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| Petalidium mannheimerae Swanepoel, Nanyeni & A.E.van Wyk, in Swanepoel, Nanyen et van Wyk, 2022. Photographs by L. Nanyeni, M. Koekemoer & N. Jürgens. |
Abstract
Petalidium mannheimerae, here described as a new species, is only known from the Gariep Centre of Plant Endemism in southern Namibia and northwestern South Africa. Apparently first collected in 1961, it grows on arid hillsides and in drainage lines. Diagnostic morphological characters for P. mannheimerae include the rigid, cylindrical distal stems, grey-brown or yellow-brown bark, puberulent vegetative parts with sparsely scattered but robust glandular and eglandular trichomes, and the solitary white flowers with the unexpanded part of the corolla tube shorter than the expanded part. A comparison of some of the more prominent features to differentiate Petalidium mannheimerae from its morphologically most similar relatives is provided. Based on IUCN Red List categories and criteria, a conservation assessment of Vulnerable VU D1 is recommended for the new species. Herbarium specimens of P. mannheimerae have long been misidentified as P. parvifolium, a rarely collected species from east-central Namibia and west-central Botswana. Lectotypes are designated for three taxa, namely P. parvifolium, P. parvifolium var. angustifolium (a synonym of P. linifolium), and P. wilmaniae (a synonym of P. parvifolium).
Keywords: desert, endemism, flora, Gariep Centre, lectotypification, Ruellieae, taxonomy
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| Petalidium mannheimerae, habitat and habit. A. Plant in flower during a particularly dry season (Aussenkjer [Aussenkehr] 147 Farm, Kharas Region, Namibia). B. Plant in full flower (Kosies, Richtersveld, Northern Cape, South Africa). Photographs by L. Nanyeni (A) & M. Koekemoer (B). |
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| Petalidium mannheimerae, morphology of flowers from different localities in the Richtersveld, Northern Cape, South Africa (A–D), and leaf morphology (E). A. Flower in lateral view (Sun Valley). B. Flower in front view (Sun Valley). C. Flower in front view (Kosies). D. Flowers (Umdaus). E. Branchlet showing leaves being semi-succulent, the blade subconduplicate to conduplicate, recurved towards apex, the margins with isolated, robust, stalked glandular trichomes. Scale bar = 5 mm. Photographs by M. Koekemoer (A–C), N. Jürgens (D), & W. Swanepoel (E). |
Petalidium mannheimerae Swanepoel, Nanyeni & A.E.van Wyk, sp. nov.
Diagnosis:—A woody dwarf shrub up to 1.5 m tall, morphologically most similar to Petalidium lucens and P. parvifolium differing from both in having the lamina semi-succulent, subconduplicate to conduplicate, recurved towards apex, and with lateral veins indistinct or absent (vs. lamina not succulent, flat, lateral veins distinct); from P. lucens in having indumentum on vegetative parts puberulent and on leaf margins having widely spaced, robust, stalked glands (vs. dense short simple or sessile stellate trichomes [with 2 or 3 branches], usually with isolated dendritic trichomes in addition, glabrescent), corolla lobes white (vs. mauve or light blue); from P. parvifolium in having indumentum on vegetative parts puberulent (vs. scattered sessile glandular trichomes), leaf margins with widely spaced, robust, stalked glandular trichomes (vs. trichomes absent), corolla white (vs. mauve, white or white with lilac tinge).
Etymology:—The specific epithet honours Mrs Coleen Anne Mannheimer [1957– ], renowned Namibian botanist and former curator of the National Herbarium of Namibia (WIND). She has collected widely in Namibia and has authored or co-authored several publications on the Namibian flora. Coleen introduced one of us (LN) to botany and botanical fieldwork and has since been a mentor to him in the herbarium and in the field. Her knowledge of the Namibian flora is exceptional and her ability to develop and encourage future Namibian botanists has been profound.
Distribution and habitat:—At present, Petalidium mannheimerae is known in Namibia from the vicinity of
Aussenkehr in the Orange River Valley and in South Africa from ca. 30 km south of Vioolsdrif to near Eksteenfontein
in the Richtersveld (Fig. 2). It occurs 92–110 km from the Atlantic Ocean on hillsides and drainage lines, at elevations
of 134–187 m in Namibia and in South Africa at 551–700 m a.s.l. Average annual rainfall in the area is less than 50
mm (Mendelsohn et al. 2002).
Wessel Swanepoel, Leevi Nanyen and Abraham E. van Wyk. 2022. Petalidium mannheimerae (Acanthaceae), A New Species from Namibia and South Africa, with Notes on the Taxonomic Identity of P. parvifolium. Phytotaxa. 561(1); 1-13. DOI: 10.11646/phytotaxa.561.1.1
