Costus

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 : 288-290

Costus

Costus L. (1753) 2. — Type: Costus arabicus L.

Perennial, rhizomatous herbs, terrestrial or epiphytic, rarely gigantic, tall, low or shootless herbs; shoot erect, unbranched, generally forming a spiral, composed entirely of sheathing leaf bases forming conspicuous nodes and internodes. Leaves few or many, spirally arranged along the shoot; shootless species with few leaves rosulate; leaf sheaths closed around the shoot; ligule present or rarely absent, membranous to chartaceous, tubular at the base and truncate to 2-lobed at the apex, sur-rounding the shoot above the proximal lobe as a continuation of the sheathing leaf base; petiole present or sometimes absent; lamina generally green, sometimes shiny, or plicate, herbaceous to coriaceous, generally elliptic with acuminate apex and acute base, upper side and lower side with various indument. Inflorescence a many-, several- or few-flowered spike, either terminating the leafy shoot, or terminating a separate leafless shoot, or lateral in the axil of a leaf, ellipsoid to ovoid or globose; bracts spirally arranged, each carrying 1 or 2 flowers, green, red or brownish, membranous, chartaceous or coriaceous, generally ovate to triangular or elliptic, imbricate, callus linear, nectariferous, absent or present; foliaceous appendages ab-sent or present, generally coloured as the bracts, ascending, horizontally spreading or reflexed, broadly ovate to narrowly triangular; each flower enclosed by 1 bracteole, generally col-oured as the bracts, boat-shaped or tubular and 1-keeled or sometimes 2-keeled, callus absent or 1 or 2 calli present. Flowers epigynous, perfect, zygomorphic; calyx generally coloured as the bracts, tubular at the base, 2(–3)-lobed, lobes erect, horizontally spreading, or reflexed, callus absent or present; corolla white, yellow, orange, pink, lilac or reddish brown, or a combination of these colours, tubular, 3-lobed, lobes erect, narrowly obovate to elliptic, rarely hyaline; labellum large, longer than or as long as the corolla, white, yellow, orange, pink, lilac, dark red, reddish brown, purple, or a combination of these colours, sometimes with darker lateral parts and/or striped margin and central yellow nectar guide, horizontally flattened, funnel-shaped, or rarely tubular, obovate to subcylindrical when spread out, margin undulate to crenate, sometimes fimbriate; stamen 1, petaloid, generally yellow or white, apex recurved, anther longitudinally placed in the middle, composed of two narrowly elliptic 2-sporangiate thecae; base of stamen and labellum joined into a tube; gynoecium composed of a single ovary, style and stigma; ovary inferior, 3-locular, placentation axile, ovules many, organized in 2 rows, anatropous, septal nectaries 2 at the base of the floral tube; style 1, terminal, cy-lindrical, filamentous, held between the thecae of the anther; stigma 1, 2-lamellate, hooked between the apices of the thecae by a dorsal 2-lobed to rounded appendage. Fruit capsular, 3-locular, generally obovoid, crowned by the persistent calyx, dehiscing loculicidally by three longitudinal slits or indehiscent and irregularly breaking when old. Seeds numerous, black, shiny, irregularly angular; aril white, lacerate.

Distribution — Tropical to subtropical sub-Saharan Africa.

Habitat & Ecology — Along with Marantaceae and Zingiberaceae , Costaceae form a significant part of the understory of the African tropical and subtropical rainforests. All three families form part of important herbaceous communities along forest margins, in forest gaps, and in the regrowth of disturbed forests (Dhetchuvi 1996).

KEY TO THE AFRICAN SPECIES OF COSTUS

1. Herbs acaulescent.............................. 2

1. Herbs with shoots 0.5–6 m tall.................... 3

2. Calyx 35–75 mm long; anthers (6–)7–12(–15) mm long..................................19. C. macranthus

2. Calyx 15–30 mm long; anthers 4–7(–12) mm long...................................... 22. C. spectabilis

3. Epiphytic plants with inflorescences lateral in the axil of a leaf, or terminating a leafy or leafless shoot.......... 4

3. Terrestrial plants with inflorescences terminating a leafy or leafless shoot................................. 6

4. Bracts 1-flowered; flowers yellow or pale lilac with yellow nectar guide................................... 5

4. Bracts 2-flowered; flowers white to dark pink with yellow nectar guide......................... 23. C. talbotii

5. Flowers completely yellow........... 12. C. lateriflorus

5. Flowers pale lilac with yellow nectar guide 14. C. lilaceus

6. Inflorescence generally terminating a separate leafless shoot, emerging directly from the rhizome............ 7

6. Inflorescence generally terminating a leafy shoot..... 18

7. All bracts provided with appendages ( Fig. 1e )......... 8

7. Bracts generally without appendages (often the lowermost bracts appendaged but all the other ones not)....... 11

8. Flowers white with yellow nectar guide.............. 9

8. Flowers differently coloured...................... 10

9. Ligule 1–2 mm long; petiole 1–3 mm long; flowers 2 per bract, appendages ascending; inflorescence globose (6–8 by 6–8 cm) on a separate leafless shoot 10–30 cm long................................. 1. C. acutissimus

9. Ligule 10–30 mm long; petiole 5–10 mm long; flower 1 per bract, appendages horizontally spreading; inflorescence ovoid (4–12 by 3–8 cm) on a separate leafless shoot 10–50 cm long............................. 3. C. albiflos

10. Flowers with white to purplish grey or reddish brown label- lum; plant glabrous; bracts green; bracteole with an incon-spicuous callus; leaf sheaths green with reddish margin; apex of leaves acuminate (acumen15–40 mm long)............................... 18. C. maboumiensis

