Cones and scales of Keteleeria spec. from the Middle Miocene (~15 million years ago) and of Pinus spec. 1 from the Eemian Interglacial (~120,000 ya). Credit: Poppinga et al. 2017

Three pine cones preserved in coal were plucked from the earth by German miners in the 1960s. Little did they imagine that their finds were still operational.

You may not have realized pine cones can move because they do so slowly, and only in response to humidity change. Pine cones, it turns out, discovered the bimetallic strip many millions of years before John Harrison, the clockmaker who famously claimed the British Longitude prize, did so in his third marine chronometer, finished in 1759.

Harrison was looking for a way to protect his clocks and watches from the temperature changes that would inevitably mar their accuracy at sea (and hence their ability to correctly give the ship's longitude by a simple comparison of the watch's time to local time). Heating and cooling cause the metal innards of clocks to expand and contract. Because metals differ in their temperature responses, he riveted together strips of two metals that counteracted the effects of each other's changes. Problem solved.

Bimetallic strips are also common components of thermostats, where they are used to measure the temperature rather than to buffer its effects.

Bimetallic stripe.svg
How a bimetallic strip works. Credit: Patrick87 Wikimedia (CC BY-SA 3.0)

A pine cone uses two different types of woody cells to harness the same principle, but these cells respond to changes in humidity rather than temperature. The cells in the lower layer of a cone scale expand 20% when wet. The upper layer hardly expands at all. As a result, in wet weather the cone scales curls up and close the cone, preventing the windborne seeds from escaping when they're unlikely to get far. When the scales dry out, they open to let the seeds to make a break for it.

Until now, the oldest known wood to retain the ability to respond hygroscopically was around 1,300 years old. Since wood is the food of choice for a bewildering array of insects, fungi, and microbes, it is not surprising so little of it has lasted long enough to be tested. Because pine cones are shed by their parents and left to decay on the ground, they rarely survive either.

The trio of superannuated cones that somehow did manage to survive deep beneath Germany belong to two pine family groups: the wildly successful genus Pinus and Keteleeria, an unusual group of conifers whose three living species inhabit southeast Asia. Based on the age of the rock it was found in, one Pinus cone grew during the last interglacial around 120,000 years ago. Another Pinus cone and a Keteleeria cone grew during the Middle Miocene around 15 million years ago.

Capitalizing on the happy accident of the cones found preserved in coal, a team of German scientists decided to test the hygroscopic and structural resilience of this ancient wood. They soaked the cones in water and simply watched to see what happened and recorded the result in a paper published in Scientific Reports in January. For comparison, they also soaked a similarly-sized fresh Scots pine (Pinus sylvestris) cone.

To their presumed delight, they found that the old cone scales still curled up in water, though only about half as much as the scales of the fresh cone did.

Hydration and angular change over time of coalified, separated seed scales (red: Keteleeria sp., blue: Pinus sp. 1, grey: P. sylvestris).Credit: Poppinga et al. 2017

They attributed the loss of cone scale spring to structural deterioration or to the separation of the two layers of the cone scale as a result of being, you know, older than dirt.

They also scanned the cones to see if mineralization – fossilization proper, where wood turns to stone and not just coal -- might have contributed to this loss of function. Though some extremities of the cones had small spots of mineralization, the overall mineral content of the ancient cones was quite low. The fact that the cones were ‘coalified’ rather than fossilized is what has preserved their ability to function, the authors say.

For millions of years and without any outside maintenance (we must presume), these scales have retained their passive-hydraulic ability to function -- far in excess of the results achieved so far by “ man-made movable flaps”, the authors note. They hope to use these results to improve our own far less time-tested designs.


Poppinga, Simon, Nikolaus Nestle, Andrea Šandor, Bruno Reible, Tom Masselter, Bernd Bruchmann, and Thomas Speck. "Hygroscopic motions of fossil conifer cones." Scientific Reports 7 (2017): 40302.