NARRATOR：

Listen to part of a lecture in a botany class.

FEMALE 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.

So this'll be a great time to talk about how and why some of these leaves turn one color in particular—and that's bright red.

Well, before we discuss why leaves turn red, 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.

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

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.

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

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.

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.

And since it's not being replaced by the new chlorophyll, the green color of the leaves gradually fades away.

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

According to the classic theory, this is true for the red pigment as well. It was there in the leaf all along but it was hidden by the green chlorophyll.

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

Why do I say "partially"? Well, it's probably true for pigments like yellow or orange, but it doesn't seem to hold for the red pigment.

Let's back up a bit. Just what produces this red color in leaves? It's a red pigment called anthocyanin.

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

What's interesting is that during the summer, there was very little if any anthocyanin in the leaves, but in the weeks before a tree is about to drop its leaves, the production of anthocyanins increases significantly.

In other words, unlike those other pigments, anthocyanins are not just unmasked by the breakdown of chlorophyll in autumn; they're actually created at this time.

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

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

On the surface, this doesn't make sense. It'd be like 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.

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

But remember, nature is very economical with its resources, so that means anthocyanin must be serving some function that's important for the tree.

Today there are some theories about what that function might be. One of them involves predatory insects; another involves fungi.

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

So I was really glad to learn about a totally different theory... a new one. It seemed to come with research and data that give a full explanation.

So here it is. Remember I said the chlorophyll breaks down? Well, in autumn, a whole lot of other chemical constituents of the leaf break down as well.

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

Now, this reabsorption process is very important for the tree, and—here's the key—it's sensitive to light, meaning that too much exposure to sunlight can interfere with this process.

So where does anthocyanin fit in here?

Well, anthocyanin's more stable than chlorophyll. It's not harmed as easily by the Sun or the cold. So it's still working long after the chlorophyll breaks down.

But what does it do? The theory is that anthocyanin protects the reabsorption process from the sunlight.

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.

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.