Costus macranthus K.Schum.

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 : 308

19. Costus macranthus K.Schum. — Plate 4a ; Map 13

Costus macranthus K.Schum.(1901) 279, t. 7, 8; (1904) 421 — Type: Goetze 1488 (holo B destroyed; lecto E, designated here; isolecto BM, BR, EA), Tanzania, T7, ‘ Rungwe District, Kondeland, im Kivirithal , am Kasimulo hügel im Buschwald bei 600 m’, Nov. 1899 .

Terrestrial shootless rosulate herb 0.1–0.2 m tall; rhizomes vertically directed, to c. 10 cm deep; horizontal runners to c. 40 cm long, 3–5(–7) mm diam, both rhizomes and runners covered with pale brown, membranous, imbricate sheaths 1.3–2 cm long; roots with side roots to c. 13 cm long. Leaves (3–)4(–5) per shoot, spreading horizontally and forming a cruciform rosette flat on the ground; ligule rarely seen, c. 0.5 mm long; petiole absent; lamina pale to yellowish green above, greenish purple below, with hyaline, red-purple to pink margins <1 mm wide, distinctly thickened and fleshy, elliptic, broadly elliptic, obovate, or suborbicular, 7–20(–30) by 5–22(–25) cm, upper side glabrous to rather densely covered with soft erect hairs <1 mm long, lower side glabrous to rather densely, rarely densely, covered with soft, erect hairs <1 mm long, margin densely ciliate, base acute, extreme base of leaf 20–30 mm long, at an angle of 90° with the lamina, surrounding the inflo- rescence, apex obtuse and mucronate (mucro c. 1 mm long). Inflorescence terminal, formed in the centre of the rosette, few-flowered, basally enclosed by the more or less overlapping thickened extreme base of the leaves and by the uppermost 4–6 sheaths; sheaths 2–6.5 by 1–6 cm, apex obtuse to round- ed, together forming a cup of 1–3 cm diam around the inflo- rescence; upper part of uppermost sheaths rather densely covered with soft erect hairs <1 mm long with thickened base; upper part of bracts, bracteoles, calyx and ovary rather densely covered with soft erect hairs <1 mm long, capsule glabrous. Flowers 1 per bract; bracts whitish green, membranous, nar- rowly ovate-triangular, 1.7–4 by 0.4–0.8 cm, callus absent; appendages absent; bracteole boat-shaped, c. 15 mm long, callus absent; calyx 35–75 mm long, split on one side, lobes 2, broadly triangular, 2–5 mm long, callus absent; corolla (pale) yellow, 90–130 mm long, glabrous, tube 50–90 mm long, lobes narrowly ovate-triangular, 40–80 mm long; labellum bright yel- low to orange, horizontally flattened, basally funnel-shaped, obovate to suborbicular when spread out, 70–110 by 60–110 mm, margin crenate; stamen yellow, 60–80 by 7–10 mm, anther (6–)7–12(–15) mm long. Capsule subterranean, subglobose, c. 10 by 9 mm. Seeds not seen.

Distribution — East Africa (Tanzania, Uganda); Southern Tropical Africa (Malawi, Mozambique, Zambia, Zimbabwe).

Habitat & Ecology — In non-inundated forest, open grass land, often on steep, rocky soil, sometimes around ant hills, plants growing in colonies, locally common, at elevations of (250–) 500–1700 m. Flowering and fruiting: November and December and rarely from January to March.

IUCN Conservation Status — Least Concern.

Notes — Costus macranthus can be recognized by four horizontally spreading leaves forming a cruciform rosette flat on the ground and a central inflorescence producing large yellow flowers, in this aspect very much resembling C. spectabilis . Lock (1984) differentiates the two species based on the length of several flower parts. Of these, the length of the anther is the most reliable in herbarium material. In C. macranthus the anther is (6–)7–12(–15) mm long (we recorded a length of 10 mm for 15 specimens studied) and in C. spectabilis 4–7(–12) mm long (we recorded 5 mm for a total of 17 specimens). Among other distinguishing characters are the length of calyx, corolla tube, corolla lobes and labellum. These are all larger in C. macranthus than in C. spectabilis . Many other possible distinctive characters still have to be studied in the field, e.g. difference in flower colour, time of floral and vegetative development (if flowers emerge before leaf emergence, at the same time as leaf development, or after leaf dehiscence) and morphology of the underground parts. Both C. macranthus and C. spectabilis occur in the region between E28–35° and S6–15° in Malawi, Mozambique, Tanzania, Uganda, Zambia and Zimbabwe. All collections of C. macranthus have been found in an area where C. spectabilis has also been collected. In this area the majority of the (c. 50) herbarium specimens of both species available for this study were collected flowering in November to December. Both species were found between 500 and 2000 m altitude. In this geographic range, at this altitude and with flower, specimens of both species were collected either with very young leaves or with senescent leaves. For C. macranthus we also recorded 5 specimens collected without any leaves. Lock wrote in Flora Zambesiaca (Lock & Diniz 2010: 117): “Inflorescence developing with the leaves or just before, but flowering continu- ing until leaves are almost fully developed”.

Figures

Plate 4 a. Costus macranthus K.Schum. Habit, flower and details of stamen, stigma and style. – b. Costus nimba H.Maas & Maas. Basal inflorescence. – c. Costus spectabilis (Fenzl) K.Schum.Habit with flowers. – d. Costus lucanusianus J.Braun & K.Schum. Inflorescence with 4 flowers.– e. Costus talbottii Ridl. Flower. – f. Costus louisii H.Maas & Maas. Inflorescence (a: Wright 1905: t 7992; b: Jongkind 11301; c: photographed in Rumonge, SW Burundi, no specimen collected; d: Maas et al. 10000; e: Maas et al. 9800; f: Maas et al. 10339). — Photos: a, b, d–f: P.J.M.Maas; c: E. Fischer.

Map 13 Distribution of Costus macranthus K.Schum. (▲) and C. spectabilis (Fenzl) K.Schum. (●).