48 ON STEMS. 



in an aged tree are no longer distinguishable towards the 

 base of the stem. 



BARK. The vegetation of the bark is precisely the in- 

 verse of that of the wood ; that is to say, it is endogenous, 

 its layers growing internally like those of the palm-tree ; 

 the new soft coat of bark therefore lies immediately in 

 contact with the new soft layer of wood. 



Emily. But if a fresh layer of bark grows every year, 

 why is the bark so much thinner than the wood ? I 

 should have supposed that they would have been of equal 

 dimensions ? 



Mrs. B. The outer coats of bark, when they become 

 too hard to be further distended by the pressure of the in- 

 ternal layers, crack, and becoming thus exposed to the in- 

 jury of the weather, fall oifin pieces : it is this which pro- 

 duces the ruggedness of the bark of some trees. In oth- 

 ers, the rind, though smooth, peels off, after cracking, like 

 that of the cherry, the birch, and particularly the plane- 

 tree. Those trees whose external coat of bark is least li- 

 . able to peel off, such as the oak and the elm, become 

 more scarred and rugged, in proportion as the tree grows 

 older, and is longer exposed to the action of air, water, 

 insects, and parasitical plants : sooner or later these va- 

 rious causes effect the destruction of the outer bark ; and 

 the other layers, as they become external and exposed to 

 the same sources of injury, experience, in due course of 

 time, the same fate ; whilst the layers of wood, being pro- 

 tected and sheltered by the bark, vegetate in security. 



Caroline. Yet it is not uncommon to see the trunks 

 of very old trees in a state of total decay, whilst the bark 

 remains uninjured. 



Mrs. B. That is the case when the wood is, by any 

 accidental circumstances, exposed to the inclemencies of 

 the weather, which it is not calculated to resist. This 

 happens, sometimes, by the lopping or breaking off of 

 large branches, considerable pieces of bark falling off, or 

 any circumstance by which the rain can gain admittance 

 to the wood. 



234. In what manner does bark grow 1 ? 235. What produces the 

 luggedness of bark on some trees'? 236. What takes place in others 

 that are smooth! 237. In what proportion do those trees whose 

 bark does not peal off become scarred and rugged'? 238. Why then 

 does the wood of a tree sometimes decay and the bark remain sound'! 



