Lecture on Gymnosperms

Introduction

• Third installment in a series on plant evolution.
• Focus on gymnosperms and their life cycle.
• Recap of alternation of generations will be included.

What are Gymnosperms?

• Definition: Plants that produce seeds, specifically "naked seeds".
• Progression from spore formation to seed formation.
• Key feature: Seeds are not enclosed in fruits.

Characteristics of Gymnosperms

• Vascular Tissue: Highly developed xylem and phloem, allowing large growth (e.g., pine trees).
• Leaf Structure: Thick cuticle and needle-like leaves to reduce water loss and deter herbivores.
• Reproduction:
  • Produce cones (male and female) for gamete production.
  • Rely on wind pollination, producing large amounts of light pollen.

Alternation of Generations

• Cycle Overview:
  • Alternation between diploid (sporophyte, 2n) and haploid (gametophyte, n) generations.
• In Gymnosperms:
  • Hard to distinguish due to single structure (e.g., adult pine tree is a sporophyte).
  • Cones contain sporangium (mega and micro) for meiosis.

Life Cycle of Gymnosperms

1. Sporophyte Generation
   • Mature sporophyte (e.g., pine tree) produces male and female cones.
2. Male Reproductive System
   • Male cones have microsporangia producing microspores (pollen grains).
   • Pollen grains carry sperm cells.
3. Female Reproductive System
   • Female cones with megasporangia and ovules.
   • Megaspores develop into eggs within ovules.
4. Pollination and Fertilization
   • Pollen grains land on female cones, grow pollen tubes to transport sperm to egg.
   • Fertilization occurs, forming a diploid zygote.
5. Seed Development and Germination
   • Zygote develops into seeds, eventually dispersed by wind.
   • Germination leads to new sporophyte (pine tree).

Terminology Recap

• Vascular Tissue: Supports plant structure.
• Cuticle and Leaves: Protection and water conservation.
• Pollination: Wind-based for gymnosperms.
• Naked Seeds: Lack of fruit covering.
• Alternation of Generations: Sporophyte (2n) and gametophyte (n).
• Pollen and Ovule: Reproductive structures.
• Fertilization and Pollen Tube: How sperm reaches the egg.

Conclusion

• Gymnosperms are well-adapted for terrestrial life with distinct reproductive strategies.
• Important for understanding plant evolution.
• Next topic will focus on angiosperms and further developments.