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Bract

A bract is a specialized, often reduced leaf-like structure in plants that subtends reproductive organs such as flowers, inflorescences, or cones, typically differing in form, size, or color from the plant's vegetative leaves.^[1]^[2] Bracts serve multiple essential functions in plant reproduction and survival, including protecting developing flowers and seeds from environmental stressors like ultraviolet radiation, extreme temperatures, and drought, as well as deterring herbivores through physical barriers, camouflage, warning coloration, or toxic compounds.^[2]^[3] In many species, bracts enhance pollinator attraction by mimicking petals with vibrant pigments such as anthocyanins (producing red, purple, or blue hues), betalains (reds and yellows), or carotenoids (yellows and oranges), thereby signaling nectar or pollen rewards—either honestly or deceptively.^[2]^[3] Some bracts retain photosynthetic capacity, contributing carbohydrates to support fruit and seed development, particularly in late-season growth.^[2]^[3] 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.^[4] Their coloration and form are genetically regulated by pathways involving genes like CHS, DFR, and MYB transcription factors, influenced by environmental cues.^[2] 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.^[2]^[5] In Bougainvillea species, colorful bracts (often mistaken for flowers) rely on betalain pigments for vivid displays that aid pollination in tropical environments.^[2] In gymnosperms, bracts form part of cone structures, such as the microsporophylls in male pine cones that bear pollen sacs.^[6] Overall, bracts represent an evolutionary adaptation that balances protection, attraction, and resource allocation in diverse plant lineages.^[3]

Definition and Characteristics

Definition

In botany, a bract is defined as a modified or specialized leaf, 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 perianth.^[7] 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 leaf of gold, a descriptor that aptly captures the scale-like or membranous appearance of many bracts.^[8] This etymological root highlights the structure'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 structure was formalized by Carl Linnaeus in his 1751 work Philosophia Botanica, where he described bractea as a floral leaf differing from ordinary leaves and explicitly differentiated it from sepals and stipules based on its position and form.^[9] Linnaeus' classification emphasized bracts' role in inflorescence architecture, building on earlier morphological observations to establish them as a key category in plant taxonomy during the 18th century.^[10] 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.^[11] 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.^[12]^[13] Their venation generally mirrors that of leaves but is simplified, featuring fewer veins and a less complex network to support their subordinate role.^[13] Variations in texture range from dry and papery in mature forms to fleshy or leathery in others, while coloration spans green shades in photosynthetic types to membranous whites or vivid hues such as red in non-photosynthetic variants.^[14]^[15] Anatomically, bracts may bear stomata on their surfaces for gas exchange, particularly in green forms, and often feature trichomes or glandular structures that can secrete mucilage or other substances, as observed in specialized tissues like internal secretory layers.^[16] 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.^[17] 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.^[18]^[19]

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.^[4] Unlike larger bracts that subtend inflorescence branches, bracteoles are closely associated with the flower base, sometimes appearing to arise from the calyx tube itself.^[20] 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.^[20] This reduction distinguishes them from main bracts by their immediate proximity to the floral organs and subdued appearance.^[4] 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.^[21] In these arrangements, they arise in the axils of higher-order branches, contributing to the precise positioning of flowers within the overall inflorescence.^[20] 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.^[21] Similar paired bracteoles are observed in other genera like Pultenaea and Sesbania, aiding in species identification through their attachment at the calyx base.^[20]

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 inflorescence, typically creating a cup- or disk-like structure around the base.^[22] This arrangement provides a composite envelope that safeguards the developing reproductive structures, distinguishing it from individual bracts associated with single flowers.^[23] 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.^[24]^[25]^[26] In inflorescences, these bracts play a key role in enclosing compact flower heads, such as the capitula in the Asteraceae family where they are known as phyllaries—as seen in sunflowers (Helianthus spp.)—offering structural support and protection to the clustered florets.^[27] Similarly, in the Apiaceae family, they surround umbels, for example in carrots (Daucus carota), stabilizing the branching arrangement of flowers.^[23] Developmentally, involucral bracts originate from sterile bracts at the inflorescence apex and have evolved to mimic the protective function of calyces in single flowers.^[28]

