Sesamothamnus leistneri P.Craven ex Swanepoel & A.E.van Wyk, 2023

Swanepoel, Wessel & Van Wyk, Abraham E., 2023, A new species of Sesamothamnus (Pedaliaceae) from Namibia and Angola, Phytotaxa 626 (3), pp. 145-158 : 149-156

Sesamothamnus leistneri P.Craven ex Swanepoel & A.E.van Wyk , sp. nov. ( Figs 1–5 )

Diagnosis:— Single or usually multi-stemmed succulent tree, morphologically most similar to Sesamothamnus rivae , differing by being up to 8 m tall with high succulence of main stems (vs. small multi-stemmed tree up to 3 m tall, low succulence of main stems), flowers lacking a spur (vs. spur present), and flowering during the rainy season with developed leaves (vs. flowering before onset of rainy season when plants still leafless).

Type:— NAMIBIA. Kunene Region: Kaokoveld, 5 Meilen oestlich Otjihende. Auf Berghang [5 miles east of Otjihende, on mountain slope], (1712DB), 23 April 1966, Giess 9381 (holotype WIND: 48985 !; isotype WIND: 48986!) .

“ Sesamothamnus sp. nov. ” in Coates Palgrave (2002: 1015); Curtis & Mannheimer (2005: 607); Van Wyk et al. (2011: 615). Sesamothamnus leistneri nom. inval. in Mannheimer & Curtis (2009: 454, 2018: 468).

Illustrations:— Curtis & Mannheimer (2005: 608, Sesamothamnus sp. nov., photographs of habit and flower); Mannheimer & Curtis (2009: 455; 2018: 469, Sesamothamnus leistneri nom. inval, photographs of habit, bark, leaves, flower, and fruit); Ihlenfeldt (2010: 153, fig. 1; 158, fig. 7; 159, fig. 13, Sesamothamnus leistneri nom. inval, habit, flower, and mucilaginous hairs); Becker et al. (2021: 58, Sesamothamnus leistneri ined., habit); Leistner (2022: 64 & 65, Sesamothamnus leistneri ined., habit and flowers).

Single or usually multi-stemmed succulent tree up to 8 m tall; stems often fluted at base ( Fig. 3A ), branching fairly high, spiny; spines 1–3 per node, produced by modification of petioles ( Fig. 3E ), straight or slightly recurved, central one up to 30 mm long, laterals up to 10 mm long or absent. Bark cream-yellow, smooth, often peeling in papery strips ( Fig. 3B ). Leaves simple, petiolate (petiole 10–30 mm long) and spirally arranged on long shoots, subsessile (petiole 1–10 mm long) and fasciculate on dwarf lateral shoots in axils of spines ( Fig. 3C ), deciduous; lamina narrowly ovate, narrowly obovate or elliptic, 40–70 × 15–40 mm, green, often shiny ( Fig. 3D ), both sides sparsely covered with quadrangular-headed mucilage glands; apex obtuse, rounded or emarginate, base decurrent; margins entire, midrib and lateral veins prominent abaxially, sparsely covered with quadrangular-headed mucilage glands. Inflorescences racemose, terminal, up to 12-flowered, dark green, with ca. rectangular headed mucilage glands, glandular hairs and villose hairs. Flowers large, lacking a spur; pedicels 5–6 mm long; calyx 5-partite, dark green, indumentum similar to that of inflorescences, ca. 6 mm long, subequally lobed, posterior lobe smaller, lobes deltate, ovate or narrowly triangular, ca. 5 mm long, ciliate with villose hairs; corolla with developing tube straight at first, then recurved, green or black-green, eventually long, cylindrical, straight or only slightly curved, 95–150 mm long, 4–6 mm diam., slightly widened at base with posterior side slightly gibbous, inside glabrous, bright yellow ( Fig. 4D ), contrasting with white filaments (fused with tube), outside yellow-green to cream-white; limb spreading at first, later reflexed, 50–55 mm diam., white to cream-white, outside of corolla with indumentum similar to that of inflorescences; lobes 5, subcircular, 24–30 mm diam. Stamens 4, equal, inserted in mouth of corolla tube; filaments adnate to tube, free part ca. 1.5 mm long; anthers included in the corolla mouth, orange-yellow, ca. 5.5 mm long; pollen white. Ovary oblong, sometimes slightly widening towards apex, ca. 10 mm long, laterally compressed, indumentum similar to that of inflorescences. Style slender, equalling corolla tube; stigma reaching corolla mouth or exserted by up to 10 mm, broadly bi-lobed. Fruit a rigid woody capsule, narrowly obovate, 65–90 × 25–32 mm, laterally compressed, apex rounded to tapering or emarginate, apiculate, green, yellowish brown when ripe. Seeds flat, obovate, broadly winged, 16–24 × 10–14 mm (including wings).

