凝聚力英语作文.doc

第页共NUMPAGES2页
凝聚力英语作文

Atmosphericpressurecansupportacolumnofwaterupto
10metershigh.But
plantscanmovewatermuchhigher;thesequoiatreecanpumpwatertoitsverytopmorethan
100metersabovetheground.Until
theendofthenieenthcentury,themovementofwaterintreesandothertallplantswasamystery.Some
botanistshypothesizedthatthelivingcellsofplantsactedaspumps.But
manyexperimentsdemonstratedthatthestemsofplantsinwhichallthecellsarekilledcanstillmovewatertoappreciableheights.Other
explanationsforthemovementofwaterinplantshavebeenbasedonrootpressure,apushonthewaterfromtherootsatthebottomoftheplant.But
rootpressureisnotnearlygreatenoughtopushwatertothetopsoftalltrees.Furthermore,
theconifers,whichareamongthetallesttrees,haveunusuallylowrootpressures.Ifwaterisnotpumpedtothetopofatalltree,andifitisnotpushedtothetopofatalltree,thenwemayask:howdoesitgetthere?Accordingtothecurrentlyacceptedcohesion-tensiontheory,waterispulledthere.The
pullonarisingcolumnofwaterinaplantresultsfromtheevaporationofwateratthetopoftheplant.As
waterislostfromthesurfaceoftheleaves,anegativepressure,ortension,iscreated.The
evaporatedwaterisreplacedbywatermovingfrominsidetheplantinunbrokencolumnsthatextendfromthetopofaplanttoitsroots.The
sameforcesthatcreatesurfacetensioninanysleofwaterareresponsibleforthemaintenanceoftheseunbrokencolumnsofwater.When
waterisconfinedintubesofverysmallbore,theforcesofcohesion(theattractionbetweenwatermolecules)aresogreatthatthestrengthofacolumnofwaterpareswiththestrengthofasteelwireofthesamediameter.Thiscohesivestrengthpermitscolumnsofwatertobepulledtogreatheightswithoutbeingbroken.