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| Petalidium engoense in Swanepoel, Becker Van Wyk, 2025. |
Abstract
Petalidium engoense, first collected in May 2025, is here described as a new species. It is a range-restricted species, only known from the western edge of the escarpment zone (Great Escarpment of southern Africa), in the hilly area bordering the Engo Valley (northwestern Namibia), and to the north of Espinheira (southwestern Angola), in the Kaokoveld Centre of Endemism. It grows amongst boulders on arid hillsides, and along an ephemeral riverbed. Diagnostic characters for P. engoense include the pale greyish green appearance of the plants, ashy-grey to white fissured bark on older stems, vegetative parts with a dense white indumentum of very small stellate (stellulate) and dendritic trichomes appearing tomentose, semi-succulent leaves, burgundy flowers borne in short few-flowered dichasia with inflorescence axis becoming spiny with age, and narrowly ovate, attenuate to acute, prominently reticulate bracteoles. The flowers of P. engoense are distinctive in having all lobes coloured burgundy, and the anterior lobe with two yellow ensiform nectar guides. A comparison of key morphological features distinguishing P. engoense from P. namibense, its closest relative in appearance, is provided. Based on IUCN Red List criteria, a provisional conservation assessment of Critically Endangered (CR) is recommended for the new species. The paper also summarises current knowledge of phenotypic plasticity within Petalidium in response to environmental variation. This contribution forms part of ongoing taxonomic studies on the genus.
endemism, Engo River, Engo Valley, flora, Hartmann Valley, Iona National Park, Kaokoveld Centre of Endemism, Kunene Region, Namib Desert, Namibe Province, Ruellieae, taxonomy, variation, Eudicots
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| Petalidium engoense. Habit, vegetative and reproductive features. A. Woody stem with fissured bark at the base of a mature plant. B. Flowering shoot with semi-succulent leaves, which are covered in matted, greyish white indumentum and typically ca. folded upward along the midrib for their full length. C. Portion of a plant showing an open flower along with several old inflorescences; the inflorescence axes usually persist and have become spiny with age. Photographs by W. Swanepoel. |
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| Petalidium engoense. Morphology of inflorescences and flowers. A. Distal part of a shoot showing both an open flower and a faded, brownish one, along with several bracteoles. B. Old inflorescences with dried, shriveled bracteoles and persistent axes that have become spiny (indicated by arrows). C, E. Frontal view of a flower showing uniformly coloured corolla lobes; the anterior lobe with two yellow, ensiform nectar guides. Visiting ants were feeding on scale insect larvae. D, G. Flower and bracteoles in lateral view. F. Flower with bracteoles, viewed from above. Photographs by W. Swanepoel. |
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| Petalidium engoense. Habitat and habit. A. Several plants growing among rocks along a seasonal drainage line in the Engo Valley, Kunene Region, Namibia. B. A mature plant (ca. 800 mm tall) displaying a densely branched structure with greyish green foliage. Photographs by W. Swanepoel. |
Wessel SWANEPOEL, Rolf W. BECKER and Abraham E. VAN WYK. 2025. Petalidium engoense (Acanthaceae), A New Species from Angola and Namibia, with notes on phenotypic plasticity in the genus. Phytotaxa. 734(1); 9-19. DOI: doi.org/10.11646/phytotaxa.734.1.2 [2025-12-16]
