General Info

Tree up to 17m in height.  Leaves bipinnate with spinescent stipules.  The spherical inflorescences with yellowish, actinomorphic Flowers on short spur branchlets.  Fruit a constricted pod with greenish Seeds.

Description

Acacia kosiensis, Vachellia kosiensis.

RSA Tree No. 172.2

Common names: Dune Thorn, Duinesoetdooring, Dune Acacia, Dune Sweet 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 Tree genera include Acacia (Vauchellia, Senegalia), Albizia, Bauhinia, Bolusanthus, Burkea, Calpurnia, Colophospermum, Cyclopia, Dichrostachys, Erythrina, Erythrophleum, Faidherbia, Indigofera, Mundulea, Peltophorum, Philenoptera, Schotia and Xanthocercis 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.  This 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 (formed like a head) inflorescence and stipules that are spinescent.  kosiensis from Kosi Bay – situated on the East coast of the RSA close to Mozambique.  Plants in the genus Vachellia usually do not possess spinescent stipules.

Conservation Status: L C. (Least Concern).  2009 (Raimondo et al.).

Tree

The long stems on this tree Tree are slender.  The crown is sparsely branched and up to 17m high.  Trees may form overlapping canopy stands.  The Bark is light brown or greyish white to white, smooth or with shallow longitudinal fissures revealing red underlining tissue.  The bark is low in tannin.  Round spots of Lichens may be visible on stems.  Young Stems are bright green or greenish-white to almost all white.  With time, they become olive green or cream coloured.  The Branches may zigzag.  Large paired spines may persist on the stem.

Leaves

The Leaves are bright green and bipinnate (compound: twice pinnate.  The central axis or rachis, has lateral “branches” not leaflets and the leaflets are on these “side branches”).  Primary leaves have 5-7 pairs of pinnae, each nearly 3,5cm long, bearing 15 pairs of jugate (in pairs) leaflets.  Each hairless leaflet is up to 5,5 x 1,6mm.  The Rachis is densely velvety and is up to 2,6cm long.  The Petiole (leaf stalk) is usually about 7mm long.  Cataphylls (rudimentary scale-like leaves, as bud scales, that precedes the foliage leaf of a plant) are in tufts of 8-11 grouped together on short spur branchlets in the axils of the spines.  These leaves are smaller than the primary leaves.  The white to cream-coloured Stipules (basal appendage of the petiole) are spinescent and up to 20cm+ long. They are sharp and usually occur on the main trunk as well as the branches.

Flowers

Flowers are yellow and sparse.  The inflorescences are in groups of 1-4 on short spur branchlets hidden amongst the leaves (V. karroo has flowers at the ends of branches)Peduncles (stalk of flower cluster, or of a single flower when that flower is the remaining member of an inflorescence) are relatively long: up to 3,5cm with glands and 1 or 2 sterile flowers in the involucres (one or more whorls of small leaves or bracts standing underneath a flower or flower-cluster).  There are up to 102 flowers per spherical inflorescence.  Flowers are bisexual, actinomorphic (Regular, symmetrical.  The Perianth, the calyx and corolla, can be divided into 3 or more identical sectors) with a few male flowers larger than the others.  There are many exserted (sticking out; projecting beyond) Stamens that are free and distinct.  They provide the yellow colour to the flower.  Below the anther and attached to the filament is a multicellular gland which is easily detached and may be a means of rewarding visiting insects.  It may also inhibit ants from interfering with pollinators. The pollen in each flower is shed before the stigma becomes receptive.  A single Pistil (the female element of the flower composed of the ovary, style and stigma) with a short stalk is present.  The single filiform (thread or filament like) Style extends moving the small, terminal and concave Stigma past the stamens.  The late female development and the extended stigma are 2 adaptations that help to promote cross-pollination.  The Ovary is superior (said of an ovary that is free from the calyx or perianth) and contains one locule (compartment).  (Nov-Apr +).

Fruit

The ripening Fruit turns a reddish brown.  The slightly sickle-shaped Pod is up to 19 x 1cm.  It is prominently constricted between seeds.  Pods open on both sides.  There are 10-12 Seeds per pod.  They are egg-shaped, olive-green, glossy and relatively large.  The pod reaches full size before the seed development is complete. (Feb-Aug).

Distribution & Ecology

These Trees are pioneer species and help stabilizing dune areas. They are especially evident in deep, sandy soils.  They are common in high rainfall areas. These trees usually occur from Tugela River up the coast and into southern Mozambique on the sandy coastal soils.  Because of its water use efficiency and growth rate, this tree may be planted in dry areas.

Ethnobotany

The plant grows quickly from seeds. There are flavonoids (a group of plant metabolites thought to provide health benefits through cell signalling pathways and antioxidant effects) in the Wood.

References

Boon, R. 2010. Pooley’s Trees of eastern South Africa. Flora and Fauna Publications Trust, Durban.

Coates Palgrave, M. 2002. Keith Coates Palgrave Trees of Southern Africa, edn 3. Struik, Cape Town.

Lawrence, G. H. M, 1951. Taxonomy of Vascular Plants, The Macmillan Company, New York. Tenth Printing 1965.

 

http://redlist.sanbi.org/species.php?species=15470-14

http://www.acacia-world.net/index.php/africa-me/south-africa/the-vachellia-karroo-complex

http://repository.up.ac.za/bitstream/handle/2263/44976/Robbertse_Phenology_2014.pdf;sequence=1

http://posa.sanbi.org/flora/browse.php?src=SP

http://www.acacia-world.net/index.php/africa-me/south-africa/acacia-kraussiana/