Costus giganteus Welw. ex Ridl. — Plate, 1887

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

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.

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.

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.