Wood under the microscope
To examine wood under a microscope, we need to prepare the wood first. First, very thin sections are made using a microtome. This device uses an extremely sharp blade to cut very fine slices from the wood (care should be taken during this step to protect one's fingers). The sections are then bleached to prepare them for staining. To make the wood cells visible, they are stained with safranin and astral blue. Safranin stains all lignin-containing cells, which are the ones that have become woody, red, while astral blue stains all non-woody cells blue. This allows for the observation of cell structures under the microscope and distinguishes living from dead, woody cells.
Wood appears different under the microscope depending on the direction it was cut. It can be cut transversely, radially, or tangentially, and different cell structures become visible depending on the cutting direction.
cross-section tangential radial
Furthermore, hardwood and softwood trees differ in their wood anatomy, and different tree species can even be distinguished based on their cell structure under the microscope. In softwood, there are two types of cells. First, tracheids, which serve as versatile cells responsible for water transport and structural support, giving the tree the necessary stability to grow tall. Second, parenchyma cells, which conduct nutrients and store fats and starch. Hardwood trees, on the other hand, are evolutionarily younger, and their cells have specialized for specific functions. Fiber cells provide structural support, while vessel cells (tracheae) specialize in water transport. In oak trees, for instance, these vessel cells are particularly large, sometimes even visible to the naked eye. Hardwood trees can be further classified as ring-porous (like oak or ash) or diffuse-porous (like beech). As the name suggests, in these species, the water-conducting vessels are arranged in rings (ring-porous) or scattered (diffuse-porous) within the annual ring.
pine beech oak
Differences in cell size also reveal the distinction between earlywood and latewood. Cells formed in the spring are larger to transport more water, as the tree experiences its most significant growth in the spring. Cells formed in late summer, on the other hand, have thicker cell walls for increased stability. These size differences contribute to the recognition of annual rings.
In wood from conifers, resin canals can also be observed under the microscope, which traverse the wood. When the tree is injured or infested by pests, resin flows through these canals to seal the wound.
Former injuries that have healed can also be detected under the microscope, as in the case of this hornbeam. The cells around the wound overgrow the dead cells, closing the wound. Even though the injury is no longer visible from the outside, it can still be seen in the wood, much to the chagrin of the wood processing industry.
Finally, not a wood image but a cross-section of a spruce needle. The green of the chloroplasts in the cells, which are needed for photosynthesis, can be clearly seen
pictures taken during the workshops: 32nd Dendroecological Fieldweek in Heiligenstadt, Germany
Gärtner, Holger, and Fritz Hans Schweingruber. "Microscopic preparation techniques for plant stem analysis." (2013).