1. Why are pine cones closed after the rain?
Sometimes, when you pick up a pine cone in the park after the rain, it is tightly closed. If you leave it in the sunlight, it might open up without you noticing. Pine cones open on dry days and close when they are wet [2]. When they open, seeds come out and are carried by the wind to distant places. It is explained that they release seeds when conditions are good for wind transport, and stay closed on wet days when they cannot rely on the wind [3]. But a pine cone that has fallen from a tree has no eyes to see the weather and no muscles to produce force. How does it "know" the weather?
2. How do pine cones move without muscles?
The scales of a mature pine cone are made of dead cells [1]. Therefore, this movement is not caused by living muscles generating force. It is a passive movement that happens because the cell walls and structure react to humidity [1]. For a long time, a two-layer model has been used to explain the bending. It was thought that the scales consist of a layer that swells when it absorbs water (thick-walled cells) and a layer that does not swell much (fiber tissue), and the difference in swelling causes the bend [4]. This is similar to how two sheets of thin paper glued together will curl if only one side gets wet. However, real scales are not paper; the bending happens due to the structure of the cell walls. So, when it rains, where does the water go through the scales?
3. How do raindrops flow from the outer scales to the inner scales of a pine cone?
When it rains, raindrops first hit the small outer scales (bract scales) of the pine cone. According to reports, many of the raindrops move from there to the inner scales [1]. Because water collected on the outside flows inward, it is explained that the closing movement begins before the whole pine cone is soaked [1]. The center of the closing movement is the scales near the tip and middle. The scales at the base (root) hardly move at all. The initial speed at the tip is about 1.5 mm per second, which is about the thickness of a 1-yen coin. This speed is the initial value, and it slows down by 20 to 25% after 1 minute [1]. Can the two-layer model alone explain all of this movement?
4. Why is the two-layer model of pine cone scales not enough?
A 2022 study reported that the two-layer model is too simple. Brown tissue and bundles of fibers also absorb a lot of water and become soft. Then, the bending that started from the swelling layer becomes easier to move (it is "released") [4]. This is not a study saying the two-layer explanation is wrong, but one that supplements it. There are also conditions regarding the length of time. In the same study's experiment, when relative humidity rose from 30% to 80% (at 23°C), it took about 7 hours for the scales to fully swell and settle [4]. This is the time to settle, which is different from the movement speed when starting to close after hitting raindrops (the speed in the previous section). It is also not a value that applies to all pine cones. How are the initial closing speed on a rainy day and the long time to settle connected? With only the materials in this article, we cannot decide the answer. However, you can observe the initial movement when wetting a pine cone at home.
5. How can you investigate pine cone opening and closing yourself?
What you can do today: Dry a picked pine cone to open it, then spray water from the outside little by little using a mist spray. Compare how much the tip, middle, and base scales move. It might be different depending on the type of pine cone. Do not use too much water, and wash your hands after touching it. What you can do when you go out: In a park after the rain, look for closed pine cones and pine cones that are open in the sunlight, and look at them side by side. Official information: On the Hokkaido University website "like Hokudai", you can read an explanation of pine cones. It is in Japanese and explains why they open.