The Sesame Family, Pedaliaceae, contains eleven genera of shrubs and trees, including the True Sesames, Sesamum spp.. Eight of these genera are confined to Africa, where the family is thought to have originated, while one, Uncarina spp., if found only on Madagascar. Of the remaining two genera, Pedalium (which contains only a single species, Pedalium murex) is found across much of Africa and South and Southeast Asia as far east as Java, while Sesamum spp. has a near global distribution, although it is likely to have been introduced to many places by Humans, due to the use of its seeds as a food crop.
Phylogenetic studies of the Family Pedaliaceae have suggested that the basalmost genera is Sesamothamnus, the Sesame Trees or Sesame Bushes, a group found in arid areas of Africa, split into two groups with a bimodal distribution. The first group of species is found in East Africa, from Ethiopia and Somalia south into Kenya and Tanzania, and are large, tree forming plants, the second group has a Southern African distribution, being found in Angola, Namibia, Botswana, Zambia and Zimbabwe, and are smaller, shrubby plants, with a bulbus base from which a single, taperinc stem arises.
The most recent species of Sesamothamnus to have been described was Sesamothamnus lugardii in 1906, however, in 1957 the South African botanists Bernard de Winter and Otto Albrecht Leistner recorded seeing a tree-like species of Sesamothamnus while on an expedition to the Kaokoveld Region of northwestern Namibia, and in addition brought back sterile samples which were stored in the collections of the South African National Biodiversity Institute in Pretoria and the National Botanical Research Institute in Namibia, where they were tentatively identified as Sesamothamnus benguellensis, a shrub-like form found in Namibia and Angola. In 1966 the Namibian botanist Heinrich Johann Wilhelm (Willy) Giess collected fertile specimens of the tree, which were stored in the National Botanical Research Institute in Namibia under the name 'Sesamothamnus leistneranus' apparently in honour of Otto Albrecht Leistner, although he never published a description of this material.
In a paper published in the journal Phytotaxa on 14 November 2023, Wessel Swanepoel, an independent researcher from Windhoek and Abraham van Wyk, from the H.G.W.J. Schweickerdt Herbarium at the University of Pretoria, formally describe the Kaokoveld Sesame Tree as a new species.
The species is described as Sesamothamnus leistneri, rather than Sesamothamnus leistneranus, as this implies that it was discovered by Leistner, rather than simply named in his honour. Wessel Swanepoel and first observed an unfamiliar Sesame Tree while visiting the Kaokoveld in 1990, although at that time the tree was not in flower or fruit, making identification very hard. In 2005 Swanepoel and van Wyk observed another Sesame Tree in the Otjitanda area of the Kaokoveld. This time the tree was producing both flowers and fruit, and samples were taken which enabled it to be compared to the material previously collected by de Winter, Leistner, and Giess.
Sesamothamnus leistneri is a single or (more-commonly) multi-trunked succulent tree, reaching about 8 m in height. Branches are formed high on these trunks, and covered with sharp spines up to 30 mm in length, formed from modified petioles (leaf stalks). The bark of the tree is creamy yellow, and peels away from the trunks in papery strips. The trees are deciduous, producing waxy green leaves 40-70 mm in length and 15-40 mm. Flowers are produced from January to June and are large and cylindrical, reaching 150 mm in length and 55 mm in width, white or cream on the outside and bright yellow on the interior. Flowers of this type are likely to be pollenated by Long-tongued Hawkmoths, as is the case with other members of the genus. The fruit, produced from February onwards is a green-to-yellow rigid woody capsule, 65-90 mm in length and 25-32 mm in width, which splits open during the dry season releasing a large number of small, winged seeds.
Sesamothamnus leistneri was found growing in mountainous regions on both sides of the Kunene River, which forms the border between Namibia and Angola. In Namibia it was found from the Ehomba Mountain and Zebra Mountains in the east throughout the Baynes Mountains to the Otjihipa Mountains in the west, and south as far as Sesfontein. In Angola Sesamothamnus leistneri was observed near the summit of Serra Tchamalindi in the Iona National Park, and is thought likely to be fairly widespread.
Sesamothamnus leistneri grows on rocky mountain and valley slopes, in kloofs (gorges), less often on plateaus and at the base of hills, in Colophospermum-Commiphora woodland at elevations of 1000–1600 m, Between 83 and 220 km from the Atlantic Ocean. Rainfall in this area is typically 100-300 mm per year. This area is sparsely populated by Humans, and the tree does not appear to be utilised by Humans in any way, though it is browsed by livestock during periods of drought. As such Swanepoel and van Wyk assess that it should be classified as being of Least Concern under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species.
The tree-like general habit of Sesamothamnus leistneri suggests that it is more closely related to the Sesame Trees of northern East Africa than the shrubby forms of Southern Africa. This is not altogether surprising, as while these areas are now separated by humid tropical forests where these Plants are unlikely to be able to survive, it is thought that the two zones would have been connected by bands of arid land several times during the Pleistocene glacial phases, when the climate of Africa would have been cooler and dryer.
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