Tree up to 20m high. Bipinnate Leaves have spinescent stipules. Flowers in capitate spikes are white due to the exserted stamens. Fruit is broad pod up to 15cm long with up to 14 shiny Seeds.
Acacia robusta subsp. robusta, Vachellia robusta. subsp. robusta, Acacia clavigera, Acacia sambesiaca.
RSA Tree No. 183.
Common names: Ankle Thorn, Narrow-pod Splendid Acacia, Enkeldoring, Engelsedoring, River thorn.
Family: Fabaceae or Leguminosae (Pea, bean or legume family). After the Orchidaceae and the Asteraceae, the Fabaceae is the third largest Angiosperm (flowering plants) family with 700+ genera and close to 20 000 species. Local genera include Acacia (Vauchellia, Senegalia), Albizia, Bauhinia, Bolusanthus, Burkea, Calpurnia, Colophospermum, Cyclopia, Erythrina, Erythrophleum, Faidherbia, Indigofera, Philenoptera and Schotia. The Fabaceae are recognisable by their fruit and by their pinnately compound Leaves. Leaves may also be simple and usually have stipules – some of which may be spinescent. Leaflets are usually entire. Flowers are bisexual and bracteate. Regular flowers usually have 4-5 sepals and the same number of petals. Irregular flowers have 4-5 sepals and 5 or less petals. Stamens have anthers that have 2 pollen sacs and there are usually at least twice the number of stamens as petals – often 10. The superior Ovary has one locule that may contain 1 or more ovules. The Stigma and Style are simple. The single carpel develops into the Fruit, which is usually a pod. The pod dehisces on both sides and may break into segments. Seeds vary.
Name derivation: Acacia – typically thorny. Vachellia – named after George H. Vachell (1789 – 1839), chaplain and plant collector in China. These plants have a capitate inflorescence and stipules that are spinescent. robusta – robust: appearance of tree.
Conservation Status: L C. (Least Concern). 2009 (Raimondo et al.).
This Tree has a slightly flattened, rounded or irregular spreading Crown. It is usual single stemmed to about 2m and then branches almost vertically reaching a maximum of 20m (usually half this height). The tree is usually taller than wide. The branches may be hairy. Lenticels (a usually raised corky oval or elongated area on the plant that allows the uncontrolled interchange of gases with the environment) are clearly visible on younger branches. Stems and branches are noticeably thick. Nodes are swollen. Bark is grey, dark brown or black. The initially smooth bark becomes deeply fissured and rough with age. It does not flake but fissures may reveal the red inner layer.
This deciduous tree has Leaves that are located on woody cushions above the spines and tightly clustered around the branches. They are bipinnate (compound: twice pinnate. The central axis – rachis has lateral “branches” not leaflets and the leaflets are on these pinnae “side branches”). Leaves may be densely hairy and up to 10cm long. Each leaf has up to 5 pairs of Pinnae. Each pinna is up to 8cm long and has 10-27 pairs of Leaflets that have distinct spaces between them. Leaflets are glossy dark green above, lighter below and each is up to 13 x 5mm. The Petiole (leaf stalk) and Rachis (main axis bearing flowers or leaflets) are hairless, or nearly so, and grooved along the top. The Petiole is usually up to 2cm long. Glands are usually present on the rachis between the top 1 or 2 pairs of pinnae, but seldom on the petiole. The Stipules (basal appendage of the petiole) are spinescent. These paired Spines are white or greyish and may be straight or slightly curved. Each spine is up to 12cm long on new growth but is much shorter on older growth.
The impressive creamy white or yellowish Flowers are capitate (formed like a head) balls. These balls are much reduced Spikes (simple indeterminate inflorescence with sessile flowers on a single unbranched stalk). Each ball of flowers is up to 1,8cm in diameter and supported by a hairless Peduncle (stalk of flower cluster). Individual flowers are very small and usually bisexual and actinomorphic (regular, symmetrical. Perianth, the calyx and corolla is divisible into 3 or more identical sectors). The Calyx is up to 3,2mm long and the hairless Corolla is up to 4mm long. The colour of the flower is due to the mass of free exserted (sticking out) Stamens. They surround a single Pistil (a unit of the Gynoecium, the female element of the flower, composed of the Ovary, Style and Stigma). The pistil contains a single superior Ovary, with a single, thin Style, which ends in a small, single concave Stigma. After the pollen is shed, the style elongates and protrudes beyond the anthers and the stigma becomes receptive. This development helps prevent self-pollination.
The straight or slightly curved Fruit is a broad – up to 3cm wide, flattish, grey to dark brown or reddish brown or a blackish, dehiscent Pod. It may reach a length of 15cm. The pod is glabrous (hairless) but is not constricted between the seeds. Most pods are dehiscent but some remain on the trees with their seeds. Up to 14 shiny Seeds developin each pod. (Oct-Aug).
Distribution & Ecology
Rhizobium is a genus of gram-negative bacteria, which develop in the root nodules. They fix atmospheric nitrogen and convert it to ammonia and organic nitrogen compounds like glutamine for the plants (a natural fertilizer). In this Symbiotic relationship, the bacteria use organic compounds supplied by the plant for their own benefit. Brown or black seed beetles in the family Chrysomelidae, live most of their lives within a single seed and destroy it. Birds hunt these beetles. The trees are reasonably frost tolerant. They extend up rivers in drier areas and are found in savanna (is a rolling grassland scattered with shrubs and isolated trees, which can be found between a tropical rainforest and desert biome) and wooded grassland. Trees grow in the Eastern and Northern Cape, Free State, northwards through KwaZulu-Natal, Gauteng, North-West Province, Swaziland, Mozambique, Mpumalanga, Namibia, Botswana, Zimbabwe and Malawi. They usually occur below 1 500m. This Tree is considered invasive in Australia. Rhino eat the Bark and Pods. Herbivorous animals browse the Leaves. Gum (A water-soluble sugary polysaccharide is exuded. Its function is to seal wounds and prevent infection by bacteria and fungi. It also helps to prevent trees from freezing) is eaten by monkeys and baboons. Birds consume the Seeds.
The Leaves are reported to be slightly toxic to stock but Leaves but some animals like the kudu graze the Pods. Pods have a crude protein value of 12%. The Wood is hard and light coloured. The heartwood is dark and has a moderately coarse texture. It is subject to borer attack. A Fungus may also discolour the wood. Cut wood tends to warp. It is thus not a good timber or fuel. Scarifying the Seeds will help their otherwise poor ability to germinate. It grows relatively quickly but should be kept away from buildings because of its aggressive Roots. The tree is drought and cold resistant.
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