TIMBER AND TIMBER TREES. [CHAP. 



the alburnum, while a portion of the sap being absorbed 

 by the bark increases its elasticity and assists the de- 

 velopment by the addition of a new layer on the inside. 



Thus the bark of trees is expansive in character, 

 and this accounts for the differences observable on the 

 surface, which varies from great smoothness, as in the 

 Beech, to extreme ruggedness in the Chestnut, and to 

 strips and flakes in some other kinds. 



It is upon the pith and its sheath that the first year's 

 growth of lignine, or woody matter, is formed, and the 

 whole structure of the stem raised. The several con- 

 centric rings or layers which surround, and are, as it 

 were, moulded upon it to form the cone, are generally 

 well formed and uniform in thickness, seldom varying 

 except when the pith is excentrically placed, or deviates 

 somewhat from the centre. Whenever this is the case, 

 the thinner layers will be found upon the side having the 

 smallest semi-diameter ; while on the reverse side, owing 

 to the annual supply of ligneous matter having been 

 drawn in that direction by atmospheric influences, they 

 are found to be compensatingly thicker, but are a little 

 less dense in texture. 



The yearly growth or increase is thus defined by 

 concentric circles outside the medullary sheath (c, Fig. 3). 

 These are generally clear to common observation in a 

 transverse section of a stem, the outer portion of each 

 being of a firm and dense texture, while the inner part 

 is perceptibly vascular and more or less porous ; the 

 quality of the wood, and its fitness for architectural or 

 engineering purposes, depending, to a great extent, upon 

 the degree of firmness and solidity of the annual layers. 

 These are all very plainly marked in the Oak and Fir, 

 and in most woods ; but in the Maple and Lime, and in 

 some others, as also in many trees of tropical growth, 



