Bark

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]

Contents 1 Botanic description 2 Periderm 3 Rhytidome 4 Uses 5 Bark removal 6 Bark repair 7 Gallery 8 See also 9 References

Botanic description

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.

Periderm

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.

Within the periderm are lenticels, which form during the production of the first periderm layer. Since there are living cells within the cambium layers that need to exchange gases during metabolism, these lenticels, because they have numerous intercellular spaces, allow gaseous exchange with the outside atmosphere. As the bark develops, new lenticels are formed within the cracks of the cork layers.

Rhytidome

The rhytidome is the most familiar part of bark, it is the outer layer that covers the trunks of trees. It is composed mostly of dead lignified cells and is produced by the formation of multiple layers of periderm tissue. It is generally thickest and most distinctive at the trunk or bole (The area from the ground to where the main branching starts) of the tree.

Uses

Cork, sometimes confused with bark in colloquial speech, is the outermost layer of a woody stem, derived from the cork cambium. It serves as protection against damage from parasites, herbivorous animals and diseases, as well as dehydration and fire. Cork can contain antiseptics like tannins, that protect against fungal and bacterial attacks that would cause decay.

In some plants, the bark is substantially thicker, providing further protection and giving the bark a characteristically distinctive structure with deep ridges. In the cork oak (Quercus suber) the bark is thick enough to be harvested as a cork product without killing the tree. Some barks can be removed in long sheets; the smooth surfaced bark of birch trees has been used as a covering in the making of canoes, the most famous example of this is the birch canoes of North America.^[8]

The bark of some trees is edible; in Finland, pine bread is made from rye to which the toasted and ground innermost layer of pine bark is added, the Sami people of far northern Europe used large sheets of Pinus sylvestris bark that were removed in the spring, prepared and stored for use as a staple food resource and the inner bark was eaten fresh, dried or roasted.^[9] Bark contains strong fibres known as bast, and there is a long tradition in northern Europe of using bark from coppiced young branches of the small-leaved lime (Tilia cordata) to produce cordage and rope, used for example in the rigging of Viking age longships.^[10]

Among the commercial products made from bark are cork, cinnamon, quinine^[11] (from the bark of Cinchona)^[12] and aspirin (from the bark of willow trees). The bark of some trees notably oak (Quercus robur) is a source of tannic acid, which is used in tanning. Bark chips generated as a by-product of lumber production, are often used in bark mulch in western North America. Bark is important to the horticultural industry since in shredded form it is used for plants that do not thrive in ordinary soil, such as epiphytes.

Wood Adhesives from Bark-Derived Phenols: Wood Bark has lignin content and when it is pyrolyzed (subjected to high temperatures in the absence of oxygen), it yields a liquid bio-oil product rich in natural phenol derivatives. The phenol derivatives are isolated and recovered for application as a replacement for fossil-based phenols in phenol-formaldehyde (PF) resins used in Oriented Strand Board (OSB) and plywood.

Bark removal

Cut logs used for the production of lumber or even log cabins generally have the bark removed, either just before cutting or for curing. Such logs and even trunks and branches found in their natural state of decay in forests, where the bark has fallen off, are said to be decorticated.

A number of living organisms live in or on bark, including insects,^[13] fungi and other plants like mosses , algae and other vascular plants. Many of these organisms are pathogens or parasites but some also have symbiotic relationships.

Bark repair

The degree to which trees are able to repair gross physical damage to their bark is very variable. Some are able to produce a callus growth which heals over the wound rapidly, but leaves a clear scar, whilst others such as oaks do not produce an extensive callus repair.

Gallery

See also

• Bark painting
• Bark beetle
• Trunk (botany)

References