AS

gymnosperms_and_angiosperms_rev

Seeds and Plant Evolution

  • Seeds transformed plant evolution, leading to the dominance of seed plants in terrestrial ecosystems.

  • Seed plants originated approximately 360 million years ago.

  • A seed consists of an embryo, nutrients, and a protective coat.

  • Seeds can be dispersed over long distances by wind or other means.

Key Adaptations in Seed Plants

  • Seed and Pollen Grains:

    • Seed and pollen are vital adaptations for terrestrial life.

    • Common features of seed plants include:

      • Reduced gametophytes.

      • Heterospory.

      • Ovules.

      • Pollen.

Reduced Gametophytes

  • The gametophytes of seed plants develop within spore walls, retained in parent sporophyte tissues, providing nutritional support.

Heterospory in Seed Plants

  • Seed plant ancestors were likely homosporous, but modern seed plants are typically heterosporous:

    • Megasporangia: Produce megaspores leading to female gametophytes.

    • Microsporangia: Produce microspores leading to male gametophytes.

Ovules and Eggs Production

  • An ovule consists of:

    • Megasporangium.

    • Megaspore.

    • One or more protective integuments.

  • Gymnosperm ovules have one integument while angiosperms typically have two.

Pollen and Sperm Production

  • Microspores develop into pollen grains containing male gametophytes.

  • Pollination is the process of transferring pollen to the ovule-containing structure of the plant.

  • Pollen allows for long-range dispersal without the need for water.

  • Germination of pollen grains gives rise to pollen tubes that deliver sperm to female gametophytes.

Evolutionary Advantages of Seeds

  • Seeds emerge from ovules and consist of a sporophyte embryo with a food supply in a protective coat.

  • Advantages of seeds over spores include:

    • Dormancy for extended periods until conditions favor germination.

    • Nutrient supply.

    • Capability for long-distance dispersal.

Gymnosperms

  • Gymnosperms, meaning "naked seeds," typically bear seeds exposed on cones.

  • Angiosperms differ by producing seeds within fruits, derived from ovaries.

  • Gymnosperms, particularly conifers, were predominant during the Mesozoic era.

Life Cycle Features of Gymnosperms

  • The gymnosperm life cycle includes:

    • Miniaturization of gametophytes.

    • Seed development from fertilized ovules.

    • Sperm transfer via pollen.

  • The pine tree serves as a model species illustrating these features.

Gymnosperm Diversity

  • Comprises four main phyla:

    • Cycadophyta: Includes cycads, usually large cones and palm-like leaves.

    • Ginkgophyta: Ginkgo biloba is the sole surviving species; known for air pollution tolerance.

    • Gnetophyta: Includes three genera: Gnetum, Ephedra, and Welwitschia, found in various habitats.

    • Coniferophyta: The largest phylum, features evergreen trees like pines and firs.

Angiosperms: The Flowering Plants

  • Angiosperms represent the most diverse group of seed plants, characterized by flowers and fruits:

    • The flower facilitates sexual reproduction, attracting pollinators.

    • Fruits protect seeds and assist in dispersal.

Flower Structure

  • Flowers consist of modified leaves, or floral organs:

    • Sepals: Enclose the flower.

    • Petals: Brightly colored to attract pollinators.

    • Stamens: Produce pollen.

    • Carpels: Contain ovules.

Symmetry and Function of Flowers

  • Flowers may have radial or bilateral symmetry affecting pollinator movement and speciation rates.

  • Flowers with all four organs are complete; those lacking one are incomplete.

Fruit Development and Seed Dispersal

  • Fruits are created from thickening ovary walls, protecting seeds and aiding dispersal.

  • Various adaptations enable seed dispersal via wind, water, or animals.

Angiosperm Life Cycle

  • Involves the formation of male and female structures within a flower.

  • Pollen grains germinate on stigmas, leading to the development of pollen tubes to ovules.

  • Double fertilization occurs, forming both embryo and endosperm.

Human Impact and Diversity of Seed Plants

  • Seed plants are crucial for food, medicine, and materials.

  • Conservation of biodiversity is vital for human welfare.