Leaves are the organs of vascular plants and the main lateral appendages of stems. Leaves and stems form buds together. Leaves are collectively referred to as "autumn leaves".

A picture of a simple leaf.

Apex midvein (main vein) secondary vein. Laminar leaf margin petiole bud stem

Although many leaves of different shapes, sizes and textures can be seen, the leaf is usually a thin, flat dorsal organ, carried on the ground and dedicated to photosynthesis. In most leaves, the main photosynthetic tissue is the leaf blade or the upper side of the leaf blade, but in some species, including the mature leaves of Eucalyptus, the leaf blade is said to be equilateral. Most of the leaves have unique upper (front) and lower (back) surfaces, with different colors, hairiness, number of pores (holes for gas suction and discharge), number and structure of epidermal wax, and other characteristics.

Broad and flat leaves with complex vessels are called giant mesophylls, and the species with them (mostly) are broad-leaved or giant leafy plants. In other species, such as bryophytes, there are different evolutionary origins. The leaves are very simple and have only one vein, which is called microphylla.

Some leaves, such as scales, bulbs, scales, are not on the ground, and in many aquatic species, the leaves are submerged in the water. Succulent plants usually have thick, juicy leaves, but some of them do not have major photosynthetic functions and may die when they mature, such as some chlorophyll and spinous processes. In addition, some of the leaflike structures found in vascular plants are not completely homologous with them. Examples include flat plant stems, known as petioles and branches, and flat leaf stems, known as petioles, which differ in structure and origin from leaves. Many structures of non vascular plants, such as the thallus of bryophytes and Artemisia, and even some leaflike lichens, are not plants at all (in the plant kingdom), and their appearance and function are very similar to leaves.