All Things Considered

Why Leaves Really Fall Off Trees

Published October 30, 2009 6:30 PM

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You think you know why leaves fall off trees. Well, you're wrong. It's not the wind. It's not the cold.

It's because trees use "scissors" to cut their leaves off.

We call this season the "fall" because all around us right now (if you live near leaf-dropping trees in a temporal zone), leaves are turning yellow and looking a little dry and crusty. So when a stiff breeze comes along, those leaves seem to "fall" off, thus justifying the name "fall."

Sounds reasonable, no?

But the truth is much more interesting.

According to Peter Raven, president of the Missouri Botanical Garden and a renowned botanist, the wind doesn't gently pull leaves off trees. Trees are more proactive than that. They throw their leaves off. Instead of calling this season "The Fall," if trees could talk they'd call this the "Get Off Me" season.

Here's why.

Around this time of year in the Northern Hemisphere, as the days grow shorter and colder, those changes trigger a hormone in leaf-dropping trees that sends a chemical message to every leaf that says, in essence, "Time to go! Let's part company!"

Once the message is received, says Raven, little cells appear at the place where the leaf stem meets the branch. They are called "abscission" cells. They have the same root as the word scissors, meaning they are designed, like scissors, to make a cut.

And within a few days or weeks, every leaf on these deciduous trees develops a thin bumpy line of cells that push the leaf, bit by bit, away from the stem. You can't see this without a microscope, but if you looked through one, you'd see those scissors cells lined right up.

That's where the tree gives each leaf a push, leaving it increasingly dangling. "So with that very slender connection, they're sort of ready to be kicked off," says Raven, and then a breeze comes along and finishes the job.

So the truth is, the wind isn't making the leaves fall. It's the tree.

The tree is deeply programmed by eons of evolution to insist that the leaves drop away. Why? Why not let the leaves stick around? Why drop?

Raven explains that leaves are basically the kitchen staff of a tree. During the spring, summer and early fall they make the food that helps the tree grow and thrive and reproduce. When the days get short and cold, food production slows down, giving the tree an option: It can keep the kitchen staff or it can let it go.

If trees kept their leaves permanently they wouldn't have to grow new ones, but leaves are not the brightest of bulbs (sorry!). Every so often, when the winter weather has a break and the days turn warm, Raven says leaves will start photosynthesizing. "They get some water up and they start operating and making food and then it freezes again."

When the cold snap's back on, the leaves will be caught with water in their veins, freeze and die. So instead of a food staff that's resting, the tree is stuck with a food staff that's dead. And when spring comes, the permanent help will be no help. The tree will die.

That's why every fall, deciduous trees in many parts of North America get rid of their leaves and grow new ones in the spring. It's safer that way.

So for leaves, falling in the fall isn't optional. The trees are shoving them off.

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ROBERT SIEGEL, host:

Sometimes, what everybody says is true is not true - it's wrong. Here's a case in point from our science correspondent Robert Krulwich.

ROBERT KRULWICH: Why do we call this season the fall? Well, you know why, because at this time of the year, the days get shorter and colder, the trees get kind of tired and dry, and they lose hold of their leaves. So a breeze comes by and then the leaves just fall off. That's why we call it fall.

Well, that sounds logical, but it's wrong, says Peter Raven, director of Missouri's Botanical Garden. Trees don't just drop their leaves.

Dr. PETER RAVEN (Director, Missouri's Botanical Garden): The tree is getting rid of them.

KRULWICH: So it's like throwing the leaves off?

Dr. RAVEN: Discarding them, discarding them when they become non-functional.

KRULWICH: So this, it isn't really like a fall. It's more like a shove.

Dr. RAVEN: Exactly.

KRULWICH: And here's how it works. When the days get sufficiently short, that triggers the release of a hormone inside the tree.

Dr. RAVEN: So they're chemical signals...

KRULWICH: That run like messengers from leaf to leaf to leaf all over the tree, saying - and what is the message that the...

Dr. RAVEN: It says, time to go. Let's part company.

(Soundbite of laughter)

KRULWICH: I see.

Dr. RAVEN: And when the signal comes...

KRULWICH: Leaf after leaf, a little layer of cells will appear. They're called abscission cells. Abscission sounds like it's related to the word - is it related with scissors?

Dr. RAVEN: Yeah. Ab means away and scindere means to cut, so abscission means to cut away.

KRULWICH: So if I put a leaf in front of you and I say, wait, where would we see this little line of cells...

Dr. RAVEN: Right at the main stem.

KRULWICH: Right at the bottom there of the leaf? Right where it's...

Dr. RAVEN: Normally, yeah. Right where it hits the stem.

KRULWICH: So right around this time of year, if you look at every leaf on a deciduous tree right where it connects to the branch, at the very bottom of the stem, you'll see a little, thin, bumpy line. If you had a microscope it would show you cells pushing that leaf away from the branch. So within - oh, I don't know, days, or maybe a week...

Dr. RAVEN: They're joined to that parent only by a couple of thin veins going from the stem up into the leaf. So with that very slender connection, you know, they will be kicked off, but they're sort of they're ready to be kicked off, so a wind will finish up the job.

KRULWICH: So you see, it's the tree that pushes the leaves off. The wind is just the garbage collector...

(Soundbite of wind howling)

KRULWICH: ...which raises the question, why are deciduous trees so determined, so programmed by evolution, to kick off their leaves?

Dr. RAVEN: Let's put it this way. It's more efficient to get rid of your food production than it is to just keep it all there.

KRULWICH: In other words, leaves during the spring and the summer and the early fall provide food for the tree. They're kind of like the chefs or the tree's kitchen staff. But when it gets cold and food production stops, the tree now has a choice. It can fire the staff - drop the leaves - or it can make them permanent employees. If you keep your leaves, then you don't have to grow any new ones in the spring. But a leaf in winter, that can be a problem.

Dr. RAVEN: If you have the leaves sitting there during the winter and it warms up and they start photosynthesizing, you know, they get some water up and then they start operating and making food and then it freezes again. They just die. And the whole plant can be killed better than (unintelligible).

KRULWICH: Because if you've kept your leaves and you can't get new ones and now they're dead, come spring you'll starve.

Dr. RAVEN: It's better to shed them first, then you're not in any danger at all from freezing.

KRULWICH: Which is why even though we people call this season the fall, if we can think of it from a tree's point of view, if trees could talk, they'd probably call it the get-off-me season.

Dr. RAVEN: Sure.

KRULWICH: okay.

Dr. RAVEN: If you have talking trees, though, you've got real problem.

(Soundbite of laughter)

KRULWICH: Robert Krulwich, NPR News.

SIEGEL: You're listening to ALL THINGS CONSIDERED from NPR News. Transcript provided by NPR, Copyright National Public Radio.

Source: NPR

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