Phenology:— Flowers were recorded from January to June. Fruit development is quick and mature ones may be encountered from February onwards. Seeds are released shortly after fruit maturation, usually well before the next rainy season.

Distribution, habitat and ecology:— Sesamothamnus leistneri is known from several localities in the mountainous region on both sides of the Kunene River, northwestern Namibia and southwestern Angola ( Fig. 6 ), where it is localized and usually solitary. In Namibia it occurs from the Ehomba Mountain and Zebra Mountains in the east throughout the Baynes Mountains to the Otjihipa Mountains in the west and southwards to near Sesfontein. In Angola the species has been recorded near the summit of Serra Tchamalindi, Iona National Park ( Becker et al. 2021), but it may eventually prove to be present on other mountains in the vicinity, as what appears to be suitable habitat is not limited to this specific mountain. 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, 83– 220 km from the Atlantic Ocean. Average annual rainfall in the area is 100–300 mm ( Mendelsohn et al. 2002).

Conservation status:— Sesamothamnus leistneri is rare and localised but widespread in uninhabited to sparingly inhabited parts of the Kunene Region of Namibia, and the far southeastern part of Namibe and most probably bordering southwestern parts of Cunene provinces in Angola. The species does not seem to be utilised by humans, but during times of drought is heavily browsed by domestic stock, mainly goats and cattle ( Fig. 1A ), which may result in considerable damage to plants. It should be considered as Least Concern (LC) since it is not endangered or vulnerable currently and is widespread in the extensive mountainous area on both sides of the Kunene River in Namibia and Angola ( IUCN 2012).

Etymology:— The specific epithet honours Otto Albrecht Leistner, who, together with Bernard de Winter, first collected Sesamothamnus leistneri in 1957 during an epic collecting trip to the Kaokoveld ( Leistner 2022). In our choice of name, we have chosen to use the epithet as a noun in the genitive case ( “ leistneri ”) rather than an adjective ( “ leistnerianus ”). Either of these options have been used to designate the taxon in the past. We consider the genitive case to be more appropriate, as it indicates that this species is Leistner’s particular discovery, not anybody else’s.

Notes:— Sesamothamnus leistneri differs from the three other southern African members of the genus in habit, leaf, indumentum, and flower characters. It is morphologically more similar to the two northeast African species (hence our choice of S. rivae to compare it with in the Diagnosis) with which it shares an indumentum of mucilage hairs with quadrangular heads; the other three southern African species have mucilaginous hairs with distinctly stellate heads (Ihlenfeld 2010: 159, figs 13 & 14). It has also been suggested that S. leistneri may well be the most primitive member of the genus ( Ihlenfeldt 1994).

The suggested closer affinity of Sesamothamnus leistneri with the disjunct northeast African S. rivea , rather than the three other southern African members of the genus, is not unexpected. There are well-known floristic (and faunal) links, often involving discontinuities in distribution, between arid areas in southern Africa and arid areas in East Africa, the Horn of Africa, and Arabia. These disjunct patterns indicate that at least the semi-arid habitats have been connected by an arid corridor, the periodic existence of which was presumably related to patterns of climate change, with arid episodes accompanied by, among other causes, periods of glaciation. Abundant botanical and zoological evidence support the existence of such a periodical arid corridor (e.g. Van Wyk & Smith 2001 and references therein, Thiv et al. 2011, Bellstedt et al. 2012, Linder 2014).

Apart from the differences in habit and morphological characters, the distribution of the three southern African taxa differs, with Sesamothamnus leistneri being confined to the mountainous areas immediately to the north and south of the Kunene River in the Kaokoveld Centre of Endemism in Angola and Namibia, and S. benguellensis to the northern part of the Kaokoveld Centre in Angola from the Benguella, Namibe, and Cunene Province southwards crossing the Kunene River into Namibia as far as the Rooidrom area (just south of the Marienfluss Valley). Sesamothamnus guerichii is confined to the southern part of the Namibian Kaokoveld from the Rooidrom area southwards to Omatjette near Omaruru in the Erongo Region. Only a single locality is known where S. leistneri and S. guerichii grow together ( Ihlenfeldt 2010). Some of the more prominent morphological features to differentiate amongst all the currently recognised species of Sesamothamnus are supplied in Table 1 .

The floral traits of S. leistneri suggest that it is specialised for pollination by long-tongued hawkmoths (flowers display the sphingophilous syndrome), as is the case in all the other members of the genus ( Ihlenfeldt 2004). Flowers open in the later afternoon and in the evening, are sweetly scented, and contain abundant nectar. They last only a single night before the corollas are shed. Further study is needed to investigate the significance of variation in the degree to which the style protrudes from the corolla tube in flowers. This variation may well result from differences in the timing of maturation between the male and female floral parts. According to Ihlenfeldt (2004), protogyny has been observed in Sesamothamnus , and it is possible that this type of dichogamy is present in S. leistneri .

