Leaves

Leaves are the flat, primarily photosynthetic organs of most vascular plants. They develop on the shoots and function to capture light, drive photosynthesis, exchange gases, and regulate water loss through transpiration. Most leaves are green due to chlorophyll, though some pigment variation occurs.

Typically a leaf consists of a blade (lamina) attached to the stem by a petiole, though many leaves are sessile. The blade contains a midrib with a network of veins and is bounded by an epidermis coated with a waxy cuticle. The interior mesophyll usually includes palisade tissue where most photosynthesis occurs and spongy tissue that facilitates gas exchange. Leaves may have specialized structures such as stomata for gas exchange and guard cells that regulate opening and closing.

Leaves are classified by structure as simple (single blade) or compound (divided into leaflets, such as pinnate or palmate). They also vary in arrangement on the stem: alternate, opposite, or whorled. Venation patterns differ: net-like in most dicots and parallel in many monocots.

Lifespan and shedding vary: many deciduous species lose leaves seasonally; evergreen species retain them for multiple years. Abscission occurs at specialized zones that form a separation layer before leaf drop. Adaptations include xerophytic features such as a thick cuticle and reduced stomata in dry environments, and spines or reduced leaves in some arid taxa; in succulent plants leaves may store water, and in many cacti spines are modified leaves.

Ecologically, leaves are primary producers that form the base of most terrestrial food webs and contribute to litter and nutrient cycling. Many leaves have culinary, medicinal, or ornamental value, and leaf chemistry affects herbivory and community dynamics. The study of leaves encompasses morphology, anatomy, physiology, and evolutionary variation across plant groups.