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1、www.ylongedu.com新托福TPO25阅读原文(三):TheEvolutionaryOriginofPlantsTPO25-3:TheEvolutionaryOriginofPlants Theevolutionaryhistoryofplantshasbeenmarkedbyaseriesofadaptations.Theancestorsofplantswerephotosyntheticsingle-celledorganismsthatgaverisetoplantspres
2、umablylackedtrueroots,stems,leaves,andcomplexreproductivestructuressuchasflowers.Allofthesefeaturesappearedlaterintheevolutionaryhistoryofplants.Oftoday’sdifferentgroupsofalgae,greenalgaeareprobablythemostsimilartoancestralplants.Thissuppositionstems
3、fromtheclosephylogenetic(naturalevolutionary)relationshipbetweenthetwogroups.DNAcomparisonshaveshownthatgreenalgaeareplants’closestlivingrelatives.Inaddition,otherlinesofevidencesupportthehypothesisthatlandplantsevolvedfromancestralgreenalgaeusedthes
4、ametypeofchlorophyllandaccessorypigmentsinphotosynthesisasdolandplants.Thiswouldnotbetrueofredandbrownalgae.Greenalgaestorefoodasstarch,asdolandplantsandhavecellwallsmadeofcellulose,similarincompositiontothoseoflandplants.Again,thegoodstorageandcellw
5、allmoleculesofredandbrownalgaearedifferent. Todaygreenalgaelivemainlyinfreshwater,suggestingthattheirearlyevolutionaryhistorymayhaveoccurredinfreshwaterhabitats.Ifso,thegreenalgaewouldhavebeensubjectedtoenvironmentalpressuresthatresultedinadaptation
6、sthatenhancedtheirpotentialtogiverisetoland-dwellingororganisms. Theenvironmentalconditionsoffreshwaterhabitats,unlikethoseofoceanhabitats,arehighlyvariable.Watertemperaturecanfluctuateseasonallyorevendailyandchanginglevelofrainfallcanleadtofluctuat
7、ionsintheconcentrationofchemicalinthewateroreventoperiodinwhichtheaquatichabitatdriesup.Ancientfreshwatergreenalgaemusthaveevolvedfeaturesthatenablethemtowithstandextremesoftemperatureandperiodsofdryness.Theseadaptationsservedtheirdescendantwellasthe
8、yinvadedland. Theterrestrialworldisgreennow,butitdidnotstartoutthatway.Whenplantsfirstmadethetransitionashoremorethan400millionyearsago,thelandwasbarrenwww.ylongedu.comanddesolate,inhospitabletolife.Fromaplant’sevolutionaryviewpoint,however,itwasals