Calyx

botany

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Britannica Editors

Last updated

Nov. 14, 2026 •History

*Abutilon*A conspicuous red calyx tube envelops the closed yellow petals of the bell-shaped *Abutilon megapotamicum*.

Calyx, the usually green outer whorl of a flower consisting of separate or fused sepals. The calyx serves primarily to protect the developing flower bud and, in some cases, to support the flower once it has bloomed. In some species, the calyx persists even when the fruit matures, as seen in persimmon, eggplant, strawberry, and tomato. Given that it is not directly involved in reproduction, the calyx is considered an accessory part of the flower.

Flower anatomyDiagram of the flower of a typical flowering plant (angiosperm).

There are commonly four distinct whorls of flower parts: (1) an outer calyx consisting of sepals; (2) the corolla, within the calyx and consisting of petals; (3) the male androecium, comprising the pollen-bearing stamens; and (4) the female gynoecium, at the center and consisting of the pistils, which hold ovules and one or more ovaries. A flower’s calyx and corolla (e.g., all the sepals and petals together) form the perianth, which serves to protect the reproductive organs and attract pollinators. The arrangement of floral organs, including the calyx, is typically in concentric whorls, with the sepals forming the outermost layer, though not every flower has every whorl. The numbers and arrangements of flower parts—including the number and degree of fusion of the sepals of the calyx—are key characteristics botanists use to identify flowering plant species.

The calyx exhibits a wide range of morphological diversity across different angiosperm species. In some flowers, the sepals are free and distinct, a condition known as aposepalous or polysepalous. In other species, the sepals are fused together, forming a tubular structure with lobes or teeth at the end, a condition referred to as synsepalous. The number of sepals in a calyx often corresponds to the number of petals, either directly (e.g., five sepals and five petals) or in multiples (e.g., four sepals and eight petals). In some species, the sepals are brightly colored and function as petals—for example, Clematis and Bougainvillea. In others, the petals and sepals are both present but are similar in color and appearance, as in the tulip tree (Liriodendron tulipifera) and Easter lily (Lilium longiflorum); in such cases, they are known as tepals.

Calyx

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