Petalidium ovatum (Schinz) Clarke (1899: 90)

Swanepoel, Wessel, Dexter, Kyle G., Loiseau, Oriane & Van Wyk, Abraham E., 2023, Reinstatement of the name Petalidium ovatum (Acanthaceae), with an amplified description of the species, Phytotaxa 626 (3), pp. 159-169 : 161-166

Petalidium ovatum (Schinz) Clarke (1899: 90) ( Figs 1–3 )

Basionym:— Pseudobarleria ovata Schinz (1890: 198) .

Type: — NAMIBIA. Kunene Region: Kaokofelt auf Korikas [Kaokoveld at Khorixas], fruct. – 0.60 m [shrublet – 0.60 m], 18 March 1885, Belck 20 [lectotype Z+ZT, Z-000000110 photo!, designated here (or perhaps holotype)]. Reasons for the lectotypification are supplied under “Typification” below.

Erect woody dwarf shrub to 1 m tall, single- or multi-stemmed from just above ground level; all vegetative parts with a dense white indumentum of sessile or short-stalked stellate and longer bottle brush-like dendritic trichomes, glabrescent on older stems and leaves. Stems: main stem up to 120 mm diam., bark fissured, grey-white; bark on distal stems cream-brown or brown, peeling in long, thin, narrow strips; young stems quadrangular, white or grey-white. Leaves opposite and decussate, younger leaves clustered in axils, petiolate; lamina ovate, rotund or subrotund, 10–51 × 6–38 mm, rarely glabrescent, white to grey-white to dark greyish green, cystoliths inconspicuous, linear-oblong or linear-oblanceolate; apex acute, rounded, emarginate or truncate, sometimes minutely apiculate, base cuneate, rounded, subcordate or truncate, shortly decurrent onto petiole, margins entire; midrib slightly prominent above and below, principal lateral veins 2–5 each side, slightly prominent below; petiole 2–15 mm long. Flowers in short dichasia, bracts foliaceous, oblanceolate, 4–13 × 1.4–2.5 mm, apex acute or obtuse, sessile; pedicel (below bracteoles) up to 5 mm long; bracteoles broadly ovate, usually asymmetric, coriaceous, apex acute, sometimes slightly acuminate, base rounded or truncate, cream to creamy brown, sometimes light grey-green, reticulation prominent on both sides, pale green, cream-brown when dry, ca. 10–15 × 7–14 mm, indumentum abaxially similar to vegetative parts, adaxially with additional short-stalked glandular trichomes or glabrous, margin lanate towards apex, cystoliths visible, especially adaxially, straight or curved. Calyx ca. 5.5 mm long including basal tube ca. 1.4 mm deep, with scattered sessile or subsessile glandular trichomes, sparingly puberulous towards apex and strigose towards base adaxially; lobes 4, lanceolate, acute, 3.1–4.2 mm long. Corolla 14.8–18.3 mm long with lobes straightened, narrow unexpanded portion cylindrical or slightly widening towards throat, laterally compressed, 7.2–9.1 mm long, 2.8–3.5 mm diam., expanded portion 2.1–2.4 mm long, corolla glabrous outside except narrow tube distally and expanded portion sometimes sparsely puberulous with short simple trichomes in addition, inside puberulous on anterior side of narrow portion, long patent eglandular trichomes on anterior side of expanded portion; anterior lobes yellow, dotted burgundy at insertion of trichomes, burgundy towards margins or on lateral margins only, other lobes burgundy, lower lobe obovate, patent, sometimes recurved, 4.7–5.1 × 5.1 mm, upper lobes narrowly obovate or oblong, connate for 45–50% of their length, erect or suberect, ca. 4.7–5.6 × 3.1 mm, lateral lobes narrowly obovate or oblong, patent, ca. 4–5 × 3 mm, lobe apices rounded, truncate or widely retuse, all lobes with long, stiff, patent, white eglandular trichomes, all lobe margins entire; palate prominently transversely 4-ribbed. Filaments didynamous, inserted dorsally in throat, each pair comprising a long and short filament connate for 1.3–1.6 mm at base, connate part prominent, adnate to tube, with scattered short-stalked glandular and few bifurcate trichomes, long filament 3.7–4.0 mm long, short filament 2.3–2.8 mm long; filament curtain reduced (sensu terminology of Manktelow 2000); anthers 2-thecous, thecae oblong with minute spurs at base, ca. 1.7 mm long, sparsely puberulous with in addition scattered short-stalked glandular and bifurcate eglandular trichomes. Gynoecium ca. 13.6 mm long; ovary ovoid, laterally compressed, ca. 2.6 ×1.7 × 1.1 mm, situated in fleshy disc, glabrous, ovules flattened-ovoid, 0.6–0.7 mm long; style filiform, ca. 10.4 mm long, puberulous, stigma lobes linear, slightly flattened, subequal, longer lobe ca. 0.6 mm long, shorter lobe 0.3 mm long. Capsule flattened, ellipsoid or ovoid, 7.5–8.6 × 4.3 × 2.5–3.0 mm, chestnut, glossy, sides smooth, glabrous. Seeds cordate, ca. 3.2–3.9 × 2.8 mm, densely covered with white long hygroscopic trichomes.

