48 THE INNER ORGANIZATION OF TREES. 



those situated nearest the bark, which are, on the contrary, 

 pale, and of a looser texture. The pale wood nearest the 

 bark is called the alburnum, or sap-wood, as the sap still 

 continues to circulate there to a greater or less extent ; the 

 older, more compact, deeply-colored, and more solidified 

 wood, which occupies the more interior part of the stem, is 

 called duramen or heart- wood. This wood consists of dead, 

 indurated cells, the cavities of which have been filled up 

 with earthy matter or lignine, and is the part chiefly valued 

 by workmen as most suitable for manufacturing purposes. 

 The various fancy-colored woods employed by the turner 

 and cabinet-maker, consist of the heart-wood only, which 

 assumes different colors in different species, being black in 

 the ebony, bright yellow in the barberry, purplish red in 

 the cedar, and dark brown in the black walnut. The 

 alburnum in all these trees, even in the ebony itself, is 

 always white, and is chipped off with the axe before the 

 wood is shipped, as a part of little or no value. 



The years' rings are manifest not only in the wood, but 

 also in the bark ; and in the pine and other trees those last 

 formed continue to be unbroken for a number of years. But 

 whilst every new layer of wood is deposited on the exterior 

 surface of the last year's wood, the diameter of the wood is 

 a constantly increasing quantity, each ring of wood remain- 

 ing unaltered in its dimensions and position until its cells 

 finally decay ; each new layer of bark is deposited on the 

 interior surface of the ring of bark of the previous year ; 

 hence, the bark-rings or layers, previously deposited, are 

 subjected to gradual but incessant distention, and are finally 

 fissured and rent. The interior growth of the bark, com- 

 bined with the annual development of the subjacent wood- 

 rings, thus prevents an accumulation of bark to any very 

 great extent, on the exterior of the stem. Hence it is that, 

 on the cross-section, the bark bears but a small proportion 

 in thickness to the wood. In the common plane tree, after 

 the eighth or tenth year, all the old layers of bark fall away 

 entirely in the form of brittle plates. The innermost layers 

 of wood, and the outermost layers of bark, are therefore, 



