1. Why Pine Cones Open on Dry Days
You may have seen a pine cone picked up in a park open up wide after sitting in the sun. Pine cones open when dry and close when wet [1]. When the scales open, seeds can escape and be carried far away by the wind. This behavior is thought to help spread seeds on dry days [1]. But is this movement caused by living muscles or energy from the tree? No. The movement is caused by materials responding passively to humidity.
2. It Is Not Living Cells That Move Them
The movement does not come from living cells. A 1997 study in the journal *Nature* examined pine cones from the radiata pine and reported that the cells in mature cones are dead [2]. The opening and closing happen passively as the material reacts to changes in humidity, not because living cells are working. The reaction depends on the structure of the scales and the cell walls surrounding the cells [2]. Many studies on this topic have been done on pines common in Europe and North America, such as the radiata pine, the Scots pine, and the Jeffrey pine. So, what part of this structure creates the bending force?
3. Two Layers with Different Swelling Bend the Scales
The part of the scale that bends at the base is made of two layers. The lower layer consists of thick-walled cells, which swell when wet. One study found that wet cells were about 20% longer than when they were dry [3]. The upper layer is made of fibers and does not swell as much [3]. Because these two layers are stuck together, when only one swells significantly, the whole scale bends. This is why it is compared to a "bimetal" strip that bends with heat, but here it works with humidity [3]. The movement of closing when wet and opening when dry comes from this bending. Note that the 20% figure refers to the length of the cells, not the entire scale. Another explanation suggests that the cellulose fibers in the layers have different orientations, with one layer having fibers perpendicular to the axis and the other parallel [4]. Does this explanation cover everything? Before checking, let us look at how long the movement takes.
4. Closing Takes About 1 Hour, Opening Takes About 2 Days
Observations from Hokkaido University showed that when a pine cone is submerged in water, it closes in about 1 hour, but drying out and reopening takes about 2 days [1]. Closing is faster than opening. However, this timing applies to submersion in water; rain or room humidity may not produce the same results. Another study noted that the time for swelling and drying varies from minutes to hours depending on the thickness of the tissue, and this motion can repeat many times [4]. When drying, the scales near the base open first, followed by those toward the tip. Water enters and exits through the back surface of the scales; blocking this surface greatly increased the time needed to open [7]. This observation was made on Scots pines and may not apply to all pine species. Where does the water go inside the scales? Can the two-layer model explain the path of the water?
5. The Two-Layer Model Was Not Enough: Recent Studies
A 2022 study showed that the "two-layer" explanation is too simple. Water enters through the epidermis on the back of the scale, passes through brown tissue and fiber bundles, and reaches the swelling thick-walled cells. Dry fibers are stiff and hold the bend in place, but they soften when wet, releasing the bend [5]. In this study, when relative humidity rose from 30% to 75%, the Young's modulus (a measure of stiffness) of the fiber bundles dropped by about 75%, from about 800 MPa to about 200 MPa. The authors explain that this softening is involved in the scale's movement [5]. In 2025, a study on Jeffrey pines showed that a multilayer model, where fiber bundles are embedded in brown tissue, better reproduces the bending angles seen in experiments [6]. The authors also discuss "hysteresis," where the reaction differs when absorbing water versus drying, which may act as a buffer so the cone does not open suddenly if humidity drops briefly [6]. This is still a hypothesis. The two-layer analogy is good for a general understanding, but the details of water paths, fiber softening, and multilayer models are still being rewritten by research. What if this material property works for a long time?
6. Fossil Pine Cones from Over 10 Million Years Ago Also Moved
A 2017 study examined fossil pine cones from about 126,000 years ago (the last interglacial period) and from about 11.5 to 16.5 million years ago (the middle Miocene). The fossil pine cones kept their ability to open and close with humidity [3]. This is considered the oldest evidence of this passive hydraulic movement in plant parts [3]. If the movement is determined by tissue structure rather than cell life, it makes sense that fossils can still move. This principle is used in biomimetics, where biological systems inspire artificial devices. An artificial flower made of two layers of paper and plastic that opens when wet and closes when dry has been created [4]. Fabrics that open with sweat or blinds that bend with sunlight are also being considered, though they are not yet finished products [8].
7. Try It Yourself
If you want to try this, take one pine cone you found and submerge it in water. Choose one that is already open. After about 1 hour, check if the scales have closed. Then, dry it in a well-ventilated place and wait for it to reopen. Since Hokkaido University observations showed it takes about 2 days, you need a plan to wait [1]. Wash your hands after touching pine cones. When you go outside, look for open and closed pine cones in a park and note the weather and time to compare them. Drying conditions may differ depending on where they are located. For more information in Japanese, the Hokkaido University CoSTEP article "Opening and Closing of Pine Cones" is a good place to start.