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Topic 16 Gymnosperms and Seed Plants

Dec 13, 2024

BIOL 108 Introduction to Biological Diversity

Topic 16: Gymnosperms

Introduction to Seed Plants

  • Originated about 360 million years ago.
  • Seed: An adaptation with an embryo, food store, protective coat.
  • Domestication began 8,000 years ago, enabling permanent settlements.

Phylogeny of Embryophyta

  • Phylogeny of land plants and strobili in gymnosperms.

Shared, Derived Traits of Seed Plants

  1. Reduced Gametophytes

    • Develop within spore walls retained in parent sporophyte tissues.
    • Reduced size; dependent on sporophyte (dominant and independent in seed plants).
  2. Heterospory

    • All seed plants are heterosporous.
    • Megasporangia: produce megaspores for female gametophytes.
    • Microsporangia: produce microspores for male gametophytes.
    • Spores are retained within sporophytes.
  3. Presence of Ovules

    • Contain megasporangium (2n), megaspore (1n), protective integuments (2n).
    • Gymnosperms: one integument; angiosperms: usually two integuments.
    • Ovules develop into seeds after fertilization.
  4. Pollen

    • Microspores develop into male gametophytes in pollen grains.
    • Pollen grains: protective sporopollenin layer.
    • Male gametophytes transported to ovule; no external sperm release.
  5. Seeds

    • Develop from fertilized ovules, contain embryo, nutrient supply, seed coat.
    • Seeds vary in size, affected by nutrient storage.

Pollination

  • Transfer of pollen to seed plant parts containing ovules.
  • Significant evolutionary shift: pollen grains dispersed, pollen tube developed.
  • Advantages: protection, long-distance transport.

Seed Dispersal

  • Seeds, not spores, used for dispersal.
  • Advantages: protection, energy supply, germination in dark.
  • Trade-off between seed size and quantity.

Phylogeny and Diversification of Seed Plants

  • Two clades: Gymnosperms (800 species) and Angiosperms (250,000 species).
  • Earliest gymnosperms fossil evidence from Carboniferous Period.
  • Angiosperms began replacing gymnosperms in Cretaceous Period.

Introduction to Gymnosperms

  • Vascular plants with naked seeds.
  • Dominant sporophyte generation; develop seeds from fertilized ovules.
  • Four phyla: Cycadophyta, Ginkgophyta, Gnetophyta, Coniferophyta.

Phylum Cycadophyta

  • 130 species, ancient lineage.
  • Dioecious, slow-growing, long-lived.

Phylum Ginkgophyta

  • Ginkgo biloba: only extant species, associated with human habitats.
  • Dioecious, deciduous, fan-shaped leaves.

Phylum Gnetophyta

  • 75 species in three genera: Ephedra, Gnetum, Welwitschia.
  • Found in tropical/desert environments.

Phylum Coniferophyta

  • 600 species, mostly evergreen.
  • Dominant in boreal/alpine regions, rely on wind pollination.

Reflective Questions

  • Synapomorphies of seed plants?
  • Shared traits between seed and seedless plants?
  • Life cycle comparison between gymnosperms and other land plants?
  • Evolutionary history fit of gymnosperms?
  • Interpretation of vessel tissues in gnetophytes?