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| Petalidium saxatile Swanepoel, K.G.Dexter, E.Tripp & A.E.van Wyk, in Swanepoel, Dexter, Adamo, Manzitto-Tripp et van Wyk. 2025. |
Abstract
Petalidium saxatile, hitherto confused with P. canescens, and the widespread P. setosum, is here described as a new species. It is a range-restricted species, only known from the vicinity of Palmwag and southwards to the Bergsig area with an outlier population to the south of Khorixas in the Kaokoveld Centre of Endemism, northwestern Namibia. It grows mainly among basaltic rocks of the Etendeka Group on arid hillsides and along ephemeral riverbeds and drainage lines. Diagnostic characters for P. saxatile include the pale green appearance of the plants, vegetative parts with a dense white indumentum of relatively short dendritic trichomes appearing matted on young leaves, flowers borne in compact dichasia, and long simple eglandular trichomes on the bracts visible to the naked eye. The flowers of P. saxatile are distinctive in having the corolla lobes inside (adaxially) distinctly bicolorous: the upper and lateral lobes are pink or brown-pink, while the anterior lobe is yellow. Additionally, all lobes are discolorous in being much paler outside (abaxially). Furthermore, analyses of genetic data across many nuclear loci, generated using a ddRADseq approach, show the species to be genetically distinct from P. canescens. A comparison of key morphological features distinguishing P. saxatile from P. canescens and P. setosum, is provided. Based on IUCN Red List criteria, a provisional conservation assessment of Endangered (EN) is recommended for the new species. It is suggested that the Etendeka Tableland, to which P. saxatile is largely confined, may serve as a local subcentre of plant endemism within the larger Kaokoveld Centre of Endemism, at least for species in the genus Petalidium.
Bergsig, endemism, Etendeka Group, flora, genetics, Kaokoveld Centre of Endemism, Khorixas, Kunene Region, Oberholzeria, Palmwag, RADseq, Ruellieae, taxonomy, Eudicots
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| Petalidium saxatile, morphology of flowers. A, B. Newly opened and faded flowers. Anterior corolla lobe inside (adaxially) bright yellow and without nectar guides. C. Flower viewed from above, showing puberulous abaxial surface of posterior corolla lobes. D. Flowers viewed obliquely from above, all corolla lobes discolorous with the outside (abaxial) surfaces puberulous and notably paler in colour. The long white trichomes next to the flower on the right do not belong to the plant, but is a wind-blown feathery awn (arrowed) of a member of the grass genus Stipagrostis. Photographs by W. Swanepoel. |
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| Petalidium saxatile, habitat and habit. Note relatively narrow leaves ca. folded upwards along the midrib (B & C). A. Several plants (some arrowed) growing in typical habitat among stones comprising Etendeka Group basalt on an arid hillside. B. Plant with pale grey-green foliage due to a more persistent whitish indumentum, and inflorescences. C. Plant in flower and with greenish foliage resulting from a sparser indumentum. Photographs by W. Swanepoel. |
Petalidium saxatile Swanepoel, K.G.Dexter, E.Tripp & A.E.van Wyk, sp. nov.
Diagnosis:—A woody dwarf shrub up to 0.5 m tall, morphologically most similar to Petalidium canescens and P. setosum, differing by having indumentum on vegetative parts consisting of simple (weak and robust), bifurcate, stellate-dendritic, dendritic and stalked glandular trichomes, lacking sessile glands (vs. indumentum strigose with in addition widely spaced long, robust, simple trichomes [P. canescens]; long, robust, simple and stalked glandular trichomes, sessile glands present [P. setosum]); leaf lamina conduplicate (vs. flat or subconduplicate [P. canescens]; flat or irregularly curved, twisted, widely undulate, recurved or incurved towards margins [P. setosum]), usually narrower, up to 22 mm wide (vs. up to 100 mm [P. canescens (broad-leaved form)]; up to 50 mm [P. setosum]), with 3 or 4 principal lateral veins each side (vs. 4–6 [P. canescens]; 3–7 [P. setosum]); corolla expanded portion longer, 7.7–9.1 mm long (vs. ca. 7.5 mm [P. canescens]; ca. 3.4 mm [P. setosum]), anterior lobe inside (adaxially) bright yellow, lateral and upper lobes pink or brown-pink, darker towards bases (vs. all lobes similarly coloured: violet-red but anterior lobe darker [P. canescens]; purple, burgundy or carmine, anterior lobe sometimes yellow towards apex [P. setosum]), nectar guides absent (vs. present).
Wessel SWANEPOEL, Kyle G. DEXTER, Martino ADAMO, Erin A. MANZITTO-TRIPP, Abraham E. VAN WYK. 2025. Petalidium saxatile (Acanthaceae), A New Species from Namibia. Phytotaxa. 716(3); 161-174. DOI: doi.org/10.11646/phytotaxa.716.3.1 [2025-09-03]
