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Sepal

A sepal is a leaf-like floral organ that forms part of the calyx, the outermost whorl of a flower in angiosperms, typically serving to enclose and protect the developing bud before it opens.^[1]^[2]^[3] Sepals are usually green and resemble miniature leaves, often performing photosynthesis in addition to their protective role, though they can be brightly colored or petal-like in certain species.^[2]^[1]^[4] Collectively, sepals form the calyx at the base of the flower's receptacle, with their number varying by plant group: typically three in monocots and four or five in dicots.^[2] In many flowers, sepals are distinct from the inner petals, but in some cases—such as lilies or tulips—they merge indistinguishably with petals and are termed tepals.^[2]^[3] Sepals may be fused at their base (gamosepalous) or free (polysepalous), and while they are present in complete flowers, they can be absent in incomplete ones, such as in many wind-pollinated flowers (e.g., oaks or grasses), or replaced by other structures in specialized inflorescences, such as the spathe in calla lilies.^[1]^[4]^[2] Beyond protection, sepals contribute to the flower's overall attractiveness in petaloid forms, aiding in pollinator attraction, and their evolution reflects adaptations in angiosperm reproductive strategies.^[2]^[1]

Etymology and Terminology

Etymology

The term sepalum, the Latin precursor to the modern English "sepal," was coined in 1790 by Noël Martin Joseph de Necker, a Belgian physician and botanist known for his contributions to plant classification. De Necker introduced the neologism in his work Elementa Botanica, aiming to provide a precise descriptor for the leaf-like components of a flower's outer whorl, distinct from the inner petals. This innovation reflected the growing need for specialized vocabulary in botany during the late 18th century, when scholars built upon Carl Linnaeus's binomial nomenclature system—established in works like Species Plantarum (1753)—to describe floral structures more accurately amid the era's explosion of taxonomic studies.^[5]^[6] De Necker formed sepalum as a portmanteau of the Latin sēparātus ("separate") and petalum ("petal"), emphasizing the sepals' identity as independent, protective elements akin to but differentiated from petals. This etymological construction highlighted the sepals' role within the calyx, the collective term for these structures that enclose the flower bud. The adoption of sepalum marked a shift toward hybrid Latin-Greek derivations in botanical terminology, facilitating clearer international communication among naturalists exploring plant morphology.^[5] In contrast, the broader term "calyx" predates de Necker's contribution, tracing back to classical antiquity. Borrowed into Latin as calyx from the Ancient Greek kályx (meaning "bud," "husk," or "pod"), it evoked the enclosing, sheath-like nature of the floral envelope. By the late 18th century, "calyx" had become standard in European botanical texts, underscoring the protective function of sepals in Linnaean descriptions of flower anatomy.^[7]

Terminology

In botanical terminology, the calyx refers to the collective whorl of sepals that forms the outermost part of a flower.^[8] Sepals are the individual segments of this whorl, typically leaf-like and protective in function, though the term emphasizes their structural role.^[8] Sepals and petals together comprise the perianth, the sterile outer envelope of the flower that encloses the reproductive organs.^[8] While sepals form the outer perianth whorl (calyx) and petals the inner (corolla), the distinction is positional rather than always morphological, as sepals are generally less showy.^[9] The fusion state of sepals is described using specific terms: polysepalous indicates free, separate sepals, whereas gamosepalous denotes sepals that are fused, at least basally, forming a tube or cup.^[8] In cases where sepals and petals are morphologically indistinguishable, they are collectively termed tepals, as seen in the Liliaceae family, such as in Tulipa species where the perianth consists of six similar tepals.^[10] Floral merosity refers to the number of parts in each whorl, with sepals typically numbering four or five in eudicots, and three or multiples of three in monocots and palaeodicots.^[11] This pattern aids in classifying angiosperm flowers taxonomically.^[12]

