10.2.1 Micrographs of wood structure

This section shows a collection of scanning electron microscope images relating to wood and woodworking. All the microscopy was carried out by Claire Barlow.

First, a few pictures to show a bit more detail of the cell structure of Norway spruce, following on from the brief description in section 10.2. Figure 1 shows a cut in the LR plane in the top half and one in the RT plane at the bottom, separated by a corner. The landscape is dominated by tracheids, and there is an annual ring boundary in the centre of the image.

Figure 1. Specimen of Norway spruce, cut to reveal the LR plane (top half) and the RT plane (bottom half).

Looking carefully at the top half of this image, we can see some more details. Dotted around, for example towards the top right-hand corner, you may be able to make out a lot of small doughnut shapes. These are called bordered pits. At the centre of each doughnut there is a hole through the cell wall, connecting one tracheid with the one next door. These allow fluids to travel through the wood, essential in the growing tree. But they are quite complicated: each bordered pit has a kind of flap valve at its centre, allowing the tree to close the holes. When the tree is cut down and the wood dries out, these flap valves usually stick themselves permanently to one face or the other, effectively sealing the hole. That is why dry wood cannot easily be soaked with water. Water can only get into tracheids that have been exposed on the surface, but it can’t get into the others because all the bordered pits are closed.

Figure 2 shows a close-up view of a group of bordered pits. They are all sealed shut, but two of them are sealed against the upper surface while two are sealed against the lower surface, so they look further away. Figure 3 shows an example of a bordered pit which has been split in half by the process of preparing the specimen. The valve flap, with its “spider’s web” support network, is sealed against the lower surface. Figure 4 shows a similar example, except that this is a different species of softwood and in this case the valve has not sealed shut but remained open in the dried wood.

Figure 2. A group of bordered pits in Norway spruce
Figure 3. A bordered pit that has lost its top half, so that the membrane forming a flap valve is visible
Figure 4. A bordered pit in a different species of softwood, in which the valve has remained open rather than sealing shut

Figure 5 shows a closer image of an “end grain” surface cut in the RT plane. You are looking into the “tunnels” of a lot of tracheids. The two horizontal lines running across the image are rays. If you look careful at the lower one of these, then look into the tracheids just above it, you can make out ridges running across the floor of those tracheids, where the other ray cells of this stack are passing underneath. You may also be able to see some small holes: these are piceoid pits, which connect tracheids to rays and allow fluids to travel radially in the growing tree.

Figure 5. Typical view of a cut in the RT plane in Norway spruce.

Figure 6 shows an even closer view of a similar RT cut. This time you can see the joins between the tracheids. Each tracheid has a wall made up of helical windings of cellulose (and other stuff). Adjacent tracheids are glued together by a thin layer called the middle lamella. When wood is pulped to make paper, the intention of the process is to keep individual tracheids (“fibres”) intact, while ungluing them from their neighbours. The fibres can then be persuaded to glue themselves together in a kind of mat: that is your sheet of paper. In Fig. 6 you can also see a couple of triangular holes, just above and below the ray running across the lower part of the picture. These are probably the very last bits of the pointed ends of tracheids that have been cut off during the preparation of the specimen.

Figure 6. Close-up of a similar cut to Fig. 5.

Figure 7 shows a different view, cut approximately in the LR plane. Figure 8 shows a corresponding view of a cut in the LT plane. In combination, these two pictures give a good idea of the distribution of rays among the tracheids. In Fig. 7 you can see parts of several rays, exposed on the surface and then diving down under the next layer of tracheids. Figure 8 shows the columns of ray cells in end view, as lines of small holes among the tracheid tubes.

Figure 7. Norway spruce, cut approximately in the LR plane.
Figure 8. Norway spruce, cut in the LT plane

Figures 9 and 10 give a glimpse of the denser and more complicated cellular structure of a hardwood. These are two views of the kind of maple (Acer platanoides) normally used for the backs and sides of violins and their relatives. A conspicuous feature in Fig. 9 is the set of large pores, distributed rather uniformly throughout the volume for this timber. Some other hardwood species, such as oak, are called “ring porous” because they have pores concentrated in bands parallel to the annual growth rings.

Figure 9. Typical structure of violin-quality maple Acer platanoides.
Figure 10. Closer view of maple, as in Fig. 6.

The remaining pictures are intended to give an impression of what happens to the cellular structure as a result of common woodworking operations. Figure 11 shows something that can go wrong. A piece of Norway spruce has been clamped in a vice, too tightly for the structure to withstand. A band of collapsed cells runs across the picture. The intact tracheids in this image also show a good selection of bordered pits and piceoid pits.

Figure 11. A sample of Norway spruce that has been clamped too hard in a vice, causing a line of cells to collapse.

Figure 12 shows a related effect, brought on by a different operation. The LR surface of the wood, in the top half of the picture, has been smoothed with a heavy cabinet scraper. The end-grain part of the picture, in the lower half, shows that the pressure of the scraper has collapsed the tracheids with thin walls, in the first few layers below the surface. However, the denser cells near the annual ring boundary have been cut rather than collapsing.

Figure 13 shows what happens when wood like this is moistened with water. This is a process described by woodworkers as “raising the grain”, and the picture shows you why. The water has made the collapsed cells pop back more or less into their original shape. The result is a step at the annual ring boundary: the cells that had been squashed now stand proud of the surface. This would be repeated at every annual ring, giving the wood surface a ridged texture.

Figure 12. A sample of Norway spruce, showing LR and RT planes as in Fig. 1. The top surface was prepared using a heavy cabinet scraper, and this has collapsed the thin-walled tracheids in the top two layers or so.
Figure 13. A similar Norway spruce sample to the one in Fig. 12, but this has been moistened to “raise the grain”. The collapsed cells have recovered their shape, making a step on the top surface at the boundary of the annual ring.

Figure 14 shows the different kind of surface texture which is generated by smoothing the wood with abrasive paper with a very fine grit. There are some fuzzy strands where the cell walls have been shredded by the abrasive particles, but mostly the surface is rather smooth. Figure 15 shows the result of using another kind of “abrasive”. Traditionally-minded violin makers sometimes use the dried skin of a dogfish, a kind of small shark. Dogfish skin indeed feels very abrasive to the touch. But the picture reveals that the effect is rather different from the abrasive paper. The cell-wall material is soft enough that it has been “smeared” by the action of polishing with the dogfish skin: the action is more like burnishing that abrasion: somewhat similar to the effect of the heavy scraper in Fig. 12.

Figure 14. The LR surface of a Norway spruce sample which has been prepared with fine-grit abrasive paper.
Figure 15. The LR surface of a Norway spruce sample which has been prepared by rubbing with dogfish skin, a natural “abrasive”.

We can see what is going on in Figs. 16 and 17. Figure 16 shows the surface of pristine dogfish skin. It is covered with structures called denticles: they are thought to give the living shark lower drag as it swims through the water. Figure 17 shows what has happened after the dogfish skin has been used extensively as an “abrasive”. The denticles have worn down to stubs. The material of the denticles, unlike the grit in abrasive paper or the steel of a cabinet scraper, is no harder than the cell-wall material of wood. So when dogfish skin is used for smoothing a surface, both the wood and the dogfish skin are worn away.

Figure 16. The surface of a new sample of dogfish skin, showing the “denticles”.
Figure 17. The surface of dogfish skin which has been used to smooth wood, wearing the denticles down.