Thin Air

What is a plant made of? The answer may surprise you.

Plants grow. Everyone knows that. But what do they grow from?

Animals take material from the food they eat: part of it comes out the other end, but the rest becomes a part of them. They use bits to fix themselves, or to make themselves bigger. You are what you eat, as the saying goes.

But what do plants eat? Surely they can’t get their nutrients just out of thin air!

In the mid 17th century, that’s what Dutch alchemist Jan Baptist van Helmont set to find out. He weighed out exactly 200 pounds of earth, kept it in a large pot, and planted a willow sapling which he weighed as well. He attached a hole-filled metal cover to the prevent extra dust from coming. Then, he got ready for a long wait.

Five years later, von Helmont took out the willow. It weighed 169 pounds and 3 ounces. Had it taken stuff from the ground? No — the earth had lost barely 2 ounces since the beginning. Von Helmont decided plants must be made from water, since that’s the only other thing he’d added in. Today, we know better.

Plants, it turns out, really do get their food from thin air.

The little fish was very worried. “What is this ‘water’ that people keep talking about? They say we will die if there is no water, but I can’t find it anywhere!”

“Don’t worry,” said the big fish, who was also older and wiser. “The water is everywhere. It is both inside and outside of you. You were born in water, and, chances are, you will die in water. Your whole life is in water; that is why you do not notice its presence.”

This is a Zen story reminding us that, just as fish forget they live in the water, so we humans forget we live in the Tao. The parable has been retold in various other contexts, such as explaining why we don’t notice the culture we’re brought up in, and how our senses get used to stimuli over time.

In our case, the analogy is a bit more literal.

We don’t give it much thought, because it’s always around us, but air is a thing. It’s made up of stuff, and does stuff. As 17th-century mathematician Evangelista Torricelli put it, “Noi vi viamo sommersi nel fondo dun pelago d’aria.” We live submerged at the bottom of an ocean of air.

Have you ever sipped on a Frooti — or better still, a fresh tender coconut? You wouldn’t be able to do that if it wasn’t for air.

When you suck through a straw, all you do is remove air from one side of the straw. There’s an empty space left behind. But why would juice come to fill that space? Why would it climb all the way up the straw, when it’s already settled happily down below?

The reason is: all those kilometres of atmosphere pressing down from above. Applying pressure from all sides, the force of thin air pushes juice up the straw, just as you would squeeze toothpaste from a tube.

If you want to drink without any atmosphere, you’d have to use space-food: specifically, the kind that comes in a tube. Space food has different reasons for being the way it is — something to do with momentum and zero-gravity — but if there’s no atmosphere to help you squeeze, it’ll let you do the squeezing yourself.

How hard does the atmosphere push, exactly? There’s one way to find out.

Fill a plastic tube with water. Dip one end in a filled bucket, and let the other end hang out. Water will begin to flow.

So how does the water climb the wall out of the bucket? As usual, the air is pushing down on the bucket’s water, making it climb in through the tube to flow out. It’s getting pushed by the weight of the Earth’s atmosphere.

Such siphons can go beyond buckets. I use one for my Sintex tank at home, and you can put it over a small hill to drain a lake. But there’s a limit: as Genoese philosopher Giovanni Baliani found out, siphons don’t work if water has to climb more than eighteen Florentine ells — or about thirty feet. That’s because, to send water up, it must be pushed with something heavy.

And, for heights over thirty feet, the atmosphere’s simply not heavy enough.

A fish may not notice the water, but a whale would notice its absence.

The reason whales can grow so big is, they have water to support them. That’s why beaches whales, stuck on land, soon die if they can’t make it back. It’s not that they can’t breath on land — they can — but that their bodies are simply too heavy to hold themselves up.

A whale, if beached for too long, would collapse under its own weight.

We land-dwellers have a different problem. If you were to rise out of the air — or even go to a place where the air is thinner, you wouldn’t be able to breathe. Not because there’s less air, but because your lungs can’t pull it in, without air pushing from outside.

Breathing, it turns out, works the same way as sipping from a straw.

That’s also why some people have difficulty breathing when they go climbing mountains. The air pressure is less over there (less air sitting on top of them, you see) so their lungs need to work harder to breathe. Those who live in the mountains have no such problems, because their lungs are naturally stronger.

Air pressure is how coconut-water gets into your mouth. But how does water get into the coconut?

“Capillarity” is the ability of water — and other liquids — to rise up a tube. It’s a combination of two things: adhesive forces, that make water molecules stick to the tube, and cohesion, that makes water stick to itself.

A tree has tiny tubes, or capillaries going all the way up from its roots to its stem. These tubes are coated with cellulose, something that water likes to stick to. So water sticks to the tube, climbing a little way up the sides — something you can see in a glass of juice as well.

If the tube is thin enough, the water in the middle sticks to that on the sides, so that bit climbs up too. And so it goes on, all the way up. The thinner the tube, the higher capillary action can take you — but there’s still a limit.

That’s where thin air comes into play.

How do you help water rise up? By sucking it away, so there’s a gap in the middle. Actually, you don’t have to suck it: you just need some way to take the water away — and that’s what evaporation does. As water rises up and away, from the top of the plant, to merge with the air, it leaves the very gap we need.

When land-plants grow, they lose a lot of water to evaporation. It simply escapes through their leaves. They’d rather not lose this water, but they need to keep their leaves open to catch an even more important substance: air.

Water isn’t the only thing that merges with the air. What we call “air” is actually many different substances: most of it is nitrogen and oxygen, but it also has carbon, ozone, ocean salt, and various other substances in small amounts.

It’s a bit like the ocean, which has not just water, but also salts, seaweed, and a small but rapidly increasing amount of microplastics.

As plants grow, they collect carbon-dioxide from the air and turn it into oxygen. Or rather, they collect everything in the air to build their bodies, and let off extra oxygen as a waste product. Carbon is most of what makes up plants — and, indeed, animals, fungi, and bacteria too.

But where do you get your carbon from?

You get it from food — which is plants, or something that eats plants. In fact, every living thing gets made from the same substances. We may pick up extra nutrients from the ground or from the water, but most of our material ultimately comes from just one source.

What we’re made up of is mostly thin air.

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Want to know more? This article was inspired by An Ocean of Air, a book by Gabrielle Walker. More sources can be found here.