Prota 7(1): Timbers/Bois duvre 1
Repert. Spec. Nov. Regni Veg. 2: 15 (1906).
Papilionaceae (Leguminosae - Papilionoideae, Fabaceae)
2n = 16, 18
Elephant wood, tree wisteria, Rhodesian wisteria tree (En). Glycine arbre (Fr).
Origin and geographic distribution
Bolusanthus speciosus occurs from Malawi and Zambia south to north-eastern South Africa and Swaziland. It has been introduced elsewhere, e.g. in Kenya, Uganda, India and Australia.
The wood is popular for the production of high-quality furniture, household articles and implements. The stems are used as fence posts. In Zimbabwe a leaf decoction is drunk to stop vomiting, whereas a root decoction is applied as emetic. In Malawi a root decoction is drunk to treat abdominal complaints. In South Africa the roots are used to treat stomach-ache and the dried inner bark to treat abdominal cramps. Bolusanthus speciosus is a beautiful ornamental and roadside tree, and it is a bee forage.
The heartwood is reddish brown, often streaked, and distinctly demarcated from the narrow, whitish yellow sapwood, that becomes grey-brown in the course of time. The wood is heavy, with a density of about 930 kg/m³ at 12% moisture content. It is hard, but turns well. It is durable, and resistant to termites and pinhole borers.
Flavonoids (several pterocarpans and many isoflavonoids) with antimicrobial activity against Bacillus subtilis, Candida mycoderma, Escherichia coli, Saccharomyces cerevisiae and Staphylococcus aureus have been isolated from the root and stem bark and the root wood. Flavonoids from the root wood also have shown antioxidant properties. Saponins with molluscicidal activity have also been isolated from Bolusanthus speciosus.
Small deciduous tree up to 12(18) m tall, frequently multistemmed; stems usually straight, sometimes branching low, up to 40(100) cm in diameter; bark deeply fissured, pale grey to dark grey or dark brown; crown narrow, dense, with upwards growing branches and drooping foliage. Leaves alternate, clustered at ends of twigs, pendent, imparipinnately compound with 37 pairs of leaflets, up to 28 cm long; stipules awl-shaped, minute; leaflets opposite, stalked, lanceolate, 48 cm Χ 12 cm, asymmetrical and slightly curved, tapering to a sharp point, margins minutely toothed, glabrescent. Inflorescence a terminal raceme up to 30 cm long, pendent, many-flowered. Flowers bisexual, papilionaceous, almost scentless; pedicel c. 1.5 cm long; calyx broadly campanulate, c. 8 mm long, 5-lobed; corolla c. 2 cm long, violet-blue with a white spot inside standard; stamens 10, free; ovary superior, oblong-linear, densely hairy, 1-celled, style curved. Fruit an oblong-linear pod 710 cm long, flattened, pale brown, later becoming grey or blackish, tardily dehiscent, 38-seeded. Seeds ovoid, flattened, c. 6 mm Χ 4 mm, smooth and shiny, yellow-brown.
Bolusanthus comprises a single species and appears related to Dicraeopetalum and Platycelyphium.
Growth rates of up to 80 cm/year have been recorded for Bolusanthus speciosus. The tree is deciduous for a short period. Under optimal conditions, young trees start to flower after 57 years. It flowers from August to January, but the flowering period of a single tree is short. The flowers are pollinated by bees. Wild animals including monkeys, giraffes and antelopes eat the pods and leaves and may disseminate the seeds. The roots develop nodules containing nitrogen-fixing bacteria.
Bolusanthus speciosus occurs in wooded grassland up to 1100 m altitude, in regions with an annual rainfall of 450700 mm. It can be found on a variety of soils, but is most common on heavy alkaline soils. It can survive periods of drought and moderate frost.
Seeds should be placed in hot water and soaked for about 12 hours before sowing. They can be sown in river sand and should be covered with a thin layer of finer sand. Germination starts 45 days after sowing, but it may take up to 30 days, and the germination rate of the seeds may be up to 95%. The seedlings can be transplanted into a soil mixture of equal parts of sand, loam and compost when the second leaf has emerged.
Genetic resources and breeding
Although Bolusanthus speciosus has a fairly large distribution area, it rarely occurs gregariously. It is protected in Swaziland, where it is considered seriously threatened because of overharvesting for building material.
Although the wood is highly valued, Bolusanthus speciosus will remain of limited importance as a timber tree because of its comparatively small size. It seems to have a good future as ornamental tree, having the splendour of Jacaranda mimosifolia D.Don from tropical regions and Wisteria spp. from temperate regions.
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Correct citation of this article:
Lemmens, R.H.M.J., 2007. Bolusanthus speciosus (Bolus) Harms. In: Louppe, D., Oteng-Amoako, A.A. & Brink, M. (Editors). Prota 7(1): Timbers/Bois duvre 1. [CD-Rom]. PROTA, Wageningen, Netherlands.