Costaceae (Specht, 2006)

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

Costaceae

Costaceae Nakai (1941) 203. — Type: Costus L.

The African Costaceae ( Costus spp. and Paracostus englerianus ) are perennial, rhizomatous herbs, terrestrial or epiphytic, rarely gigantic, tall (max. 6 m), low or acaulescent herbs; shoot erect or prostrate, unbranched, generally spirally shaped, composed of nodes and internodes; leaf sheaths, petioles and lig-ules originating at the nodes. Leaves 1, or few to many, spirally arranged along the shoot; shootless species with few leaves ro-sulate; prostrate species with 1 leaf per shoot ( Paracostus ); leaf sheaths fully closed around the shoot; ligule present or rarely absent, membranous to chartaceous, tubular at the base and truncate to 2-lobed at the apex; petiole present or sometimes absent; lamina generally green, sometimes shiny, or plicate, herbaceous to coriaceous, generally elliptic with acuminate apex and acute base, sometimes extreme base of leaf fleshy surrounding the inflorescence ( Paracostus ). Inflorescence a many-, several- or few-flowered spike, either terminating a leafy shoot, or terminating a separate leafless shoot, or lateral in the axil of a leaf, sometimes partially enclosed by the overlapping margins of the bases of the leaf lamina and the uppermost 1–3 sheathing leaf bases ( Paracostus ); inflorescence ellipsoid to ovoid or globose when terminating a leafy shoot, loosely arranged when axillary; bracts spirally arranged, each carrying 1 or 2 flowers, yellow, green, red or brownish, membranous, chartaceous or coriaceous, generally ovate to triangular or elliptic, imbricate, callus linear, nectariferous, absent or present; foliaceous appendages absent or present, generally coloured as the bracts, ascending, horizontally spreading, or reflexed, broadly ovate to narrowly triangular; each flower enclosed by 1 bracteole, generally coloured as the bracts, boat-shaped or tubular and 1-keeled or sometimes 2-keeled, callus absent or 1 or 2 calli present. Flowers epigynous, bisexual, zygomorphic; calyx generally coloured as the bracts, tubular, 3-lobed or rarely 2-lobed, lobes erect, horizontally spreading, or reflexed, cal-lus 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 upper margin and central yellow nectar guide, horizontally flattened, funnel-shaped or rarely tubular, narrowly elliptic, ovate, obovate or subcylindrical when spread out, rarely lateral margins curved upwards ( C. giganteus ), margin undulate to crenate, sometimes fimbriate; stamen 1, petaloid, generally yellow or white, apex recurved or erect and cucullate ( C. giganteus ), anther longitudinally centred, composed of 2 narrowly elliptic 2-sporangiate thecae; base of stamen and labellum joined into a tube; gynoecium composed of a single ovary with attached style and stigma; ovary inferior, 3-locular, placentation axile, ovules many, organized in 2 rows per locule, anatropous; septal nectaries 2 at the apex of the ovary secreting nectar into the base of the floral tube; style 1, filamentous, supported 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. In Paracostus englerianus the stigma is composed of a funnel-shaped upper part and a reflexed lamellate part, and the appendage is absent. Fruit capsular, 3-locular, placentation axile, 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 reflecting tight packing in fruit; aril white, lacerate.

KEY TO THE AFRICAN GENERA OF COSTACEAE

1. Plants erect, leaves generally more than two, occasion-ally in a basal rosette; inflorescence many-flowered, bracts conspicuous and often brightly coloured........ Costus

1. Plants prostrate, leaves solitary, never in a basal rosette; inflorescence few-flowered, bracts inconspicuous and not brightly coloured....................... Paracostus