Petalidium hoarusibense Swanepoel & A.E.van Wyk, 2025

Swanepoel, Wessel & Van Wyk, Abraham E., 2025, Petalidium hoarusibense (Acanthaceae), a new species from Namibia, Phytotaxa 681 (1), pp. 1-10 : 2-8

Petalidium hoarusibense Swanepoel & A.E.van Wyk , sp. nov. ( Figs 1 , 2 & 4A )

Diagnosis: —A woody dwarf shrub up to 1.2 m tall, morphologically most similar to Petalidium kaokoense , from which it differs in having indumentum on the bracteoles cobwebbed, consisting of a mixture of loosely entangled simple and dendritic trichomes up to 2.5 mm long, with in addition scattered short-stalked glandular trichomes (vs. matted/compact, stellate and dendritic, shorter, up to 0.5 mm long, lacking glandular trichomes); corolla glabrous outside (vs. exposed part strigose); nectar guides on anterior lobe narrowly triangular, separate (vs. linear-oblong, confluent or nearly so).

Type: — NAMIBIA. Kunene Region: 1812 (Sanitatas), Hoarusib River Gorge, 23 km south of Okandjombo (–BD), 458 m a.s.l., 17 May 2024, Swanepoel 644 (holotype WIND!; isotypes PRE!, PRU!) .

Erect woody dwarf shrub up to 1.2 m tall; all vegetative parts with dense white indumentum of short dendritic trichomes with three or more lateral branches, sometimes rebranching, usually some trichomes longer and bottlebrush-like, also simple or bifurcate trichomes in addition, glabrescent, branches of trichomes on stems and petioles irregular, trichomes on leaf lamina shorter with lateral branches straight. 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, white, brownish white or greyish white; older distal stems terete, bark smooth or peeling in short flakes or in long, thin, narrow strips, cream-white, or grey-white; young stems quadrangular, pale green or yellow-green, becoming white or cream-white with age, cystoliths visible, short, circular, elliptic or oblanceolate. Leaves opposite and decussate on new shoots, fascicled on older stems; petiole 5–13 mm long; lamina narrowly ovate to ovate, lanceolate (sensu Lindley) or elliptic, flat, or subconduplicate towards apex, up to 40 × 22 mm, green but appearing ashy grey due to dense indumentum, apices acute, rarely obtuse or rounded, bases cuneate, decurrent, margins entire, often sub-undulate, midrib prominently raised on both sides, 3–5 principal lateral veins slightly prominent adaxially, cystoliths visible adaxially, linear. Flowers in short axillary dichasia; bracts foliaceous, linear, oblanceolate or lanceolate (sensu Lindley), sessile, 7–12 × 1–2 mm, indumentum of dendritic trichomes (sometimes lacking adaxially) with scattered small, stalked glandular trichomes in addition; pedicels (below bracteoles; “peduncle” of some authors) up to 1 mm long; bracteoles narrowly ovate or elliptic, deeply concave, coriaceous, ca. 9 × 5 mm, connate proximally for up to 1.2 mm, apex acute, pale green, cream-brown when dry, venation reticulate, indumentum appearing cobwebby abaxially, consisting of a mixture of loosely entangled irregular simple and dendritic trichomes up to 2.5 mm long, the latter with few lateral branches towards apex, with in addition scattered small, stalked glandular trichomes with the more robust ones multicellular, indumentum adaxially of scattered short-stalked glandular trichomes, strigose towards apex, margin lanate towards apex, cystoliths visible both sides, linear or curved, dense. Calyx ca. 6 mm long including basal tube of ca. 1.1 mm deep, lobes 4, regular, narrowly triangular, lanceolate when flattened, acute, unequal, 4.0– 4.9 mm long, 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, 14–15 mm long with lobes straightened, narrow portion ca. 7 mm long, 2.1–2.4 mm diam., expanded portion at slight angle to anterior side of narrow portion, 3.5–4.9 mm long, outside glabrous, inside of anticous portion towards mouth puberulous and with few long stiff white simple trichomes, otherwise glabrous; palate prominently transversely 5–7-ribbed; lobes patent with respect to corolla tube axis, anterior lobe ovate, obovate or rectangular (subrotund when flattened), 4.0–5.5 × 4.0– 5.5 mm, lateral lobes rectangular (obovate or subrotund when flattened), 3.6–4.8 × 2.8–3.2 mm, upper lobes obovate or rectangular (elliptic when flattened), 3.8–4.8 × 2.9–3.5 mm, connate for ca. 45% of their length, overlapping, lobe margins entire, apices retuse, irregularly crenulate, lobes magenta, lateral and upper lobes sometimes slightly darker shaded than anterior lobe, lobes sometimes discolorous then abaxially (outside) whitish magenta, anterior lobe adaxially (inside) with two narrowly triangular yellow nectar guides, other lobes sometimes with short triangular maroon nectar guides, lobes glabrous except for few long stiff white simple trichomes towards bases adaxially (inside). Stamens didynamous, inserted dorsally in throat, fused portion ca. 1.2 mm long, free parts slightly tapering towards apex, with few to scattered simple and short stalked glandular trichomes, long filaments 3.0– 3.4 mm long, short filaments 1.6–1.8 mm long, outer filament with basal ridge (“trace”) from point of insertion on corolla decurrent to ca. 4.9 mm from base of tube, puberulous; filament curtain reduced (sensu terminology of Manktelow 2000); anthers 2-thecous, thecae linear-elliptic or linear-oblong, equal, ca. 1.6 mm long including short basal spur, maroon-white with widely spaced short-stalked glandular trichomes. Gynoecium 9.7–11.6 mm long; ovary ovoid, laterally compressed, ca. 1.3 × 1.3 mm, inserted in fleshy disc, glabrous; style filiform, ca. 8.2 mm long, puberulous, stigma lobes linear, unequal, longer lobe 0.4–1.3 mm long, shorter lobe 0.3–0.9 mm long. Capsule flattened ovoid or ellipsoid, ca. 5.5 × 3.5 mm, tawny, glossy, glabrous; seeds cordate, ca. 3.4 × 2.6 mm, densely covered with white hygroscopic trichomes.

