In botany, a leaf is a plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin, to expose the chloroplast containing cells (chlorenchyma) to light over a broad area, and to allow light to penetrate fully into the tissues. Leaves are also the sites in most plants where respiration, transpiration, and guttation take place. Leaves can store food and water, and are modified in some plants for other purposes.
A structurally complete leaf of an angiosperm consists of a petiole (leaf stem), a lamina (leaf blade), and stipules (small processes located to either side of the base of the petiole). Not all species produce leaves with all of these parts. In some species, stipules are not obvious; or a petiole may be absent. The blade is not always laminar (flattened). External leaf characteristics (shape, margins, hairs, etc.) are important for identifying plant species. The point at which the petiole attaches to the plant stem is called the leaf axil.
A leaf typically consists of the following tissues:
* An epidermis that covers the upper and lower surfaces;
The epidermis is the outer layer of cells covering the leaf blade. The layer is usually transparent (cells lack chloroplasts) and coated on the outer surface with a waxy cuticle that prevents water loss. The cuticle may be thinner on the lower epidermis than on the upper epidermis; and is thicker on leaves from dry climates as compared with those from wet climates.
The epidermis is covered with pores called stomata (sing., stoma) that enable oxygen and carbon dioxide to move in and out of the leaf. These pores are more numerous over the lower epidermis than the upper epidermis in most leaves. Water vapor also passes out of the stomata during transpiration. To conserve water, the guard cells at the opening of the stomata may force the opening to close up during the night.
Trichomes or hairs grow out from the epidermis in many species.
Most of the interior of the leaf between the upper and lower layers of epidermis is a parenchyma or chlorenchyma tissue called the mesophyll. This is the primary photosynthetic tissue of the plant. It is divided into two layers: an upper palisade layer of tightly packed, vertically elongated cells, one to two cells thick. Beneath the palisade layer is the spongy layer. The cells of the spongy layer are more rounded and not so tightly packed. The pores or stomata of the epidermis open into the spaces between the spongy layer cells.
Leaves are normally green in color, which comes from chlorophyll found in plastids in the chlorenchyma. Leaves in Temperate, Boreal, and seasonally dry zones may be seasonally deciduous (falling off or dying for the inclement season). In cold autumns they sometimes turn yellow, bright orange or red as various accessory pigments (carotenoids and anthocyanins) are revealed when the tree responds to cold and reduced sunlight by curtailing chlorophyll production.
The veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll. The veins are made up of xylem, which brings water from the stem into the leaf, and phloem, which usually moves sap out, the latter containing the glucose produced by photosynthesis in the leaf. The xylem typically lies over the phloem, and both are embedded in a dense parenchyma with usually some structural collenchyma tissue present.