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1、BOX:transfercells(转移细胞)Therateatwhichmaterialcanbeactivelytransporteddependsonthenumberofmolecularpumpspresent,whichinturndependsonthesurfaceareaoftheplasmamembrane:Thelargerthemembrane,themoremolecularpumpsitcanhold.Incertainspecializedtransfercells,thewallsaresmoothontheoutersurfacebuthave
2、numerousfinger-likeandridgelikeoutgrowthsontheinnersurface.Theplasmamembraneispressedfirmlyagainstalltheconvolutionsandthushasamuchlargersurfaceareathanitwouldifthewallwereflat.Consequently,roomisavailableformanymolecularpumps,andhigh-volumetransportcanoccuracrossthesetransferwalls.Transferc
3、ellsarefoundinareaswhererapidshortdistancetransportisexpectedtooccur:inglandsthatsecretesalt,inareasthatpassnutrientstoembryos,andinregionswheresugarisloadedintooroutofphloem.(a)Oncereleasedfromacell,amoleculediffusesinaseriesofrandomshortpathsasitcollideswithandbouncesoffothermolecules.Prob
4、abilityfavorsitsenteringaneighboringcell(molecules1,2,and3),butitcandiffuselaterallyalongthewallaswell(molecules4and5).(b)Inmanyglands,theapopIastislarge,somovementbetweencellsmaybefasterandeasierthanmovementwithincells.Suchglandsoftenhavealiningofcompacttissuethatisolatesthegland,preventing
5、thesecretedmaterialfrompermeatingthewholeregion.6.1.2longdistancetransport (长距离运输)6.1.2.1pathway(途径)：phloem(韧皮部)。Girdlingexperiment(环割试验)、radioactivetracer(同位素示踪技术)及谚语“树怕扒皮”等可以证明，同化物运输的途径是维管组织的韧皮部。水分和同化物运输的途径比较Diagramshowingbasicelementsinthecirculationofwater,inorganicions,andassimilatesint
6、heplant.Waterandinorganicionstakenupbytherootmoveupwardinthexyleminthetranspirationstream.Somemovelaterallyintotissuesoftherootandstem,whileothersaretransportedtogrowingplantpartsandmatureleaves.Intheleaves,substantialamountsofwaterandinorganicionsaretransferredtothephloemandareexportedwiths
7、ucroseintheassimilatestream.Asearlyastheseventeenthcentury,MarcelloMalpighiofItalynoticedthat,whenaringofbarkwasremovedfromastem(a),thetissuesabovetheringbecameswollen(b).Hecorrectlyinterpretedthisphenomenonasnewgrowthofwoodandbarktissuesstimulated
