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Bract
Bract
Bract
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Papery (upper) and leafy bracts on Rhinanthus minor (hay rattle). All the "leaves" in this image are bracts.

In botany, a bract is a modified or specialized leaf, associated with a reproductive structure such as a flower, inflorescence axis or cone scale. Bracts are usually different from foliage leaves in size, color, shape or texture. They also look different from the parts of the flower, such as the petals or sepals.

A plant having bracts is referred to as bracteate[1] or bracteolate, while one that lacks them is referred to as ebracteate[2] or ebracteolate.

Variants

[edit]

Some bracts are brightly coloured which aid in the attraction of pollinators, either together with the perianth or instead of it. Examples of this type of bract include those of Euphorbia pulcherrima (poinsettia) and Bougainvillea: both of these have large colourful bracts surrounding much smaller, less colourful flowers.[3]

In grasses, each floret (flower) is enclosed in a pair of papery bracts, called the lemma (lower bract) and palea (upper bract), while each spikelet (group of florets) has a further pair of bracts at its base called glumes. These bracts form the chaff that is usually removed from cereal grain during threshing and winnowing.[4]

Bats may detect acoustic signals from dish-shaped bracts such as those of Marcgravia evenia.[5]

A prophyll is a leaf-like structure, such as a bracteole, subtending (extending under) a single flower or pedicel. The term can also mean the lower bract on a peduncle.

The frequently showy pair of bracts of Euphorbia species in subgenus Lacanthis are the cyathophylls.

Bracts subtend the cone scales in the seed cones of many conifers, and in some cases, such as Pseudotsuga, extend beyond the cone scales.

Bracteole

[edit]

A small bract is called a bracteole or bractlet. Technically this is any bract that arises on a pedicel instead of subtending it.

Involucral bracts

[edit]
Beggar-tick (Bidens comosa)

Bracts that appear in a whorl subtending an inflorescence are collectively called an involucre. An involucre is a common feature beneath the inflorescences of many families, including the Apiaceae (carrot family), Asteraceae (sunflower or the daisy family), Dipsacaceae, and Polygonaceae. Each flower in an inflorescence may have its own whorl of bracts, in this case called an involucel. They can be called chaff, paleas or receptacular bracts and are usually minute scales or bristles. Many asteraceous plants have bracts at the base of each inflorescence.[6]

The term involucre is also used for a highly conspicuous bract or bract pair at the base of an inflorescence. In the family Betulaceae, notably in the genera Carpinus and Corylus, the involucre is a leafy structure that protects the developing nuts. Beggar-tick (Bidens comosa) has narrow involucral bracts surrounding each inflorescence, each of which also has a single bract below it. There is then a pair of leafy bracts on the main stem and below those a pair of leaves.[6]

Epicalyx of Hibiscus rosa-sinensis

Epicalyx

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An epicalyx (which forms an additional whorl around the calyx of a single flower) is a modification of bracteoles.[7] In other words, the epicalyx is a group of bracts resembling a calyx or bracteoles forming a whorl outer to the calyx.[8][page needed] It is a calyx-like extra whorl of floral appendages. Each individual segment of the epicalyx is called an episepal because they resemble the sepals in them.[9] They are present in the hibiscus family, Malvaceae. Fragaria (strawberries) may or may not have an epicalyx.

Anthurium scherzerianum inflorescence with spathe and spadix

Spathe

[edit]

A spathe is a large bract or a pair of bracts that forms a sheath to enclose the flower cluster of plants such as palms, arums, irises,[10] crocuses[11] and dayflowers (Commelina). Zephyranthes tubispatha in the Amaryllidaceae derives its specific name from its tubular spathe. In many arums (family Araceae), the spathe is petal-like, attracting pollinators to the flowers arranged on a type of spike called a spadix.

