Acacia nilotica (PROTA)
Acacia nilotica (L.) Willd. ex Delile
- Protologue: Descr. Egypte, Hist. nat.: 79 (1813).
- Family: Mimosaceae (Leguminosae - Mimosoideae)
- Chromosome number: 2n = 52, 104, 208
- Acacia arabica (Lam.) Willd. (1806),
- Acacia scorpioides (L.) W.Wight (1905).
- Babul acacia (trade name), scented thorn, scented-pod acacia (En).
- Babla, gommier rouge, nebneb (Fr).
- Tchanga, goma da Índia (Po).
- Mjungu, msemehi, mgunga, mtetewe (Sw).
Origin and geographic distribution
Acacia nilotica is native to the drylands of tropical Africa and western Asia, eastwards as far as India, Myanmar and Sri Lanka. In Africa it occurs from Senegal to Egypt and southwards through eastern Africa to Mozambique and South Africa (Natal) and the Indian Ocean islands. It has been distributed throughout the tropics and became naturalized in many areas, including Cape Verde, Jamaica, Nepal, Indonesia, Vietnam and Australia. It is widely cultivated in the Indian subcontinent.
The pods of Acacia nilotica are used traditionally in Nigeria and other sub-Saharan countries for tanning leather and as a source of khaki-to-brown dyes if used without mordant, or grey and black dyes for cotton combined with a mordant of iron-rich mud. The latter process gives the typical dark grey stripes in the ‘hile’, ‘sampak’ and ‘siole’ traditional cotton textiles of Cameroon and in the ‘langtang’ traditional cotton weaving of the Mumuye of eastern Nigeria. These textiles were locally and regionally used as currency well into the 20th century and still have a great cultural importance. Now they are mostly used as presents from a man to his future wife and in major religious festivals, clan gatherings and funerals. Black dyes combining tannins most probably obtained from ‘sant’ (acacia pods) with iron mordants have been identified in ancient Egyptian textiles dating from the 18th Dynasty ( 1542–1305 BC) onwards. The dried mature pods used in local tanneries in Sudan produce a pinkish white leather of good quality. The tan stuff extracted from the pods is internationally called by its Hausa name: ‘bagaruwa’.
In contrast in India and Pakistan it is the bark, a by-product from timber plantations, that is used for tanning and dyeing leather. The tannin produces a heavy leather which is firm, durable and hard, but combined with myrobalans (from Terminalia species) it produces excellent leather. Bark tannin is used especially in Bengal and Bangladesh for dyeing cotton and wool into various shades of brown and into black by combining it with a mordant of iron sulphide or iron-rich mud.
Acacia nilotica is a truly multipurpose tree, widely used as a timber, source of fodder, tannin and gum, and as a fence, shade and fuel tree. The dark brown heartwood is nearly twice as hard as teak and very shock resistant, and is used widely in constructions, for railway sleepers, mine props, tool handles and carts. The trees make effective live fencing, are a good host for the semi-parasitic sandalwood (Santalum album L.) and are important trees for lac insects (shellac) in the Indian subcontinent. Gum collected from the trunk and branches was formerly used in paints and medicines. It has properties similar to true gum arabic from Acacia senegal (L.) Willd. and is frequently used in calico printing and dyeing as a thickening agent. It is also used as sizing material for silk and cotton, and in paper manufacture in India.
The leaves and pods are an excellent fodder, rich in protein. The flowers yield a honey of good quality. The Hausa people use roasted seeds as a food flavouring. In Tanzania the inner bark and the thick fruit pulp are boiled in water and drunk as a tea. In eastern Java (Indonesia) sprouted seeds are consumed as a vegetable, and well-roasted seeds are mixed with coffee. The bark of slender branches yields a fibre which is used for the manufacture of paper or is made into coarse ropes in India. In Africa twigs are used as a toothbrush. Acacia nilotica is also a popular ornamental avenue tree in India. The tannins contribute to its many medicinal uses, the plant acting as a powerful astringent. A decoction of the fruit is considered a febrifuge, and the seeds have antimalarial, antidiabetic, antihypertensive and antispasmodic activities. The pods have molluscicidal and algicidal properties, and are added to ponds in Sudan to kill snail species that carry schistosomiasis without affecting the fish. The bark, gum, leaves and pods are used in many traditional medicines, possessing anti-inflammatory properties.
