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| Petalidium namibense Swanepoel & A.E.van Wyk, in Swanepoel et van Wyk. 2024. |
Abstract
Petalidium namibense, hitherto confused with P. englerianum, P. rossmannianum, and the widespread P. variabile, is here described as a new species. It is a range-restricted species, only known from the area to the southwest, west and northwest of Puros in the Kaokoveld Centre of Endemism, northwestern Namibia, where it grows at the base of rocky outcrops, on arid hillsides, and along ephemeral riverbeds and drainage lines. Diagnostic characters for P. namibense include the pale grey appearance of the plants, corky bark on older stems, vegetative parts with a dense white indumentum of relative long dendritic trichomes appearing lanate, flowers borne in short few-flowered dichasia, and bracteoles narrowly ovate. The flowers of P. namibense are distinctive in having the two upper corolla lobes discolorous, abaxially light brown, adaxially vermillion, without nectar guides, and connate towards the base for 25–40% of their length. The anterior lobe is adaxially magenta with two relative long, narrowly triangular yellow nectar guides. The two lateral lobes are adaxially vermillion, or distally magenta, grading to vermillion towards the throat, and lack conspicuous nectar guides. A comparison of key morphological features distinguishing P. namibense from P. sesfonteinense, its closest relative in appearance, as well as from P. englerianum, P. rossmannianum, and P. variabile, is provided. Based on IUCN Red List criteria, a provisional conservation assessment of Vulnerable (VU) is recommended for the new species.
endemism, flora, Kaokoveld Centre of Endemism, Kunene Region, Namib Desert, Ruellieae, Puros, taxonomy, Eudicots
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Petalidium namibense, habitat and habit. A. Mature plant (ca. 0.5 m high) with greyish appearance, growing among rocks. B. Multiple stems (thickest ca. 80 mm in diam.) from base of a relatively old plant, each covered with thick corky bark. Petalidium namibense, habitat and habit. A. Several plants (dark grey dwarf shrubs) growing in the bed of an ephemeral drainage line. B. Ancient windswept plant sprouting from persistent woody stems with corky bark. Photographs by W. Swanepoel. |
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| Petalidium namibense, morphology of leaves and flowers. A. Flower, reduced shoots, and congested leaves; leaves with dense greyish indumentum. B. Flower in the process of fading as indicated by its pale colours. C. Newly opened flower with bracteoles in oblique lateral view; note brownish abaxial colour of posterior corolla lobes. D. Flower in front view. E. Flower with bracteoles in lateral view. F. Flower in dorsal view. Photographs: W. Swanepoel. |
Petalidium namibense Swanepoel & A.E.van Wyk, sp. nov.
Diagnosis:—A woody dwarf shrub up to 1 m tall, morphologically most similar to Petalidium sesfonteinense, differing by having the leaf lamina ovate, elliptic, suborbicular or orbicular (vs. ovate, elliptic or oblanceolate); bracteoles with trichomes dendritic, interspersed with glandular ones (vs. trichomes glandular with in addition appressed simple ones towards base); corolla shorter, 15.0–17.5 mm (vs. 20–24 mm long), upper lobes rectangular, smaller, 4.9–5.2 × 2.9–3.2 mm (vs. obovate, 5.8–8.2 × 3.5–4.3 mm), upper and lateral lobes differently coloured than anterior lobe with nectar guides absent or inconspicuous (vs. all lobes similarly coloured, nectar guides on upper and lateral lobes conspicuous).
Etymology:—The specific epithet refers to the Namib Desert to which Petalidium namibense is endemic. The Namib Desert in its broadest definition, stretches along the Atlantic Ocean from Saõ Nicolau (Bentiaba) in Angola through Namibia to the Olifants River in South Africa (Seely 2004, Goudie & Viles 2015).
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| Petalidium sesfonteinense, morphology of leaves and flowers. A. Flowers, shoots, and leaves; greyish leaves are still densely covered in trichomes; green leaves almost glabrous. B–E. Flowers in front view showing variation in corolla colour and lobe margins, with two yellow nectar guides. Photographs: W. Swanepoel. Republished from Swanepoel & Manzitto-Tripp (2022). |
Wessel Swanepoel and Abraham E. van Wyk. 2024. Petalidium namibense (Acanthaceae), A New Species from Namibia. Phytotaxa. 671(2); 128-138. DOI: doi.org/10.11646/phytotaxa.671.2.2