leaf

Leaf shapes and arrangements on the stem are many and varied; in cross section, a leaf is a complex arrangement of cells surrounded by the epidermis. This is pierced by the stomata through which gases enter and leave.

The veins of a maple (Acer) leaf. As in most plants, the leaf is the primary food-manufacturing organ of the maple. The veins act as a skeletal structure and transport materials to and from the leaf tissue. In the autumn, the green chlorophyll breaks down, revealing the striking yellow, orange, and red pigments in the leaf.

Lateral outgrowth on the stem of a plant, and in most species is the primary organ of photosynthesis (the process in which the energy from absorbed sunlight is used to combine carbon dioxide and water to form sugars). The chief leaf types are cotyledons (seed leaves), scale leaves (on underground stems), foliage leaves, and bracts (in the axil of which a flower is produced).

Typically, leaves are composed of three parts: the sheath or leaf base, the petiole or stalk, and the lamina or blade. The lamina has a network of veins through which water and nutrients are conducted. A leaf contains several layers of tissue. Structurally the leaf is made up of inner tissues known as mesophyll cells, surrounded by the epidermis and usually an additional waxy layer, termed the cuticle, which prevents excessive evaporation of water from the leaf tissues by transpiration.

The epidermis is formed of a single layer of cells on the outside of the leaf. It is interrupted by small pores, or stomata, through which gas exchange between the plant and the atmosphere occurs for photosynthesis and respiration. Water is also lost through the stomata by transpiration. The mesophyll can be divided into two layers. The upper half of the mesophyll has elongated cells densely packed with chloroplasts that carry out photosynthesis – this is the palisade tissue. The lower half has more irregularly arranged cells with fewer chloroplasts – this is called spongy mesophyll. The mesophyll includes many air spaces, which allow carbon dioxide and oxygen to diffuse to and from the leaf cells (see diffusion). Outside the epidermis is a waxy layer, called the cuticle, which prevents evaporation of water directly from epidermal cells.

A simple leaf is undivided, as in the maple or oak. A compound leaf is composed of several leaflets, as in the blackberry, horse chestnut, or ash (the latter being a pinnate leaf). Leaves that are shed in the fall are termed deciduous, while evergreen leaves are persistent.