2.1 - Structure of a Leaf

External Structure of a Leaf

• A leaf is the main plant organ that carries out photosynthesis.
• Its external structure consists of the lamina and petiole.

Lamina

• Flat, thin, smooth, and green part of the leaf.
• Flat shape provides a wide surface so cells containing chloroplasts receive maximum sunlight.
• Thin shape allows gases involved in photosynthesis to diffuse efficiently into the leaf.

Petiole

• Leaf stalk that connects the lamina to the plant stem.
• Extends into the lamina to form a network of veins that supports the lamina.

Leaf Mosaic

• In most plants, leaf laminae are arranged without overlapping.
• This leaf mosaic arrangement enables the leaves to receive optimum light for photosynthesis.

Internal Structure of a Leaf Lamina

Cuticle

• Waxy, waterproof, and transparent layer covering the upper and lower epidermis.
• Reduces excessive water loss through evaporation.
• Allows sunlight to pass through it.

Upper Epidermis

• Located on the upper surface of the leaf, beneath the cuticle.
• Cells contain no chloroplasts and are transparent, allowing sunlight to pass through.

Palisade Mesophyll

• Cells are arranged vertically and closely packed to receive maximum light.
• Contains many chloroplasts and is a site of photosynthesis.

Spongy Mesophyll

• Irregularly shaped cells increase the internal surface area for gaseous exchange.
• Cells are loosely arranged with many intercellular air spaces, allowing gases to diffuse efficiently.
• Contains fewer chloroplasts than the palisade mesophyll and carries out photosynthesis.

Lower Epidermis

• Located on the lower surface of the leaf.
• Contains stomata; each stoma is surrounded by a pair of guard cells.

Vascular Bundle

• Xylem transports water and mineral salts absorbed by the roots to the leaves.
• Thick, lignified xylem walls provide support and mechanical strength.
• Phloem transports organic substances produced during photosynthesis from the leaves to other parts of the plant.