A Pine Cone Taught a Sunshade to Open and Close by Itself

Leave a pine cone on a windowsill and it will keep an eye on the weather for you. On a dry, bright day its woody scales stand wide open, like a small wooden flower. Bring on a spell of rain and they fold in tight, until the cone looks like a closed fist. Nobody winds it up and nothing inside it is alive, yet it goes on doing this for years.

How a dead cone moves

For the tree, the movement has a clear job. A team of plant scientists writing in the journal Frontiers in Plant Science explains that cones open in dry conditions, because that is when the wind can carry their light seeds a good distance, and close when it is wet. The surprising part is that the cone needs no help. Two physicists writing in the Journal of the Royal Society Interface noted that dead cones lying on the ground go on opening and closing every time they pass between dry and damp air.

Their explanation is that each scale is built from two layers. One is made of tightly packed, thick-walled cells that grow longer when they take up moisture and shorter when they dry. The other hardly responds at all. Because the two are joined, the scale cannot simply stretch. It has to bend, one way when wet and the other way when dry. The physicists compared it to a bimetallic strip, the sandwich of two metals that curls when it is heated.

What people often get wrong

It is tempting to say that the cone forecasts the weather. It does not. It answers to the moisture that is already in the air around it, so it tells you about now, not about tomorrow. Nor is anything in it deciding. There are no muscles and no nerves, only materials that swell by different amounts.

The neat two-layer picture is a simplification, too. The plant scientists list four kinds of tissue in a scale, each swelling and letting water through in its own way, and report that the movement still works in coalified cones millions of years old. They also found that a fresh cone's very first opening is not gentle at all. Resin seals the scales together, and they suggest that drying builds up force until the seal breaks. When it does, the scales crack apart in about 1.5 thousandths of a second.

What engineers borrowed

A moving part that needs no power is hard for an engineer to ignore. The two physicists made a flower of paper and plastic that blooms in water and closes again when it dries. Researchers at the Universities of Freiburg and Stuttgart went a good deal further and built a sunshade. Their Solar Gate is made of plant-based cellulose fibres, printed on a standard 3D printer in two layers modelled on a pine cone scale.

It sits on the south-facing skylight of a research building in Freiburg, and it was tested in real weather for more than a year first. In damp air the printed elements take up moisture and curl open, letting in the winter sun. In dry summer air they flatten shut and keep the heat out. That is the cone's trick turned the other way round, since a cone shuts when it is wet. The University of Freiburg describes it as the first weather-responsive shading system that works without electrical energy. There is no motor, no sensor and no wiring, because the material is all three. A pine cone would not be impressed. It has been doing this for a very long time.