Key Concepts in Seed Plant Evolution and Development

Learning Objectives

  • Describe two major innovations that allowed seed plants to reproduce in the absence of water.
  • Explain when seed plants first appeared and when gymnosperms became the dominant plant group.
  • Discuss the purpose of pollen grains and seeds.
  • Describe the significance of angiosperms bearing both flowers and fruit.

Moving Toward Seed Plants

  • Dominant Sporophyte Generation: Seed plants have a dominant diploid sporophyte generation compared to the gametophytes.
  • Reduction of Gametophyte: Gametophytes are reduced to microscopic sizes and are not free-living, unlike in seedless plants.
  • Heterosporous Nature: Seed plants produce two types of spores:
  • Megaspores: Develop into female gametophytes that produce eggs.
  • Microspores: Develop into male gametophytes that produce sperm.
  • Seeds and Pollen: These features are key distinctions of seed plants compared to non-seed plants.

First Seed Plants

  • Origin: Distinct seed plants appeared around 350 million years ago (MYA).
  • Gymnosperms: Became prominent around 319 MYA during the Pennsylvanian period, dominated the Early Triassic (240 MYA) and mid-Jurassic (205 MYA).
  • Angiosperms: Dominated starting in the mid-Cretaceous period (around 100 MYA).
  • Evolution Timeline: Review figure 26.2 for detailed timeline of plant evolution.

Gymnosperm Evolution

  • Seed Ferns: The earliest seed plants, such as Elkinisia polymorpha, existed around 400 MYA and produced seeds in protective structures called cupules.
  • Diversity: These seed ferns thrived during the Carboniferous period in coal swamp environments.
  • Progymnosperms: Originated in the Devonian period (390 MYA) and were early plant structures that led to gymnosperms.
  • Ginkgoales: First gymnosperms, such as Ginkgo biloba, appeared in the Jurassic period.
  • Expansion: Gymnosperms thrived during the Mesozoic era (starting around 240 MYA) and remain dominant in environments such as the Taiga (Northern Boreal Forest) and the Alpine regions.

Seeds and Pollen as Evolutionary Adaptations

  • Seeds: Comprised of an embryo encased in a protective coating that contains nutrient stores, allowing for:
  • Dormancy for thousands of years.
  • Prevention of desiccation.
  • Dispersal across distances and over time via wind, animals, and water, which helps avoid competition.
  • Pollen Grains: These are male gametophytes, consisting of haploid cells protected from desiccation. They form a pollen tube upon contact with a female gametophyte, facilitating reproduction without water.

Evolution of Angiosperms

  • Origins: Angiosperms did not evolve from gymnosperms, instead evolving alongside insects, with a close relationship observed.
  • Classification:
  • Monocots
  • Eudicots
  • Basal Angiosperms (e.g., Water Lily)
  • Innovations: Introduced critical features such as flowers and fruits, which serve as protected sites for fertilization and seed development.
  • Fertilization: After fertilization, the ovary thickens to form fruit, which aids in seed dispersal and protection.