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Bract

View on Grokipedia
from Grokipedia
A bract is a specialized, often reduced leaf-like structure in that subtends reproductive organs such as flowers, inflorescences, or cones, typically differing in form, size, or color from the plant's vegetative leaves. Bracts serve multiple essential functions in and survival, including protecting developing flowers and seeds from environmental stressors like , extreme temperatures, and , as well as deterring herbivores through physical barriers, , warning coloration, or toxic compounds. In many , bracts enhance pollinator attraction by mimicking petals with vibrant pigments such as anthocyanins (producing red, purple, or blue hues), betalains (reds and yellows), or (yellows and oranges), thereby signaling or rewards—either honestly or deceptively. Some bracts retain photosynthetic capacity, contributing carbohydrates to support and development, particularly in late-season growth. Bracts exhibit diverse morphologies and are classified based on their position and structure; for instance, floral bracts directly subtend individual flowers, while involucral bracts form protective whorls around entire inflorescences, and smaller bracteoles arise on pedicels or near the calyx. Their coloration and form are genetically regulated by pathways involving genes like CHS, DFR, and MYB transcription factors, influenced by environmental cues. Notable examples include the bright red bracts of the poinsettia (Euphorbia pulcherrima), which attract pollinators and overshadow the tiny true flowers, and the petal-like white or pink bracts of dogwoods (Cornus spp.), functioning similarly to draw insects. In Bougainvillea species, colorful bracts (often mistaken for flowers) rely on betalain pigments for vivid displays that aid pollination in tropical environments. In gymnosperms, bracts form part of cone structures, such as the microsporophylls in male pine cones that bear pollen sacs. Overall, bracts represent an evolutionary adaptation that balances protection, attraction, and resource allocation in diverse plant lineages.

Definition and Characteristics

Definition

In , a bract is defined as a modified or specialized , often reduced in size, that is associated with reproductive structures such as flowers, cones, or inflorescences, typically arising from the axil and subtending them without forming part of the . Unlike sepals or petals, which constitute the floral envelopes, bracts serve a supportive role in relation to these organs and are generally positioned external to the flower proper. The term "bract" originates from the Latin bractea, meaning a thin metal plate or of , a descriptor that aptly captures the scale-like or membranous appearance of many bracts. This etymological root highlights the 's flattened, often delicate form, which distinguishes it visually from more robust foliage in numerous plant species. The concept of the bract as a distinct botanical was formalized by in his 1751 work Philosophia Botanica, where he described bractea as a floral differing from ordinary leaves and explicitly differentiated it from sepals and stipules based on its position and form. Linnaeus' classification emphasized bracts' role in architecture, building on earlier morphological observations to establish them as a key category in during the . Bracts are classified as phyllomes—homologous to leaves in developmental origin—but they diverge from typical foliage leaves through their specific positioning adjacent to reproductive organs and adaptations such as size reduction or altered texture to facilitate reproductive processes. This homology underscores their evolutionary derivation from leaf-like primordia, while their modifications, including frequent diminishment in size compared to vegetative leaves, reflect specialization for non-photosynthetic support.

Morphological Features

Bracts are typically reduced in size compared to foliage leaves, often appearing scale-like, membranous, or herbaceous, and they may be either sessile or possess a short petiole. Their venation generally mirrors that of leaves but is simplified, featuring fewer veins and a less to support their subordinate role. Variations in texture range from dry and papery in mature forms to fleshy or leathery in others, while coloration spans shades in photosynthetic types to membranous whites or vivid hues such as red in non-photosynthetic variants. Anatomically, bracts may bear stomata on their surfaces for , particularly in forms, and often feature trichomes or glandular structures that can secrete or other substances, as observed in specialized tissues like internal secretory layers. Their homology to foliage leaves is supported by developmental studies demonstrating origination from similar leaf primordia, with bracts exhibiting comparable early patterning genes and structures. Bracts develop from axillary primordia positioned adjacent to reproductive meristems, where growth is typically suppressed to prevent overshadowing of flowers, resulting in their compact form.

