|MadSci Network: Botany|
The website cited below explains that pine cones open when dry because that is the most favorable weather to disperse the winged seeds, which do not travel as far when they are wet. Some cones may remain closed for several years before opening and some need to be exposed to fire to make them open and release their seeds. It also says that the reason why cones open when they dry is because the outer halves of the scales shrink more than the inner halves, and they pull away from the cone. When dry cones get wet, the outer halves swell more than the inner halves and they close. The Nature article cited say cones close due to differences in the characteristics, including water absorbing ability, of the two types of woody tissue found inside pine cones. References Why do pine cones open and close? Colin Dawson, Julian F.V. Vincent, Anne-Marie Rocca, Nature 390 (1997), 668 ("How pine cones open").
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