This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
This may be sufficiently explained in a few words. A cross section of an exogenous (" outward growing ") tree, which class includes all timbers used in construction, shows it to be made up of several concentric rings, called " annual," from their being generally deposited at the rate of 1 a year; at or near the centre is a column of pith, whence radiate thin lines called "medullary rays," which, in some woods, when suitably cut, afford a handsome figure termed "silver grain." As the tree increases in ago, the inner layers are filled up and hardened, becoming what is called duramen or "heartwood"; the remainder, called alburnum or "sapwood," is softer and lighter in colour, and can generally be easily distinguished. The heartwood is stronger and more lasting than the sapwood, and should alone bo used in good work. The annual rings are generally thicker on the side of the tree that has had most sun and air, and the heart is therefore seldom in the centre.