PLANTS FOR CONSTRUCTION
The lightness, strength and durability of the lignified xylem vessels that make wood have made it vitally important in the construction of buildings, furniture, ships and ornaments. It can be preserved for centuries, but fungi and bacteria can digest it in damp conditions.
Other uses of wood
Wood can be broken down into fiber to be used for paper, one of its most important current uses. Refined fibers are used as clothing fabrics, notably viscose. It was the most important fuel before fossil fuels were exploited commercially.
Wood use and the Environment
Throughout history woods have been felled for pasture and agriculture and for their products. Most have not been replaced, but some regeneration of desirable species has been encouraged at the expense of less useful ones. Fuel wood has in some places been harvested by coppicing, a sustainable practice though leading eventually to reduced soil fertility. Plantations are usually conifers or eucalyptus for paper. These are usually poor in wildlife and, though the soil is retained, it is often acidified through slow leaf decay.
Useful plant fibers are strings of sclerenchyma or collenchyma cells either from the leaves or stems of herbaceous plants or seed appendages. Many species have been used such as flax, hemp and sisal. The most important plant fiber is cotton, long unicellular seed appendages spun to fine thread. Demand for this had major historical repercussions. Nylon and ‘artificial’ fibers are made from oil derived from fossil plants.
Wood is made of elongated hollow xylem cells thickened with lignin and cellulose . Lignin in particular is strong and durable and only some fungiand bacteria are capable of digesting it. In addition, many trees secrete tanninsor resinous compounds into the xylem in the heartwood of a tree trunk that haslost its function as water conducting cells; this acts as a preservative. Many ofthe vessels remain hollow and wood is light in weight considering its strength.These properties make wood excellent material for the construction of buildings,
furniture, tools, etc, and it has been used for numerous purposes since paleolithic times. It is (or was) abundantly available across much of the habitable world. Most woods float in water and its use in building boats and ships gradually increased in importance until the early 19th century when metal ships were introduced.
The ‘grain’ of some woods, formed mainly by annual growth rings and its ability to be smoothed and stained have made it an important medium for sculptures and other ornaments and qualities of particular species are used to make many musical instruments, e.g. maple and pine or spruce for violins, African blackwood for clarinets. If kept dry and adequately treated, wood will last well and can last for centuries in the right conditions, although if kept permanently damp it will decay within a few months to a few decades depending on the type of wood, its size and the environment.
There is a minor but significant and constant demand for corks made from the spongy bark of the Mediterranean cork oak, Quercus suber.
Fungi and bacteria can digest the lignin and other compounds that form wood and are the organisms responsible for wood decay. All decay organisms require moisture (including the so-called ‘dry-rot’ fungus) and wood can be kept for centuries if it is kept dry, or treated with preservative. In the tropics termites are specialist feeders on dead wood and digest it by means of symbiotic microorganisms making it difficult to preserve wood in the wet tropics.
Wood may be broken down into fibers to make paper and this is now one of the largest uses for wood. Paper can be made from almost any tree species. The wood from conifers and broad-leaved trees is pulped mechanically in a water and chemical solution to extract the fibers which are then glued, pressed and bleached, the whole operation being highly commercialized. Many other fibers are suitable for paper-making and the earliest paper was made from the papyrus, Cyperus papyrus, from African swamps. Wood pulp is also used to make fine clothing material after chemical treatment that modifies the cellulose. One such product, sodium cellulose xanthate, is the best known fiber from wood pulp, known commercially as viscose.
Wood has been the most important fuel for burning throughout most of human history, either burned directly or first charred to charcoal to be used as a hotter and more efficient fuel later. Only with the mining of large quantities of coal, oil and gas since the 19th century has its importance diminished. It is well to remember that these mined fuels are all partially decayed fossil plants, including some wood in the coal, preserved by crushing and, in coal, petrifying over millions of years.
