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Stipule

A stipule is one of a pair of small, usually leaf-like appendages borne at each side of the base of the petiole in many seed plants.^[1] These outgrowths arise from the petiole or leaf base and are typically present in pairs on either side of the leaf attachment to the stem.^[2] Stipules can vary widely in size and form, from inconspicuous scales to prominent structures, and are a key morphological feature in plant taxonomy.^[2] Stipules exhibit diverse morphology and functions across angiosperms, serving primarily as protective structures for young leaves and axillary buds during early development.^[3] In some species, such as legumes, they are foliaceous and contribute significantly to photosynthesis, with examples like Lathyrus aphaca where stipules act as the principal photosynthetic organs, featuring stomata, palisade, and spongy mesophyll tissues.^[4] Modifications include spine-like forms for defense against herbivores, as seen in certain Acacia species, or glandular structures that secrete protective substances in families like Apocynaceae.^[3] Tendril-like stipules aid in climbing, while their development is genetically regulated by conserved modules involving genes like NOOT and SGL in legumes.^[2] The presence or absence of stipules distinguishes stipulate from exstipulate leaves, influencing plant identification and evolutionary studies, with interpetiolar configurations considered ancestral in some orders like Gentianales.^[3] Their vascularization and ontogeny vary, often losing direct connections to the stem in derived lineages, highlighting adaptive diversity in response to environmental pressures.^[2]

Definition and Morphology

Etymology and Historical Recognition

The term stipule derives from the Latin stipula, meaning "straw" or "stalk," referring to the slender, appendage-like nature of these structures.^[5]^[6] Swedish botanist Carl Linnaeus, often regarded as the father of modern taxonomy, coined the term in the mid-18th century. He first introduced it in his Philosophia Botanica (1751), a foundational text outlining botanical principles, where stipules were described among various foliar modifications.^[7] Linnaeus expanded its use in Species Plantarum (1753), his comprehensive catalog of plant species, employing the term to denote the small, often paired outgrowths at the leaf petiole base in systematic descriptions.^[8] This marked the formal adoption of "stipule" into scientific nomenclature, standardizing its application across botanical classifications. In modern botany, the term has evolved to precisely distinguish stipules—typically the lateral appendages of vegetative leaf petioles—from similar structures like bracts, which subtend flowers or inflorescences rather than foliage.^[9] This differentiation, rooted in Linnaean morphology, underscores stipules' role as integral leaf components, commonly paired in dicotyledons, while avoiding confusion with reproductive-associated scales or sheaths.^[10]

Anatomical Structure and Occurrence

Stipules are defined as paired, leaf-like appendages that arise from the base of the petiole or the leaf sheath in many flowering plants, particularly dicotyledons, where they form small outgrowths flanking the leaf axis.^[11] These structures are typically positioned opposite each other and develop early in leaf primordia formation, often serving as initial protective coverings for emerging leaves. In eudicots, stipules exhibit a range of forms but consistently originate from the leaf base, distinguishing them from other appendages like bracts or scales.^[2] Anatomically, stipules are composed of tissues analogous to those in leaves, including an outer epidermis that may bear trichomes or glands and an internal mesophyll layer of parenchyma cells, sometimes containing chloroplasts for limited photosynthesis. Vascular supply to stipules is provided by branches from the lateral leaf traces, commonly associated with trilacunar nodes in dicotyledons, where three gaps in the vascular cylinder correspond to the median and two lateral traces supplying the petiole and stipules. This vascularization ensures nutrient and water transport, with bundles often embedded in the mesophyll and connecting directly to the stem's stele. In some cases, such as colleter-like stipules, vascular tissue may be reduced or absent, consisting primarily of secretory parenchyma.^[11]^[3] Stipules occur predominantly in eudicots, appearing in numerous families across orders like Gentianales and Fabales, while they are rare in monocotyledons, where analogous structures like ligules may substitute in grasses. For instance, in the eudicot genus Artocarpus (Moraceae), stipules can reach lengths of up to 8 cm, forming large, ovate sheaths around buds before becoming caducous. In contrast, Rosa species (Rosaceae) feature small, scale-like stipules, often fringed and glandular, measuring just a few millimeters and adnate to the petiole base. Variability in stipules includes differences in size from minute scales to foliaceous expansions, textures ranging from membranous and hyaline to leathery or glandular, and consistent bilateral symmetry in opposite pairs, reflecting adaptations to diverse habitats.^[3]^[12]^[13]

