This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Knots are more or less common in all timber, and consist of small pieces of dead wood which occupy a place in the body of the log with sound wood all around them. These bits of dead wood have no connection with the living wood about them, so that in the course of time they work loose, and when the log is sawed up into boards the pieces of dead wood fall out leaving round or irregular-shaped holes. Knots are formed at the juncture of the main tree trunk with branches or limbs, while such branches are still young and green. At such points the fibers of the main trunk, near the place where the branch comes in, do not follow straight along up the trunk, but are turned aside so as to follow along the branch as shown in Fig. 6. Frequently such a branch is broken off Dear the trunk of the tree when it is still young, while the tree itself continues to grow and the trunk increases in size until the end of the branch which was left buried in the main trunk is entirely covered up. Meanwhile the end of the branch dies and a knot is formed. The presence of a limited number of knots will not harm a piece of timber which is subjected to a compressive stress so long as they remain in place and do not drop out, but they very greatly weaken a piece subjected to a tension stress or used as a beam. Knots always spoil the appearance of woodwork which is to be polished.
The defects heretofore considered result from the natural growth of the tree and are not attributable to the handling of the timber after it has been cut, but there are several classes of defects which are caused by the seasoning of the timber and which have little or nothing to do with the growth of the tree. Among these are the actions known as "warping" and "check-ing."