Anatomy and Morphology

General Structure

Sepals represent the outermost whorl of floral organs in angiosperm flowers, consisting of modified leaves that collectively form the calyx.^[13] These structures are positioned exterior to the petals, stamens, and carpels, serving as the initial protective layer around the developing flower bud.^[14] In typical flowers, sepals are distinct from inner whorls and arise from the floral meristem in a spiral or whorled phyllotaxy, with the calyx often described as polysepalous when sepals are free or gamosepalous when fused.^[15] The characteristic green coloration of sepals arises from chlorophyll pigments within their tissues, enabling limited photosynthesis in many species.^[16] This pigmentation contributes to their leaf-like appearance and supports their role in enclosing and shielding the unopened flower from environmental stresses.^[17] In terms of basic tissue composition, sepals exhibit an anatomy analogous to that of foliage leaves, comprising three primary tissue systems: dermal, ground, and vascular.^[18] The dermal tissue forms the outer epidermis, often covered by a waxy cuticle for protection, while the inner adaxial epidermis lines the surface facing the flower interior.^[16] The ground tissue, or mesophyll, consists of parenchyma cells containing chloroplasts that impart the green hue and facilitate gas exchange through stomata present in both epidermal layers.^[16] Vascular tissues, including xylem and phloem, are organized into veins that run through the mesophyll, providing structural support and transport of water, nutrients, and photosynthates similar to leaf venation patterns.^[18]

Morphological Diversity

Sepals exhibit considerable variation in number across angiosperm species, often determined by the underlying floral formula and phylogenetic lineage. In many eudicot families within the rosids, such as Rosaceae, sepals typically number five and are arranged in a single whorl.^[19] In contrast, monocot families like Liliaceae commonly feature three sepals, though these are often indistinguishable from petals as undifferentiated tepals totaling six perianth parts.^[20] Sepal shapes and sizes display a broad spectrum, ranging from small, scale-like structures that are inconspicuous and protective to highly elaborate forms that dominate the flower. For instance, in most herbaceous plants, sepals are modest in size and lanceolate or ovate in shape, but in Aristolochia grandiflora, the fused sepals form a large, tubular perianth up to 60 cm long, with a flared limb resembling a pelican's pouch.^[21] Fusion of sepals, or connation, varies significantly, leading to distinct calyx architectures. In families like Solanaceae, sepals are gamosepalous, fusing to form a tubular or campanulate calyx, as seen in tomato (Solanum lycopersicum) where the five-lobed structure persists into fruit development.^[22] Conversely, in Ranunculaceae, sepals are polysepalous, remaining free and separate, often appearing as five small, petaloid appendages in species like buttercup (Ranunculus acris).^[23] While sepals are characteristically green and photosynthetic, color variations occur, particularly in petaloid forms that mimic petals in hue and texture. In Magnoliaceae, the outer tepals function as sepals and are typically green and sepaloid, while inner ones are petaloid and white or pink, as in Magnolia grandiflora.^[24] Similarly, in Iridaceae, such as iris (Iris species), the three sepals are petaloid, exhibiting vibrant blues, purples, or yellows to attract pollinators.^[25] Aestivation patterns, which describe the arrangement of sepals within the flower bud, further contribute to morphological diversity. Valvate aestivation predominates in many sepals, where margins touch without overlapping, as observed in the calyx of Aristolochia species.^[26] Imbricate aestivation, involving overlapping margins, occurs in families like Ranunculaceae, where sepals partially overlap in a quincuncial pattern for tighter bud enclosure.^[26]