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 Malvales, particularly within the family Malvaceae, it arises from the reduction of inflorescence 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 bud. 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 Malva 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 green but contribute to the overall floral appearance. In genera like Hibiscus, the epicalyx features 5–13 free or adnate bracts that are caducous, detaching early after anthesis, distinguishing them from the more persistent true calyx. This homology to reduced bracts underscores its evolutionary role in perianth modification, as seen in the sequential initiation of primordia during floral development. The epicalyx is prominently developed across six subfamilies of Malvaceae, including Malvoideae, where it enhances both protection and visual appeal in species such as Hibiscus (e.g., H. rosa-sinensis and H. sabdariffa), okra (Abelmoschus esculentus), and cotton (Gossypium spp.). In cotton flowers, for instance, it forms three large green bracts that fully enclose the bud, while in Hibiscus, 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 desiccation 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 inflorescence, particularly a spadix, functioning to guard it during early development before it splits open to expose the flowers.^[29] In monocots such as those in the Araceae and Arecaceae families, it typically forms a single, sheath-like structure that envelops the flower cluster, providing a protective enclosure that opens longitudinally as the inflorescence matures.^[30]^[31] 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 Araceae, the spathe is divided into a basal tubular portion known as the tube and a distal expanded portion called the blade, which can exhibit varied coloration to suit ecological roles. For instance, in species of the Araceae like Zantedeschia aethiopica (calla lily), the spathe is prominently white or green, enhancing visibility while maintaining structural integrity.^[30]^[32] In monocotyledons, spathes play key protective roles by enclosing spadices in the Araceae or florets in palms of the Arecaceae, 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 inflorescence growth. In palms, the spathe similarly sheathes the entire inflorescence, supporting its expansion in humid, forested environments.^[33]^[30] Evolutionarily, spathes represent modified foliar structures derived from leaf bases, an adaptation prevalent in aroid and palm lineages that facilitates inflorescence protection and development in tropical ecosystems. This homology to leaf sheaths underscores their role as specialized bracts in monocot phylogeny.^[34]

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.^[35] In such arrangements, each bract arises from the inflorescence axis and envelops the base of a single flower, as observed in families like Cyperaceae where glumes function as these subtending bracts for spikelet flowers.^[36] 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 seed dispersal.^[37] For instance, in pine (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 seed retention.^[38] Bractlets, or bracteoles, are diminutive subsidiary bracts that occur within complex inflorescence aggregates, enhancing structural complexity in certain families; in Euphorbiaceae, they are prominent in cyathia, the fused cuplike structures housing reduced unisexual flowers.^[39] In species like poinsettia (Euphorbia 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.^[40] Less common bract variants include the involucel, a partial involucre composed of small bractlets subtending secondary umbels in the compound inflorescences of Apiaceae (umbellifers), where they provide additional support to partial umbels.^[41] Spine-like bracts, adapted for defense against herbivores, appear in certain Caryophyllaceae species, such as spiny Dianthus taxa from Turkey, where these rigid, thorn-tipped structures modify typical foliaceous bracts into protective elements.^[42]

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.^[43] 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.^[44] 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.^[45] The globe artichoke (Cynara cardunculus) features fleshy, thorn-tipped bracts that physically impede herbivores, enhancing seed protection in Mediterranean environments.^[46] In orchids like Thunia alba, large bracts physically enclose the spurs, deterring nectar-robbing insects such as Bombus breviceps.^[47] Bracts also regulate the microenvironment around inflorescences by retaining moisture and moderating temperature, particularly in harsh habitats. In Hemiboea magnibracteata, womb-like bracts secrete up to 17.7 mL of fluid that immerses developing buds, preventing desiccation in karst drought conditions and buffering temperature 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.^[48] This fluid, derived from root-absorbed water via vascular transport, maintains higher leaf water potential under stress, supporting floral viability.^[48] 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 Asteraceae where involucral bracts prevent floret displacement and abortion during growth.^[44] In Hemiboea magnibracteata, the protective enclosure lowers abortion rates by sustaining a stable microenvironment, ensuring higher reproductive success in crowded or resource-limited settings.^[48]