Phenology: —Flowers and fruit have been recorded from March to June.

Distribution and habitat: — Petalidium hoarusibense is currently known only from the mountainous area along the Hoarusib River and its tributaries to the east and south of Okandjombo. This area is part of the Great Escarpment in northwestern Namibia ( Fig. 3 ). Petalidium hoarusibense occurs on arid hillsides and along drainage lines at elevations of 450–700 m a.s.l., 64–88 km inland from the Atlantic Ocean. The region receives less than 100 mm of annual rainfall, mostly during the summer months ( Atlas of Namibia Team 2022). Additionally, fog from the Atlantic Ocean may provide some moisture ( Mitchell et al. 2020), as this area experiences fog on 1–5 days per year ( Atlas of Namibia Team 2022). However, fog-derived moisture is not considered to be a significant contributor to the water requirements of this species.

Conservation status: — Petalidium hoarusibense has been recorded at several localities in an area of ca. 20 × 18 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. Many dead plants can be seen on the hill sides, probably due to prolonged droughts in the area. Petalidium hoarusibense is here provisionally ranked as Vulnerable VU B1 (a), (b).v ( IUCN 2012).

Etymology:— The specific epithet refers to the Hoarusib River and its catchment to which Petalidium hoarusibense is endemic.

Notes: — Petalidium hoarusibense is morphologically most similar to P. kaokoense , perhaps its closest relative. Hence these two species were compared in the diagnosis above. Some of the morphological features to distinguish between P. hoarusibense and P. kaokoense are also provided in Table 1. Also see Fig. 4 .

The new species can also be confused with several other species of Petalidium from the Kunene Region, Namibia, with dense, white indumentum and flowers borne in short axillary dichasia, notably Petalidium ohopohense , P. rossmannianum , P. sesfonteinense , and P. welwitschii Moore (1880: 227) . However, P. hoarusibense is readily differentiated from these species by the magenta corolla lobes [vs. mauve to violet-red ( P. ohopohense { Fig. 4E }), white ( P. rossmannianum { Fig. 4F }), white, pink, magenta, apricot, yellow or cream ( P. sesfonteinense { Fig.4C }), and cream or mauve with claret longitudinal lines ( P. welwitschii { Fig. 4C & D })]. Those forms of P. sesfonteinense with magenta flowers, as in P. hoarusibense , can be distinguished by differences in the indumentum of the bracteoles (see below). In addition, Petalidium hoarusibense has the outside of the corolla glabrous [vs. exposed part strigose ( P. rossmannianum )]. Indumentum of the bracteoles of P. hoarusibense consists of among others long eglandular dendritic trichomes (up to 2.5 mm long, appearing cobwebby) [vs. <0.5 mm, appearing matted or trichomes scattered ( P. ohopohense , P. rossmannianum , and P. sesfonteinense ), and simple, glandular ( P. welwitschii )].

