|The most crucial aspect of wood anatomy, as it governs the behavior and design of timber members and connections, is that wood is basically a bundle of tubes bound together by a natural glue. The tubes are wood cells aligned along the major axes of the tree trunk and its branches. These cells are an eighth to a half inch long with walls composed primarily of cellulose. The glue binding the cells together is lignin, a very complex and tough compound.
As a tree grows, cells are laid up in concentric layers under the bark. The bark splits and expands, making room for new wood. Once a cell is in place, it does not move within the tree. Fences nailed to trees are encased by them, but not lifted. Except in the tropics, the new cells are not formed identically throughout the year. The difference in appearance of spring and summer wood causes wood’s growth rings and grain.