The bark ofPinus thunbergii is made up of countless shiny layers.
Bark is the outermost layer ofstems androots ofwoody plants. Plants with bark includetrees, woodyvines, andshrubs. Bark refers to all thetissues outside thevascular 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 olderstems is living tissue, includes the innermost layer of the periderm. The outer bark on older stems includes the dead tissue on the surface of the stems, along with parts of the outermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees which lies external to the living periderm is also called therhytidome.[2][3][4][5][6][7][8][9][10][11][excessive citations]
Products derived from bark include bark shingle siding and wall coverings, spices, and other flavorings,tanbark fortannin,resin,latex, medicines, poisons, varioushallucinogenic chemicals, andcork. Bark has been used to make cloth, canoes, and ropes and used as a surface for paintings and map making.[12] A number of plants are also grown for their attractive or interesting bark colorations and surface textures or their bark is used as landscapemulch.[13][14]
The process of removing bark isdecortication and a log or trunk from which bark has been removed is said to bedecorticated.[15][16][17][18][19]
Cork cell walls containsuberin, a waxy substance which protects the stem against water loss, the invasion of insects into the stem, and prevents infections bybacteria and fungal spores.[21] The cambium tissues, i.e., the cork cambium and thevascular cambium, are the only parts of a woody stem wherecell division occurs; undifferentiated cells in the vascular cambium divide rapidly to produce secondaryxylem to the inside and secondaryphloem to the outside. Phloem is anutrient-conducting tissue composed of sieve tubes or sieve cells mixed withparenchyma and fibers. Thecortex is the primary tissue ofstems and roots. In stems the cortex is between theepidermis layer and the phloem, in roots the inner layer is not phloem but thepericycle.[22][23][24][25][26][27][28][29][3][excessive citations]
As the stem ages and grows, changes occur that transform the surface of the stem into the bark. Theepidermis 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 treetrunks and other stems.
Cork, sometimes confused with bark in colloquial speech, is the outermost layer of a woody stem, derived from thecork cambium. It serves as protection against damage fromparasites,herbivorous animals and diseases, as well as dehydration and fire.
Often a secondary covering called the periderm forms on small woody stems and many non-woody plants, which is composed of cork (phellem), thecork cambium (phellogen), and the phelloderm. The periderm forms from the phellogen which serves as a lateral meristem. The periderm replaces the epidermis, and acts as a protective covering like the epidermis. Mature phellem cells havesuberin in their walls to protect the stem from desiccation and pathogen attack. Older phellem cells are dead, as is the case with woody stems. The skin on the potato tuber (which is an underground stem) constitutes the cork of the periderm.[30][31]
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. A limited number of cell layers may form interior to the cork cambium, called the phelloderm.As the stem grows, the cork cambium produces new layers of cork which are impermeable to gases and water and the cells outside the periderm, namely the epidermis, cortex and older secondary phloem die.[32]
Within the periderm arelenticels, 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.
The rhytidome is the most familiar part of bark, being the outer layer that covers the trunks of trees. It is composed mostly of dead cells and is produced by the formation of multiple layers ofsuberized periderm, cortical and phloem tissue.[33] The rhytidome is especially well developed in older stems and roots of trees. In shrubs, older bark is quicklyexfoliated and thick rhytidome accumulates.[34] It is generally thickest and most distinctive at the trunk orbole (the area from the ground to where the main branching starts) of the tree.
Bark tissues make up by weight between 10 and 20% of woody vascular plants and consists of variousbiopolymers,tannins,lignin,suberin andpolysaccharides.[35] Up to 40% of the bark tissue is made of lignin, which forms an important part of a plant, providing structural support by crosslinking between different polysaccharides, such as cellulose.[35]
Condensedtannin, which is in fairly high concentration in bark tissue, is thought to inhibitdecomposition.[35] It could be due to this factor that the degradation of lignin is far less pronounced in bark tissue than it is in wood. It has been proposed that, in the cork layer (the phellogen), suberin acts as a barrier to microbial degradation and so protects the internal structure of the plant.[35][36]
Analysis of thelignin in the bark wall during decay by the white-rot fungiLentinula edodes (Shiitake mushroom) using13C NMR revealed that the lignin polymers contained more Guaiacyllignin units than Syringyl units compared to the interior of the plant.[35] Guaiacyl units are less susceptible to degradation as, compared to syringyl, they contain fewer aryl-aryl bonds, can form a condensed lignin structure, and have a lowerredox potential.[37] This could mean that the concentration and type of lignin units could provide additional resistance to fungal decay for plants protected by bark.[35]
Bark can sustain damage from environmental factors, such asfrost crack andsun scald, as well as biological factors, such aswoodpecker andboring beetle attacks. Maledeer and other male members of theCervidae (deer family) can cause extensive bark damage during the rutting season by rubbing their antlers against the tree to remove theirvelvet.
The bark is often damaged by being bound to stakes or wrapped with wires. In the past, this damage was called bark-galling and was treated by applyingclay laid on the galled place and binding it up withhay.[38] In modern usage, "galling" most typically refers to a type ofabnormal growth on a plant caused by insects or pathogens.
