Bruguiera sexangula (Lour.) Poir.

Duke, N. C. & Ge, X. - J., 2011, Bruguiera (Rhizophoraceae) in the Indo-West Pacific: a morphometric assessment of hybridization within single-flowered taxa, Blumea 56 (1), pp. 36-48 : 46

4. Bruguiera sexangula (Lour.) Poir. — Fig. 5 ; Map 1

Etymology. ‘sex-angula’ means ‘six-angled’ (in Latin), and refers to the angular sides of the hypocotyl of this species. Common Australian name is ‘upriver orange mangrove’.

Tree or shrub to 15 m, evergreen, columnar or multi-stemmed, branching mostly sympodial, stem base with short, sinous buttresses, to 0.3 m high. Exposed root stilts often low on stem, pneumatophores knee-like, to 15 cm. Bark grey to pale-brown, with vertical fissures, with few corky lenticels to 1 cm diam. Foliage comprised of compact rosettes of paired leaves, clustered at 2 – 5 leaf scar nodes down from apical shoot, terminal, spicate, prominent, red-green, 5–11 cm long. Interpetiolar stipules paired, lanceolate, green to yellowish, occasional pinkish tinge, enclose terminal bud to 8 cm long. Leaves opposite, simple, blade elliptic-oblong, smooth, glossy green, 7–16(– 20) cm long, 3– 6(–7) cm wide, 4 – 9(–11) cm shape length, length/ width ratio 2.3 – 3.5, length/shape ratio 1.8 – 2.1, margin entire, apex acute, base cuneate; petiole green, 1.9 – 3.5 cm long. Inflorescence axillary, 1-flowered, buds generally nodding, maturing within leafy rosette; peduncle 4.4 –11(–12) mm long, 1.5– 2.5(–3.4) mm wide; mature buds present at 1–2 internodal segments below apical shoot; mature hypocotyls present at 3 – 6 internodal segments below apical shoot. Mature flower buds pink-orange to pale yellowish green, 27.6– 34 mm long, 3 – 8.3 mm wide around calyx tube, 6.8 –9.9 mm wide at sepal lobes, distil tip acute; calyx tube turbinate, ribbed, with 10–12 lobes, slender pointed, longer than tube, 15.6– 20 mm long, margins on closed bud raised; petals 10–12, creamy white, turning orange brown at anthesis, 13.2–16 mm long, 1.9 –2.6 mm closed width, bilateral folded, 3.4 – 4.6 mm open width, bilobed; lobes 3.6 – 6.4 mm long, densely fringed with hairs along outer margins, apices rounded with 0 –3 bristles near top, 0 –1.3 mm long, sinus between lobes with hair-like spine, 1.9– 4 mm long, not exceeding lobes, spine/petal lobe length ratio 0.4 –0.8; stamens 20– 24, creamy white turning orange brown at anthesis, 10.5–14 mm long, 0.1–0.5 mm wide, compressed pair within closed petal, dehiscing precociously when triggered, anthers linear, creamy pale yellow turning brown at anthesis, 4.1–6.2 mm long, 0.4–0.7 mm wide; style filiform, smooth, pale green, 16.8 –21 mm long, to 0.6 –1.5 mm wide, stigma minutely 3(– 4)-lobed at tip, mounted centrally within calyx bowl 2.2– 4 mm wide, 2.4 – 5.1 mm deep. Mature fruit cryptic within slightly enlarged calyx tube, turbinate, ribbed, 28.6–38 mm long, 4.1–17 mm wide, lobes reflexed; germination viviparous, hypocotyl emergent from calyx with maturation. Mature hypocotyl cigar-shaped, stout, green, 2 –11 cm long, 7–17 mm at widest point, 10– 62 mm shape length, 3 – 6 mm width at plumule end, length/width ratio 3.4 –7.8, length/shape ratio 1.5– 2.4, slight longitudinal ribbing, distal end blunt, plumule 0.1– 3.2 mm long, buoyant dispersal agent.

Phenology — In the southern hemisphere, notably Australia, flowering peaks in August and September, and maturation of propagules occurs in September to December. In the northern hemisphere, this is expected to differ by around six months.

Ecology and local influences — Bruguiera sexangula is usually found in upstream reaches of river-dominated estuaries in high rainfall areas, in mid intertidal, upstream estuarine positions. Other Bruguiera , especially B. gymnorhiza , dominate lower and intermediate sections of such estuaries. As with other Bruguiera , this species has a distinctive explosive pollen release mechanism. Honey-eaters frequently visit the flowers to gather nectar.

Distribution — Bruguiera sexangula is distributed from India to Asia, through the Indonesian Archipelago to New Caledonia and the northern coast of Australia. In Australia (Duke 2006), the species occurs in estuaries along the northern coastline from the Peter John River,Arnhem Bay, Northern Territory (12°15' S, 136°22'E) in the west,to the Herbert River, Queensland (18°31' S, 146°19' E) in the east.

Note — Bruguiera sexangula is distinguished by open flowers showing petals with no (or minute) bristles at lobe tips. Bruguiera sexangula is distinguished from other Bruguiera by: large single-flowered inflorescences and petals having a spine slightly shorter than the paired-lobes, as distinct from those of B. cylindrica , B. hainesii and B. parviflora ; and blunt petal lobes with single minute or absent bristles, and relatively short hypocotyls, as distinct from B. gymnorhiza and B. × rhynchopetala .

Figures

Map 1 Global distributions of the three large, single-flowered Bruguiera species, B. exaristata, B. gymnorhiza and B. sexangula. Note that B. sexangula was introduced to the Hawaiian islands (Allen et al. 2000).

Fig. 5 Bruguiera sexangula (Lour.) Poir. a. Habit, Daintree River, Queensland, Australia; b. bark, Hainan, China; c. trunk and prop-like buttresses; d. foliage; e. leafy rosette with flowers; f. closed mature flower buds; g. open mature flower buds showing petals in one; h. diagram of open petal; i. immature fruit; j. mature hypocotyl. — Scale bars: a–h = 10 mm.