托福备考阅读原文及题目分享

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MineralsAndPlants

Researchhasshownthatcertainmineralsarerequiredbyplantsfornormalgrowthanddevelopment.Thesoilisthesourceoftheseminerals,whichareabsorbedbytheplantwiththewaterfromthesoil.Evennitrogen,whichisagasinitselementalstate,isnormallyabsorbedfromthesoilasnitrateions.Somesoilsarenotoriouslydeficientinmicronutrientsandarethereforeunabletosupportmostplantlife.So-calledserpentinesoils,forexample,aredeficientincalcium,andonlyplantsabletotoleratelowlevelsofthismineralcansurvive.Inmodernagriculture,mineraldepletionofsoilsisamajorconcern,sinceharvestingcropsinterruptstherecyclingofnutrientsbacktothesoil.

Mineraldeficienciescanoftenbedetectedbyspecificsymptomssuchaschlorosis(lossofchlorophyllresultinginyelloworwhiteleaftissue),necrosis(isolateddeadpatches),anthocyaninformation(developmentofdeepredpigmentationofleavesorstem),stuntedgrowth,anddevelopmentofwoodytissueinanherbaceousplant.Soilsaremostcommonlydeficientinnitrogenandphosphorus.Nitrogen-deficientplantsexhibitmanyofthesymptomsjustdescribed.Leavesdevelopchlorosis;stemsareshortandslender,andanthocyanindiscolorationoccursonstems,petioles,andlowerleafsurfaces.Phosphorus-deficientplantsareoftenstunted,withleavesturningacharacteristicdarkgreen,oftenwiththeaccumulationofanthocyanin.Typically,olderleavesareaffectedfirstasthephosphorusismobilizedtoyounggrowingtissue.Irondeficiencyischaracterizedbychlorosisbetweenveinsinyoungleaves.

Muchoftheresearchonnutrientdeficienciesisbasedongrowingplantshydroponically,thatis,insoillessliquidnutrientsolutions.Thistechniqueallowsresearcherstocreatesolutionsthatselectivelyomitcertainnutrientsandthenobservetheresultingeffectsontheplants.Hydroponicshasapplicationsbeyondbasicresearch,sinceitfacilitatesthegrowingofgreenhousevegetablesduringwinter.Acroponics,atechniqueinwhichplantsaresuspendedandtherootsmistedwithanutrientsolution,isanothermethodforgrowingplantswithoutsoil.

Whilemineraldeficienciescanlimitthegrowthofplants,anoverabundanceofcertainmineralscanbetoxicandcanalsolimitgrowth.Salinesoils,whichhavehighconcentrationsofsodiumchlorideandothersalts,limitplantgrowth,andresearchcontinuestofocusondevelopingsalt-tolerantvarietiesofagriculturalcrops.Researchhasfocusedonthetoxiceffectsofheavymetalssuchaslead,cadmium,mercury,andaluminum;however,evencopperandzinc,whichareessentialelements,canbecometoxicinhighconcentrations.Althoughmostplantscannotsurviveinthesesoils,certainplantshavetheabilitytotoleratehighlevelsoftheseminerals.

Scientistshaveknownforsometimethatcertainplants,calledhyperaccumulators,canconcentratemineralsatlevelsahundredfoldorgreaterthannormal.Asurveyofknownhyperaccumulatorsidentifiedthat75percentofthemamassednickel,cobalt,copper,zinc,manganese,lead,andcadmiumareothermineralsofchoice.Hyperaccumulatorsruntheentirerangeoftheplantworld.Theymaybeherbs,shrubs,ortrees.Manymembersofthemustardfamily,spurgefamily,legumefamily,andgrassfamilyaretophyperaccumulators.Manyarefoundintropicalandsubtropicalareasoftheworld,whereaccumulationofhighconcentrationsofmetalsmayaffordsomeprotectionagainstplant-eatinginsectsandmicrobialpathogens.

