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1、關(guān)于秋的美文2Whydoleaveschangecolorinthefall?asksPatriciaBrown,ofNewYorkCity.Autumn'scooldaysaretrimmedwithdeepblueskiesandgoldenlight,andbrilliantleavesofyellow,orangeandred.Leaveschangingcolorinthefallareatree'swayofpreparingforlongwinter,ratherlikeweputupstormwindowsandpullwarmclothes
2、andblanketsoutofstorage.Insummer,theleavesontreeslikepinoaksandsugarmaplesaregreenbecausetheyarechock-fullofthegreenpigmentchlorophyll.Treesneedsunlighttoproducechlorophyll.Inturn,chlorophyllusessunlight'senergytosplitwater(H2O)intohydrogenandoxygen.Meanwhile,leavesalsoabsorbcarbon
3、dioxidegasfromtheair.Theendproductsofleafchemistry:carbohydrates(homemadeplantfoodforthetree),andoxygen,releasedintotheair(thegasweneedtobreathe).Thewholeprocessiscalledphotosynthesis.Alongwithgreenchlorophyll,mostleavesalsocontainyellow,orangeandred-orangepigmentscelledcarotenoids
4、.Treesdon'tneedlighttomakecarotenoids.Botanistscallthem"helperpigments,"becausecarotenoidsabsorbsomesunlightand(nicely)passtheenergyalongtochlorophyll.Wedon'tseemuchofthesedeputypigments(carotene,lycopene,andxanthophyll)insummer,becausetheyaremaskedbyabundantgreenchlorophyll.Butthe
5、ever-shorteningdaysoffallmeanlessdaylightandcolderweather.Theaveragetreeisrushingtosaveallthenutrientsitcanforitswinterhibernation.Nitrogenandphosphorusarepulledfromleavesforstorageinbranches.Alayerofcorkycellsgrowsbetweentheleaves'stemsandtheirbranches,reducingtheleaves'supplyofnu
6、trientsandwater.Withdiminishedsunlight,water,andnutrients,chlorophyllsynthesisslows.Old,worn-outchlorophyllbreaksdownattheusualrate--ironically,sunlightdestroysit--soeachleaf'sstockgraduallydwindles.Andasthegreenfades,yellowandorangeemergefromhiding.Unlikethegreenandyellowpigments,
7、redandpurplepigments(anthocyanins,partoftheflavonoidclass)actuallyforminleavesintheautumn,tintingleavesscarletandburgundy.Botanistshavelongwonderedwhysometreesaregeneticallyprogrammedtomanufactureanthocyaninsinthefall.Newresearchindicatesthatanthocyaninsmaybeatree'sownsunscreen.Ant
8、hocyaninsaremadeinaleaf'ss