Development

Ontogeny

Sepals initiate as small outgrowths, or primordia, on the flanks of the floral meristem (FM), which is the first morphologically visible stage of floral organ formation in the outermost whorl.^[27] In model plants like Arabidopsis thaliana, this occurs during stage 3 of floral development, shortly after the FM emerges as a hemispherical bulge from the inflorescence meristem, with sepal founder cells recruited sequentially starting from the abaxial position and proceeding to the lateral and adaxial sides.^[27] This outward positioning ensures sepals form a protective enclosure around the developing bud from the earliest stages. Following initiation, sepal growth proceeds through distinct phases of enlargement and differentiation, transforming the primordia into flattened, leaf-like structures. Early growth involves rapid cell division and expansion, particularly along the longitudinal axis, leading to a basipetal gradient where the tip matures first while the base continues proliferating.^[28] As the flower bud develops, sepals differentiate further, developing features such as vascular tissues and epidermal layers that contribute to their green, photosynthetic appearance, all while enclosing and protecting inner whorls.^[29] This process is timed sequentially relative to other floral organs, with sepal primordia appearing before petal initiation at stage 5, ensuring orderly whorl assembly in an acropetal manner across the inflorescence.^[27] Recent studies have highlighted the robustness of sepal morphogenesis in Arabidopsis, where organ size and shape remain consistent despite genetic or environmental perturbations, driven by emergent properties from cellular interactions, mechanical feedback, and compensatory growth mechanisms.^[16] For instance, analyses of cell wall mutants and live imaging reveal that highly expressed cell wall genes contribute to size stability, while spatial transcriptomic atlases as of 2025 uncover intra-cell-type heterogeneity in sepal epidermal cells during elongation.^[30]^[31] Environmental factors significantly influence sepal growth, particularly size and the degree of bud enclosure. Light exposure promotes chlorophyll accumulation and sepaloid characteristics, such as domed epidermal cells, in species like waterlilies, enhancing photosynthetic capacity during development.^[10] Temperature variations also affect sepal morphology; for instance, higher ambient temperatures can reduce intersepal region size in Cardamine hirsuta, leading to tighter packing and altered enclosure of inner buds, while lower temperatures may promote expansion.^[32] Abnormal sepal development is evident in various mutants, highlighting the precision of ontogenetic processes. In Arabidopsis thaliana ap2 mutants, defective APETALA2 function causes homeotic transformation of sepals into leaf-like or mildly carpeloid structures, altering protection of inner organs.^[33] Fused sepal anomalies occur in ptl (PETAL LOSS) mutants, where sepals occasionally merge at the base due to disrupted boundary growth inhibition, and in double mutants like ant-4 rbe-3, where multiple sepals fuse along their margins, compromising bud protection.^[34]^[35] These examples, often linked to genes like PTL that briefly reference molecular involvement, underscore the genetic underpinnings of sepal integrity without delving into regulatory details.

Molecular Mechanisms

In the ABC(DE) floral organ identity model, sepals are specified in the outermost whorl of the flower by the activity of A-class genes, which act in combination with E-class genes to establish perianth identity. In Arabidopsis thaliana, the MADS-box transcription factor APETALA1 (AP1) serves as a key A-class gene, promoting sepal formation by activating downstream targets that define the sepal primordium while repressing inner whorl identities.^[36] Similarly, the APETALA2 (AP2) gene, an A-class regulator without a MADS domain, contributes to sepal specification by antagonizing C-class genes like AGAMOUS (AG), thereby preventing ectopic reproductive organ development in the outer whorls.^[37] Mutations in AP2, such as in apetala2 loss-of-function alleles, disrupt this balance, leading to homeotic transformations where sepals convert to carpel-like structures due to derepression of AG and expansion of C-class activity into whorl 1.^[37] These genetic interactions highlight how combinatorial transcription factor activity ensures sepal distinctiveness from inner floral organs. Hormonal regulation further refines sepal development through auxin signaling, which establishes gradients that direct the initiation of sepal primordia from the floral meristem. Auxin maxima, mediated by the polar transport inhibitor PIN-FORMED1 (PIN1), create localized response peaks that trigger the expression of primordium founder cell markers, such as those regulated by the MONOPTEROS (MP) transcription factor, thereby positioning and initiating sepal outgrowth in a unidirectional pattern around the meristem periphery.^[38] This auxin-directed process ensures precise spacing and timing of sepal formation, with disruptions in auxin biosynthesis or transport leading to irregular primordia initiation and altered sepal numbers.^[39] At the cellular level, microtubule orientation in epidermal cells plays a critical role in shaping sepals by guiding anisotropic cell expansion and wall reinforcement. Cortical microtubules align perpendicular to the primary growth axis in sepal epidermal cells, directing cellulose microfibril deposition and thus restricting expansion in certain directions to achieve the organ's characteristic flattened, concave form.^[40] Mechanical feedback loops amplify this process, where stress-induced reorientation of microtubules at the sepal tip senses tissue tension and modulates growth rates, preventing overexpansion and maintaining overall sepal curvature.^[40] The mosaic theory posits that the evolutionary distinction between sepal and petal identity arose early in angiosperm history through compartmentalized gene expression patterns, allowing flexible regulation of perianth traits without fixed organ boundaries. Under this framework, sepalness and petalness represent mosaic assemblages of genetic programs—such as A-class dominance for sepals versus A+B-class overlap for petals—that can be environmentally modulated, enabling secondary losses or gains of petaloid features in sepals across lineages while preserving core identity mechanisms.^[10] This compartmentalization, evident in variable B-class gene expression boundaries, underscores how subtle shifts in regulatory domains contribute to perianth diversity.^[10]

