Prota 7(2): Timbers/Bois d’œuvre 2
Engl. & Prantl, Nat. Pflanzenfam., II–IV Nachtr. 1: 161 (1897).
Isolona bruneelii De Wild. (1909), Isolona seretii De Wild. (1909).
Origin and geographic distribution
Isolona hexaloba occurs from eastern Ghana east to the Central African Republic, and south to DR Congo and the extreme northern part of Angola.
In DR Congo the wood is used for shields. Pieces of wood serve as torches. In traditional medicine the bark is used as purgative and bark decoctions are administered to treat abdominal pain, constipation and wounds.
Production and international trade
The timber of Isolona hexaloba is probably only used locally and not traded on the international market. The bark is regularly sold on local markets, e.g. in Yaoundé (Cameroon), where in 2000 the price was on average 1500 FCFA/kg.
The heartwood is creamy white, and indistinctly demarcated from the white sapwood, which becomes reddish upon exposure. The bisbenzylisoquinoline alkaloids curine and cycleanine have been isolated form the root bark. Both compounds showed significant trypanocidal activity in mice infected with strains of Trypanosoma cruzi, a protozoan causing Chagas’ disease. The sesquiterpene derivative cazolobine has also been isolated from the roots.
Medium-sized tree up to 30 m tall; bole branchless for up to 25 m, deeply fluted, up to 60(–100) cm in diameter; bark surface smooth to fissured, grey to greenish brown, inner bark fibrous, yellowish, becoming reddish upon exposure, with blackish outer layer and aromatic smell; twigs glabrous. Leaves alternate, simple and entire; stipules absent; petiole 2–4 mm long, grooved; blade ovate to elliptical, 7.5–30 cm × 2.5–11.5 cm, cuneate at base, acuminate at apex, leathery, glabrous, pinnately veined with 8–16 pairs of lateral veins. Flowers usually solitary in the leaf axils, bisexual, regular, 3-merous, pendulous, sweet-scented; pedicel 1–2.5(–3) cm long; sepals free, elliptical to triangular, 1–4 mm long, slightly hairy at margins; corolla tube 4–10 mm long, lobes 6, elliptical to ovate, 0.5–2.5 cm long, fleshy, spreading, glabrous, white or yellowish becoming dark red; stamens numerous, c. 2 mm long, anthers sessile; ovary superior, ovoid to ellipsoid, c. 1 mm long, 1-celled, glabrous, stigma head-shaped, 2-lobed. Fruit a broadly ovoid to conical, non-dehiscing capsule 3–8 cm × 2.5–5 cm, wall 3–4 mm thick, irregularly ribbed and lumpy, glabrous, pale green becoming purple-red when mature, many-seeded. Seeds ellipsoid, 8–15 mm × 4–6 mm, slightly rough, pale brown, with ruminate endosperm.
Flowers and fruits can be found throughout the year. Pollination of the flowers is probably by insects, possibly beetles. The fruits are possibly eaten by large mammals such as gorillas, chimpanzees and elephants, which may serve as seed dispersers.
Isolona comprises about 20 species and is limited to the forest zones of West, Central and East Africa, but it occurs also in Madagascar with 5 endemic species. It is most closely related to Monodora, which has the same fruit structure but differs in its larger, multi-coloured flowers with differentiated outer and inner corolla lobes.
Isolona congolana (De Wild. & T.Durand) Engl. & Diels is a small to medium-sized tree up to 20(–30) m tall with bole up to 45 cm in diameter, occurring from western Cameroon east to western Uganda. It seems related to Isolona hexaloba, but its twigs, young leaves and flowers are short-hairy. Its rather soft wood is used for joinery. In DR Congo an infusion of the inner bark is used as wash to treat eye complaints. Crushed seeds have been used as soap.
Isolona thonneri (de Wild. & T.Durand) Engl. & Diels is a small tree with bole up to 25 cm in diameter, occurring from southern Nigeria east to DR Congo. It is characterized by its long and narrow, glabrous corolla lobes. The wood is reportedly hard and used for construction and for making xylophones.
Isolona hexaloba occurs in evergreen and semi-deciduous forest, in primary as well as secondary forest, up to 700 m altitude. It is often found along rivers.
In Cameroon the mean number of Isolona hexaloba trees with boles above 15 cm diameter has been recorded as 1.3/ha, with a mean wood volume of 1.9 m³/ha.
Genetic resources and breeding
Isolona hexaloba is widespread in Central Africa and is locally common, also in protected areas. It is therefore considered not to be threatened by genetic erosion at present.
Isolona hexaloba is not considered a timber tree of commercial importance, and this will probably remain so because of the deeply fluted bole.
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Correct citation of this article:
Lemmens, R.H.M.J., 2009. Isolona hexaloba (Pierre) Engl. In: Lemmens, R.H.M.J., Louppe, D. & Oteng-Amoako, A.A. (Editors). Prota 7(2): Timbers/Bois d’œuvre 2. [CD-Rom]. PROTA, Wageningen, Netherlands.