Types of Bracts

Bracteole

A bracteole is a small, secondary bract subtending an individual flower, typically positioned on the pedicel or at the base of the calyx, often occurring in pairs directly below the flower proper. Unlike larger bracts that subtend inflorescence branches, bracteoles are closely associated with the flower base, sometimes appearing to arise from the calyx tube itself. Morphologically, bracteoles are usually minute and inconspicuous, reduced in size compared to primary bracts, and exhibit shapes such as linear, ovate, or scale-like forms without prominent coloration or showy features. They may be simple, bilobed, or trilobed, varying in complexity but generally lacking the foliaceous or petaloid traits of other bract types. This reduction distinguishes them from main bracts by their immediate proximity to the floral organs and subdued appearance. Bracteoles commonly occur in inflorescences such as racemes or cymes, where they support the pedicels of individual flowers, providing localized structural association at the flower level. In these arrangements, they arise in the axils of higher-order branches, contributing to the precise positioning of flowers within the overall inflorescence. In the Fabaceae family, bracteoles are exemplified in pea flowers (Pisum sativum), where they appear as small basal scales in pairs on the pedicel, subtending the zygomorphic blooms in racemose inflorescences. Similar paired bracteoles are observed in other genera like Pultenaea and Sesbania, aiding in species identification through their attachment at the calyx base.

Involucral Bracts

Involucral bracts collectively form an involucre, defined as a whorl or ring of bracts that encircles and protects a group of flowers or florets within an , typically creating a cup- or disk-like structure around the base. This arrangement provides a composite that safeguards the developing reproductive structures, distinguishing it from individual bracts associated with single flowers. Morphologically, involucral bracts are often larger and more rigid than typical bracts, arranged in one or more overlapping (imbricate) series where inner bracts may be longer than outer ones, or occasionally fused at their bases to form a more unified cup-shaped structure; they are generally green and sepal-like, though some species feature spine-tipped margins for added defense. In inflorescences, these bracts play a key role in enclosing compact flower heads, such as the capitula in the family where they are known as phyllaries—as seen in sunflowers ( spp.)—offering and to the clustered florets. Similarly, in the family, they surround umbels, for example in carrots (), stabilizing the branching arrangement of flowers. Developmentally, involucral bracts originate from sterile bracts at the apex and have evolved to mimic the protective function of calyces in single flowers.

Epicalyx

The epicalyx is defined as a whorl of bract-like structures positioned immediately below and external to the true calyx, subtending the flower and often resembling an additional set of sepals. In the order , particularly within the family , it arises from the reduction of bracts, integrating into the floral structure as a distinct outer layer. This positioning separates it from the calyx by a short distance, typically 1–3 mm, allowing it to envelop the developing . Morphologically, the epicalyx consists of multiple segments or lobes that are frequently fused proximally and divided distally, with a trimerous (three-lobed) configuration representing the ancestral state in lineages such as the generic alliance. These lobes vary in shape—ranging from linear and narrow to ovate or lanceolate—and are often larger than the calyx, bearing stellate trichomes for a tomentose texture; they are generally but contribute to the overall floral appearance. In genera like , the epicalyx features 5–13 free or adnate bracts that are caducous, detaching early after , distinguishing them from the more persistent true calyx. This homology to reduced bracts underscores its evolutionary role in modification, as seen in the sequential initiation of primordia during floral development. The epicalyx is prominently developed across six subfamilies of , including , where it enhances both protection and visual appeal in species such as (e.g., H. rosa-sinensis and H. sabdariffa), ( esculentus), and ( spp.). In flowers, for instance, it forms three large green bracts that fully enclose the bud, while in , the variable lobe count and shape aid in species delineation and phylogenetic grouping. Functionally, it serves as a secondary calyx, bolstering durability during the bud stage by shielding sensitive reproductive parts from and herbivores, and later integrating into the floral display to attract pollinators through its structural contribution.

Spathe

A spathe is defined as a large bract that subtends and often encloses an , particularly a spadix, functioning to guard it during early development before it splits open to expose the flowers. In monocots such as those in the and families, it typically forms a single, sheath-like structure that envelops the flower cluster, providing a protective that opens longitudinally as the inflorescence matures. Morphologically, spathes are often elongated and lanceolate in shape, with a texture ranging from membranous to leathery or fleshy, and they may be either caducous, falling away after flowering, or persistent on the plant. In the , the spathe is divided into a basal tubular portion known as the tube and a distal expanded portion called the , which can exhibit varied coloration to suit ecological roles. For instance, in species of the like (calla lily), the spathe is prominently white or green, enhancing visibility while maintaining structural integrity. In monocotyledons, spathes play key protective roles by enclosing spadices in the or florets in palms of the , thereby retaining humidity around the developing flowers and shielding them from excessive rainfall in tropical habitats. This enclosure also offers mechanical protection against physical damage and potential herbivory during vulnerable stages of growth. In palms, the spathe similarly sheathes the entire , supporting its expansion in humid, forested environments. Evolutionarily, spathes represent modified foliar structures derived from bases, an prevalent in aroid and palm lineages that facilitates protection and development in tropical ecosystems. This homology to leaf sheaths underscores their role as specialized bracts in monocot phylogeny.

