🌱 Photosynthesis
The tree that grew from thin air
A whole oak, grown almost entirely out of the sky. No, really.
Ask anyone where all that wood came from.
They’ll point at the ground.
Nearly all of it floated in on the air.
The puzzle
So what is a tree made of?
If a tree barely touches the soil, where does all that wood come from? Nearly four hundred years ago a man called Jan van Helmont got tired of guessing and weighed it.
His plan was simple: plant the willow, weigh it and weigh the soil, then give it nothing but rain for five years and weigh both again. Whatever the tree gained had to come from somewhere, and a scale doesn’t lie.
Your turn. Drag the years below and watch the tree and the soil drift apart.
The willow, year 0
2.3 kg
Soil: 90.00 kg
Tree has gained: +0.0 kg
Nothing but rainwater for five years. The tree put on weight, and plenty of it. Watch what the soil does while it happens.
The trap
So where did 75 kilogrammes come from?
Five years on, the willow had put on about 75 kg: five times its old weight, and more. The soil? Practically untouched. All that new wood, leaf and root had to come from somewhere. Before you read on, pick one. No changing your mind after.
The 75 kg of new tree came mostly from…
↑ that thin brass edge on the right is all the soil gave: about 57 grammes gone. The weight of a slice of bread. Over five years.
Here’s the part that should feel impossible: most of a tree comes out of an invisible gas. Van Helmont proved it wasn’t the soil. He guessed the rest was water (a fair guess, and half right). The full answer took later science, and it’s stranger than he knew.
A leaf reaches up…
and drinks the sky.
The why
A tree drinks the sky
The air is full of a gas called carbon dioxide: CO₂. Using sunlight, a leaf grabs it, keeps the carbon to build itself with, and throws the oxygen back out. Wood is, mostly, captured air. Tap the button and feed the tree, molecule by molecule.
0
carbon banked
0
oxygen let go
CO₂ + water + sunlight → sugar (the stuff a tree is built from) + O₂
Every molecule you pull in was floating in the sky a second ago. Now it’s solid wood you could knock on.
The why
The oxygen you just breathed
Here’s the loop that closes it. That oxygen the tree throws out? That’s the air you breathe. And the CO₂ you breathe back out is exactly what the tree wants. You and the tree are passing the same atoms back and forth. Take a breath and watch.
You give the tree the gas it eats. The tree gives you the gas you can’t live without. Round and round, all day.
Try the smaller version
You don’t need five years and a willow
Van Helmont’s trick took five years. Here’s the fast, cheap version anyone can check: grow one seed in soil and one in nothing but water, no soil at all. If the tree ate soil, the water-only one should starve. Drag the days and see.
Day 0: both just planted
Growing seeds in water alone works: it’s called hydroponics, and growers raise a lot of supermarket lettuce and tomatoes that way, no soil ever. They grow perfectly well, which tells you the bulk never came from the ground.
But building all that takes energy.
A tree never plugs in anywhere.
It runs on sunlight, and nothing else.
The real magic
A tree is bottled sunlight
Snapping gas out of the sky and turning it into wood takes energy, loads of it. A tree never plugs in. Every leaf is a solar panel, catching sunlight and locking that energy inside the sugar it builds. So a tree isn’t only air made solid. It’s sunlight, bottled. And you can always let bottled energy back out.
Sunlight stored in the wood
That glow is years of caught sunlight, sitting perfectly still. Tap to let it out.
Same trick, hiding everywhere. Tap one:
Every one of these is sunlight in disguise. Pick one and see how.
A tree is the sky,
made solid…
…with a piece of the sun locked inside.
Built out of invisible gas, powered by a star ninety-odd million miles away. Every plank of wood is a bit of sky and a bit of sunlight, held still.
One more thing
So next time someone says a plant “eats the soil” to grow, you’ve got them. Tell them about the willow: 75 kg gained, 57 grammes of soil gone. Ask where the rest came from, and watch their face when you say the air. Then tell them the other half: the campfire they toast marshmallows on is sunlight from a few summers back, handed straight out again as heat. Air and sunshine: that’s the whole recipe. This one’s yours now.