Classification of Stipules

Based on Persistence

Stipules are classified into three main types based on their persistence or duration relative to the leaf's development and lifespan: caducous, deciduous, and persistent. Caducous stipules abscise early, typically before the leaf fully expands, thereby offering brief protection to the emerging leaf bud during its initial vulnerable stage. This type is common in certain climbing or vining plants where rapid leaf development is advantageous.^[14] Deciduous stipules persist until after the leaf has unfolded but detach prior to the onset of leaf senescence, allowing them to contribute to early functions like protection or photosynthesis before being shed. This category is prevalent in leguminous plants adapted to varied habitats. An example occurs in the Fabaceae family, particularly in genera like Canavalia, where stipules are small, often peltate, and deciduous, as observed in Canavalia rosea and Canavalia gladiata.^[15] Persistent stipules remain attached for the duration of the leaf's life or beyond, potentially transitioning into modified structures that provide ongoing support or defense. Such longevity is seen in some woody or thorny species where stipules enhance structural integrity. For example, in Zapoteca aculeata (Fabaceae), the stipules develop into spinescent forms that persist, deterring herbivores even after leaf fall.^[16] The varying persistence of stipules influences biological processes, including resource allocation in response to seasonal environmental pressures; caducous or deciduous forms enable plants to reallocate nutrients efficiently during dormancy or stress, while persistent types support prolonged adaptations in stable or perennial growth strategies.

Based on Form and Position

Stipules exhibit considerable morphological diversity in their form and position relative to the petiole and stem, reflecting adaptations to various ecological niches across plant taxa. These variations include distinct attachment types and specialized shapes that influence their structural integration with the leaf. In dicotyledonous plants, stipules typically occur as paired structures at the petiole base.^[17] Lateral free stipules are independent outgrowths positioned on either side of the petiole base without fusion to it, often appearing as small, separate appendages. This form is exemplified in Hibiscus species (Malvaceae), where the stipules are tiny and freely attached, contributing to the leaf's basal symmetry.^[17] Adnate stipules, in contrast, are fused to the base of the petiole, creating a continuous winged or sheathing extension along its margins. In the Polygonaceae family, such as Polygonum species, the stipules are adnate and connate around the stem, forming a characteristic membranous structure known as an ochrea that encircles the node.^[11] Interpetiolar stipules arise from the fusion of stipules from adjacent opposite leaves, positioning them between the petioles rather than directly on a single petiole. This configuration is a defining feature in the Rubiaceae family, as seen in genera like Mussaenda and Gardenia, where the small, lanceolate stipules develop intrapetiolary or interpetiolary, enhancing nodal protection.^[17]^[3] Beyond these attachment-based forms, stipules display specialized shapes adapted for specific functions. Foliaceous stipules are expanded and leaf-like, enabling photosynthesis through their green, flattened blades, as observed in Lathyrus aphaca and Pisum sativum (Fabaceae). Spiny stipules modify into sharp, defensive thorns, deterring herbivores; for instance, in Prosopis species (Fabaceae), the paired stipular spines measure 0.5 to 3.5 cm and are straight, providing mechanical protection. Glandular stipules feature secretory structures that produce resins or nectar, such as in Euphorbia species, where glands at the stipule bases aid in defense or attraction. Tendrillar stipules elongate into coiled tendrils for climbing support, exemplified by Smilax (Smilacaceae), facilitating attachment to substrates.^[17]^[18]^[19]^[17]

