leaf

A leaf is the primary site of photosynthesis in most plants and is typically the green, blade-like organ that grows from the plant’s stem. It consists of a flattened blade (lamina) that is sometimes attached by a stalk (petiole) and, in some species, lacks a petiole entirely. Leaves vary widely in size, shape, and margin, and may be simple (a single blade) or compound (divided into leaflets).

Internal structure includes an epidermal and cuticular layer, stomata for gas exchange, and a photosynthetic mesophyll arranged into palisade and spongy tissues. In most dicots, veins form a reticulate network; monocots typically show parallel venation. Veins supply water and carry sugars; the arrangement supports the leaf’s surface area for light capture.

Leaves develop from primordia at the shoot apical meristem and can be deciduous or evergreen. They participate in photosynthesis, transpiration, and temperature regulation, and may store nutrients. Adaptations include thick cuticles in arid environments, trichomes or hairs for protection, and various forms such as needles in conifers or succulent leaves in arid species.

Leaves show extensive ecological and evolutionary diversity. Seasonal changes can cause color shifts and abscission in deciduous species. Leaf morphology often reflects environment, such as sun versus shade, humidity, and herbivory. Leaves have played a central role in studies of plant physiology and taxonomy.

Beyond botany, the word leaf also refers to a leaf of paper in a book, the action of turning pages (to leaf through), and, in computer science, a leaf node at the end of a tree data structure.