the alburnum, while a portion of the sap being absorbed by the bark increases its elasticity and assists the development 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 concentric 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,
