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A tree being built out of gas drawn from the air A tree stands on the right with a thin band of soil under it. Its roots reach only a little way into that band. Streaming in from the left is a crowd of small rings, drifting toward the tree along two long dashed arrows. The rings are the carbon dioxide the leaves take out of the air, and they are what the wood is built from, which is why the soil band under the tree is drawn so thin.

A Tree Is Mostly Built From Air

About 11 minutes

You already know what a plant takes in and what it gives out. Sunlight, water and carbon dioxide go in. Sugar and oxygen come out.

Now stand next to an old oak tree and look up. That is tonnes of wood. Every branch, every root, the whole trunk. It was not there when the acorn dropped, so it had to come from somewhere.

Nearly everybody guesses the soil. It is the obvious answer, and it is wrong.

The gardener who weighed his dirt

About four hundred years ago a Flemish scientist named Jan Baptist van Helmont decided to check. He did something simple that nobody had bothered to do.

He dried out a pot of soil and weighed it. He planted a small willow shoot in it. Then for five years he did nothing but add water.

At the end of five years he had a young tree, far heavier than the shoot he had put in. So he dried the soil out again and weighed it a second time.

Build a tree out of the only two things it really uses. One button opens the leaves, and every press turns a hollow square of carbon dioxide up in the sky into a solid square of wood in the crown. The pot lends a square of water for each square of wood, and that water leaves again through the leaves, so the pot needs filling. Watch the block of soil while you do it: it is drawn at exactly the same size every single turn, and it never gets smaller.

Then the tree goes under a glass jar, where the air runs out and growing stops even though the pot below is still full of soil and still holds water. And then you live out the five years of the 1648 willow experiment yourself, guessing first how much soil the pot will have lost.

The soil had lost a few ounces. That was all.

If the tree had been built out of soil, the pot would have been nearly empty. It was not even dented. Whatever the tree was made of, it had not come up out of the ground.

Van Helmont decided it must be the water. He was wrong too, but he had already done the important part: he had ruled out the answer everyone else believed, by weighing it.

Wood is mostly carbon

Take a piece of wood and dry all the water out of it. What is left is about half carbon, by weight.

So the real question is not "where did the wood come from" but "where did all that carbon come from". And there is only one place a leaf can get it.

Carbon dioxide. The gas. The one in the list you already know.

Every carbon atom in that oak tree floated in through a tiny hole in a leaf, as part of a gas, and was fastened into place. The trunk is not made of ground. It is made of air that has been caught and would not be let go.

Air is not nothing

That sounds impossible, because air feels like nothing. But air is not nothing. It is stuff, it has weight, and if you could pile it up you could weigh it.

Carbon dioxide is only a small part of it: about four molecules in every ten thousand. So a leaf is not scooping up mouthfuls. It is sieving. It holds itself open in the sunlight and takes the right molecules out of the air that drifts past, one at a time, for as long as it lives.

An oak takes a hundred years over it. That is why it takes a hundred years.

The price of leaving the door open

Here is the part that explains why plants are so thirsty.

Carbon dioxide cannot get into a leaf through a solid wall. It comes in through pores on the leaf's underside. To feed at all, a leaf has to open them.

But the inside of a leaf is wet, and the air outside usually is not. The moment a pore opens, water vapour pours out of it. Not a trickle either — for every one molecule of carbon dioxide a leaf catches, hundreds of water molecules escape the other way.

The trade a leaf makes at an open pore A leaf drawn from the side as a long flat slab. There is a gap in its underside, which is a pore. One arrow comes up through the gap into the leaf, marked as carbon dioxide arriving one molecule at a time. A fan of five arrows leaves through the same gap going down and away, marked as water vapour escaping hundreds of molecules at a time. The pore cannot be open for one without being open for the other, so every gulp of carbon dioxide a plant takes costs it a great deal of water. leaf, seen from the side pore carbon dioxide in one at a time water out hundreds at a time The pore cannot be open for one and shut for the other.
The pore is one hole, and it works in both directions at once. There is no setting where carbon dioxide comes in and water stays put.

That is the trade, and no plant has ever got out of it. It is why a big tree can pull hundreds of litres of water up out of the ground on a hot day, and why almost none of that water is still in the tree by evening. Most of it was never building material. It was the cost of holding the door open.

It also explains something you may have seen. When a plant runs short of water it shuts its pores to stop the leak — and the instant it does, it stops eating too. Same hole. A plant in a drought is not only thirsty. It is shut.

Van Helmont's willow gained many pounds over five years while the soil in the pot lost only a few ounces. So what was the wood built from?

Next time you put your hand on a tree trunk, press a bit harder. You are leaning on about a hundred years of air.