英语散文:Why do leaves change color in the fall? 秋天叶子为什么会变颜色?

Why do leaves change color in the fall?

Why do leaves change color in the fall? asks Patricia Brown, of New York City.

　　Autumn's 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 tree's 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 sunlight's 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 don't need light to make carotenoids. Botanists call them "helper pigments," because carotenoids absorb some sunlight and (nicely) pass the energy along to chlorophyll. We don't 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 leaf's 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 tree's own sunscreen.

　　Anthocyanins are made in a leaf's 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 leaf's 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.