經典英語作文:秋天葉子為什麼會變顏色
2024-03-27 12:42:05
Whydoleaveschangecolorinthefall?asksPatriciaBrown,ofNewYorkCity.
Autumn'scooldaysaretrimmedwithdeepblueskiesandgoldenlight,andbrilliantleavesofyellow,orangeandred.Leaveschangingcolorinthefallareatree'swayofpreparingforlongwinter,ratherlikeweputupstormwindowsandpullwarmclothesandblanketsoutofstorage.
Insummer,theleavesontreeslikepinoaksandsugarmaplesaregreenbecausetheyarechock-fullofthegreenpigmentchlorophyll.
Treesneedsunlighttoproducechlorophyll.Inturn,chlorophyllusessunlight'senergytosplitwater(H2O)intohydrogenandoxygen.Meanwhile,leavesalsoabsorbcarbondioxidegasfromtheair.Theendproductsofleafchemistry:carbohydrates(homemadeplantfoodforthetree),andoxygen,releasedintotheair(thegasweneedtobreathe).Thewholeprocessiscalledphotosynthesis.
Alongwithgreenchlorophyll,mostleavesalsocontainyellow,orangeandred-orangepigmentscelledcarotenoids.Treesdon'tneedlighttomakecarotenoids.Botanistscallthem"helperpigments,"becausecarotenoidsabsorbsomesunlightand(nicely)passtheenergyalongtochlorophyll.Wedon'tseemuchofthesedeputypigments(carotene,lycopene,andxanthophyll)insummer,becausetheyaremaskedbyabundantgreenchlorophyll.
Buttheever-shorteningdaysoffallmeanlessdaylightandcolderweather.Theaveragetreeisrushingtosaveallthenutrientsitcanforitswinterhibernation.Nitrogenandphosphorusarepulledfromleavesforstorageinbranches.Alayerofcorkycellsgrowsbetweentheleaves'stemsandtheirbranches,reducingtheleaves'supplyofnutrientsandwater.
Withdiminishedsunlight,water,andnutrients,chlorophyllsynthesisslows.Old,worn-outchlorophyllbreaksdownattheusualrate--ironically,sunlightdestroysit--soeachleaf'sstockgraduallydwindles.Andasthegreenfades,yellowandorangeemergefromhiding.
Unlikethegreenandyellowpigments,redandpurplepigments(anthocyanins,partoftheflavonoidclass)actuallyforminleavesintheautumn,tintingleavesscarletandburgundy.
Botanistshavelongwonderedwhysometreesaregeneticallyprogrammedtomanufactureanthocyaninsinthefall.Newresearchindicatesthatanthocyaninsmaybeatree'sownsunscreen.
Anthocyaninsaremadeinaleaf'ssugarysap,withthehelpoflotsofsunandcooltemperatures.Botaniststhinkthatanthocyaninsshieldtheleaves'fadingphotosynthesisfactoriesfromtoomuchsunlight,ratherlikethepigmentmelaninprotectsourskinfromthesun.Whiletheredpigmentsactasashield,thetreefeverishlybreaksdownandpullsnutrientsoutofleavesandintoitslimbsandtrunkbeforeleavesdropordie.
AnthocyaninsmayalsoactlikeVitaminCorE,scavengingso-called"freeradicals"beforetheycandooxidizingdamagetoafallleaf'sfragilestructure.
Upperandouterleavestendtobereddest,sincetheyaremostexposedtosunlightandcold.Insometrees,likesugarmaples,theredsoftheanthocyaninscombinedwiththeyellowsofthecarotenoidsmakeespeciallybrilliantorangeleaves.
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