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 green pigment
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 leaves disintegrates.
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 pigment carotene,
the same pigment that gives carrots their bright orange color.
Red, wine-red, and purple leaves contain the pigments anthocyanins, which also lend their color to
radishes, cabbage, roses, and geraniums. 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 thin veins that, 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 by tannins, 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.
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note:
pigment: 色素 chlorophyll: 葉綠素
disintegrate: 分解
carotene: 胡蘿卜素
anthocyanins: 花青素 radish: 蘿卜 geraniums:
天竺葵
vein: 紋理
tannis: 單寧酸
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