SECT. V, THE LIGNEOUS LAYERS. 339 



only different in their external structure, as has been 

 already seen, but also in their degree of tenacity 

 even in the same plant. This is obvious from the 

 well known fact of the splitting of the wood which 

 you can always easily effect in the direction of the 

 longitudinal fibres, but never in the direction of the 

 transverse fibres, owing evidently to the strong and 

 tough fabric of the former ; and to the slender and 

 delicate contexture of the latter. 



Hence also the firmness or hardness of any par- 

 ticular species of wood depends upon the number 

 and compactness of its longitudinal fibres. The 

 longitudinal fibres of Guaiacum and Mahogany are 

 numerous and closely united, and the wood very 

 hard ; while the longitudinal fibres of the Fir-tree 

 and Poplar are few and scattered, and the wood 

 very soft ; the hardest wood being always of slowest 

 growth, as in the case of the Oak and Box ; 

 and the softest wood of quickest growth, as in the 

 case of the Horse-chesnut and Ash. 



If the fibres of the bark or wood are examined And ap- 

 individually with a good glass, they will not be composed 

 found to present, as might perhaps have been ex- cession of 

 pected, a smooth and uniform surface ; but rather rin 3 * 

 an appearance as if, composed of a succession of 

 rings, giving them a sort of slight resemblance to 

 the awn of Barley.* In the liber and alburnum 

 they are soft and gelatinous ; but in the interior 

 layers of the wood, and exterior layers of the bark, 

 * Seneb. Phys. Veg. vol. i. p. 59. 

 Z 2 