Functions

Pre-Flowering Functions

Sepals primarily function to enclose and protect the developing flower bud during pre-flowering stages, shielding the inner floral organs from environmental stresses such as desiccation and physical damage. By forming a tough outer layer, often with thicker cuticles and trichomes, sepals reduce water loss and provide a physical barrier against mechanical injury.^[41] In addition to protection, sepals contribute to photosynthesis through the presence of chloroplasts and stomata, enabling carbon fixation during bud development. However, sepal photosynthetic rates are typically lower than those of leaves, achieving approximately 20-50% of leaf rates due to reduced stomatal density—often 70-80% lower—and about half the chlorophyll content. For instance, in Helleborus viridis, sepals exhibit a maximum net photosynthetic rate of 2.3 μmol CO₂ m⁻² s⁻¹ compared to 10.6 μmol CO₂ m⁻² s⁻¹ in leaves, with stomatal densities of 23 mm⁻² versus 111 mm⁻².^[42] During flower opening (anthesis), the interlocking edges of sepals and petals maintain bud integrity, with uneven growth rates creating internal strain that facilitates controlled expansion and opening.^[43] Sepals also contribute to thermoregulation by insulating the bud against temperature fluctuations, adjusting lipid compositions in their membranes to enhance stability under mild heat stress (e.g., 27°C) and protect inner organs from thermal damage. This includes increasing saturated triacylglycerols to maintain membrane fluidity, a mechanism observed in Arabidopsis sepals that safeguards reproductive development.^[44] A representative example is the green sepals in rose (Rosa spp.), which enclose the bud to provide early protection against desiccation and physical harm before petal emergence, leveraging their leaf-like structure for robust shielding.^[45]

Post-Flowering Roles

After anthesis, sepals exhibit diverse persistence patterns across plant species. In many eudicots, such as Arabidopsis thaliana, sepals typically senesce, wither, and abscise shortly after fertilization to redirect resources toward fruit and seed development.^[28] In contrast, sepals in certain species enlarge or become accrescent to enclose and safeguard the developing fruit; for instance, in eggplant (Solanum melongena), the persistent, often prickly calyx remains attached and partially covers the fruit, reducing physical damage and water loss during maturation.^[46]^[47] Persistent sepals frequently serve a protective role for fruits and seeds by forming an enclosure that deters herbivores and environmental stressors. In Hibiscus trionum, the calyx inflates dramatically post-anthesis into a papery, bladder-like structure that fully envelops the capsule, creating a physical barrier that limits access by seed predators and reduces desiccation.^[48]^[49] In species with persistent green sepals, these organs continue to perform photosynthesis, supplying energy and carbohydrates to support fruit growth and seed filling. For example, in Helleborus foetidus, the green sepals persist for months after flowering and contribute to seed development through photosynthetic activity, with removal reducing seed mass by approximately 10%.^[50] Similarly, in Paris polyphylla, leafy sepals provide photosynthates that enhance fruit expansion and seed viability, compensating for reduced leaf contributions during reproduction.^[51] Modifications of persistent sepals can extend their utility into seed dispersal phases. In fuchsias (Fuchsia spp.), the colorful sepals often remain vibrant around the maturing berry, visually attracting avian dispersers that consume the fruit and excrete seeds, thereby facilitating wider propagation.^[52] Sepal senescence and abscission are tightly regulated by hormonal signals that coordinate tissue breakdown and detachment. Ethylene and jasmonic acid promote sepal abscission by activating cell wall-modifying enzymes in the abscission zone, while auxin gradients inhibit premature shedding to ensure timely resource allocation; for instance, in Korla fragrant pear, declining auxin and rising ethylene levels in sepal abscission zones trigger ultrastructural changes leading to separation.^[53]^[54]^[55]