Bark damage can have several detrimental effects on the plant. Bark serves as a physical barrier to disease pressure, especially from fungi, so its removal makes the plant more susceptible to disease. Damage or destruction of the phloem impedes the transport of photosynthetic products throughout the plant; in extreme cases, when a band of phloem all the way around the stem is removed, the plant will usually quickly die. Bark damage in horticultural applications, as in gardening and public landscaping, results in often unwanted aesthetic damage.
The degree to which woody plants are able to repair gross physical damage to their bark is quite variable across species and type of damage. 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.Sap is sometimes produced to seal the damaged area against disease and insect intrusion.[citation needed]
A number of living organisms live in or on bark, including insects,[39] 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 of maturemango (Mangifera indica) showinglichen growth
The patterns left in the bark of a Chinese Evergreen Elm after repeated visits by a Yellow-Bellied Sapsucker (woodpecker) in early 2012.
The self-repair of the Chinese Evergreen Elm showing new bark growth, lenticels, and other self-repair of the holes made by a Yellow-Bellied Sapsucker (woodpecker) about two years earlier.
Alder bark (Alnus glutinosa) with characteristic lenticels and abnormal lenticels on callused areas.
The inner bark (phloem) of some trees is edible. In hunter-gatherer societies and in times of famine, it is harvested and used as a food source. In Scandinavia,bark bread is made fromrye to which the toasted and ground innermost layer of bark ofscots pine orbirch is added. TheSami people of far northern Europe use large sheets ofPinus sylvestris bark that are removed in the spring, prepared and stored for use as a staple food resource. The inner bark is eaten fresh, dried or roasted.[40]
Bark ofpine was used as emergency food in Finland duringfamine, last time during and aftercivil war in 1918.
Bark can be used as a construction material, and was used widely in pre-industrial societies. Some barks, particularlyBirch bark, can be removed in long sheets and other mechanically cohesive structures, allowing the bark to be used in the construction of canoes, as the drainage layer in roofs, for shoes, backpacks, and other useful items.[41] Bark was also used as a construction material in settler colonial societies, particularly Australia, both as exterior wall cladding and as a roofing material.[42][43]
In the cork oak (Quercus suber) the bark is thick enough to be harvested as acork product without killing the tree;[44] in this species the bark may get very thick (e.g. more than 20 cm has been reported[45]).
Somestem barks have significantly differentphytochemical content from other parts. Some of these phytochemicals havepesticidal, culinary, or medicinally and culturally importantethnopharmacological properties.[46]
Among the commercial products made from bark arecork,cinnamon,quinine[48] (from the bark ofCinchona)[49] andaspirin (from the bark ofwillow trees). The bark of some trees, notably oak (Quercus robur) is a source oftannic acid, which is used intanning. Bark chips generated as aby-product oflumber production are often used in barkmulch. Bark is important to the horticultural industry since in shredded form it is used for plants that do not thrive in ordinary soil, such asepiphytes.[50]
Wood bark containslignin which whenpyrolyzed yields a liquidbio-oil product rich innatural phenol derivatives. These are used as a replacement for fossil-based phenols in phenol-formaldehyde (PF) resins used inOriented Strand Board (OSB) and plywood.[51]
^Taylor, Luke. 1996.Seeing the Inside: Bark Painting in Western Arnhem Land. Oxford Studies in Social and Cultural Anthropology. Oxford: Clarendon Press.
^Sandved, Kjell Bloch, Ghillean T. Prance, and Anne E. Prance. 1993.Bark: the Formation, Characteristics, and Uses of Bark around the World. Portland, Or: Timber Press.
^Vaucher, Hugues, and James E. Eckenwalder. 2003.Tree Bark: a Color Guide. Portland: Timber
^Artschwager, E (1924). "Studies on the potato tuber".J. Agr. Res.27:809–835.
^Peterson, R.L.; Barker, W.G. (1979). "Early tuber development from explanted stolon nodes of Solanum tuberosum var. Kennebec".Botanical Gazette.140 (4):398–406.doi:10.1086/337104.S2CID85217835.
^Mauseth, James D. (2003),Botany: an Introduction to Plant Biology, Jones & Bartlett Learning, p. 229,ISBN0-7637-2134-4
^Katherine Easu (1977).Anatomy of Seed Plants. Plant Anatomy (2nd ed.). John Wiley & Sons. p. 185.ISBN0-471-24520-8.
^abcdefVane, C. H.; et al. (2006). "Bark decay by the white-rot fungus Lentinula edodes: Polysaccharide loss, lignin resistance and the unmasking of suberin".International Biodeterioration & Biodegradation.57 (1):14–23.Bibcode:2006IBiBi..57...14V.doi:10.1016/j.ibiod.2005.10.004.
^Vane, C. H.; et al. (2001). "Degradation of Lignin in Wheat Straw during Growth of the Oyster Mushroom (Pleurotus ostreatus) Using Off-line Thermochemolysis with Tetramethylammonium Hydroxide and Solid-State 13C NMR".Journal of Agricultural and Food Chemistry.49 (6):2709–2716.Bibcode:2001JAFC...49.2709V.doi:10.1021/jf001409a.PMID11409955.
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