Costus giganteus Welw. ex Ridl.

Kamer 1, H. Maas-van de, Maas 1, P. J. M., Wieringa 1, J. J. & Specht, C. D., 2016, Monograph of African Costaceae, Blumea 61 (3), pp. 280-318 : 299-300

9. Costus giganteus Welw. ex Ridl. — Plate 2c ; Map 6

Costus giganteus Welw. ex Ridl. (1887) 131; K.Schum. (1904) 407. — Type: Welwitsch 6465 (holo BM 2 sheets [ BM000617218 , BM000617219 ]; iso LISU, NY), São Tomé & Principe, ‘ Ilha de S. Thomé, Monte Caffé’ , 600– 850 m (‘2000–2800ft’), Dec. 1860 .

10. Costus gracillimus Maas & H.Maas , nom. nov. — Plate 2d ; Map 6

Costus gracillimus Maas & H.Maas. — Costus pulcherrimus A.Chev. (1917) 304, nom. illeg., non Kuntze (1898) 301. — Type: Chevalier 19568 (holo P), Ivory Coast, Tabou, ‘ Bassin du Cavally, pays des Tépos , entre Toula et Nekaougnié’, 25 July 1907 .

Terrestrial herb 1.5–8 m tall. Leaves many; sheaths to c. 2 cm diam; ligule chartaceous, truncate, 15–25 mm long; petiole 15–30 mm long; sheaths, ligule and petiole subglabrous; lamina shiny above, narrowly elliptic to narrowly obovate, rarely ovate, 28–52 by 7–16 cm, upper side glabrous, lower side initially densely covered with soft, erect hairs 1–1.5 mm long, soon glabrous, base acute to obtuse, apex acuminate (acumen 10–15 mm long). Inflorescence many-flowered, ovoid, 12–20 by 7–10 cm, terminating a separate leafless shoot 100–150(–300) cm long; bracts, bracteoles, calyx, ovary and capsule glabrous. Flowers 1 per bract; bracts red, coriaceous, broadly ovate to ovate, 3.5–6 by 3–5 cm, callus 5–10 mm long; appendages absent; bracteole boat-shaped, 28–45 mm long, callus (3–) 5–10 mm long; calyx basally white, apically pinkish red, 13–25 mm long, lobes shallowly ovate-triangular, 3–5(–7) mm long, callus absent; corolla yellow, 75–85 mm long, glabrous, tube (25–) 35–40 mm long, lobes narrowly elliptic, 40–45(–50) mm long, sometimes bending towards each other forming a kind of hood over the stamen; labellum completely yellow, tubular, narrowly elliptic to ovate when spread out, 40–45 by 15–20 mm, lateral marginal parts curved upwards, upper margin crenulate; stamen erect, not bending downwards and not closing the throat, yellow, 40–50 by 5–10 mm, apex cucullate, anther 10–12 mm long. Capsule obovoid to subglobose, 15–20 by 10–20 mm. Seeds 3–3.5 by 2–2.5 mm.

Terrestrial herb 1–3 m tall. Leaves many; sheaths 0.2–0.6 cm diam; ligule chartaceous, obliquely truncate, 10–15(–20) mm long; petiole 5–10 mm long; sheaths, ligule and petiole glabrous or rarely sparsely covered with erect hairs <1 mm long; lamina greyish to brown on upper side, lower side paler, coriaceous, narrowly elliptic to elliptic, 13–21 by 4–8 cm, slightly 4–6-pli- cate, glabrous on both sides, but margins and apex rarely covered with some hairs, base acute, apex acuminate (acumen 10–15 mm long). Inflorescence few-flowered, broadly ovoid to globose, 1.5–5 by 1.5–5 cm, terminating the leafy shoot; bracts, bracteoles, calyx, ovary and capsule glabrous, rarely rather densely covered with erect hairs <1 mm long. Flowers 1(–2) per bract; bracts red, orange-red to red-purple, coriaceous, broadly ovate to ovate, 1.5–2.5 by 1.5–2.5 cm, callus mostly absent or present and then c. 1 mm long; appendages gener- ally absent; bracteole boat-shaped, 9–14 mm long, callus 1–2 mm long; calyx 7–10 mm long, lobes very shallowly triangular, c. 1 mm long, callus absent; corolla fleshy, 20–29 mm long, red, orange or yellow, glabrous, tube 5–7 mm long, lobes narrowly ovate to elliptic, 15–22 mm long; labellum fleshy, red, orange or yellow, tubular, broadly obovate when spread out, 15–23 by 15–20 mm, margin crenulate; stamen yellow, 13–15 by 6–8 mm, anther 7–8 mm long. Capsule obovoid, 9–10 by 5–6 mm. Seeds 1.5–2 by 1.5 mm.

