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大学英语六级样题RainforestsTropicalrainforestsarethemostdiverseecosystem生态系统onEarthandalsotheoldest.Todaytropicalrainforestscoveronly6per__ntoftheEarth’sgroundsu_____buttheyarehometooverhalfoftheplanet’splantandani__lspecies.WhatIsaRainforestGenerallyspeakingarainforestisanenviro__entthatre__iveshighrainfallandisdominatedbytalltrees.Awiderangeofecosystemsfallintothiscategoryofcourse.Butmostofthetimewhenpeopletalkaboutrainforeststheymeanthetropicalrainforestslocatedneartheequator.Theseforestsre__ivebetween160and400inchesofrainperyear.Thetotalannualrainfallisspreadprettyevenlythroughouttheyearandthetemperaturerarelydipsbelow60degreesFahrenheit.Thissteadycli__teisduetothepositionofrainforestsontheglobe.BecauseoftheorientationoftheEarth’saxistheNorthernandSouthernhemisphereseachspendpartoftheyeartiltedawayfromthesun.Sin__rainforestsareatthemiddleoftheglobelocatedneartheequatortheyarenotespeciallyaffectedbythischange.Theyre__ivenearlythesameamountofsunlightandthereforeheatallyear.Consequentlytheweatherintheseregionsre__insfairlyconstant.Theconsistentlywetwarmweatherandamplesunlightgiveplantlifeeverythingitneedstothrive.Treesh__etheresour__stogrowtotremendousheightsandtheyliveforhundredseventhousandsofyears.Thesegiantswhichreach60to150ftintheairformthebasicstructureoftherainforest.Theirtopbranchesspreadwideinordertocapture__ximumsunlight.Thiscreatesathickcanopy(树冠)levelatthetopoftheforestwiththinnergreenerylevelsunderneath.Somelargetreesgrowsotallthattheyeventoweroverthecanopylayer.Asyougolowerdownintotherainforestyoufindlessandlessgreenery.Theforestflooris__deupofmossfungianddecayingplant__tterthathasfallenfromtheupperlayers.Thereasonforthisdecreaseingreeneryisvery______:Theoverabundan__ofplantsgatheringsunlightatthetopoftheforestblocksmostsunlightfromreachingthebottomoftheforest__kingitdifficultforrobustplantstothrive.TheForestfortheTreesTheamplesunlightandextremelywetcli__teof__nytropicalareasencouragethegrowthoftoweringtreeswithwidecanopies.Thisthicktoplayeroftherainforestdictatesthelivesofallotherplantsintheforest.Newtreeseedlingsrarelysurviveto__keittothetopunlesssomeoldertrees___creatinga“hole”inthecanopy.Whenthishappensalloftheseedlingsonthegroundlevelcompeteintenselytoreachthesunlight.__nyplantspeciesreachthetopoftheforestbyclimbingthetalltrees.Itismucheasiertoas__ndthiswaybecausetheplantdoesn’th__etoformitsownsupportingstructure.Someplantspeciescalledepiphytesgrowdirectlyonthesu_____ofthegianttrees.Theseplantswhichincludeavarietyoforchidsandferns__keupmuchoftheunderstorythelayeroftherainforestrightbelowthecanopy.Epiphytesarecloseenoughtothetoptore__iveadequatelightandtherunofffromthecanopylayerprovidesallthewaterandnutrients(养分)theyneedwhichisimportantsin__theydon’th__eac__sstothenutrientsintheground.StranglersandButtressesSomeepiphyteseventuallydevelopintostranglers.Theygrowlongthickrootsthatextenddownthetreetrunkintotheground.Astheycontinuetogrowtherootsformasortofwebstructureallaroundthetree.Atthesametimethestranglerplant’sbranchesextendupwardspreadingoutintothecanopy.Eventuallythestrangler__yblocksomuchlightfromaboveandabsorbsuchahighper__ntageofnutrientsfromthegroundbelowthatthehosttree___s.Competitionovernutrientsisalmostasintenseascompetitionforlight.Theex__ssiverainfallrapidlydissolvesnutrientsinthesoil__kingitrelativelyinfertileex__ptatthetoplayers.Forthisreasonrainforesttreerootsgrowoutwardtocoverawiderarearatherthandownwardtolowerlevels.This__kesrainforesttreessomewhatunstablesin__theydon’th__everystronganchorsintheground.Sometreescompensateforthisbygrowingnaturalbuttresses.Thesebuttressesarebasicallytreetrunksthatextendoutfromthesideofthetreeanddowntothegroundgivingthetreeadditionalsupport.Rainforesttreesaredependentonbacteriathatarecontinuallyproducingnutrientsintheground.Rainforestbacteriaandtreesh__eaveryclosesymbiotic共生的relationship.Thetreesprovidethebacteriawithfoodintheformoffallenle__esandother__terialandthebacteriabreakthis__terialdownintothenutrientsthatthetreesneedtosurvive.Oneofthemostre__rkablethingsaboutrainforestplantlifeisitsdiversity.ThetemperaterainforestsofthePacificNorthwestare__inlycomposedofadozenorsotreespecies.Atropicalrainforestontheotherhandmighth__e300distincttreespecies.AllCreaturesGreatand__allRainforestsarehometothe__jorityofani__lspeciesintheworld.Andagreatnumberofspecieswhonowliveinotherenviro__entsincludinghu__nsoriginallyinhabitedtherainforests.Researchersesti__tethatinalargerainforestareathere__ybemorethan10milliondifferentani__lspecies.Mostofthesespeciesh__eadaptedforlifeintheupperlevelsoftherainforestwherefoodismostplentiful.Insectswhichcaneasilyclimborflyfromtreetotree__keupthelargestgroupantsarethemostabundantani__lintherainforest.Insectspeciesh__eahighlysymbioticrelationshipwiththeplantlifeinarainforest.Theinsectsmovefromplanttoplantenjoyingthewealthoffoodprovidedthere.Astheytr__eltheinsects__ypickuptheplants’seedsdroppingthemsomedistan__away.Thishelpstodispersethepopulationoftheplantspeciesoveralargerarea.Thenumerousbirdsoftherainforestalsoplaya__jorpartinseeddispersal.Whentheyeatfruitfromaplanttheseedspassthroughtheirdigestivesystem.Bythetimetheyexcrete排泄theseedsthebirds__yh__eflown__nymilesawayfromthefruit-bearingtree.Therearealsoalargenumberofreptilesand__m__lsintherainforest.Sin__theweatherissohotandhumidduringthedaymostrainforest__m__lsareactiveonlyatnightduskordawn.The__nyrainforestbatspeciesareespeciallywelladaptedforthislifestyle.Usingtheirsonarbatsn__igateeasilythroughthe__ssoftreesintherainforestfeedingoninsectsandfruit.Whilemostrainforestspeciesspendtheirlivesinthetreesthereisalsoalotoflifeontheforestfloor.Greatapeswildpigsbigcatsandevenelephantscanallbefoundinrainforests.Thereareanumberofpeoplewholiveintherainforestsaswell.Thesetribes—whichupuntilre__ntlynumberedinthethousands—arebeingfor__doutoftherainforestsatanalarmingratebecauseofdeforestation.DeforestationInthepasthundredyearshu__nsh__ebegundestroyingrainforestsatanalarmingrate.Todayroughly
1.5acresofrainforestaredestroyedeverysecond.Peoplearecuttingdowntherainforestsinpursuitofthree__jorresour__s:LandforcropsLumberforpaperandotherwoodproductsLandforlivestockpasturesInthecurrenteconomypeopleobviouslyh__eaneedforalloftheseresour__s.Butalmostallexpertsagreethatovertimewewillsuffermuchmorefromthedestructionoftherainforeststhanwewillbenefit.Theworld’srainforestsareanextremelyvaluablenaturalresour__tobesurebutnotfortheirlumberortheirland.Theyarethe__incradleoflifeonEarthandtheyholdmillionsofuniquelifeformsthatweh__eyettodiscover.Destroyingtherainforestsiscomparabletodestroyinganunknownplanet—weh__enoideawhatwe’relosing.Ifdeforestationcontinuesatitscurrentratetheworld’stropicalrainforestswillbewipedoutwithin40years.
1.Virtuallyallplantandani__lspeciesonEarthcanbefoundintropicalrainforests.
2.Thereisnotmuchchangeintheweatherinthetropicalrainforestsalltheyearround.
3.ThelargestnumberofrainforestsintheworldarelocatedontheAfricancontinent.
4.Belowthecanopylevelofatropicalrainforestgrowsanoverabundan__ofplants.
5.Newtreeseedlingswillnotsurvivetoreachthecanopylevelunless___________.
6.Epiphyteswhichformmuchoftheunderstoryoftherainforestgetalltheirwaterandnutrientsfrom__________.
7.Stranglersaresocalledbecausethey________byblockingthesunlightandcompetingforthenutrients.
8.Sin__rainforestbacteriaandtreesdependoneachotherforlifetherelationshiptheyformistermed_____.
9.Plantspeciesaredispersedoveralargeareawiththehelpof________.
10.Aswearestillignorantofmillionsofuniquelifeformsintherainforestdeforestationcanbecomparedtothedestructionof___________.
1.N
2.Y
3.NG
4.N
5.someoldertrees___
6.thecanopylayer
7.killhosttree
8.symbiotic
9.insectsbirds/insects/birds/ani__ls
10.anunknownplanet。