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Bark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines and shrubs. Bark refers to all the tissues outside of the vascular cambium and is a nontechnical term.^[1] It overlays the wood and consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost area of the periderm. The outer bark in older stems, includes the dead tissue on the surface of the stems, along with parts of the innermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees is also called the rhytidome. Products used by people that are derived from bark include spices and other flavorings, tannin, resin, latex, medicines, poisons, various hallucinatory chemicals and cork. Bark has been used to make cloths, canoes, ropes and used as a surface for paintings and map making;^[2] A number of plants are also grown for their attractive or interesting bark colorations and surface textures.^[3][4]

What is commonly called bark, includes a number of different tissues. Cork is an external, secondary tissue that is impermeable to water and gases, it is produced by the Cork cambium which is a layer of cells that are in a persistent meristematic state. The cork cambium, which is also called the phellogen, is normally only one or two cell layers thick and the outside surface produces cork. The phelloderm, which is not always present in all barks, is a layer of cells formed from the inner cells of the cork cambium, it contains suberin, a waxy substance which protects the stem against water loss, the invasion of insects into the stem, and prevents infections by bacteria and fungal spores.^[5] The cambium tissues are the only parts of a woody stem where cell division occurs; undifferentiated cells in the cambium divide rapidly to produce secondary xylem to the inside and secondary phloem to the outside. Phloem which is a nutrient-conducting tissue composed of sieve tubes or sieve cells mixed with parenchyma and fibers. The Cortex is the primary tissue of stems and roots. In stems the cortex is between the epidermis layer and the phloem, in roots the inner layer is not phloem but the pericycle. From the outside to the inside of a mature woody stem, these layers are arranged (1) Cork (Phellem) (2) Cork cambium (Phellogen) (3) Phelloderm (4) Cortex (5) Phloem (6) Cambium (7) Xylem. The bark includes (1) through (5), and is composed of periderm and phloem and the cells that produce these tissues.^[6] The periderms includes (1),(2) and (3).

In young stems, which lack what is commonly called bark, the tissues are from the outside to the inside epidermis, periderm, cortex, primary phloem, secoundary phloem, vascular cambium and then xylem. As the stem ages and grows, changes occur that transform the surface of the stem into the bark. The epidermis, which is a layer of cells that cover the plant body, including the stems, leaves, flowers and fruits, that protects the plant from the outside world. In old stems the epidermal layer, cortex, and primary phloem become separated from the inner tissues by thicker formations of cork. Due to the thickening cork layer these cells die because they do not receive water and nutrients. This dead layer is the rough corky bark that forms around tree trunks and other stems.

In smaller stems and on typically non woody plants, sometimes a secondary covering forms called the periderm, which is composed of cork, the cork cambium, and the phelloderm. It replaces the epidermis, and acts as a protective covering like the epidermis, it too is made up of mostly dead tissue. The skin on the potato is a periderm. In woody plants the epidermis of newly grown stems is replaced by the periderm later in the year. As the stems grow a layer of cells form under the epidermis, called the cork cambium, these cells produce cork cells that turn into cork. The single cell layer of cork cells is called the phelloderm, it is produced inside the cork cambium layer (also called the phellogen). As the stem grows, the phelloderm produces new layers of cork which are impermeable to gases and water and the cells outside of the periderm, namely the epidermis, cortex and older secondary phloem die. ^[7] As the stem grows wider, the periderm cannot effectively seal the stem from the outside world, thus the formation of cork becomes the new protective surface for the stem.

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