A giant redwood can lift water more than a hundred metres into the air, yet it has no heart and no pump pushing it up. So how does the water get there? The trick happens at the leaves. Sunshine and wind dry the leaves out, and water keeps evaporating away into the air. Now here is the clever part: water molecules cling tightly to one another, like a chain of people holding hands. When the top of the chain leaves as vapour, it tugs on the molecule below, which tugs on the next, and so on, all the way down to the roots. The whole unbroken thread of water gets pulled upward at once. It is just like sipping a drink through a very, very long straw, except the Sun does the sipping. Open the simulator and watch the chain rise.
Most people think a tree pumps water up from its roots like a heart pushing blood. In fact there is no pump at all; the water is pulled up from the top, by sunlight evaporating it from the leaves.
What's actually happening
It is one of the quietest miracles in nature, happening above your head every sunny day, and it took scientists a surprisingly long time to believe it. A tree as tall as a cathedral hauls tonnes of water from the soil all the way to its topmost leaves, against gravity, without a single moving part. There is no muscle, no beating pump, no engine of any kind. For a while people assumed roots must push the water up under pressure, but the sums never worked: root pressure can lift water a few metres at most, nowhere near the height of a forest giant.
The real answer is stranger and more elegant. It begins at the very top, in the leaves. On a warm or windy day, water evaporates from inside the leaves and escapes into the air through tiny pores called stomata. This is transpiration, and it is essentially the leaf drying out and being constantly refilled. The crucial fact is that water molecules are extraordinarily sticky to one another. Each water molecule has a slightly positive end and a slightly negative end, and these snap onto neighbouring molecules in weak bonds called hydrogen bonds. So the water filling the thin pipes of the tree, the xylem, is not a loose liquid sloshing about. It is a continuous, unbroken thread running from the roots to the leaves.
Now the picture clicks together. When a water molecule evaporates from a leaf, it pulls on the one beneath it, which pulls on the next, and the tug travels straight down the whole thread to the roots. The roots are sitting in damp soil, ready to draw in more. So the entire column is hauled upward, molecule by molecule, powered not by the tree at all but by the Sun evaporating water at the top. The tree spends no energy lifting the water; it just keeps the pipes intact and the pores open. The danger is that you can pull a thread of water surprisingly hard before it snaps, but not infinitely hard. On a brutally hot, dry day a tree under huge tension can suffer cavitation, where the water column breaks and an air bubble blocks the pipe. That is why drought can be deadly for tall trees: not because they run dry at the roots, but because the climbing thread itself can break.
A tree lifts water by evaporation pulling on a chain of clinging molecules, so the Sun, not any pump, does the work of climbing.
- 1Stand a stick of celery (with leaves if you can) or a pale flower in a glass of water with a few drops of food colouring.
- 2Leave it somewhere warm and bright for a few hours, the warmer and airier the better, so water evaporates faster from the leaves.
- 3Come back and look: the colour has climbed up the stalk into the veins and leaves. The evaporation at the top pulled the coloured water all the way up, with no pump anywhere.
Common questions
Evaporation at the leaves. As water escapes through tiny pores, it tugs on the water below it. Because water molecules stick together by hydrogen bonds, the pull travels down an unbroken thread all the way to the roots, hauling the whole column up.
Only a little. Root pressure can lift water a few metres at most, nowhere near the height of a tall tree. The real lifting force is the tension created by evaporation at the leaves, called the transpiration pull.
If the tension grows too high on a hot, dry day, dissolved gas can flash out of solution and break the water column. This is called cavitation; an air bubble blocks the pipe, and that vessel can no longer carry water.