According to Colorado State University, the parts of a leaf can be divided into internal features and external features. The internal features of a leaf include the epidermis, the cuticle, leaf hairs, the palisade layer, chloroplasts, the vascular bundle, spongy mesophyll, the stomata and guard cells. The external features include the leaf blade, the petiole and the stipules.
The epidermis of the leaf is the outer layer of tissues. This tissue includes the cuticle, a waxy protective outer layer of the epidermis that prevents water loss, and the leaf hairs. The palisade layer is a tightly packed layer of parenchyma tissues, which are filled with chloroplasts and used for photosynthesis. Chloroplasts themselves contain chlorophyll, a pigment that captures energy from light and starts the process of turning that energy into sugars.
The vascular bundle consists of the xylem and phloem tissues, which act as veins for the leaf. The spongy mesophyll is a layer of parenchyma tissues arranged loosely to help with the movement of oxygen, carbon dioxide and water vapor. The stomata are natural openings in leaves that allow for gas exchange. The guard cells are kidney-shaped cells that open and close the stomata.
The leaf blade is the flat part of the leaf, and the petiole is the stalk of the leaf which connects it to the branch. The stipules are small leaf-like appendages that grow where the petiole connects to the branch.