Costaceae

Growth Form and Habit

Perennial, rhizomatous herbs, often large and robust (0.5 to 4+ meters tall). Stems (true stems or pseudostems formed by rolled leaf sheaths) are typically erect, unbranched or sparsely branched, and frequently exhibit a pronounced spiral growth pattern, resulting in a spiral arrangement of the leaves. Rarely epiphytic. Contain aromatic essential oils.

Leaves

Leaves are arranged alternately, often distinctly spirally, along the stem. They are simple, usually large, with entire margins, typically elliptic, ovate, or obovate. The base of the leaf forms a closed tubular sheath that surrounds the stem. At the junction of the sheath and the blade (petiole often short or absent) is a prominent membranous or leathery appendage called the ligule. Venation is pinnate-parallel, with a strong midrib and parallel lateral veins.

Inflorescence

The inflorescence is typically a dense, ovoid or cylindrical, cone-like terminal spike or head (sometimes borne on a separate leafless shoot). It is composed of tightly overlapping, often leathery or fleshy, frequently brightly colored (red, orange, yellow, green) bracts. Flowers emerge individually or in small groups from the axils of these bracts over a prolonged period.

Flowers

Flowers are usually large, showy, bisexual (perfect), strongly zygomorphic (bilaterally symmetrical), epigynous (borne above the ovary), and based on a 3-merous plan.

• Calyx: Consists of 3 sepals fused into a tube, typically with 3 lobes at the apex, usually shorter than the corolla tube, and persistent in fruit.
• Corolla: Consists of 3 petals fused below into a tube that is usually longer than the calyx tube. The 3 corolla lobes are typically unequal, often thin-textured.
• Androecium: Highly modified and diagnostically unique. There is only one functional, fertile stamen. This stamen itself is broad and petaloid (petal-like), often forming the upper part of the flower; the two pollen sacs (thecae) are located on the inner surface of this structure, usually about midway along its length. The remaining staminodes (typically 2 or more, representing the inner androecial whorl) are fused together to form a single large, showy, petal-like labellum. The labellum is usually the largest and most conspicuous part of the flower, often trilobed, typically positioned below the fertile stamen, serving as a landing platform for pollinators. Lateral staminodes (present in Zingiberaceae) are absent or vestigial.
• Gynoecium: The ovary is inferior, composed of 3 (less commonly 2 in Dimerocostus) fused carpels, forming a 3-locular (or 2-locular) ovary. Placentation is axile, with numerous ovules per locule. A single, slender style runs up from the ovary, typically passing through a groove or channel in the filament of the petaloid fertile stamen. The stigma is small, often cup-shaped, funnel-like, or slightly lobed, sometimes with small appendages, emerging near the level of the anther sacs. Septal nectaries are typically present in the ovary wall, releasing nectar often near the flower base.

Fruits and Seeds

The fruit is most commonly a loculicidal capsule, which is often somewhat fleshy, typically white or reddish, crowned by the persistent calyx, and splits open via 3 valves. Less frequently, the fruit is indehiscent and berry-like. It contains numerous seeds. Seeds are typically small, hard, black or brown, and possess a conspicuous, fleshy, usually white aril (an outgrowth associated with the hilum), which often aids in dispersal (e.g., by ants or birds). Both endosperm and starchy perisperm are present within the seed.

Chemical Characteristics

Plants contain aromatic essential oils and resins, contributing to their scent. Starch is stored in the rhizomes. Saponins and calcium oxalate crystals (raphides) are often present.