LESSON 25.] EXOGENOUS STEK. 153 



the Sweet Gum-tree, and the White and the Paper Birch. But it 

 all dies after a while ; and the continual enlargement of the wood 

 within finally stretches it more than it can bear, and sooner or later 

 cracks and rends it, while the weather acts powerfully upon its 

 surface ; so the older bark perishes and falls away piecemeal year 

 by year. 



430. But the inner bark, or liber, does make a new growth an- 

 nually, as long as the tree lives, inside of that formed the year before, 

 and next the surface of the wood. More commonly the liber occurs 

 in the form of thin layers, which may be distinctly counted, as in 

 Bass wood : but this is not always the case. After the outer bark 

 is destroyed, the older and dead layers of the inner bark are also 

 exposed to the weather, are riven or split into fragments, and fall 

 away in succession. In many trees the bark acquires a considerable 

 thickness on old trunks, although all except the innermost portion is 

 dead ; in others it falls off more rapidly ; in the stems of Honey- 

 suckles and Grape-vines, the bark all separates and hangs in loose 

 shreds when only a year or two old. 



431. Sap-WOOd, In the wood, on the contrary, owing to its 

 growing on the outside alone, the older layers are quietly buried 

 under the newer ones, and protected by them from all disturbance. 

 All the wood of the young sapling may be alive, and all its cells 

 or woody tubes active in carrying up the sap from the roots to the 

 leaves. It is all Sap-wood or Alburnum, as young and fresh wood 

 is called. But the older layers, removed a step farther every year 

 from the region of growth, or rather the zone of growth every 

 year removed a step farther from them, soon cease to bear much, 

 if any, part in the circulation of the tree, and probably have long 

 before ceased to be alive. Sooner or later, according to the kind of 

 tree, they are turned into 



432. Ilcart-WOOd, which we know is drier, harder, more solid, and 

 much more durable as timber, than sap-wood. It is generally of a 

 different color, and it exhibits in different species the hue peculiar 

 to each, such as reddish in Red-Cedar, brown in Black- Walnut, 

 black in Ebony, &c. The change of sap-wood into heart-wood re- 

 sults from the thickening of the walls of the wood-cells by the depo- 

 sition of hard matter, lining the tubes and diminishing their calibre ; 

 and by the deposition of a vegetable coloring-matter peculiar to each 

 species. 



433. The heart-wood, being no longer a living part, may decay, 



