新托福阅读背景知识分享 托福阅读

新托福阅读背景知识：植物适应沙漠，托福阅读想要拿到高分，大家在复习的过程中，还是需要多进行各种知识的储备。今天介绍的是新的托福阅读的背景知识，这样能够更好地了解托福阅读的文章内容。

Plantadaptationtothedesert(背景材料)

Cactusadaptations.

Thesecrettothesuperiorenduranceofcactiliesintheiradaptations.Overmillionsofyears,throughnaturalselection,onlythestrongestandbestadaptedspeciessurvived.

Asyouknow,itisverydryinthedesert.Plantsthatadapttothisareknownasxerophytes(fromzeros,dryandpython,plant).Thereareplantsthatavoidthedryseasonbysproutingfromseedjustafterthespringrainandgrowingveryfastsothatbythetimethedryseasoncomes,theyhavealreadyproducedalotofseedsanddied.Theseseedslieonthesoilforthedryseasonandsproutagaininspringandthecyclerepeats.Otherxerophytessimplydroptheirleavesandstaydormantforthewinter.Butthereisanotherspecialtypeofxerophyteswhichstoreswaterinitsfleshytissues.Suchplantsarecalledsucculents(fromsuccess,juicy).Thecactusisatypicalexampleofasucculent.

Ifyoucutacactusopen,youseeajuicy,slimytissue.Thisiswherethemoistureisstoredforthedryseason.Thepartbetweenthemiddlecircle(andpith)andjustundertheverygreenpartoftheplant(orpalisadeparenchyma)justundertheskinisallocatedforthestorageofwaterandfoodfortheplant.Thisisatypeofspongyparenchymaandcantakeupto85%oftheplant'svolume.Thisisamajoradaptationinthedesert.Becausetheplantremainscompletelyaliveduringthedryseasonandthereisnoneedforittodryupandloseeverything,makesitpossiblefortheplanttogrowtolargesizes.Anotheradvantageisthattheplantretainssupplies(intheformofstarch)forthewintersothatitcanflowerrightawayinspringwithoutaccumulatingmoresupplies(asmostplantsneedtodoinspring).Thewholepurposeofstoringsuppliesforthewinterismostlytoenergizefloweringinspringbutitalsoletsthecactusstartgrowingmuchsooner.

Floweringplantsbreatheandtranspire(evaporatewaterfromtheirsurface)throughcloseablemicroscopicporescalledstoatsontheleavesorstems.Todothis,theirporeshavetobeopen.Inmostplantstheseareopenalldayandonwarmnights.Butforcactithisisinconvenientasindaytimeitisveryhotandthustheplantwouldlosealotofwaterthroughevaporation.Sothecactusmustclosetheminthedaytime.Butthenitcannotbreatheorphotosynthesize(theprocesswheresugarsaremadefromcarbondioxideandwaterandreleasingoxygenusingthesun'senergy).Succulentshaveanadaptationtothat.Theirstoatsareclosedduringthedayandareopenatnight,whenitisnotthathotandstorecarbondioxideinitstissuesascrassleanacidandthenturnitbacktocarbondioxideinthedaytime.ThisprocessiscalledcrassleanacidmetabolismorCAManditisaverysmartwayofrespiringinthedesert.

Ifwelookattheoutsideoftheplant,wenoticethatthereisatoughleatheryskincoveringtheplant,wecanalsonoticethepresenceofribsandspinesandsometimesfur.Theseareallverysmartadaptations.Theyservemainlyforsurvivingheatbutarealsousedasdefense.

Thetoughleatheryskinisveryimpermeabletowater,thusreducingevaporationfromthesurfaceoftheplant.Thisskinoftenhasalayerofplantwaxonitwhichisoftenlightlycolored(Pilosocereusazuresisanexampleofaplantwithsuchwax),whiteorblue.Thisreflectslightandalsoreducesevaporationfromtheinside.