Euphorbia melanohydrata Nel subsp. conica Swanepoel, 2012

Swanepoel, W., 2012, Euphorbia melanohydrata subsp. conica (Euphorbiaceae), a new subspecies from Namibia, with notes on the identification of similar medusoid euphorbias, Phytotaxa 62 (1), pp. 33-38 : 34-38

Euphorbia melanohydrata Nel subsp. conica Swanepoel , subsp. nov. ( Fig. 1 )

Differs from the typical subspecies in the plants being distinctly conical or ovoid and nearly always solitary (not clumpforming); lateral branches usually shorter (15–30 mm); peduncular bracts elliptic or obovate; involucre glabrous inside, involucral lobes glabrous, gland apices not bifid; style apices dilated.

Type: — NAMIBIA. Karas Region: Plain below Namuskluft Mountain , 2716DB, 1340 m, 18 November 2006, Swanepoel 263 (holotype WIND!; isotype PRE!) .

Erect, spineless, dwarf succulent, conical, smaller plants also ovoid, up to 300 mm high, 210 mm diam. at ground level, often with small, tuberculate, branchless area at apex; plants nearly always solitary, very rarely with one or two rhizomatous stems. Main stem single, trullate or ovoid, surface irregularly horizontally folded or with convex projections, folds spaced at 4–10 mm; stem partly buried, broadest at ground level, up to 280 mm long, 170 mm diam., prolonged at base into tap root. Bark white-grey, glabrous, smooth. Branches perpendicular all over stem at slight intervals from one another, usually cylindric, few somewhat clavate or terete, tuberculate, glabrous, often with secondary branchlets, especially towards stem base, green with thin whitish waxy coating, often reddish green towards branch apices, 15–30 mm long, 7–20 mm diam., ultimately withering, exposing bare stem, persisting, new replacement branches often developing at exposed areas. Tubercles unequally rhombic, penta- or hexagonal, usually longer than broad, laxly helical, rather scattered, glabrous, 3.0–6.6 × 1.7–5.7 mm long, 1.5–5.6 mm wide, prominent; tip slightly recurved, located at centre or from above centre to near apex; leaf scar whitish. Leaves simple, borne on tubercles, deciduous, somewhat fleshy, blue-green with whitish bloom, greener abaxially especially on midrib, glabrous; lamina linear, linearelliptic, rarely oblanceolate, channelled adaxially, apex acute, mucronate, base cuneate, 2–16 × 1.4–2.2 mm long; margin cherry-red, ± pellucid, entire or with irregularly spaced, small, deltoid, pellucid teeth; petiole short, up to 1 mm long. Inflorescences: cyathia solitary or in simple cymes, bisexual or male, produced near apex of branches, pedunculate. Peduncles in axils of tubercles, simple or branching into 2–5 short, stellate cyme branches, bracteate, straight or curved downwards, cylindric or terete, fleshy, glabrous, up to 20 mm long, 2.3–3.5 mm diam., ultimately withering, spine-like and persisting with prominent bract scars; bracts elliptic or obovate, apex denticulate-fimbriate, glabrous, soon deciduous, ± 1.0 × 1.3 mm. Involucre broadly infundibular, pentagonal from below, glabrous outside and within, 1.8–3.2 mm long, 3.6–5.6 mm diam. including the glands; glands 5, fleshy, glabrous, distant, spreading, distinctly stalked, transversely rectangular, quadrate or obovate, 0.8–1.3 × 0.8–1.7 mm, with deep transverse depression in front of inner margin, convex towards outer margin, inner margin and sides revolute, outer margin irregular with 1–5 subulate, somewhat recurved, pale green processes, apices not bifid, 0.3–1.6 mm long; upper surface finely pitted, pale pinkish yellow, lower surface pinkish green; lobes 5, erect, transversely rectangular, glabrous, 1.3–1.8 × 0.7–0.9 mm, apex irregularly denticulate-fimbriate. Male flowers 25, developing in 4 successions, arranged in 5 fascicles subtended by fascicular bracts, 5 flowers per fascicle; fascicular bracts fan-shaped, irregularly fimbriatelaciniate, widening towards apex, ± 2 mm long, laterally adnate to involucre; pedicels with longish hairs towards joint with filament, cylindric, pale green or pinkish green, when fully developed ± 2 mm long, 0.4 mm diam.; filaments terete, glabrous, pale green or pinkish green, ± 0.8 mm long; anther thecae oblongelliptic from above, reniform in lateral view, green, when fully developed 0.5–0.7 × 0.5 mm, pollen bright yellow. Female flowers erect, shortly stipitate, ovary 3-locular, obtusely trigonous, elliptic in lateral view, glabrous, 0.8–1.1 mm high, 1.0– 1.3 mm diam., included in involucre; pedicel stout with few scattered hairs near joint with ovary, ± 0.8 mm long, ± 0.6 mm diam.; ovary rudimentary in male cyathium, 0.4 mm long, 0.4 mm diam., perianth obtusely triangular or circular, ± 0.7 mm diam; styles three, exerted from involucre, united at base into column 0.7–0.9 mm long, free portion with inner face deeply grooved, spreading-recurved, 0.7–0.9 mm long, glabrous, dilated at apex, entire. Capsule khaki-coloured when dry with slightly prominent veining, otherwise smooth, glabrous, obtusely trigonous, broadly ovate or elliptic-ovate in lateral view, apex obtuse, base emarginate, 4.6–5.1 mm high, 4.3–4.8 mm diam., halfway exerted when mature, on pedicel ± 0.8 mm long. Seed ± ovoid, obscurely 4-angled, apex acute, 3.2–3.5 × 2.3–2.6 × 2.0– 2.5 mm, smooth or with few slight scattered tubercles, khaki-coloured.