Other Variants

Floral bracts are modified leaves that directly subtend individual flowers, particularly in simple inflorescences like spikes where the flowers are sessile and lack pedicels, providing structural support and protection to the reproductive units. In such arrangements, each bract arises from the axis and envelops the base of a single flower, as observed in families like where glumes function as these subtending bracts for spikelet flowers. In gymnosperms, cone scales are specialized bracts that bear ovules on megasporophylls or microsporangia on microsporophylls, forming the protective and reproductive units of cones; these scales are typically woody, overlapping, and persistent after . For instance, in (Pinus) cones, each scale consists of a fertile ovuliferous scale fused with a sterile bract, which emerges below the scale during early development and contributes to the cone's rigidity and retention. Bractlets, or bracteoles, are diminutive subsidiary bracts that occur within complex aggregates, enhancing structural complexity in certain families; in , they are prominent in , the fused cuplike structures housing reduced unisexual flowers. In species like ( pulcherrima), these tiny, often hairy bractlets surround the male flowers within the cyathium, numbering typically five and aiding in the enclosure alongside the more conspicuous colored involucral bracts. Less common bract variants include the involucel, a partial involucre composed of small bractlets subtending secondary umbels in the compound inflorescences of (umbellifers), where they provide additional support to partial umbels. Spine-like bracts, adapted for defense against herbivores, appear in certain species, such as spiny taxa from , where these rigid, thorn-tipped structures modify typical foliaceous bracts into protective elements.

Functions and Roles

Protective Functions

Bracts serve as a primary physical barrier for reproductive structures, shielding developing buds and flowers from environmental stressors such as desiccation, excessive rainfall, wind, and mechanical damage. In the dove tree (Davidia involucrata), large white bracts function like an umbrella during the rainy flowering season, preventing water from reaching the capitula and reducing pollen loss; experimental removal of bracts resulted in significant pollen dispersal and viability decline, with 86.3% of pollen grains bursting after 4 hours of immersion in water compared to only 37.5% in a sucrose solution mimicking nectar. Similarly, in Asteraceae species, rigid involucral bracts enclose the immature florets within the capitulum, protecting them from physical abrasion and exposure during early development stages. Anti-herbivory defenses are another key protective role, where bracts deter grazing through structural toughness, spines, or chemical secretions. In Passiflora foetida, glandular bracts produce sticky exudates containing sugars (0.106%) and enzymes like proteases (52-47 n katalas/μg protein) and acid phosphatases (27.3 n katalas/μg protein), which trap and digest insects from 10 families, thereby reducing bud damage from 55.82% in bract-removed plants to 17.65% in intact ones. The globe artichoke (Cynara cardunculus) features fleshy, thorn-tipped bracts that physically impede herbivores, enhancing seed protection in Mediterranean environments. In orchids like Thunia alba, large bracts physically enclose the spurs, deterring nectar-robbing insects such as Bombus breviceps. Bracts also regulate the microenvironment around inflorescences by retaining moisture and moderating , particularly in harsh habitats. In Hemiboea magnibracteata, womb-like bracts secrete up to 17.7 mL of fluid that immerses developing buds, preventing in karst conditions and buffering fluctuations by up to 3.6°C on hot days (>35°C); drained bracts led to higher bud abortion rates compared to fluid-filled controls. This fluid, derived from root-absorbed water via vascular transport, maintains higher under stress, supporting floral viability. In terms of developmental support, bracts stabilize pedicels and reduce flower abortion in dense inflorescences by providing structural reinforcement. Bracts provide structural reinforcement, stabilizing pedicels and reducing flower abortion in dense inflorescences, as in the compact capitula of where involucral bracts prevent floret displacement and abortion during growth. In Hemiboea magnibracteata, the protective enclosure lowers abortion rates by sustaining a stable microenvironment, ensuring higher in crowded or resource-limited settings.

