Stylochaeton glaucophyllum Struwig, S.J.Siebert & A.E.van Wyk, 2023

Struwig, Madeleen, Le Roux, Precious T., Siebert, Stefan J. & Van Wyk, Abraham E., 2023, Two new species of Stylochaeton (Araceae: Aroideae) from South Africa, Phytotaxa 620 (1), pp. 1-17 : 4-11

Stylochaeton glaucophyllum Struwig, S.J.Siebert & A.E.van Wyk , sp. nov. ( Figs 2–5A )

In habit approaching forms of S. borumense with sagittate leaf blades, but differs in the spadix having male and female flowers contiguous and the filaments not thickened (vs. male and female flowers separated by a zone of few to 10 sterile flowers and filaments thickened towards apex). Morphologically most similar to S. sekhukhuniense , differing in having a glabrous petiole (vs. scattered trichomes on petiole immediate distal to petiolar sheath); petiolar sheath not ligulate (vs. distinctly ligulate); leaf blade long, 112–332 mm (vs. shorter, 44–180 mm) with basal and apical lobes linear (vs. elliptic), greenish blue (vs. green); spadix protruding somewhat from spathe-tube (vs. entirely enclosed by spathe-tube).

Type: — SOUTH AFRICA, Limpopo: Manyaka, Mashishi , ridge behind Makgamathu Secondary School , 2430AC, 5 December 1999, Van Wyk & Siebert 1332 (holotype PRU!; isotypes PRE!, K!) .

Geophyte, up to 450 mm high, with vertical rhizome, ca. 30 × 15 mm; roots stout, not tuberous. Cataphylls ca. 18–55 × 6–10 mm, membraneous, ensiform, acuminate with mucro ca. 2–4 mm long. Leaves usually 2–6; petiole 200–300 mm long, basal ¼ covered with white and/or purple-green transverse bands/mottling, upper ⅔ green, glabrous throughout; petiolar sheath up to 85 mm long, with bands/mottling as described; blade narrow sagittate, apical lobe 58–172 × 5–27 mm, linear, mucro up to 15 mm long, basal lobes 54–160 × 4–20 mm, linear, tips rounded; greenish blue, glabrous, margin entire, veins prominent, 1 midvein with 2 principal lateral veins parallel to midvein, veins end in submarginal collective vein, coriaceous. Inflorescence appears before leaves; semi-subterranean; peduncle ca. 18–34 × 6–8 mm; spathe ca. 100–118 mm long, glabrous, thickened, inside (ventrally) creamish white, cream-coloured or pale buff (dull yellowish brown), rarely with dull vinaceous blotch in centre, outside (dorsally) brownish, yellowish brown or purplish brown, distinctly or vaguely longitudinally striped with creamish lines (veins), veins somewhat ribbed on outer surface; basal tube erect, 22–26 mm long, upper part cylindrical, 10–12 × 8–9 mm, lower part inflated, globose, ca. subterranean, 12–15 × 8–17 mm; upper limb 75–92 × 26–30 mm, oblong, acuminate, apex with cusp ca. 7 mm long. Spadix ca. 37 mm long, protrudes somewhat from spathe-tube; monoecious; staminate part cylindric, ca. 29 × 4 mm, many-flowered, contiguous with pistillate part, pistillate part hemispherical, ca. 8 × 9 mm, ≥ 10-flowered, flowers in 2 or 3 rows. Staminate flowers with perigone cupular, rhomboid, connate; stamens 4 or 5, filaments filiform, ca. 2 mm long; anthers globose, basifixed, extrorse; central pistillode lacking. Pistillate flowers with perigone cupular, contracted at the mouth, subregular, 4–6-sided, thickened, connate; stamens 0; ovary subglobose, 2-locular; style stout, ca. 2 mm long, exserted, stigma discoid, 1 mm broad. Infructescence: peduncle ca. 35 × 5 mm; globose, partially below the ground, 18–27 × 14–30 mm; a cluster of 10–25 berries, ripening white to yellowish or greenish cream, above-ground parts exposed to sunlight often with greenish to dark purplish brown surface, globose, 5–10 × 6–9 mm, styles persistent, 1 or 2-seeded. Seeds ovoid, ca. 4 mm long.

Phenology:— Flowers appear from September to November (spring) and ripened fruit can be observed from December to January (summer).

