Tuesday, June 7, 2016

Pinecone Motility

Recently, I carried a found pine-cone into my work and set it on my desk. After a couple days, I noticed how much it had changed shape after drying. The cone was elongated and tightly closed when fresh. After drying, it was short and had its scales opened in a way that made it readily sit upright on its end.

I decided to do an observational experiment. I went and retrieved two more fresh pine-cones and set them at my desk, where I could watch them dry as I worked. Upon arriving at work the second day, one of the cones was sitting on its end. The second cone was mostly open and was on its side. I assumed that nobody had been messing with my desk and that the first cone had righted itself, but was annoyed I had missed the action. Around 3pm, while I was looking at the computer monitor above the pine cones, the second cone rolled over and righted itself. (Ah hah! I caught you!)

Closed pinecone in cross-section.
Now that I know for certain these pine-cones can right themselves, I decided to make a proper time-lapse of a cone as it dried to try and determine how they can do this. I soaked one of the dry cones in water overnight to get it to close up again, then I cut it in half length-wise (with a flat plunge-cutting oscillating tool) and placed the two halves on a flatbed scanner. The cut surface of the cone [at left] was slightly charred while cutting, but this shouldn't be a problem for the experiment.

I wrote up a script to control the scanner, automating it to take one image every hour for up to four days. From the resulting images, I constructed the animation shown below. The flat surface of the halved cone kept it from moving around [too much] as it dried.

The images could hypothetically be used to calculate how the center-of-mass changed as the pinecone dried and thus concisely describe why the pinecones rolled onto their ends. Even without any actual calculations, it is apparent from the animation that the center-of-mass is shifting towards the stem-end during the drying process. (Each scale carries its mass stemwards as it dries, thus the cumulative center-of-mass shifts stemwards as the pinecone dries.) At some point, the center-of-mass shifted past the tipping point and the pinecones rolled over on their end.

time-lapse of drying pinecone in cross-section.
100 hour time-lapse of drying pinecone halves.

Something is evident in the animation that I wasn't expecting. Towards the end of the sequence, the animation seems to stall a couple of times as well as move backwards a bit at the very end. We were having a very humid couple of days during the time-lapse and this interfered with the final drying of the pinecone. If I do another time-lapse, I'll have to have it stretch over a longer time-span than the 100 hours I used for this one.

Pinecones, upright and laying down.
I don't expect this will happen with every type of pinecone, but it seems to happen routinely with the pinecones produced by this tree. I took a picture of the ground beneath the tree. Of the twelve pinecones in the frame, eight are more-or-less upright. This is a much higher percentage than I would have expected from simple random falls. I didn't do an exhaustive survey of all the pinecones under this tree, but the photo is basically representative of what I saw. With the previous experiments showing the cones can right themselves during drying... it isn't unreasonable to infer that (across wet and dry spells) the pinecones in this photo have been reorienting themselves to point upwards after their random orientations upon falling from the tree.

What benefit might the tree get in having its pinecones reorient themselves? Maybe the winged seeds are spread better by the wind from upright pinecones. I really don't know. I do know I'm not planning to do the experiments needed to come to a conclusive answer to the question. I'm also pretty sure this behavior isn't seen in the pinecones produced by all types of pine trees, so it is likely a behavior that evolved under selection (even if the behavior is incidental to selection on the pinecone shape for unrelated reasons).