Functions and Adaptations

Protective and Structural Roles

Stipules often enclose and shield developing buds and young leaves from environmental stresses such as desiccation and herbivory, particularly in their caducous form where they abscise shortly after leaf expansion. In many angiosperms, these early-formed structures wrap around the apical meristem and nascent foliage, providing a physical barrier that reduces water loss and deters small herbivores during vulnerable growth stages. For instance, in temperate plants like those in the Rosaceae family, caducous stipules offer transient protection before shedding, allowing the mature leaf to emerge unimpeded.^[3]^[20] In woody species, stipules contribute to structural support by functioning as bud scales or protective sheaths. Modified into membranous, scale-like coverings, they encase terminal and axillary buds during dormancy, safeguarding against mechanical damage and extreme weather while the plant invests resources in overwintering. Examples include Magnolia and Ficus species, where paired stipules fully ensheath the bud, deriving vascular support from lateral traces to maintain integrity. Additionally, in plants like Rosa, adnate stipules integrate with the petiole base to form stabilizing sheaths that reinforce the leaf attachment and prevent petiole buckling under wind or weight.^[11]^[17] Defensive adaptations of stipules enhance mechanical deterrence against grazing animals, with spiny forms prominent in arid-adapted species. In Acacia, stipules evolve into paired, rigid spines that physically impede large herbivores, reducing browse damage. Glandular stipules, often bearing colleters, secrete resins, mucilage, and compounds like terpenoids and tannins that act as irritants or adhesives, trapping or repelling insects and microbial pathogens while forming a barrier against desiccation. Such secretions in Rubiaceae species like Pentas lanceolata provide layered defense, with iridoids and phenolics concentrated in colleter tips for targeted release.^[21]^[22] In climbing plants, tendrillar stipules facilitate attachment and ascent by coiling around supports, exemplifying structural adaptation for habitat exploitation. In Smilax aspera, stipules modify into sensitive, filiform tendrils derived from leaf sheaths, capable of extending up to 91 cm to grasp substrates via helical growth and epicuticular prickles, enabling the vine to reach light in dense vegetation. This mechanism underscores stipules' role in mechanical stability without relying on stem twining alone.^[23]

Physiological and Ecological Roles

In foliaceous stipules, photosynthetic activity contributes significantly to the plant's early carbon fixation and overall energy production, particularly during leaf development or in species with reduced leaf blades. For instance, in the papilionoid legume Lathyrus aphaca, stipules function as the primary photosynthetic organs, possessing stomata, palisade and spongy mesophyll tissues, and vascular bundles analogous to those in leaves; they account for over 95% of photosynthate production at each node in adult plants, while the tendril-like leaves contribute less than 5%.^[24] Similarly, in pea (Pisum sativum), stipules support a functional photosynthetic electron transport chain, enabling non-photochemical quenching and redox regulation, although their CO₂ fixation capacity is lower than that of leaflets due to anatomical constraints. This photosynthetic role is especially vital in shaded or early growth stages, supplementing the plant's metabolic demands before full leaf expansion. Glandular stipules facilitate nutrient-related processes through storage and secretion, enhancing plant resilience and interactions with biotic agents. These structures can temporarily store metabolic reserves, such as sugars or proteins, to support rapid growth or stress responses, though specific quantification varies by species. More prominently, they secrete nectar or other compounds to attract mutualistic organisms; in broad bean (Vicia faba), stipular extrafloral nectaries produce sugar-rich nectar via specialized trichomes, drawing predatory insects that reduce herbivory and improve plant fitness in agricultural settings. This secretion mechanism underscores stipules' biochemical contributions to plant defense and nutrient cycling, without relying on floral pollinators. Ecologically, stipules mediate key interactions that influence community dynamics and plant survival. In ant-plant mutualisms, they often host or produce rewards for symbiotic ants, fostering protection against herbivores and competitors. For example, in myrmecophytic Piper species such as P. cenocladum, small, lipid-rich food bodies are generated within domatia formed by petiole bases and leaf sheaths; these nourish the ant Pheidole bicornis, which aggressively defends the plant, with food body production tightly regulated by ant presence to optimize the symbiosis. Such interactions highlight stipules' role in nutrient exchange and ecosystem signaling, promoting plant establishment in tropical understories. In many taxa, stipules occur as vestigial structures, reduced in size and function, illustrating evolutionary trade-offs where energetic costs of elaboration exceed adaptive benefits in open or specialized habitats. These remnants, often mere scales or scars, persist from ancestral forms but contribute minimally to physiology or ecology. Within the legume tribe Indigofereae, for instance, numerous species exhibit inconspicuous and non-photosynthetic stipules, reflecting a shift toward reliance on other leaf modifications for survival amid varying selective pressures. This reduction underscores stipules' plasticity across lineages, balancing historical constraints with contemporary environmental demands.^[25]