Production and international trade
In Africa production and trade in tannin, timber and other products of Acacia nilotica is important locally and nationally, but few products enter international trade to any extent and recent statistics for domestic consumption and exports are lacking. In India annual production of babul acacia bark for tannin was estimated in 1994 at 22,000 t, worth 1.2 million US$.
In India tannin contents of entire pods of Acacia nilotica vary from 12–19% and from 18–27% after removal of the seeds. In Sudan de-seeded pods can reach tannin contents of up to 50%. Seeds are usually removed because of their high content of sugar-like components which tend to cause the tannin liquor to ferment. The pods produce a light-coloured, soft leather, ideal for book binding. Tannins from fallen pods are hydrolyzed in the soil to gallic and ellagic acids which are allelopathic to the herb layer. Two highly active flavanol molluscicides, epigallocatechin-7-gallate and epigallocatechin-5,7-digallate, have been isolated from pods. Bark of babul acacia constitutes one of the most important tanning materials of northern India; tannin content varies from 7–20% on a dry weight basis, with a tannin/non-tannin ratio of 1.5. The tannins, which belong to the proanthocyanidin type, increase in content in the bark as the trees age. The bark also contains colouring matters of the ‘phlobaphene reds’ type, giving a dark colour to the leather. Improved results for tanning leather are obtained by mixing bark with myrobalans (two parts to one) or controlling the pH of the tan liquor.
Tests in Kenya showed that the tannin contents of Acacia nilotica bark and bark extract are low (11.5–13% and 49.5–55%, respectively) in comparison to Acacia mearnsii De Wild. The tannins are less astringent and weakly binding, and penetrate hide rather extensively, leading to a comparatively soft product, which may however be brittle and non durable. Mixtures of an Acacia nilotica bark extract with the highly astringent bark extract of Acacia senegal in a ratio 3 : 1 give good results in tanning and may be used as a substitute for Acacia mearnsii bark extracts.
The heartwood is pale red to pinkish brown, often darkening upon exposure, and distinctly demarcated from the yellowish white sapwood. The wood is strong, tough and heavy, with a density of 650–830 kg/m3 at 15% moisture content, but a density of 1170 kg/m3 has been reported for wood of South African origin. At 15% moisture content, the modulus of rupture is 88 N/mm2, modulus of elasticity 11,060 N/mm2 and the compression parallel to grain 52.5 N/mm2. The wood has a high content of silica which rapidly blunts cutting tools but is very durable if seasoned well. The wood is an excellent fuel, the energy value of moisture free heartwood being 20,710 kJ/kg. It yields a high grade charcoal with an energy value of 27,950–28,600 kJ/kg.
The nutritional composition of the leaves on dry weight basis is approximately: crude protein 13.9%, crude fibre 9.2%, N-free extract 69.8%, ash 7–10%, Ca 2.6%, Mg 0.4% and P 0.1% and their palatability is good. The pods are also used as fodder containing about 11–16% crude protein and 8–21% crude fibre, even though they also contain tannins. The seeds have low levels of the proteinase inhibitors trypsin and chymotrypsin.
The gum of Acacia nilotica varies in colour from very pale yellowish brown to dark reddish brown depending on the quantity of tannins in the sample. The lighter, more highly valued gums are soluble in water and very viscous; the tannins in the darker gum reduce the solubility. The gum has a moisture content of about 13% and is slightly dextrorotary. It is composed of galactoaraban which gives on hydrolysis L-arabinose, D-galactose, L-rhamnose, D-glucuronic acid and 4-O-methyl- D-glucuronic acid.
- Single-stemmed, evergreen or deciduous tree up to 15(–25) m tall; root system deep and extensive; trunk straight, up to 100 cm in diameter; bark rough and longitudinally fissured, 1–1.5 cm thick, grey to brownish black, younger stems grey-brown and smooth; crown flattened or rounded; branchlets with paired stipular spines 1–5 cm long, straight but usually characteristically directed downwards.
- Leaves alternate, bipinnately compound, with 2–14 pairs of pinnae; petiole 0.5–2.5 cm long; rachis 1–10 cm long, with glands at the base of each pair of pinnae or top few pairs only; leaflets in 7–36 pairs per pinna, elliptical or narrowly oblong, 1.5–7 mm × 0.5–2 mm, rounded and oblique at base, apex obtuse, glabrous to pubescent.