The use of wood through history has had a profound effect on the world’s plant communities. Many forests were destroyed to open land for pasture and agriculture, and forests had disappeared from large areas of temperate Eurasia by 0 AD. Wood used for building was usually cut without any replanting and the large quantity of wood required for ships in western Europe, mainly during the 1700s, destroyed many more. Throughout the world, forests diminished in extent through human intervention and many other wooded areas changed in character; for example, in many areas, such as much of North America, the understorey was burned to make a more open environment. Some areas of forest remained in most places for use as a source of wood. Trees were allowed to regenerate naturally and often desirable tree species were encouraged and other species removed. Useful species were increasingly planted to replace felled woods, particularly in the 18th and 19th centuries. The domination of many western European woods by the oak,Quercus robur, and in places beech, Fagus sylvatica, and the absence of the lime, Tilia cordata, is mainly due to human encouragement of the good timber crops. Many forests now dominated by a single species would naturally be mixed.
Fuel wood is frequently harvested on a partially sustainable basis. Many trees will regenerate new stems from a cut base and this ability has been used in coppices for many centuries. The understorey trees are cut on a rotational basis with a cycle of a few years or decades depending on the fertility of the site and species involved. This will periodically open the coppice to light after which it will gradually become shadier until the next cut; many plants at a woodland edge are adapted to this and have colonized coppice woodlands. Numerous small and some large wooded areas remain, particularly in northern Europe, owing to the need for coppice wood as fuel and for some construction purposes. Over centuries, soil fertility declines and it takes increasingly longer for the trees to regrow in a coppice woodland. The cork oak woods of south-western Europe have been kept for their valuable product and remain ecologically rich.
Plantations of trees have increased in the 20th century with the decline of ancient woods and with the huge demand for paper. These have largely been plantations of conifers, most originating in North America, in the cool temperate zones, often on acidic soils not suitable for other crops, andeucalyptus, originally from Australia, and other conifers in the warmer parts of the world. These plantations suffer from the problems of monoculture described and can deplete the soil of nutrients. With the dense plantings, few other plants can live except in the young early stages. Many of these trees are planted in continents different from their place of origin. This has sometimes led to growth problems because of a lack of mycorrhizal infection and, frequently, a limited wildlife community compared with plantations of native species. Despite this, wildlife is often more common than on open agricultural land and the soil structure and depth may be retained, though the soil may become increasingly acidic since the leaves of many trees are resistant to decay.
Useful plant fibers come from many species, the fibers themselves consisting mainly of sclerenchyma cells , sometimes with collenchyma or conducting cells. Some of these have lignified walls but, for flexibility, the lignin is usually digested out to leave cellulose walls for the fiber. A few derive from seed appendages but most others are strands of cells within the vegetative tissue that provide support and usually have no cell contents at maturity. In a few plants the fibers are long and at least partially separate and can be used directly for clothing (e.g. the underbark of a few trees), but more frequently they must be treated before use. This usually involves soaking and beating to remove surrounding cells and sometimes treatment with chemicals. The finer fibers must then be spun to make long strands.
Most fibers are extracted from the stems of dicots or the leaves of monocots. The most important among the dicots are flax (Linum spp.) that gives us linen, used widely before cotton, and hemp (Cannabis spp.) and jute (Corchorus spp.), both of which are used mainly for rope. Among monocots,sisal (Agave spp.) and Manila hemp from a banana, Musa textilis, are still widely used. Many other species have been used and still have local uses, such as nettle species, Urtica, pineapple fiber, date palms, etc. With the large demand for paper some of these crops are being tested for suitability in paper manufacture.
The world’s most important fiber is cotton, the feathery appendage of seeds of the genus Gossypium (Malvaceae, the mallow and hibiscus family), native to Asia and the Americas. Cotton has been used for thousands of years in India and South America. These appendages are each a single elongated cell, the longest making the finest cotton, and now most commercial cotton comes from a South American species, Gossypium hirsutum. One kilogram of cotton consists of approximately 200 million seed hairs. Exploration of Asia, and later South America, introduced the fine qualities of cotton to Europe during the 15th century after which it became much sought after in Europe where there were only coarser fabrics. The huge plantations of cotton, particularly those involving slavery in the southern USA, made Liverpool the richest port of Europe and most northern English towns rich through manufacture of cotton clothing. The serious inequalities that developed between northern American states that marketed and exported cotton and southern states that grew it, was one of the major factors leading to the American civil war.
Kapok fiber derives, like cotton, from seed appendages and was used as light stuffing for mattresses and life jackets. It comes from a giant forest tree, Ceibapentandra, of the Bombacaceae, related to the cotton family. Nylon and other ‘artificial’ fabrics derive from oil, itself derived from fossil plants.
Copyright © 2018-2020 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.