Sexual Repro. Flowering Plants

Cheatsheet Content

### Flower: A Fascinating Organ of Angiosperms - **Flower:** Reproductive part of angiosperms. - **Parts of a flower:** - **Calyx (Sepals):** Outermost whorl, green, protects bud. - **Corolla (Petals):** Brightly coloured, attracts pollinators. - **Androecium (Stamens):** Male reproductive part. - **Gynoecium/Pistil (Carpels):** Female reproductive part. ### Androecium: Stamen, Microsporangium, and Pollen Grain - **Stamen:** Consists of filament (stalk) and anther (bilobed structure). - **Anther:** Typically dithecous (two lobes), each lobe has two theca (chambers), thus four microsporangia. - **Microsporangium (Pollen Sac):** - **Structure:** Surrounded by 4 wall layers: Epidermis, Endothecium, Middle layers, Tapetum. - **Tapetum:** Innermost layer, nourishes developing pollen grains. - **Sporogenous tissue:** Homogenous cells occupying the center of microsporangium. - **Microsporogenesis:** Process of formation of microspores from a pollen mother cell (PMC) through meiosis. - PMC (2n) $\xrightarrow{Meiosis}$ Microspore tetrad (n) $\xrightarrow{Differentiation}$ Pollen grains (n). - **Pollen Grain (Male Gametophyte):** - **Structure:** Spherical, 2-layered wall. - **Exine:** Outer layer, hard, made of sporopollenin (most resistant organic material known), has germ pores. - **Intine:** Inner layer, thin, continuous, made of cellulose and pectin. - **Cytoplasm:** Contains a nucleus, surrounded by plasma membrane. - **Development:** Matures into a 2-celled stage (vegetative cell, generative cell) before shedding. - **Vegetative cell:** Larger, irregular nucleus, abundant food reserve. - **Generative cell:** Smaller, spindle-shaped, floats in vegetative cell cytoplasm. Divides mitotically to form two male gametes. ### Gynoecium: Pistil, Megasporangium, and Embryo Sac - **Pistil/Carpel:** Female reproductive unit. May be monocarpellary (one carpel) or multicarpellary (many carpels). - **Syncarpous:** Fused carpels. - **Apocarpous:** Free carpels. - **Parts of Pistil:** - **Ovary:** Basal swollen part, contains ovules. - **Style:** Elongated tube connecting ovary to stigma. - **Stigma:** Receptive surface for pollen grains. - **Ovule (Megasporangium):** - **Funicle:** Stalk attaching ovule to placenta. - **Hilum:** Junction between ovule and funicle. - **Integuments:** Protective envelopes, usually 1 or 2. - **Micropyle:** Small opening at the tip of integuments, allows entry of pollen tube. - **Chalaza:** Opposite to micropylar end, basal part of ovule. - **Nucellus:** Mass of parenchymatous cells, rich in food reserve, enclosed by integuments. - **Megasporogenesis:** Process of formation of megaspores from a megaspore mother cell (MMC) through meiosis. - MMC (2n) $\xrightarrow{Meiosis}$ 4 haploid megaspores. - Usually, only one functional megaspore (chalazal end) develops, others degenerate (micropylar end). - **Embryo Sac (Female Gametophyte):** - **Development:** Functional megaspore undergoes 3 successive mitotic divisions to form 8 nucleate, 7-celled embryo sac. - **Monosporic development:** From a single functional megaspore. - **Structure:** - **Egg apparatus (micropylar end):** 1 egg cell (female gamete) + 2 synergids. - **Synergids:** Have filiform apparatus (cellular thickenings at micropylar tip) which guides pollen tube. - **Antipodals (chalazal end):** 3 cells. - **Central cell:** Large, contains 2 polar nuclei (fuse to form diploid secondary nucleus). ### Pollination: Transfer of Pollen Grains - **Pollination:** Transfer of pollen grains from anther to stigma. - **Types of Pollination:** - **Autogamy (Self-pollination):** - **Geitonogamy:** Pollen from anther of one flower to stigma of another flower on the same plant. Genetically similar to autogamy. - **Cleistogamy:** Flowers never open, anthers and stigma lie close, always self-pollinated (e.g., *Viola*, *Oxalis*). Assured seed set. - **Chasmogamy:** Flowers open, exposed anthers and stigma. - **Allogamy (Cross-pollination/Xenogamy):** Pollen from anther of one flower to stigma of another flower on a different plant of the same species. Genetically distinct. - **Agents of Pollination:** - **Abiotic:** - **Wind (Anemophily):** Common in grasses. Pollen grains light, non-sticky, often winged. Stigma large, feathery. Flowers often with single ovule, numerous flowers packed into an inflorescence. - **Water (Hydrophily):** Rare, mostly in monocots. Pollen grains protected by mucilaginous covering. - **Epiohydrophily:** Pollen floats on water surface (e.g., *Vallisneria*). - **Hypohydrophily:** Pollen