- Inflorescence a globose head, 1–6 per leaf axil, with a pair of small bracts in basal half of peduncle; head c. 50-flowered.
- Flowers bisexual or male, 4–6-merous, bright or golden-yellow, sweetly scented; calyx lobes 1–2 mm long, glabrous or pubescent; corolla lobes 2.5–3.5 mm long, glabrous or pubescent; stamens numerous, free, up to 6 mm long, glandular; ovary superior, 1-celled, style long and slender.
- Fruit an oblong to linear, flattened pod, 4–22 cm × 1–2 cm, straight or curved, margins entire or deeply constricted between the seeds, the position of each seed clearly marked by a distinct raised bump in the pod valves, dark brown to grey, glabrous or velvety, indehiscent, 6–17-seeded.
- Seeds elliptical to circular in outline, flattened, 6.5–9 mm × 5–8 mm, dark brown to brownish black.
- Seedling with epigeal germination; cotyledons circular-ovate, stalked.
Other botanical information
Acacia is a large pantropical genus, comprising more than 1300 species; most of them are found in Australia (more than 900), more than 200 in America, and about 130 in Africa. Acacia nilotica belongs to subgenus Acacia, which accommodates all the African Acacia species with straight spinescent stipules. Nine subspecies are distinguished in Acacia nilotica separated by pod and twig characteristics and tree shape. In tropical Africa the following 7 subspecies occur:
- subsp. adstringens (Schumach. & Thonn.) Roberty (synonym: Acacia adansonii Guill. & Perr.): young branchlets densely tomentose; pods not necklace-like, 13–21 mm wide, margins distinctly, often irregularly crenate, surface densely tomentose. In Africa from Senegal and Gambia to Somalia; also in western Asia to India.
- subsp. indica (Benth.) Brenan: similar to subsp. nilotica but the pods densely white-hairy. In Africa naturalized in Ethiopia, Somalia, Kenya, Tanzania and Angola; indigenous from Yemen to India and Myanmar, cultivated in Iran and Vietnam, and naturalized in Australia.
- subsp. kraussiana (Benth.) Brenan: similar to subsp. adstringens but pods with margins shallowly crenate, pubescent all over at first but the raised part over the seeds becoming glabrescent and shiny black when dry. From Angola and Tanzania to South Africa.
- subsp. leiocarpa Brenan: young branchlets glabrous to shortly hairy; pods not necklace-like, narrow (10–13 mm wide), margins straight or slightly crenate, almost glabrous. Restricted to coastal eastern Africa from Ethiopia southwards to Mozambique.
- subsp. nilotica: young branchlets glabrous to shortly hairy; pods necklace-like, narrowly and regularly constricted between the seeds, almost glabrous. From Mali and Burkina Faso to Sudan and Egypt; it has been cultivated in Tanzania, Iraq and the Arabian peninsula.
- subsp. subalata (Vatke) Brenan (synonym: Acacia subalata Vatke): similar to subsp. adstringens but pods with straight or only slightly crenate margins. In Africa in Sudan, Ethiopia, Somalia, Kenya, Uganda, Tanzania and Madagascar; possibly also in Pakistan, India and Sri Lanka.
- subsp. tomentosa (Benth.) Brenan: similar to subsp. indica but also young branchlets densely white hairy. Restricted to Africa, from Mauritania and Senegal to Sudan, Ethiopia and Egypt.
Subspecies with necklace-like pods (subsp. indica, nilotica, tomentosa) tend to be tall riverine trees or occur in seasonally flooded areas, while those with straighter margins to the pods (subsp. adstringens, kraussiana, leiocarpa, subalata) occur in drier sites. Occasionally intermediates between the subspecies are found and several questions about the pattern of variation within Acacia nilotica are still unsolved.
Wood-anatomical description (IAWA hardwood codes):
- Growth rings: 2: growth ring boundaries indistinct or absent.
- Vessels: 5: wood diffuse-porous; 13: simple perforation plates; 22: intervessel pits alternate; 23: shape of alternate pits polygonal; 26: intervessel pits medium (7–10 µm); 29: vestured pits; 30: vessel-ray pits with distinct borders; similar to intervessel pits in size and shape throughout the ray cell; 42: mean tangential diameter of vessel lumina 100–200 µm; 46: = 5 vessels per square millimetre; 47: 5–20 vessels per square millimetre; 58: gums and other deposits in heartwood vessels.
