60 COMPOUND ORGANS OF PLANTS. 



be broadest on the side of the tree where there is the most 

 wood deposited, and this is invariably on the side which 

 contains the greatest number of branches and leaves. In a 

 solitary tree, other conditions being favorable, the rings are 

 generally the broadest on thje south side of the stem. 



The Duramen or Heart-wood. We have already stated 

 that the walls of the fully developed fibre-cells through which 

 the sap circulates, become thickened by the deposition of 

 matter in layers on their interior surface, until at length the 

 cavities of the cells are almost entirely closed; when this 

 happens, the sap can no longer permeate the walls of the cells, 

 and their vital functions cease. 



This solidification of the wood-cells is usually connected 

 with a change in the color of the wood, more or less marked. 

 Sometimes this change is made suddenly and without any 

 intermediate shades, as in the wood of the ebony and logwood, 

 of which the heart-wood is black or deep red and the albur- 

 num almost white ; but it frequently happens, that there exists 

 no sensible difference of color between the alburnum and heart- 

 wood, as for example, in trees with white wood, the color of 

 the heart-wood never changing except from incipient decay. 



This older, more solidified, and harder wood, which occupies 

 the centre of the trunk, is the part principally valued by work- 

 men as most suitable for economic purposes. The various fancy 

 colored woods employed by the turner and cabinetmaker con- 

 sist 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 wood, and dark-brown in the 

 black walnut. The alburnum in all these trees, even in the 

 ebony itself, is always white, and is regarded by workmen as 

 a part of the tree of little or no value. 