Attractive and Signaling Functions

Bracts often serve visual cues to attract pollinators, functioning similarly to petals by displaying vibrant colors that draw or birds to the reproductive structures. In s (Euphorbia pulcherrima), the red bracts act as pseudo-petals, evolving to accumulate anthocyanins that shift their primary role from to pollinator attraction, thereby guiding to the small central flowers. Some bracts incorporate (UV) patterns or reflectance to further aid bee guidance, enhancing visibility in the pollinators' spectral range and directing them toward or sources. Olfactory signaling via bracts supplements visual appeal, particularly in families like , where the spathe—a specialized bract—hosts scent-producing glands that emit volatile compounds to lure specific pollinators such as flies or beetles. These odors, often mimicking decaying matter or pheromones, emanate from glandular tissues on the inner spathe surface, trapping temporarily to ensure transfer. In species like , such scents from bract-associated glands play a in attracting non-rewarding pollinators despite the absence of . Bracts can also facilitate and , imitating female flowers or mating sites to exploit male in deceptive syndromes. In aroids, the colorful spathe combined with the spadix mimics a female or brood site, drawing males that attempt and inadvertently pollinate the . The of showy bracts involves trade-offs, as their emphasis on pigmentation and signaling reduces photosynthetic capacity compared to typical foliage, yet this is offset by heightened success. In Saururus chinensis, reversible bract coloration prioritizes pollinator attraction during , incurring only a minor photosynthetic cost while significantly increasing fruit set and seed production. This balance underscores how selective pressures for reproductive enhancement drive bract diversification, with non-photosynthetic showiness yielding net fitness gains in pollinator-dependent .

Occurrence and Examples

In Monocot Families

In the family , a characteristic feature of monocot bracts is the spathe, a large, modified leaf-like bract that encloses and subtends the spadix, a fleshy spike of minute flowers. This structure provides protection to the developing while often serving an attractive role through coloration and, in some species, fragrance to draw pollinators. For instance, in , the spathe is typically heart-shaped and glossy, appearing in vibrant hues such as red, pink, or white, which enhances its dual protective and signaling functions. Within the family, bract-like structures manifest as glumes and lemmas in the , the basic unit of the grass . Glumes are paired, sterile basal bracts that enclose the , while lemmas are the subtending bracts around each floret, usually dry, chaffy, and membranous. In numerous species, lemmas bear awns—bristle-like extensions that assist by enabling hygroscopic movement in response to moisture changes or by attaching to animal fur. In Orchidaceae, bracts typically subtend individual flowers along racemose inflorescences, often emerging from pseudobulbs in epiphytic or lithophytic species. These bracts are frequently colorful, such as in certain Cylindrolobus species with vivid hues, and deciduous, falling after to reduce weight on the slender peduncle. They support the structure while sometimes contributing to attraction through visual cues. Monocots exhibit a notable evolutionary prevalence for large, spathe-like bracts.

In Dicot Families

In the family, commonly known as the aster or daisy family, bracts referred to as phyllaries form a protective involucre surrounding the compact called a capitulum. These phyllaries are typically arranged in several imbricate series, overlapping to shield the developing florets from environmental damage and herbivores, as observed in common examples like daisies (). The arrangement of these involucral bracts often varies in texture and layering, contributing to the family's characteristic head-like inflorescences. In the family, an epicalyx composed of multiple segmented bracts positioned exterior to the calyx provides additional layers of enclosure and aesthetic enhancement to the flowers. These bracts can exhibit vibrant coloration, aiding in attraction while offering mechanical protection to the delicate petals and reproductive structures, particularly evident in cotton plants ( species) where the epicalyx segments fuse partially at the base. The family showcases specialized cyathial involucres in genera like , where fused bracts form a cuplike structure enclosing reduced male and female flowers, often adorned with colorful petaloid appendages. These appendages attract pollinators along with nectar glands on the , as seen in species like Euphorbia pulcherrima (). Across dicotyledonous families, bracts display notable variation in coloration—from green and protective to vividly petaloid—and in fusion patterns, such as partial connation in epicalyces or complete envelopment in cyathia, which adapt to diverse types for protection and attraction.

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  75. https://psfaculty.plantsciences.ucdavis.edu/courses/plb102/lab1.html
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