Influence of Water on Microfibril Structure in Wood
Cellulose chains are arranged (threaded) into long, thin microfibrils.
The cell wall of wood contains microfibrils arranged in a spiral fashion.
Water adsorption and desorption displace the cellulose chains altering their angle relative to the cell wall.
The larger the microfibril angle, the more elongation or shrinkage occur per change in moisture content of wood.
Because cell walls are not perfectly lined up in a 3D matrix, wood products that are cut into rectangular prisms such as sawn lumber experience inconsistent dimensional changes when moisture changes occur.
Reference: Ylinen, A., and P. Jumppanen. 1967. Theory of shrinkage of wood. Wood Sci. Tech. 1(4): 241-252