凝聚力张力理论(Cohesion-tension Theory)

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atmosphericpressurecansupportacolumnofwaterupto10metershigh.butplantscanmovewatermuchhigher;thesequoiatreecanpumpwatertoitsverytopmorethan100metersabovetheground.untiltheendofthenieenthcentury,themovementofwaterintreesandothertallplantswasamystery.

somebotanistshypothesizedthatthelivingcellsofplantsactedaspumps.butmanyexperimentsdemonstratedthatthestemsofplantsinwhichallthecellsarekilledcanstillmovewatertoappreciableheights.otherexplanationsforthemovementofwaterinplantshavebeenbasedonrootpressure,apushonthewaterfromtherootsatthebottomoftheplant.butrootpressureisnotnearlygreatenoughtopushwatertothetopsoftalltrees.furthermore,theconifers,whichareamongthetallesttrees,haveunusuallylowrootpressures.

ifwaterisnotpumpedtothetopofatalltree,andifitisnotpushedtothetopofatalltree,thenwemayask:howdoesitgetthere?aordingtothecurrentlyaeptedcohesion-tensiontheory,waterispulledthere.thepullonarisingcolumnofwaterinaplantresultsfromtheevaporationofwateratthetopoftheplant.

aswaterislostfromthesurfaceoftheleaves,anegativepressure,ortension,iscreated.theevaporatedwaterisreplacedbywatermovingfrominsidetheplantinunbrokencolumnsthatextendfromthetopofaplanttoitsroots.thesameforcesthatcreatesurfacetensioninanysampleofwaterareresponsibleforthemaintenanceoftheseunbrokencolumnsofwater.

whenwaterisconfinedintubesofverysmallbore,theforcesofcohesion(theattractionbetweenwatermolecules)aresogreatthatthestrengthofacolumnofwaterpareswiththestrengthofasteelwireofthesamediameter.thiscohesivestrengthpermitscolumnsofwatertobepulledtogreatheightswithoutbeingbroken.