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?