When a pinecone gets wet, its scales fold shut. They open correctly in dry weather. That is quite a handy design of such a pinecone. Between those scales are seeds. If they fall out in wet weather, they won’t get that far. They immediately rain down.
Open pine cones blow out their seeds
On sunny days, the scales are open so that the seeds can blow out. The farther seeds get, the better a species can spread and the more likely it will survive. When all the seeds fall directly under the parent tree, they have to fight for space and food. This reduces their chance of growing into a large tree. Under moist conditions they can therefore better remain where they are, safely locked up in the pinecone.
Wetness makes scales swell
But how do those pinecone scales full of dead cells open and close? When it’s wet outside, moisture soaks into the pinecone’s scales. Cellulose is contained in the cell walls of plants. It consists of a long chain of small molecules. Water sits between these cellulose chains and pushes them apart.
Such a pinecone scale consists of two layers. The cellulose chains lie in different directions in both layers. This ensures that especially the outer layer swells and stretches in wet weather. The scales then close. When the water evaporates again, the tissue on the outside contracts. The scales fold open. The seed is then free to blow away.