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When a pine cone is wet, it remains closed. However, when it's dry it opens again.

From the perspective of physics or biomechanics, what is the mechanism that allows a pine cone to open and close as I've described?

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From How Do Pine Cones Open (Nature):

The scales of seed-bearing pine cones move in response to changes in relative humidity. The scales gape open when it is dry, releasing the cone's seeds. When it is damp, the scales close up. The cells in a mature cone are dead, so the mechanism is passive: the structure of the scale and the walls of the cells composing the scale respond to changing relative humidity.

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As to the reasons why:

Pine cones usually appear open because this is considered a more favourable condition for the seed dispersal and germination. This usually happens when the weather is warm and when they touch water or in cold conditions, they tend to close to avoid destruction.

As to the physics/biomechanics of the pine cone opening/closing, there is an interesting article titled Hygromorphs: from pine cones to biomimetic bilayers that discusses objects that respond to environmental humidity to change their shape.

Using the pine cone as an example that opens when dried and closes when wet, we quantify the geometry, mechanics and dynamics of closure and opening at the cell, tissue and organ levels, building on our prior structural knowledge.

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