Neotropical Costaceae

Description

Perennial often large-sized herbs, with rhizomes. Stems terete, straight or spirally contorted, containing an acid juice. Leaves spirally arranged, with closed sheaths and a truncate or 2-lobed ligule. Inflorescence a spike, terminal on a leafy stem or sometimes on a separate leafless, basal shoot, rarely flowers solitary in the axils of the upper leaves (Monocostus K.Schum.); bracts often brightly coloured, coriaceous to herbaceous, imbricate, each subtending 1 flower, with a linear callus just below the apex; bracteole 1, folded or tubular. Flowers zygomorphic; calyx tubular, 3-lobed; petals 3, connate into a distinct tube; fertile stamen 1, petaloid, anther 1, usually attached at the middle; labellum equaling or much exceeding the corolla, tubular or horizontally spreading; style 1, filiform, lying close to the stamen and embraced by the thecae, stigma 1, 2-lamellate with a 2-lobed appendage or cup-shaped and unappendaged, margins ciliate; ovary inferior, 2-3-locular, with 2 septal nectarial glands towards the apex, placentation axile, ovules many, anatropous. Fruit a white capsule, 3- or 2-locular, crowned by the persistent calyx, often irregularly and tardily dehiscent. Seeds many, angular-ovoid to 4-sided, with a white aril, glossy black or brown.

Notes on delimitation

• Costaceae were formerly treated as a subfamily of Zingiberaceae (Costoideae), but recent research revealed that they should be treated as a family of its own. The family belongs in the Zingiberales.

Distribution in the Neotropics

Family: From Mexico in the North to S Brazil and N Paraguay in the South, also present in the West Indian Islands.

Genera:

• Chamaecostus C.D.Specht & D.W.Stev. - Colombia, Venezuela, the Guianas, and Amazonian and SE Brazil, and Bolivia.
• Costus L. - throughout he Neotropics.
• Dimerocostus Kuntze (2 spp.) - from Honduras in he North to Peru and Bolivia in the South.
• Monocostus (1 sp.) - Eastern Peru.
  

Distinguishing characters (always present)

• Stems with an acid juice.
• Leaves spirally arranged, with a ligule and a closed sheath.
• Inflorescence a spike, covered with often brightly coloured bracts (except for Monocostus).
• Flowers zygomorphic.
• Stamen 1, petaloid.
• The largest part of the flower is the tubular to spreading lip (labellum).
• Calyx tubular and persistent on top of the fruit.

Other important characters

• Inflorescence terminal on the leafy stem or on a separate, leafless, basal shoot.
• Most species have thick, coriaceous bracts; in some species the bracts are chartacous to herbaceous.

Key differences from similar families

Costaceae have various flower characters in common with Zingiberaceae, but they differ by having:

• Non-aromatic leaves.
• Closed leaf sheaths.

Number of genera

• Chamaecostus (7 spp.)
• Costus (ca. 50 spp.)
• Dimerocostus (2 spp.)
• Monocostus (1 sp.)

Useful tips for generic identification

Key to genera of the Neotropical Costaceae

1. Flowers solitary in the axils of the upper leaves...Monocostus
1. Flowers in a terminal spike... 2

2. Ovary 2-locular; bracteole tubular...Dimerocostus 
2. Ovary 3-locular; bracteole folded or tubular... 3

3. Low or very small, occasionally acaulescent plants, never exceeding 1 m in height; bracts herbaceous to chartaceous, green or yellow; bracteole tubular, bicarinate at the abaxial side; stigma cup-shaped...Chamaecostus
3. Tall, gigantic, or low plants, usually over 1 m tall; bracts coriaceous, rarely chartacous, red, orange, yellow or green; bracteole folded; stigma 2-lamellate, always provided with a dorsal 2-lobed appendage...Costus

Notable genera and distinguishing features

• Monocostus is unique in the family in having solitary, axillary flowers; it is endemic to Eastern Peru, where it grows in forests on limestone.
• Chamaecostus has been recently separated from Costus by Specht et al. (2001, 2006) based on molecular evidence. Formerly it had been placed (see Maas, 1972) in Costus subgenus Cadalvena Fenzl.

Status

• All genera are native.

General notes

The petaloid labellum of Costaceae has been supposed to consist of 5 staminodes.

Important literature

Larsen, K. 1998. Costaceae. In: K. Kubitzki  (ed.), The families and genera of vascular plants 4: 128-132.

Maas, P.J.M. 1972. Costoideae (Zingiberaceae). Flora Neotropica Monograph 8: 1-140.

Maas, P.J.M. 1972. Renealmia (Zingiberaceae- Zingiberoideae). Costoideae (Additions) (Zingiberaceae). Flora Neotropica Monograph 18: 162-219.

Specht, C.D. 2006. Systematics and evolution of the tropical monocot family Costaceae (Zingiberales): A multiple dataset approach. Syst. Bot. 31: 89-106.

Specht, C.D., & Stevenson, D.W. 2006. A new phylogeny-based generic taxonomy for the monocot family Costaceae (Zingiberales). Taxon 555: 153-163.

Specht, C.D., Kress, W.J., Stevenson, D.W., & DeSalle, R. 2001. A molecular phylogeny of Costaceae (Zingiberales). Mol. Phyl. Evol. 21: 333-345.