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Ovule

An ovule is a female reproductive structure in seed plants that develops into a seed after fertilization, serving as the site for the female gametophyte and embryo formation.^[1] It consists of a central nucellus (megasporangium) surrounded by one or more protective integuments, attached to the placenta by a funiculus, with a micropyle providing access for pollen tubes.^[2] In angiosperms, ovules are typically enclosed within the ovary of the flower and exhibit diverse orientations, such as anatropous (curved toward the placenta) or orthotropous (straight), with most featuring two integuments (bitegmic).^[1] The nucellus houses the megasporocyte, which undergoes meiosis to produce megaspores; one develops into the embryo sac containing the egg cell, synergids, central cell, and antipodals.^[2] Upon double fertilization—one sperm uniting with the egg to form the zygote and another with the central cell to produce endosperm—the ovule matures into a seed, with integuments forming the seed coat.^[2] Ovules trace their evolutionary origins to early seed plants around 400 million years ago, evolving from gymnosperm precursors with adaptations like bitegmy and curvature enhancing protection and pollination efficiency in angiosperms.^[1] Diversity in ovule morphology, including nucellus thickness (crassinucellar or tenuinucellar) and integument number (unitegmic in some lineages), reflects phylogenetic trends and ecological adaptations across seed plant groups.^[1]

Overview

Definition and Characteristics

In seed plants, the ovule is defined as a megasporangium enclosed by one or more protective integuments, within which the nucellus houses the developing female gametophyte that produces the egg cell; following fertilization, this structure matures into a seed.^[3] The ovule represents a key synapomorphy of seed plants, distinguishing them from earlier vascular plants by providing enclosure and protection for the female reproductive process.^[3] Key characteristics of the ovule include its typical possession of one or two integuments that surround the nucellus—a mass of diploid maternal tissue—and form a small opening called the micropyle, through which pollen can access the interior.^[4] Ovules arise from placental tissue, a meristematic region within the reproductive organ, ensuring their attachment and nutrient supply during development.^[5] These features set ovules apart from other plant reproductive structures, such as free spores or pollen grains, by their integumentary protection and role in containing the haploid female gametophyte within a diploid sporophytic framework.^[6] The basic anatomy of the ovule, including its integuments and internal tissues, was first systematically described by the English botanist Nehemiah Grew in his 1672 work The Anatomy of Vegetables Begun, where he observed microscopic details such as the micropyle in young seeds using early compound microscopes.^[7] This foundational observation laid the groundwork for later understandings of ovule structure across gymnosperms and angiosperms.^[7]

Role in Reproduction

The ovule serves as the primary site for female gamete production and fertilization in seed plants, housing the development of the megagametophyte, which contains the egg cell and central cell essential for sexual reproduction.^[8] In angiosperms, the ovule facilitates double fertilization, a unique process where two sperm cells from the pollen tube participate: one fuses with the egg cell to form the diploid zygote that develops into the embryo, while the other fuses with the central cell to produce the triploid endosperm, providing nourishment for the embryo.^[9] This mechanism ensures efficient resource allocation, as endosperm development is triggered only upon successful egg fertilization.^[10] Post-fertilization, the ovule's tissues encase and protect the developing embryo, transforming into the seed coat to shield it from environmental stresses.^[11] The evolutionary significance of the ovule lies in its role in enabling seed plants to thrive in terrestrial environments by decoupling reproduction from water dependence. Unlike earlier plant groups, ovules allow for pollen-mediated sperm delivery, eliminating the need for free-swimming sperm and permitting fertilization in dry conditions, which facilitated the colonization of diverse habitats and the dominance of seed plants over non-seed lineages.^[12] Additionally, ovules support embryo dormancy within protective structures, enhancing survival during adverse periods and enabling long-distance dispersal via seeds, a key adaptation that contributed to the ecological success of gymnosperms and angiosperms.^[13] In contrast to non-seed plants such as ferns and mosses, where female gametophytes are free-living and vulnerable to desiccation due to external exposure and reliance on water for sperm motility, ovules in seed plants enclose and nourish the reduced female gametophyte internally, minimizing water loss and predation risks.^[2] This internalized protection represents a pivotal evolutionary innovation that reduced desiccation threats and supported the transition to fully terrestrial reproduction.^[14]

