Xylotomy is the cutting of thin sections of wood specimens for microscopic examination. Wood splinters, chips, or fragments can be important items of trace evidence in some crimes. Typically, the forensic botanist will cut the sections from the wood sample with a very sharp knife mounted on a jig called a microtome. The sections are then stained so that features such as cells and grain direction can be seen under the high-powered microscope.
It is not always possible to identify a tree species from this kind of fragment examination. However, comparison with known samples will give an idea of the type of timber involved. If a suspect has used a piece of wood to assault someone, the suspect’s clothing may carry splinters. The forensic investigator can compare these splinters with the weapon to try to make an association. Similarly, doors and windows may be damaged on entering or leaving a crime scene. Splinters found on the suspect’s clothes can be compared with samples from the entry and exit sites of the scene.
Wood analysis played a crucial part in one famous case, the kidnapping and murder of the infant son of aviator Charles Lindbergh from his New Jersey home in 1932. The ransom note left at the scene suggested the kidnapper was poorly educated and of German descent. This was not much to go on, given there were no fingerprints on the note. However, a homemade wooden ladder was left at the scene and had been used to gain access to the child’s nursery. Arthur Koehler, an expert in wood and wood products, examined the ladder and determined it was made of Ponderosa Pine, North Carolina Pine, birch, and fir. He suggested that the fir section was actually a piece of flooring. Microscopic examination revealed marks made by a planing machine. Planed wood samples from mills around the country were compared to the ladder samples. The timber was tracked down to a company in the Bronx. Lindbergh had paid out a ransom and bills with the corresponding serial numbers also turned up in this location, narrowing down the search for the kidnapper.
Bruno Richard Hauptmann, a carpenter of German descent, was later arrested in connection with the crime. Examination of his home revealed a missing floorboard and nail holes corresponding to those in the piece of fir used in the ladder. The final piece of incriminating evidence was the presence of a wood plane which made smoothing marks matching those found on the ladder.