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listen to part of a lecture in a botany class.
请听植物学课上的部分内容。
It’s autumn and as you know in most parts of the United States, the leaves on the trees are changing color from green to yellow, orange and lots of other colors.
现在是秋天，正如你们所知，在美国的大部分地区，树上的树叶正从绿色变成黄色、橘色和很多其他颜色。
So this will be a great time to talk about how and why some of these leaves turn one color in particular and that’s bright red.
所以现在是个绝佳的时机来谈论一些树叶是如何以及为什么会变成一种特别的颜色，那就是亮红色。
Well, before we discuss why leaves turn red,
在我们讨论树叶为什么会变红之前，
first, let’s, um, look, I know this is very old material, but just to play it safe, let’s first go over why leaves are usually green.
我们首先......我知道这是非常老的材料，但是为了保险起见，我们先来过一遍树叶为什么通常是绿色的。
It’s chlorophyll, right? Leaves get their green color from chlorophyll, the chemical that’s responsible for photosynthesis.
是叶绿素对吧？树叶从叶绿素中获得它们的绿色，这种化学物质是产生光合作用的原因。
The chlorophyll in the leaves collects energy from the Sun in the form of sunlight and it converts this energy into sugar which is food for the plants.
树叶中的叶绿素从太阳的阳光中收集能量，然后将这种能量转化成植物的食物——糖。
It’s chlorophyll that makes leaves green most of the time.
让树叶在大部分时候保持绿色的正是叶绿素。
Now, the classic explanation for why leaves change color is this: in autumn, the leaves start preparing for the winter and stop synthesizing new chlorophyll.
对树叶为什么会变颜色的传统解释是：在秋天，树叶开始为冬天做准备，停止合成新的叶绿素。
Since chlorophyll is sensitive to sunlight and to cold temperatures, both of which you get in autumn, the existing chlorophyll in the leaves breaks down.
因为叶绿素对阳光和低温很敏感，而在秋天这两个因素都有，所以树叶中存在的叶绿素开始分解。
And since it’s not been replaced by the new chlorophyll, the green color of the leaves gradually fades away.
由于没有新的叶绿素来代替它，树叶的绿色渐渐消退。
As this happens, the other pigments present in the leaf become visible.
这种现象发生时，树叶中的其他色彩开始显现。