Attractive and Signaling Functions

Bracts often serve visual cues to attract pollinators, functioning similarly to petals by displaying vibrant colors that draw insects or birds to the reproductive structures. In poinsettias (Euphorbia pulcherrima), the red bracts act as pseudo-petals, evolving to accumulate anthocyanins that shift their primary role from photosynthesis to pollinator attraction, thereby guiding insects to the small central flowers.^[49]^[50] Some bracts incorporate ultraviolet (UV) patterns or reflectance to further aid bee guidance, enhancing visibility in the pollinators' spectral range and directing them toward nectar or pollen sources.^[51] Olfactory signaling via bracts supplements visual appeal, particularly in families like Araceae, 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 insects temporarily to ensure pollen transfer.^[32]^[52] In species like Zantedeschia aethiopica, such scents from bract-associated glands play a critical role in attracting non-rewarding pollinators despite the absence of nectar.^[53] Bracts can also facilitate mimicry and deception, imitating female flowers or mating sites to exploit male pollinators in deceptive pollination syndromes. In aroids, the colorful spathe combined with the spadix mimics a female insect or brood site, drawing males that attempt pseudocopulation and inadvertently pollinate the plant.^[32]^[54] The evolution 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 pollination success. In Saururus chinensis, reversible bract coloration prioritizes pollinator attraction during anthesis, incurring only a minor photosynthetic cost while significantly increasing fruit set and seed production.^[55] This balance underscores how selective pressures for reproductive enhancement drive bract diversification, with non-photosynthetic showiness yielding net fitness gains in pollinator-dependent species.^[50]

Occurrence and Examples

In Monocot Families

In the family Araceae, 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 inflorescence while often serving an attractive role through coloration and, in some species, fragrance to draw pollinators. For instance, in Anthurium, 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.^[56]^[57]^[58] Within the Poaceae family, bract-like structures manifest as glumes and lemmas in the spikelet, the basic unit of the grass inflorescence. Glumes are paired, sterile basal bracts that enclose the spikelet, 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 seed dispersal by enabling hygroscopic movement in response to moisture changes or by attaching to animal fur.^[59]^[60]^[61] 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 anthesis to reduce weight on the slender peduncle. They support the inflorescence structure while sometimes contributing to pollinator attraction through visual cues.^[62]^[63]^[64] Monocots exhibit a notable evolutionary prevalence for large, spathe-like bracts.^[65]^[66]

In Dicot Families

In the Asteraceae family, commonly known as the aster or daisy family, bracts referred to as phyllaries form a protective involucre surrounding the compact inflorescence 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 (Bellis perennis).^[24]^[25] The arrangement of these involucral bracts often varies in texture and layering, contributing to the family's characteristic head-like inflorescences.^[67] In the Malvaceae 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 pollinator attraction while offering mechanical protection to the delicate petals and reproductive structures, particularly evident in cotton plants (Gossypium species) where the epicalyx segments fuse partially at the base.^[68]^[69]^[70] The Euphorbiaceae family showcases specialized cyathial involucres in genera like Euphorbia, 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 cyathium, as seen in species like Euphorbia pulcherrima (poinsettia).^[71]^[72]^[73]^[74] 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 inflorescence types for protection and attraction.^[75]

