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Costus Linnaeus

Type: Costus arabicus

Description:
     Rhizomatous perennial herbs of moderate to large structure, 2-5 m, stems unbranched. Stems erect from rhizome, covered near base with leafless sheaths, leafy higher up. Leaves spirally arranged with closed, ligulate sheaths. Ligule typically large and pronounced. Inflorescence terminal on leafy stem or on special leafless shoots directly from the rhizome, spiciform, cone-like, with conspicuous persistent imbricated bracts. Bracts coriaceous, narrowly to braodly ovate, often terminating in a foliaceous appendage. Calyx more or less equal lobed, rarely exceeding the bracts; stamen 1, petaloid. Labellum equaling or surpassing the corolla. Ovary inferior, trilocular, containing numerous ovules in rows of 2. Fruit fleshy, globose, tardily dehiscent or indehiscent in which case seeds are released by the decaying of the fruit wall. Seeds black, elliptical, with large white laciniate aril.

Geographical Distribution:
     Costus as currently circumscribed is restricted to the tropical moist forests of Africa and America with species diversity centered in the neotropics. Asian species formerlly in Costus have been moved to Cheilocostus.

Phylogeny and Classification:
     Schumann organized the large genus into 5 subgenera, with Eucostus being the largest and comprising the majority of the species heretofore included in the genus Costus. Schumanns Eucostus is not completely congruent with Costus, however, as Shumann (1904) included all Asian taxa currently placed in Cheilocostus and several taxa now included in Chamaecostus. Schumann also distinguished subgenera Epicostus and Metacostus which for the most part include those taxa found in the basal grade. While there is no support for the monophyly of these subgenera, the characters used to define them were either plesiomorphic (open labellum, small stature) or are found in more than one lineages within the grade (ephiphytic habit). Additional monographical work in this group is necessary to clearly delimit species so that a more complete phylogenetic analysis can be developed. Current results show that the characters that once defined subgenera Epicostus and Metacostus are not sufficient for recognition in an evolutionary classification, however these characters do separate the taxa from those in the Costus radiation clade and the African melittophilous clade.
     
     Maas divided the Neotropical species of Costus subgenus Costus into two sections, Costus and Ornithophilus, based on labellum characters (1977) or, more succinctly, upon the pollination syndrome (with sect. Costus being bee-pollinated and sect. Ornithophilus being hummingbird-pollinated). The first section he characterized as having a labellum with a short, rather broad tube, and a distinct, exposed limb; its color varies from white to yellow, but the lateral lobes are often striped with red to purple. The bracts of this group are typically green, and the section appears to be adapted to pollination by bees. The second section is comprised of species with a small, tubular labellum of yellow, orange, or reddish colour: the bracts are of the same colour, or rarely green. This group is adapted to hummingbird pollination, and was thus referred to as section Ornithophilus (bird-loving). The two sections were considered to be natural sections of subgenus Costus within the new world, but relationships of these sections to old world species were not discussed except to infer a potential relationship between section Ornithophilus and the genus Tapeinochilos. The results reported here show that these two sections do not actually form distinct sections within the new world taxa, but rather that ornithophily and melittophily are homoplasious when viewed as a single characters. However, together these two groups form a strongly supported clade that is sister to a clade of African melittophilous taxa. This clade is informally referred to as the Costus radiation clade but will not be formally named in the current treatment due to the paraphyly of the remaining Costus taxa excluded from this clade.

Taxonomic Diversity:
     The genus Costus is by far the largest genus in the family Costaceae and has the broadest circumscription, including most of the morphological diversity in the family. The genus is comprised of about 100 species.

Ecology and Pollination Biology:
     Within the New World taxa, species are either hummingbird pollinated or primarily bee pollinated with floral morphology strongly reflecting the pollination syndrome. The African taxa are divided into a bee pollinated group (eg. Costus dubius) and a basal group that is generalist in form (e.g. Costus lateriflorus) and is pollinated by bees, flies, and beetles. There are no bird pollinated taxa in Africa.

Common Names, Uses and Notes:
     Known widely as CaƱa Agria throughout Latin America. The stems are crushed and applied as a poultice for treating inflammation or drank cold or hot for cough. The juice of the stems can be sucked to reduce thirst while working in the tropics. The flowers are edible and make a beautiful and delicious salad (see Alan Carle's article in The Heliconia Society International's Newsletter).

A stem of Costus vargasii demonstrates why the Costaceae are known as the 'spiral gingers.'
Costus afer growing at the Lyon Arboretum in Hawaii.  This species is found natively in tropical Africa.
A species native to the west coast of Africa, this plant is growing in the living collection of Alan Carle, Mossman, Australia.
Costus guanaiensis from Bolivia
Costus lateriflorus, an epiphytic species from tropical Africa found throughout the Congo basin.
Costus woodsonii, native to coastal areas of Costa Rica.
Costus leucanthus from Lyon Arboretum
Costus villosissimus from Peru.
A potentially new species from the Yungas of Bolivia.
The spiral stem and flower of Costus tappenbeckianus
Costus bracteatus
Costus phaeotrichus from Africa growing at the Lyon Arboretum

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