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1、www.ylongedu.com新托福TPO25阅读原文(三)：TheEvolutionaryOriginofPlantsTPO25-3：TheEvolutionaryOriginofPlants Theevolutionaryhistoryofplantshasbeenmarkedbyaseriesofadaptations.Theancestorsofplantswerephotosyntheticsingle-celledorganismsthatgaverisetoplantspresumab
2、lylackedtrueroots,stems,leaves,andcomplexreproductivestructuressuchasflowers.Allofthesefeaturesappearedlaterintheevolutionaryhistoryofplants.Oftoday’sdifferentgroupsofalgae,greenalgaeareprobablythemostsimilartoancestralplants.Thissuppositionstemsfromthec
3、losephylogenetic(naturalevolutionary)relationshipbetweenthetwogroups.DNAcomparisonshaveshownthatgreenalgaeareplants’closestlivingrelatives.Inaddition,otherlinesofevidencesupportthehypothesisthatlandplantsevolvedfromancestralgreenalgaeusedthesametypeofchl
4、orophyllandaccessorypigmentsinphotosynthesisasdolandplants.Thiswouldnotbetrueofredandbrownalgae.Greenalgaestorefoodasstarch,asdolandplantsandhavecellwallsmadeofcellulose,similarincompositiontothoseoflandplants.Again,thegoodstorageandcellwallmoleculesofre
5、dandbrownalgaearedifferent. Todaygreenalgaelivemainlyinfreshwater,suggestingthattheirearlyevolutionaryhistorymayhaveoccurredinfreshwaterhabitats.Ifso,thegreenalgaewouldhavebeensubjectedtoenvironmentalpressuresthatresultedinadaptationsthatenhancedtheirpo
6、tentialtogiverisetoland-dwellingororganisms. Theenvironmentalconditionsoffreshwaterhabitats,unlikethoseofoceanhabitats,arehighlyvariable.Watertemperaturecanfluctuateseasonallyorevendailyandchanginglevelofrainfallcanleadtofluctuationsintheconcentrationof
7、chemicalinthewateroreventoperiodinwhichtheaquatichabitatdriesup.Ancientfreshwatergreenalgaemusthaveevolvedfeaturesthatenablethemtowithstandextremesoftemperatureandperiodsofdryness.Theseadaptationsservedtheirdescendantwellastheyinvadedland. Theterrestria
8、lworldisgreennow,butitdidnotstartoutthatway.Whenplantsfirstmadethetransitionashoremorethan400millionyearsago,thelandwasbarrenwww.ylongedu.comanddesolate,inhospitabletolife.Fromaplant’sevolutionaryviewpoint,however,itwasals
