1.1 The Leaf

External Structure of a Leaf

• Petiole: Stalk attaching leaf to stem.
• Leaf Blade (Lamina): Broad, flat part; photosynthesis.
• Midrib & Veins: Central and branching vascular tissues.
• Apex: Leaf tip.
• Margin: Leaf edge.
• Base: Part attached to petiole.

Kinds of Leaves

• Simple Leaf: Single, undivided blade (e.g., Mango, Guava).
• Compound Leaf: Blade divided into smaller leaflets (e.g., Neem, Rose).

Types of Venation

• Reticulate Venation: Veins form a net-like pattern (e.g., most dicots like Peepal).
• Parallel Venation: Veins run parallel to each other (e.g., most monocots like Grass, Maize).

Functions of a Leaf

1. Photosynthesis: Primary function; produces food for the plant.
2. Transpiration: Water vapor loss; cools plant, pulls water up roots.
3. Respiration: Takes in O₂, releases CO₂ to release energy.
4. Storage: Some leaves store food or water (e.g., Onion, Aloe Vera).

Modifications of Leaves

• Tendrils: Help plant climb (e.g., Pea).
• Spines: Protection, reduce water loss (e.g., Cactus).
• Storage Leaves: Fleshy, store food/water (e.g., Onion).
• Phyllodes: Flattened petioles functioning as leaves (e.g., Australian Acacia).

Insectivorous Plants

• Derive nutrients (especially nitrogen) from trapping and consuming insects.
• Grow in nutrient-poor soils.
• Examples:
  • Pitcher Plant: Leaves modified into pitcher-like traps.
  • Venus Flytrap: Leaves form a trap with sensitive hairs.

Vegetative Propagation in Leaf

• Some plants reproduce asexually from their leaves.
• Example: Bryophyllum (Mother of Thousands): Adventitious buds develop in leaf notches, growing into new plantlets.