Costus

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 : 289-290

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 labellum; plant glabrous; bracts green; bracteole with an inconspicuous callus; leaf sheaths green with reddish margin; apex of leaves acuminate (acumen 15–40 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; bracteole 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 yellow 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; ligule 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, ascending; flowers not aromatic............... 4. C. aureus

20. Ligule 5–35 mm long; appendages of bracts pinkish red to purplish brown, strongly reflexed; flowers strongly aromatic.......................... 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 many-flowered, 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 yellow 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