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3.8 Plant Reproduction

Changes in Environment Affecting Organisms

  • Interaction of temperature and humidity with organisms.

  • Sexual reproduction leads to variations in offspring.

  • Importance of gametes from two different parents for variation.

Comparison of Reproduction in Angiosperms and Gymnosperms

  • Main focus on angiosperms due to complexity of their reproductive processes involving flowers and fruits.

  • Overview of gymnosperms:

    • Microstrobili (male cones) and megastrobili (female cones).

    • Gymnosperms produce ‘naked seeds’ without covering.

    • Contrast with angiosperms that have fleshy outer layers surrounding seeds.

    • Fertilization Types:

    • Gymnosperms: Single fertilization.

    • Angiosperms: Double fertilization.

    • Pollination:

    • Gymnosperms: Predominantly wind pollination (especially conifers).

    • Angiosperms: Diverse pollinators (insects, birds, mammals).

Flower Structure

  • Flower Types: Hermaphroditic (perfect flowers) vs monoecious (separate male and female flowers) vs dioecious (male and female plants).

  • Flowers produced by mature sporophytes housing gametophyte generation.

  • After fertilization, structures develop into sporophyte generation (seeds).

  • Morphology of Flowers:

    • Outer to inner whorls:

    1. Calyx: Outermost whorl made up of sepals (modified leaves).

    2. Corolla: Next whorl containing petals.

    3. Androecium: Male reproductive structures (stamen composed of filament and anther).

    4. Gynoecium: Female reproductive structures (carpals).

  • Whorl Identification: Start identifying from the outermost whorl.

Female Reproductive Structures (Gynoecium)

  • Carpal Structure:

    • Ovary: Contains ovules which develop into seeds when fertilized.

    • Stigma: Sticky surface that traps pollen.

    • Style: Connects stigma to ovary.

  • Gynoecium may consist of multiple carpals, termed collectively as gynoecium.

Male Reproductive Structures (Androecium)

  • Stamen structure:

    • Filament: Support structure for anther.

    • Anther: Location of pollen grains.

Flower Completeness and Perfection

  • Complete Flowers: All four whorls present.

  • Incomplete Flowers: Missing at least one whorl.

  • Perfect Flowers: Have both male and female reproductive structures.

  • Imperfect Flowers: Missing one reproductive structure (staminate = male; carpolate = female).

  • Relation: A complete flower is always perfect, but a perfect flower does not have to be complete.

Angiosperm Reproduction Overview

  • Male Gamete Production: Begins with microsporogenesis via diploid microsporocyte going through meiosis resulting in pollen tetrad.

    • Pollen grains develop through gametogenesis (two sperm cells produced from generative cell).

  • Female Gamete Production: Begins with megasporogenesis via diploid megasporocyte undergoing meiosis.

    • One large megaspore formed, leading to a seven-cell structure with eight nuclei after several mitotic rounds (egg, synergids, central cell, antipodal cells).

  • Double Fertilization: Refers to fertilization of egg (producing embryo) and fertilization of central cell (resulting in triploid endosperm).

Pollination vs. Fertilization

  • Pollination: Occurs when pollen reaches stigma; may not always lead to fertilization.

  • Fertilization: Occurs within the ovary post-pollination.

Pollination Mechanisms

  • Self-Pollination: Occurs in perfect flowers that mature male and female parts simultaneously.

  • Animal Pollination: Involves insects, birds, and mammals; requires energy investment in nectar and attraction mechanisms.

  • Wind Pollination: Less energy-intensive compared to animal pollination.

Seed Structure

  • Seed Layers:

    • Seed coat: Protective outer layer.

    • Endosperm: Nutritional support for embryo.

  • Embryo Composition:

    • Cotyledons (first leaves).

    • Radicle (develops into roots).

    • Hypocotyl (develops into stems).

  • Micropyle and Hilum: Micropyle allows for gas exchange; hilum marks seed attachment in ovary.

Seed Development and Comparison

  • Germination process initiated when conditions are favorable (water, sunlight, warmth).

  • Growth stages include:

    • Absorption and swelling of seed.

    • Rupture of the seed coat.

    • Oxygen availability triggers respiration and growth.

  • Embryonic tissue leads to root and shoot development.

Asexual Reproduction in Plants

  • Occurs via mitosis, resulting in genetic clones with no variation.

  • Methods include:

    • Stolons/Runners: E.g., strawberries.

    • Plantlets: E.g., succulents.

  • Apomixis: Seeds produced asexually bypassing meiosis, cloning without fertilization.

Plant Lifespans

  • Senescence: Aging process in plants.

  • Types based on lifecycle:

    • Annuals: One growing season.

    • Biennials: Two seasons (vegetative in first, reproductive in second).

    • Perennials: Long-lived species.

  • Monocarpic: Flowering once before dying.

  • Polycarpic: Flowering multiple times throughout lifespan.

Overview of Plant Reproductive Mechanisms

  • Sexual reproduction involves complex processes like flowers, pollination, and seed development.

  • Asexual reproduction promotes rapid spread and colonization without genetic variation.

  • Importance of differentiating between types of fruit (true, accessory, simple, aggregate, multiple) and their development from floral structures.