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Chapter 38: Plant Reproduction and Development Flowers Sexual organs of Angiosperms. Develop from compressed shoots with four whorls of modified leaves. Flower Whorls 1. Sepals 2. Petals 3. Stamens 4. Carpels Sepals Whorl of sterile leaf-like structures. May be brightly colored. Function Protect other flower parts. Attract pollinators. Petals Whorl of sterile flower parts. Often brightly colored. Function Attract pollinators. Stamens Male reproductive flower structure. Function Produce pollen (sperm). Stamen Structure Anther - pollen producing sac. Filament - stalk. Carpel Female reproductive flower structure. Also called pistil. Function Carpel Structure Produce embryo sac (eggs). Stigma – receives the pollen. Style – stalk. Ovary – contains the ovules. Ovule A rudimentary seed before fertilization. Flowers Are highly variable in form, shape, and color. Not all flowers contain the same combination of whorls. Flower Variations Complete Flowers - have all four whorls. Incomplete Flowers – less than all four whorls. Perfect Flowers - have stamens and carpels. Imperfect Flowers - have stamens or carpels. Monoecious Plants Staminate and pistillate flowers are on the same plant. Ex: corn Tassel – staminate flowers Ears – carpellate flowers Dioecious Plants Have staminate or pistillate flowers, but not both. Ex: Holly, Ginkgo Comment - "Seedless" plants may be a staminate plant. Ex: male Ginkgo Pollen Development Pollen is the male gametophyte. Starts with a 2N cell called a microsporocyte. Microsporocyte Undergoes meiosis and produces 4 Microspores (1N). Each Microspore undergoes mitosis and produces a pollen grain, which is the male gametophyte. Mature Pollen Grain Special cell wall. Tube Nucleus (cell). Generative Nucleus (cell) which will divide and produce two sperm nuclei. Embryo Sac Development The Embryo Sac is the female gametophyte. Starts with a megasporocyte (2N). Megasporocyte Undergoes meiosis and produces 4 Megaspores (1N). Three of the megaspores abort, leaving only one to develop into the embryo sac. Megaspore Undergoes three rounds of mitosis to produce a “sac” with 8 nuclei. The nuclei may wall off or may remain as part of a large cell. Embryo Sac 3 Antipodal cells. 2 Polar nuclei (1 cell) 2 Synergid cells 1 Egg cell Pollination: The transfer of pollen from a stamen to the stigma. Pollen Vectors Bees Flies Butterflies Moths Birds Beetles Bats Wind Comment The flower is usually highly adapted to the pollen vector. Ex: Colors Rewards Scents Shape Fertilization The union of egg and sperm to produce a zygote (2N). Angiosperms Have double fertilization. 1. Egg + sperm zygote 2. Polar nuclei + sperm Endosperm Endosperm Is Triploid (3N) tissue that will be used as a nutrition source for the embryo. After Fertilization Zygote Embryo Ovule Seed Ovary Fruit Embryo Development Root/shoot polarity set with 1st cell division. Mature embryo has all three primary tissues, apical meristems etc. Endosperm Monocots - large tissue. Dicots - transfer the energy over to the embryo's two cotyledons. Seed Coat Formed from the integuments of the ovule. Hard layer to protect the seed. Fruit A mature ovary. Sometimes includes other plant parts. Ex: apple Fruit Functions Protect the seeds. Aid in seed dispersal. Fruit Types Fleshy: soft ovary walls. Dry: hard ovary walls. Fruit Types Simple Fruit: from a single ovary. Aggregate Fruit: from a flower with multiple carpels. Ex: Peach, Cherry Ex: Raspberry Multiple Fruit: develops from several flowers into one structure. Ex: Pineapple Seeds Contain a miniature plant. Main dispersal mechanism for plants. Embryo is often “dormant” when the seed is mature. Seed Dormancy When a seed is not actively growing. Used to increase the chances that the plant will develop when conditions are favorable. Usually controlled by plant hormones. May require changes in temperature, moisture, etc. before growth will continue. Imbibition The absorption of water by a seed. First step in seedling growth. Causes “swelling” which ruptures the seed coat. Starts metabolism to resume growth. Germination The continuation of growth of the plant within a seed. Root usually emerges first. Shoot tip must break through the soil surface. Role of Light Light is the usual clue that the shoot has broken above ground. Light causes many growth changes in the shoot Ex. Leaves to expand Stem elongation rate decreases Asexual Reproduction Offspring produced by mitosis. Also called “cloning”. Normal Method Fragmentation of the plant body so that new plants are formed. Ex: Cuttings Grafting Offshoots High-Tech Methods Tissue Culture. Clumps of cells grow into embryoids which can be used to regenerate whole plants. Often used in genetic engineering of plants. Protoplast Fusion. A "naked" plant cell (no cell wall). Used to create new hybrids Plants Frequently use both sexual and asexual reproduction depending on the environment. Asexual - stable Sexual - unstable Plant Development Growth: increase in size Development: changes in body form and structure. Plant Problems Determining the direction of cell growth because of the cell wall. Control Mechanism Cytoskeleton: determines the direction of cell expansion. Cellulose Microfibrils: are arranged in parallel strips as guided by microtubules in the cell membrane. Cell Expansion Increase in cell size is usually caused by turgor pressure. Direction of cell increase is at right angles to the cellulose mircofibrils. Cell Differentiation Depends on the control of gene expression. (review previous chapters on this topic) Pattern Formation The development of specific structures in specific locations. Important in plants since cells don’t usually migrate. Juvenile Mature Positional Information Gradients of chemicals that provide clues of position of cells to each other. Ex: why some cells develop into shoots and others into roots. Organ-Identity Genes Genes for normal organ development. Used in positional information to determine which organ the cells should develop into. Example The combination of three genes that give rise to the flower parts. A sepals A + B petals B + C stamens C carpels Mutations Cause other floral parts to form in the flowers. (review Chapter 21). Summary Know the general structures of flowers. Know the general life cycle of flowers. Review the items on plant development.