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　　Why do leaves change color in the fall?

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

　　Autumn's cool days are trimmed with deep blueskies and golden light, and brilliant leaves of yellow,orange and red. Leaves changing color in the fall area tree's way of preparing for long winter, rather likewe put up storm windows and pull warm clothes andblankets out of storage.

　　In summer, the leaves on trees like pin oaks andsugar 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 tosplit water (H2O) into hydrogen and oxygen. Meanwhile, leaves also absorb carbon dioxidegas from the air. The end products of leaf chemistry: carbohydrates (homemade plant food forthe tree), and oxygen, released into the air (the gas we need to breathe). The whole process iscalled photosynthesis.

　　Along with green chlorophyll, most leaves also contain yellow, orange and red-orangepigments celled carotenoids. Trees don't need light to make carotenoids. Botanists call them"helper pigments," because carotenoids absorb some sunlight and (nicely) pass the energyalong to chlorophyll. We don't see much of these deputy pigments (carotene, lycopene, andxanthophyll) in summer, because they are masked by abundant green chlorophyll.

　　But the ever-shortening days of fall mean less daylight and colder weather. The averagetree is rushing to save all the nutrients it can for its winter hibernation. Nitrogen andphosphorus are pulled from leaves for storage in branches. A layer of corky cells growsbetween the leaves' stems and their branches, reducing the leaves' supply of nutrients andwater.

　　With diminished sunlight, water, and nutrients, chlorophyll synthesis slows. Old, worn-outchlorophyll breaks down at the usual rate--ironically, sunlight destroys it--so each leaf's stockgradually 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 theflavonoid class) actually form in leaves in the autumn, tinting leaves scarlet and burgundy.

　　Botanists have long wondered why some trees are genetically programmed tomanufacture anthocyanins in the fall. New research indicates that anthocyanins may be a tree'sown sunscreen.

　　Anthocyanins are made in a leaf's sugary sap, with the help of lots of sun and cooltemperatures. Botanists think that anthocyanins shield the leaves' fading photosynthesisfactories from too much sunlight, rather like the pigment melanin protects our skin from thesun. While the red pigments act as a shield, the tree feverishly breaks down and pulls nutrientsout 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" beforethey 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 andcold. In some trees, like sugar maples, the reds of the anthocyanins combined with the yellowsof the carotenoids make especially brilliant orange leaves.