Chinese solid wood furniture factories use a lot of solid wood. So what is the structure of solid wood?
The structure of wood includes macrostructure and microstructure.
The macrostructure of wood refers to the characteristics that can be seen with the naked eye or under a magnifying glass.
The microstructure of wood refers to the structure of wood as seen under a microscope.
To help us identify and use wood correctly, here we mainly introduce the macroscopic structure of wood.
1. Cut section of wood
Usually, we observe and study the structure of the wood on three cut sections: cross-section, radial section and tangential section.
(1) Cross section
Cross section refers to the section cut perpendicular to the central axis of the trunk or the direction of the wood grain.
In this section, we can see annual rings and wood rays concentrically distributed around the pith.
The cross-section of wood reflects the structure of timber relatively wholly. It is the most crucial section for identifying wood.
(2) Radial section
The radial section refers to the section cut through the pith along the central axis of the trunk.
On the radial section, the growth rings are parallel strips. They show the length direction of the longitudinal cells and the length and height directions of the transverse tissues.
(3) Tangential section
The tangential section refers to the section parallel to the central axis of the trunk but not passing through the pith.
On this cut section, the annual rings are V-shaped. They show the length direction of longitudinal and horizontal cells or tissues’ height and width directions.
2. Annual rings, earlywood and latewood
2.1. Annual rings(Growth rings)
On the cross-section, the concentric circles around the pith centre are growth rings.
Only one growth ring is formed a year during the growth period of trees in temperate and frigid zones. This growth ring is the annual ring.
In the tropics, the climate changes little during the year. And the growth of trees is affected by the rainy and dry seasons. The trees grow continuously in four seasons. So several growth rings can be formed in a year.
Two or more growth rings formed in a year are called double rings.
For tree species in temperate or frigid zones, usually in the early growing season, the cell division speed is fast, and the volume is also large. The cell wall is relatively thin, the texture is soft, and the colour is light. This is called earlywood.
In autumn, the flow of nutrients weakens, and cell division slows down. And cells with small cavities and thick walls are formed. This part of the tree, dark in colour and dense in tissue, is called latewood.
The annual growth of earlywood and latewood forms an annual ring. Due to the different structures of earlywood and latewood, their properties are also different.
3. Sapwood, heartwood and mature wood
In many species of wood, the part near the bark in a lighter colour and with more moisture is called sapwood.
The surrounding part of the pith core, in a darker colour and with less moisture, is called heartwood, as shown in Figure 1-21.
3.3. Mature wood
For some tree species, there is no difference in wood colour between the central and peripheral parts of the trunk. But the water content is different. The wood with less water in the central part is called mature wood.
Tree species with mature wood are called mature wood species or hidden heartwood species, such as fir, aspen, etc.
For some tree species, there is no colour difference between the trunk’s centre and periphery, and the water content is equal. They are called sapwood species, such as birch and pine.
The tree species with noticeable differences between heartwood and sapwood are called heartwood species.
|Tree Species Types
|Colour difference between the trunk’s centre and periphery
|Water content difference between the trunk’s centre and periphery
|Birch, pine, poplar, and podocarpus imbricates, etc
|Larch, red pine, Masson pine, cherry wood, ash and Chinese toon
Generally, the moisture content of the sapwood is greater than that of the heartwood in coniferous wood, while less in some broad-leaved wood.
The tree species with noticeable differences between heartwood and sapwood include larch, red pine, Masson pine, cherry wood, ash, Chinese toon, etc.
Tree species with no apparent difference between heartwood and sapwood include birch, poplar, and podocarpus imbricates.
Heartwood is transformed from the sapwood. The transformation process is a complex biochemical process.
Different tree species have different widths of sapwood.
Theoretically speaking, any tree species has heartwood or sapwood. Some tree species have noticeable differences between heartwood and sapwood, while others do not.
Figure 1-2 Heartwood and sapwood of wood
4. Top-wood, root-wood and middle-wood
In the actual use of wood, we will subdivide the longitudinal section of the wood.
Top wood: The part of the wood close to the treetop is called the top wood.
Root wood: The part of the wood close to the tree’s root is called the root wood.
Middle wood: The part of the wood between the top wood and the root wood is called the middle wood.
