8 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 (¢, Fig. 3). 
These are generally clear to common observation ina 
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, 