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: Kaoko Otavi–Sanitatas , 27 miles to S [Sanitatas], 1812BD, 8 June 1963, Kers 1198 ( WIND!) ; Tributary to Hoarusib River , 8 km S of Okandjombo, 1812BD, 733 m, 17 May 2024, Swanepoel 641 ( WIND!) ; Tributary to Hoarusib River , 9 km S of Okandjombo, 1812BD, 623 m, 17 May 2024, Swanepoel 642 ( WIND!) ; Tributary to Hoarusib River , 14 km S of Okandjombo, 1812BD, 540 m, 17 May 2024, Swanepoel 643 ( WIND!) ; 10.5 miles W of Otjihu , 1813 AC, 3 May 1957, De Winter & Leistner 5667 ( PRE!) ; 2 Meilen W Omutati , 1713 CC [1813 AC], 8 June 1963, Giess & Leippert 7409 ( WIND!) ; 6 km from Okomutati towards Okandjombo on District Road 3707, 1813 AC, 679 m, 24 March 2024, Swanepoel 640 ( WIND!) .

The locality of Giess & Leippert 7409 is indicated as “ 2 miles westlich Omutati” [two miles west of Omutati] in quarter degree square 1713 CC, with the habitat described as a mountain gorge. A site visit revealed that a point two miles (3.2 km) west of Omutati is at the village of Etanga, one of the major villages to the west of Opuwo. The topography here is rather flat, lacking any gorges or mountains nearby. In quarter degree square 1813 AC is another place called Omutati (also called Okomutati) and here the habitat corresponds with the description on the label. This point is less than 3 km from the locality of Swanepoel 640. Hence, for the purposes of the distribution map, the quarter degree square of Giess & Leippert 7409 is thus taken as 1813 AC.

Figures

FIGURE 1. Petalidium hoarusibense. Habitat and habit. A. Mature plant ca. 800 mm high (greyish shrub in foreground), growing among dark grey sedimentary rocks of the Swakop Group along the banks of the Hoarusib River near Okandjombo, Namibia. B. Base of plant showing several branches arising from the main stem just above ground level, each covered with rough, fissured, greyish white bark. Photographs by W. Swanepoel.

FIGURE 2. Petalidium hoarusibense. Morphology of leaves and flowers. A. Shoot with flower and leaves, the latter with dense, greyish white indumentum. B, C. Flower in front (B) and side (C) view; note bracteoles (some of spent flowers) with cobwebby indumentum. D, E, F, G. Flowers in front view, each from a different plant to show variation. Note all corolla lobes of a flower being ca. similarly coloured in various shades of magenta, or lateral and upper lobes slightly darker shaded than the anterior lobe, sometimes (as in D, E, G) with darker magenta or maroon triangular nectar guides; anterior lobe adaxially with two narrowly triangular yellow nectar guides. Photographs by W. Swanepoel.

FIGURE 3. Known distribution of Petalidium hoarusibense (black dots) and P. kaokoense (open circles). Based on herbarium specimens in Herbs. PRE, PRU, and WIND.

FIGURE 4. Petalidium hoarusibense; flowers, compared with those of other species with leaves covered in a rather similar-looking whitish grey indumentum of mainly dendritic trichomes. A. P. hoarusibense; corolla lobes adaxially magenta, with yellow nectar guides on anterior lobe narrowly triangular and separate. B. P. kaokoense; corolla lobes adaxially maroon (vinaceous), with yellow nectar guides on anterior lobe linear-oblong, confluent or nearly so. C. P. sesfonteinense; colour of corolla lobes very variable: white, pink, magenta, apricot, yellow or cream (more images in Swanepoel & Manzitto-Tripp 2022). D. P. welwitschii; corolla lobes cream or mauve, with claret longitudinal lines. E. P. ohopohense; corolla lobes mauve to violet-red. F. P. rossmannianum; corolla white, with pinkish centre. Photographs by W. Swanepoel.