36 NUTRITION. 



28. The Wood immediately surrounds the 

 central pith, and is formed of concentric layers 

 of vessels, or ducts, and of fibre, annually depo- 

 sited outside each other. It consists of two 

 parts, namely : 



1. The central layers which are harder, 

 more coloured, and evidently older than those 

 near the circumference : these form what 

 workmen call the heart of the wood, and na- 

 turalists, true wood, or lignum. 



2. The external layers, which being incom- 

 pletely formed, are softer, whiter, and younger 

 than the former, and constitute what is called 

 the Alburnum. 



In some trees, especially in those which are 

 not very hard, the line of demarcation between 

 the true wood and the alburnum is not very 

 perceptible ; in the hard woods it is well marked, 

 both by texture and colour, as in ebony, in 

 which the wood is jet black and the alburnum 

 white. 



Every layer both of the wood and alburnum, 

 if we except the medullary portion, is composed 

 of vessels and fibres intermixed with elongated 

 cellular tissue. The sole organic difference be- 

 tween the wood and the alburnum, is, that in 

 the former, the interior of the cells and perhaps 



