Sexual Reproduction in Flowering Plants

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1. Flower: A Reproductive Organ Definition: Modified stem meant for sexual reproduction. Parts of a typical flower: Calyx (Sepals): Outermost whorl, green, protective. Corolla (Petals): Brightly colored, attracts pollinators. Androecium (Stamens): Male reproductive part. Gynoecium/Pistil (Carpels): Female reproductive part. 2. Pre-fertilisation Events 2.1. Androecium (Male Reproductive Whorl) Consists of stamens . Each stamen has: Filament: Stalk. Anther: Bilobed, dithecous structure (two lobes, each with two theca). Microsporangium (Pollen Sac): Located inside anther lobes. Develops into pollen sacs. Walls: Epidermis, Endothecium, Middle layers, Tapetum (innermost, nourishes developing pollen). Microsporogenesis: Formation of microspores from PMC (Pollen Mother Cell) via meiosis. PMCs are diploid ($2n$). Form haploid ($n$) microspore tetrads. Pollen Grain (Male Gametophyte): Haploid, unicellular structure. Exine: Outer layer, made of sporopollenin (most resistant organic material). Has germ pores. Intine: Inner layer, thin, continuous, made of pectin and cellulose. Cytoplasm: Contains two cells: Vegetative cell: Larger, contains food reserve, irregular nucleus. Generative cell: Smaller, floats in vegetative cell cytoplasm, produces two male gametes. 2.2. Gynoecium (Female Reproductive Whorl) Consists of carpels/pistils . Each carpel has: Stigma: Receptive surface for pollen. Style: Elongated tube connecting stigma to ovary. Ovary: Basal swollen part containing ovules. Ovule (Megasporangium): Attached to placenta via funicle. Integuments: Protective coverings. Micropyle: Small opening at the tip of integuments. Chalaza: Opposite to micropyle, basal part of ovule. Nucellus: Mass of nutritive cells, enclosed by integuments. Megasporogenesis: Formation of megaspores from MMC (Megaspore Mother Cell) via meiosis. MMC is diploid ($2n$). Usually, one MMC in nucellus. Forms four haploid ($n$) megaspores, typically three degenerate, one functional. Embryo Sac (Female Gametophyte): Developed from functional megaspore. Undergoes three mitotic divisions without cytokinesis. Result: 8 nucleate, 7-celled structure. Egg apparatus: 3 cells at micropylar end (1 egg cell, 2 synergids). Synergids have filiform apparatus. Antipodals: 3 cells at chalazal end. Central cell: Large central cell with 2 polar nuclei (fuses to form secondary nucleus $2n$ before fertilisation). 3. Pollination Definition: Transfer of pollen grains from anther to stigma. Types: Autogamy (Self-pollination): Pollen from same flower (e.g., wheat, rice). Cleistogamous flowers: Never open, always self-pollinated (e.g., Viola, Oxalis). Geitonogamy: Pollen from different flower on same plant (genetically self-pollination, ecologically cross-pollination). Xenogamy (Cross-pollination): Pollen from different flower on different plant of same species. Agents of Pollination: Abiotic: Wind (anemophily), Water (hydrophily). Biotic: Animals (zoophily) - insects (entomophily), birds (ornithophily), bats (chiropterophily), etc. Outbreeding Devices: Mechanisms to discourage self-pollination and encourage cross-pollination. Dichogamy (anther and stigma mature at different times). Herkogamy (physical barrier between anther and stigma). Self-incompatibility (genetic mechanism preventing self-pollen germination). Unisexuality of flowers (dioecy). 4. Post-pollination Events 4.1. Pollen-Pistil Interaction Recognition of compatible pollen. Pollen germinates on stigma, forming a pollen tube. Pollen tube grows through the style towards the ovule, entering through the micropyle. Pollen tube discharges two male gametes into one of the synergids. 4.2. Double Fertilisation Unique to flowering plants. Syngamy (True Fertilisation): One male gamete ($n$) fuses with the egg cell ($n$) to form a diploid zygote ($2n$). Triple Fusion: The other male gamete ($n$) fuses with the diploid secondary nucleus ($2n$, formed by fusion of two polar nuclei) to form the triploid Primary Endosperm Nucleus (PEN, $3n$). 4.3. Post-fertilisation Changes Zygote develops into Embryo . Primary Endosperm Nucleus (PEN) develops into Endosperm (nutritive tissue). Types: Nuclear, Cellular, Helobial. Ovule develops into Seed . Ovary develops into Fruit . Ovary wall develops into Pericarp (fruit wall). Other floral parts (sepals, petals, stamens, style, stigma) usually wither and fall off. 5. Seed and Fruit Development 5.1. Embryo Development (Embryogeny) Occurs at the micropylar end of the embryo sac. Zygote $\rightarrow$ Proembryo $\rightarrow$ Globular $\rightarrow$ Heart-shaped $\rightarrow$ Mature embryo. Dicot Embryo: Embryonal axis, two cotyledons. Epicotyl: Above cotyledons, terminates in plumule (shoot tip). Hypocotyl: Below cotyledons, terminates in radicle (root tip), covered by root cap. Monocot Embryo: Single cotyledon (scutellum). Protective sheaths: Coleoptile (plumule), Coleorhiza (radicle). 5.2. Seed Fertilised ovule. Contains embryo, food reserve, protective seed coat. Non-albuminous (Exalbuminous) seeds: No residual endosperm (e.g., pea, groundnut). Food stored in cotyledons. Albuminous seeds: Retain a part of endosperm (e.g., wheat, maize, castor). Perisperm: Remnant of nucellus in some seeds (e.g., black pepper, beet). Dormancy: State of metabolic inactivity allowing seed survival through unfavorable conditions. 5.3. Fruit Mature or ripened ovary. True fruits: Develop only from the ovary (e.g., mango, tomato). False fruits (Accessory fruits): Develop from ovary along with other floral parts (e.g., apple, cashew, strawberry - thalamus contributes). Parthenocarpic fruits: Develop without fertilisation, hence seedless (e.g., banana). Can be induced by growth hormones. 6. Apomixis and Polyembryony Apomixis: Production of seeds without fertilisation (a form of asexual reproduction mimicking sexual reproduction). E.g., Asteraceae, grasses. Types: Recurrent (diploid embryo sac from MMC without meiosis, or from nucellar cell), Non-recurrent (haploid embryo sac from MMC by meiosis, egg develops parthenogenetically), Adventive embryony (embryo from nucellus/integuments), Apospory (embryo sac from nucellar cell). Polyembryony: Occurrence of more than one embryo in a seed. E.g., Citrus, mango. Can arise from different sources: more than one egg cell fertilised, multiple nucellar cells developing into embryos.