MONOCOTYLEDONS. These species are of little or no technological interest; only the palms form a proper trunk and often reach considerable heights. They are slender plants with a cylindrical stem, branched or not. It is worth stressing that a palm is not a woody species but a giant herbaceous plant, so its body is necessarily unlike the trunk we are used to. Put simply, its structure is made of parenchyma tissue with vascular bundles (phloem and xylem) embedded in no particular order. Growth comes from a meristematic ability the parenchyma acquires, so that the tissue as a whole takes part in cell division.
The palms have very limited technological use; only a few species have a workable stem, and they are more often used in Latin America and subtropical Africa as material for rustic building or as poles for power lines.
GYMNOSPERMS (conifers) and DICOTYLEDONS (broadleaves).
The bodies of these species are alike. Look at a cross-section of a trunk and you will see the tissues differentiate; moving from the centre outward, you find:
- the PITH
- the HEART OR HEARTWOOD
- the SAPWOOD
- the CAMBIUM
- the PHLOEM
- the BARK
There are also MEDULLARY RAYS, arranged — as the name suggests — along the radii of the circle.
Bark. In a young, still-green plant, protection falls to a thin cuticle called the epidermis; when this tears and sheds as the trunk grows in diameter, a special meristem forms — the PHELLOGEN, or cork cambium — which produces a parenchyma (phelloderm) and, on the outside, cork (phellem) in greater quantity. Before long the phellogen dies under the compression caused by the growing diameter; a new one then forms further in, and so on, so the bark ends up layered, its innermost part made of newly formed, still-living cells. As the tree ages, the bark, lacking elasticity, either flakes away (birch and plane) or, where it does not, becomes fissured with cracks and grooves. Technologically the bark matters for several reasons:
- it shields the wood from impacts, damage, fungi and the effects of heat and moisture.
- it resists the penetration of tools.
- it yields useful products of its own (cork, extractives...).
Phloem. Made up of SIEVE tubes, it is the tissue that carries the processed sap downward — from the leaves to the plant's living, non-green parts, above all the roots. The phloem also contains sclerenchyma and parenchyma, and sometimes secretory channels.
Cambium. Purely meristematic tissue; its job is to divide its cells, which then differentiate into the definitive adult tissues.
Sapwood. Young wood not yet fully formed. It consists of living and dead cells, chiefly parenchyma and the vessels that carry sap upward. Because of the cell juices and sap it holds, it is easily attacked by fungi and parasites.
Heart or Heartwood. The most interesting part technologically. Here even the parenchyma cells are dead, and the reserves they held have been removed or turned into heartwood substances (tannins). In many woody species the heart stands out clearly from the sapwood because it is darker, but a lack of colour difference does not mean heartwood formation has failed to occur — in other words, the parenchyma cells can die and their contents be transformed or removed with no change of colour at all. Where the heartwood is not set off from the sapwood, there is in practice no difference in working; where it is differentiated, some differences do appear:
a) a deeper, darker colour to the heartwood; sometimes the colour changes completely (ebony, with yellow or pinkish sapwood), with obvious difficulties in use. But do not be fooled by the oxidation of the juices seeping from the cut of a freshly felled tree; cleaning the section restores the normal differentiation.
b) an increase in the heartwood's specific weight as the heartwood substances are laid down.
c) greater durability of the heartwood than the sapwood against fungi and insects, since the sapwood's juices carry sugars while the heartwood's tannins act as a strong antiseptic.
d) lower permeability of the wood.
e) lower moisture content of the wood.
Between heartwood and sapwood there is often an intermediate zone of a different colour.
One peculiarity of conifer wood is the possible presence of a "core": a fast-growth zone at the centre of the trunk, concentric with the pith, of coarse texture. Its technological properties are poor, so it counts as defective wood.
Pith - Rays - Ray fleck. Running along the axis of the trunk is a cylinder of parenchyma tissue called the PITH: a loose zone, brownish-pink, circular or polygonal in shape. Technologically it brings more drawbacks than benefits, though its position can reveal how regularly the trunk grew, since an off-centre pith points to growth anomalies. Once sawn, the boards that contain it — especially those where it shows on the surface — are of little value. Its size varies by species, and it can occasionally be striking by its total absence, leaving a hollow pith cylinder.
More important is the horizontal parenchyma tissue that forms ribbon-like clusters running radially, hence called RAYS. They grow from cells called ray initials, and it is wrong to call them medullary, since they are not made of cells like the pith's, nor do they always reach it. Because the rays fan out as the tree needs them and cannot sit too far apart, the cambium has to start new ones every year; as a result the rays are not all the same length, even though they all end at the cambium on the outside.
In cross-section they are usually easy to make out, at least in woods with differentiated heartwood; but they show up even more in radial sections, with their typical ribbon-like, shiny appearance, often against a different background colour — which is why they are called "ray fleck".
Often, thanks to the wood's irregular texture, the rays in radial section do not run straight but very tortuously — so much so that, because of their sheen, they look like the silvery trail left by a snail; for this reason they go by the untechnical but very apt name of "snail trails".