Bound to the cellulose molecules, it's part of the structure of the material. As bound water leaves, wood continues to lighten, increases in strength and stiffness, while the cellulose shrinks. As cell walls shrink, wood's diameter decreases, not its length. This cross-grain shrinkage does, however, influence the timber's behavior and appearance. Checks in Timbers Cross-grain shrinkage opens gaps at the connections, and will open checks on the surfaces.
Checks occur when wood shrinks and the timber separates along grain lines.
We have methods of handling the movement of green timbers at their connections, including housing the end of most timbers into the face of their receiving timber. Surface checking is a function of several variables, including wood species, sawing orientation, and environmental conditions as the wood dries. The basic material making up all species of wood is almost identically dense. Balsa wood weighs less than oak because its cell walls are much thinner. A thinner cell wall means less diametric decrease in the cell as bound water leaves.