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Dec 20, 2015 · These bracts evolved to have the red color to attract insects for pollination of the small flowers. Through modern breeding, the cyanthia ...
[50]
Hybrid de novo transcriptome assembly of poinsettia (Euphorbia ...
Nov 27, 2019 · The bracts of poinsettia are leaves changing their photosynthetic function into pollinator attraction (i.e. by accumulating anthocyanins) ...
[51]
White Bracts of the Dove Tree (Davidia involucrata) - Arnold Arboretum
Feb 15, 2011 · Green bracts function photosynthetically, while mature white bracts absorb UV light and attract pollinators, especially bees, who prefer them.Missing: types | Show results with:types
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Floral Scents of a Deceptive Plant Are Hyperdiverse and Under ...
This appendix produces and releases the scent for pollinator attraction, e.g., Lack and Diaz (1991) and Scheven (1994). At the base of the spadix, female ( ...
[53]
Floral biology of Zantedeschia aethiopica (L.) Spreng. (Araceae)
The question that arises is by what means are insects attracted to spathes of the non-scent producing species of Zante- deschia'? ... The role of scent glands in ...
[54]
Reconstructing the origin and elaboration of insect-trapping ... - NIH
The insects are trapped by slippery spathe surfaces and sterile flowers on the spadix that partially occlude the spathe chamber. ... protect developing fruits ( ...
[55]
Sexual Mimicry in Mormolyca ringens (Lindl.) Schltr. (Orchidaceae
Background and Aims Pollination through sexual mimicry, also known as pseudocopulation, has been suggested to occur in some genera of the Neotropical orchid ...
[56]
Reversible colour change in leaves enhances pollinator attraction ...
Jan 9, 2018 · The reversible leaf colour change in S. chinensis appears to be adaptive because it enhances pollination success during flowering, with a small ...
[57]
Araceae - FNA - Flora of North America
Nov 5, 2020 · Araceae are best characterized by the inflorescence, a fleshy cylindric or ovoid, unbranched spadix subtended or surrounded by a spathe. True ...
[58]
ENH1102/EP366: Tropical Foliage Plant Development: Breeding ...
Anthurium and Spathiphyllum produce bisexual flowers (Figures 2 and 3). Their inflorescences consist of a spadix enclosed by a spathe. The spadix is a fleshy ...
[59]
Anthurium - North Carolina Extension Gardener Plant Toolbox
Their blooms may be red, pink, orange, or white inflorescence that consists of a shiny, leathery spathe and rod-like spadix. The habit, leaf size, and shape may ...
[60]
PLB 102 | Lab 8 - UC Davis Plant Sciences
In some species of Juncus, the bract subtending the inflorescence is round in cross-section and looks like a continuation of the stem; in such cases, the ...Missing: botany | Show results with:botany
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Poaceae - Michigan Flora
Except in the rare instances when one or both are absent, there is a pair of scales or bracts called glumes at the base of each spikelet. In addition, at the ...
[62]
Diverse ecological functions and the convergent evolution of grass ...
The awns of grasses may function in dispersal in three ways: hygroscopically active awns move in response to changes in moisture, moving the diaspore (the seed ...
[63]
Floral convergence in Oncidiinae (Cymbidieae; Orchidaceae)
Apr 4, 2009 · ... deciduous. Inflorescences produced laterally from the base of the pseudobulb, subtended by a sheathing bract, much longer than leaves ...
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Two new species of Cylindrolobus (Orchidaceae) from the eastern ...
Aug 29, 2019 · ... colorful bracts. These two new species are distributed in a narrow area in the border region between China and Myanmar. We found there are ...<|control11|><|separator|>
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Dendrobium
Floral bracts scarious, small, subtending the base of the pedicel. Pedicel relatively long, thin, merging with the ovary. Ovary short, straight or curved.
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[PDF] Diversity and Evolution of Monocots
Striking modifications & bracts: grasses, pulpits, orchids, spadices & more! Monocots have usually been considered as derived out of basal angiosperms - Ranales ...
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13.2: Monocot Leaves - Biology LibreTexts
May 3, 2022 · Monocot leaves tend to have parallel venation, as opposed to the branching patterns seen in eudicots. A monocot leaf showing parallel venation.
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[PDF] Diversity of Wisconsin Asterids
Apr 3, 2018 · The head is surrounded by special bracts called the involucre or phyllaries. The involucre is important in the classification and ...
[69]
malvaceae - Florida Plant Atlas
Leaf blades 4-15 cm wide; flowers bisexual, with an epicalyx, calyx, and petals; fruit a loculicidal capsule; seeds comose (cotton). Gossypium. 4. Herbs to ...
[70]
Family: Malvaceae
epicalyx = whorl of bracts below the calyx showy flowers with nectar, seeds often covered in fine hair. Vegetative Features. Leaves: alternate, often ...
[71]
Digital Flowers
Malvaceae. Flowers: Perfect, actinomorphic; sepals often subtended by bracts (epicalyx); petals distinct, adnate to the staminal tube, stamens many ...
[72]
Comparative ontogeny of the cyathium in Euphorbia ...
The cyathium is a cuplike structure of fused bracts, often with distinct extrafloral nectar glands and showy petaloid appendages, enclosing a single terminal ...
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Euphorbia - Michigan Flora
Around the margin of the cyathium are (1-) 4-5 glands, and in some species these have petaloid appendages, so that the whole cyathium mimics a single flower.
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Euphorbia - Florida Plant Atlas
Euphorbia sl is a large genus of ca. 2000 species characterized by its cyathium (an involucre of clustered, reduced male and female flowers).
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PLB 102 | Lab 1 - UC Davis Plant Sciences
The small leaves that occur in the inflorescence (the flowering parts) are often called bracts. Often, in the angle between the leaf base and the stem (the leaf ...