6 OF THE WOOD. 



broadest, on the side most favourable to vegetation, 

 and where there are most branches and leaves. This 

 in a solitary tree is generally towards the south ; but it 

 is easy to perceive the occasional variations 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 per- 

 ceptible, and in most the external rings are much less 

 firm, compact, and durable than the rest, retaining 

 rnore 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 un- 

 derstood 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. Malpighi and Grew 

 thought it was formed by the bark, and the best ob- 

 servations have confirmed their opinion. Hales sup- 

 posed the wood added a new layer to itself externally 