Distribution — Central Africa (both islands São Tomé and Principe and one specimen (Wrigley & Melville 270) from Annobon (Equatorial Guinea)).

Habitat & Ecology — In rainforest. At elevations of 0– 1450 m. Flowering and fruiting: October to January.

Field observations — Unlike most species of Costus, many flowers of the same inflorescence can be at anthesis at the same time.

IUCN Conservation Status — Based on an EOO of 3764 km 2 and an AOO of 32 km 2 from three locations which all are only partially protected, while the remainder of these locations show severe habitat degradation (most records are actu-ally very old and the species has not recently been found in those localities), we assess this species as Endangered (EN) B1ab(ii,iii)+2ab(ii,iii).

Notes — Costus giganteus is unique among the African species of Costus by the combination of separate flowering shoots, combined with red bracts and yellow, tubular flowers. In these aspects it resembles some Neotropical species like C. erythrocoryne K.Schum. The only other African species of Costus with the same type of yellow, tubular flowers is C. gracillimus, a species of much smaller stature from the African continent. The flowers of C. giganteus have a relatively narrow labellum with upcurved lateral margins and an erect cucullate stamen. The flower presents an open throat to pollinators, in contrast with all other species of African Costus where the single fertile stamen closes the throat and visitors have to force their way in. The filament is not flat but rolled inwards lengthwise along its margins, especially at its base. The corolla lobes bend towards each other forming a kind of hood over the stamen and the apex of the stamen is cucullate. This type of flower is unique to this species. The style of C. giganteus is often persistent after flowering. Young leaf lamina are shiny above, while older ones are dull green. The aril is relatively large, measuring up to 2 times the length of the seed.

Distribution — West Africa (Ghana, Guinea, Ivory Coast, Liberia, Sierra Leone).

Habitat & Ecology — Mostly in rainforest, along paths or in open places and/or disturbed areas, often in wet places, at elevations of 0– 850 m. Flowering and fruiting: all year through.

IUCN Conservation Status — Based on all collections C. gracillimus has an AOO of 184 km 2 from about 20 locations of which only four are partially protected, while the remainder of these locations are already lost or face severe habitat degradation. If we only use records collected since 1970, this species has only 10 locations left (with an AOO of 68 km 2), of which the majority is lost or threatened. We therefore assess this species as Vulnerable (VU) A2; B2ab(i,ii,iii,iv,v).

Notes — Costus gracillimus is a relatively small herb with a slender spiralling shoot (0.2–0.6 cm diam), a long ligule (10– 15 mm long) and relatively small, tubular, fleshy flowers. The leaves of C. gracillimus have relatively narrow and long petioles. The labellum is about as long as the corolla, forming a tube with its longitudinally incurved margins. The majority of plants studied have red bracts with red-orange flowers. However, Jongkind et al. 11941 from Liberia has green bracts and yellow flowers. Based on the overall similarities of this single collec- tion, we do not believe it to be a separate species but rather a lighter-coloured form of C. gracillimus . Costus gracillimus has often been misidentified as C. deistelii K.Schum. That species, known only from the type collection Deistel 498 (B destroyed), now placed in the synonymy of C. afer , differs from C. gracillimus by a shorter ligule, longer inflorescence and bracteole, and the presence of a rim at the base of the ligule not completely encircling the shoot.

Figures

Plate 2 a. Costus dubius (Afzel.) K.Schum. Basal inflorescence. – b. Costus gabonensis Koechlin. Inflorescence. – c. Costus giganteus Welw. ex Ridl. Inflo- rescence. – d. Costus gracillimus Maas & H.Maas. Inflorescence with red bracts (a: photographed in Burgers’ Bush, Arnhem, The Netherlands, no specimen collected; b: Maas et al. 10465; c: Scharf 221; d: Maas et al. 10571). — Photos: a, b, d: P.J.M.Maas; c: L.Y.T.Westra.