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Nature and farming · How do plants live?

Water and roots: how does water reach the top of a tree?

How does a plane tree with no pump lift water thirty metres? Follow water's path from root hairs and osmosis to the xylem and transpiration from the leaves, and find out why overwatering drowns roots and how rice survives in a flooded field.

  • 16 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

Some of Istanbul's century-old plane trees stand nearly thirty metres tall, and the tallest coast redwoods in California rise more than a hundred metres. Getting water up to the tank on the roof of a block of flats takes powerful electric pumps. Trees have neither a heart nor a pump. So how does water climb from the soil to the very top leaf? And where is the engine that drives this journey?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Plant growth

Set up a rice paddy

Move the light, water and temperature sliders. Look at each factor's score and at the limiting factor. Try the water slider all the way to the end. When growth reaches its highest level, press Check.

Prediction

Predict first

What do you think happens to the roots when a rice paddy stays under water for weeks?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Ordering

A water molecule's journey

Order the cards by dragging them or moving them with the keyboard, then press Check.

Prediction

Predict first

Where do you think most of the water a root takes from the soil ends up?

It doesn’t matter whether you’re right; the simulation opens once you choose.

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Frequently asked

5 questions

Is water pumped up to the top of a tree?

Plants have no pump. When water evaporates from the leaves (transpiration), the water molecules behind it are pulled along. Because water molecules cling to each other, the water in the xylem is pulled up like an unbroken rope. The energy comes from the Sun; the plant spends none of its own.

Can't capillary action or root pressure explain it?

Not on their own. Even in the narrowest tubes, capillary action can lift water only a few metres, and root pressure is good for only a few metres as well. In a thirty-metre plane tree, the real force lifting the water is the pull of transpiration from the leaves.

How can a plant go thirsty in wet soil?

If the soil is very salty, osmosis can run the other way: the root cells cannot take in water and may even lose it. In overwatered soil the roots run out of air, rot and can no longer carry water, so the plant wilts as if it were dry.

Should I water a wilting plant right away?

First push your finger two or three centimetres into the soil. If the soil is still wet, the problem may be too much water, and waiting is the better choice. If water is standing in the saucer, empty it and stop watering until the top of the soil dries out.

Where does most of the water a plant takes up go?

The vast majority, more than 90% according to sources, goes back into the air through transpiration. Only a small share is used in photosynthesis and growth. Transpiration still carries minerals upward and cools the leaves on hot days.