126 THE STEM. 



before ceased to take part in any vital operations. The cells of the 

 older layers, also, commonly have thicker walls and smaller calibre 

 than those of the newer, — as here shown in Fig. 198, 199, conii)ared 

 with Fig. 196, 197, — owing to the greater amount of thickening or- 

 ganic materials (43) mingled with encrusting mineral matters intro- 

 duced with the water imbibed by the roots (93) which have been de- 

 posited upon them from within. This older, more solidified, and harder 

 wood, which occupies the centre of the trunk, and is the part princi- 

 pally valuable for timber, &c., is called Heart-wood, or Duramen : 

 while the neAver layers of softer, more open, and bibulous wood, more 

 dr less charged Avith sap, receive the name of Sap-wood, or Alkuu- 

 NUM. The latter name was given by the earlier physiologists in allu- 

 sion to its white or pale color. lu all trees which have the distinction 

 between the sap-wood and heart-wood well marked, the latter acquires 

 a deeper color, and that peculiar to the species, such as the dark brown 

 of the Black Walnut, the blacker color of the Ebony, the purplish-red 

 of Red Cedar, aftd the bright yellow of the Barberry. These colors 

 are owing to special vegetable products mixed with the incrusting 

 matters ; but sometimes the hue appers to be rather an alteration of 

 the lignine with age. In the Red Cedar, the deep color belongs 

 chiefly to the medullary rays. In many of the softer woods, there is 

 little thickening of the cell-walls, and little change in color of the 

 heart-wood, except from incipient decay, as in the White Pine, Pop- 

 lar, Tulip-tree, «fec. The heart-wood is no longer in any sense a 

 living part ; it may perish, as it frequently does, without affecting the 

 life or health of the tree. 



225. The Bark is much more various in structure and growth than 

 the wood : it is also subject to grave alterations with advancing age, 

 on account of its external position, viz. to distention from the con- 

 stantly increasing diameter of the stem within, and to abrasion and 

 decay from the influence of the elements without. It is never entire, 

 therefore, on the trunks of large trees ; but the dead exterior parts, 

 no longer able to enlarge with the enlarging wood, are gradually 

 fissured and torn, and crack off in layei's, or fall away by slow decay. 

 So that the bark of old trunks bears but a small proportion in thick- 

 ness to the wood, even when it makes an equal amount of annual 

 growth. 



226. The three parts of the bark (214-217), for the most part 

 readily distinguishable in the bark of young shoots, grow indepen- 

 dently, each by the addition of new cells to its inner face, so long as 



