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A sepal (// or //) is a part of the flower of angiosperms (flowering plants). Collectively the sepals are called the calyx (plural calyces), the outermost whorl of parts that form a flower. The word calyx adopted from the Latin calyx, not to be confused with calix, a cup or goblet. Calyx derived from the Greek καλυξ a bud, a calyx, a husk or wrapping, from Sanskrit kalika, a bud. Usually green, sepals typically function as protection for the flower in bud, and often as support for the petals when in bloom. After flowering, most plants have no more use for the calyx which withers or becomes vestigial, however, some plants retain a thorny calyx, either dried or live, as protection for the fruit or seeds. Examples include species of Acaena, some of the Solanaceae, and the water caltrop, Trapa natans. In some species the calyx not only persists after flowering, but instead of withering, begins to grow actively until it forms a bladder-like enclosure around the fruit. This is an effective protection against some kinds of birds and insects, for example in Hibiscus trionum and the Cape gooseberry.
Morphologically, both sepals and petals are modified leaves. The calyx (plural, calices, the sepals) and the corolla (the petals) are the outer sterile whorls of the flower, which together form what is known as the perianth.
The term tepal is usually applied when the parts of the perianth are difficult to distinguish, e.g. the petals and sepals share the same color, or the petals are absent and the sepals are colorful. Examples of plants in which the term tepal is appropriate include genera such as Aloe and Tulipa. In contrast, genera such as Rosa and Phaseolus have well-distinguished sepals and petals.
In plants that have tepals, such as Tulipa, the sepals are often not much more robust than the petals. Such plants either come from warm temperate zones, or flower mainly in mild seasons, such as late spring.
The number of sepals in a flower is its merosity. Flower merosity is indicative of a plant's classification. The merosity of a eudicot flower is typically four or five. The merosity of a monocot or palaeodicot flower is three, or a multiple of three.
The development and form of the sepals vary considerably among flowering plants. They may be free (polysepalous) or fused together (gamosepalous). Often, the sepals are much reduced, appearing somewhat awn-like, or as scales, teeth, or ridges. Most often such structures protrude until the fruit is mature and falls off.
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 (as in the Lythraceae family), a floral tube that can include the petals and the attachment point of the stamens.