Location and Arrangement

In Flowering Plants

In flowering plants, ovules are located within the ovary of the carpel, the female reproductive organ, where they are attached to the placenta, a specialized tissue on the inner ovary wall that nourishes and anchors them.^[1] This positioning integrates ovules into the flower's reproductive system, allowing pollen tubes to reach them post-pollination for fertilization. The number of ovules per ovary varies widely among species; for instance, mango (Mangifera indica) typically has a single ovule, while tomato (Solanum lycopersicum) features hundreds, influencing potential seed yield.^[15]^[16] Ovules arise developmentally from meristematic tissue on the placenta during early flower initiation, emerging laterally as primordia that differentiate into mature structures.^[15] Their arrangement, known as placentation, follows several patterns that determine spatial organization within the ovary. In parietal placentation, ovules attach directly to the ovary wall in a unilocular or multilocular ovary, as seen in mustard (Brassica). Axile placentation positions ovules along a central axis in a multilocular ovary, common in tomato. Free central placentation features ovules on a free-standing central column without septa, exemplified by carnations (Dianthus). Basal placentation confines ovules to the ovary base, as in sunflowers (Helianthus).^[1]^[1] These placentation types significantly affect fruit and seed development by dictating seed distribution and dispersal mechanisms; for example, axile placentation in tomato leads to centrally clustered seeds in the mature berry, enhancing uniform ripening and seed packing.^[1] The enclosing ovary further protects ovules from desiccation and herbivores until fertilization.^[15]

In Non-Flowering Seed Plants

In non-flowering seed plants, collectively known as gymnosperms, ovules are located on specialized structures called megasporophylls or modified scales, which are arranged within ovulate cones or strobili, and they remain exposed without enclosure by an ovary, unlike the protected position within carpels in flowering plants.^[17] In conifers such as Pinus species, ovules are borne exposed on the adaxial (upper) surface of ovuliferous scales that form part of the megasporophylls in female cones, with typically two ovules per scale positioned for direct wind pollination.^[18] By contrast, in Ginkgo biloba, ovules develop in pairs at the tips of elongated stalks (peduncles) arising from leaf axils on short shoots, providing a degree of enclosure by surrounding tissues during maturation, though they lack a compact cone structure.^[19] The arrangement of ovules in gymnosperms varies by group but generally occurs in compact ovulate cones or strobili that facilitate pollination and seed dispersal. In cycads, such as species of Cycas and Zamia, megasporophylls are loosely or tightly arranged in large, often massive cones, with multiple ovules (up to six pairs per megasporophyll) that develop into large, fleshy seeds attractive to animal dispersers.^[14] Conifers, including pines and firs, feature tightly packed ovulate cones where ovules on seed scales mature into smaller, often winged seeds adapted for wind dispersal, enhancing their spread across diverse habitats.^[20] Evolutionarily, the ovules of modern gymnosperms originated from heterosporous megasporangia in progymnosperms during the Late Devonian, with the earliest known ovules appearing in primitive seed plants such as Elkinsia around 360 million years ago. These structures, often on fern-like fronds and partially enclosed by cupules, represented a key transitional stage toward fully enclosed seeds that provided enhanced protection and nutrition for embryos.^[21] This shift marked a key innovation in seed plant evolution, allowing survival in terrestrial environments without reliance on water for fertilization.^[21]

