Structure of Bark. 



73 



260. In the bark itself, an annular layer of growth is also formed 

 next to the wood, and in some cases we may count the distinct lay- 

 ers of growth of recent years ; but in other cases they become a 

 homogeneous mass, as seen by the naked eye, and the outer layers 

 are gradually cast off. In some trees, like the birch and the cherry, 

 the outer layers consist of strong fibers, running horizontally around 

 the trunk and branches. These may exfoliate in loose shreds. . This 

 tendency to exfoliation is shown in the plane tree, where the outer 

 bark falls off in hard masses, leaving a fresh surface, at first white, 

 but becoming greenish, and in the Scotch pine {Pinus sylvestris) , in 

 which the outer bark peels off in thin flakes from the upper part of 



61. Longitudinal Section of the Bark 62. Transverse Section of the Bark of the 

 of the Linden: showingtheminute Linden: showing the outer bark, A; 



interlocking fibers of the bast-bun- the green cellular layer, g; the bast- 



dlcs. crossing the longitudinal layer, h; and the edse of contact with 



fiters.Cl) the wood.srr; with the medullary rays 



of the bark, m, m, m, m, m, extend- 

 ing into the green cellular layer. (1) 



the trunk, and from the branches, leaving a smooth and fresh green 

 surface. 



261 . When the bark and wood of a tree are cut or wounded by 

 accident, as by the marking-hammer of the forester, or the ax of a 



1 From Eossmassler. 



