A sepal is a part of the flower

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

 of angiosperms (flowering plants). Collectively the sepals form the calyx, which is the outermost whorl of parts that form a flower. Usually green, sepals have the typical function of protecting the petal

Petal

Petals are modified leaves that surround the reproductive parts of flowers. They often are brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals lying...

s when the flower is in bud. Another common function of sepals is protection from insect attack, and when there is no such protection, there often is some other ecological strategy, such as poison.

Morphologically

Morphology (biology)

In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 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

Tepal

Tepals are elements of the perianth, or outer part of a flower, which include the petals or sepals. The term tepal is more often applied specifically when all segments of the perianth are of similar shape and color, or undifferentiated, which is called perigone...

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

Aloe

Aloe , also Aloë, is a genus containing about 500 species of flowering succulent plants. The most common and well known of these is Aloe vera, or "true aloe"....

and Tulipa. In contrast, genera such as Rosa

Rosa

The name Rosa, Latin and botanic name of the flowering shrub Rose, could refer to:-Places:*223 Rosa, an asteroid*Rosa, Alabama, USA*Rosa, Germany, in Thuringia, Germany*Roşia Nouă village, Petriş Commune, Arad County, Romania-Other uses:...

and Phaseolus

Phaseolus

Phaseolus is a genus in the family Fabaceae of about fifty plant species, all native to the Americas.At least four of the species have been domesticated since pre-Columbian times for their beans. Most prominent among these is the common bean, P...

have well-distinguished sepals and petals.

Commonly in plants such as Tulipa, that have tepals, the sepals are hardly more robust than the petals, and 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

Merosity

Merosity is the number of component parts in each whorl of a plant structure. It is most commonly used in the context of flowers, in which case it refers to the number of sepals in the calyx, the number of petals in the corolla, and the number of stamens in each whorl of the androecium...

. Flower merosity is indicative of a plant's classification. The merosity of a eudicot

Eudicots

Eudicots and Eudicotyledons are botanical terms introduced by Doyle & Hotton to refer to a monophyletic group of flowering plants that had been called tricolpates or non-Magnoliid dicots by previous authors...

 flower is typically four or five. The merosity of a monocot

Monocotyledon

Monocotyledons, also known as monocots, are one of two major groups of flowering plants that are traditionally recognized, the other being dicotyledons, or dicots. Monocot seedlings typically have one cotyledon , in contrast to the two cotyledons typical of dicots...

 or palaeodicot

Basal angiosperms

The basal angiosperms are the first flowering plants to diverge from the ancestral angiosperm. In particular, the most basal angiosperms are the so-called ANITA grade which is made up of Amborella , Nymphaeales and Austrobaileyales...

 flower is three, or a multiple of three.

The forms of the sepals vary considerably among flowering plant

Flowering plant

The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...

s. Often, the sepals are much reduced, appearing somewhat awn-like, or as scales, teeth, or ridges. Most often such structures protrude until the fruit

Fruit

In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...

 is mature and falls off. Some however, remain as tough or thorny protection for the fruit. Although the sepal withers in most plants, or at any rate does not grow after the fruit has been fertilised, in some species, such as the cape gooseberry

Physalis

Physalis is a genus of plants in the nightshade family , native to warm temperate and subtropical regions throughout the world. The genus is characterised by the small orange fruit similar in size, shape and structure to a small tomato, but partly or fully enclosed in a large papery husk derived...

, the fused part of the calyx grows and encloses the ripening fruit practically entirely, forming a protective papery cape

Cape

Cape can be used to describe any sleeveless outer garment, such as a poncho, but usually it is a long garment that covers only the back half of the wearer, fastening around the neck. They were common in medieval Europe, especially when combined with a hood in the chaperon, and have had periodic...

.

Examples of flowers with much reduced perianths are found among the grasses

Poaceae

The Poaceae is a large and nearly ubiquitous family of flowering plants. Members of this family are commonly called grasses, although the term "grass" is also applied to plants that are not in the Poaceae lineage, including the rushes and sedges...

. In some flowers, the sepals are fused towards the base, forming a calyx tube (as in the Lythraceae

Lythraceae

Lythraceae are a family of flowering plants. It includes about 620 species of mostly herbs, with some shrubs and trees, in 31 genera. Major genera include Cuphea , Lagerstroemia , Nesaea , Rotala , and Lythrum...

 family), a floral tube that can include the petals and the attachment point of the stamen

Stamen

The stamen is the pollen producing reproductive organ of a flower...

s.