Etymology:— The specific epithet is a compound word derived from the Greek glaucos = greenish blue or seagreen, dull green, passing into greyish blue + phyllon = leaf ( Stearn 1992). It refers to the greenish blue color of the leaves. The generic name is treated as neuter, as this is how it was originally published ( Nicolson & Mayo 1984). The local vernacular name in English for S. natalense ( Fig. 1 ) is “bushveld arum” and in Afrikaans bosveldvarkoor. As English and Afrikaans names for the new species we propose “blue bushveld arum” and bloubosveldvarkoor, respectively.

Distribution:— Stylochaeton glaucophyllum is endemic to the Sekhukhuneland Centre of Plant Endemism ( Fig. 6 ), Limpopo Province, South Africa. It grows in bushveld (savannah) with well-drained stony, clay soil along gentle to moderate slopes.

Habitat: —The core area of the Sekhukhuneland Centre of Endemism is more or less congruent with surface outcrops of the Rustenburg Layered Suite, one of the stratigraphic units of the eastern Bushveld Complex ( Scoon & Viljoen 2019). This igneous complex is characterised by mafic and ultramafic rocks, the latter that give rise to soils that are often rich in heavy metals ( Adhikari et al. 2022). As is the case with many plant species largely confined to the Sekhukhuneland Centre, S. glaucophyllum grows mainly on pyroxenite and norite outcrops and hillsides ( Siebert et al. 2001), in closed Sekhukhune Mountain Bushveld (mapping unit: SVcb 28) ( Mucina & Rutherford 2006) characterized by the trees Combretum apiculatum Sonder (1850: 45) , Kirkia wilmsii Engler (1897: 25) and Senegalia nigrescens ( Oliver 1871: 340) Hurter (2008: 1021) , and commonly associated with Corbichonia decumbens ( Forsskål 1775: 103) Exell (1935: 80) , Hibiscus coddii subsp. barnardii ( Exell 1959: 177) Leistner & Winter (2005: 95) , Cyphocarpa angustifolia Lopriore (1899: 45) and Melhania prostrata Burchell (1824: 263) in the forb layer. Populations are usually small and localized, and may be found in close proximity to, or intermixed with, populations of Stylochaeton natalense ( Fig. 1 ).

Stylochaeton glaucophyllum grows in slightly acidic (pH 5.5) ultramafic soil (Ca/Mg ratio 0.68) rich in metals such as Al and Fe, and to a lesser extent Cr, Mn, Ni and Ti ( Table 1 ). Although nutrient levels are above 4000 mg /kg for essential elements, such as K, the EC is low (40 mS/m) suggesting that nutrients are not readily available in the soil ( Table 1 ) and therefore contributing to the “serpentine soil effect” typical for ultramafic areas ( O’Dell et al. 2006).

Conservation status:— Stylochaeton glaucophyllum is not known from any statutory conservation areas. It is subjected to especially human population pressure, subsistence and commercial farming and extensive mining for platinum and chromium. It is assessed as Endangered according to IUCN Red List categories and criteria ( IUCN 2012) due to the <5000 km 2 (i.e., 869 km 2) extent of occurrence and <10 km 2 (i.e., 4.6 km 2) area of occupancy, and the existence of only nine known populations ( Siebert et al. 2002).

Notes:— The inflorescences of S. glaucophyllum are cryptically coloured and easily overlooked as they tend to blend in with the colours of the associated dry grass, rocks, and soil during the flowering period in spring ( Fig. 3A–E ). When ripe the berries in an infructescence ( Fig. 4 ) become soft and emit a strong but pleasant scent reminiscent of that of a ripe mango fruit. The colour of infructescences does not change significantly upon ripening (whitish underground, greenish or purplish above-ground), although the pulp of the berries becomes slightly yellowish. The strong scent suggests that the fruit is most probably consumed by an animal that also facilitates seed dispersal.