Phenology: —Flowers and fruit have been recorded from February to May (late summer to autumn).

Typification: —In the protologue of the name Pseudobarleria ovata, Schinz (1890) cited a single gathering, namely Belck 20, collected in 1885 from near Khorixas, Namibia, but did not specify a particular specimen or the institution housing it. As pointed out by McNeill (2014) if, prior to 1958, a single gathering is indicated as the basis of a new taxon, “...there will be a holotype only if it can be established that the author used no other element and if the gathering is represented by a single specimen—because the specimens that comprise the gathering are syntypes ” ( Turland et al. 2018: Art. 40.2, Note 1). We were able to locate only one specimen of Belck 20, which is currently held in Herb. Z+ZT. However, Obermeijer (1936) mentioned that she had seen Belck 20, but did not specify Herbarium Z when acknowledging various herbaria from which she studied specimens. This suggests the possibility of a duplicate of Belck 20 existing elsewhere, most likely in Herb. B, which she explicitly mentioned. In fact, it has been documented ( Urban 1916: 328) that Herb. B received 226 numbers from Waldemar Belck between 1881 and 1885. Thus, circumstantial evidence suggests that there may have been multiple sheets of Belck 20, possibly also in Herb. B. Nevertheless, no specimens from this collection are extant in Herb. B, leading to the assumption that the original specimen(s) were likely destroyed during a World War II bombing raid in 1943.

Swiss botanist Hans Schinz (1858–1941) was based in Zürich, where he served as the Director of the Botanical Garden and held the position of Professor of Botany at the University of Zürich ( Glen & Germishuizen 2010). Therefore, it is highly likely that Schinz had access to the specimen of Belck 20 currently housed in Herb. Z+ZT when he described Pseudobarleria ovata . This particular specimen also includes a determinavit label on which Schinz has written “ Petalidium latifolium (Schinz) C.B. Clarke ” and “ Petalidium ovatum Schinz ,” dated March 1920.Additionally, a determinavit slip by P.G. Meyer dated 1957 is affixed to this sheet, and on it is written “ Petalidium englerianum (Schinz) C.B. Clarke var. ovatum (Schinz) Hainz ex P.G. Meyer comb. nov. ined.” However, it should be noted that this designation by Meyer was never validly published. Given the likelihood of duplicates of Belck 20 existing at the time of the original publication of the name Pseudobarleria ovata , we have designated the specimen held in Herb. Z+ZT as a lectotype, but with the additional qualification “or perhaps holotype,” as suggested by McNeill (2014).

Diagnostic characters: — Petalidium ovatum is a woody dwarf shrub up to 1 m tall, morphologically most similar to P. englerianum from which it differs in having an indumentum on vegetative parts of both stellate and dendritic trichomes (vs. stellate only or rarely few dendritic in addition); lamina ovate, rotund or subrotund (vs. narrowly to broadly elliptic, oblong-elliptic, oblanceolate or rarely ovate), lamina length:width ratio of 1.0–1.5:1.0 (vs. 1.7–3.1:1.0); bracteoles broadly ovate (vs. ovate, narrowly obovate, oblanceolate, lanceolate or oblong-elliptic); corolla upper lobes connate for 45–50% of their length (vs. 20–40%), anterior lobe yellow, or yellow with burgundy in places (vs. always yellow), other lobes burgundy (vs. pale yellow or orange, usually fading to brownish orange).

