Narrator: Listen to part of a lecture in a botany class.

旁白：请听一部分植物学的讲座。

Professor: 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.

教授：现在是秋天，正如你们所知，在美国大部分地区，树叶正由绿色变成黄色，橘色以及很多其他颜色。

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，这时候我们正好讨论下有些树叶怎么变化，而且为什么就变化成某种特定的颜色，也就是亮红色。

Well, before we discuss why leaves turn red, first, let's, urn, look, I know this is very old material, but just to play it safe, let's first go over why leaves are usually green.

在我们讨论为什么树叶会变红之前，首先，让我们来看一下，嗯……我知道这是一个老生常谈的话题，但是为了谨慎起见，我们首先来复习一下为什么树叶通常是绿色的。

It's chlorophyll, right?

叶绿素，是吗？

Leaves get their green color from chlorophyll, the chemical that's responsible for photosynthesis.

树叶是绿色通常是因为叶绿素这种物质，它有助于光合作用。

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 plant.

叶子中的叶绿素以太阳光的形式汲取能量，将这能量转化为糖，糖又为植物提供食物。

It's chlorophyll that makes leaves green most of the time.

是叶绿素使树叶大多时候是绿色的。

Now, the classic explanation for why leaves change color is this.

关于树叶为什么会变色的经典解释是这个。

In autumn, the leaves start preparing for the winter and stop synthesizing new chlorophyll.

秋天，树叶开始准备过冬，不再合成新的叶绿素。

Since chlorophyll is sensitive to sunlight and to cold temperatures, both of which you get in autumn,

由于叶绿素对秋天里的阳光和冷空气非常敏感，

the existing chlorophyll in the leaves breaks down. And since it's not been replaced by the new chlorophyll.

现存的叶绿素在树叶里分解，而且没有新的叶绿素来替换。

the green color of the leaves gradually fades away.

树叶的绿色就好渐渐地退去。

As this happens, the other pigments present in the leaf become visible.

此时，树叶中的其他色素就会显现出来。

According to the classic theory, this is true for the red pigment as well.

根据这个理论， 它也适用（树叶中的）红色色素。

It was there in the leaf all along but it was hidden by the green chlorophyll.

树叶中红色一直都有的，只是被绿色隐藏了。

OK, so that's the classic explanation and it's partially right.

这是一个经典的解释，它只是部分正确。

Why do I say partially?

为什么我说部分呢？

Well, it's probably true for pigments like yellow or orange, but it doesn't seem to hold for the red pigment.

它可能对于黄色和橘色的解释比较正确，似乎不太适用于红色。

Let's back up a bit.

让我们再回顾一下。

Just what produces this red color in leaves?

树叶中的红色是什么产生的？

It's a red pigment called anthocyanin.

是一种红色色素叫花青素。

Here is where the classic explanation doesn't seem to apply to red.

这就是为什么这个经典理论不适合解释红色。

What's interesting is that during the summer there was very little if any anthocyanin in the leaves.

有趣的是，在夏天，树叶中的花青素即使有，数量也很少。

But in the weeks before a tree is about to drop its leaves, the production of anthocyanins increases significantly.

但是在树落叶之前的几个星期里，花青素的产量会大量增加。

In other words, unlike those other pigments, anthocyanins are not just unmasked by the breakdown of chlorophyll in autumn.

换句话说，不像其他的颜料，花青素不仅仅是因为秋天叶绿素分解而显现。

They are actually created at this time.

而实际上是在这个时候产生的。

So that raises a question, why would a tree produce more anthocyanin just before dropping its leaves?

所以，有一个问题，为什么一棵树在它落叶之前会产生更多花青素呢？

Why does the tree spend so much of its resources doing this just before the leaves fall off?

为什么树在落叶之前会消耗很多能量来做这个呢？

On the surface, that doesn't make sense.

表面上，这解释不通。

It'd be likes spending money to... I don't know... to have your old car repainted when you know the car's not going to last more than a couple of months.

这就好像，你花了很多钱给旧车涂新染料，而你知道这个车用不了几个月了。

All this extra anthocyanin in the autumn seems like a waste.

秋天，这些额外的花青素似乎是一种浪费。

But remember nature is very economical with its resources.

但是要记住，大自然对它的资源是非常节省的。

So that means anthocyanin must be serving some function that's important for the tree.

这意味着花青素对树木而言，一定有某种重要的作用。

Today, there are some theories about what that function might be.

今天，关于这种功能可能是什么，有一些理论。

One of them involves predatory insects; another involves fungi.

其中的一个理论涉及到捕食性昆虫；另一个涉及到真菌。

You know, the more I read about these theories and the related research, it always created more questions for me than answers.

我读得越多这些理论和相关研究，它总是给我带来更多的问题而不是答案。

So I was really glad to learn about a totally different theory, a new one.

所以我很开心了解完全不同的新的理论。

It seems to come with research and data that give a full explanation.

这个理论有研究和数据，给出了一个完整的解释。

So here it is.

是这样的。

Remember I said the chlorophyll breaks down?

记得我说过叶绿素分解吗？

Well, in autumn, a whole lot of other chemical constituents of the leaf break down as well.

在秋天，很多叶子中的其他化学成分也被分解。

I don't mean they are totally destroyed 'cause actually they break down into other different chemicals that the tree can reabsorb from the leaves and reuse later.

我并不是说它们被完全摧毁了，因为实际上它们会分解成其他不同的化学物质， 树木可以从叶子中重新吸收这些化学物质并稍后重复使用。

Now, this reabsorption process is very important for the tree and here is the key.

这个重新吸收的过程对树木来说非常重要，关键来了。

It's sensitive to light, meaning that too much exposure to sunlight can interfere with this process.

它对光很敏感，暴露于光下太久会干扰这个过程。

So where does anthocyanin fit in here?

那么花青素在这里起什么作用呢？

Well, anthocyanin is more stable than chlorophyll.

花青素比叶绿素更稳定。

It's not harmed as easily by the Sun or the cold.

它不容易被阳光和低温破坏。

So it's still working long after the chlorophyll breaks down.

所以在叶绿素分解后很久，它仍然在起作用。

But what does it do?

但是它能做什么呢？

The theory is that anthocyanin protects the reabsorption process from the sunlight.

这个理论是花青素能保护再吸收过程免受阳光的破坏。

For example, if you look closely at a red leaf on a tree, you'll notice that most of the red pigment is on the upper side of the leaf, the side facing the sun.

比如，你如果仔细看树上的红叶，你就会注意到大多数红色在树叶正面对着太阳。

This new theory suggests that what the anthocyanin is doing there on top is shielding the rest of the leaf from the sunlight, and more importantly, allowing those important chemicals to be reabsorbed by the tree.

这个新理论表明，花青素正保护着余下的树叶免受阳光的破坏，更重要的是，它使得那些重要的化学物质能被树叶再吸收。