Definitions

secondary tissue

Bark

[bahrk]

Bark, also known as periderm, is the outermost layer of stems and roots of woody plants such as trees. It overlays the wood and consists of three layers, the cork or phellem, the phelloderm and the cork cambium or phellogen. 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; A number of plants are also grown for their attractive or interesting bark colorations and surface textures.

Botanic description

  • Cork - an external, secondary tissue impermeable to water and gases.
  • Cork cambium - A layer of cells, normally one or two cell layers thick that is in a persistent meristematic state that produces cork.
  • Phelloderm - (not always present) A layer of cells formed in some plants from the inner cells of the cork cambium (Cork is produced from the outer layer).
  • Cortex - 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.
  • Phloem - nutrient-conducting tissue composed of sieve tube or sieve cells mixed with parenchyma and fibers.

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 made up of cork cambian, cork and phelloderm. It replaces the dermal layer and acts as a covering much like the corky bark, it too is made up of mostly dead tissue. The skin on the potato is a periderm.

Definitions of the term can vary. In another usage, bark consists of the dead and protective tissue found on the outside of a woody stem, and does not include the vascular tissue.

The vascular cambium is the only part of a woody stem where cell division occurs. It contains undifferentiated cells that divide rapidly to produce secondary xylem to the inside and secondary phloem to the outside.

Along with the xylem, the phloem is one of the two tissues inside a plant that are involved with fluid transport. The phloem transports organic molecules (particularly sugars) to wherever they are needed.

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, parasites and diseases, as well as dehydration and extreme temperatures. Cork can contain antiseptics like tannins. Some cork is substantially thicker, providing further insulation and giving the bark a characteristic structure. In the cork oak (Quercus suber) the bark is thick enough to be harvestable as cork product without killing the tree. Bark has been used a covering in the making of canoes, the most famous example of this is the birch canoes of North America.

The bark of some trees is edible, and in Finland, pine bread is made from rye to which the toasted and ground innermost layer of pine bark is added. 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.

Among the commercial products made from bark are cork, cinnamon, quinine (from the bark of Cinchona) 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, 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.

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