External Structure

Integuments and Associated Features

The integuments of an ovule consist of one or more layers of sterile tissue that enclose and protect the nucellus, originating from the dermal layer of the ovule primordium through periclinal divisions in the epidermal cells.^[22] In gymnosperms, ovules are typically unitegmic, featuring a single integument that surrounds the nucellus and contributes to the protective seed coat after fertilization.^[23] Angiosperms, by contrast, possess bitegmic ovules with two integuments—an inner one homologous to the gymnosperm integument and an outer one that enhances enclosure and curvature.^[1] These integuments develop from the chalazal region of the ovule and provide mechanical protection against desiccation and pathogens while facilitating the transformation into the seed coat post-fertilization.^[1] The micropyle represents a critical opening at the apex of the ovule, formed by the incomplete enclosure of the nucellus by the integument(s), which allows the pollen tube to enter during fertilization.^[1] In gymnosperms, the single integument creates a simple micropylar canal that also serves for pollen capture via a pollination drop.^[23] Angiosperm micropyles vary: endostomic types form solely from the inner integument, amphistomic from both, and exostomic rarely from the outer alone, with the configuration influencing pollen tube guidance and ovule orientation.^[1] This pore ensures targeted delivery of male gametes to the female gametophyte while minimizing exposure.^[1] In anatropous or curved ovules, common in many angiosperms, the funicle attaches along the ovule's side, forming a raphe—a ridge-like structure through which the vascular bundle extends from the funicle to the chalaza. The raphe arises from the adnate fusion of the funicle and outer integument, providing structural support and nutrient conduction without altering the primary protective role of the integuments.^[1] This feature is absent in orthotropous ovules but enhances stability in inverted orientations.^[24]

Hilum and Funicle

The funicle, or funiculus, is a stalk-like structure that attaches the ovule to the placenta on the inner wall of the ovary in angiosperms. It consists of a multicellular filament containing one or more vascular bundles composed of xylem and phloem, which transport water, minerals, and organic nutrients from the parent plant to support ovule development and maturation. This vascular supply is essential for sustaining the energy demands of megasporogenesis and female gametophyte formation within the ovule.^[25] The hilum represents the junction where the funicle merges with the ovule body, typically near the chalazal region. In the mature seed, the hilum manifests as a distinct scar on the seed coat (testa), indicating the former attachment site and remnants of vascular tissue that facilitated nutrient influx during embryogenesis. This scar serves as a key morphological marker for seed identification and is critical for post-fertilization nutrient exchange between the seed and the developing fruit. Variations in funicle and hilum morphology occur across plant species, reflecting adaptations to ovule orientation and environmental pressures. In sessile ovules, the funicle is greatly shortened or absent, enabling direct placental attachment and minimal vascular extension. Conversely, pendulous ovules feature an elongated funicle, which suspends the ovule within the ovarian locule and may incorporate multiple or twisted vascular bundles for enhanced transport efficiency. In curved ovule types, such as anatropous forms common in angiosperms, the funicle aids in the inversion of the ovule body relative to the micropyle. Vascular configurations in the funicle, including collateral or amphicribral arrangements, further diversify to optimize nutrient delivery in families like Leguminosae.^[26]^[25]

Internal Structure

Nucellus and Megasporangium

The nucellus constitutes the central, multi-layered tissue within the ovule, derived from the sporophyte and maintaining a diploid (2n) chromosome complement. It forms as part of the ovule primordium and typically consists of several cell layers that surround and enclose the megaspore mother cell, providing structural support during early developmental stages.^[1] This tissue is essential for housing reproductive processes and varies in thickness across plant groups, such as being more robust (crassinucellar) in basal angiosperms like Magnoliids compared to thinner (tenuinucellar) forms in derived groups like Asterids.^[1] Functionally, the nucellus serves as the megasporangium, an indehiscent structure where the diploid megaspore mother cell undergoes meiosis to produce four haploid megaspores, one of which typically develops further.^[1] Unlike dehiscent sporangia in ferns, the nucellus retains the megaspores internally, facilitating the subsequent formation of the female gametophyte without dispersal.^[27] This role underscores its evolutionary adaptation in seed plants for protected spore production. In addition to its sporogenic function, the nucellus acts as a nutritive tissue, supplying essential nutrients and substances to the developing female gametophyte through direct cellular contact or specialized layers like the endothelium in certain ovules.^[1] In some angiosperm seeds, particularly within the Caryophyllales, the nucellus persists post-fertilization as perisperm, a starchy storage tissue that accumulates reserves from the maternal sporophyte; for example, in sugar beet (Beta vulgaris), the undigested nucellus forms the central perisperm, serving as a primary food reserve for the embryo.^[28]^[29]

