Namib desert petal-bush (Petalidium namibense), previously confused with P. englerianum, P. rossmannianum and P. variabile

NEWS - Namib desert petal-bush (Petalidium namibense Swanepoel & A.E.van Wyk, sp. nov.), previously confused with Petalidium englerianum, Petalidium rossmannianum and Petalidium variabile was established as a new species with a restricted range in the southwest, west and northwest of Puros in the Kaokoveld Centre of Endemism, northwestern Namibia.

Currently, 41 species of Petalidium Nees von Esenbeck (1832) have been described in Africa. The main centre of diversity for the genus is in northwest Namibia and adjacent southwest Angola. Namibia is home to 31 species, while 13 species have been recorded in Angola, 6 in South Africa, and 33 species have been recorded in South Africa, Namibia, Botswana, Eswatini and Lesotho.

During several expeditions to the Puros region, Wessel Swanepoel discovered an unknown Petalidium characterized by a dwarf shrubby habit, many stems from below or above the ground, white flaking bark (corky on older stems), long dendritic trichomes and flowers in short dichasia.

P. namibense has an erect, hemispherical or procumbent woody dwarf shrub habit up to 0.8 m tall; all vegetative parts with a dense white indumentum of dendritic trichomes, the trichomes sparsely branched or bottlebrush-like, sometimes rebranching, appearing lanate, interspersed with few or scattered short-stalked glandular trichomes, the latter often concealed by the much longer dendritic ones.

Stems single or multi-stemmed from just below or above ground level from thick rootstock or main stem, up to 120 mm in diam., bark rough and fissured, corky in older plants, gray or blackish grey; older distal stems cylindrical, bark smooth, often longitudinally fissured or rough and corky, cream-white, cream-brown or grey; young stems quadrangular, green, becoming cream-white with age, glabrescent, cystoliths visible.

Leaves opposite and decussate on new shoots, fascicled on older stems; petiole 1–14 mm long; lamina ovate, elliptic, suborbicular or orbicular, flat, often sub-conduplicate towards apex, up to 42 × 26 mm, green, apices rounded or acute, usually apiculate, bases cuneate, truncate or rounded, margins entire, often subinvolute, midrib and 3–6 principal lateral veins prominently raised adaxially, cystoliths not visible; indumentum on leaves with short bifurcate trichomes in addition.

Flowers in short axillary dichasia; bracts foliaceous, lanceolate, linear-lanceolate or oblanceolate, sessile, 6–13 × 1–2 mm; pedicels up to 1 mm long; bracteoles narrowly ovate, slightly asymmetrical, coriaceous, ca. 11 × 5 mm, connate proximally for up to 2.5 mm, apex acute or acuminate, pale green, cream-brown when dry, venation reticulate, indumentum abaxially similar to vegetative parts but trichomes longer and more robust, adaxially strigose towards apex and with additional short-stalked glandular trichomes or glabrous, margin lanate towards apex, cystoliths visible both sides, linear or curved, dense.

Calyx 5.9–6.5 mm long including basal tube of 0.9–1.5 mm deep, lobes 4, regular, narrowly triangular, acute, unequal, 3.9–5.6 × 0.6–1.4 mm, anticous lobe indistinctly bifid; strigose both sides, scattered short-stalked glandular trichomes in addition abaxially.

Corolla with narrow unexpanded portion of tube cylindrical, laterally slightly flattened, 15.0–17.5 mm long with lobes straightened, narrow portion 7.4–8.2 mm long, ca. 2.3 mm diam., expanded portion at a slight angle to anterior side of narrow portion, 3.9–4.5 mm long, outside glabrous, sometimes sparingly puberulous on posterior side, inside of anticous portion towards mouth puberulous and with few long stiff white simple trichomes, inside otherwise glabrous.

