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Tetramerous flower of the Primrose Willowherb (Ludwigia octovalvis) showing petals and sepals

A sepal (from Latin separatus "separate" + petalum "petal") is a part of the flower of angiosperms or flower plants. Sepals in a "typical" flower are green and lie under the more conspicuous petals. As a collective unit the sepals are called the calyx of a flower.

The calyx is part of the perianth of the flower which is composed of the sepals collectively called the calyx, which is the outer part of the flower with the inner part of the perianth composed of the petals, which are collectively called the corolla. The petals and sepals are usually differentiated into colorful petals and green sepals. But many flowers have colorful sepals and lack petals or the sepals and petals look similar and are often called tepals. The term tepal is usually applied when the petals and sepals are not differentiated and look similar or the petals are absent and the sepals are colorful. When the flower is in bud, the sepals enclose and protect the more delicate floral parts within. Morphologically they are modified leaves.

The number of sepals in a flower (called merosity) is indicative of the plant's classification: eudicots having typically four or five sepals and monocots and palaeodicots having three, or some multiple of three, sepals.

There exists considerable variation in form of the sepals among the flowering plants. Often the sepals are much reduced, appearing somewhat awn-like, or as scales, teeth, or ridges. Examples of flowers with much reduced perianths are found among the grasses. In some flowers, the sepals are fused towards the base, forming a calyx tube. This floral tube can include the petals and the attachment point of the stamens.