Next will be noted the annual rings forming concentric circles round the pith. They are so called because in temperate climates a new ring is added every year by the rising and falling of the sap. As the tree ages, the first-laid rings harden and become what is known as duramen or heartwood. The later rings are known as alburnum or sapwood. The distinction between the two is in most trees easily recognised, the sapwood being lighter and softer than the heartwood, which is the stronger and more lasting. The bark forms the outer covering of the tree.
Defects in the living tree.—Shakes or splits in the interior of the wood are the most common defects in the living tree, and are known as star or radial and cup or ring shakes. The cause of these defects is imperfectly understood. They are rarely found in small trees, say those of under 10 inches diameter. Stevenson, in his book on wood, puts forward the following reason, which, up to now, has not been refuted by any practical writer. ‘In the spring, when the sap rises, the sapwood expands under its influence and describes a larger circle than in winter. The heartwood, being dead to this influence, resists, and the two eventually part company, a cup or ring shake being the result’ (see [fig. 55], where the cup shake is shown in its commonest form).
Fig. 55
Fig. 56
The star or radial shake is a variant of the same defect. In this case the cohesion between the sapwood and the heartwood is greater than the expansive forces can overcome, the result being that the heartwood breaks up into sections as shown in [fig. 56].
The star shake may have two or more arms. More than one cup shake, and sometimes both cup and star shakes are found in a single tree. The radial shake is probably the most common.
The branches suffer in this respect in like manner as the trunk, the same shakes being noticeable throughout
The development of these defects is the forerunner of further decay in the tree, giving, as they do, special facilities for the introduction of various fungi, more especially that form of disease known as the rot. Wet rot is found in the living tree and occurs where the timber has become saturated by rain.