Internal Structure of Leaf.

Leaves are the principle structure, produced on stems, where photosynthesis takes place. Cacti are an exception. The leaves are reduced to spines, and the thick green, fleshy stems are where photosynthesis takes place.

The leaf blade is composed of several layers as follows:

Epidermis – Outer layer of tissues

Cuticle – Waxy protective outer layer of epidermis that prevents water loss on leaves, green stems, and fruits. The amount of cutin or wax increases with light intensity.
Leaf hairs – part of the epidermis

Palisade layer – A tightly packed layer of parenchyma tissues filled with chloroplasts for photosynthesis.

Chloroplasts – Sub-cellular, photosynthetic structures in leaves and other green tissues. Chloroplasts contain chlorophyll, a green plant pigment that captures the energy in light and begins the transformation of that energy into sugars.

Vascular bundle – Xylem and phloem tissues, commonly known as leaf veins.

Spongy mesophyll – Layer of parenchyma tissues loosely arranged to facilitate movement of oxygen, carbon dioxide, and water vapor. It also may contain some chloroplasts.

Stomata – Natural openings in leaves and herbaceous stems that allow for gas exchange (water vapor, carbon dioxide, and oxygen).

Guard cells – Specialized kidney-shaped cells that open and close the stomata.