shown in Figs. 1 and 2 as a substance quite different from the wood that lies within, and is protected by it. Growth of the stem of ordinary trees takes place in a continuous zone just beneath the bark, the latter being also supplied with new material, as it may be needed to supply the same formative layer. As the years roll on, the wood first made, while the stem was small, and
Fig. 4.—Cross Section of Pin Oak. Positive.
now situated near the center, changes its appearance by taking on some color, the shade being determined by the kind of wood. In some of the "precious woods," so called because of their great value for special purposes and possibly their variety, the central or heart wood is nearly jet black, as in the ebony. There is usually a marked difference in the color between the latest formed sap wood lying close under the bark and that formed many years before and now covered by later layers.
We have come now to consider another point of structure previously hinted at and plainly shown in the negravings, namely, the rings of wood. The tree as it enlarges from year to year leaves in its structure the evident record of its life. Each growing season is marked by a ring of wood, and only under the most adverse circumstances is this deposit omitted, and likewise extraordinary events only can lead to the formation of two rings. Therefore with a fair degree of certainty the age of a forest giant can be determined by the number of annual deposits of wood in rings around the common center.
These deposits become manifest to the naked eye, because of the difference in structure between the spring and autumn deposits, speaking of course for tree growth in the temperate regions. Glance at the papaw stem in Fig. 1, and it will be seen that the lower portion includes the heartwood nearly to the cen-
