ascular
tissue--crossing each other. Now, after the portion is once formed,
which is woven every year by the wondrous machinery set to work for
this purpose, it receives no fresh texture, yet each fibre remains a
conducting tube to transmit the sap upwards, or, in the course of
time, becomes charged with various principles, prepared, as already
stated, by the leaves, and returned to the central part by that
apparatus or system of canals for their transit inwards, the medullary
rays, and at last are obstructed, so that no passage of fluid is
effected through the inner layers of wood. But for every layer that is
thus blocked up, a new one, which will continue pervious, is formed
exterior to those already existing, so that a constant provision is
made for carrying on the vital processes; to accomplish which, a free
channel from the points of the roots to the surface of the leaves is
absolutely necessary. The outer strata, produced by a tree of
considerable age, are observed to be thinner than those formed at an
earlier period, and become successively thinner and thinner, so that
ultimately, if accident should not have previously caused it, the
death of the tree is inevitable. The portions which are obstructed
constitute the _duramen_ or heartwood, the pervious portion the
_alburnum_ or sapwood. The original tissue is colourless; but
according to the nature of the secretions deposited in it, the
heartwood is either of a deeper colour, sometimes party-coloured, or
at least of a much greater specific gravity than the sapwood. The
removal of the juices by any solvent restores the wood to its
primitive hue, and renders it again light. The difference of weight of
a cubic foot of wood depends not merely on the different quantity of
vegetable tissue compressed into a given space, in the first
construction of the tree, but also on the quantity and quality of the
secretions ultimately lodged in it. The same species of tree will
present a difference in this respect, according to the country or
situation where it grew, and also according to the character of the
seasons during the time it flourished. According to the nature of the
tree, if placed in favourable circumstances in reference to soil and
weather, it invariably prepares and lodges in the stem those
principles which it was designed to elaborate--the oak preparing
tannin--the sugar-maple preparing its saccharine juice. That the
primary object of these was some advantage to the