Acrosanthes Ecklon & Zeyher 1837: 328; Adamson 1959: 23–28; Hartmann 2001: 25–26 Lectotypus, designated by Adamson 1959: 23 as “type species”, A. anceps (Thunberg) Sonder = Aizoon L. subg. Acrosanthes (Ecklon & Zeyher) D.Dietrich 1842: 130 LT, designated by Hartmann 2001: 25, Aizoon fistulosum (Ecklon & Zeyher) D.Dietrich ≡ A. fistulosa Ecklon & Zeyher = A. anceps (Thunberg) Sonder Etym Gk akros, point, top, apex anthos, flower, referring to the terminal flowers above a forking = Didaste E. Meyer ex Sonder in Harvey & Sonder 1862: 472 nomen nudum.

Mat-forming or sprawling with long branches, I woody, at least at their bases; L terete to flat, glabrous; Fl solitary, seemingly lateral, but in reality terminal and overtopped by one of the side branches developing below, perigon 5-lobed, lobes shorter than the basal tube, white inside, stamens 8-many, mostly in groups, ovary perigynous, divided basally only incompletely by low septa into two locules, a single ovule per locule in basal position; Fr a xerochastic capsule with a parchment-like wall persisting in the drying perigon; S compressed and reniform or orbicular, testa rugose; Ecol winter rainfall >400 mm p.a. (Fig. 1); Distr W districts of WC, S Africa.

Fig. 1
figure 1

Distribution of Acrosanthes; note that the genus does not extend beyond the south-eastern boundary of the winter rainfall region (Copyright H.E.K. Hartmann)

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Note: The genus has received little attention since its establishment in 1837, hence no photographs are available; instead, drawings are presented here. The present treatment follows Adamson 1959: 23–28 essentially, including the lectotypification of Acrosanthes, but with an additional lectotypification of Aizoon subg. Acrosanthes. A new study based on fresh, growing material is highly desirable, focussing in particular on the constancy or variation in growth forms and establishing whether indeed the longitudinal slits seen in capsules on herbarium material represent a mode of opening entirely different from the circumscissile fruits in most genera of the subfamily Sesuvioideae. The epidermis of the leaves should also be examined in order to assess whether the genus does indeed belong in the Aizoaceae. On herbarium sheets, the needle-shaped leaves of some species appear in a shiny olive green quite unlike any other leaves in the family. The complete lack of any expanding tissue and the basal 1–2 ovules separate the genus well from Aizoon, where it had been placed forming a subgenus (Dietrich 1842: 130, not 1847, as given by Adamson 1959: 23). The name Didaste E. Meyer appears in Sonder (Harvey & Sonder 1862: 472, 473) without any description and only in three synonyms for three species of Acrosanthes, excluding the type species of the latter genus, A. anceps.

Key to the Species of Acrosanthes

1.

Leaves terete, stamens 25 and more

A. teretifolia

–

Leaves flat, stamens always <25

2

2.

Stamens up to 10

3

–

Stamens 12–20

4

3.

Leaves yellow-green, flowers 4–6 mm long, as long as the basally connate leaves

A. humifusa

–

Leaves blackish-green, flowers ca. 2 mm long, half as long as the basal petiolate free leaves

A. microphylla

4.

Flowers 2–3 mm l

A. anceps

–

Flowers 4–6 mm l

A. angustifolia

A. anceps (Thunberg) Sonder in Harvey & Sonder 1862: 472 ≡ Trianthema anceps Thunberg 1794: 80 LT, chosen here from two syntypes, Thunberg (UPS 10399!) = A. fistulosa Ecklon & Zeyher 1837: 328 ≡ Aizoon fistulosum (Ecklon & Zeyher) D.Dietrich 1842: 130 T Ecklon & Zeyher 2146 (K?).

Mat-forming to loosely branching, older I terete, younger ones markedly compressed and with two ridges, light brown; L oblong, obovate, oblanceolate, with papulose margins, 10–20 mm l, 2–5 mm b, acuminate, basally narrowed; Fl pedicels 8–30 mm l, deflexed or spreading, perigon 5–6 mm l, two lobes longer than the other three, 15–20 stamens, anthers white; Fr rounded, mucronate, a little shorter than the tepals; S black, obtusely tuberculate; Ecol on stony or gravelly slopes; Distr Clanwilliam, Piquetberg, Tulbagh, Worcester, WC, S Africa.

Note: Length of internodes appears most variable in this species, which can form mats or appear compact on exposed places. The placement of A. fistulosa as a synonym follows Sonder (in Harvey & Sonder 1862: 472) and Adamson (1959: 24). The two sheets kept at UPS belong both with A. anceps, as comparisons show, representing two syntypes: on the sheet Thunberg 10399 flowers are clearly visible and the sheet was studied in detail at UPS (Fig. 2); further, it is this sheet that bears the inscription “e Cap. b. spei Thunberg” on the back, confirming that the collector was Thunberg himself. Sheet Thunberg 10400 (UPS!) remains a syntype; it bears the inscription “E Cap. bona spei Masson”, who undertook some travels together with Thunberg.

Fig. 2
figure 2

Acrosanthes anceps, a close-up of the lectotype of Thunberg kept under 10399 (UPS!), photographed with permission from UPS. Note the twisted long flower stalk in the middle, the flower with a very short tube and long free tepals, and the fruiting state at the right hand side, exposing the longitudinal slit from the opening of the capsule which has reached the length of the tepals enclosing it (Copyright H.E.K. Hartmann)

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A. angustifolia Ecklon & Zeyher 1837: 329 ≡ Aizoon angustifolium (Ecklon & Zeyher) D.Dietrich 1842: 130 T Ecklon & Zeyher 2147 (B) = Didaste decandra E. Meyer ex Sonder in Harvey & Sonder 1862: 472 nomen nudum in synonymy T Drege (BM?).

