sepal, any of the outer parts of a flower that enclose and protect the unopened flower bud. The sepals on a flower are collectively referred to as the calyx. They are sterile floral parts and may be either green or leaflike or composed of petal-like tissue. From their base and along most of their length, sepals remain either separate (aposepalous, or polysepalous) or marginally fused (synsepalous), forming a tube with terminal lobes or teeth. The number of calyx lobes equals the number of fused (connate) sepals, and the number of sepals is usually the same as the number of petals.
In a complete flower the sepals are one of four organs attached to the floral stalk by a receptacle; the other three organs are the petals, stamens, and carpels. The sepals and petals are accessory parts or sterile appendages; though they protect the flower buds and attract pollinators, they are not directly involved with sexual reproduction. When the colour and appearance of sepals and petals are similar, as in the tulip tree (Liriodendron tulipifera) and Easter lily (Lilium longiflorum), the perianth is said to be composed of tepals.
Floral organs are often united or fused, and the basic floral pattern consists of alternating whorls of organs positioned concentrically: from outside inward, sepals, petals, stamens, and carpels. In a complete five-merous flower (starting from the outside) there would be a whorl of five sepals, followed by an alternating whorl of five petals, followed by an alternating set of five stamens.
The calyx is commonly persistent and evident when the fruit matures (e.g., persimmon, Diospyros virginiana), in contrast to the more short-lived petals and stamens. Sepals may be brightly coloured and function as petals when true petals are missing—for example, the virgin’s bower (Clematis) and the Bougainvillea. Petaloid sepals in this case differ from tepals because the first group of stamens are on the same radii as the sepals, indicating the absence of the petals, which would normally be positioned on alternating radii in the next floral whorl.