Mr. Poppey gives a suggestive paper on this subject in this number, and offers the correct explanation. Wood is formed by the germination of new cells from the mother cells. The size or abundance of these cells depends on the vital power of the mother cells. When they are able to absorb a large amount of nutrition the result is seen in quantity, and we have a thick layer of wood; when weak, that is to say when the vital power is low, they get little food, and the woody layer is thin. The thickness of an annual woody layer then depends, first, on the vital power of the mother cell, and secondly, on the supply of food to that cell. There are so many secondary causes influencing these primary ones, that where the two classes of causes are not distinguished discussions follow. As usually discussed, the disputants are often both right and both wrong.