Leaf Structure

How is the leaf organized in cross-section?

Moving from the top of the leaf (adaxial surface) to the bottom (abaxial surface), you will observe these structures: the epidermis, the mesophyll layer, the vascular bundles.

Epidermis

  • Outer layer of cells that protects the internal stem tissues

  • A layer of cuticle coats the epidermis to protect against dessication and UV radiation

  • Stomata are dotted along the epidermis for gas exchange

    • In monocots, like grasses, stomata occur on both sides of leaf

    • In eudicots, like trees, stomata are only on the abaxial side of leaf

Mesophyll layer

  • The layer of parenchyma cells involved in photosynthesis and gas exchange

  • In Eudicots, this layer is separated into two layers: palisade layer and spongy layer

Palisade layer

  • A layer of chlorenchyma cells, where light interception for photosynthesis occurs

  • This layer is toward the adaxial, or upper, side of the leaf

Spongy layer

  • A layer of aerenchyma cells, where gas exchange for photosynthesis occurs

  • This layer is toward the abaxial, or lower, side of the leaf

Vascular Bundles or Veins

  • The veins of the leaves consists of vascular bundles

  • The vascular bundles are composed of primary xylem and primary phloem, which formed from the shoot apical meristem and the procambium

Above: Eudicot leaf in cross-section, showing epidermis, vascular bundles, palisade and spongy mesophyll layers

Below: Monocot leaf in cross-section, showing epidermis, mesophyll, and vascular bundles

What is the morphology of leaves?

Blade

  • The flattened, photosynthetic portion of the leaf

Petiole

  • The thin, stalk-like portion; sometimes called the stalk

  • The petiole connects the blade to the stem

  • Not all leaves possess a petiole (=apetiolate)

Stipules

    • The leaf-like outgrowths from petiole near stem

    • Most leaves do not have stipules (=non-stipulate)

Veins

    • Veins are the visible vascular bundles running through the leaf

Additional Resources