36 TIMBER AND TIMBER TREES. [chap. 



addition of a new layer on the inside of the cortex, 

 the cambium itself remaining between, and exerting 

 its activity at definite periods, usually in the summer in 

 our climate. Thus the bark and cortex of trees is 

 expansive in character, and it is owing to differences 

 connected with this that we find such differences ob- 

 servable on the surface, which varies from great smooth- 

 ness, as in the Beech, to extreme ruggedness in 

 the Chestnut, and to strips and flakes in some other 

 kinds.* 



It is around the pith that the first year's growth 

 of wood is formed, and upon this the whole structure 

 of the istem is, so to speak, 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 ; 

 when they do 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 various 

 influences, they are found to be thicker, but are often 

 less dense in texture. 



The yearly growth or increment is thus defined by 

 concentric circles outside the medullary sheathf [c, 

 Fig. 3). These are generally clear to common observa- 

 tion 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 



* For further information as regards the cambium, its structure and growth, 

 see " Timber and Some of its Diseases." 



t The ' ' medullary sheath " is the first formed cylindrical layer of wood. 