- Tracheids and fibres: 61: fibres with simple to minutely bordered pits; 66: non-septate fibres present; 69: fibres thin- to thick-walled; 70: fibres very thick-walled.
- Axial parenchyma: 79: axial parenchyma vasicentric; 80: axial parenchyma aliform; (81: axial parenchyma lozenge-aliform); (83: axial parenchyma confluent); (89: axial parenchyma in marginal or in seemingly marginal bands); (90: fusiform parenchyma cells); 91: two cells per parenchyma strand; 92: four (3–4) cells per parenchyma strand.
- Rays: 98: larger rays commonly 4- to 10-seriate; 104: all ray cells procumbent; 115: 4–12 rays per mm.
- Mineral inclusions: 136: prismatic crystals present; 142: prismatic crystals in chambered axial parenchyma cells.
Growth and development
Acacia nilotica is a pioneer species. A deep and extensive root system is formed on dry sites, the taproot developing first and then the laterals, which become compact and massive with age. On flooded sites however, the root system is largely lateral. Acacia nilotica nodulates and fixes nitrogen throughout its natural range and forms mycorrhizal associations with Glomus spp. It produces root suckers on rare occasions only. Acacia nilotica flowers at a relatively young age, around 3–4-years old under ideal conditions. Flowering is prolific and occurs on current-season growth mainly during the rainy season. If adequate moisture is available flowering can occur a number of times during the year.
Acacia nilotica is a tropical species, growing where average annual temperatures range from 15–28°C, being frost sensitive when young and withstanding daily maximum temperatures of 50°C. The mean maximum temperature of the hottest month is 25–42ºC and the mean minimum temperature of the coldest month 6–23°C. It occurs from sea-level to over 2000 m altitude. Acacia nilotica prefers dry conditions, with an annual rainfall of (100–) 250–1500(–2300) mm, however the extremes are only found under irrigation or where it is planted outside its natural range, e.g. in South-East Asia. In India the optimum lower rainfall limit is around 600 mm without irrigation. It occurs in both unimodal and bimodal rainfall regions with summer or winter regimes. Depending on the subspecies, it will tolerate both drought and flooded conditions for several months.
Acacia nilotica in Africa exhibits two very distinct ecological preferences. Subsp. adstringens, leiocarpa and subalata occur in wooded grassland, savanna and dry scrub forests on deep sandy to loamy soils, and also on lateritic and calcareous sites. Subsp. kraussiana also prefers dry grasslands and savannas, especially on compacted sandy loam, shallow granite or clay soils along drainage lines and rivers, but away from flooding. On the other hand, subsp. nilotica and tomentosa are restricted to riverine habitats and seasonally flooded areas on alluvial clay soils.
In the Indian subcontinent, subsp. indica occurs in dry forests at low altitudes, usually on alluvial soils subject to flooding or on black cotton soils. It is now widely planted on farms throughout the plains and will also grow on saline, sodic or alkaline soils and on soils with calcareous pans.
Propagation and planting
Acacia nilotica can be easily propagated by seed. Trees produce seed in abundance from around 5–7-years old. Seeds can be extracted from the pods while seeds ejected by sheep during rumination or those from cattle and goat droppings may be collected. The latter seeds germinate easily due to fermentation and moistening. Seed weight varies from 6, 600–11,600 seeds per kg. Germination capacity varies from 50–90%. Germination starts 1–3 weeks after sowing and is mostly complete in one month. Pre-sowing treatment is required once seed exhibits dormancy as the seed coat is very hard. Seed may be immersed in hot water (80°C) for about 30 minutes or in boiling water for 10–15 seconds and then soaked for 24 hours in cold water before sowing. Germination can also be improved by mechanical scarification, by soaking in concentrated sulphuric acid (90%) for 10–30 minutes followed by rinsing in cold water, and by fermentation in moist cow dung for 48 hours. Growth is rapid, and when planted in polytubes seedlings reach a height of 25–30 cm in 4 months. Acacia nilotica is selective in its association with Rhizobium and forms effective root nodules with only a few strains. Seed mixed with Rhizobium strain ANM 18 in a slurry and then sown in polybags has shown significantly increased seedling growth. Direct sowing in the field is the most common method of planting. Sowing in line with a seed rate of 1 kg/ha is recommended. For transplanted seedlings a spacing of 3 m × 3 m is common; when planted for the production of tannin and gum, a spacing of 4 m × 4 m is recommended.
