托福阅读真题10篇.docx

1995





年托福阅读真题

Questions1-13
Atmosphericpressurecansupportacolumnofwaterupto10metershigh.Butplantscanmovewatermuchhigher,thesequoiatreecanpumpwatertoitsverytop,morethan100metersabovetheground.Untiltheendofthenineteenthcentury,themovementofwaterintreesandothertallplantswasamystery.Somebotanistshypothesizedthatthelivingcellsofplantsactedaspumps,Butmanyexperimentsdemonstratedhatthestemsofplantsinwhichallthecellsarekilledcanstillmovewatertoappreciableheights.Otherexplanationsforthemovementofwaterinplantshavebeenbasedonrootpressure,apushonthewaterfromtherootsatthebottomoftheplant.Butrootpressureisnotnearlygreatenoughtopushwatertothetopsoftalltrees.Furthermore,theconifers,whichareamongthetallesttrees,haveunusuallylowrootpressures.

Ifwaterisnotpumpedtothetopofatalltree,andifitisnotpushedtothetopofatalltree,thenwemayask,Howdoesitgetthere?Accordingtothecurrentlyacceptedcohesion-tensiontheory,waterispulledthere.Thepullonarisingcolumnofwaterinaplantresultsfromtheevaporationofwateratthetopoftheplant.Aswaterislostfromthesurfaceoftheleaves,anegativepressure,ortension,iscreated.Theevaporatedwaterisreplacedbywatermovingfrominsidetheplantinunbrokencolumnsthatextendfromthetopofaplanttoitsroots.Thesameforcesthatcreatesurfacetensioninanysampleofwaterareresponsibleforthemaintenanceoftheseunbrokencolumnsofwater.Whenwaterisconfinedintubesofverysmallbore,theforcesofcohesion(theattractionbetweenwatermolecules)aresogreatthatthestrengthofacolumnofwatercompareswiththestrengthofasteelwireofthesamediameter.Thiscohesivestrengthpermitscolumnsofwatertobepulledtogreatheightswithoutbeingbroken.

1.Howmanytheoriesdoestheauthormention?(A)One
(B)Two(C)Three(D)Four

2.Thepassageanswerswhichofthefollowingquestions?(A)Whatistheeffectofatmosphericpressureonfoliage?(B)Whendodeadcellsharmplantgrowth?
(C)Howdoeswatergettothetopsoftrees?
(D)Whyisrootpressureweak

3.Theword"demonstrated"inline5isclosestinmeaningto
(A)ignored(B)showed(C)disguised
(D)distinguished

4.Whatdotheexperimentsmentionedinlines