Sindora stipitata is very similar to S. leiocarpa from Malesia, but it is easily distinguished by the following characters: a smaller stature (3-5 m high), 6-foliolate paripinnate leaves, falcate persistent stipules, presence of a petal auricle, absence of a petal claw, stipitate ovary and capitate stigma.

Figure 1. 

Sindora stipitata. A habit and habitat B branches and inflorescences C branch with leaves and stipules D branch with fruits. Photographs of the type specimen by W. Chatan.

Figure 2. 

Line drawing of Sindora stipitata Chatan & Promprom, sp. nov. A a branch with leaves and inflorescence B floral bud C floret D posterior sepal (abaxial side) E posterior sepal (adaxial side) F one of the remaining narrower sepal (abaxial side) G one of the remaining narrower sepal (adaxial side) H petal (adaxial side) I free staminode J fused stamen K pistil. Illustration by W. Chatan (based on type specimen).

THAILAND. Nakhon Phanom Province: Phu Langka National Park, elevation 250–350 m, 17°59'18.7"N 104°07'50.1"E (Fig. 3), 20 April 2012, W. Chatan 1231 (Holotype: BKF!; Isotype: K!).

Figure 3. 

Distribution of Sindora stipitata Chatan & Promprom (*) in Phu Langka National Park, Nakhon Phanom Province, Thailand.

Small tree, 3–5 m high. Stem diameter 3–5 cm. Stipules falcate, 23–25 × 1.0–1.3 mm, coriaceous, glabrous on both surfaces, persistent, venation distinctly reticulate. Leaves 6-foliolate, paripinnate, leaflets opposite; petioles 3.0–5.5 cm long, with sparse minute hairs or glabrescent; rachis 7–11 cm long, with sparse minute hairs or glabrescent. Leaflets rigidly coriaceous, elliptic or ovate or obovate or oblong, 7.5–11.5 × 3.5–5.3 cm; apex obtuse or sometime shallowly emarginate; base obtuse or cuneate, slightly asymmetric, glabrous on both surfaces, sometimes with a few minute hairs on the blade margin; abaxial side dull. Petiolules 3–6 mm long; glabrescent or with sparse minute hairs. Inflorescences paniculate, lax, up to 20 cm long, lateral branches up to 6 cm long, both rachises and lateral braches densely pubescent; bracts and bracteoles ca. 4.5 × 1.5 mm, puberulous; pedicels 11–12 mm long, densely puberulous. Buds ca. 15 × 10 mm, elliptic. Pedicel 10–11 mm long, densely puberulous. Hypanthium asymmetrically short and funnel-like, 0.5–1.0 mm long, brownish-yellow. Calyx lobes 4, thick, brownish-yellow, without any spiny outgrowth on the outer surface, densely puberulous outside, appressed hairs inside; the posterior lobe boat-shaped and obovate, 14–20 × 10–11 mm, apices acuminate; the other three lobes narrower, 15–20 × 4–6 mm, narrowly oblong or elliptic, apices narrowly acute. Corolla comprised of a solitary boat–shaped petal, lanceolate or narrowly obovate, thick, 15–17 × 5–6 mm, apices acute or acuminate, bases auriculate, puberulous outside, glabrous inside. Stamens 10, dorsifixed; upper stamen free, staminodal, 10–11 mm long, lower 2/3 of filament length densely puberulous, upper 1/3 of filament length sparsely hairy and glabrous near apex; remaining nine stamens, fertile, joined at the base into a sheath; sheath ca. 8–9 × 4.0–4.5 mm, densely puberulous on both surfaces; the two largest stamens are fertile, free parts of filament ca. 22–25 mm, lower half of the free parts of filament sparsely hairy and the upper half glabrous; anthers 5.5–6.0 × 1.5–1.6 mm, glabrous; other fertile stamens seven, free parts of filament 7–8 mm, lower 1/3 of the free parts densely puberulous, upper 2/3 of the free parts sparsely hairy and glabrous near apex; anthers, 2.5–3.0 × 1.0–1.2 mm, glabrous. Pistils with 5–6 mm stipe length; stipes densely puberulous; ovary asymmetrically elliptic; 7–8 × 4.5–5.0 mm, densely puberulous, no spiny outgrowth; style 9–11 mm long, densely puberulous on lower parts and sparsely hairy on the anterior side of upper parts; stigma capitate, glabrous. Pods circular or ellipsoid to obovate, 3.3–9.0 × 3.5–5.5 cm diameter, flattened, slightly smooth, unarmed, obscurely veined, with 1.5–5.0 mm long beaks, sparsely hairy on both surfaces, densely appressed hairs on the margin; seeds 1–4.

THAILAND. Nakhon Phanom Province: Phu Langka National Park, 29 June 2013, fruiting, P. Saisaard 55 (BKF).

Flowering in March–May and fruiting April–June.

The new species is a Thai endemic and is known from only the type locality in the Phulangka National Park, Ban Pheang District, Nakhon Phanom Province, North-eastern Thailand.

This new species grows in open areas of dry deciduous forest at an elevation of 250–350 m.

Ma Kha Tae Nakhon Phanom, Mak Tae.

The specific epithet refers to its distinctly long ovary stipe. This character is one of many morphological characters that distinguishes the new species from its closely related species.

Sindora stipitata is known only from the type locality and its estimated extent of occurrence is less than 100 km². The number of mature individuals was less than 1,000 and the occupied area is continuing to decline slightly. Therefore, it should be considered as “Critically Endangered” according to the IUCN criteria B1 (IUCN 2017).

Sindora stipitata is closely related to S. leiocarpa, a plant that grows in Sumatra (Jimbi, Palembang and Riau) and Borneo (Sarawak, Brunei, Sabah and Kalimantan) (Hou 2000), but is easily distinguished from the latter by several morphological characters. Details of the differences between S. stipitata and S. leiocarpa are presented in Table 1.

The new species is also related to S. coriacea (Baker) Prain. Both have unarmed sepals, but S. stipitata is clearly different from S. coriacea by its stature of a small tree (3–5 m high) (vs. a large tree up to 50 m high), its abaxial leaflet surface dull brown (vs. shining), the two largest stamen 22–25 mm long (vs. ca. 10 mm long) and anthers 5.5–6 mm long (vs. 2–3 mm long). The new species also clearly differs from S. laotica Gagnep., a species that is distributed near the border of Thailand (in Vientiane, Phou Khao Khouay National Biodiversity Conservation Area, Lao) and Vietnam (Larsen, Larsen and Vidal 1980); the two species are different in that there is no spiny outgrowth on the outer surface of the calyx of the new species (vs. calyx spinescent). The following identification key for Sindora in Thailand and Malesia is constructed by modification of the key from the Flora Malesiana (Hou 2000).