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.