銀杏樹葉黃了，楓葉紅了……秋天的樹木都換上了新裝。在欣賞自然美景的同時，我要問大家一個問題：你知道樹葉為什么會變色嗎？

Leaves change color in the fall, because the chemical changes in the leaves, as their nutrients drain into the tree's branches, trunk, and roots for winter storage, cause the leaves to stop producing the greenpigment chlorophyll.

Chlorophyll's role in the leaves, to absorb sunlight and to use the sun's energy, is no longer necessary, and the remaining chlorophyll in the leavesdisintegrates.

When the pigment chlorophyll departs, secondary pigments, substances that also absorb light, emerge to take its place, and change the leaves' colors to theirs. For example, yellow and orange leaves contain the pigmentcarotene, the same pigment that gives carrots their bright orange color.

Red, wine-red, and purple leaves contain the pigmentsanthocyanins, which also lend their color toradishes, cabbage, roses, andgeraniums. The main difference between carotene and anthrocyanins is that the anthrocyanins only form in the leaves in the fall when the weather becomes cooler, especially when the temperature falls to between 32 degrees to 45 degrees Fahrenheit.

One characteristic of the leaves' color is that, for the most part, it is inherited. The variations in the color, however, are determined by weather conditions. The richest and most brilliant foliage appears after weeks of cool, sunny weather in places in the United States such as New England.

The color of the leaves fade as fall turns to winter, and the stems that secure them to the branches begin to loosen with the change in weather. The cells at the ends of the stems fall apart, leaving the leaves attached to the branches by the thinveinsthat, in warmer weather, transported water and nutrients to the leaves. At this point, virtually any movement can break the veins and the leaves tumble to the ground.

Although the yellow and red pigments last for a few days once on the ground, they soon disintegrate just as the green chlorophyll pigment did. All that remains are brown leaves colored bytannins, which also give tea its brown color. Without a water supply, the brown leaves carpeting the ground become dry and brittle, and the trees bare.

（來源：coolquiz.com 英語點津 Annabel 編輯）