资源描述：
《新托福tpo25阅读原文及译文(三)》由会员上传分享，免费在线阅读，更多相关内容在行业资料-天天文库。
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
