Petalidium mannheimerae Swanepoel, Nanyeni & A.E.van Wyk, 2022

Swanepoel, Wessel, Nanyeni, Leevi & Van Wyk, Abraham E., 2022, Petalidium mannheimerae (Acanthaceae), a new species from Namibia and South Africa, with notes on the taxonomic identity of P. parvifolium, Phytotaxa 561 (1), pp. 1-13 : 5-11

Petalidium mannheimerae Swanepoel, Nanyeni & A.E.van Wyk , sp. nov. (Figs 3 & 4)

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).

Type: — NAMIBIA. || Kharas Region : Karasburg District , Aussenkjer [Aussenkehr] 147 Farm : The mountain on the bank of the Orange River less than a km from the turn off to the right as you come in from the Sjambok entrance, 2817 CB, 14 September 2014 , Nanyeni 936 (holotype WIND!; isotypes PRE!, PRU!) .

Petalidium sp. A in Snijman (2013: 164).

Dense, woody, dwarf shrub to 1.5 m tall. Stems multi-stemmed from just below or above ground level from a thick rootstock or main stem, up to 140 mm in diam., bark smooth, cream, grey-white or grey-black; older distal stems rigid, cylindrical, bark smooth or longitudinally fissured, cream, grey-brown or yellow-brown; young stems quadrangular, green, puberulent, usually with widely spaced, long, robust, multi-cellular trichomes, glabrescent, cystoliths visible, linear or linear-oblanceolate. Leaves opposite and decussate, subsessile or with petioles up to 1.5 mm long; lamina narrowly obovate, oblanceolate, elliptic or suborbicular, subconduplicate to conduplicate, recurved towards apex, 10– 24 × 4–16 mm, green, semi-succulent, puberulent, apex acute, rounded or emarginate, base attenuate, decurrent onto petiole almost to stem, margin entire with widely spaced, robust, stalked glandular trichomes; midrib conspicuous, prominent abaxially, lateral veins indistinct or absent, cystoliths conspicuous on both sides, especially in herbarium material. Flowers solitary, axillary, supporting leaf (“bract”) similar to foliage leaves; pedicels (below bracteoles) 3–4 mm long; bracteoles ovate, usually asymmetrically, membranaceous, apex attenuate, usually with 1–5, long, sharp, robust, multicellular trichomes, base cordate or truncate, pale green, yellow-green or mauve-green, when dry also buff or brown, venation usually conspicuous, pale green, dark green or mauve-green, prominent especially adaxially, 13–15 × 7–9 mm, puberulent both sides, margins lanate and sometimes with widely spaced short stalked glandular and or long, robust multicellular trichomes, cystoliths visible on midrib towards base. Calyx 5–6 mm long including basal tube of 1–2 mm, puberulous abaxially towards base and with scattered very short stalked glandular trichomes abaxially, strigose adaxially, margins puberulous-ciliate; lobes 5, regular, linear-triangular, acute, 3.3–4.0 × 1.0– 1.2 mm. Corolla with narrow unexpanded portion of tube cylindrical, slightly widening towards throat, slightly flattened, 22–25 mm long with lobes straightened, narrow portion ca. 5–7 mm long, 2.0– 2.5 mm diam., expanded portion 8–10 mm long, corolla white, inside of expanded portion pale maroon, chestnut or yellow, distal half sometimes white, lobe traces (veins) including herringbone pattern on anticous portion white, front lobe with two pale yellow or cream narrowly triangular nectar guides or guides absent, other lobes sometimes with pale maroon markings towards base, outside puberulous, towards mouth with few long stiff white hairs, throat puberulous, otherwise glabrous; lobes patent, obovate, apices rounded, often retuse, margins entire or irregularly denticulate to crenate, upper lobes overlapping, free, ca. 6 × 9 mm, lateral lobes ca. 8 × 8 mm, front lobe ca. 9 × 7 mm; palate prominently transversely ca. 7-ribbed. Filaments didynamous, inserted dorsally in throat, filaments fused for ca. 1.8 mm at base, fused part prominent, adnate to tube, free parts tapering towards apex, flattened, sparingly puberulous with few short stalked glandular trichomes, long filament 6.1–7.3 mm long, short filament 4.3–4.8 mm long, outer filament trace decurrent to base of tube, puberulous; filament curtain phaulopsoid ( Manktelow 2000); anthers 2-thecous, thecae oblong, equal or subequal, 2.1–2.3 mm long with scattered short stalked glandular trichomes, apex rounded, with minute spurs at base, 0.2 mm long. Gynoecium ca. 14 mm long; ovary ovoid, laterally compressed, 2.0 × 1.3 mm, inserted on fleshy disc, glabrous; style filiform, ca. 10 mm long, puberulous, stigma lobes linear, slightly flattened, unequal, longer lobe ca. 0.8 mm long, shorter lobe ca. 0.4 mm long. Capsule elliptic, 6.5–7.0 × 3.4–5.0 mm long, tawny, glossy, glabrous.