10. Flowers with white labellum with pink to purple margin; upper and lower side of leaves densely covered with soft erect hairs to c. 3 mm long; bracts dark brown-red; brac-teole with a distinct callus; leaf sheaths dark red; apex of leaves acute to acuminate (acumen c. 10 mm long)..................................... 13. C. ligularis

11. Bracts green................................ 12

11. Bracts red to brown........................... 15

12. Sheaths, ligule, petiole and leaves glabrous........ 13

12. Sheaths, ligule, petiole and leaves very densely covered with brown hairs 0.5–2 mm long................. 14

13. Flowers white to purplish grey or reddish brown................................... 18. C. maboumiensis

13. Flowers white with yellow nectar guide.... 6. C. dubius

14. Corolla lobes salmon-coloured; labellum white with 2 dark pink patches at the base and a yellow nectar guide, horizontally flattened..................... 20. C. nimba

14. Corolla lobes whitish; labellum white to pale pink with yel-low nectar guide, funnel-shaped........ 5. C. dinklagei

15. Bracts brown to reddish brown, 1- or 2-flowered; labellum not tubular; separate leafless shoot 1–9 cm long.... 16

15. Bracts red, 1-flowered; labellum tubular; separate leafless shoot 100–150 cm long.............. 9. C. giganteus

16. Bracts 2-flowered; flowers white to dark pink with yellow nectar guide................................. 17

16. Bracts 1-flowered; flowers completely yellow.......................................... 11. C. kupensis

17. Labellum white to dark pink with yellow nectar guide; ligule 40–55 mm long; leaves not bullate nor plicate, base obtuse to acute........................... 23. C. talbotii

17. Labellum dark pink to white with yellow nectar guide; lig-ule 1–5 mm long; leaves bullate and 5–10-plicate, base cordate................... 24. C. tappenbeckianus

18. All bracts provided with appendages.............. 19

18. Bracts generally without appendages (often the lowermost bracts appendaged but all the other ones not)...... 23

19. Flowers completely yellow..................... 20

19. Flowers differently coloured.................... 21

20. Ligule 1–5 mm long; appendages of bracts green, ascend-ing; flowers not aromatic............... 4. C. aureus

20. Ligule 5–35 mm long; appendages of bracts pinkish red to purplish brown, strongly reflexed; flowers strongly aro-matic.......................... 8. C. gabonensis

21. Bracts soon falling apart into separate fibers ( Fig. 4c ), leaves obovate, 4, c. 20-plicate; petiole 0–3 mm long.................................. 7. C. fenestralis

21. Bracts not falling apart into separate fibers, leaves not obovate, not plicate, generally more than 4; petiole 3–15 mm long................................... 22

22. Bracts apically dark brown-red; labellum white with pink to purple, without nectar guide; callus on bracteole distinct, 1.5–2 mm long; inflorescence few- to several-flowered, 1–4 by 1–4 cm; upper side of leaves covered with soft erect hairs to c. 3 mm long............ 13. C. ligularis

22. Bracts green to reddish purple; labellum pink to dark pink or white, with yellow nectar guide; callus on bracteole present, 2–3.5 mm long ( Fig. 1e ); inflorescence manyflowered, 2.5–10 by 3–7 cm; upper side of leaves generally glabrous.................... 21. C. phyllocephalus

23. Bracts red to reddish brown.............. ... ... 24 23. Bracts completely green................. ... ... 26 24. Labellum horizontally flattened, either completely yellow or dark pink; leaf sheaths 6–11 mm diam; calyx 11–20 mm long................................. .. .... 25

24. Labellum tubular; flowers red or yellow to orange; leaf sheaths 2–6 mm diam; calyx 7–10 mm long........................................ 10. C. gracillimus 25. Herbs 1.5–2.5 m tall; leaves distinctly shiny, glabrous; calyx 17–20 mm long; flowers white and pink... 16. C. louisii

25. Herbs to 0.6 m tall; leaves not shiny, densely covered with erect to half-appressed hairs 1.5–2 mm long on both sides; calyx 11–12 mm long; flowers completely yellow..................................... 15. C. loangensis

26. Corolla lobes not hyaline, white; labellum white with yel-low and pink or red; ligule 1–11 mm long, with distinct rim around its base.............................. 27

26. Corolla lobes hyaline, salmon-coloured; labellum white with salmon and 2 dark pink zones at the base; ligule 14–23 mm long without a distinct rim........... 20. C. nimba

27. Flowers white with yellow and pink; calyx in fruit about as long as the bracts, lobes erect; ligule 4–11 mm long, generally with a glabrous incomplete rim around its base; leaves glabrous........................ 2. C. afer

27. Flowers white with yellow, orange and dark red; calyx in fruit exceeding the bracts, lobes horizontally spreading to reflexed; ligule 1–4 mm long, with a complete hairy rim around its base; lower side of leaves often silvery (covered with mainly erect hairs to c. 2 mm long).......................................... 17. C. lucanusianus

Figures

Fig. 1 Schematic details (* = node of shoot). a. Ligule shapes (the utmost right one: C. lucanusianus); b. ligule lengths (left: C. lateriflorus); c. 1-flowered bract with 1 bracteole and 1 flower; d. 2-flowered bract with 2 bracteoles and 2 flowers; e. drawing of bract with nectary, 2 appendaged bracts, bracteole with nectary, and ovary and calyx with nectary on apex of lobes. — Drawing by Hendrik Rypkema.

Fig. 4 Costus fenestralis Maas & H.Maas a. Habit showing few leaves in an apical rosette; b. detail of sheaths dilacerating into fibers; c. detail of withered bract; d. detail of bracteole and calyx; e. inflorescence (all: Jongkind et al. 5811, WAG). — Drawing by Hendrik Rypkema.