Stenostelma ligulatum Bester & Nicholas, 2018

Bester, Stoffel P. & Nicholas, Ashley, 2018, New combinations in Stenostelma (Apocynaceae-Asclepiadoideae) and two novel species from South Africa, Phytotaxa 361 (1), pp. 41-55 : 44-49

Stenostelma ligulatum Bester & Nicholas spec. nov.

Closely related to Stenostelma umbelluliferum in all vegetative features, florally very distinct from the former species in that the corolla lobes are erect, tightly adpressed to form a pseudo-tube in S. ligulatum compared to sub-erect loose lobes in S. umbelluliferum . Further the corona lobes are flat and elongated with only the apical tips recurved where they are exerted between the corolla lobes, compared to small rounded corona lobes in S. umbelluliferum .

Type:— SOUTH AFRICA, Limpopo Province, 2428 (–CD), Wonderboom District, just off Bela-Bela / Codrington road to Hammanskraal , Farm : Bath , section Avon , alt. 1085 m, 7 Nov. 2007 (fl.), S.P. Bester 8159 (holotype, PRE!; isotypes, E!, K!, MO!, NH!, NU!). ( Figs. 1–5 ) .

Geophytic perennial herb, all parts with milky latex ( Fig. 3A ). Tuber carrot-shaped 50–160 × 9–25 mm, dry fibrous, white in colour; neck 36–81 mm long, usually with long, fine fibrous roots; ending abruptly in ± 1 mm diam. basal tip to 300 mm long. Stems (100–) 230–280 mm tall, re-sprouting annually, sparingly branched from the base, sometimes stems bifariously hairy with pilose to tomentose hairs from the base towards the tips, but less persistent on old stems; older stems more pronouncedly grooved, furrowed, pleated or ribbed and pilose all round. Leaves sessile, opposite throughout, 14–134 × 1–6 mm, narrowly lanceolate or linear, base attenuate, apex acute or attenuate, margins revolute; main vein prominent below, paler than blade, sometimes tinged purple-maroon above (usually only the basal leaves), leaves usually much longer than internodes; sparsely pilose above, usually glabrous below, but if very dense above (in few specimens) then sparingly pilose below along veins and margin, seemingly wanting in mature leaves. Inflorescences axillary, umbellate, 3–7 per stem or branch, pendant in bud, erect at anthesis, 5–16-flowered; peduncle 14–43 mm long in flower (13–34 mm long in fruit, contorted), bifariously pilose; pedicels 3–6.5 mm long, bifariously pilose; bracts linear to narrowly triangular, (0.5)2.5–3.6 × 0.4–0.5 mm ( Fig. 3C ). Calyx narrowly triangular, 2.3–3.5 × 0.5–1.3 mm; glabrous above, below evenly pilose or with a patch towards the apex. Flowers 2.5–4.9 × 2.2–4.4 mm; corolla 3.4–4.9 × 0.8–1.2 mm, strap-shaped with an abruptly dilating and rounded apex; inside green, maroon, cream, pink or greenish-cream, usually paler inside; glabrous, but papillose on the recurved apex; very rarely sparsely pilose towards apex on outside, green, or cream-brownish, usually darker outside. Corona 2.2–2.5 × 0.8–0.9 mm, strap-shaped or slightly broader above base, furrowed, tongue-like, extending beyond the “tube” (formed by the tight-packed basal sections of the corolla) bending out- and downwards between the diverging corolla lobes, yellowish, yellowish-green, dark yellow to orange; extending beyond the corolla tube at which point they curve downwards extending between the corolla lobes. Gynostegial column 2.4 × 1.7–1.8 × 1.5 mm in diam. Gynostegial head conical and involute at the apex, 0.61–0.62 × 0.91–0.92 mm, with prominent stype. Anthers: anther-wing 0.5–0.6 mm long, extending 0.1–0.2 mm from column, notch at base of wing. Pollinaria : pollinia 0.72–0.77 × 0.24–0.26 mm; caudicle 0.30–0.33 × 0.05–0.08 mm; corpuscle ovate, 0.31–0.40 × 0.1–0.2 mm. Fruit mostly solitary per inflorescence due to abortion of others, (20–)90–132 × 6–8 mm with an elongated beak ending in an abrupt snout; green, two shades of green in longitudinal direction alternating ( Fig. 3B ); densely tomentose when immature to finely pubescent when mature. Seed ovate, 4.5–7 × 2–4 mm, brown; coma 11–24 mm long, attached at blunt end of seed.

Habitat:—Growing on level plains of Acacia -savanna in full sun, in well-drained black turf soils. The known habitat is restricted to the Springbokvlakte Thornveld vegetation unit in the Central Bushveld bioregion ( Mucina & Rutherford 2006). The altitudinal range is narrowly restricted between 1064 and 1105 m.

Distribution:— Stenostelma ligulatum is known only from Limpopo Province in the Springbok Flats of the Warmbad, Waterberg and Wonderboom districts ( Fig. 5 ).

