How Do Plants Drink Water? How Trees Lift Water 100 Metres Without a Pump

The tallest tree on Earth, a coast redwood in California, stands about 116 metres high. Every day, water has to travel from its roots to the leaves at the very top, higher than a 30-storey building. There is no heart, no pump and no moving parts. So how does a plant drink?

It starts at the roots

Water enters a plant through its roots, mainly through millions of microscopic root hairs near the tips. These hairs greatly increase the surface area in contact with moist soil. Water moves into the root cells and then into the plant’s plumbing.

The plumbing: xylem

Running from the roots, through the stem or trunk and out into every leaf, are bundles of thin tubes called xylem. In trees, wood is mostly xylem. The tubes are made of dead cells whose ends and walls have become pipes, forming long continuous channels.

Water in these pipes forms an unbroken column from the roots to the leaves. The question is what moves it.

The engine: evaporation from the leaves

The surface of a leaf is dotted with microscopic pores called stomata. They open to let in carbon dioxide for photosynthesis, but they also let water vapour escape. This loss of water from leaves is called transpiration.

When a water molecule evaporates from a leaf, it leaves a tiny gap. Water molecules are strongly attracted to each other (cohesion) and to the walls of the xylem (adhesion). So the molecule behind is pulled forward to fill the gap, which pulls the one behind it, and so on, all the way down to the roots.

This is called the cohesion-tension theory. The water in the xylem is under tension, a little like a rope being pulled from above. The energy comes from the Sun, which drives evaporation. The plant itself spends almost nothing to lift its water.

Why so much is lost

It seems wasteful: more than 95% of the water taken up by the roots simply evaporates. But transpiration does useful work. It keeps the water column moving, carries dissolved minerals from the soil to the leaves and cools the leaves on hot days, much as sweating cools us.

On hot, dry days plants can close their stomata to save water. The price is that less carbon dioxide enters, so growth slows down. Managing this trade-off is one of the main challenges of a plant’s life.

The limit to tree height

Pulling water ever higher is hard. The higher the leaves, the greater the tension in the water column and the harder it is for the top leaves to get enough water. In 2004 a team led by George Koch climbed the tallest redwoods and measured their leaves at different heights. Leaves near the top were smaller and more water-stressed. Extrapolating, they estimated that the physics of water transport limits trees to a maximum height of about 122–130 metres.

Too much tension can also break the water column. Air bubbles can form in the xylem, an event called embolism, blocking the pipe. Drought makes this more likely, and it is one way that severe droughts kill trees.

In short

• Plants absorb water through their roots, mostly via tiny root hairs.
• Water travels up through thin pipes called xylem.
• Transpiration, evaporation from pores in the leaves, pulls the water up.
• Because water molecules cling together, the column is pulled up without breaking.
• More than 95% of absorbed water evaporates, but it carries minerals and cools the leaves.
• The physics of lifting water limits trees to about 122–130 metres.