Sepal

File:Petal-sepal.jpg showing petals and sepals]]

File:Terengganu roselle.jpg expand into an edible accessory fruit.]]

File:Vicia February 2008-1.jpg

File:20170919Crataegus germanica2.jpg fruit is the source of its vulgar nicknames.]]

The term sepalum was coined by Noël Martin Joseph de Necker in 1790, and derived {{etymology|grc|{{wikt-lang|grc|σκέπη}} ({{grc-transl|σκέπη}})|covering}}.Stearn, William T. (2000). Botanical Latin, 4th ed.: 38–39. {{ISBN|0-88192-321-4}}Necker, N.J. de (1790). [https://www.biodiversitylibrary.org/page/3589030 Corollarium ad Philosophiam botanicam Linnaei 18], [https://www.biodiversitylibrary.org/page/3589043 31]

Collectively, the sepals are called the calyx (plural: calyces),{{Cite book |title=Shorter Oxford English dictionary, 6th ed. |publisher=Oxford University Press |year=2007 |isbn=978-0199206872 |location=United Kingdom |pages=3804}} the outermost whorl of parts that form a flower. The word calyx was adopted from the Latin {{wikt-lang|la|calyx}},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 {{wikt-lang|la|calix}} 'cup, goblet'.John Entick, William Crakelt, Tyronis thesaurus, or, Entick's new Latin English dictionary. Publisher: E.J. Coale, 1822 The Latin calyx is derived from Greek {{wikt-lang|grc|κάλυξ}} {{Transliteration|grc|kalyx}} 'bud, calyx, husk, wrapping' ({{abbr|cf.|compare with}} Sanskrit {{Transliteration|sa|kalika}} 'bud'),{{Cite book |last=Tucker, T. G. |title=A Concise Etymological Dictionary of Latin |publisher=Max Niemeyer Verlag |year=1931 |location=Halle (Saale)}} while {{lang|la|calix}} is derived from Greek {{wikt-lang|grc|κύλιξ}} {{Transliteration|grc|kylix}} 'cup, goblet'; both words have been used interchangeably in botanical Latin.Stearn, William T. (2000). Botanical Latin, 4th ed.: 38

The term tepal is usually applied when the parts of the perianth are difficult to distinguish,{{Harvnb|Beentje|2010|p=119}} 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, an alternative name is lilioid monocots. 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. {{citation needed|date=April 2014}}

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.Sattler, R. 1973. Organogenesis of Flowers. A Photographic Text-Atlas. University of Toronto Press. {{ISBN|0-8020-1864-5}}. They may be free (polysepalous) or fused together (gamosepalous).{{sfn|Beentje|2010|pp=51 & 91}} 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,{{Cite web |last=Carr |first=Gerald |title=Lythraceae |url=http://www.botany.hawaii.edu/FACULTY/CARR/lythr.htm |url-status=dead |archive-url=https://web.archive.org/web/20081205132651/http://www.botany.hawaii.edu/faculty/carr/lythr.htm |archive-date=5 December 2008 |access-date=20 December 2008 |publisher=University of Hawaii}} and Fabaceae). In other flowers (e.g., Rosaceae, Myrtaceae), a hypanthium includes the bases of sepals, petals, and the attachment points of the stamens.

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.{{Cite journal |last1=Hervieux |first1=Nathan |last2=Dumond |first2=Mathilde |last3=Sapala |first3=Aleksandra |last4=Routier-Kierzkowska |first4=Anne-Lise |last5=Kierzkowski |first5=Daniel |last6=Roeder |first6=Adrienne H.K. |last7=Smith |first7=Richard S. |last8=Boudaoud |first8=Arezki |last9=Hamant |first9=Olivier |date=April 2016 |title=A Mechanical Feedback Restricts Sepal Growth and Shape in Arabidopsis |journal=Current Biology |language=en |volume=26 |issue=8 |pages=1019–1028 |doi=10.1016/j.cub.2016.03.004 |pmid=27151660 |doi-access=free|bibcode=2016CBio...26.1019H |hdl=11858/00-001M-0000-002B-1620-2 |hdl-access=free }}

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.{{Cite book |last1=Davis |first1=P.H. |title=The identification of flowering plant families, including a key to those native and cultivated in north temperate regions |last2=Cullen |first2=J. |publisher=Cambridge University Press |year=1979 |isbn=0-521-29359-6 |location=Cambridge |pages=106}} In some plants, such as Aristolochia, the calyx is the primary whorl, forming a flower up to {{convert|20|in|cm}} wide, with one sepal growing to a length of {{convert|13|ft|m}} {{ndash}} Aristolochia grandiflora, the largest of all calyces.{{ cite book | last= Rohwer | first= Jens G. | date= 2002 | title= Tropical Plants of the World | location= New York | publisher= Sterling | page= 208 }}{{ cite journal | last=Pfeifer | first= HW | date= November 1966 | title= Revision of the North and Central American Species of Aristolochia | journal= Annals of the Missouri Botanical Garden | volume= 53 | issue= 2 | page=164| doi= 10.2307/2394940 | jstor= 2394940 | url= https://www.biodiversitylibrary.org/part/4247 }}

Similarly to ordinary leaves, sepals are capable of performing photosynthesis. However, photosynthesis in sepals occurs at a slower rate than in ordinary leaves due to sepals having a lower stomatal density which limits the spaces for gas exchange.{{Cite journal |last1=Aschan |first1=G. |last2=Pfanz |first2=H. |last3=Vodnik |first3=D. |last4=Batič |first4=F. |date=1 March 2005 |title=Photosynthetic performance of vegetative and reproductive structures of green hellebore (Helleborus viridis L. agg.) |journal=Photosynthetica |language=en |volume=43 |issue=1 |pages=55–64 |doi=10.1007/s11099-005-5064-x |s2cid=24426595|doi-access=free |bibcode=2005Phsyn..43...55A }}

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 (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 (for example the Tomatillo, 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.

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