released inside water (e.g., *Zostera*). - **Biotic:** Animals (Zoophily) - Insects (Entomophily), Birds (Ornithophily), Bats (Chiropterophily), etc. - Flowers are large, colourful, fragrant, rich in nectar/pollen to attract pollinators. - Pollen grains are sticky. - **Outbreeding Devices:** Mechanisms to prevent self-pollination and encourage cross-pollination. - **Dichogamy:** Anther and stigma mature at different times (e.g., protandry - anthers mature earlier; protogyny - stigma matures earlier). - **Hermaogamy:** Anthers and stigma placed at different positions. - **Self-incompatibility:** Genetic mechanism preventing germination of pollen on stigma of the same flower/plant. - **Unisexuality:** Flowers are either male or female. - **Monoecious:** Both male and female flowers on the same plant (e.g., castor, maize). Prevents autogamy but not geitonogamy. - **Dioecious:** Male and female flowers on different plants (e.g., papaya). Prevents both autogamy and geitonogamy. ### Pollen-Pistil Interaction - All events from pollen deposition on stigma until pollen tube entry into ovule. - Pistil has ability to recognize compatible/incompatible pollen. - Compatible pollen germinates on stigma, producing a pollen tube through a germ pore. - Pollen tube grows through stigma and style, reaches ovary, enters ovule (usually through micropyle). - Filiform apparatus in synergids guides pollen tube entry into embryo sac. ### Double Fertilization - Unique to angiosperms. - Pollen tube releases two male gametes into embryo sac (via synergids). - **Syngamy (Generative fertilization):** One male gamete (n) fuses with egg cell (n) to form diploid zygote (2n). - **Triple Fusion (Vegetative fertilization):** Other male gamete (n) fuses with diploid secondary nucleus (2n) of central cell to form triploid Primary Endosperm Nucleus (PEN, 3n). - **Post-fertilization Events:** Events after double fertilization. ### Post-Fertilization Events: Development of Seed and Fruit - **Endosperm Development:** - PEN divides repeatedly to form endosperm (3n), a nutritive tissue for developing embryo. - **Types:** - **Free nuclear endosperm:** Nuclei divide, but cell wall formation does not occur immediately (e.g., coconut water). - **Cellular endosperm:** Cell wall formation occurs after each nuclear division (e.g., coconut meat). - **Persistence:** - **Albuminous seeds:** Endosperm persists in mature seed, providing nutrition (e.g., castor, maize, wheat). - **Non-albuminous/Exalbuminous seeds:** Endosperm completely consumed by developing embryo (e.g., pea, groundnut). - **Embryo Development (Embryogeny):** - Zygote (2n) $\xrightarrow{Mitosis}$ Proembryo $\xrightarrow{Differentiation}$ Mature embryo. - **Dicot Embryo:** - **Embryonal axis:** Consists of plumule (future shoot tip), radicle (future root tip). - **Cotyledons:** Two, store food. - **Epicotyl:** Part of embryonal axis above cotyledons, terminates in plumule. - **Hypocotyl:** Part of embryonal axis below cotyledons, terminates in radicle. - **Monocot Embryo:** - **Single cotyledon:** Called scutellum (lateral position). - **Coleoptile:** Sheath enclosing plumule. - **Coleorhiza:** Sheath enclosing radicle. - **Seed:** Fertilized ovule. - **Seed coat:** From integuments. - **Micropyle:** Remains as small pore in seed coat, facilitates water and oxygen absorption during germination. - **Hilum:** Scar on seed coat. - **Seed dormancy:** Period of suspended growth. - **Fruit:** Fertilized and ripened ovary. - **Pericarp:** Fruit wall, develops from ovary wall. - **Types:** - **True fruits:** Develop only from ovary (e.g., mango, tomato). - **False fruits:** Develop from ovary + other floral parts (thalamus, e.g., apple, strawberry; cashew). - **Parthenocarpic fruits:** Develop without fertilization, seedless (e.g., banana). Induced by growth hormones. ### Apomixis and Polyembryony - **Apomixis:** Production of seeds without fertilization (asexual reproduction mimicking sexual reproduction). - **Types:** - **Apospory:** Diploid sporophytic cells (nucellus) develop into embryo sac without meiosis. - **Diplospory:** Diploid MMC develops into embryo sac without meiosis. - **Adventive embryony:** Embryo develops directly from nucellar or integument cells (e.g., citrus, mango). - **Significance:** Maintains hybrid vigour, useful in hybrid seed industry. - **Polyembryony:** Occurrence of more than one embryo in a seed (e.g., citrus, mango, onion). - Can arise from: - Multiple nucellar embryos (adventive embryony). - More than one egg cell in embryo sac. - Cleavage of proembryo.