Turnera oculata
Story var. oculata

Family : Turneraceae
Common name : Namib Turnera

Close-up of the Namib Turnera

The Namib Turnera with striking yellow, Hibiscus-like flowers with a conspicuous dark centre occurs in Kaokoveld in the Namib Desert in northern Namibia and in southern Angola.

Description
Erect evergreen woody and sparingly branched shrub up to 2.5 m tall. The main branch is about 30 mm in diameter at the base, the side branches are short. The shrub is densely covered with grey-green velvety hairs. The hairs are sometimes branched in star-like manner (stellate) or simple. The leaves are leathery, alternately arranged on the nodes about 10–12 mm apart, but often crowded at the tip where the internodes are very short. The leaf blades are ascending-spreading, reversed egg-shaped (obovate), up to 50 mm long and 25 mm broad and bear 5–7 pairs of shallow, saw-like teeth. The veins on the lower leaf surface are conspicuous, radiating from the central vein in a menorah-like (7-branched candelabrum) fashion. All parts of the leaves are densely covered with short velvety grey-green hairs. The leaf point is sharp to blunt and the base is wedge-shaped. The petiole is short to about 15 mm long and each mature leaf bears two whitish oval glands (opposite each other) near the base of the leaf blade. Each mature gland has a conspicuous orange-coloured rim, and produces a droplet of nectar (extra-floral nectaries). The flowers are solitary and their stalks (pedicels) are fused at their base to the upper side of the leaf stalks (petioles).Each flower bears two thread-like bracts about 7 mm long. The calyx is tubular, about 12 mm long with narrowly pointed lobes. The 5 petals are at first rolled up lengthwise (convolute), but when the flower opens during the day they spread out (flower up to 50 mm in diameter when fully opened). The petals are reversed egg-shaped (obovate) and about 25 mm long. They are bright yellow but dark brown and black towards the centre. The fruiting capsule is egg-shaped, about 9 mm long, with three chambers. The seeds are slightly curved, cylindrical and 4 mm long.

Close-up of the extrafloral nectaries on the leaf petioles of the Namib Turnera. The nectar produced feeds ants which protect the plant.

The genus Turnera consists of about 100 species and is mainly confined to South America, with only two members in Africa, one species in Namibia and Angola and the other confined to tropical Africa. Turnera oculata has two varieties: var. oculata, a tall erect shrub, and var. paucipilosa, a small spreading shrub confined to the mountains adjacent to the Kunene.

Turnera belongs to the family Turneraceae a small family with about hundred species most of which are confined to South America but with a few members in southern Africa. According to Bredenkamp (2003) the family Turneraceae is represented in South Africa and Namibia by 4 genera and 10 species. Turnera is the largest genus with more than 50 species of which only one species, T. oculata occurs in Namibia and southern Angola. All Turnera species have extra-floral nectaries at the ends of their leaf petioles near the bases of their attachment to the leaf blade.

Distribution and habitat
Adult plant of the Namib Turnera on the Otjihipa Mountains, N. Namibia. The Namib Turnera (Turnera oculata var. oculata) occurs localised in dry sandy and rocky streambeds at the eastern foot of the escarpment mountains (eastern margin of the Namib Desert) in the northern Namib Desert, both in western Kaokoveld and south-western Angola. The climate is hot and dry and the rainfall occurs mainly during summer and early autumn, and is below 100 mm per annum. Plants grow scattered in full sun and are not very common. Associated vegetation includes mopane (Colophospermum mopane), oak-leaved corkwood (Commiphora wildii), purple-stemmed corkwood (C. multijuga), C. anacardiifolia, Ficus glumosa, Boscia tomentosa, Tamarix usneoides, Adenolobus gariepensis, Rogeria adenophylla and Petalidium coccineum.

Derivation of name and historical aspects
The genus and family name (Turnera, Turneraceae) honour William Turner (died 1586), a British physician and botanist (Jackson 1990). The specific epithet ‘oculata ', pertains to the central dark ‘eye-like' marking of the flower. A distinctive feature of the genus Turnera are its flowers, which are partially fused to the upper side of the leaf petioles. Turnera oculata was first discovered in Namibia by Dr Robert Story (1913–), formerly of the Botanical Research Institute in Pretoria, in August 1956, some 20 km south of the Kunene River. Nine months later it was again collected by Dr Bernard de Winter (1924–) former director of the Botanical Research Institute in Pretoria and Dr Otto A. Leistner (1931–) on an expedition to the Kaokoveld. They found plants at Otjinungua on the banks of the Kunene River (near the confluence of the Marienfluss). The species was named by Story in the botanical magazine ‘Bothalia' in 1961, based on the specimen collected by De Winter and Leistner.

The plant depicted here was collected and introduced into cultivation by the author on an expedition to the Kaokoveld exploring cliffs in the region. The collected plants were found growing in a dry sandy riverbed in July 2004 east of Otjinungua at the foot of the escarpment mountains (Kirstenbosch National Botanical Garden 121/2005).

Ecology
Turnera oculata occurs well within the tropics and is well adapted to the very hot dry sandy streambeds. Its slender, erect growth, greyish-green velvety hairs on the leathery leaves and stems are all strategies for coping with the heat. The pair of oval glands (extra-floral nectaries) on the petiole near the base of the blade of mature leaves have a conspicuous orange-coloured margin. Nectar glands are usually associated with flowers, however in this case they have a different purpose. The glands provide nectar to local ants in return for protection of the flowers and other parts of the plant. In cultivation at the Botanical Society conservatory the invasive Argentinean ants rapidly discovered and regularly feed from the glands. Nectaries on other parts than the flowers are not confined to this family but are also found in families such as the Passifloraceae (e.g. in Adenia). Flowering time is summer, and flowers open during the day. The conspicuous yellow flowers with their dark centre are visited by insects such as butterflies.

Uses and cultural aspects
No uses have been recorded.

Branch of the Namib Turnera on the Otjihipa Mountains, N. Namibia

Growing the Namib Turnera (Turnera oculata)

Propagation
Best from cuttings during summer, each cutting about 50 to 100 mm long, the lower leaves removed, planted in sand or a mixture of peat and polistyrene and kept moist. They root particularly well in a heated mist unit.

Cultivation
Plants thrive in cultivation but are best grown in warm desert or semi-desert gardens. Plant in a well drained site in full sun, the plants grow rapidly and should flower in the second or third year of planting. Plants are cultivated in the Botanical Society conservatory at Kirstenbosch National Botanical Garden where they can be seen flowering regularly during summer months.

Acknowledgements
The author is thankful to James Deacon for propagating the plants from cuttings.

References

  • Bredenkamp, C.L. 2003. Turneraceae. In G. Germishuizen & N.L. Meyer (eds.), Plants of southern Africa: an annotated checklist. Strelitzia 14: 938.
  • Gunn, M. & Codd, L.E. 1981. Botanical Exploration of Southern Africa. A.A. Balkema, Cape Town.
  • Jackson W.P.U. 1990. Origins and meanings of names of South African plant genera. UCT Ecolab, Botany Department, Private Bag 7799 Rondebosch.
  • Obermeyer, A.A. 1976. Turneraceae. Flora of southern Africa. 22. Botanical Research Institute, Pretoria.

 

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Kirstenbosch National Botanical Garden
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