Lecture on Plant Structure: Leaves

Overview

• Focus on leaf structure and specialized leaves.
• Leaves are flat with a blade shape for sunlight absorption and gas exchange.

Leaf Structure

• Blade: Contains veins (vascular tissue: xylem and phloem).
  • Microphylls: One vein.
  • Megaphylls: Multiple veins.
• Petiole: Stem connection in eudicots.
  • Contains collenchyma cells for rigidity and flexibility.

Leaf Types

• Simple Leaves: Single structure blade.
• Compound Leaves: Blade divided into leaflets.
  • Identified by a single petiole from one node.
  • Advantages: large surface area, wind resistance, disease isolation.

Leaf Arrangement

• Spiraling Nodes: Common in vascular plants, usually at 137.5° (Fibonacci spiral).
  • Alternate Arrangement: One leaf per node.
  • Opposite Arrangement: Two leaves per node.
  • Whorled Arrangement: Multiple leaves per node.

Internal Structure of Leaves

• Epidermis: Covers leaf, stomata on lower epidermis, trichomes on upper.
• Mesophyll: Middle layer, contains veins.
  • Bundle Sheath: Surrounds veins.
• Differences in monocots vs. eudicots:
  • Monocots: Undifferentiated mesophyll, large bundle sheath (C4 photosynthesis).
  • Eudicots: Palisade (upper) and spongy (lower) mesophyll, varied stomata distribution.

Specialized Leaves

• Floral Leaves/Bracts: Leaves mimic flowers (e.g., poinsettias).
• Spines: Non-photosynthetic, deter herbivores, protect in dry climates.
• Vegetative Reproduction: Adventitious plantlets.
• Window Leaves: Found in arid plants, reduce evaporation, underground photosynthesis.
• Shade vs. Sun Leaves: Different adaptations on the same plant for sunlight efficiency.
• Insectivorous Leaves: Carnivorous plants (e.g., sundew), capture nitrogen-rich prey.

Key Takeaways

• Leaves come in various forms and arrangements, adapted for specific environmental challenges and roles.
• Specialized leaves demonstrate incredible diversity and adaptation in plant biology.

This lecture completes the discussion on plant structure focused on leaves.