Petalidium namibense Swanepoel & A.E.van Wyk, 2024

Swanepoel, Wessel & Van Wyk, Abraham E., 2024, Petalidium namibense (Acanthaceae), a new species from Namibia, Phytotaxa 671 (2), pp. 128-138 : 129-134

Petalidium namibense Swanepoel & A.E.van Wyk , sp. nov. ( Figs 1–3 )

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).

Type: — NAMIBIA. Kunene Region: 1812 (Sanitatas), Namib Desert, 3 km north of Khumib River, 4 km east of Skeleton Coast Park boundary, gneiss ridge (–DA), 377 m a.s.l., 23 May 2022, Swanepoel 636 (holotype WIND!; isotypes PRE!, PRU!) .

Erect, hemispherical or procumbent woody dwarf shrub 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, grey 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 (sensu Lindley), linear-lanceolate or oblanceolate, sessile, 6–13 × 1–2 mm; pedicels (below bracteoles; “peduncle” of some authors) 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 colour than that of the associated lobes, lobes glabrous except for 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 (sensu terminology of Manktelow 2000); 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.

Phenology: —Flowers and fruit have been recorded from February to November.

Distribution and habitat: —At present, Petalidium namibense is only known from the area to the southwest, west and northwest of Puros, between the lower Sechomib and Hoarusib rivers below the Great Escarpment in northwestern Namibia ( Fig. 4 ). The specimen Moss & Jacobsen K312 from the Hoanib River Valley (southernmost point in Fig. 4 ) is located ca. 70 km to the south of the known core range of P. namibense . It morphologically seems to conform in all respects to P. namibense and probably represents a second, outlier population of the species. Petalidium namibense occurs at the base of rocky outcrops, on arid hillsides, and along drainage lines at elevations of 280–560 m a.s.l., 20–55 km inland from the Atlantic Ocean. Average annual rainfall in the area is less than 100 mm and falls mainly in summer ( Atlas of Namibia Team 2022). In certain parts of the Namib Desert, fog from the Atlantic Ocean is a regular and vital source of moisture, particularly for smaller animals ( Mitchell et al. 2020). Although the area in the distribution range of P. namibense experiences fog about 5–10% of the year ( Atlas of Namibia Team 2022), the poor condition of the plants during droughts suggests that they may not benefit significantly from this moisture source.

Conservation status: — Petalidium namibense has been recorded at several localities in an area of ca. 55 × 20 km where it is occasional to locally 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. Even though it is protected within the Skeleton Coast National Park, where its range slightly extends, a recent survey of its distribution area revealed numerous recently dead plants, likely due to extended droughts. Petalidium namibense is here provisionally ranked as Vulnerable VU B1a(iii), (b)iii ( IUCN 2012).

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).

Notes: — Petalidium namibense is morphologically most similar to P. sesfonteinense , perhaps its closest relative. Hence these two species were compared in the diagnoses above. Some of the morphological features to distinguish between P. namibense and P. sesfonteinense are provided in Table 1; also see Fig. 5 . An identification key to distinguish between these two species, as well as P. englerianum , P. rossmannianum , and P. variabile , is provided below.

When not in flower and especially in the case of herbarium specimens, the new species can also be confused with several other species of Petalidium from the Kunene Region, Namibia, with dense indumentum and similar inflorescences, notably Petalidium kaokoense Swanepoel (2020: 237) , P. ohopohense Meyer (1973: 108) , P. rossmannianum , and P. welwitschii Moore (1880: 227) . However, the indumentum of P. kaokoense consists of stellate trichomes interspersed with a few dendritic ones (vs. dendritic trichomes interspersed with scattered short-stalked glandular trichomes); some branches of the dendritic trichomes of P. ohopohense bear a terminal gland and the corollas are pilose on the outside (vs. trichome branches eglandular and corolla glabrous 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 expanded part of the corolla tube is puberulous (vs. dendritic trichomes larger, longer branched, interspersed with scattered short-stalked glandular trichomes, corolla glabrous outside); the bracteoles of P. welwitschii are covered abaxially with scattered villose trichomes up to 4 mm long (vs. bracteoles appearing lanate due to dense layer of dendritic trichomes).

All the mentioned taxa are from the group composed of plants with irregular, four-parted calyces ( Obermeijer 1936, Tripp et al. 2017).

Additional specimens examined (paratypes): — NAMIBIA, Kunene Region: 7 km southwest of Ogams Fountain , 1812 CB, 406 m, 5 November 2023, Swanepoel 639 ( WIND!) ; Namib-Kiesfläche 14 Meilen nördlich Sarusas , 1812DA, 9 June 1963, Giess & Leippert 7453 ( PRE!, WIND!) ; 20 Meilen nordoestlich Sarusas Ost , 1812DA, 26 April 1966, Giess 9416 ( PRE!, WIND!) ; 30 km west-northwest of Puros along track to Skeleton Coast National Park , 1812DA, 456 m, 30 May 2018, Swanepoel 635 ( WIND!) ; 8 km southeast of Ogams Fountain , 1812DA, 551 m, 5 November 2023, Swanepoel 638 ( WIND!) ; Puros, rocky terrain opposite Hoarusib River , 1812 DD, 292 m, 3 November 2023, Swanepoel 637 ( WIND!) .

Figures

FIGURE 1. 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.

FIGURE 2. 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. Photographs by W. Swanepoel.

FIGURE 3. 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.

FIGURE 4. Known distribution of Petalidium namibense (black dots; ●) based on specimens in Herbs PRE and WIND.Also depicted are the distribution range of P. sesfonteinense (shaded grey), based on Swanepoel & Manzitto-Tripp (2022).

FIGURE 5. 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).