Study

The transpiration of water at negative pressures in a synthetic tree

Wheeler, T. D.; Stroock, A. D.

Nature, 2008

Peer-reviewedcontrolled laboratory experimentlaboratory

What they did

A tree was built out of hydrogel. Two chambers — one standing in for leaves, one for roots — were joined by liquid-filled channels a few micrometres across, and water was allowed to evaporate from the leaf side into unsaturated air. Pressure in the channels was measured directly while the device ran.

Sample size: microfluidic devices, run repeatedly

What they found

The artificial leaf pulled water through the device continuously, and the liquid in the channels sat at pressures well below zero — down to about −1 MPa — while remaining liquid rather than boiling. Water was drawn from the root chamber, against gravity, with nothing pumping anywhere in the system.

What it means

The authors' reading, and ours. Where they differ, that difference is the point.

How the authors put it

A synthetic system reproduces transpiration, negative-pressure transport and cavitation, showing that the physics of the cohesion–tension mechanism requires no biology beyond the right geometry and wettability.

How NatureHQ reads it

This is the cleanest available answer to the reader who suspects a tree must be pumping. The device has no cells, no membranes and no metabolism, and it lifts water anyway, because the mechanism was never biological in the first place — it is evaporation, cohesion and a narrow enough pipe. It also reproduces the failure mode: push the tension far enough and the column snaps into vapour, in a plastic chip exactly as it does in a trunk.

Limitations

A model system, not a tree; it demonstrates the physics is sufficient, not that a particular tree does nothing else as well.
Heights and tensions achieved are far smaller than those in a tall tree.
Says nothing about how a living tree manages, repairs or avoids cavitation.