The leaf shape of Stylochaeton glaucophyllum is very similar to that of S. borumense Brown (1901:191) , S. hypogeum Leprieur (1834: 185) and S. oligocarpum Riedl (1990: 297) (although these three species do not occur in South Africa). These species differ in that the basal lobes of the leaf blade are shorter (25–120 mm) than the apical lobe (63–200 mm), at a ratio of ca. 1:2, whereas in S. glaucophyllum the basal lobes are almost the same length (54–160 mm) as the apical lobe (58–172 mm). The basal lobes of S. oligocarpum are characteristically spread horizontally. The spathe of S. borumense is glaucous green to yellow green and the staminate and pistillate flowers are separated by an area (2–12 mm) with few to 10 sterile flowers. The spathe of S. glaucophyllum is various shades of brown with usually longitudinally creamish stripes and the staminate flowers are contiguous with the pistillate flowers. There are only two pistillate flowers in the inflorescence of S. oligocarpum whereas S. glaucophyllum has 10 or more pistillate flowers. The style in S. hypogeum is bottle-shaped, but in S. glaucophyllum the style is not thickened below the middle. The leave blades of S. natalense ( Fig. 1 ) are wide (ca. 150 mm) and variable in shape (ovate to triangular to sagittate-hastate or hastate-cordate) while those of S. glaucophyllum are narrow (ca. 27 mm) and sagittate. The spathe of S. natalense ( Fig. 1B ) is yellowish or purplish, whereas in S. glaucophyllum it is cream-coloured or brownish with paler longitudinal stripes ( Table 2 ). For notes on the distinction between the two new species, see “Differences between Stylochaeton glaucophyllum and S. sekhukhuniense ” under S. sekhukhuniense .

This species was referred to as Stylochaeton sp. S 1332 in Siebert et al. (2002: 138).

Additional specimens examined (paratypes):— SOUTH AFRICA. Limpopo: Mecklenburg: Thokane, near Vodacom tower on summit of mountain (2430 AC), 12 December 2000, Van Wyk 13596 ( PRU!) ; Burgersfort, banks of Steelpoort River (2430 CB), 20 November 2009, Siebert 4452 ( PUC!) ; Steelpoort, Driekop (2430 AC), 27 January 2012, Siebert & Rajakaruna 4462 ( PRU!) ; Manyaka, hill behind primary school, Ga-Mashishi (2430 AC), 29 September 2000, Siebert 1845 ( PRU!) ; Burgersfort, Getlane Lodge, on road between Burgersfort and Lydenburg (2430 CD), 5 December 1999, Van Wyk & Siebert 1374 ( PRU!) .

Figures

FIGURE 1. Stylochaeton natalense, general morphology. A. Plants with ca. sagittate leaves. B. Inflorescence. C. Plants with ca. hastate leaves. D. Developing infructescence. Photographs: W. McCleland (A), R.G.C. Boon (B), G. Nichols (C) and A.E. van Wyk (D).

FIGURE 2. Stylochaeton glaucophyllum. A. Plant with mature infructescence borne partly below-ground; dashed line indicates soil level. B. Vertical rhizome with sturdy, though non-tuberous roots, and mature infructescence, the latter intact (left) and as seen in longitudinal section (right); horizontal band of stippling indicates soil level. Scale bar = 10 mm. Artists:Anne Stadler (A) and Gillian Condy (B).

FIGURE 3. Stylochaeton glaucophyllum, morphology of inflorescence. A. Inflorescence at start of anthesis. B. Inflorescence partly open; note dark central blotch. C. Fully opened inflorescence, with dark central blotch and spadix protruding somewhat from the spathe-tube. D. Inflorescence with spathe in dorsal view to show cryptic coloration among dry grass. E. Inflorescence with spathe in dorsal view to show coloration; flowering in a recently burned area. F. Inflorescences removed from plants and with soil cleaned from basal parts, seen from different angles; inflorescence on far-left still in bud. G. Upper part of spadix with staminate flowers and protruding from the spathe-tube. Photographs: S.J. Siebert.

FIGURE 4. Stylochaeton glaucophyllum, morphology of infructescence. A. Plant with mature infructescence, the latter borne partly below-ground; some soil has been removed from around the infructescence to show the upper purplish surface visible above-ground and the lower whitish part that were below-ground. B. Infructescence removed from the soil; the bulk of it was below-ground, with only the area tinged greenish purple at the top exposed above-ground. C. Infructescence with purplish colouration indicating that most of the berries were borne above-ground. D. Infructescence that were mainly borne below-ground as indicated by the broad whitish lower part. Photographs:A.E. van Wyk (A, C & D) and S.J. Siebert (B).

FIGURE 5. Stylochaeton glaucophyllum and S. sekhukhuniense, habit and habitat. A. S. glaucopyllum, two plants, each with two leaves, growing among norite rocks. B. S. sekhukhuniense, plant growing among norite rocks and with black burned plant material indicating that the plant has sprouted after a recent wild fire. Photographs: A. Klopper (A) and S.J. Siebert (B).

FIGURE 6. Topographical map showing the known distribution (black dots) of Stylochaeton glaucophyllum. The insert shows a map of southern Africa with names of countries; the grey rectangle indicates the area depicted by the topographical map.