For some reason, the heartwood of certain species is prone to decay. Sycamore is the best example. The largest trunks are generally hollow. The heart has disappeared, leaving only the thin shell of sapwood, and this is required not only to maintain the tree’s life and activities, but to support the trunk’s weight. In most instances the substances deposited in the heartwood, and associated with the coloring matter, tend to preserve the wood from decay. For that reason heart timber lasts longer than sap when exposed in damp situations. The dark and variegated shades of the heartwood of some species give them their chief value as cabinet and furniture material. The sapwood of black walnut is not wanted by anybody, for it is light in color and is characterless; but when the sap has changed to heart, and its tones have been deepened by the accumulation of pigments, it becomes a choice material for certain purposes. The same is true of many other timbers, notably sweet and yellow birch, black cherry, and several of the oaks. It sometimes happens that when sapwood is transformed into heart, a physical change, as well as a coloring process, affects it. Persimmon and dogwood are examples, and hickory in a less degree. The sapwood of persimmon and dogwood makes shuttles and golf heads, but after the change to heartwood occurs, it is considered unsuitable. Handle makers and the manufacturers of buggy spokes prefer hickory sapwood, but use the red heartwood if it is the same weight as the sap. _Annual Rings_--The trunks of both hardwoods and softwoods are made up of concentric rings. In most instances the eye easily detects them. They are more distinct in a freshly cut trunk than in weathered wood, though in a few instances weathering accentuates rather than obliterates them. A count of the rings gives the tree’s age in years, each ring being the growth of one year. An occasional exception should be noted, as when accident checks the tree’s growth in the middle of the season, and the growth is later resumed. In that case, it may develop two rings in one year. A severe frost late in spring after leaves have started may produce that result; or defoliation by caterpillars in early summer may do it. Perhaps not one tree in a thousand has that experience in the course of its whole life. Trees in the tropics where seasons are nearly the same the year through, seldom have rings. Imitations of mahogany are sometimes detected by noting clearly marked annual rings. It is difficult for the woodfinisher to obliterate the annual rings, but some of the French woodworkers very nearly accomplish it.