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.