Science Puzzle
How Does Water Reach the Top of the Tallest Tree?
Hyperion, a coast redwood in California, is the tallest tree anyone has found: about 116 metres, taller than the tower that holds Big Ben. Every day it lifts hundreds of litres of water from the ground to its highest leaves.
A tree has no heart and no pump. A suction pump, the kind that sucks water up a pipe, can't lift water more than about 10 metres. So how does the water get to the top?
The Answer
It's pulled up from the top. Water evaporating from the leaves drags the water below it up the trunk, in unbroken threads that run all the way down to the roots.
A suction pump can't do better than 10 metres because it doesn't really pull at all. It lowers the pressure at the top of the pipe, and the air pressing down on the water outside pushes the water up. The weight of the air can only hold up about 10 metres of water, so that's the limit.
A tree works differently. Its wood is packed with narrow tubes, called xylem, filled with water from root to leaf with no air gaps. Water molecules cling to each other strongly. On a sunny day water evaporates from tiny pores in the leaves, and as each molecule escapes it tugs on the ones behind it, which tug on the ones behind them, all the way down. The water in the tubes is under tension, stretched like a rope being hauled upwards, and it climbs the trunk.
The danger is an air bubble. A bubble breaks the thread and that tube stops working, which is one reason a tree has so many tubes side by side. The taller the tree, the harder the pull and the greater the risk. A study in 2004 worked out that this limits trees to somewhere around 122 to 130 metres, so Hyperion is not far from the ceiling.
The principle: Transpiration pull. Water evaporating from leaves pulls a continuous column of water up through narrow tubes, because water molecules cling together strongly enough to be stretched without snapping.