tions which must arise from local exposure, soil, moisture and other causes.
In some trees, a number of the outermost rings differ greatly in colour from the innermost, and are called by workmen the sap. In the Laburnum the former are yellow, the latter brown. In the Oak and many other trees a similar difference, though less striking, is perceptible, and in most the external rings are much less firm, compact and durable than the rest, retaining more vital principle, and more of the peculiar juices of the plant. Such rings are all comprehended by Du Hamel under the name of Aubier, alburnum; and he rightly observes that this difference often extends to a greater number of rings on one side of a tree than on another. It seems that the more vigour there is in a tree, or side of a tree, the sooner is its alburnum made perfect wood. By this term, however, is properly understood only the layer of new unhardened wood of the present year. When the word alburnum is used in the following pages, it applies to this part only.
Physiologists have long differed and do still differ about the origin of the wood.
