How Trees Pull Water

Trees are incredible organisms, and the physics behind them baffled scientists until relatively recently. Inside trees, there is a constant flow of water up from the roots, through the xylem and out the leaves. We often think of atmospheric pressure and capillary action as the mechanisms for pushing water up against the force of gravity, but this is not how trees work. Instead, the evaporation of water from the tree’s leaves actually pulls the entire water column up the tree. Water molecules really like sticking to one another, which actually allows them to hold together under this tension. 

The result of all this pulling is a negative pressure inside the tree, and, with some clever manipulation, it’s possible to measure just how negative the pressure inside a tree is using a device called a pressure bomb. You can see the whole process in action in the Science IRL video below. The magnitude of a tree’s negative pressure fluctuates over a day, depending on how quickly it’s losing water, but typical values can range from 2-3 atmospheres of negative pressure to 17 or more! To get the equivalent (positive) pressure, you’d have to be nearly 2.7 kilometers under water. (Image and video credit: Science IRL)

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