Shrubs with woody branches, young I with two opposite ridges, older terete; L linear-obovate, narrowing basally, margins rough, 4–12 mm l, ca. 2 mm b, acute; Fl pedicels 3–10 mm l, perigon ca. 3 mm l, lobes of ± equal length, 12–20 stamens; Fr a little shorter than the perigon, opening with a longitudinal slit (Fig. 3); S black, finely rugose; Ecol in sand or between stones; Distr Ceres, Clanwilliam, Piquetberg, Tulbagh, Worcester, WC, S Africa.

Fig. 3
figure 3

Acrosanthes angustifolia, drawing of an old flower of a sheet of Ecklon & Zeyher 2147 kept at OXF, showing the preformed longitudinal line along which the capsule will open and the small groups of stamens; drawn with permission (Copyright H.E.K. Hartmann)

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Note: A sheet of Ecklon & Zeyher 2147 kept at OXF (!) shows the almost free tepals and the stamens/filaments in small bundles and exhibits a dark line along the side of the almost ripe capsule, indicting that a longitudinal slit will be developed (Fig. 3). The species has been confused with A. anceps, from which it differs in having more woody stems. Yet, the two species are very similar in their long pedicels, their tepals only a little longer than the capsule, and the moderate number of stamens in small groups.

  • A. decandra = A. humifusa

  • A. fistulosa = A. anceps

A. humifusa (Thunberg) Sonder in Harvey & Sonder 1862: 472 ≡ Trianthema humifusa Thunberg 1794: 80 T Thunberg (UPS?) = A. decandra Fenzl 1839: 270 T Drege 2989 (BM) = Didaste pentandra E.Meyer ex Sonder in Harvey & Sonder 1862: 472 T Drege (BM?).

Flat mats of yellow-green colour, stems woody, short shoots very numerous; L in secund arrangement, crowded, persisting for a long time on the stems, narrowing to the base, 4–6 mm l, 1–3 mm b; Fl pedicels 0.5–3 mm l, perigon 4–6 mm l, the lobes acute or acuminate, keeled on the back, ca. 10 stamens (Fig. 4); Fr as long as the perigon; S flat; Ecol in flats below the mountains; Distr Ceres, Clanwilliam, Worcester, WC, S Africa.

Fig. 4
figure 4

Acrosanthes humifusa, drawn from a sheet of Didaste pentandra collected by Drege, annotated by E.Meyer, cited by Sonder (1862: 472), kept at OXF; note the very short pedicel, typical of the species, and the relatively few stamens. Drawn with permission (Copyright H.E.K. Hartmann)

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Note: Distinguished from other species with compact growth by the densely placed, erect leaves, the short-stalked almost sessile flowers, and the keeled perigon lobes; the yellowish-green colour of the mats appears to be typical of the species, too. No material of this species has been found at UPS. Material of Didaste pentandra E. Meyer collected by Drege s.n. kept at OXF (!) confirms the very short pedicel and number of only 10 stamens per flower.

A. microphylla Adamson 1959: 26 T Compton 7988 (NBG).

Forming flat mats of blackish-green colour, short shoots very numerous; L crowded, elliptical to ovate, basally narrowing into a petiole ca as long as the blade, 4–6 mm l, 1–2 mm b; Fl pedicels 4–6 mm l, perigon 2–3 mm l, rounded on the back, 8–10 stamens, usually in pairs; Fr unknown; S unknown; Ecol rocks on mountain summits; Distr Ceres, Clanwilliam, Laingsburg, Montagu, Worcester, WC, S Africa.

Note: Very similar to A. humifusa, differing most obviously from that species by the blackish-green leaves and the occurrence on mountain tops.

A. teretifolia Ecklon & Zeyher 1837: 329 ≡ Aizoon teretifolium (Ecklon & Zeyher) D.Dietrich 1842: 130 T Ecklon & Zeyher 2148 (B) = Didaste icosandra E. Meyer ex Sonder in Harvey & Sonder 1862: 473 nomen nudum in synonymy T Drege (BM?).

Sprawling shrubs with stems to 80 cm l, young I terete; L linear to cylindrical, often secund in arrangement, mucro often bent backwards, 10–20 mm l; Fl pedicels 5–15 mm l, ascending or spreading, often deflexed when the fruit is ripe, perigon 4–8 mm l, stamens >25, not in groups, filaments dark, anthers white (Fig. 5); Fr rounded, shorter than the perigon; S dark brown, rugose; Ecol sandy or stony flats and lower slopes; Distr from Piquetberg to Bredasdorp, WC, S Africa.

Fig. 5
figure 5

Acrosanthes teretifolia, drawn from a sheet of the isotype of Ecklon & Zeyher 2148 kept at OXF, showing a virtual longitudinal section through the flower with the dome-shaped gynoecium half as high as the tepals and two of the five distinct bundles of stamens between them, each with five elements; drawn with permission (Copyright H.E.K. Hartmann)

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Note: The species differs from all others of the genus in the high number of stamens never arranged in groups and in the terete leaves.

Didaste = Acrosanthes

The name Didaste E.Meyer was used by Sonder (in Harvey & Sonder 1862: 472–473) first, who listed three species as synonyms in the genus Acrosanthes, attributing the names to E. Meyer, who had probably named sheets collected by Drege. Didaste E.Meyer ex Sonder lacks any description and is, therefore, a nomen nudum.

  • D. decandra = Acrosanthes angustifolia

  • D. pentandra = Acrosanthes humifusa

  • D. icosandra = Acrosanthes teretifolia