Evolutionary and Taxonomic Perspectives

Evolutionary Origins and Development

Stipules originate developmentally from the margins of leaf primordia, particularly within the lower leaf zone of eudicots, where they form as paired lateral outgrowths at the petiole base.^[26] This outgrowth is regulated by the antagonistic interaction between class I KNOX genes, which maintain meristematic activity and promote marginal blastozone formation, and ARP genes (such as AS1 in Arabidopsis), which repress KNOX expression to enforce leaf polarity and differentiation.^[27] In species exhibiting compound leaves, ectopic or prolonged KNOX expression in primordia margins enables the development of complex structures, including stipule-like appendages, by delaying differentiation and sustaining growth potential.^[27] Evolutionary hypotheses posit that stipules derived from marginal tissues of proto-leaves in early vascular plants, representing an adaptation for basal protection and structural support during leaf unfolding.^[28] These structures are considered homologous to basal leaflets in compound leaves, with historical analyses suggesting that lateral leaflets in taxa like walnut (Juglans) and hickory (Carya) evolved through modification of ancestral stipular tissues.^[27] This homology underscores a shared developmental pathway involving marginal blastozones, where small clusters of founder cells (1–4 per outgrowth) initiate laminar expansion akin to leaflet formation.^[27] Genetic models further elucidate stipules' integration with axillary meristem formation, as their proximal positioning in leaf axils facilitates hormone-mediated signaling, including auxin maxima that suppress meristem initiation until stipule maturation provides a niche for stem cell establishment.^[29] In monocots, stipule loss correlates with evolutionary shifts toward parallel venation, driven by expanded sheathing leaf bases that homologize to lateral stipular regions and alter KNOX expression patterns to encircle the shoot apical meristem, promoting radialized proximal growth over discrete appendages.^[26] This transition reflects broader phylogenetic modifications in leaf zonation, where ARP-mediated repression adapts to support uniform venation without stipular outgrowths.^[26]

Distribution Across Plant Taxa

Stipules are a characteristic feature of many eudicot families within angiosperms, where they occur ubiquitously at the base of petioles, often as paired, leaf-like appendages. For instance, in Rosaceae, stipules are generally present and considered a primitive trait, free or fused to the petiole base and persistent or deciduous depending on the genus.^[30] Similarly, Fabaceae exhibit compound leaves with well-defined stipules, which may be modified into glands or spines in some taxa, contributing to the family's recognition across diverse habitats.^[31] Overall, approximately one-third of angiosperm families possess stipules, with the highest diversity concentrated in the rosid clade, reflecting an ancestral presence in this group while absent in the common angiosperm ancestor.^[32] In basal angiosperms, stipule occurrence is more variable and generally limited, absent in the most primitive lineages such as Amborellales, Nymphaeales, and Austrobaileyales, but present in some Magnoliales (e.g., certain Degeneriaceae and Himantandraceae). This patchy distribution highlights early evolutionary experimentation before their proliferation in core eudicots. In contrast, stipules are absent in most monocots, including the Poaceae (grasses), where leaf sheaths and ligules have evolved as alternative basal structures, representing a derived loss associated with the clade's adaptation to grassy habitats.^[33] Among non-angiosperm groups, stipules are rare in gymnosperms, though some cycads (e.g., Zamia and Ceratozamia) display well-developed stipule-like outgrowths or winged bases at the leaf and cataphyll attachments, potentially homologous to angiosperm stipules in function if not origin.^[34] In ferns, stipules are present in the Marattiales, including the extant family Marattiaceae and the extinct Psaroniaceae, providing protective roles similar to stipules in seed plants. Recent studies (as of 2025) suggest that stipule-like structures evolved in Permo-Carboniferous ferns of the Psaroniaceae, long predating their widespread occurrence in angiosperms, indicating multiple evolutionary origins.^[35] Phylogenetic patterns link stipule presence to specific node anatomies, particularly the trilacunar condition—characterized by three leaf gaps—predominant in stipule-bearing families, while unilacunar nodes correlate with exstipulate taxa, as established in early comparative studies of dicotyledonous vascular patterns.^[36] These studies highlight ongoing research gaps in tracing regulatory networks for stipule absence across monocot lineages.

References

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Stipule originates from Latin stipula (stalk, straw), dating to 1793, meaning a small appendage at a leaf's petiole base.
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Bract: A reduced leaf or leaflike structure at the base of a flower or flower cluster. ... Stipules: A pair of leaflike structures found at the base of a leaf.
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Bract – Specialized, often highly colored leaf ... Tendril – Modified leaf, stipule, or other plant part used for climbing or as an attachment mechanism.
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