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Hardwood iswood fromangiospermtrees. These are usually found in broad-leaved temperate andtropical forests.[1] Intemperate andboreallatitudes they are mostlydeciduous, but intropics andsubtropics mostlyevergreen. Hardwood (which comes fromangiosperm trees) contrasts withsoftwood (which is fromgymnosperm trees).
Hardwoods are produced byangiosperm trees that reproduce by flowers, and have broad leaves. Many species are deciduous.[2] Those of temperate regions lose their leaves every autumn as temperatures fall and are dormant in the winter, but those of tropical regions may shed their leaves in response to seasonal or sporadic periods of drought. Hardwood from deciduous species, such as oak, normally shows annualgrowth rings, but these may be absent in sometropical hardwoods.[3]
Hardwoods have a more complex structure than softwoods and are often much slower growing as a result. The dominant feature separating "hardwoods" from softwoods is the presence of pores, orvessels.[4] The vessels may show considerable variation in size, shape of perforation plates (simple, scalariform, reticulate, foraminate), and structure of cell wall, such as spiral thickenings. Several specific microscopic features are used in the identification process of a hardwood species.[5]
As the name suggests, the wood from these trees is generally harder than that of softwoods, but there are significant exceptions. In both groups there is an enormous variation in actual wood hardness, with the range in density in hardwoods completely including that of softwoods; some hardwoods (e.g.,balsa) are softer than most softwoods, whileyew is an example of a hard softwood.
The structural polymers of hardwoods arecellulose,hemicellulose, andlignin.[6] The constituents of hardwood lignin differs from those included in softwood.Sinapyl alcohol andconiferyl alcohol are the main monomers of hardwood lignin.[7]
Hardwoods contain a smaller quantity of non-structural constituents, namedextractives, than softwoods.[8] These extractives are usually categorized into three broad groups:aliphatic compounds,terpenes andphenolic compounds. Aliphatic compounds found in hardwoods includefatty acids,fatty alcohols and theiresters withglycerol, fatty alcohols (waxes) and sterols (steryl esters),hydrocarbons, such asalkanes,sterols, such assitosterol,sitostanol andcampesterol.[9] The terpene content of the hardwood significantly differs from the softwood, and mainly consists oftriterpenoids,polyprenols and other higher terpenes. Triterpenoids commonly purified from hardwoods includecycloartenol,betulin andsqualene. Hardwood polyterpenes arerubber,gutta percha, gutta-balatá and betulaprenols.[8][9] Although in small quantities, hardwoods also containmono-,sesqui- andditerpenes, such asα- andβ-pinenes,3-carene,β-myrcene,limonene,hinokitiol,δ-cadinene,α- andδ-cadinols,borneol. Hardwood is rich in phenolic compounds, such asstilbenes,lignans, norlignans,tannins,flavonoids.[9]
Hardwoods are employed in a large range of applications, includingfuel,tools,construction,boat building,furniture making,musical instruments,flooring,cooking,barrels, and manufacture ofcharcoal. Solid hardwood joinery tends to be expensive compared to softwood. In the past, tropical hardwoods were easily available, but the supply of some species, such as Burmateak andmahogany, is now becoming scarce due to over-exploitation. Cheaper "hardwood" doors, for instance, now consist of a thin veneer bonded to a core of softwood, plywood ormedium-density fibreboard (MDF). Hardwoods may be used in a variety of objects, but are most frequently seen in furniture or musical instruments because of their density which adds to durability, appearance, and performance. Different species of hardwood lend themselves to different end uses or construction processes. This is due to the variety of characteristics apparent in different timbers, including density, grain, pore size, growth and fibre pattern, flexibility and ability to be steam bent. For example, theinterlocked grain ofelm wood (Ulmus spp.) makes it suitable for the making of chair seats where the driving in of legs and other components can cause splitting in other woods.[citation needed]
There is a correlation between density and calories/volume. This makes the denser hardwoods likeoak,cherry, andapple more suited for camp fires, cooking fires, and smoking meat, as they tend to burn hotter and longer than softwoods likepine or cedar whose low-density construction and highly-flammablepitch make them burn quickly and without producing quite as much heat.[citation needed]
