Ordinarily wood fibres are parallel with their pith. They are straight grained. Exceptions to this rule are constantly encountered. The chief cause of variation is the fact that tree trunks branch. Limbs have their origin in the pith of the stems that bear them. Any stem is normally one year older than the branch it bears. So the base of any branch is a cone quite buried in the parent stem. A cross-section of this cone in a board sawed from the trunk is a _knot_. Its size and number of rings indicate its age. If the knot is diseased and loose, it will fall out, leaving a _knot hole_. The fibres of the wood of a branch are extensions of those just below it on the main stem. They spread out so as to meet around the twig and continue in parallel lines to its extremity. The fibres contiguous to those which were diverted from the main stem to clothe the branch must spread so as to meet above the branch, else the parent stem would be bare in this quarter. The union of stem and branch is weak above, as is shown by the clean break made above a twig when it is torn off, and the stubborn tearing of the fibres below down into the older stem. A half hour spent at the woodpile or among the trees with a jack-knife will demonstrate the laws by which the straight grain of wood is diverted by the insertion of limbs. The careful picking up and tearing back of the fibres of bark and wood will answer all our questions. Basswood whose fibres are tough is excellent for illustration.
When a twig breaks off, the bark heals the wound and the grain becomes straight over the place. Trees crowded in a forest early divest themselves of their lower branches. These die for lack of sun and air, and the trunk covers their stubs with layers of straight-grained wood. Such timbers are the masts of ships, telegraph poles, and the best bridge timbers. Yet buried in their heart wood are the roots of every twig, great or small, that started out to grow when the tree was young. These knots are mostly small and sound, so they do not detract from the value of the lumber. It is a pleasure to work upon such a "stick of timber."
A tree that grows in the open is clothed to the ground with branches, and its grain is found to be warped by hundreds of knots when it reaches the sawmill. Such a tree is an ornament to the landscape, but it makes inferior, unreliable lumber. The carpenter and the wood chopper despise it, for it ruins tools and tempers.