NC EOG Practice Test 7

The Hidden Colors of Fall

¹Every fall, the mountains of western North Carolina turn into a patchwork of red, orange, and gold. Visitors drive for hours just to see it. But those colors are not brand new. Many of them were inside the leaves all summer long, hidden. In a typical year, the color starts on the highest peaks in early October and creeps down into the lower valleys over the next few weeks.

²A leaf is like a tiny food factory. Inside it are pigments, substances that give the leaf its color. The most important pigment is chlorophyll, which is green. Chlorophyll captures energy from sunlight, and the tree uses that energy to turn water and carbon dioxide from the air into sugar, which is the tree's food. This process is called photosynthesis. During spring and summer, leaves are packed with so much chlorophyll that its green color covers up everything else.

³Leaves also contain yellow and orange pigments called carotenoids. These are the same kind of pigments that make carrots orange. Carotenoids help the leaf collect light, too. They are in the leaf all summer, but the green hides them.

⁴As fall comes, the days get shorter and the nights get longer. Trees sense this change. Without enough sunlight to make much food in winter, many trees get ready to drop their leaves. They stop making new chlorophyll, and the chlorophyll already in the leaves breaks down. As the green fades, the yellows and oranges that were there all along finally show.

⁵Reds and purples are different. They come from pigments called anthocyanins, which most leaves do not have during the summer. In some trees, such as red maples and sourwoods, leaves start making anthocyanins in late summer and fall, using sugar trapped in the leaf. Weather makes a big difference. A fall with many sunny days and cool nights that stay above freezing helps leaves make more red pigment, so the reds are brighter. A warm, cloudy fall usually brings duller colors, and an early hard frost can end the show quickly.

⁶Brown, often the last color of fall, comes from waste materials left in the leaf after the other pigments break down. Scientists call these materials tannins. Many oak leaves turn brown and may cling to the branches for months.

⁷Finally, a layer of special cells grows where each leaf stem meets its branch. This layer slowly cuts the leaf off from the tree, and the leaf falls. The tree rests through the winter. In spring it grows a fresh set of green leaves, ready to start making food again. The fallen leaves are not wasted, either. As they rot on the forest floor, they return nutrients to the soil that will help the forest grow.