A sepal () is a part of the
flower
Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants ( angiosperms). Typically, they are structured in four circular levels, called whorls, around the end of a stalk. These whorls include: calyx, m ...
of
angiosperms
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed within a fruit. T ...
(flowering plants). Usually green, sepals typically function as protection for the flower in bud, and often as support for the
petal
Petals are modified leaves that form an inner whorl surrounding the reproductive parts of flowers. They are often brightly coloured or unusually shaped to attract pollinators. All of the petals of a flower are collectively known as the ''corol ...
s when in bloom.
[, p. 106]
Etymology

The term ''sepalum'' was coined by
Noël Martin Joseph de Necker in 1790, and derived .
Collectively, the sepals are called the ''calyx'' (plural: calyces),
the outermost
whorl
A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs).
In nature
File:Photograph and axial plane floral diagra ...
of parts that form a flower. The word ''calyx'' was adopted from the Latin ,
[Jackson, Benjamin, Daydon; A Glossary of Botanic Terms with their Derivation and Accent; Published by Gerald Duckworth & Co. London, 4th ed 1928] not to be confused with 'cup, goblet'. The Latin ''calyx'' is derived from Greek 'bud, calyx, husk, wrapping' ( Sanskrit 'bud'),
while is derived from Greek 'cup, goblet'; both words have been used interchangeably in botanical Latin.
Description
The term ''
tepal
A tepal is one of the outer parts of a flower (collectively the perianth). The term is used when these parts cannot easily be classified as either sepals or petals. This may be because the parts of the perianth are undifferentiated (i.e. of very ...
'' is usually applied when the parts of the
perianth
The perianth (perigonium, perigon or perigone in monocots) is the non-reproductive part of the flower. It is a structure that forms an envelope surrounding the sexual organs, consisting of the calyx (sepals) and the corolla (petals) or tepal ...
are difficult to distinguish,
e.g. the petals and sepals share the same color or the petals are absent and the sepals are colorful. When the undifferentiated tepals resemble petals, they are referred to as "petaloid", as in
petaloid monocots, orders of monocots with brightly colored tepals. Since they include
Liliales
Liliales is an order (biology), order of monocotyledonous flowering plants in the Angiosperm Phylogeny Group and Angiosperm Phylogeny Web List of systems of plant classification, system, within the lilioid monocots. This order of necessity includ ...
, an alternative name is lilioid monocots. Examples of plants in which the term tepal is appropriate include genera such as ''
Aloe
''Aloe'' (; also written ''Aloë'') is a genus containing over 650 species of flowering plant, flowering succulent plant, succulent plants.WFO (2022): Aloe L. Published on the Internet;http://www.worldfloraonline.org/taxon/wfo-4000001341. Acc ...
'' and ''
Tulipa
Tulips are spring-blooming perennial herbaceous bulbiferous geophytes in the ''Tulipa'' genus. Their flowers are usually large, showy, and brightly coloured, generally red, orange, pink, yellow, or white. They often have a different coloured ...
''. In contrast, genera such as ''
Rosa'' and ''
Phaseolus
''Phaseolus'' (bean, wild bean) is a genus of herbaceous to woody annual and perennial vines in the family Fabaceae containing about 70 plant species, all native to the Americas, primarily Mesoamerica.
It is one of the most economically importan ...
'' have well-distinguished sepals and petals.
The number of sepals in a flower is its
merosity Merosity (from the greek "méros," which means "having parts")) refers to the number of component parts in a distinct whorl of a plant structure. The term is most commonly used in the context of a flower where it refers to the number of sepals in a ...
. 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
Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are flowering plants whose seeds contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. A monocot taxon has been in use for several decades, but ...
or
palaeodicot flower is three, or a multiple of three.
The development and form of the sepals vary considerably among
flowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...
s. 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 family
Lythraceae
Lythraceae is a family (biology), family of flowering plants, including 32 genus, genera, with about 620 species of Herbaceous plant, herbs, shrubs, and trees. The larger genera include ''Cuphea'' (275 spp.), ''Lagerstroemia'' (56), ''Nesaea (pl ...
, and
Fabaceae
Fabaceae () or Leguminosae,[International Code of Nomen ...](_blank)
). In other flowers (e.g., Rosaceae, Myrtaceae), a
hypanthium
In angiosperms, a hypanthium or floral cup is a structure where basal portions of the Sepal, calyx, the petal, corolla, and the stamens form a cup-shaped tube. It is sometimes called a floral tube, a term that is also used for corolla tube and cal ...
includes the bases of sepals, petals, and the attachment points of the
stamen
The stamen (: stamina or stamens) is a part consisting of the male reproductive organs of a flower. Collectively, the stamens form the androecium., p. 10
Morphology and terminology
A stamen typically consists of a stalk called the filament ...
s.
Mechanical cues may be responsible for sepal growth and there is a strong evidence suggesting that microtubules are present and determine the tensile strength and direction of growth at a molecular level.
Morphology
Morphologically, both sepals and petals are modified leaves. The calyx (the sepals) and the corolla (the petals) are the outer sterile whorls of the flower, which together form the ''perianth''.
In some plants, such as ''
Aristolochia'', the calyx is the primary whorl, forming a flower up to wide, with one sepal growing to a length of
Aristolochia grandiflora, the largest of all calyces.
Function
Similarly to ordinary leaves, sepals are capable of performing
photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
. However, photosynthesis in sepals occurs at a slower rate than in ordinary leaves due to sepals having a lower
stoma
In botany, a stoma (: stomata, from Greek language, Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the Epidermis (botany), epidermis of leaves, stems, and other organs, that controls the rate of gas exc ...
tal density which limits the spaces for gas exchange.
After flowering, most plants have no more use for the calyx which withers or becomes vestigial, although in a few plants such as
Lodoicea and
eggplant
Eggplant (American English, US, Canadian English, CA, Australian English, AU, Philippine English, PH), aubergine (British English, UK, Hiberno English, IE, New Zealand English, NZ), brinjal (Indian English, IN, Singapore English, SG, Malays ...
(''Solanum melongena'') the calyx grows along with the fruit, possibly to protect the attachment point. 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
Solanaceae (), commonly known as the nightshades, is a family of flowering plants in the order Solanales. It contains approximately 2,700 species, several of which are used as agricultural crops, medicinal plants, and ornamental plants. Many me ...
(for example the
Tomatillo
The tomatillo (''Physalis philadelphica'' and ''Physalis ixocarpa''), also known as the Mexican husk tomato, is a plant of the nightshade family bearing small, spherical, and green or green-purple fruit. Tomatillos originated in Mexico and were ...
, ''Physalis philadelphica''), and the
water caltrop, ''Trapa natans''. In some species, the calyx not only persists after flowering but instead of withering, begins to grow 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. In other species, the calyx grows into an
accessory fruit
An accessory fruit is a fruit that contains tissue derived from plant parts other than the Ovary (botany), ovary. In other words, the flesh of the fruit develops not from the floral ovary, but from some adjacent tissue exterior to the carpel (f ...
.
See also
*
Plant morphology
Phytomorphology is the study of the physical form and external structure of plants.Raven, P. H., R. F. Evert, & S. E. Eichhorn. ''Biology of Plants'', 7th ed., page 9. (New York: W. H. Freeman, 2005). . This is usually considered distinct from pl ...
References
{{Authority control
Plant morphology