In some woods the difference between the heartwood and sapwood is very marked. In ebony, for instance, the heartwood is coal black and the sapwood white.
The sketch shows half a log, the annual rings being indicated, and also the radial lines, called medullary rays.
Fig. 73. Log cut lengthwise, showing how “Grain” is formed
Looking at the length of the log we see that the lines in a board, which we call the grain, are really the edges of the annual rings.
Fig. 74. Showing Weather Checks and “Shake”
It often happens in the forest that the wind sways the trees to such an extent that the annual rings separate and slide one within the other; this produces a defect in the wood called a shake (see s, [Fig. 74]).
There are other characteristics of wood known as warping and shrinkage.