Onlyrecentlyhaveinvestigatorsconsideredusingtheseplantstocleanupsoilandwastesitesthathavebeencontaminatedbytoxiclevelsofheavymetals–anenvironmentallyfriendlyapproachknownasphytoremediation.Thisscenariobeginswiththeplantingofhyperaccumulatingspeciesinthetargetarea,suchasanabandonedmineoranirrigationpondcontaminatedbyrunoff.Toxicmineralswouldfirstbeabsorbedbyrootsbutlaterrelocatedtothestemandleaves.Aharvestoftheshootswouldremovethetoxiccompoundsoffsitetobeburnedorcompostedtorecoverthemetalforindustrialuses.Afterseveralyearsofcultivationandharvest,thesitewouldberestoredatacostmuchlowerthanthepriceofexcavationandreburial,thestandardpracticeforremediationofcontaminatedsoils.Forexamples,infieldtrials,theplantalpinepennycressremovedzincandcadmiumfromsoilsnearazincsmelter,andIndianmustard,nativetoPakistanandIndia,hasbeeneffectiveinreducinglevelsofseleniumsaltsby50percentincontaminatedsoils.

考试科目：

1.AccordingtoParagraph1,whatistrueofplantsthatcangrowinserpentinesoil?

A.Theyabsorbmicronutrientsunusuallywell.B.Theyrequirefarlesscalciumthanmostplantsdo.C.Theyareabletoabsorbnitrogeninitselementalstate.D.Theyaretypicallycropsraisedforfood.

2.Thewordexhibit"inthepassageisclosestinmeaningto

A.fightoffB.showC.causeD.spread

3.AccordingtoParagraph2,whichofthefollowingsymptomsoccursinphosphorusdeficientplantsbutnotinplantsdeficientinnitrogenoriron?

A.ChlorosisonleavesB.ChangeinleafpigmentationtoadarkshadeofgreenC.Short,stuntedappearanceofstemsD.Reddishpigmentationontheleavesorstem

4.AccordingtoParagraph2,asymptomofirondeficiencyisthepresenceinyoungleavesof

A.Deepreddiscolorationbetweentheveins.B.Whiteoryellowtissuebetweentheveins.C.Deadspotsbetweentheveins.D.Characteristicdarkgreenveins.

5.Thewordfacilitates"inthepassageisclosestinmeaningto

A.slowsdownB.affectsC.makeseasierD.focuseson

6.AccordingtoParagraph3,whatistheadvantageofhydroponicsforresearchonnutrientdeficienciesinplants?

A.Itallowsresearcherstocontrolwhatnutrientsaplantreceives.B.Itallowsresearcherstoobservethegrowthofalargenumberofplantssimultaneously.C.Itispossibletodirectlyobservetherootsofplants.D.Itisunnecessarytokeepmistingplantswithnutrientsolutions.

7.Thewordsuspended"inthepassageisclosestinmeaningto

A.grownB.protectedC.spreadoutD.hung

8.Whydoestheauthormentionherbs,shrubs,andtrees?

A.Toprovideexamplesofplanttypesthatcannottoleratehighlevelsofharmfulminerals.B.Toshowwhysomanyplantsarehyperaccumulators.C.Tohelpexplainwhyhyperaccumulatorscanbefoundinsomanydifferentplaces.D.Toemphasizethathyperaccumulatorsoccurinawiderangeofplanttypes.

9.Thewordafford"inthepassageisclosestinmeaningto

A.offerB.preventC.increaseD.remove

10.Whichofthesentencesbelowbestexpressestheessentialinformationinthehighlightedsentenceinthepassage?Incorrectchoiceschangethemeaninginimportantwaysorleaveoutessentialinformation.

A.Beforeconsideringphytoremediation,hyperaccumulatingspeciesofplantslocaltothetargetareamustbeidentified.B.Theinvestigationbeginswithanevaluationoftoxicsitesinthetargetareatodeterminetheextentofcontamination.C.Thefirststepinphytoremediationistheplantingofhyperaccumulatingplantsintheareatobecleanedup.D.Minesandirrigationpondscanbekeptfrombecomingcontaminatedbyplantinghyperaccumulatingspeciesintargetedareas.