Evolutionary and Ecological Aspects

Evolutionary Origins

Sepals originated in the early angiosperms approximately 140 million years ago during the Early Cretaceous period, evolving from leaf-like bracts or sterile appendages that subtended reproductive structures in ancestral plants.^[56] In gymnosperms, such as conifers, equivalent structures include bracts that enclose ovules on seed cones, providing a protective role similar to that later assumed by sepals in flowers.^[57] This transition reflects a broader evolutionary shift toward enclosed seeds and more complex floral organization in angiosperms, where sepals formed as the outermost protective layer.^[58] The perianth, comprising sepals and petals, likely emerged as an undifferentiated structure of tepals in the most recent common ancestor of angiosperms, with sepals representing the ancestral outer whorl derived from modified bracts.^[56] In some lineages, petals evolved secondarily from sepals through modifications for pollinator attraction, while in others, they arose from stamens, leading to diverse perianth configurations across angiosperm clades.^[59] This diversification underscores sepals' conserved role as the foundational protective envelope, predating petal specialization.^[60] The mosaic theory posits that the genetic distinction between sepal and petal identity arose early in angiosperm evolution, initially through environmental influences on perianth organs in basal lineages like water lilies (Nymphaeales).^[10] In these groups, individual organs exhibit mosaic identities—sepal-like in exposed regions and petal-like in sheltered areas—suggesting an ancestral mechanism where physical interactions with neighboring organs and light exposure regulated differentiation, later becoming genetically fixed in whorl-specific patterns.^[61] This theory aligns with observations in basal angiosperms, where B-class genes contribute to partial petaloid traits without full whorl separation.^[10] Fossil evidence for sepal-like structures appears in Early Cretaceous deposits, such as those of Archaefructus from approximately 125 million years ago in China, where bract-like appendages enclose reproductive organs in a manner akin to primitive sepals.^[62] Although Archaefructus lacks distinct petals or fully differentiated sepals, its foliar bracts provide early indications of perianth precursors, supporting the gradual evolution of protective outer whorls from gymnosperm-like sterile parts.^[63] More controversial Jurassic fossils, like Nanjinganthus (~174 million years ago), suggest possible earlier perianth with sepal- and petal-like elements, though their angiosperm affinity remains debated; recent analyses as of 2023 provide further evidence supporting its status as an early angiosperm.^[64]^[65]

Ecological Significance

Sepals play a crucial role in defending plants against herbivores through their structural toughness and, in some cases, chemical properties that deter feeding. In species like Physalis angulata, expanded persistent sepals form a protective barrier around developing fruits, providing a structural refuge that shields against generalist herbivores and reduces parasitoid attacks on specialist larvae, thereby enhancing overall reproductive success. This mechanical protection is complemented by bitter or toxic compounds often present in calyces, such as alkaloids or phenolics, which discourage herbivory in various angiosperms. ^[67] In certain lineages, sepals contribute to pollination by adopting petaloid characteristics that attract insects. For instance, in Aquilegia species, the ovate, flat, and colored sepals of the outer perianth whorl contribute to pollinator attraction, sharing the ecological function with petals. ^[68] Sepals also facilitate environmental adaptations, particularly in challenging habitats. In arid regions, thicker or inflated sepals, as seen in Physalis species, create a humid microclimate around fruits by buffering temperature fluctuations and retaining moisture, aiding seed development under water-limited conditions. ^[69] This structure indirectly supports water retention for the enclosed reproductive organs, improving survival in dry environments. Beyond protection and attraction, photosynthetic sepals significantly contribute to plant fitness, especially in shaded understories. In Helleborus foetidus, persistent green sepals perform photosynthesis at rates comparable to 20-60% of mature leaves, supplying assimilates that increase seed mass by approximately 10% and enhance germination rates, thereby elevating seedling establishment and overall reproductive output in low-light forest floors. ^[50] Sepal color polymorphism in wild populations further influences ecological interactions, particularly pollinator preference. In Aquilegia coerulea, variation in sepal color (e.g., blue versus white morphs) affects visitation by bumblebees and other insects, with blue sepals often preferred under certain conditions, maintaining polymorphism through differential attraction and potentially balancing reproductive success across morphs. ^[70]