Phenology: —Cyathia were recorded on subsp. conica from November to February.

Distribution and habitat: —At present Euphorbia melanohydrata subsp. conica is only known from the type locality and its immediate vicinity, on two plains, separated by a series of low hills, in the northern part of the Namus Mountains to the northeast of Rosh Pinah, southwestern Namibia ( Fig. 2 ). This area falls within the Gariep Centre of Endemism ( Van Wyk & Smith 2001). The subspecies is found ± 95 km from the coast at elevations of 1300–1360 m. This is a very arid region with an annual rainfall of 50–100 mm ( Mendelsohn et al. 2002), most of which is received in winter. Plants are locally uncommon and grow as isolated colonies in open succulent scrubland on stony-sandy soil derived from sedimentary rocks of the Nama Group ( Mendelsohn et al. 2002).

Conservation status: —Subspecies conica is vulnerable as it occurs on commercial farm land which is from time to time subject to grazing and trampling by livestock. Potentially the greatest threat to the species is the illegal collecting of plants for the succulent plant trade. Due to the small known population of less than 250 mature individual plants, an IUCN Red List category of Endangered (EN) is proposed ( IUCN 2001, Red List criterion D). In South Africa, subspecies melanohydrata has earlier been evaluated as Least Concerned ( Raimondo et al. 2009).

Etymology: —The subspecific epithet refers to the distinctly conical growth form of the plants.

Additional specimen examined (paratype): — NAMIBIA. Karas Region: Farm Kolke 84, on road to farm Zebrafontein , 2716DB, 1300 m, 18 November 2006, Swanepoel 264 ( WIND!) .

The subspecies conica, with its neat conical or ovoid (when young) habit, is an attractive and conspicuous plant. It differs from the typical subspecies in habit and some floral characters. Subspecies conica has a single stem whereas subsp. melanohydrata is often clump-forming due to the formation of rhizomatous branches, or the central stem dividing into two or more stem-like branches. In addition, the primary aerial side branches in subsp. conica usually are shorter (15–30 mm) than in subsp. melanohydrata (30–100 mm).

