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
The mystery of the origin of the leaf sheath (SciTechDaily 9Dec2021; Richardson et al. 2021)