Conceptual Overview

Leaves, the lateral organs of a shoot, typically have a more-or-less flattened form, and a dorsiventral or bifacial structure. The upper (or adaxial) and lower (abaxial) sides of the leaf with respect to the stem are easily distinguished. In most plants, leaves are dorsiventral in their internal structure. This is in contrast to usually cylindrical, radially-symmetrical axial organs, i.e. stems and roots. However, some monocots have isobilateral, or unifacial leaves, and some have cylindrical (centric) leaves. The flattened, relatively thin, plate-like form of the leaf provides a maximal surface area with respect to the volume unit of its tissues. This in turn, facilitates absorption of light energy and carbon dioxide, both of which are required for the main function of a typical green leaf, photosynthesis. Leaf anatomy is also well adapted to the second most important leaf function, transpiration. The main part of the leaf is its lamina (or leaf blade) to which the above characteristics belong. In addition, leaves often have a petiole and a pair of stipules. A petiole attaches the lamina to the stem. Stipules are usually paired flaps of projections of tissue found beneath the petiole, if present at all.

The lamina consists of an adaxial and abaxial epidermis enclosing several layers of mesophyll cells interspersed with a network of vascular bundles. The role of supporting tissues in leaves may be performed by sclerenchyma fibers, individual sclereids,or collenchyma strands. Fibers are most often associated with large veins either surrounding them completely, or forming abaxial or (to a lesser extent) adaxial caps. Collenchyma is often found at the periphery of major veins and on the leaf margin where it helps to prevent rupturing of the leaf. The anatomy of the lamina is normally best shown in cross-sectional views.

Mesophyll is, as a rule, a multi-layered parenchyma (or chlorenchyma) consisting of cells which are similar in their structure and functional specialization for photosynthesis. Their most important specific feature is the extraordinary development of plastids. There is no other tissue where there are so many chloroplasts or such a high level of organization. Characteristic of mesophyll is the presence of a highly developed anastomosing system of intercellular spaces in which more than half of the cell surfaces are in contact with the gas phase. The remaining surfaces are in cell-to-cell contact and have numerous plasmodesmata that allow for symplastic continuity among mesophyll cells and the vascular system. The mesophyll is usually divided into palisade and spongy parenchyma. The palisade parenchyma is just below the adaxial epidermis and consists of one to several layers of compactly arranged vertically oriented cylindrical cells. The spongy parenchyma extends from the palisade parenchyma to the lower epidermis. The cells of the spongy parenchyma are irregular in shape and loosely arranged. Idioblasts such as oil cells, crystal-containing idioblasts, and mucilage cells, as well as resin ducts and laticifers may be interspersed among the mesophyll of some plants.

The epidermis consists of a single (or sometimes multiple) layer of cells that covers the entire leaf surface and is continuous with the surface of the stem. In addition, the epidermis contains a variety of cells whose description has been given in the units of study on "Epidermis" and "Secretory Structures." Adaxial and abaxial epidermises often differ from each other in the size and shape of ground epidermal cells, and in the frequency of trichomes. In leaves which are oriented with the adaxial epidermis to the light, stomata mostly occur within the abaxial epidermis (i.e. designated as hypostomatous leaves). Such a location allows for the reduction of water loss during transpiration. In leaves without a preferable orientation with respect to light, the stomata may be found in both epidermises (i.e. designated as amphistomatous leaves). In rare cases, such as floating leaves of water plants or xeromorphic leaves of grasses, the stomata are confined to the upper epidermis (i.e. designated as epistomatic leaves).

The vascular system of leaves consists of veins which branch and form a continuous network extending throughout the leaf and passing into the stem. The arrangement of veins in a leaf is called venation. There are several types of venation, but most common are parallel venation being broadly typical of monocots, and reticulate venation characteristic of dicots.

In cross sections of the lamina, veins are usually in a single row. Two main types of veins are recognized according to their size and function, major veins and minor veins. The functions of the major veins are to conduct water and photosynthates, and to support the delicate mesophyll. Major veins are typically composed of vascular bundles, bundle sheaths, undifferentiated parenchyma, sclerenchyma and collenchyma. They function in the transport of substances in and out of the leaf, and in the strengthening of thin mesophyll. Most major veins in dicots usually appear as ribs on the abaxial side of the lamina. It is the major veins that determine the venation pattern of a leaf. By contrast, minor veins are completely enclosed in the mesophyll. The functions of the minor veins are to distribute the transpiration stream through the mesophyll, and to load the phloem with photosynthates produced by the chlorenchyma cells. Accordingly, no mesophyll cell is more than a few cells away from a minor vein. A minor vein consists of a small vascular bundle enclosed in a bundle sheath. A characteristic feature of vascular bundles in minor veins is the prominence of parenchyma cells, especially in phloem. Two main types of minor veins are distinguished, open (or symplastically connected to the mesophyll), and closed (or symplastically isolated from the mesophyll). Lamina bundles are usually collateral and closed (with no secondary growth), with the adaxial xylem and abaxial phloem.

In dicots, the vascular bundles collect at the base of the lamina into several strands and pass through the petiole into the node of the stem where they are called leaf traces.

The vascular bundles of a leaf do not directly contact intercellular spaces or mesophyll cells. They are delimited from them by parenchymatous bundle sheaths which lack intercellular spaces. In some monocots, there are two sheaths, the outer parenchymatous and inner sclerenchymatous. In C₃ plants the cells of the parenchymatous sheath are small and contain only a few chloroplasts and other organelles. Alternatively, in C₄ plants bundle sheath cells are large with many chloroplasts and other organelles. Therefore, they should be included in chlorenchyma tissue. Most commonly, palisade mesophyll cells are like a wreath (German = Kranz) radially arranged around vascular bundles in such plants (a so-called Kranz syndrome or C₄ syndrome).

The autumnal fall of leaves from deciduous trees occurs due to the formation of a specialized separation layer called an abscission zone. Abscission zones are found at the site of connection of petioles with the stem. This layer of cells defines a transverse rupture zone. Similar zones are also found at the sites of connection for flowers and fruits. It is a region that has parenchyma cells with thin walls, tracheids instead of vessels, and a near lack of fibers. Differentiation of this zone may be early, or it starts just before activation. Activation involves the action of the plant growth regulator ethylene (a gas), to stimulate degradation of the middle lamella and hydrolysis of the cellulosic wall. Since lignin is not affected, everything at the juncture site is severed except for xylem. Physical force, in time, destroys the xylem. The formation of the abscission zone prevents injury to the living tissues in the stem and protects the newly exposed surface from desiccation and infection.