Impatiens horizontalis M. M. Latt, B. B. Park & Nob. Tanaka 2023, sp. nov.
Creators: name: Latt, Myo Min
name: Tanaka, Nobuyuki
name: Park, Byung Bae
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Plantae
Tracheophyta
Magnoliopsida
Ericales
Balsaminaceae
Impatiens
Impatiens horizontalis

description: Impatiens horizontalis M.M. Latt, B.B. Park & Nob. Tanaka, sp. nov. (Figs 4 & 5)
Type: — MYANMAR. Sagaing Region: Htamanthi Wildlife Sanctuary, Khamti Township, Khamthi District, ca. 173–196 m elevation, N 25 33 59 and E 95° 49’ 40” on 6 January 2021, Myo Min Latt MY5939 (holotype-RAF, isotype-TNS).
Diagnosis: Impatiens horizontalis is morphologically similar to Impatiens stenantha Hook. f. but can be distinguished by being horizontally creeping stem on the surface of gravel soil along the stream, elliptic to linear leaf, coriaceous, serrate margin with bristles, axillary inflorescence and pure golden yellow flower without marks; in spite of similar creeping habit, it is distinguished from I. prostrata Hook. f. by having narrowly elliptic to linear leaf, serrate, antrorse bristle between teeth, glandular stipulate, twisted spur with abruptly capitate tip, linear and glabrous ovary and spheroid, ex-arillate, glabrous seed; from I. repens Moon by having not succulent and stout stem and racemose inflorescence; from I. procumbens Franch and I. reptans Hook. f. having not erect stem and branches, narrowly elliptic to linear, glabrous leaf, glandular stipulate, oblong sepals, deeply bucciniform lower sepal, twisted spur with abruptly capitate tip, lanceolate, twisted, obtuse lower later petal, triangular upper lateral petal, linear ovary, oblanceolate—linear fruit (Fig-4&5).
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Rheophyte creeping on gravel stony island in the stream. Stem horizontally creeping, 1 m or longer, red, slender, glabrous, glandular on the streak line on stem, internode up to 5 cm long, 3 mm in diam.; roots from each node, glabrous, pink. Leaves alternate, petiolate 1–1.5 cm long, glandular stipulate, glabrous, coriaceous, narrowly elliptic to linear, 0.8–1 × 12–13.5 cm, margin serrate, dark purple antrorse bristle at the ends of lateral veins between the teeth, mid and lateral veins conspicuous on abaxial surface, tip acuminate, attenuate; petiole dull red, glabrous, up to 1.5 cm long. Inflorescence axillary, raceme, up to 8.5 cm long, 2–5 flowers, indefinite, peduncle glabrous, dull red, swollen at the base, tapering to tip, up to 5.5 cm long, 2.5 mm in diam. at base, 1 mm in diam. at top. Flower golden yellow, up to 4 cm long, up to 2.5 cm across; 2 floral bracts, elliptic, acuminate, 0.5 x 1.5mm, glabrous; pedicel up to 2 cm long, glabrous, dull red. Lateral sepals 2, persistent, dull red, oblong, 2 × 5 mm, acuminate, both surfaces glabrous, united at base each other. Lower sepal bucciniform, 3.5 cm long, 6 × 8 mm across the mouth, funnel 1.5 cm long, cuspidate, tongue at mouth, pointed, 3 × 3 mm, yellow, distal part gradually tapering into a twisted spur with curved end, dull red tip, capitate. United lateral petals connate, asymmetric, distorted and twisted, lobes free; lower petal golden yellow, red mark at base, like a pair of rabbit’s ears, knife shape, lanceolate, twisted, asymmetrical, 4 × 25 mm, margin undulate, obtuse, both surface glabrous, united with upper petal, veins clear; Upper petal golden yellow, liver shape or triangular, asymmetrical, 5 × 17 mm, margin undulate, acuminate, base united with base of lower petal, both surface glabrous. Dorsal petal golden yellow with red marks on abaxial surface, obovate to orbicular, 7 × 7 mm, round tip, tapering base, both surface glabrous. Androecium erect, yellow styles, 1 mm long, united base. Gynoecium ca. 3 mm long, yellow stigma. Ovary green, 2 mm long, 1 mm in diam. Fruit straight, slender, oblanceolate to linear, 5 lobed, tapering to base, swollen tip, green with dull red strikes, glabrous, 2.5 cm long, 3 mm in diameter. Seeds spheroid like chickpea, ex-arillate, glabrous, 2–3 seeds, occupy at top of fruit.
Habitat and ecology: Impatiens horizontalis horizontally grow and a creeper on gravel stony island frequently flooded along the perennial stream in evergreen forest and is light demander. It hardly survives on other types of soil.
Phenology: Flowering in January–February before summer season.
Distribution: Myanmar. Thus far, limited populations were found on less than 5 gravel islands only along the Nam Phet La stream in Htamanthi Wildlife Sanctuary.
Vernacular name: “Nam-phet-la Dan-pan”, meaning Dan-pan of Nam-pha-lar stream.
Note: Impatiens horizontalis is assignable to subgenus Impatiens sect. Racemosae Hook. f. based on its racemose inflorescence, linear capsule and 4 lateral sepals with 2 reduced inner ones. We proposed the local name of the species as “Nam-phet-la Dan-pan” because it was firstly found only along Nam Phet La stream in Htamanthi Wildlife Sanctuary.
IUCN Red Data Category: Impatiens horizontalis could be seriously threatened due to the gold mining along the stream of the habitat (Fig-3-E). Therefore, based on the latest IUCN criteria, Impatiens horizontalis is proposed to be Critically Endangered (CR) (IUCN, 2022).
Hooker (1872) noted that there were two varieties under I. stenantha and one of them perhaps a different species. I. horizontalis is also allied to I. prostrata Hook. f., I. repens Moon, I. procumbens Franch and I. reptans Hook. f.. However, I. horizontalis distinctly differ from all such four species by the key characters described in several literatures (Moon, 1824; Wight, 1840; Arechaveleta, et al., 1886; Franchet, 1887; Trimen, 1893; Hooker, 1908; Hooker, 1913; Wu, et al., 2007)(Table 2).
description: Published as part of Latt, Myo Min, Tanaka, Nobuyuki & Park, Byung Bae, 2023, Two New species of Impatiens (Balsaminaceae) from Myanmar, pp. 141-152 in Phytotaxa 583 (2) on pages 146-151, DOI: 10.11646/phytotaxa.583.2.2, http://zenodo.org/record/7609266