专业外语讲稿-第2课-Structure-of-Wood

2StructureofWood-1-2StructureofWoodThefibrousnatureofwoodstronglyinfluenceshowitisused.Woodisprimarilycomposedofhollow,elongate,spindleshapedcellsthatarearrangedparalleltoeachalongthentrunkofatree,Whenlumberandotherproductsarecutfromthetree,thecharacteristicsofthesefibrouscellsandtheirarrangementaffectsuchpropertiesasstrengthandshringkageaswellasthegrainpatternofthewood.Thischapterbrieflydescribessomeelementsofwoodstructure.Bark,Wood,BranchesandChambiumAcrosssectionofatree(Fig.2-1)showsthefollowingwelldefinedfeatures(fromoutsidetocenter):bark,whichmaybedividedintoanoutercorkydeadpart,whosethicknessvariesgreatlywithspeciesandageoftreesandaninnerthinlivingpart,whichcarriesfoodfromtheleavestothegrowingpartsofthetree;wood,whichinmerchantabletreesofmostspeciesisclearlydifferentiatedintosapwoodandheartwood;andpith,asmallcoreoftissuelocatedatthecenteroftreestems,branches,andtwigsaboutwhichinitialwoodgrowthtakesplaced.2StructureofWood-2-Sapwoodcontainsbothlivinganddeadtissueandcarriessapfromtherootstotheleaves.Heartwoodisformedbyagradualchangeinthesapwoodandisinactive,Thewoodrays,horizontallyorientedtissuethroughtheradialplaneofthetree,varyinsizefromonecellwideandafewcellhightomorethan15cellswideandseveralcentimetershigh.Theraysconnectedvariouslayersfrompithtobarkandformswoodandbarkcells,canbeseenonlywithamicroscopel.Asthrtreegrowsinheight,branchingisinitiatedbylateralbuddevelopment.Thelateralbranchesareintergrownwiththewoodofthetrunkaslongastheyarealive.Afterabranchdies,thetrunkcontinuestoincreaseindiameterandsurroundsthatportionofthebranchprojectingfromthetrunkwhenthebranchdied.2StructureofWood-3-Ifthedeadbranchesdropfromthetree,thedeadstubsbecomeovergrownandclearwoodisformedSapwoodandHeartwoodSapwoodislocatedbetweenthecambiumandheartwood(Fif.2-1).Sapwoodcontainsbothlivinganddeadcellsandfunctionsprimarilyinthestorageoffood;intheouterlayersnearthecambium,sapwoodhandlesthetransporofwaterorsap.Thesapwoodmayvaryinthicknessandnumberofgrowthrings.Sapwoodcommonlyrangesfrom4to6cminradialthickness.Asarule,themorevigorouslygrowingtreeshavewidersapwood.Ingeneral,heartwoodconsistsofinactivecellsthatdonotfunctionineitherwaterconditionorfoodstorage.2StructureofWood-4-Thetransitionfromsapwoodtoheartwoodisaccompaniedbyanincreaseinextractivecontent.Frequently,theseextractivesdarkentheheartwoodandgivespeciessuchasblackwalnutandcherrytheircharacteristiccolor.Lightercoloredheartwoodoccursinsomespecies.Heartwoodextractivesmayalsoaffectwoodby(a)reducingpermeability,makingtheheartwoodslowertodryandmoredifficulttoimpregnatewithchemicalpreservatives,(b)increasingstabilityinchangingmoistureconditions,and(c)increasingweight(slightly).GrowthRingsInmostapeciesintemperateclimates,thedifferencebetweenwoodthatisformedearlyinagrowingrings(Fig.2-1).However,ifthegrowthindiameterisinterrupted,bydroughtordefoliationbyinsectsforexample,morethanoneringmaybeformedinthesameseason,2StructureofWood-5-Insuchanevent,theinnerringsusuallydonothavesharplydefinedboundariesandaretermedfaslerings,Treesthathaveonlyverysmallcrownsorthathaveaccidentallylostmostoftheirfoliagemayformanincompletegrowthlayer,sometimescalledadiscontinuousring.Growthringsaremostreadilyseeninspecieswithsharpcontrastbetweenlatewoodformedinoneyearandearlywoodformedinthefollowingyear,suchasinthenativeringporoushardwoodsashandoak,andinsoftwoodslikesouthernpines.Insomeotherspecies,suchaswatertupelo,aspen,andsweetgum,differentiationofearlywoodandlatewoodisslightandtheannualgrowthringsaredifficulttorecognize.Inmanytropicalregions,growthmaybepracticallycontinuousthroughouttheyear,andnowelldefinedgrowthringsareformed.WoodCellsWoodcells-----thestructuralelementsofthewoodtissue----areofvarious2StructureofWood-6-sizesandahapesandquitefirmlycementedtogether.Drywoodcellsmaybeemptyorpartlyfilledwithdeposits,suchasgumsandresins.Themajorityofwoodcellsareconsiderablyelongatedandpointedattheends;thesecellsarecustomarilycalledfibersortracheids.Thelengthofwoodfibersishighlyvariablewithinatreeandamongspecies.Hardwoodfibersaverageabout1mminlength;softwoodfibersrangefrom3to8mminlength.Inadditiontofibers,hardwoodshavecellsofrelativelylargediameterknownasvesselsorpores.Bothhardwoodsandsoftwoodshavecells(usuallygroupedintostructuresortissues)thatareorientedhorizontallyinthedirectionfrompithtowardbark.Thesegroupsofcellsconductsapradiallyacrossthegrainandarecalled2StructureofWood-7-raysorwoodrays.Theraysaremosteasilyseenonedgegrainedorquartersawsurface,andtheyvarygreatlyinsizeindifferentspecies.ChemicalCompositionDrywoodisprimarilycomposedofcellulose,lignin,hemicelluloses,andminoramounts(5%to10%)ofextraneousmaterials.Cellulose,themajorcomponent,constitutesapproximately50%ofwoodsubstancebyweight.Itisahigh–molecular-weightlinearpolymerconsistingofchainsof1tomorethan4–linkedglucosemonomers.Ligninconstitutes23%to33%ofthewoodsubstanceinsoftwoodsand16%to25%inhardwoods.Althoughligninoccursinwoodthroughoutthecellwall,itisconcentratedtowardtheoutsideofthecellsandbetweencells.2StructureofWood-8-Ligninisoftencalledthecementingag