leaf anatomy

The leaf is where most photosynthesis takes place inside a plant.

Outer Layers

  • Cuticle: A thin waxy covering (A) on the outermost surfaces of the leaf. Its primary function is to restrict the evaporation of water from the leaf. Plants that live entirely within water do not have a cuticle.

  • Epidermis: A single layer of tightly-packed cells (B) found directly underneath the cuticle on the upper and lower sides of the leaf. It forms a protective barrier and transmits light.

Mesophyll Layer

  • The internal layer of cells within the leaf, whose name means "middle leaf" in Greek. It is divided into two layers:

    • Palisade mesophyll layer (D): Composed of column-like cells located just under the upper epidermis. These cells contain many chloroplasts and carry out most of the photosynthesis.

    • Spongy mesophyll layer (F): Located between the palisade layer and the lower epidermis. It contains more rounded, loosely packed cells with fewer chloroplasts, separated by larger, airy spaces that allow for the diffusion of oxygen, water vapor, and carbon dioxide.

Vascular Tissues (Veins)

  • Veins are extensions that run from the roots to the edges of the leaves, containing vascular tissues.

    • Bundle sheath cells (E): Form the outer layer of the vein, creating a circle around the xylem and phloem.

    • Xylem (G): The upper layer of cells within the vein, usually larger than phloem cells. It transports water and minerals from the roots and stems into the leaf.

    • Phloem (H): The lower layer of cells within the vein. It transports sugar (food) made in the leaves to the rest of the plant.

Gas Exchange Structures

  • Stomata (I): Tiny holes, also known as "little mouths," found within the lower epidermis. They regulate the intake of carbon dioxide and the release of oxygen during photosynthesis, and water vapor also exits through them.

  • Guard cells (C): Specialized cells that surround the stomata, shaped like two cupped hands. Changes in water pressure cause the stoma to open or close. If guard cells are full of water, they swell and bend away from each other, opening the stoma. During dry times, guard cells close to limit water loss.