Page:Popular Science Monthly Volume 45.djvu/335

Rh more extensive. It is likely that the root and leaf surface may not increase in the same ratio as that of the cambium or growing layer.

Let us now confine our attention to any one ring—the one, for example, near the middle of the engraving. It is bounded upon the inner and outer side by a dark line. Starting at the dark inner line, the ring of wood is very porous, as shown by the multitude of small holes giving a light appearance to this portion of the ring. Farther out the wood in the ring becomes more dense, until it ends in the almost solid outer dark band. This dense layer is in fact the last portion of the annual ring to be formed, and is laid down toward the end of the growing season. The next spring a new ring begins to form just outside this dense layer, and is often produced rapidly and with many large ducts and vessels among the woody fibers. In short, the ring of wood increases in density from the inside to the outside, and this being followed up year after year, the most dense or autumn wood is brought close to that which is the most porous, and the ring structure when seen in mass inevitably results.

It is not unusual for one side of a stem to grow faster than another, and then after a few years the center is toward one side of the middle, and the stem is called excentric. This is quite uniformly the case with all climbing stems, and the writer has a vivid recollection of a microscopic study of this subject of stem eccentricty in the poison ivy, for the work was interrupted by the swelling and closing of the eye most engaged in the task. Fig. 1 is still a fertile subject, and gives the observer a view of both this eccentricity and an irregularity not uncommon in stems. For some reason—and it might have been one of many—when the stem was about ten years old a defect developed, as shown upon the lower right-hand side, when each succeeding ring formed quite an angle that was gradually outgrown during the subsequent ten years. This blemish is shown perhaps to less advantage in the positive (Fig. 2).

The points that have been brought out in the papaw stem are also shown in the section of the ash. From what has been said it is evident that the lower side of the picture represents the inner side of the section. The center of the tree was where two pencils would intersect if held with their tips to the right and left side respectively of the lower edge of the engraving and at right angles to the curvature shown by the rings of growth. The tree from which the section used in the engraving was cut must needs have been at least a foot in diameter, but how much more can not be determined, for there is no means of knowing how far it is from the outermost ring shown to the bark. This could be determined in a general way from a knowledge of the ratio