Forensic Botany

There are at least two different ways plant materials can be useful as forensic evidence. One way—perhaps the most obvious—is as “trace” materials. Trace evidence can be many things, including hairs, fibers, glass, soil, and cosmetics, as well as plant materials that might consist of leaves, stems, flowers, seed pods, tree bark, or pollen. Many plants reproduce by producing pollen or spores, which are released to and carried by the wind. Seeds may also be windborne, and the wind can carry leaves, stems, or other plant structures as well. Plant materials often occur on or fall to the ground and can easily be transferred to a person’s shoes or clothing or to a vehicle.

Most plants have a defined geographical habitat, a limited range of territory in which they live. The presence of traces of a given plant on someone or something can help show that the person or object was within the plant’s habitat area. This type of information could provide valuable investigative leads and circumstantial evidence: For example, a suspect in a crime might be associated with a crime scene in this way; or a suspect’s vehicle could be associated with a crime scene; or a suspect could be associated with a crime victim by finding similar plant materials on both their clothing.

With DNA technology there may also be ways to take the presence of plant materials on a person or vehicle a step further in placing the person or vehicle at a location than is possible just using visual botanical comparisons. For instance, in a real-life case the presence of pods from a paloverde tree in the bed of a pickup circumstantially placed the vehicle at the scene of a homicide in Arizona in 1992 (see sidebar “The Denise Johnson Case”).

A second way plant materials can become forensic evidence is when they are ingested by people as food. Medical examiners can sometimes judge how long someone has been dead by the extent of food digestion in the stomach. Food digests in the stomach at a known rate, and forensic doctors know about this. Sometimes the food is recognizable and can be correlated with a witness statement about the decedent’s (the dead person’s) last meal, helping establish time of death. Medical examiners might be able to recognize various plant materials in stomach contents, like seeds, leafy structures, or the skin from a fruit. If someone was with a decedent a short time before death, and especially if they were eating together, the witness can help the medical examiner confirm the contents of a last meal. This information can help a medical examiner determine when the person died. Plant materials that pass through the digestive system but are not well digested, and thus not very much changed, can be recognized and identified in fecal material by forensic plant experts. Forensic botany experts can recognize a plant from the structures they can see when looking at it through a microscope. Although one does not hear about this very often, fecal matter can be evidence at scenes and on clothing, and once in a while a forensic botanical examination can become important in one of these cases.