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Lecture+12+slides

Lecture Overview

  • Lecture Topic: Seed Bearing Plants (Gymnosperms)

  • Course: Bio 80

  • Chapter: 30

  • Instructor: E771

Lecture Objectives

  • Understand the structure, function, and significance of the five derived traits of gymnosperms:

    • Seeds

    • Ovules

    • Pollen

    • Dependent gametophyte

    • Heterospory

  • Describe the gymnosperm life cycle.

  • Identify the first gymnosperms.

Gymnosperms

  • Definition: Gymnosperms are a group of seed-bearing plants that have seeds exposed on sporophylls, usually forming cones.

  • Evolutionary Background: Evolved after vascular, non-seed-bearing plants.

  • Derived Traits:

    1. Reduced, dependent gametophyte

    2. Heterospory

    3. Ovules

    4. Pollen

    5. Seeds

Derived Traits of Gymnosperms

1. Reduced, Dependent Gametophytes

  • In gymnosperms, gametophytes are typically microscopic and develop from spores retained within sporophyte sporangia.

  • Benefits: Protects gametophytes from environmental stresses and allows them to receive nutrients from the parent sporophyte.

2. Heterospory

  • Gymnosperms produce two types of spores:

    • Megaspores form female gametophytes.

    • Microspores form male gametophytes (pollen).

  • Contrast with homosporous plants like ferns.

3. Ovules

  • Consist of a megaspore within a megasporangium, surrounded by protective integuments.

  • Gymnosperms typically have one integument.

4. Pollen

  • Microspores develop into pollen grains (male gametophytes) enclosed in a pollen wall.

  • Key Points:

    • Pollination does not require water.

    • Pollen can be transferred long distances.

5. Seeds

  • Development of seeds from ovules allows for protection and nourishment for the embryo.

  • Seeds can disperse over long distances, enhancing survival.

  • Contrast with spores:

    • Seeds are multicellular, while spores are single-celled.

    • Seeds can remain dormant until favorable conditions arise.

Life Cycle of Gymnosperms

  • The life cycle is sporophyte-dominated, with microscopic gametophytes.

  • Reproduction typically occurs through cones (male and female).

  • Pollination leads to fertilization, resulting in seed development.

Key Reproductive Adaptations

  1. Miniaturization of gametophytes.

  2. Seed production as a resilient and dispersible life stage.

  3. Pollen as an airborne means for gamete transfer.

Gymnosperm Diversity

  • Four primary phyla:

    • Cycadophyta: Palm-like gymnosperms.

    • Ginkgophyta: Includes Ginkgo biloba.

    • Gnetophyta: A diverse group including Gnetum.

    • Coniferophyta: Cone-bearing trees like pines and firs.

  • Gymnosperms thrived during the Mesozoic era and were significant in terrestrial ecosystems.

  • While angiosperms dominate today, gymnosperms remain ecologically and economically important.