Costus.
Plants of the World Online recognises 70 species while others say 80 to around 100.
Australia has 1 native and 1 naturalised species.
They are clump-forming, mostly erect plants from 50 cm to 4 m tall.
The usually unbranched cane-like stems grow from underground stems or rhizomes.
The sometimes spirally twisted stems can be green or tinted red or purple.
Leaves, on a very short to long petiole are spirally arranged in 1 rank.
Basal leaves are reduced to a sheath and the upper lanceolate to elliptic ones, up to around 40 cm long have a pointed tip.
The upper surface is usually smooth but the lower may have a few to dense hairs.
The green or blue-green leaves may be red underneath and the pinnate veins silvery.
The often prominent ligule is a flat-topped membranous flap of tissue at the top of the sheath.
There may be hairs on the sheath and the edge of the ligule.
Inflorescences can be terminal on the leafy stems or on short side shoots.
Some are on a short leafless stem off the rhizome.
Up to around 20 cm long the inflorescence is on a stalk or peduncle up to 13 cm long.
They are spikes that can be condensed into a head or a longer cone-like structure with few to many flowers.
The spirally arranged flowers may have a small smooth or hairy bracteole at the base.
Each flower, or pair of flowers is subtended by a bract up to 5 cm long and 4.5 cm wide.
The overlapping, leathery or woody bracts may have a sharp point and may be hairy.
Commonly red they can also be green, yellow or orange.
The bisexual flowers can be small or up to 10 cm long.
The green, red or yellow calyx has 3 wide often pointed lobes that are a few mms long.
It is sometimes split down one side to the base and it may have hairs.
The bases of the 3 white, cream, yellow, red, pink or orange petals are fused.
There are 6 stamens in 2 whorls but only 1, in the inner whorl is fertile.
The fertile stamen has a petal-like filament around 1 cm wide.
The 2 pollen sacs in the anther open inwards through longitudinal slits.
The 5 infertile staminodes are fused into an obovate petal-like labellum or lip with a crinkly edge.
The sides curve in to form a narrow open funnel up to around 9 cm long with a usually widely flaring top.
The base of the funnel section is fused to the base of the stamen.
Usually the largest and most prominent part of the flower it can be white, yellow, red, pink or orange
or it can be white with a yellow center or have red markings down the sides.
The smooth or hairy ovary, up to around 1cm long has 3 locules each with numerous ovules with axile placentation.
The thread-like style passes between the anther sacs of the fertile stamen with the stigma just above the anther.
There are nectaries on top of the ovary and sometimes on the bracts.
The fruit are mostly capsules with the calyx still attached but are occasionally a berry.
Capsules, up to 2.5 cm long open into the chambers (loculicidal) or irregularly.
Smooth or with hairs it contains numerous hard black seeds.
J.F.