Megaspore and Functional Megaspore

Megasporogenesis is the process by which a diploid megaspore mother cell, located within the nucellus of the ovule, undergoes meiosis to produce a tetrad of four haploid megaspores.^[30] This reduction division halves the chromosome number, ensuring that the resulting megaspores are haploid and capable of giving rise to a haploid female gametophyte.^[31] The meiotic divisions typically occur in a linear sequence, but the arrangement of the tetrad can vary across seed plants, including linear, T-shaped, or tetrahedral configurations depending on the orientation of the meiotic spindles.^[32] These variations are observed in both angiosperms and gymnosperms, with linear tetrads being most common in many angiosperm species, while T-shaped and tetrahedral forms appear in specific taxa such as certain gymnosperms like Taxus.^[33] Among the four megaspores in the tetrad, typically only one survives to become the functional megaspore, while the other three degenerate.^[30] In most cases, the chalazal-most megaspore—the one farthest from the micropyle and closest to the chalaza—is selected as the functional one due to its advantageous position for nutrient uptake from the surrounding nucellus.^[34] This selection process ensures that the surviving haploid megaspore can proceed to develop into the female gametophyte, maintaining the genetic reduction necessary for sexual reproduction in seed plants.^[35] The degeneration of the micropylar megaspores often involves programmed cell death, preventing competition and conserving resources within the ovule.^[36]

Female Gametophyte Development

Formation of the Embryo Sac

The formation of the embryo sac, also known as megagametogenesis, begins with the functional megaspore in angiosperms and involves a series of mitotic divisions that develop the female gametophyte within the ovule.^[30] In the most common pattern, the Polygonum type or monosporic development, which occurs in approximately 70% of angiosperm species, the process starts immediately after meiosis when the chalazal megaspore survives and the others degenerate.^[37] The functional megaspore first undergoes two rounds of mitosis without cytokinesis, resulting in a binucleate stage followed by a tetranucleate coenocyte during the free nuclear phase.^[30] This coenocytic stage features free nuclei divided by a large central vacuole, with two nuclei migrating to the micropylar pole and two to the chalazal pole.^[38] A third mitosis then produces eight nuclei, after which cellularization occurs through the formation of cell walls, yielding the mature seven-celled, eight-nucleate embryo sac.^[30] During cellularization, the nuclei organize into specific domains: at the micropylar end, the egg apparatus forms, consisting of one egg cell and two synergid cells; centrally, the two polar nuclei define the binucleate central cell; and at the chalazal end, three antipodal cells develop.^[39] The polar nuclei in the central cell often fuse to form a secondary nucleus before fertilization, though this varies slightly among species.^[30] Throughout development, the embryo sac depends on the surrounding nucellus for nutrients and structural support, with antipodal cells sometimes enlarging to facilitate nutrient transfer in certain taxa.^[38] This process is highly conserved in angiosperms but contrasts with gymnosperm variations, where multiple archegonia may form instead of a single embryo sac.^[40]

Cellular Organization in Angiosperms

In angiosperms, the mature embryo sac typically exhibits a cellular organization consisting of seven cells and eight nuclei, known as the Polygonum-type, which is the most common configuration. This structure includes the egg apparatus at the micropylar end, comprising the egg cell and two synergid cells, as well as three antipodal cells at the chalazal end and a large central cell containing two polar nuclei. The egg cell is positioned adjacent to the synergids, featuring a polarized structure with a prominent vacuole and its nucleus located toward the chalazal side. The synergid cells play crucial roles in facilitating fertilization by secreting chemical attractants that guide the pollen tube toward the embryo sac and by controlling the pollen tube's arrest and discharge of sperm cells.^[41] The antipodal cells, often highly active and sometimes proliferating, are involved in nutrient absorption and transport from surrounding nucellar tissue to support the developing embryo sac.^[42] The central cell serves as the precursor to the endosperm, where its two polar nuclei fuse with one sperm nucleus during double fertilization to form the triploid endosperm that nourishes the embryo.^[43] While the Polygonum-type dominates, rarer bisporic and tetrasporic embryo sacs occur in certain angiosperm lineages, characterized by modified nuclear arrangements derived from two or four megaspores, respectively, leading to variations such as shared cytoplasm or altered cell numbers without the standard three mitoses.^[44] For instance, bisporic types like the Allium pattern involve two contributing megaspores with three free nuclear divisions, while tetrasporic types, such as the Adoxa pattern, incorporate all four megaspores and feature two mitotic divisions. These atypical organizations highlight evolutionary diversity in female gametophyte development among angiosperms.^[44]

