332 COMPOSITE ORGANS. CHAP. II. 



interior layers, but acquiring strength and solidity 

 with age. It does not however acquire its utmost 

 degree of solidity till after a number of years, as 

 is plain from the regular gradation observable in 

 the solidity of the different layers. But if a tree 

 is barked a year before it is cut down, then the 

 Alburnum is converted into wood, in the course of 

 that year.* 



SUBSECTION II. 



Divergent Layers. The divergent layers (PI. 

 VIII. Fig. 13.), which intersect the concentric 

 layers in a transverse direction, constitute also a 

 considerable proportion of the wood, as may be 

 Discover- seen in a horizontal section of the Fir or Birch, 

 horizontal or f almost any woody plant, on the surface of 

 section. w hich they present an appearance like that of 

 the radii of a circle. But if the wood is split lon- 

 gitudinally, in a direction passing through the centre 

 of the stem, fragments of the divergent layers will 

 be seen adhering to the surface of the fracture, in 

 the form of large and smooth plates, which cross 

 the concentric layers and form a sort of binding 

 and cement to the whole, exhibiting a slight re- 

 semblance to a fine but irregular wicker-work. 

 This appearance is peculiarly conspicuous in the 

 trunk of the Elm-tree and Oak, if riven in the 

 above direction ; and in the latter even after having 

 been planed, the irregular fragments of the trans- 

 * Du Hamel, Mem. de 1'Acad. 1737- 