Phenology:— Flowers have been recorded in August and September; fruits throughout the year.

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).

Conservation status:— Petalidium mannheimerae has been recorded at seven localities in an area of ca. 60 × 15 km where it is locally occasional to common. Although a brief search at various other localities with seemingly suitable habitat did not reveal any plants, it is probably more widespread than currently known. Although protected in the Aussenkehr Nature Park in Namibia, small stock farming on the banks of the Orange River might lead to overutilization by domestic goats belonging to local inhabitants. In South Africa, grazing by small stock and even a heavily grazed population is noted on herbarium sheets by two collectors (Jürgens 22952, Steyn 2127). At Swartkop, mining activities might pose a threat as P. mannheimerae occurs very close to and even in the mining area. A recent visit to the area of distribution revealed many dead woody plants, recently died, probably due to prolonged droughts in the area. It is possible that some of these might be referred to the new species. Petalidium mannheimerae is here provisionally ranked as Vulnerable VU D1 ( IUCN 2012).

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.

Notes:— Some of the morphological features to distinguish among Petalidium mannheimerae , P. lucens , and P. parvifolium are provided in Table 1; also see Figs 4–6.

Petalidium mannheimerae can also be confused with P. cymbiforme and P. linifolium due to similarities in habit and flower morphology. However, these species have a different indumentum and linear leaves, with the corolla of P. linifolium pale mauve (vs. white) and the venation of the bracteoles of P. cymbiforme inconspicuous (vs. usually conspicuous).

All the mentioned species are members of Petalidium sect. Petalidium (sensu Meyer 1968), a group composed of plants with regular, five-parted calyces ( Obermeijer 1936; Tripp et al. 2017).

Additional specimens examined (paratypes):— NAMIBIA. || Kharas Region: Karasburg District, Aussenkjer [Aussenkehr] 147 Farm : The mountain on the bank of the Orange River less than a km from the turn off to the right as you come in from the Sjambok entrance, 2817 CB , 12 August 2018, Nanyeni 1371, 1372 ( WIND!); Mountain slope , 250 m downstream of Sambok River Mouth, Farm Aussenkehr 147, 2817 CB , 134 m, 23 February 2022, Swanepoel 571 ( WIND!) . SOUTH AFRICA. Northern Cape Province: Richtersveld. Kahams area east of Stinkfonteinberge, east of kloof running down from Cornellsberg , 2817 CA , 600 m, 5 September 1977, Oliver, Tölken & Venter 665 ( PRE!); 12 miles from Stinkfontein on way to Vioolsdrif in Richtersveld , 2917 CB , ca. 700 m, 15 November 1971, Werger 1511 ( PRE!); Richtersveld. Vanzylsrivier , 2817 CB , 580 m, 25 September 1987, Jurgens 22952 ( PRE!); Namaqualand District. Richtersveld, southeast of Sunvalley camp site, 2817 CB , 551 m, 8 August 2011, Steyn 1847 ( PRE!); Richtersveld. Near Stinkfontein, 2817CD, 14 September 1961, Van Breda 1497 ( PRE!); Richtersveld. Road between Stinkfontein & Modderdrift, 20 miles from Stinkfontein, 2817CD, 15 September 1961, Hardy 675 ( PRE!); Vioolsdrif, Klein Helskloof, 20 km northeast of Eksteenfontein, 2817CD, 9 September 2002, Venter & Venter 9800 ( PRE!); Namakwaland. 53 km vanaf Steinkopf pad na Vioolsdrif, 2817DC, 27 August 1983, Van Wyk 6513 ( PRE!, PRU!); Vioolsdrif District . Nababeepsberg (Swartberge) in deep gorge below Helshoogte on eastern side of main road, 2817DC, 27 August 1988, Williamson 3912 ( PRE!); Namakwaland. Swartbergmyn tussen Steinkopf en Vioolsdrif , 2817 DD , 27 August 1983, Van Wyk 6532 ( PRE!, PRU!); Swartkop Mine, 2817 DD , 643 m, 7 August 2018, Nanyeni 1370 ( PRE!, WIND!); Namaqualand District. Wyepoort River Valley , 2917 BA , 598 m, 9 October 2015, Steyn 2127 ( PRE!) .