Scientific name: Eustigma honbaense H.Toyama, Tagane & V.S.DangEnglish name: Vietnamese name: Chân thư hòn bàOther name:
Trees 8–10 m tall. Branches yellow-green when young, turning brown when mature; indumentum sparsely to densely brown stellate hairy when young (denser on the uppermost branch), glabrescent; lenticels more distinct on young branches when dry. Terminal buds naked, narrowly ovoid, ca. 5 mm long, ca. 2 mm wide, having 2 opposite stipules incompletely covering young leaves, linear, ca. 4.5 mm long, ca. 0.5 mm wide, brown stellate hairy. Axillary buds scaled, narrowly ovoid, ca. 3.5 mm long, ca. 1.2 mm wide; bud scales 2, opposite, incompletely covering young leaves, narrowly ovate-oblong, 3–4 mm long, 0.5–1 mm wide, densely brown stellate hairy. Leaf blades elliptic to oblong, 6.5–21.5 cm long, 2.2–8.5 cm wide, coriaceous, lustrous and glabrous on both surfaces except veins below, base obtuse to cuneate, apex acuminate to rounded, margin entire; midribs sunken and glabrous above, prominent and sparsely brown stellate hairy below; secondary veins 5–9 pairs, slightly sunken and glabrous above, prominent and sparsely brown stellate hairy below; tertiary veins reticulate, slightly sunken and glabrous above, prominent and sparsely brown stellate hairy below; petiole 9–14 mm long, sparsely brown stellate hairy; stipules 2, opposite, caducous, linear, 5–8 mm long, 0.5–1 mm wide, brown stellate hairy. Flowers not seen. Infructescences terminal and axillary, racemose, 5–10.3 cm long; peduncles 1.2–4.5 cm long, brown stellate hairy, with 0–2 basal leaves; basal bracts not seen; bracts and bracteoles caducous, narrowly ovate, 1.8–3 mm long, 0.5–1 mm wide (but see note), densely brown stellate hairy; fruiting pedicels 5–8 mm long, densely brown stellate hairy. Floral cups in young fruits, turbinate, 2–3 mm in diameter, densely brown stellate hairy; ovary 2-locular; ovules 1 per locule. Capsules ovoid-globular, 12–16 mm long, 9–10 mm in diameter, woody, dehiscing loculicidally by two 2-lobed valves, sparsely lenticellate, sparsely brown stellate hairy, the length above tepal scar 6.5–9 mm; endocarp loose from woody exocarp. Seeds 2 per capsule, narrowly ovoid, ca. 10 mm long, ca. 4.5 mm wide, ca. 3 mm thick, brownish black, smooth, hilum ca. 4 mm long, ca. 3 mm wide for bigger side (the basal side on the placenta).
Distribution and habit
This species is only known from Hon Ba Nature Reserve of southern Vietnam. We found only three individuals in the evergreen forest along a stream at ca. 400 m alt.
Preliminary conservation status
Eustigma honbaense was collected from a single locality in the Hon Ba Nature Reserve, where only three individuals (one reproductive tree and two young trees) were found in evergreen forest along a streamside. The forest around the habitat was frequently logged and disturbed. Therefore, this species is assessed as Critically Endangered (CR) using the criterion D of the Red List Categories (IUCN 2012), although more individuals could be discovered by more thorough surveys.
Eustigma honbaense has terminal naked buds and axillary scaled buds that are in contrast with the description of Eustigma having naked-buds as given in the Flora of China (Zhang et al. 2003). The naked bud is covered either with immature leaves that develop to foliage leaves or with their stipules, while the scaled bud is covered with cataphylls or stipules that are highly modified to protect the shoot tip (Nitta and Ohsawa 1998). The terminal buds of E. honbaense are classified as naked buds because they are incompletely covered by 2 stipules (Fig. 4A, B, E & F), while the axillary buds are classified as scaled buds because they are covered by 2 cataphylls that are subsequently shed or remained at the tip of previous shoot without further growth (Fig. 4C, D, E & F). The terminal bud of Fig. 4B shows alternate leaf arrangement from the axillary bud, but same direction is also observed.