INTEBNAL STRUCTURE. 81 



In 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, and the bright yellow of the Barberry. These colors are 

 owing to special vegetable products, or sometimes to alterations 

 resulting from age. In the Red Cedar, the deep color belongs 

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

 is little change in color of the heart- wood, except from incipient 

 decay, as in the White Pine, Poplar, Tulip-tree, &c. 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. 



148. The Growth and Duration of the Bark, also the differences 

 in structure, are much more various than of the wood. Moreover, 

 the bark is necessarily subject to grave alterations with advanc- 

 ing age, on account of its external position ; to distention from 

 the constantly 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 en- 

 larging wood, are gradually fissured and torn, and crack off in 

 strips or pieces, or disappear by slow decay. So that the bark 

 of old trunks bears only a small proportion in thickness to the 

 wood, even when it makes an equal amount of annual growth. 



149. The three parts of the bark (142), for the most part 

 readily distinguishable in the bark of j'oung shoots, grow inde- 

 pendentty, each by the addition of new cells to its inner face, so 

 long as it grows at all. The green layer commonly does not 

 increase after the first }-ear ; the opaque corky layer soon excludes 

 it from the light ; and it gradually perishes, never to be renewed. 

 The corky layer usually increases for a few years only, by the 

 formation of new tabular cells : occasionally it takes a remarkable 

 development, forming the substance called Cork, as in the Cork 

 vJak, and the thin and parchment- like layers of the White and 

 Paper Birches. 



150. The liber, or inner bark, continues its growth through- 

 out the life of the exogenous tree, by an annual addition from 

 the cambium-laver applied to its inner surface. Sometimes this 

 growth is plainly distinguishable into layers, corresponding with 

 or more numerous than the annual layers of the wood : often, 

 there is scarcely any trace of such layers to be discerned. In 

 composition and appearance, the liber varies greatly in different 

 plants, especially in trees and shrubs. That of Basswood or 



