How can a pine cone open without muscles?

A mature pine cone can open in dry air and close in damp air even though its scales have no muscles. The cells in a mature scale are dead. Their remaining cell walls still take up and lose moisture, and that is enough to make the scale move.

Different parts of a scale change size by different amounts as they absorb water. If those parts were separate, each could swell on its own. But they remain joined. One part cannot lengthen freely without pulling on its neighbor, so the connected structure bends instead. Small changes inside the tissues can therefore change the position of a whole scale.

When moisture leaves, the tissues shrink differently and the scale bends the other way. In dry conditions, the cone's scales spread apart. Gaps open through which its seeds can leave. In damp conditions, the scales move inward again. The response comes from material structure interacting with water, rather than a brain deciding whether the weather is suitable.

This is a moving structure that works without living muscles or an electrical motor. The same joined tissues can respond as their moisture changes, although real scales contain more complicated tissue arrangements than a simple two-layer strip. Their opening angle and speed depend on conditions; the useful connection is between invisible moisture uptake, unequal size changes and a visible opening.