Phenology:—All specimens were collected in November, mostly in flower, but some specimens also in fruit. It is deduced from the current material available that the plants flower quickly and have a short flowering period currently known as to be restricted to November.

Etymology:—The specific epithet is in reference to the shape of the corona lobe which is long and thin and resembles a tongue sticking out of a person’s mouth.

Conservation assessment:—Plants have a restricted distribution in soils with high vertic clay content which is highly sought for agriculture. In populations, plants are not numerous, but this may be an under-estimation due to their small habit which might result in them being overlooked easily. For example, at the Codrington collection site (Bester 8601) there were only 18 plants and at the population between Marble Hall and Settlers (Bester 11424) only a single specimen was encountered.

The habitat of this species is degraded and the population has therefore probably declined in the recent past. Since the agricultural potential of the habitat is high, it is likely to decline further in the future. These plants have specialised pollination mechanisms adapted to insects, but it is not precisely known what insects pollinate this species. It is likely to be small flies or midgets ( Culicoides spp. , personal observation by first author)—flies have been measured to fly between 1.6–3.2 km ( Townsend 2016). Whatever insect pollination mechanism is used, cross pollination is not likely to take place between subpopulations that are more than 10 km apart. Furthermore, the seeds, although wind-dispersed, are unlikely to travel far as the coma detaches relatively quickly after the seed is released (personal observation by the first author) and due to the small stature of the plants amongst much taller and dense grasses in the habitat. The two localities that are more than 10 km apart are therefore regarded different subpopulations, and by this reasoning there are three subpopulations. Within one of the three subpopulations this species has been recorded from four locations. All subpopulations occur in areas which are highly transformed. Inferring from the increasingly transformed habitat in which these plants grow, it is likely to be undergoing a continuing decline over time. With this, and an extent of occurrence (EOO) <5000 km 2 (here only ± 223km 2), the criteria for category Endangered (EN) according to IUCN (2001) are met (Assessors: S.P. Bester & J.E. Victor 17/02/2016). Summary of criteria met:

• Six locations within three subpopulations, within highly transformed habitat; population probably declining continuously over time.

• EOO= 223 km 2

• EN B1ab,i,ii,iii

Specimens examined:— SOUTH AFRICA. Limpopo Province. Nylstroom (2428): (–CD), Wonderboom District, just off Bela-Bela / Codrington road to Hammanskraal , Bath farm, section Avon , alt. 1 085 m, 8 November 2007 (fl. & fr.), S.P. Bester 8156 ( PRE!, UDW!) and (fl.), S.P. Bester 8158 ( PRE!, UNIN!) ; 15 km W of Settlers when turning from the R516 between Settlers and Warmbad to Codrington , alt. 1064 m, 9 Nov. 2005 (fl. & fr.), S.P. Bester 6049 ( PRE!, UNIN!; UDW!) ; Warmbad District, Het Bad 465 KR farm, 4 km SSE of Warmbad, alt. 1105 m, 11 November 2011 (fl.), S.P. Bester 10821 ( PRE!). (–DC), Waterberg District , 12 km ENE from Settlers on road R516 to Marble Hall, 35.1 km SE from Modimolle, Hopefield 675 KR farm, alt. 1090 m, 28 November 2012, S.P. Bester 11424 ( PRE!). Pretoria (2528): (–AB), Warmbad District , 16 km SW from Settlers on R576, alt. 1085 m, 23 November 2008, S.P. Bester 8601 ( PRE!, PRU!, UNIN!) ; on road from Hammanskraal to Codrington , alt. 1085 m, 8 November 2007 (fl.), S.P. Bester 8152 ( PRE!) .

Figures

FIGURE 1. Stenostelma ligulatum. A, Habit. B, Tuber. C, Flowers side view. D, Fruit. Scale bars: A, B, 10 mm; C, 0.5 mm; D, 10 mm. Voucher:A–C, S.P. Bester 8601 (PRE); D, S.P. Bester 8156 (PRE). Artist: G.S. Condy.

FIGURE 2. Stenostelma ligulatum. A, Flower side view, calyx partly removed; 2 corolla and 3 corona lobes removed. B, Flower vertical view. C, Pollinarium. Corona lobe: D, dorsal view; E, side view; F, ventral view. Scale bars: A, B, 0.5 mm; C, 0.2 mm; D, E, 0.5 mm. Voucher: S.P. Bester 8159 (PRE). Artist: G.S. Condy.

FIGURE 3. Stenostelma ligulatum: A, Habit. B, Inflorescence. C, Flowers in angled side-view; D, Flowers in side-view. Stenostelma umbelluliferum: E, flowers in face view. Scale bars: A, B, 10 mm; C, D, 5 mm; E, 1 mm. Photos: S.P. Bester.

FIGURE 4.—Scan of holotype of Stenostelma ligulatum, Bester 8159 (PRE).

FIGURE 5. Known distribution of Stenostelma ligulatum (z) and Stenostelma urceolatum („).Map created in ArcGIS10.1.