Acanthus polystachyus Delile

Acanthaceae (part 1), Kaj Vollesen, Flora of Tropical East Africa, 2008

Morphology General Habit

Shrub to 4 m tall; young stems puberulous to tomentellous (rarely glabrous).

Morphology Leaves

Leaves with petiole 0.3–2.2(–2.5) cm long; lamina ovate to elliptic in outline, largest 11–35 × 6–16 cm, deeply lobed with large triangular spine-tipped lobes, each lobe with 1–2 spines on antrorse side or on both sides, apex acute to acuminate, spine-tipped, base truncate to cordate, beneath sparsely puberulous to tomentellous (rarely glabrous) on midrib and larger veins, glabrous to puberulous on lamina, above sparsely to densely pubescent on midrib and glabrous or with scattered long hairs on lamina (rarely uniformly puberulous); stipule-like interpetiolar leaves present, but often falling quickly, with lamina, up to 1.5 cm long, spiny.

Morphology Reproductive morphology Inflorescences

Cymes solitary or also 2–4 from upper axils, 4–25(–30) cm long; rachis puberulous to tomentose, with several pairs of sterile bracts at base; bracts pale green to straw-coloured, elliptic to slightly obovate, puberulous to densely pubescent (rarely almost glabrous but for ciliate margin), 1.4–2.8 cm long in middle of cyme, spine-tipped and with (5–)10–25 large and small teeth per side, longest 3–5 mm long, small teeth inserted between large, teeth hairy near base; bracteoles linear-lanceolate, 1–2.5 cm long, toothed in apical part.

Morphology Reproductive morphology Flowers Calyx

Calyx green or tinged purplish, glabrous to densely puberulous or pubescent, conspicuously ciliate; dorsal and ventral sepals ovate or broadly so, dorsal 1.4–2.2 cm long, obtuse or with 1–3-toothed tip and without or with 1(–4) weak lateral teeth per side, ventral 1.1–1.8 cm long, obtuse or with spiny tip or with 2 small additional apical teeth or with 2 triangular spine-tipped lobes, lateral ovate, 0.8–1.4 cm long, obtuse or acuminate.

Morphology Reproductive morphology Flowers Corolla

Corolla pale pink to pink or rose (more rarely white); tube 3–5 mm long below thickened rim which is 3–5 mm long; limb 5-lobed, 2.5–4 × 3–4 cm, below densely puberulous, above puberulous (rarely glabrous); callus with broad central groove and two narrow lateral ridges.

Morphology Reproductive morphology Flowers Androecium Stamens Filaments

Filaments 1.4–2.4 cm long, glabrous or 2 dorsal hairy on inside near base; anthers 3.5–5 mm long, glabrous or with long curly hairs on sides and back.

Morphology Reproductive morphology Fruits

Capsule 1.7–2.4 cm long.

Morphology Reproductive morphology Seeds

Seed ellipsoid to round, 7–9 mm long, glabrous.

Figures

Fig. 15.

Distribution

Range: Kenya. Range: Uganda. Range: Sudan, Ethiopia, Congo-Kinshasa, Rwanda, Burundi Range: Tanzania. Range: Sudan, Ethiopia, Congo-Kinshasa, Rwanda, Burundi Flora districts: U2 U 3, U 4; K3 K5 T1 T4

Ecology

Tall fireswept seasonally wet grassland, often forming dense stands at forest margins, rocky hills; 1100–2300 m

Note

Ups. 29, 3: 160 (1991) reports A. pubescens from both E and W Usambara Mts. It often forms a 5–10 m wide belt between the forest and the tall grassland, thus protecting the forest from the grassfires and eventually allowing the forest to expand. Iversen in Symb. Occasionally these are cultivated in Kenya. No species of Acanthus has been collected in the Usambaras; it is unclear what Iversens records refer to. White-flowered forms are apparently much more common in Uganda than elsewhere. One of the most characteristic forest margin species in Uganda and NW Tanzania.

Vollesen, K. (2007). Synopsis of the Species of Acanthus (Acanthaceae) in Tropical East and Northeast Africa and in Tropical Arabia. Kew Bulletin, 62(2), 233-249. Retrieved from http://www.jstor.org/stable/20443348

Type

Sudan, Singué [Singa], Cailliaud s.n. (holotype P).

Distribution

Sudan, Ethiopia, Congo (Kinshasa), Uganda, Kenya, Rwanda, Burundi, Tanzania.

Ecology

Margins of upland Podocarpus forest and forest/grassland mosaics, tall fireswept grassland, sometimes seasonally inundated, often forming dense stands at forest margins, rocky hills, also in secondary forest, scrub and bushland and in old fields; 1100 - 2500 m.

Conservation

Acanthus polystachius is the most widespread of the species treated here. It often occurs in large stands and is usually common where it occurs. In W Tanzania (pers. obs.) the species often forms a belt between tall Hyparrhenia grassland and forest or woodland, and acts as a firebreak preventing the often fierce grassfires from entering the forest. Even after severe late season burns it seems to fully recover in the following growing season. In N Tanzania it is also unpalatable to browsing animals and acts as a natural fence keeping cattle out of the forest. In Ethiopia it tolerates moderate disturbance and occasionally seems to flourish as a "weed" in old fields and in secondary vegetation. With the exception of the Studain and Congo there are recent collections from all parts of the species' distribution area. Up to the middle of last century the species was widespread in the eastern part of Congo but due to prolonged civil wars there is very little recent material from there and it is impossible to evaluate its present status in this area. Over its total area it cannot in any way be considered under threat. Least concern (LC).

Note

The excellent illustration in Delile, made from the type material at Paris (P), leaves no doubt as to its identity. C. B. Clarke cites three collections (C. F. Elliott 244, Evan James s.n. and Dawe 237) when describing Acanthus ugandensis, all three are kept at Kew (K). Most botanists have tried to keep Acanthus polystachius and A. pubescens separate on the basis of bract and corolla size. It is true that material from northern Ethiopia and Sudan (A. polystachius sensu stricto) have generally larger bracts with more teeth, larger calyces and larger corollas than material from further south. However a closer study of even this material and comparison with material from western and southern Ethiopia and Uganda reveal a total gradation with typical "A. pubescens". A superficial glance at the distribution map of Acanthus polystachius shows that the species has a disjunct distribution between Ethiopia/ eastern Sudan and Uganda/ Kenya/Tanzania. This might, contrary to what has just been stated, seem to indicate that two distinct taxa are involved. However, if we look at the ecology of the species it becomes clear that this disjunction is really an illusion. A. polystachius is a species of fairly wet intermediate or upland grassland. The areas in northern Kenya, northern Uganda and southwestern Ethiopia, where it is absent, are much drier and on the whole covered with Acacia-Commiphora bushland. The reason A. polystachius does not occur here is simply that the climate is far too dry.