Lobes patent with respect to corolla tube axis, anterior lobe obovate, margins entire, apices retuse or truncate, 5.0–5.7 × 4.7–5.5 mm, lateral and upper lobes rectangular (upper lobes obovate when flattened), margins often revolute, apices retuse or truncate, lateral lobes 4.0–4.9 × 2.6–3.5 mm, upper lobes 4.9–5.2 × 2.9–3.2 mm, connate for 25–40% of their length, slightly overlapping, discolorous, abaxially light brown, adaxially vermillion, anterior lobe adaxially magenta with two narrowly triangular yellow nectar guides, the two lateral lobes adaxially vermillion or carmine distally, grading into vermillion towards throat, lacking nectar guides or nectar guides inconspicuous, slightly darker in color than that of the associated lobes, glabrous lobes except for a few long stiff white simple trichomes towards bases adaxially; palate prominently transversely 4- or 5-ribbed.

Stamens didynamous, inserted dorsally in throat, fused portion 1.1–1.7 mm long, free parts slightly tapering towards apex, glabrous, long filaments ca. 4.0 mm long, short filaments ca. 2.6 mm long, outer filament with basal ridge from point of insertion on corolla (“trace”) decurrent to 4.2–5.7 mm from base of tube, puberulous; filament curtain reduced; anthers 2-thecous, thecae linear-elliptic, equal, ca. 2.3 mm long including short basal spur, cream-brown with scattered short-stalked glandular trichomes.

Gynoecium 12.0–13.8 mm long; ovary ovoid, laterally compressed, 1.1–1.7 × 0.8–1.4 mm, inserted in fleshy disc, glabrous; style filiform, 10.0–11.8 mm long, puberulous, stigma lobes linear, unequal, longer lobe ca.1.0 mm long, shorter lobe ca . 0.7 mm long. Capsule flattened ovoid, ca. 5.5 × 3.1 mm, tawny, glossy, glabrous; seeds cordate, ca. 3.1 × 2.4 mm, densely covered with white hygroscopic trichomes.

Flowers and fruits recorded from February to November. Currently , the new species is known only from the southwest, west and northwest of Puros, between the lower Sechomib and Hoarusib rivers below the Great Escarpment in northwest Namibia. P. namibense at the base of rocky outcrops, arid hillsides and along drainage lines at elevations of 280-560 m.

Average annual rainfall is less than 100 mm and mostly in the summer. Although areas within the distribution range experience fog from the Atlantic Ocean about 5–10% of the year, poor conditions during the dry season suggest the plant may not benefit from this water source.

The species has been recorded at several sites in an area of about 55 × 20 km, and is rare to common locally. Although protected in the Skeleton Coast National Park, recent surveys have revealed that many plants have recently died, likely due to prolonged drought. The researchers have proposed a provisional status of Vulnerable (VU).

The specific designation refers to the Namib Desert where 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.

P. namibense is most similar to P. sesfonteinense. When not in flower, the new species can also be confused with several other Petalidium species from the Kunene Region, Namibia, with dense indumentum and similar inflorescences, especially P. kaokoense Swanepoel (2020), P. ohopohense Meyer (1973), P. rossmannianum and P. welwitschii Moore (1880).

However, the indumentum of P. kaokoense consists of stellate trichomes interspersed with a few dendritic trichomes (vs. dendritic trichomes interspersed with scattered short-stalked glandular trichomes). Some branches P. ohopohense dendritic trichomes have terminal glands and hairy corollas on the outside (vs. eglandular trichomes branching and glabrous corollas on the outside).

The dendritic trichomes on the leaves and bracteoles of P. rossmannianum are very small with short lateral branches, interspersed with scattered stellate trichomes, and the widened corolla tube portion is pubescent (vs. the dendritic trichomes are larger, more branched, interspersed with short-stemmed glandular trichomes). scattered, crown glabrous on the outside).

P. welwitschii bracteoles are covered abaxially with scattered villous trichomes up to 4 mm long (vs. bracteoles that appear lanate due to a dense layer of dendritic trichomes).

Original research

Wessel Swanepoel & Abraham E. Van Wyk (2024). Petalidium namibense (Acanthaceae), a new species from Namibia. Phytotaxa 671 (2): 128–138, DOI:10.11646/phytotaxa.671.2.2

Dlium theDlium