Evolution of Seed Plants

Introduction to Seed Plant Evolution

• Focus: Evolution of seeds from ferns.
• Previous Topic: Ferns and spore production.
  • Spores are haploid cells that divide into gametophytes via mitosis.
  • Ferns are mostly homosporous (one type of spore), but some are heterosporous (two types of spores).

Homosporous vs. Heterosporous Life Cycles

Homosporous Life Cycle

• Sporophyte: Multicellular diploid stage.
  • Produces haploid spores via meiosis in sporangia.
• Spores: Develop into multicellular haploid gametophytes.
  • Produce eggs and sperm via mitosis.
• Fertilization: Eggs and sperm combine to form a zygote.
  • Zygote develops into a new sporophyte.

Heterosporous Life Cycle

• Sporophyte: Produces megaspores and microspores.
  • Megaspores in megasporangia, microspores in microsporangia.
• Megaspores: Develop into female gametophytes.
  • Female gametophytes produce eggs.
• Microspores: Develop into male gametophytes.
  • Male gametophytes produce sperm.
• Fertilization: Leads to a new sporophyte generation.

Evolution of Seed Plants

Role of Sporophyll

• Definition: A sporophyll is a leaf that holds sporangia.
  • Megasporophyll: Holds megasporangia.

Seed Plant Evolution Process

• Initial Stage: Megaspores are not released from the sporangia.
• Close-up of Megasporangium:
  • Contains a megaspore mother cell (sporocytes).
  • Megaspore mother cell divides by meiosis to produce four megaspores - three degenerate.
  • Surviving megaspore develops into a megagametophyte via mitosis.
  • Megagametophyte produces eggs.

Fertilization and Seed Formation

• Fertilization: One egg is fertilized by sperm, forming a zygote.
• Embryo Development: Zygote becomes an embryo through mitosis.
  • Embryo is nourished by surrounding food.
• Seed Formation:
  • Embryo with food becomes a seed.
  • Protective seed coat forms around the embryo.

Summary

• Seed plants evolved from heterosporous plants retaining megaspores on sporophylls.
• This retention led to the formation of seeds, which are crucial for plant reproduction and survival.