Wet an open, mature pine cone and its scales gradually fold inward; let it dry and they open again. The tree does not have to ‘sense’ the weather for this to happen. The relevant tissues in a mature cone are dead, with no muscles, nerves or actively powered control system.
The mechanism lies in the scale's composite structure. Each scale behaves like two joined materials whose cell walls swell and shrink by different amounts along their length as moisture enters and leaves. Just as two bonded strips with unequal expansion must bend, this mismatch curves the whole scale. Moisture bends it towards closure; drying bends it outwards. The movement is passive hygroscopic deformation, not rain physically pushing the scales shut.
The response suits seed dispersal. In many pines, winged seeds are more likely to travel away from the parent tree in dry, breezy conditions. Closing in damp weather can reduce release under poor conditions. This is an evolved structural effect, not evidence that an individual cone predicts weather or makes a decision.
A safe home observation needs only a fallen mature cone. Record it when dry, place it in water, then allow it to dry naturally. The movement takes time and varies with species, maturity and damaged scales, so one cone is not a precision humidity meter.
https://royalsocietypublishing.org/rsif/article/6/39/951/65502/Hygromorphs-from-pine-cones-to-biomimetic-bilayers
https://www.nature.com/articles/srep09963
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