Daily American organ. [volume] (Washington, D.C.) 1854-1856, 1855-01-18
Lectures at the Smithsonian Instituttou.
REpoRrED ron rHE AAxRIcAs oReAs.
WOOD.


BY Ploy. AsA GaAr, or Harvard UsrvxxsIrY
Wood consists Of hrmsr and tougher material
than cellular tissue and is found lengthwise in Il
Is woven vertically into the tissue, so as 10 give
greater strength.
The elements which form wood are proper woody
tissue, and ducts; both notifications Of cells.
The woody tissue proper, consists of elongated
cells with very thick walls, but smaller in diameter
than ordinary cells The strength which this gives
TO the shoot is owing TO the thickness and density
Of the cell WALL and the way in which these cells
are put together--their pointed ends overlapping
each other. These are combined into threads OF
bundles which run lengthwise through the stem
sometimes separately imbedded in the softer cellu
lar portions, and at others, accumulated SO as tc
make compact layers, as in the trunk of a tree
These being hollow, convey sap with more rapidly
than the short cells.
The ducts are larger and longer cells, closed Al
their ends. When the stem Is young, they convey
sap; but, at other times contain air, and appear
designed for purpose.
The stems of all towering herbs are constructed
on two general plans--the exogenous, represented
by the oak and other trees of our climate, and the
indigenous, seen in the palm, &C., and here in
the palmetto.
The endogens are formed by mere bundles o.
woody tissue being interspersed among the old
and mostly inside, this process continuing year by
year thus the parts nearest the circumference are
soonest consolidated into woody mass. The re-
suit, as Is seen in the palmetto Is that the oldest
and hardest wood is near the circumference, while
the newer and softer is near the centre. Hence
they are called endogens, or inside growers.
The exogens are formed by adding new layer
Of wood outside the frst--and an additional one Is
added each year while the tree ives--so that the
structure of the wood is series of concentrc lay
ers, appearing in rings on the cross sections, in
number as many as the tree has numbered years,
with q pth in the centre, and bark on the outside,
the new layer of being annually added between the
former wood and the bark.
The use of the pith is that, being gorged with
sap, 1t may sustain the bud, and when that has ex-
banded the pith generally becomes dry.
What the wood Is made of is already seen. But
beside the woody tissues themselves which grow
vertically, there is part that grows horizontally
viz the silver grain or medelluiary rays. This is
the woof, while the wood is the warp; and the
wood might be said to be woven.
The bark at the beginning is cellular and green;
but soon three parts can be distinguished One
portion remains green, has the structure of the
green pulp Of leaves and long as it lasts performs
the same Office Of digestion. Soon an exterior por-
ton turns gray brown, and forms the corky lay
err seen in the birch and cork trees. But the inner
portion continues to grow, by addition to its inner
face yearly 80 long as the tree lives, in the lin
den, where the inner or fbrous bark is composed
of leaves Of annual growth this part has enough
woody tissue in 1t to render 1t tough and flexible
The annual layers are most distinct in trees of
the temperate zone, where there is a prolonged
period Of repose, from the winters cold followed
by vigorous growth in the spring.
The old wood is annually covered by layer of
new, and each layer remains safe within; the older
1t is the more deeply it is covered and protected
But the bark being renewed only from within,
the parts least capable Of resistance are exposed
to the action of the elements, and the increasing
circumference of the tree stretches, splits and
finally throws 1t of, leaving the young bark to I'M
dergo the same as time advances
The wood Of the trunk >undergoes certain
changes from age--what is known as sapwood be-
comes heartwood. The sap is conveyed through
the woody tissue, and as 1t ascends to the leaves,
encrusts the tubes through which II passes until
they are nearly obiterated as in oak or hickory.
Il is this gradual filling up that makes the change.
As a necessary consequence the heart wood
ceases TO convey sap, and bears no more part In
the vital operations of the tree than the outer bark
Of an old trunk which Is dead and decaying In
fact life has long left BOTH While the old bark is
dead, and being gradually thrown off the trunk,
the old wood is dead and buried in the center.
They are equally useless to the tree except me-
chemically. The heart wood can strengthen the
trunk and the thick bark protect the living tender
parts underneath. But that neither Is necessary
Is shown by the shedding of the bark, and the de-
cay of the heart, without injuring the vitality Of
the tree.