erect, densely pubescent, 1.2-1.8 cm long; pedicels slender, densely pubescent, 1.1-1.5 cm long, bearing 1-2 bracts at base; bracts persistent, narrowly subulate or narrowly triangular, 3-5 mm long, 1-2 mm wide. Flowers whitish purple to reddish purple, large, 3-4 cm deep. Lateral sepals 2, ovate to broadly elliptic, (8)10-13(15) mm long, (4) 5-7(8) mm wide, pale greenish at apex to dark purple or red at base, inconspicuous 3-5 veins, abaxially longitudinal ribbed, sparsely puberulent, adaxially glabrous, apex acuminate to mucronate, base broadly cuneate to subrounded. Lower sepals broadly funnel- form, purple, adaxially sparsely puberulent, 1.8-2.1 cm in diam., 1.5-2.0 cm deep, gradually narrowed into an incurved spur ca. 2.0-2.2 cm long, with an acuminate tip at vertical mouth. Dorsal petals suborbicular, (1.8) 1.9-2.0(2.1) cm×(1.5)1.6-1.7(1.8) cm, with a small crest-like appendage ca. 1.5-2.0 mm high; apex emarginate, mid vein sparsely puberulent, abaxially narrowly carinate. Lateral united petals 2-lobed, 2.6- 2.8 cm×1.3-1.5 cm, glabrous; upper petals (basal lobes) overlapped on lower petals, obovate or pisiform, 1.5-1.7 cm×0.8-1.0 cm, with apex short acute or obtuse; lower petals (distal lobes) dolabriform, 1.7- 1.9 cm×1.0-1.2 cm, apex obtuse; auricle inflexed, bright yellow. Filaments linear, dull white to purplish, 5-7 mm; anthers ovoid, apex obtuse. Ovary linear, apex incurved-rostrate, glabrous, 6-8 mm long. Fruits green, glabrous when young.

Distribution: China, Guangxi, Napo County; Vietnam, Ha Giang Province, Quan Ba District, Cao Ma Po Commune.

Ecology and phenology: Impatiens napoensis grows in degraded primary and secondary evergreen broad-leaved forests on limestone mountains of Cao Ma Po Commune, at elevations of 1 600-1 750 m a.s.l. This area is characterized by the tropical climate monsoon associated with mountains, which has mean average temperature around 13℃-16℃, mean average rainfall of 2 800-3 500 mm, and mean average relative humidity of 85%-90%^[14-15]. Impatiens napoensis usually occurs in wet, humid shaded areas along watershed of mountain slopes. Population of I. napoensis in Cao Ma Po Commune is often associated with Cinnamomum parthenoxylon (Jack) Meisner, Morella esculenta (Buch.-Ham. ex D. Don) I. M. Turner, M. rubra Lour., Alnus nepalensis D. Don, Schima sp., Lyonia sp., Viburnum sp., etc., and some species of ferns and orchids to form a forest structured by three main strata (trees, shrubs and herbs). Impatiens napoensis flowers in May.

Examined materials: Vietnam. Ha Giang Province: Quan Ba District, Cao Ma Po Commune, Chin Chu Lin Village, 23°06′′ N, 104°48′ E, at elevations of 1 650-1 780 m a.s.l., May 14-15, 2017, Nguyen Sinh Khang & Truong Van Van, NSK 925, NSK 932, NSK 940.

Notes: When describing I. napoensis, Chen compared it with I. reptans. However, the differences of the two species are so obvious that they are hardly to be confused. Impatiens napoensis has pilose or puberulent stems 40-60 cm long, whitish purple to reddish purple flowers, ovate to broadly elliptic lateral sepals, ovoid anthers and linear ovary with incurved- rostrate apex, while I. reptans has glabrous stems 20 cm long, yellow flowers, falcate-ovate lateral sepals, acute anthers and fusiform ovary with erect apex^[16]. The species truly closely related to I. napoensis Y. L. Chen is I. chlorosepala Hand.-Mazz. on morphology^[12]. However, I. napoensis differs from I. chlorosepala by several characteristics such as its prostrate or procum- bent stem (vs. erect), pilose or puberulent stems and branchlets (vs. glabrous), densely dull white pilose leaf blade (vs. abaxially glabrous), lateral sepals with thickened mid-vein (vs. not thickened), suborbicular dorsal petals, (1.8)1.9-2.0(2.1) cm×(1.5)1.6-1.7 (1.8) cm (vs. orbicular, 0.8-1.2 cm), lateral united petals with upper petals obovate or pisiform (vs. suborbicular), apex short acute (vs. broadly obtuse or subrounded), filaments with obtuse apex (vs. dilated apex), and ovary linear (vs. fusiform), apex incurved- rostrate (vs. erect). Impatiens napoensis is also close to I. begoniifolia^[17], but it is distinguishable by the prostrate or procumbent stems 40-60 cm long (vs. erect, 10-20 cm long), leaves covered by dull white pilose on both surfaces (vs. glabrous), inflorescences with peduncles and pedicels densely pubescent (vs. glabrous), lateral sepals ovate to broadly elliptic, (8) 10-13(15) mm long (vs. subulate-lanceolate, 4 mm long), lower sepal broadly funnel-form, purple, 1.8- 2.1 cm in diam., 1.5-2.0 cm deep, with spur incurved (vs. navicular, 1.5 cm in diam., 1.1 cm deep, spur nearly straight), dorsal petals suborbicular, (1.8)1.9-2.0 (2.1) cm×(1.5)1.6-1.7(1.8) cm (vs. orbicular, 1.0 cm× 0.9 cm), lateral united petals with upper petals over- lapped on lower petals, obovate or pisiform, with apex short acute or obtuse (vs. not overlapped, apex rounded), lower petals dolabriform, apex obtuse (vs. ovate, apex rounded), and ovary linear (vs. fusiform).

We cordially thank authorities of Ha Giang Province for providing facilities to our field surveys. The authors are indebted to curators of Herbaria (HN, IBSC) for their kind help.