Differences in floral (cyathium) characters include the peduncular bracts that are elliptic or obovate in subsp. conica , but spatulate in subsp. melanohydrata . In subsp. conica the involucre is glabrous, whereas in subsp. melanohydrata it has a patch of hairs on the inside opposite the glands. Involucral lobes in subsp. conica lack ciliate hairs on the fimbrillae, whereas these are present in subsp. melanohydrata . In subsp. melanohydrata the processes of the glands have apices that are often bifid, whereas those of subsp. conica are always simple. The styles in the two taxa also differ: in subsp. conica they are dilated at the apex, but in subsp. melanohydrata bifid.

Although Euphorbia melanohydrata subsp. conica is probably not closely related to E. multiceps , the two taxa share a similar conical habit. However, subsp. conica differs in a number of features from E. multiceps and also has a different geographical distribution. Subspecies conica is only known from the type locality in southwestern Namibia, whereas E. multiceps is confined to South Africa where it has a wide distribution ranging from the Little Karoo in the south, northwards to Steinkopf in Namaqualand ( Court 2010).

In subsp. conica the primary side branches are much less densely crowded (there are narrow gaps between the branches through which the main stem is often visible) than in E. multiceps , of ± uniform length on individual plants, thinner, shorter and secondary branches are usually present. The branches are usually cylindric, with a few somewhat clavate or terete. In E. multiceps the branches are very densely crowded (main stem not visible) and branches are gradually decreasing in size from stem base to stem apex. Branches in E. multiceps are all somewhat clavate-cylindric and secondary branches are absent. The tubercles in E. melanohydrata subsp. conica are laxly helically arranged, rather scattered, whereas they are helical in E. multiceps .

The peduncles in subsp. conica arise from the lateral branches, are simple or branched at the tips into 2–5 conspicuous radiating branchlets (appearing like star-shaped spines). Peduncles of E. multiceps arise from both the main stem and the branches and are always simple. Cyathia in subsp. conica have the involucre glands transversely rectangular, quadrate or obovate with a deep transverse depression in front of the inner margin and are convex towards the outer margin with subulate processes. In E. multiceps the involucre glands are transversely oblong and concave with linear or oblong processes.

and three other medusoid euphorbias from the Gariep Centre of Endemism.

Euphorbia melanohydrata subsp. conica can also be confused with E. namibensis and E. friedrichiae , dwarf medusoid euphorbias which also occur in the western parts of the Gariep Centre of Endemism ( Table 1). Subspecies conica, however, can be readily distinguished from these species, both of which are globose or cylindric in habit, whereas it is conical or ovoid. It furthermore differs in several other morphological characters from these two species. In E. namibensis the primary branches are very laxly crowded and restricted to the apical half to two thirds of the main stem, the tubercles are helically arranged and the cyathia are bisexual only. The numerous dense, tapering peduncles, emerging and projecting beyond the main body of E. friedrichiae , give it a rugged, untidy appearance, whereas subsp. conica always has a neat, compact look. Euphorbia friedrichiae furthermore has the cyathia always bisexual, the involucre cup-shaped, often pubescent on the outside, the styles usually much longer (up to 5 mm), the capsules densely pubescent or tomentose and ± double the size with larger seeds.

Additional morphological features to differentiate among the above-mentioned species are supplied in Table 1.

Figures

FIGURE 1. E. melanohydrata subsp. conica: (A) habit; (B) fruit, lateral and apical views; (C) female flower; (D) bisexual cyathium from above (male flowers still immature); (E) involucral gland, dorsal view (left) and lateral view in transverse section (right); (F) involucral lobe; (G) fascicular bract; (H) male flower. Voucher: Swanepoel 263. Scale bar 10 mm (A) or 1 mm (B–H). Illustration by Lesley Deysel.

FIGURE 2. Known distribution of E. melanohydrata subsp. melanohydrata, ̝; and subsp. conica, ●.