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  • You've probably heard different explanations for why leaves change color: lower temperatures, less daylight, less rain.

  • But, the correct answer is: less daylight.

  • Temperature can affect the intensity of leaf color, which I will explain in a minute, but less daylight actually causes color change.

  • Most leaves contain chlorophyll, a green pigment that starts the process of photosynthesis.

  • Chlorophyll helps convert sunlight, carbon dioxide and water into sugars that travel to the branches and the roots, feeding the tree.

  • Chlorophyll breaks down and is replenished all summer long.

  • As daylight hours shorten, however, veins that bring water and nutrients to the leaf gradually close off.

  • The old chlorophyll disappears, and no new chlorophyll forms.

  • The green color fades.

  • Yellow and orange pigments, which were there all along, become visible.

  • But what about the red colors?

  • They actually form during the fall.

  • As veins close, sugars get trapped in the leaf.

  • They react with other chemicals to form the red pigments.

  • You'll remember I said intensity is connected to temperature.

  • If days are bright, and nights are cold, more sugars get trapped, and the reds intensify.

  • Eventually, the drier autumn weather triggers a hormone telling the tree to drop its leaves.

  • Otherwise, the leaves would use up water that the tree needs to keep its roots alive during the winter.

  • Okay, what about evergreens?

  • The needles have a waxy coating, and less surface area, which reduces water loss.

  • They also contain a chemical like anti-freeze, so the roots and the leaves can survive the winter.

  • Now that you know the science behind colorful autumn leaves, let's identify some of them:

  • Oaks turn red and brown.

  • Hickories turn bronze;

  • Aspen and poplar turn gold and yellow;

  • Sugar maples turn orange;

  • Black maples turn yellow;

  • And red maples turn -- what else?-- brilliant red.

  • For Scientific American's Instant Egghead,

  • I'm Mark Fischetti.

You've probably heard different explanations for why leaves change color: lower temperatures, less daylight, less rain.

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B1 US chlorophyll daylight autumn change color leaf yellow

Why Do Autumn Leaves Change Color? - Instant Egghead #51

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    Ashley Chen posted on 2015/09/25
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