Distribution and habitat: —At present, Petalidium ovatum is only known from the Khorixas-Bergsig area in the Kaokoveld Centre of Endemism, northwestern Namibia ( Fig. 4 ). The specimen Müller 1666 from north of Orupembe (most northwestern point in Fig. 4 ), is located ca. 300 km to the northwest of the known core range of P. ovatum . It morphologically seems to conform in all respects to P. ovatum and probably represents a second, outlier population of the species. Petalidium ovatum occurs on arid hillsides, drainage lines and along seasonally dry riverbeds at elevations of 650–1000 m a.s.l., about 70–160 km from the Atlantic Ocean. Average annual rainfall in the area is 100–250 mm ( Mendelsohn et al. 2002).

Conservation status: — Petalidium ovatum is locally common and probably more widespread in suitable habitats than currently recorded. It is here considered not in immediate conservation danger because it occurs in sparsely to unpopulated areas and does not seem to be utilised by humans. The extent of occurrence is estimated at <20000 km ² (3300 km ²) with less than 10 (7) subpopulations. However, since no decline in population size is known, it is here ranked as Least Concern (LC) ( IUCN 2012).

Notes: —Hitherto in herbaria, Petalidium ovatum has most often been confused with P. englerianum ( Fig. 5 ), a species from which it differs in indumentum, leaf, and flower characters. This confusion was further entrenched by Meyer’s (1968) treatment of P. ovatum as a synonym of P. englerianum . Both of these species were first validly described by Schinz (1890) in the genus Pseudobarleria ( Anderson 1863: 26) . The morphological similarity between these two species was already highlighted by Schinz (1890) when he did not describe the floral features of Petalidium ovatum . Instead, Schinz (1890) stated that the details in the construction of its flower correspond perfectly to those of Petalidium englerianum , hence the description of the latter should be consulted. Obviously, the conspicuous difference in corolla colour between these two species was not reflected by the limited herbarium material available to Schinz. Some of the morphological features to distinguish between P. ovatum and P. englerianum are provided in Table 1 .

The known distribution of the two species ( Fig. 4 ), does not overlap: P. ovatum occurs in northwestern Namibia from Khorixas westwards to near Bergsig, whereas P. englerianum occurs from the east of Khorixas to Outjo, Otjiwarongo, Otavi, Etosha National Park, Tsumeb, Grootfontein and further eastwards across the Kalahari Sandveld to Tsumkwe and into northwestern Botswana.

Petalidium ovatum can also be confused with P. pilosibracteolatum and P. variabile with which it shares a similar habit and pale grey, sometimes almost white, appearance. However, it can easily be distinguished from both by the indumentum on vegetative parts of P. ovatum that consists of both dense stellate and longer dendritic trichomes (vs. densely strigose) and from P. pilosibracteolatum by the bracteoles lacking long simple trichomes (vs. present). In general, P. ovatum is much more densely pubescent on the vegetative parts than the other two species, which gives it a whiter appearance.

All the mentioned species, including P. ovatum , are from the group composed of plants with irregular, four-parted calyces ( Obermeijer 1936, Tripp et al. 2017). We can confirm that the material hitherto attributed to P. englerianum used in molecular studies ( Tripp et al. 2017) is from authentic P. englerianum . The phylogenetic position of P. ovatum is therefore not known at present, but we expect that it is closely related to P. englerianum and P. ramulosum Schinz (1916: 434) , which form a clade in recent phylogenetic studies ( Tripp et al. 2017, Loiseau et al. 2023). Petalidium ramulosum has a similar indumentum and white appearance, while its flower more closely resembles that of P. ovatum , at least in the colouration of the corolla. The latter two species have a very disjunct distribution and entirely different growth forms (prostrate growing via runners in P. ramulosum versus an upright shrub in P. ovatum ).

