OP THE WOOD. 33 



tions which must arise from local exposure, 

 soil, moisture, and other causes. 



In some trees, a number of the outermost 

 rings difler greatly in colour from the inner- 

 most, 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 Aiibier, 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. 



