In-Depth Notes on Seed Plants (BIOL 109)

Seed Plants

Overview

• Seed plants, also known as Spermatophyta, are divided into two major clades:
  • Gymnosperms: Include conifers and their relatives.
  • Angiosperms: Include flowering plants.

Seed Structure

• A seed consists of three primary parts:
  • Embryo Sporophyte: The young plant.
  • Food Reserves: Nutritional tissue that supports the embryo.
  • Seed Coat: A tough protective covering.
• Seeds originate from female gametophytes and are fertilized by pollen grains, which are immature male gametophytes.

Reproductive Success of Seeds

• Seeds greatly enhance the probability of successful germination and growth of a new individual:
  • Protected by a seed coat.
  • Contains a multicellular sporophyte with embryonic root and shoot.
  • Nutritional support until the plant establishes a root and shoot system.
  • Seeds can remain dormant for varying periods based on environmental conditions.

Pollen and Pollination

• Pollen Grains: Developed from microspores, they contain the male gametophyte amidst a protective wall made of sporopollenin.
• Pollination: Transfer of pollen to female parts, crucial for seed plants, removing the dependency on liquid water for reproduction, allowing adaptation to drier environments.

Gametophyte Development

• Heterospory: Seed plants produce two types of spores:
  • Megaspores: Develop into egg-producing female gametophytes.
  • Microspores: Develop into sperm-producing male gametophytes.

Key Groups of Seed Plants

1. Gymnosperms (Phylum Coniferophyta):
   • Features exposed ovules, nonmotile sperm located in pollen, and primarily wind pollinated.
   • Example groups: Cycadophyta (Cycads), Ginkgophyta (Ginkgo), Gnetophyta (Gnetophytes), Coniferophyta (Conifers).
2. Angiosperms (Phylum Anthophyta):
   • Flowers and fruits protect ovules and seeds.
   • Adapt to diverse habitats, from dry land to aquatic environments.
   • Significant groups include monocots (e.g., grasses) and eudicots (e.g., flowering trees).

Angiosperm Structure

• Flowers: Modified shoots containing reproductive organs, typically formed of:
  • Carpels: Female part, surrounding ovules and seeds.
  • Stamens: Male part where pollen is produced.
  • Petals and Sepals: Leaf-like structures, often aiding in pollinator attraction.

Life Cycle of Angiosperms

• Features double fertilization:
  • One sperm fertilizes the egg; the other forms the endosperm (nutritive tissue for the embryo).
• Fruits: Develop from ovary and enclose seeds, aiding in their protection and dispersal to new locations.

Coevolution with Pollinators

• Angiosperms have adapted traits (flower structure, scents, nectar) that attract specific pollinators (e.g., insects, birds), creating a mutualistic relationship essential for reproduction.

Summary of Key Terms

• Spermatophyta: Seed plants.
• Gymnosperms: Naked seeds, predominantly wind-pollinated.
• Angiosperms: Seeds enclosed in fruits, more diverse and resilient.
• Heterospory: Production of two types of spores, female and male.
• Pollination: Mechanism of transferring pollen to facilitate reproduction.

Evolutionary Timeline**

• Approx. 400 million years ago: Emergence of gametophytes and sporophytes.
• Progression from seedless plants to the evolution of seed-bearing plants; development of secondary growth in progymnosperms, leading to both early gymnosperms and later angiosperms forming distinct groups with specific adaptations.