Additional specimens examined: — NAMIBIA, Kunene Region: Kaokoland, 16 km north of Orupembe waterhole, 1812 BA, May 1979, Müller 1666 ( WIND!) ; Farm Driefontein ( OU 716 ) auf Gesteinsfläche , 2013BD, 30 March 1974, Merxmüller & Giess 30612 ( WIND!) ; 1.5 km north of Nugas homestead—| Owe ||ganas Spring at eastern bank of Nugas River —on ridge, 2014AB, 18 February 1998, Loutit 145 ( WIND!) ; Farm Fonteine 717, 2.5 km northeast of springs, 2014 AC, 718 m, 19 March 2022, Swanepoel 617 ( WIND!) ; Farm Fonteine 717, 3.4 km northeast of springs, 2014 AC, 663 m, 20 March 2022, Swanepoel 618 ( WIND!) ; Grootberg , highest point on road C39 between Vrede and Bergsig , 2014 AC, 893 m, 20 March 2022, Swanepoel 619 ( WIND!) ; Farm Bergsig 714, 6 km south of Bergsig village on road C39 , 2014 AC, 997 m, 11 May 2022, Swanepoel 622 ( WIND!) ; south of Bergsig on rocky red sandstone plain with scattered Euphorbia bushes, 2014 AC, 4 May 2022, Dexter & Loiseau 7686 ( WIND!, E!) ; south of Bergsig on rocky red sandstone plain, 2014 AC, 4 May 2022, Dexter & Loiseau 7722 ( WIND!, E!) ; Petrified Forest , 2014 BC, August 1950, Strey 2652 ( PRE!) ; Farm OU 516 , Sandsteinruecken, 2014 BC, 13 April 1964, Giess & Barnard 7920 ( PRE, WIND!) ; Farm Rooiberg ( OU 517 ), Versteinerter Wald, 2014 BC, 14 May 1966, Giess 9430 ( PRE!, WIND!) ; District Outjo : Versteinerter Wald, Farm Rooiberg Outjo 724, 2014 BC, 9 April 1968, Meyer 1155 ( PRE!, WIND!) ; Farm Naauwpoort 511, 2 km northeast of Petrified Forest on road C39 , 2014 BC, 755 m, 11 May 2022, Swanepoel 621 ( WIND!) ; Khorixas Townlands ( Fransfontein Block )—outskirts of town on hillside, 2014BD, 5 March 1998, Loutit 149 ( WIND!) ; Farm Inhoek 482, 8 km from Khorixas on road C 39 in riverbed, 2014BD, 878 m, 10 May 2022, Swanepoel 620 ( WIND!) ; On top of mountain slightly southwest of parking area at Twyfelfontein, 2014 CB, 2 March 2004, Schubert, Hochobes & Lutombi SS368 ( WIND!) ; Twyfelfontein , 2014 CB, 7 April 2004, Burke 04099 ( WIND!).

Figures

FIGURE 1. Petalidium ovatum, habitat and habit. A. Plant (ca. 600 mm high) on arid hillside at Grootberg, highest point on road C39 between Vrede and Bergsig. B. Plant (ca. 500 mm high) in ephemeral riverbed, Farm Inhoek 482, 8 km from Khorixas on road C39. Photographs: W. Swanepoel.

FIGURE 2. Petalidium ovatum, habit. A. Dwarf shrub, ca. 350 mm high (at Farm Fonteine 717). B. Base of mature plant showing woody stems with fissured greyish white bark (at Farm Fonteine 717). Photographs: W. Swanepoel.

FIGURE 3. Petalidium ovatum, leaf (A) and flower morphology (B–E). A. Branchlet with leaves and flowers. B. Flower in front view. C. Flower in lateral view; note widely ovate bracteole with venation ca. visible. D. Flower in front view. E. Flower in lateral view; note bracteoles with indumentum particularly dense and venation not visible. Photographs: W. Swanepoel.

FIGURE 4. Known distribution of Petalidium ovatum (black dots; ●) and the distribution range of P. englerianum (shaded grey); based on herbarium specimens in Herbs E, PRE, WIND, and Z+ZT.

FIGURE 5. Petalidium englerianum, leaf (A) and flower morphology (B–E). A. Branchlet with leaves and flower, the latter starting to fade. B. Flower in front view. C. Flower in lateral view. D. Flower in front view. E. Relatively old flower; upper corolla lobes having turned brownish orange as they are fading. Photographs: W. Swanepoel.