经典英语作文秋天叶子为什么会变颜色

　　Whydoleaveschangecolorinthefall？asksPatriciaBrown，ofNewYorkCity。
　　Autumn39；scooldaysaretrimmedwithdeepblueskiesandgoldenlight，andbrilliantleavesofyellow，orangeandred。Leaveschangingcolorinthefallareatree39；swayofpreparingforlongwinter，ratherlikeweputupstormwindowsandpullwarmclothesandblanketsoutofstorage。
　　Insummer，theleavesontreeslikepinoaksandsugarmaplesaregreenbecausetheyarechockfullofthegreenpigmentchlorophyll。
　　Treesneedsunlighttoproducechlorophyll。Inturn，chlorophyllusessunlight39；senergytosplitwater（H2O）intohydrogenandoxygen。Meanwhile，leavesalsoabsorbcarbondioxidegasfromtheair。Theendproductsofleafchemistry：carbohydrates（homemadeplantfoodforthetree），andoxygen，releasedintotheair（thegasweneedtobreathe）。Thewholeprocessiscalledphotosynthesis。
　　Alongwithgreenchlorophyll，mostleavesalsocontainyellow，orangeandredorangepigmentscelledcarotenoids。Treesdon39；tneedlighttomakecarotenoids。Botanistscallthemhelperpigments，becausecarotenoidsabsorbsomesunlightand（nicely）passtheenergyalongtochlorophyll。Wedon39；tseemuchofthesedeputypigments（carotene，lycopene，andxanthophyll）insummer，becausetheyaremaskedbyabundantgreenchlorophyll。
　　Buttheevershorteningdaysoffallmeanlessdaylightandcolderweather。Theaveragetreeisrushingtosaveallthenutrientsitcanforitswinterhibernation。Nitrogenandphosphorusarepulledfromleavesforstorageinbranches。Alayerofcorkycellsgrowsbetweentheleaves39；stemsandtheirbranches，reducingtheleaves39；supplyofnutrientsandwater。
　　Withdiminishedsunlight，water，andnutrients，chlorophyllsynthesisslows。Old，wornoutchlorophyllbreaksdownattheusualrateironically，sunlightdestroysitsoeachleaf39；sstockgraduallydwindles。Andasthegreenfades，yellowandorangeemergefromhiding。
　　Unlikethegreenandyellowpigments，redandpurplepigments（anthocyanins，partoftheflavonoidclass）actuallyforminleavesintheautumn，tintingleavesscarletandburgundy。
　　Botanistshavelongwonderedwhysometreesaregeneticallyprogrammedtomanufactureanthocyaninsinthefall。Newresearchindicatesthatanthocyaninsmaybeatree39；sownsunscreen。
　　Anthocyaninsaremadeinaleaf39；ssugarysap，withthehelpoflotsofsunandcooltemperatures。Botaniststhinkthatanthocyaninsshieldtheleaves39；fadingphotosynthesisfactoriesfromtoomuchsunlight，ratherlikethepigmentmelaninprotectsourskinfromthesun。Whiletheredpigmentsactasashield，thetreefeverishlybreaksdownandpullsnutrientsoutofleavesandintoitslimbsandtrunkbeforeleavesdropordie。
　　AnthocyaninsmayalsoactlikeVitaminCorE，scavengingsocalledfreeradicalsbeforetheycandooxidizingdamagetoafallleaf39；sfragilestructure。
　　Upperandouterleavestendtobereddest，sincetheyaremostexposedtosunlightandcold。Insometrees，likesugarmaples，theredsoftheanthocyaninscombinedwiththeyellowsofthecarotenoidsmakeespeciallybrilliantorangeleaves。