Although other plant organs can perform photosynthesis, the leaf is the primary structure for this function. Photosynthesis is the process of capturing light energy and transforming it into chemical energy, usually stored in sugars and starches. The flattened blades of leaves provide a large surface area to effectively collect light energy.

      There are four characteristics of leaves that distinguish them from other plant organs: (1) leaves are almost always determinate, (2) leaves are lateral appendages of shoot systems, (3) leaves arise on the flanks of the shoot apical meristem, and (4) leaves are associated with axillary buds.

      Leaves may be classified as enations, microphylls, or megaphylls. Enations are small outgrowths of photosynthetic tissue from the shoot. A single vein of vascular tissue extends partway into the small "flap" but does not reach the tip. Microphylls, found in some of the lower vascular plant groups, are usually small leaves supplied with a single vascular leaf trace. Megaphylls are usually large, expanded leaves found in higher vascular plants. Megaphylls typically are supplied with more than one leaf trace and the vasculature often branches throughout the entire leaf tissue. All three classes of leaves typically have the defining characteristics of leaves, except that the association of leaves with axillary buds is uncommon in the lower vascular plants (ferns and fern allies.)

Basic Organization

      There is a multitude of terms used to describe the variable shape of leaves. The paragraph below serves as an introduction to the basic anatomy of a leaf while the subsequent sections will deal with some of the variations and specializations of different types of leaves.

      Most leaves consist of three major morphological regions: the blade, the petiole, and the leaf base. The blade region differs in morphology from the other two regions because it is specialized for photosynthesis. The blade is dorsiventral from its developmental origin. The dorsal surface usually faces upward and is known as the adaxial surface. The downward facing or ventral surface is the abaxial surface. The two surfaces usually differ in their internal organization. A transverse section of a leaf blade reveals that both adaxial and abaxial surfaces are covered with epidermal cells. Lying beneath the adaxial epidermis are palisade parenchyma cells containing chloroplasts that carry out the photosynthesis. Below the palisade parenchyma is spongy parenchyma interspersed with vascular bundles, followed by the abaxial epidermal cells. The vascular bundles have xylem (the water transporting tissue) toward the adaxial surface and phloem (the sugar transporting tissue) toward the abaxial surface. This makes perfect sense when considering that most leaves have stomata, the apertures that allow for gas exchange and transpiration, on their abaxial surface, although some leaves have stomata on both surfaces.

      The petiole region is the stalk of the leaf and it helps to orient the leaf blade so that it most efficiently intercepts the light. Leaves lacking a petiole are termed sessile.

      The leaf base region is the point of attachment of the leaf to the stem. The amount of tissue making up the leaf base varies from species to species. Many monocotyledons have sheathing leaf bases that completely surround the shoot. Many leaf bases are associated with stipules, leaf-like appendages that occur on the flanks of the leaf base. Some stipules encircle the entire stem at the leaf base, and some stipules (median stipules) are found in the middle of the petiole. Located near the bottom of the leaf base is an abscission layer of cells. These cells degrade due to chemical changes and the leaf eventually abscises, or falls away.

      Leaves of monocots differ from the leaves of eudicots. While monocots usually have veins that run parallel to each other, with cross veins connecting them, eudicots exhibit complex systems of netted or reticulate venation.

      Leaves originate on the flanks of the shoot apical meristem. Initially the new leaf appears as a clump of cells, known as a leaf buttress, at the side of the meristem. The base of this bulge grows laterally around the apical dome to form the leaf base. A leaf primordium grows upward from the leaf buttress. Initially the entire leaf primordium is flattened dorsiventrally. Later a rounded petiole may develop between the leaf base and the future leaf blade. Five or six leaf primordia may be packed into an apical bud. These embryonic leaves may have blades folded or rolled up in the bud. As development advances, these unfold or unroll until they form fully expanded leaves.

      The leaves found on one plant are not always identical to one another but vary from one node to another. The cotyledons may be specialized to store food for the developing seedling, and often are not photosynthetic. The true vegetative leaves have wide blades for optimal photosynthesis, while floral leaves (bracts) may be reduced in order not to distract pollinators away from the showy floral parts.