Re: why do pinecones close when wet?

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").