Plantations of Acacia nilotica for timber, tannin or gum production require regular weeding and thinning to maintain maximum growth. Along the Blue Nile in Sudan, thinning begins from 4–5 years and continues in 3-year cycles. An ideal stocking density at 30 years is 100 trees/ha for 28 m tall trees and 400 trees/ha for 14 m tall ones. In old stands in Sudan, used for fuel wood extraction, there is evidence of adequate natural thinning and additional thinning seems not justified. Plantations in Sindh and Punjab (India) are irrigated at 3–7 day intervals after sowing, and 2–3 weedings are sufficient to control weed growth. Spot sowings with 10–15 seeds per hole did not require weeding, but needed early thinning and then thinning at 5-yearly intervals. For 5-year-old stands a density of 1200 trees/ha is recommended. Trees pollard well and can be freely lopped for thorn fences; however heavy lopping reduces tree growth. Use of NPK fertilizers did not prove economically worthwhile in plantations grown on a 20–25 year rotation.
Diseases and pests
Several fungal diseases and insect pests attack Acacia nilotica but none limits its cultivation. Fomes badius and Ganoderma lucidum are the most damaging fungi, causing spongy rot, affecting the heartwood of the stem and branches of old trees. Removal of infected trees and sporophores, and improved soil aeration provide some protection. In India the beetles Celosterna scabrator and Psiloptera fastuosa are the most destructive insects. Celosterna scabrator is a root and stem borer, feeding on young bark. In Maharashtra, larvae are hand collected to reduce damage. Psiloptera fastuosa defoliates and gnaws the bark of thin twigs. Larvae of Ascotis sp., Cusiala sp., Hyposidra sp., Pteroma sp. and Semiothesa sp. defoliate the tree, larvae of Argyroploce illepida bore into the pods, grubs and adults of the weevil Hypolixus truncatulus damage seedlings and saplings. Seeds are often predated by bruchid beetles, that may destroy up to 70% of them. A dieback disease in Sudanese forests was due to a buprestid beetle. Powder-post beetles (Sinoxylon spp. and Lyctus africanus) attack the sapwood of felled timber. Wild animals, camels and goats can cause significant damage to the trees although light grazing by sheep may help to remove competing grass.
Plantations of Acacia nilotica planted for timber and fuel wood production are usually harvested on a 10–20 year rotation with the bark as a useful by-product. In India the bark is separated from the logs by beating them with mallets. The strips are then sun-dried, chopped into small chips before being sent to tanneries. Bark from trees up to 10 years old yields a lighter tannin which is preferred in Pakistan. In sub-Saharan Africa, pods are collected for tanning, preferably from the tree and soon after turning black, to avoid mineral contamination from soil. Pods can be harvested at different ages to vary the colour of the tanned leather. In Sudan pods that have been lightly ground to remove seeds and fibrous matter, yield up to 60% tannin. For harvesting the gum, strips of bark 5–7.5 cm wide are removed from the trunk in the dry season, the bark around the cut is bruised with a hammer, and gum nodules form where the trunk has been damaged. Gum dries on contact with air and is collected a few days later.
Productivities of plantations and natural stands of Acacia nilotica vary considerably with site conditions. Growth rate is relatively fast, with average timber yields for plantations on dry sites of 3–5 m3/ha per year using a 15–20 year rotation with 700–1000 trees/ha (10–12 m tall). Maximum yields of up to 9 m3/ha per year 10–15 years after planting have been obtained in Rajasthan and Uttar Pradesh (India). In the riverine forests of Sindh (Pakistan) a maximum mean annual increment of 13 m3/ha at 20 years, and 10.5 m3/ha at 30 years has been recorded. Estimates of standing biomass range from 36 t/ha for 5-year-old plantations to 307 t/ha for 17-year-old plantations in Haryana (India). In India 10-year-old trees yield about 35–40 kg bark, or about 6 t/ha, and plantations of about 600 trees/ha produce 12 t bark 15 years after planting. Average annual pod yield from the plantations is 8–10 t/ha. In Sudan individual trees yield 18 kg pods per year. Young trees in India yield annually 0.1–0.6 kg gum per tree, older trees yield less.