Additional collections (paratypes):— ANGOLA, Namibe Province:—1612: Serra Tchamalindi , eastern part of range, (– DD), Swanepoel, Van Jaarsveld & Gomes 622 ( LUBA!) .

NAMIBIA, Kunene Region:—1712: Kaokoveld, Otjomborombonga, main kloof to the south, (– BB), 14 July 1976, Leistner, Oliver, Steenkamp & Vorster 162 ( PRE!) ; Otjihipa Mountains , (–BD), 21 December 1999, Bruyns 8042 ( WIND!) ; Mountain slopes with large weathered boulders at Ombepera , (–DB), 11 April 1957, De Winter & Leistner 5504 ( PRE!, WIND!) ; Otjihende, where Otjihende Dam road forks ± half mile from Otjihende , (–DB), 24 February 1973, Owen-Smith & Malan 291A ( WIND!) ; 8 km oos van Otjitanda (–DB), 23 June 1977, Viljoen 432 ( WIND!) ; 7 km towards Van Zyl’s Pass from Otjitanda , 1300 m, (–DB), 24 February 1999, Bruyns 8056 ( WIND!) ; Opuwo District, Etengua along road to Otjitanda , 1413 m, (–DB), 31 January 2009, Rugheimer, Klaassen, Lutombi, Haufiku, Hasheela & Aiyambo 2633 ( WIND!) ; Marble ridge, 2 km north of Onyava [Onyuva], (–DC), 1156 m, 10 January 2004, Swanepoel SWA3/110 ( WIND!) ; Kunene, (–DC), 28 November 2004, Curtis BC 2191 ( WIND!).—1713: Hill south of camp. Ombuku River , (– AD), 12 March 2003, Curtis BC1976 ( WIND!) ; Ehomba Village , 1176 m, (–BD), 16 January 2004, Swanepoel 3/114 ( WIND!) . —1813: Ohohorwa (Poort ± 40 km south of Kaoko-Otavi ), (–DA), 17 February 1973, Owen-Smith & Malan 291 ( WIND!) ; Robbies Pass , (–DA), 1420 m, 31 March 2002, Curtis, Aronson & Le Floch CUR1659 ( WIND!) ; Robbies Pass , 1360 m, (–DA), 6 January 2004, Swanepoel SWA3/85 ( WIND!) ; Ohumbameya , 1400 m, (–DA), 6 January 2004, Swanepoel SWA3/87 ( WIND!).

Figures

FIGURE 1. Sesamothamnus leistneri, habitat and habit. A. Multi-stemmed tree with distinct browse line caused by domestic stock. B & C. Trees showing typical upright habit and relatively dense green foliage during the rainy season. Photographs by A.E. van Wyk (A, B) and W. Swanepoel (C).

FIGURE 2. Sesamothamnus leistneri, habitat and habit. A. Typical single-stemmed tree. B. Tree with foliage starting to turn yellow in autumn. C. Leafless tree during the dry season (winter), with persistent dehisced fruit. Photographs by W. Swanepoel.

FIGURE 3. Sesamothamnus leistneri, vegetative morphology. A. Single-stemmed tree showing ± fluted trunk. B. Bark peeling in papery pieces. C. Long shoot with leaves clustered on lateral short shoots. D. Mature leaves showing green colour and ± glossy upper surface. E. Long shoot with spines derived from petioles; one dried leaf blade still attached. Photographs by A.E. van Wyk (A, E) and W. Swanepoel (B–D).

FIGURE 4. Sesamothamnus leistneri, flower morphology. A. Inflorescence showing one open flower, several flowers in different stages of development, and persistent styles (whitish, tipping blackish ovaries) of faded flowers following shedding of the corolla. B. Flowers, open and in bud; note both stamens and stigma positioned in mouth of the corolla tube, also the lack of a spur at the base of the tube. C. Flower showing style with bilobed stigma exserted about 10 mm from the mouth of the corolla tube. D. Distal part of corolla tube opened out to show the yellow inner surface of the tube, white filaments, and anthers positioned at the level of the corolla mouth. Photographs by W. Swanepoel (A, B, D) and A.E. van Wyk (C).

FIGURE 5. Sesamothamnus leistneri, fruit morphology. A. Two almost mature fruit and a dehisced one. B. Old dehisced fruit persist for some time on trees. Photographs by W. Swanepoel (A) and A.E. van Wyk (B).

FIGURE 6. Known distribution of Sesamothamnus leistneri. Solid circles indicate localities supported by herbarium specimens in PRE and WIND; open circles with small central dots depict sight records only.