THAILAND, Phulangka National Park, Ban Pheang District, Nakhon Phanom Province, 17°57.087'N, 104°09.425'E, alt. 170–240 m, 28 June 2012, W. Chatan 1337 (Holotype: BK; Isotypes: MSUT).

Bauhinia nakhonphanomensis is a tendrilled liana. It differs from other closely similar species by having entire leaves, acuminate or caudate leaf apices, oblong or elliptic floral buds, floral bud 25–35 mm long, raceme or panicle inflorescences, 10–13 mm long hypanthium, and anthers opening by longitudinal slits.

Large tendrilled liana climbing on shrubs or trees or big rocks. Branch glabrous; small young branches straight and the old ones flattened forming “Monkey-Ladders”. Leaves simple; lamina ovate, 7.0–14.5 × 4.0–8.0 cm, palmately netted venation with 5 large veins near the middle and 2 short and small ones marginally; margin entire; apex acuminate or caudate; base rounded to truncate or cordate; both surfaces glabrous excepted for hairs at base of the underside of lamina; young fresh leaves pinkish and green when old. Inflorescences raceme or panicle, terminal or leaf axial; axes greenish and glabrous near base, reddish and covered by densely reddish hairs near apex. Peduncles 35–40 mm long, glabescent. Floral buds oblong or elliptic, 5-ridged, 25–35 × 7–9 mm, apex twisted, reddish-green when fresh and brown when dry. Bracts 1, insert near pedicel base, ovate or lanceolate 8–9 × 4–5 mm, reddish when fresh and brown when dry, sparse minute hairs on abaxial side, dense hairs on adaxial side. Bracteoles 2, insert at the pedicel apex, orbicular or broadly ovate, 10–13 × 9–10 mm, dense reddish-green hairs on adaxial side when fresh and brown when dry, dense hairs on abaxial side when fresh and brown when dry, hairs caducous. Pedicels 28–40 mm long, densely covered with reddish hairs when fresh, the hairs change to be brown when dry. Hypanthium funnel-form, 10–13 mm long, striated. Sepals 5, connate forming an oblong or ellipsoid shape; 5-ridged floral buds, splitting into 5 separated and recurved sepals; each sepal linear, 15–20 × 2–3 mm, abaxial side densely hairy, adaxial side sparsely hairy near apex. Petals 5, pinkish, spatulate, acute to obtuse apex; expanded portion 25–32 × 10–12 mm, sparsely covered by whitish hairs on both surfaces; margin entire, densely hairy on upper part and sparsely hairy on lower part; petal claw 15–22 mm long. Stamens 9–10; fertile stamens 3, filament 55–60 mm long, whitish to pinkish, hairy on lower part and glabrous on upper part; anther pink, sparsely hairy and 5–6 mm long and opening by longitudinal slits; sterile stamens 6–7, filament 20–24 mm long, anther 2.3–2.5 mm long, hairy and opening by longitudinal slits. Pistil flattened, reddish, hairy on the two ridges extending from base to the top of style; stipe 15–17 mm; ovary fusiform, 10–12 mm long; styles 12–14 mm long; stigma capitate, approx. 1 mm diameter, glabrous. Fruit not seen.

flowering April–July and fruiting unknown.

This new species is an endemic to Thailand and known from only one location at Phulangka National Park, Ban Pheang District, Nakhon Phanom Province, Thailand.

This species grows in a rocky and dense dry evergreen forest at an elevation of 170–240 m. It climbs on small to tall shrubs, trees or on big stones. Some plants grow along the river.

Thao Khadailing.

Bauhinia nakhonphanomensis is named after the type locality Nakhon Phanom Province, the northeastern Thailand.

In Thailand, Bauhinia species can be divided into two groups based upon their habit. The first group is comprised of trees or shrubs, while the other is tendrilled climbers. Bauhinia nakhonphanomensis belongs to the latter, but is clearly distinct from the other tendrilled species in having entire leaves with acuminate or caudate leaf apices, oblong or elliptic floral buds, floral bud 25–35 mm long, raceme or panicle inflorescences, 10–13 mm long hypanthium, and the anther opening by longitudinal slits. When comparing the new species to the other Thai species, it seems to closely resemble Bauhinia concreta Craib, Bauhinia curtisii Prain, Bauhinia scandens L., Bauhinia strychnifolia Craib and Bauhinia tubicalyx Craib based on their tendrilled climber habit, entire leaves and anther opening by longitudinal slits. Bauhinia nakhonphanomensis is distinct from these species by having long floral buds (i.e. 25–35 mm) and longer pedicels (i.e. 28–40 mm), while Bauhinia concreta Craib, Bauhinia curtisii Prain, Bauhinia scandens L., Bauhinia strychnifolia Craib and Bauhinia tubicalyx Craib have 12–15 mm floral bud lengths and shorter pedicels (2–20 mm) (Larsen et al. 1984).

When comparing this new species to the entire leaf species of Bauhinia in Indo-China, it can be distinguished from the other species based on hypanthium lengths. The hypanthium lengthof Bauhinia nakhonphanomensis is between 10–13 mm, while Bauhinia clemensiorum Merrill has 20–25 mm hypanthium length. Of the other entire leaf species, Bauhinia calycima Pierre ex Gagnep., Bauhinia cardinalis Pierre ex Gagnep, Bauhinia championii (Bentham) Bentham, Bauhinia curtisii Prainand, and Bauhinia scandens L., all have a short to very short hypanthium less than 5 mm long (Larsen et al. 1980).

Bauhinia nakhonphanomensis closely resembles Bauhinia exurrens Stapf, known only from Mt Kinabalu Malaysia (Larsen and Larsen 1996). The two species are similar to each other by having entire leaves, anthers opening by longitudinal slits, short hypanthium (approx. 10 mm), petals not recurved and long pedicels more than 25 mm. Further differences between these two species is shown in Table 1.