Handling after harvest
Both pods and bark of Acacia nilotica are usually sun dried before being transported. To increase the tannin yield of the pods, seeds and fibrous matter are removed from them. Seeds may also provoke fermentation of the tannin due to their sugar content. In Sudan the traditional process for preparing leather is carried out in pots or waterproof pits in the ground of about 1 m in depth and diameter. Hides are first soaked in a river until they are completely clean, rehydrated and soft. Then they are plunged into a lime bath to destroy or soften the epidermis whereby also the hair or wool is loosened, and to destroy loose meat, sweat glands and blood vessels in the skin to facilitate the absorption of the tanning substances. Fresh hides are generally washed for about 2 hours, dry-salted hides 12–24 hours and flint-dried hides a little longer. In the bath the skins are stirred at least once a day. After this bath the skins are steeped in pigeon or dog dung for about 24 hours and then enter a tan bath with a solution prepared from babul pods (about 15 kg pods in 40 l water tan about 30 hides). This tanning takes 24–48 hours and the skins are stirred 4–5 times per day. After tanning the skins are first steeped in a saturated salt solution and then dried over a beam. The skins are softened by rubbing with a smooth stone or piece of wood. After drying the leather may be dyed. In Djibouti hides of goats to be used as water containers are tanned by simply filling the skins with crushed wet pods for 48 hours. Among the Hausa people in Nigeria, tanned hides are dyed into ‘kuloko’ black with a mixture of uncrushed ‘sant’ pods (Acacia nilotica) and blacksmith’s iron refuse first steeped in water and left for several days to ferment and then concentrated by boiling and thickened with honey or sugar. The dye is applied with a piece of leather over the whole surface of the skin or in stencilled patterns. In Malawi dry pods are pounded and mixed with black ferruginous mud plastered on cloth to dye it black.
Acacia nilotica is very widespread and is not threatened. Subsp. hemispherica Ali & Faruqi however, endemic to a restricted area in Pakistan, could be under threat of extinction. Acacia nilotica is among a number of species selected by FAO/IBPGR in the late 1970s for exploration and conservation in arid zones. Some international collections were made from relatively few sites. Range-wide collections in Africa have been undertaken by the Oxford Forestry Institute (OFI, United Kingdom) and members of the African Acacia trials network (comprising OFI, CIRAD-Forêt and the Danida Forest Seed Centre-DFSC), co-coordinated by FAO. International provenance trials have been planted but these have yet to be evaluated. In India large provenance trials have recently been planted.
Most planting of Acacia nilotica is still based on seed from local provenances. In India data on germination, seedling growth and nitrogen fixation are available from recently planted provenance trials. Selection for products such as tannin or gum has not been undertaken. Isozyme banding patterns suggested that Acacia nilotica is autotetraploid, displaying tetrasomic inheritance. Acacia nilotica is self-compatible and approximately 60% of seeds are set through self-pollination.
Acacia nilotica is a very useful multipurpose tree and has been traditionally used and planted in Africa and Asia as a source of tannin, brown, grey and black dyes, gum, timber, fodder and fuel. The dyes and tannins extracted from the pods are excellent examples of renewable resources since they do not interfere with the plant growth. Being easy to propagate by seed, fast growing, nitrogen fixing, tolerant of poor soils and arid conditions it will always have a role to play in the reclamation of degraded areas where fuel and timber supplies are lacking.
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Sources of illustration
- Brenan, J.P.M., 1970. Leguminosae (Mimosoideae). In: Brenan, J.P.M. (Editor). Flora Zambesiaca. Volume 3, part 1. Crown Agents for Oversea Governments and Administrations, London, United Kingdom. 153 pp.
- C.W. Fagg, Departamento de Engenharia Florestal, Faculdade de Tecnologia, Campus Universitario, Universidade de Brasilia, 71910 Brasilia DF, Brasil
- James Z.A. Mugedo, Department of Chemistry, Maseno University, Maseno, Kenya
Correct citation of this article
Fagg, C.W. & Mugedo, James Z.A., 2005. Acacia nilotica (L.) Willd. ex Delile. In: Jansen, P.C.M. & Cardon, D. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. Accessed 14 February 2018.
- See the Prota4U database.