Description. Habit small evergreen trees, single- to multi-stemmed, 2.5 – 8 m tall, 10 – 60 cm GBH. Stilt roots much-branched, descending from the base of the stem, bearing numerous lenticels. Branches and branchlets decussate, growing upward at acute angles; branches often with pendulous aerial roots; branchlets terete, glabrous, with conspicuous annular stipular scars and leaf scars at the nodes. Bark grayish brown or dark brown, varies from smooth, rough or shallowly longitudinally and transversely fissured. Stipules interpetiolar, in pairs enclosing the young shoot (including the terminal bud and young leaves) and young inflorescences, pale green or green, turning whitish pale green or creamish white before the leaves emerging or before falling off, reddish brown when dry, linear-lanceolate, gradually narrowing towards the apex, 3.5 – 7.5 cm long, 2.5 – 9 mm diam. at the base, apex acute, glabrous and caducous, outline in transverse section depressed orbicular basally and suborbicular to orbicular apically, with dense colleters aggregated in a basal band, producing a sticky white exudate; colleters sessile, pale yellow, narrowly conical, 0.5 – 1.2 mm long, 0.2 – 0.5 mm diam. at the base, apex obtuse. Leaves decussate and crowded at the apical part of branchlets, elliptic, 8 – 16 × 3 – 8.5 cm, apex mucronate, a short terminal stiff point pale green, turning black before they come off, 2 – 6 mm long, base cuneate, margin entire, coriaceous, shiny dark green above, pale green below, glabrous on both surfaces, with numerous conspicuous, scattered tiny black cork warts below, midrib pale green (paler than lamina), flattened above, raised below, secondary veins 8 – 14 on each side, curving towards the margin and connected in distinct loops and united into an intramarginal vein, visible above, obscure below, with intersecondary veins, veinlets reticulate, visible above, obscure below; petioles green, 2 – 4.8 cm long, 2 – 4.5 mm diam., glabrous; fresh leaves brittle when crushed; young leaves shiny pale green; mature leaves turning greenish bright yellow and bright yellow before falling off; dry leaves yellowish brown. Inflorescences axillary, opposite, compound dichasia, dichotomously branched, 4 – 13 - flowered cymes, 5 – 12.5 cm long; peduncles pale green, 2.3 – 6 cm long, 2 – 3.5 mm diam., glabrous. Bracts 2, pale green, concave, 2.5 – 4 × 2.7 – 3.5 mm, apex rounded. Bracteoles at the base of the flower, pale green, connate at the base, bilobed, 3 – 5 × 4 – 6 mm. Flowers 4 - merous; flower buds pale green, turning pale yellow when mature, ovoid or conical-ovoid, 0.9 – 1.5 cm long, 4 – 8 mm diam., apex obtuse; fully open flowers 1.2 – 1.8 cm diam.; sepals 4, erect or patent after anthesis, pale yellow on both sides, triangular, 0.7 – 1.3 cm × 4 – 6 mm, apex acute, coriaceous, glabrous; petals 4, creamish white, narrowly elliptic or elliptic, 0.6 – 1 cm × 1.5 – 3.5 mm, thin, involute, densely long-villous along margins; stamens 8, 4 antesepalous and 4 antepetalous (each petal enclosing 1 stamen in flower buds); anthers falcate-like, 3.5 – 8.5 × 1.2 – 2.8 mm, apex mucronate, triangular in outline in transverse section; filaments very short; free part of the ovary 0.7 – 1.2 mm long, 1.2 – 1.8 mm diam.; style 3 – 5 mm long; stigma 2 - lobed; pedicels pale green, 0.6 – 1 cm long, 2 – 3 mm diam. Fruits brownish green, greenish brown or brown, ovoid or conical-ovoid, 2 – 2.7 cm long, 1 – 1.7 cm diam. at the base, apex obtuse (before seed germination); obpyriform, 2.2 – 4 cm long, 1.7 – 3 cm diam. at the basal part, 0.9 – 1.7 cm diam. at the apical part (when the hypocotyls nearly come off), roughened surface; persistent sepals reflexed, triangular, 0.9 – 1.1 cm × 5 – 8.5 mm; infructescence stalks 2 – 6 cm long, 2 – 3 mm diam.; fruit stalks 0.6 – 1.7 cm long, 3 – 5 mm diam. Seeds 1, viviparous. Hypocotyls green, slightly glossy, cylindrical-clavate, 19.5 – 40 cm long, 6.5 – 9 mm diam. at the apical part, 1 – 1.6 cm diam. at the widest part, 0.8 – 1.3 cm diam. at the basal part, acute at the basal end, roughened surface, with numerous, scattered lenticels; cotyledonous cylindrical tubes pale green or greenish pale yellow, 0.6 – 1.2 cm diam. (can be seen when the hypocotyls nearly falling off). The measurements of the vegetative and reproductive parts of Rhizophora stylosa in Thailand are presented in Table 1.
