Forensic botany

DEFINITION: Specialized field of study concerned with the examination of plant material in relation to crimes.

SIGNIFICANCE: The ubiquity of plants and plant-derived products and the potential for identifying the sources of plant material from small fragments make plants and plant parts very useful tools for tracing the movements of people and objects. Forensic botany can also determine the identities and sources of vegetable matter material to crimes, including drugs, rare plants, and stolen timber.

Because the use of botanical evidence to link a suspect to a crime scene depends on comparison of fragments, investigators need to collect reference samples at the scene, dry them to prevent deterioration, and store them in sealed containers. Bits of vegetable matter found adhering to suspects’ clothing or in suspects’ vehicles or dwellings should be similarly preserved. On-site identification of species likely to have left evidence, such as burr-producing herbs along an escape route, is helpful.

Most botanical analyses require microscopic examination. By examining the size, arrangement, and ornamentation of cell walls in a small wood fragment, an expert can often tell the genus of tree from which the fragment came. Hairs, venation, and epidermal cell arrangement are clues to the identity of leaf fragments. Pollen is extremely diagnostic. Mosses, identifiable to species based on cellular patterns, are potentially powerful diagnostic tools because many species are quite habitat-specific. Fungal spores recovered from stomach contents during autopsy can determine whether a suspicious death is attributable to mushroom poisoning.

A field survey of vegetation can pinpoint the timing of an event. Knowing the usual plant succession following disturbance in an area, a botanist can help locate a buried body or estimate the amount of time elapsed since burial. If vegetation is damaged during commission of a crime, the developmental stages of uprooted plants and broken limbs will indicate the season when the damage occurred. Annual growth rings in roots help date long-buried skeletal remains.

Dendrochronology is a useful forensic tool. The sequence of annual rings in a large piece of wood can indicate the year of harvest, geographic source, and sometimes the individual identity of the parent tree.

DNA (deoxyribonucleic acid) comparison can detect not only species but also populations and individuals, establishing, for example, that a murder suspect’s truck collided with a particular tree. In one California case, comparison of the DNA of live oak leaves from a burial site with DNA of leaves on a suspect’s truck showed the leaves originated elsewhere. As DNA analysis becomes more routine, more widely available, and less costly, its use to identify plant material in forensic cases is likely to increase.

Diatoms, a type of microscopic aquatic alga producing an ornamented silica shell, provide valuable clues in crimes involving bodies of water. The presence or absence of certain species of diatoms in the lungs of a body recovered from water will indicate whether the person drowned there or the body was dumped after death occurred elsewhere. A forensic botanist can determine how long a submerged body has been in the water by examining the colonization of the body by algae and aquatic fungi.

Types of Investigations

The uses of forensic botany in murder investigations include demonstrating that a suspect was present at the crime scene through identification of plant fragments on clothing or in vehicles, locating the crime scene if the body is dumped at a remote location, tracing the victim’s movements prior to the murder through plant material on clothing or in stomach contents, tracing wooden items used in perpetrating the crime, locating buried bodies, and reconstructing the timing and sequence of events through vegetational disturbance at the crime scene. In cases of rapes, robberies, and assaults, plant evidence can help demolish alibis by demonstrating high probability of a suspect’s having been at a particular place at a particular time. When a perpetrator flees a crime scene through a weedy field, burrs and pollen from the species growing in the field will adhere to that person’s clothing. A man who committed a in a patch of poison ivy was convicted based on the evidence presented in court concerning a rash he had on body parts not normally exposed to nature.

Drug-producing plants are usually identified through chemical testing for the active principle, but identification of the plant itself is sometimes important. Under laws that prohibited the growing and selling of Cannabis sativa (marijuana) but not other species of the Cannabis genus, a few suspects successfully defended themselves by presenting expert testimony to the effect that they had been in possession of a different species. Most states now ban the entire genus. When the chemical test is nonspecific or will detect levels of the drug found in legal materials, botanical examination of the seized material’s morphology will confirm or refute its identity as contraband—for example, by determining that a supposed shipment of marijuana has been so cut with oregano that there is insufficient drug present to support prosecution.

Wildlife biologists sometimes examine animal stomach contents for forensic purposes, such as to determine whether an animal was killed on private land or within a nature reserve, or whether an animal has been feeding on toxic plants. Cases of deliberate human poisoning by toxic plants, including mushrooms, are extremely rare; consequently, the determination that a suspicious death can be attributed to a vegetable toxin creates a strong presumption of accidental death.

Botanical evidence is important in cases of fraud and theft involving plants and plant parts. Determining the species and ages of particular pieces of wood through anatomy and dendrochronology can help to expose art forgery and sale of reproductions as antiques or indigenous folk art. In cases involving large-scale theft of agricultural crops, determining the variety or seed source may be important. DNA analysis is used to prosecute cases of patent infringement involving deliberate propagation of genetically engineered crops and to trace the inadvertent, sometimes deleterious, migration of introduced genes into weedy species.

Forensic botany can reconstruct the timing and progression of an illegal logging event for years after it occurred and can identify logs or even sawn timber by matching tree rings to those on cut stumps. By determining the identity of individual plants or restricted populations through DNA analysis, experts can distinguish between an illegal wild source and a legitimate cultivated source for rare and endangered plants such as orchids and cycads.

Many forensic applications of botany depend on the availability of investigators trained in plant morphology and taxonomy. While these disciplines had once been neglected in American universities, numerous US universities offered forensic botany classes as part of their forensic biology programs in the 2020s.

Bibliography

Bock, Jane H., and David O. Norris. Forensic Plant Science. Academic Press, 2016. eBook Collection (EBSCOhost), search.ebscohost.com/login.aspx?direct=true&db=nlebk&AN=1098998&site=ehost-live. Accessed 3 Jan. 2017.

Coyle, Heather Miller, ed. Forensic Botany: Principles and Applications to Criminal Casework. Boca Raton, Fla.: CRC Press, 2005.

Kasprzyk, Idalia. “Forensic Botany: Who?, How?, Where?, When?” Science & Justice, vol. 63, no. 2, 18 Jan. 2023, 258–275, doi: 10.1016/j.scijus.2023.01.002. Accessed 14 Aug. 2024.

Lane, Meredith A., et al. “Forensic Botany: Plants, Perpetrators, Pests, Poisons, and Pot.” Bioscience 40, no. 1 (1990): 34–39.

Nijhuis, Michelle. “Profile: Of Murder and Microscopes—How Botanist Jane Bock Became a Crime Fighter.” Sierra Magazine, May/June, 2007.

Yoon, C. K. “Botanical Witness for the Prosecution.” Science 260 (1993): 894–95.