GRE写作范文——Cohesion-tension Theory
<p> <span word="Atmospheric">Atmospheric</span> <span word="pressure">pressure</span> <span word="can">can</span> <span word="support">support</span> <span word="a">a</span> <span word="column">column</span> <span word="of">of</span> <span word="water">water</span> <span word="up">up</span> <span word="to">to</span> 10 <span word="meters">meters</span> <span word="high">high</span>. <span word="But">But</span> <span word="plants">plants</span> <span word="can">can</span> <span word="move">move</span> <span word="water">water</span> <span word="much">much</span> <span word="higher">higher</span>; <span word="the">the</span> <span word="sequoia">sequoia</span> <span word="tree">tree</span> <span word="can">can</span> <span word="pump">pump</span> <span word="water">water</span> <span word="to">to</span> <span word="its">its</span> <span word="very">very</span> <span word="top">top</span> <span word="more">more</span> <span word="than">than</span> 100 <span word="meters">meters</span> <span word="above">above</span> <span word="the">the</span> <span word="ground">ground</span>. <span word="Until">Until</span> <span word="the">the</span> <span word="end">end</span> <span word="of">of</span> <span word="the">the</span> <span word="nineteenth">nineteenth</span> <span word="century">century</span>, <span word="the">the</span> <span word="movement">movement</span> <span word="of">of</span> <span word="water">water</span> <span word="in">in</span> <span word="trees">trees</span> <span word="and">and</span> <span word="other">other</span> <span word="tall">tall</span> <span word="plants">plants</span> <span word="was">was</span> <span word="a">a</span> <span word="mystery">mystery</span>. <span word="Some">Some</span> <span word="botanists">botanists</span> <span word="hypothesized">hypothesized</span> <span word="that">that</span> <span word="the">the</span> <span word="living">living</span> <span word="cells">cells</span> <span word="of">of</span> <span word="plants">plants</span> <span word="acted">acted</span> <span word="as">as</span> <span word="pumps">pumps</span>.</p><p> <span word="But">But</span> <span word="many">many</span> <span word="experiments">experiments</span> <span word="demonstrated">demonstrated</span> <span word="that">that</span> <span word="the">the</span> <span word="stems">stems</span> <span word="of">of</span> <span word="plants">plants</span> <span word="in">in</span> <span word="which">which</span> <span word="all">all</span> <span word="the">the</span> <span word="cells">cells</span> <span word="are">are</span> <span word="killed">killed</span> <span word="can">can</span> <span word="still">still</span> <span word="move">move</span> <span word="water">water</span> <span word="to">to</span> <span word="appreciable">appreciable</span> <span word="heights">heights</span>. <span word="Other">Other</span> <span word="explanations">explanations</span> <span word="for">for</span> <span word="the">the</span> <span word="movement">movement</span> <span word="of">of</span> <span word="water">water</span> <span word="in">in</span> <span word="plants">plants</span> <span word="have">have</span> <span word="been">been</span> <span word="based">based</span> <span word="on">on</span> <span word="root">root</span> <span word="pressure">pressure</span>, <span word="a">a</span> <span word="push">push</span> <span word="on">on</span> <span word="the">the</span> <span word="water">water</span> <span word="from">from</span> <span word="the">the</span> <span word="roots">roots</span> <span word="at">at</span> <span word="the">the</span> <span word="bottom">bottom</span> <span word="of">of</span> <span word="the">the</span> <span word="plant">plant</span>. <span word="But">But</span> <span word="root">root</span> <span word="pressure">pressure</span> <span word="is">is</span> <span word="not">not</span> <span word="nearly">nearly</span> <span word="great">great</span> <span word="enough">enough</span> <span word="to">to</span> <span word="push">push</span> <span word="water">water</span> <span word="to">to</span> <span word="the">the</span> <span word="tops">tops</span> <span word="of">of</span> <span word="tall">tall</span> <span word="trees">trees</span>. <span word="Furthermore">Furthermore</span>, <span word="the">the</span> <span word="conifers">conifers</span>, <span word="which">which</span> <span word="are">are</span> <span word="among">among</span> <span word="the">the</span> <span word="tallest">tallest</span> <span word="trees">trees</span>, <span word="have">have</span> <span word="unusually">unusually</span> <span word="low">low</span> <span word="root">root</span> <span word="pressures">pressures</span>.</p><p> <span word="If">If</span> <span word="water">water</span> <span word="is">is</span> <span word="not">not</span> <span word="pumped">pumped</span> <span word="to">to</span> <span word="the">the</span> <span word="top">top</span> <span word="of">of</span> <span word="a">a</span> <span word="tall">tall</span> <span word="tree">tree</span>, <span word="and">and</span> <span word="if">if</span> <span word="it">it</span> <span word="is">is</span> <span word="not">not</span> <span word="pushed">pushed</span> <span word="to">to</span> <span word="the">the</span> <span word="top">top</span> <span word="of">of</span> <span word="a">a</span> <span word="tall">tall</span> <span word="tree">tree</span>, <span word="then">then</span> <span word="we">we</span> <span word="may">may</span> <span word="ask">ask</span>: <span word="how">how</span> <span word="does">does</span> <span word="it">it</span> <span word="get">get</span> <span word="there">there</span>? <span word="According">According</span> <span word="to">to</span> <span word="the">the</span> <span word="currently">currently</span> <span word="accepted">accepted</span> <span word="cohesion">cohesion</span>-<span word="tension">tension</span> <span word="theory">theory</span>, <span word="water">water</span> <span word="is">is</span> <span word="pulled">pulled</span> <span word="there">there</span>. <span word="The">The</span> <span word="pull">pull</span> <span word="on">on</span> <span word="a">a</span> <span word="rising">rising</span> <span word="column">column</span> <span word="of">of</span> <span word="water">water</span> <span word="in">in</span> <span word="a">a</span> <span word="plant">plant</span> <span word="results">results</span> <span word="from">from</span> <span word="the">the</span> <span word="evaporation">evaporation</span> <span word="of">of</span> <span word="water">water</span> <span word="at">at</span> <span word="the">the</span> <span word="top">top</span> <span word="of">of</span> <span word="the">the</span> <span word="plant">plant</span>. <span word="As">As</span> <span word="water">water</span> <span word="is">is</span> <span word="lost">lost</span> <span word="from">from</span> <span word="the">the</span> <span word="surface">surface</span> <span word="of">of</span> <span word="the">the</span> <span word="leaves">leaves</span>, <span word="a">a</span> <span word="negative">negative</span> <span word="pressure">pressure</span>, <span word="or">or</span> <span word="tension">tension</span>, <span word="is">is</span> <span word="created">created</span>. <span word="The">The</span> <span word="evaporated">evaporated</span> <span word="water">water</span> <span word="is">is</span> <span word="replaced">replaced</span> <span word="by">by</span> <span word="water">water</span> <span word="moving">moving</span> <span word="from">from</span> <span word="inside">inside</span> <span word="the">the</span> <span word="plant">plant</span> <span word="in">in</span> <span word="unbroken">unbroken</span> <span word="columns">columns</span> <span word="that">that</span> <span word="extend">extend</span> <span word="from">from</span> <span word="the">the</span> <span word="top">top</span> <span word="of">of</span> <span word="a">a</span> <span word="plant">plant</span> <span word="to">to</span> <span word="its">its</span> <span word="roots">roots</span>.</p>
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