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Sep 1, 2005 · This observation has fueled the traditional view that Helleborus sepals “assist in photosynthesis” (Mathew, 1989, p. 40), “helping the seed-pods ...<|separator|>
[48]
Function of leafy sepals in Paris polyphylla - CSIRO Publishing
These results indicate that photosynthates produced by sepals are involved in the fruit growth and seed development and that developing fruit and rhizomes ...
[49]
The Systematics and Evolution of Fuchsia Sect. Fuchsia (onagraceae)
On a local scale, bird dispersal is an effective means of spreading seeds to new habitats, and it probably has been instrumental in the extension of many ...
[50]
Abscission in plants: from mechanism to applications - PMC - NIH
Aug 9, 2024 · Plant hormones play a crucial role in regulating plant abscission and their crosstalk has long been a subject of research interest. A widely ...
[51]
Phytohormonal dynamics in the abscission zone of Korla fragrant ...
Oct 8, 2024 · Phytohormones play a crucial role in regulating the abscission of plant organs and tissues. In this study, the ultrastructure of the sepals ...
[52]
Molecular regulatory events of flower and fruit abscission in ...
Flower and fruit abscission is regulated by hormones, peptides, carbohydrates, polyamines, and cell wall modifying proteins, and occurs at abscission zones ( ...
[53]
The ancestral flower of angiosperms and its early diversification
Aug 1, 2017 · The most recent common ancestor of all living angiosperms likely existed ∼140–250 million years ago. In contrast, the most recent common ...
[54]
[PDF] Beyond pine Cones: An Introduction to Gymnosperms
The fact that the scale where the ovules are formed sits at the base of the bract is important because therein lies the fundamental compound nature of the cone.
[55]
Are Petals Sterile Stamens or Bracts? The Origin and Evolution of ...
I challenge the widely held classical view that petals are morphologically derived from stamens in the core eudicots, and sepals from tepals or bracts.
[56]
[PDF] Origin and evolution of petals in angiosperms
In Delosperma the staminodial petals behave as expected for stamens in the early expression of AP3, PI (B- genes) and AG (C-gene) before these genes are turned ...
[57]
Evolution of petal identity | Journal of Experimental Botany
Angiosperm flowers generally contain petals that display considerable diversity in their size, morphology, and colour. The brilliant hues and complex forms ...Missing: aestivation | Show results with:aestivation<|control11|><|separator|>
[58]
Environmental control of sepalness and petalness in perianth ...
We propose a novel theory (the Mosaic theory), in which the distinction between sepalness and petalness evolved early in angiosperm history.
[59]
Fossil Hints at First Blooms | Science | AAAS
But Archaefructus would raise a florist's eyebrows: It has no sepals or petals, and strangest of all, its stamens come in pairs rather than singly. Based on ...
[60]
Archaefructus - an overview | ScienceDirect Topics
In particular, Archaefructus does not have sterile external parts (petals and sepals). ... fossil record for those same lineages. This convergence is encouraging.
[61]
Page not found | Nature
Insufficient relevant content.
[62]
Auxin regulated metabolic changes underlying sepal retention and ...
Apr 6, 2021 · However, largely expanded sepals in Physalis angulate enhance plant fitness by providing herbivore protective function against parasitoids ...Phytohormone Signal... · Auxin Acts As A Signal And... · Discussion
[63]
Defensive functions and potential ecological conflicts of floral ...
Nov 18, 2022 · In general, our data suggest that stickiness in reproductive tissues can serve the same defensive function as stickiness in vegetative tissues ...
[64]
Evolution of novel floral organ identity in Aquilegia - Kramer Lab
The perianth is composed of two whorls of morphologically distinct petaloid organs: the first whorl petaloid sepals are ovate and flat and the second whorl ...
[65]
DSpace
Insufficient relevant content. The provided content (https://kuscholarworks.ku.edu/bitstream/handle/1808/8154/Landis_ku_0099M_11651_DATA_1.pdf) only includes metadata and a stylesheet link, with no substantive information about petaloid sepals, Rhodochiton, or pollinator attraction.
[66]
Chinese lantern in Physalis is an advantageous morphological ...
Jan 24, 2019 · The photosynthetic capability of green calyx contributes to fruit and seed development in Physalis floridana. (A) Quantification of calyx areas ...
[67]
The role of pollinators in maintaining variation in flower colour in the ...
Mar 25, 2015 · Whether a single pollinator could maintain a flower colour polymorphism was examined by testing to see if preference for a flower colour varied ...