The middle wood on the vertical section is generally good material.
If the stumps or boards are sawn in sections, in the daily use of wood, these woods are often divided into two parts: root wood and top wood.
The middle wood on the vertical section tends to have small variability and good quality.
5. Conduits holes
Conduits are the canalizing structures of most broad-leaved trees. Conduits are unique to broad-leaved wood. Therefore, broad-leaved wood is also called porous wood.
We can see many pores of different sizes on the cross-section of broad-leaved trees. They are conduits holes, or pinholes.
Different tree species have different permutations of conduits holes.
According to the permutations of the conduits holes, the broad-leaved wood can be divided into ring-porous wood, diffuse-porous wood and semi-diffuse porous wood or semi-ring porous wood.
(1) Ring porous wood
Pores in earlywood are significantly larger than those in latewood. The pores of the early wood are arranged in a ring.
Therefore, there is a clear distinction between earlywood and latewood, such as oak, ash, elm, oak and so on.
(2) Diffuse porous wood
The size of conduit holes in the annual ring is almost identical. And the distribution is relatively uniform. And the difference between earlywood and latewood is not apparent, such as poplar, cottonwood, colour wood, Xi wood, etc.
(3) Semi-diffuse or semi-ring porous wood
In a growth ring, the permutation of conduits holes of semi-diffuse or semi-ring porous wood is between ring-porous wood and diffuse-porous wood. Earlywood conduits pores are larger and slightly distributed in a ring.
Pores from earlywood to latewood change gradually, and the boundary is not apparent, such as walnut catalpa, maple poplar, black wood and so on.
When identifying wood, the presence or absence of conduit holes is one of the main differences between coniferous timber (softwood) and broad-leaved wood(hardwood).
6. Pith Rays(wood rays)
On some species of wood cross-section, we can see many thin stripes in light colours radiating from the pith centre to the bark. They are called pith rays.
The pith rays in the xylem are called wood rays.
In the organization of wood, only wood rays are transverse.
Wood rays show different shapes on the different cut sections of wood. It is short-line or spindle-shaped on the tangential section and short-ribbon-shaped on the radial section.
The wood ray width of different tree species is quite different.
Some tree species, like Chinese oak, have thicker wood rays, which are very clear to the naked eye.
Some tree species have tiny wood rays, such as pine, which can be seen with the naked eye.
Some tree species have extremely tiny wood rays, such as poplar, which cannot be seen clearly by the naked eye.
7. Resin canal
The resin canal is a kind of tissue unique to particular coniferous wood. It has the function of secreting rosin resin and is distributed in the latewood part in light-coloured spots.
According to the cause of forming resin canals, there are Normal and injured resin canals.
According to the direction of the resin canal, there are radial and axial resin canals.
(1) Normal resin canal and injured resin canal
Normal resin canals are a normal physiological phenomenon in trees.
Tree species with normal resin canals include pine, spruce, larch, boxwood, silver fir, and oil fir.
Injured resin canals result from tree injuries. They may grow in tree species with resin or without resin canals, such as fir, hemlock, cedar, redwood, metasequoia, etc.
(2) Radial resin canals and axial resin canals
The radial resin canals parallel the trunk, mainly in the latewood part. They appear in the spindle-shaped wood rays and are very small. They appear as brown dots on the tangential surface of the wood.
The axial resin canals are scattered in the annual rings on the cross-section. We can see them mostly in latewood. They are small light-coloured dots, and the large ones look like pinholes.
The presence, size, and number of resin canals are essential for distinguishing coniferous wood from each other. For example, the resin canals of red pine are small and many, and the resin canals of larch are large and few.
8. Pith fleck
On the cross-section of some tree species, we can see 1-3mm long and brown semicircular or crescent-shaped spots.
Yet, they are dark brown stripes of indefinite length on radial and tangential sections. These spots are pith fleck.
Pith fleck is an abnormal tissue of wood. It is a healing tissue formed by damage to the xylem caused by insects and other attacks on trees.
The texture of the pith fleck is soft, and the distribution is irregular. It is common in some specific tree species, such as coconut wood, tip wood, fir, cypress, etc.
The excessive presence of pith will reduce the strength of the wood.
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