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Leaf anatomy

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). The petiole attaches to the stem at a point called the "leaf axil". Not every species produces leaves with all of the aforementioned structural components. In some species, paired stipules are not obvious or are absent altogether. A petiole may be absent, or the blade may not be laminar (flattened). The tremendous variety shown in leaf structure (anatomy) from species to species is presented in detail below under Leaf morphology. After a period of time (i.e. seasonally, during the autumn), deciduous trees shed their leaves. These leaves then decompose into the soil.

A leaf is considered a plant organ and typically consists of the following tissues:

1. An epidermis that covers the upper and lower surfaces
2. An interior chlorenchyma called the mesophyll
3. An arrangement of veins (the vascular tissue).

Epidermis

The epidermis is the outer multi-layered group of cells covering the leaf. It forms the boundary separating the plant's inner cells from the external world. The epidermis serves several functions: protection against water loss, regulation of gas exchange, secretion of metabolic compounds, and (in some species) absorption of water. Most leaves show dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions.

The epidermis is usually transparent (epidermal cells lack chloroplasts) and coated on the outer side with a waxy cuticle that prevents water loss. The cuticle is in some cases 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 tissue includes several differentiated cell types: epidermal cells, guard cells, subsidiary cells, and epidermal hairs (trichomes). The epidermal cells are the most numerous, largest, and least specialized. These are typically more elongated in the leaves of monocots than in those of dicots.

The epidermis is covered with pores called stomata, part of a stoma complex consisting of a pore surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts. The stoma complex regulates the exchange of gases and water vapor between the outside air and the interior of the leaf. Typically, the stomata are more numerous over the abaxial (lower) epidermis than the adaxial (upper) epidermis.