As yet too little is known of the relation of structure and chemical composition to the mechanical and physical properties to permit more than general conclusions.
RATE OF GROWTH To understand the effect of variations in the rate of growth it is first necessary to know how wood is formed.

A tree increases in diameter by the formation, between the old wood and the inner bark, of new woody layers which envelop the entire stem, living branches, and roots.

Under ordinary conditions one layer is formed each year and in cross section as on the end of a log they appear as rings--often spoken of as _annual rings_.

These growth layers are made up of wood cells of various kinds, but for the most part fibrous.

In timbers like pine, spruce, hemlock, and other coniferous or softwood species the wood cells are mostly of one kind, and as a result the material is much more uniform in structure than that of most hardwoods.