Ovule Development and Maturation

Pre-Fertilization Stages

During ovule maturation in angiosperms, the integuments undergo progressive thickening to form protective layers around the nucellus, typically consisting of 2-3 cell layers for the inner integument and varying thickness for the outer, which can exceed two layers in groups like Magnoliids and certain monocots.^[45] This development ensures structural integrity while allowing space for internal gametophyte maturation. Concurrently, the micropyle, formed primarily by the inner integument (endostomic) or both integuments (amphistomic), opens as a narrow canal to facilitate pollen tube entry, though it may remain partially sealed by secretions in some species until pollination.^[45] The embryo sac matures alongside pollen development in the anthers, reaching a 7-celled, 8-nucleate stage in most angiosperms, with synergids producing attractants that prepare the ovule for sperm reception.^[46] Pollination initiates the entry phase, where germinated pollen tubes from the stigma traverse the style's transmitting tract before reaching the ovule's micropyle.^[46] Upon arrival, the pollen tube navigates the micropyle, guided toward the embryo sac's synergids, where it bursts to release the two sperm cells without penetrating the egg apparatus.^[46] This process is highly precise, often resulting in one-to-one pollen tube-ovule interactions to prevent polyspermy.^[47] Pre-fertilization guidance relies on chemotropism, where diffusible signals from the synergids and nucellus apex direct pollen tube growth.^[48] Key attractants include cysteine-rich peptides like LUREs secreted by synergids, which bind to receptors on the pollen tube tip to reorient growth toward the micropyle.^[49] Additional cues, such as regulated GABA levels and FERONIA-dependent signaling from the ovule's outer integument, enhance attraction and adhesion along the funiculus before micropylar entry.^[50] These mechanisms ensure efficient sperm delivery, with pollen tube progression typically completing within hours to days post-pollination depending on the species.^[51]

Post-Fertilization Changes

Following double fertilization in angiosperms, one sperm cell fuses with the egg cell to form a diploid zygote, which undergoes mitotic divisions to develop into the embryo, while the second sperm cell fuses with the central cell to produce a triploid primary endosperm nucleus that proliferates into the nutritive endosperm tissue.^[9] This process, unique to angiosperms, ensures coordinated development of both embryonic and storage tissues within the ovule.^[52] As the embryo and endosperm develop, the ovule undergoes structural transformations to form the seed; the diploid maternal integuments differentiate and lignify to create the protective seed coat, consisting of an outer testa and inner tegmen in species with two integuments.^[9] The nucellus, the central tissue surrounding the embryo sac, often partially or fully degenerates but may persist as perisperm in certain angiosperms, such as those in the Caryophyllales, providing additional nutrient reserves.^[53] Post-fertilization, the accessory cells of the embryo sac degenerate to support seed maturation; the two synergids, which guide the pollen tube, undergo programmed cell death triggered by pollen tube arrival and discharge, ensuring no further fertilization attempts.^[54] Similarly, the three antipodal cells at the chalazal end break down shortly after fertilization, often through autophagic processes, freeing space for endosperm expansion.^[9] In some taxa, such as Sapindaceae, the funicle enlarges post-fertilization to form an aril, a fleshy outgrowth that aids seed dispersal by attracting animals.^[55]

