学文网经典英语作文：秋天叶子为什么会变颜色
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英语作文：秋天叶子为什么会变颜色1
Why do leaves change color in the fall? asks Patricia Brown,of New York City.
Autumns cool days are trimmed with deep blue skies and golden light,and brilliant leaves of yellow,orange and red.Leaves changing color in the fall are a trees way of preparing for long winter,rather like we put up storm windows and pull warm clothes and blankets out of storage.
In summer,the leaves on trees like pin oaks and sugar maples are green because they are chock-full of the green pigment chlorophyll.
Trees need sunlight to produce chlorophyll.In turn,chlorophyll uses sunlights energy to split water (H2O) into hydrogen and oxygen.Meanwhile,leaves also absorb carbon dioxide gas from the air.The end products of leaf chemistry: carbohydrates (homemade plant food for the tree),and oxygen,released into the air (the gas we need to breathe).The whole process is called photosynthesis.
Along with green chlorophyll,most leaves also contain yellow,orange and red-orange pigments celled carotenoids.Trees dont need light to make carotenoids.Botanists call them "helper pigments," because carotenoids absorb some sunlight and (nicely) pass the energy along to chlorophyll.We dont see much of these deputy pigments (carotene,lycopene,and xanthophyll) in summer,because they are masked by abundant green chlorophyll.
But the ever-shortening days of fall mean less daylight and colder weather.The average tree is rushing to save all the nutrients it can for its winter hibernation.Nitrogen and phosphorus are pulled from leaves for storage in branches.A layer of corky cells grows between the leaves stems and their branches,reducing the leaves supply of nutrients and water.
With diminished sunlight,water,and nutrients,chlorophyll synthesis slows.Old,worn-out chlorophyll breaks down at the usual rate--ironically,sunlight destroys it--so each leafs stock gradually dwindles.And as the green fades,yellow and orange emerge from hiding.
Unlike the green and yellow pigments,red and purple pigments (anthocyanins,part of the flavonoid class) actually form in leaves in the autumn,tinting leaves scarlet and burgundy.
Botanists have long wondered why some trees are genetically programmed to manufacture anthocyanins in the fall.New research indicates that anthocyanins may be a trees own sunscreen.
Anthocyanins are made in a leafs sugary sap,with the help of lots of sun and cool temperatures.Botanists think that anthocyanins shield the leaves fading photosynthesis factories from too much sunlight,rather like the pigment melanin protects our skin from the sun.While the red pigments act as a shield,the tree feverishly breaks down and pulls nutrients out of leaves and into its limbs and trunk before leaves drop or die.
Anthocyanins may also act like Vitamin C or E,scavenging so-called "free radicals" before they can do oxidizing damage to a fall leafs fragile structure.
Upper and outer leaves tend to be reddest,since they are most exposed to sunlight and cold.In some trees,like sugar maples,the reds of the anthocyanins combined with the yellows of the carotenoids make especially brilliant orange leaves.
英语作文：秋天叶子为什么会变颜色2
Unknowingly,Miss Qiu came to our side.There are many fruits on the fruit trees,and there are unique scenes everywhere.In Xingyang,at this time,the most beautiful scenery is the ginkgo trees on both sides of Suohe road!
From a distance,the road is lined with ginkgo trees,solemn and majestic,just like a famous soldier guarding our city.Closer look,some ginkgo trees also hang a string of ginkgo fruit,round toot,like ice sugar gourd,lovely! Listen to my mother,ginkgo fruit or a kind of traditional Chinese medicine!
A gust of autumn wind blowing,ginkgo leaves like a beautiful butterfly flying in the air,and then slowly fall on the ground,I cant help but pick up a ginkgo leaf,carefully observe,found that its shape is very unique,like a small fan,it seems that these small fans,blowing autumn is particularly cool,and then I continue to observe other ginkgo leaves,some of them like small goldfish The tail! Some are like my brothers little hands! They are all kinds of things!
In addition to their different shapes,their colors are also different.Some are green with yellow,some are gray yellow on the edge of the leaves,and some are pure yellow.They are so beautiful! In addition to the color,if you look closely,you will find that each leaf has vertical stripes,but it feels smooth and smooth,and it smells like a fragrance!
I love autumn,more love so colorful ginkgo leaf!
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