Variations Across Plant Groups

Gymnosperm Ovules

Gymnosperm ovules are typically unitegmic, featuring a single integument that forms a protective layer around the nucellus, distinguishing them from the bitegmic structure common in many angiosperms.^[23] However, Gnetales, such as Gnetum and Ephedra, possess bitegmic ovules.^[56] This single integument often develops into a fleshy or woody seed coat post-fertilization, while the ovule itself remains exposed on megasporophylls or cone scales rather than being enclosed in an ovary.^[17] A prominent feature is the large, multicellular female gametophyte, which develops within the ovule and serves as the primary nutritive tissue for the embryo, replacing the endosperm found in angiosperms.^[17] This gametophyte contains multiple archegonia, each housing an egg cell, allowing for potential polyspermy or multiple fertilization events in some species.^[17] The development of the female gametophyte in gymnosperms begins with meiosis in the megaspore mother cell within the nucellus, producing four megaspores, of which typically one survives and undergoes free nuclear divisions.^[57] These divisions can yield thousands of nuclei—often 2,000 to 6,000 in conifers—without immediate cell wall formation, creating a coenocytic stage that expands the gametophyte volume.^[58] Cellularization follows, forming a multicellular structure with distinct tissue regions, including a nutritive corpus and peripheral layers, after which archegonia differentiate at the micropylar end.^[57] This process contrasts with the more compact, cellularized embryo sac in angiosperms, emphasizing the gymnosperm gametophyte's role in provisioning resources directly.^[58] In conifers, such as pines and spruces, ovules are borne on the upper surface of ovuliferous scales within female cones, often accompanied by resin canals that secrete protective resins to deter herbivores and pathogens.^[59] These canals run through the scale tissue, enhancing ovule defense during the extended maturation period. In cycads, like Cycas species, ovules are large and borne in pairs on modified leaves, featuring pollination drops exuded from the micropyle to capture pollen; fertilization involves motile, multiflagellated sperm delivered via pollen tubes, a primitive trait retained from earlier seed plants.^[60]^[61]

Angiosperm Ovule Types

In angiosperms, ovules exhibit diverse orientations and curvatures determined primarily by the configuration of the integuments and funicle, which influence their position relative to the placenta and the path of pollen tube growth. These variations are classified into several main types based on the degree of bending or inversion of the ovule body.^[1] The orthotropous ovule is characterized by a straight, upright orientation where the micropyle, chalaza, and funicle attachment (hilum) are aligned in a single axis, with no curvature present. This type maintains radial symmetry and is typical in more primitive or basal angiosperm lineages.^[1] In contrast, the anatropous ovule, the most prevalent type, features a complete 180-degree inversion of the ovule body, positioning the micropyle adjacent to the hilum and close to the placenta. This curvature arises from differential growth of the funicle and outer integument, resulting in the nucellus lying parallel to the funicle. Anatropous ovules predominate across angiosperm clades, occurring in the majority of families and considered the ancestral condition.^[1] The campylotropous ovule displays a partial curvature of the integuments, with the nucellus bent but the micropyle and chalaza remaining somewhat aligned, often leading to a zig-zag orientation of the micropyle. This type is more common in derived eudicot groups, such as those in Ranunculales and Fabales.^[1] Amphitropous ovules exhibit a more pronounced bend, where the ovule body forms nearly a right angle with the funicle, and the embryo sac adopts a horseshoe shape due to the curvature of both integuments and nucellus. This configuration is relatively rare and occurs sporadically in certain monocot and eudicot families, such as Alismataceae.^[1] A variant, hemianatropous (or hemitropous), represents an intermediate form with partial inversion, where the ovule bends about 90 degrees; it is observed in legumes (Fabaceae), contributing to compact seed arrangements.^[1] These ovule orientations have functional implications, particularly in pollination and seed development. The anatropous form aligns the micropyle toward the base of the ovary, facilitating direct entry of the pollen tube from the style and stigma for efficient fertilization. Such curvatures also influence post-fertilization seed shape, with anatropous ovules often yielding elongated or curved seeds due to the raphe formation along the funicle. The funicle plays a key role in mediating these curvatures through its elongation and attachment.^[1]

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Within the Ovule of Taxus baccata L. Megasporogenesis in Taxus led to a T-shaped tetrad, the mitochondria and plastids being largel to the chalazal megaspore, ...
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In living Gymnosperms s.l. megasporogenesis takes pIace in ovules which generalIy have a crassinucellate conformation; in fact a more or less developed ...
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The embryo sac is a structure within the ovule, formed from a megaspore mother cell, containing one egg cell, two synergids, three antipodals, and a central ...
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Embryo sacs are classified as monosporic (one megaspore), bisporic (two), or tetrasporic (four). Monosporic types can be 16, 8, or 4 nucleate.
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Arils are accessory seed structures present in both gymnosperms and angiosperms, being important for seed dispersal, and might possess economic importance.
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Pollination drops are produced by nucellar and/or adjacent secretory tissues within a chamber located at the archegonial pole, which typically is the exposed ...
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Two methods of sperm delivery occur in gymnosperms: zooidogamy, defined by pollen tubes with motile sperm as exhibited in cycads and Ginkgo, and siphonogamy ...