Botanical Evidence: An Overview

Pollen grains and spores are produced in enormous quantities by flowering plants, conifers, mosses, and ferns. They are microscopic (typically 10 to 100 micrometres in diameter), structurally durable, and equipped with adhesive surfaces or sculptured walls that promote attachment to fabric, hair, and skin. A person walking through a habitat accumulates pollen from the plants they contact directly and from airborne pollen that settles on them. The resulting assemblage on their clothing reflects the species composition of the location, the season, and the micro-habitats traversed.

The analytical procedure begins with solvent washes or tape lifts of the garment surface, followed by acetolysis (acid treatment that removes organic debris while preserving the sporopollenin pollen wall) and mounting on microscope slides. The palynologist identifies each pollen type under a light or scanning electron microscope and records the relative abundance of each species. The resulting profile is compared to reference profiles from the scene and from control locations that the suspect claims to have visited.

Persistence varies by fabric type and environmental conditions. Pollen embedded in woven fibres can survive months or years if the garment is stored undisturbed. Airborne pollen in transit falls off quickly, so assemblages from clothing worn at a scene tend to be richer in entomophilous (insect-pollinated) species, which are heavy and sticky, than in anemophilous (wind-pollinated) species, which are abundant in the air but weakly adherent. This asymmetry is factored into interpretation.

Seeds and fruits transfer to clothing and vehicles through several mechanisms: hooks and barbs (burs), adhesive surfaces (mucilage), and simple gravitational contact with seed heads. Unlike pollen, seeds are large enough to see and recover by direct search of clothing and vehicle interiors. They are identified by gross morphology and, where necessary, scanning electron microscopy of surface microstructure. Many seeds are species-specific to particular plant communities, giving them locality value comparable to pollen assemblages.

Plant macrofossils recovered from clandestine graves provide evidence of burial history. As a buried body decomposes, the local plant community changes: pioneer species colonise disturbed soil, roots penetrate the burial, and leaf litter accumulates at the surface. A forensic botanist can estimate the minimum time since burial from the degree of root penetration, the successional stage of vegetation over the grave, and the seasonal growth layers visible in root cross-sections. This is distinct from entomological post-mortem interval estimation but often used in conjunction with it.

Diatoms are single-celled algae enclosed in a silica frustule. More than 100,000 species have been described, each with a distinctive frustule shape and surface ornamentation that allows species-level identification under light or electron microscopy. Diatom communities differ between water bodies based on pH, conductivity, nutrient load, salinity, and temperature. A river in one catchment has a characteristic diatom assemblage that differs from a reservoir, a coastal inlet, or a paddy field a few kilometres away.

In a vital drowning, water containing diatoms is inhaled under pressure. Diatoms pass through the alveolar membrane into the pulmonary circulation and are distributed to the heart, liver, kidneys, and, critically, bone marrow. The silica frustule is not degraded by tissues, and diatoms persist in bone marrow indefinitely. The standard analytical procedure digests femoral bone marrow with concentrated nitric acid, centrifuges the residue, and mounts it on a slide for species identification. The assemblage from the marrow is compared to assemblages collected from candidate water bodies.

Diatom evidence has been accepted in courts in Japan, the United Kingdom, Germany, the Netherlands, and India. The Bombay High Court has considered diatom evidence in drowning cases where the body was recovered from a location other than the water body proposed by witnesses. In the United States, admissibility has been addressed in individual state courts rather than through a single federal standard, and expert witnesses are required to demonstrate the scientific basis of their methodology under Daubert or state equivalents.

Wood is used in crime scene analysis in three contexts. First, wood fragments or splinters recovered from a wound, a vehicle, or a weapon can be identified to species and compared to a suspected source object. Second, timber in illegal logging and CITES (Convention on International Trade in Endangered Species) enforcement cases must be identified to species and sometimes to geographic origin to establish whether trade restrictions were violated. Third, wooden tool marks or tool handles can be matched to a specific piece of wood by growth ring patterns, knot positions, or wood anatomy.

Species identification rests on wood anatomy. A thin cross-section cut from the fragment is examined under a light microscope. The key features are vessel arrangement (ring-porous or diffuse-porous), vessel grouping, ray width and height, the presence or absence of resin canals, and fibre wall thickness. These features together identify the genus in most cases and the species in many. Reference collections of known-species wood sections are maintained by institutions including the Royal Botanic Gardens Kew (UK), the Forest Products Laboratory (US), and equivalent bodies in Germany, Japan, and Brazil.

Geographic origin can be inferred from stable isotope ratios (carbon-13, oxygen-18, strontium) and from DNA extracted from wood cells, though both methods require well-developed reference databases. The International Association of Wood Anatomists maintains a global standard for wood identification terminology. In illegal timber cases, prosecutors in the EU have relied on wood anatomy evidence under the EU Timber Regulation (Regulation 995/2010), and similar frameworks operate under the US Lacey Act and analogous provisions in India under the Forest Rights Act and its enforcement regulations.

Botanical evidence is fragile at the scene but durable in the laboratory once properly preserved. The principal risks are contamination from the investigator (each officer carries pollen from their own environment), loss of surface material during transport, and degradation of DNA in fresh plant material. Clothing should be packaged in clean paper bags rather than plastic, which promotes condensation and mould. Each garment is bagged separately to prevent cross-transfer between items.

Scene samples for pollen comparison must be collected from the specific micro-habitats where contact occurred, not from the general area. A palynologist attending the scene will typically collect control samples from each distinct vegetation zone, label them with GPS coordinates, and photograph the collection points. Control samples collected too broadly produce reference assemblages that are too variable to support a meaningful comparison.

Pollen sporopollenin is chemically resistant to most acids and oxidants, so pollen persists for tens of thousands of years in peat, sediment, and dry conditions. In contrast, plant DNA degrades within weeks in warm, moist environments. The choice of analytical method therefore depends on the evidence type and storage conditions. Older exhibits may yield pollen data but no DNA; fresh material may yield both. Diatom frustules are resistant to most chemical treatments and persist indefinitely in bone.

Botanical evidence is circumstantial evidence. A matching pollen assemblage demonstrates that contact between the suspect and the scene location is consistent with the evidence, not that such contact occurred at the time of the offence. Two alternative explanations must be addressed in any case. First, secondary transfer: could the botanical material have reached the suspect via a third person or object that was at the scene? Second, background match: is the pollen assemblage common enough that it could have been acquired at many locations other than the scene?

Quantifying the probability of a background match requires systematic data on pollen assemblage variability across the study region. Such databases are well-developed in New Zealand, the United Kingdom, and parts of Europe, but remain limited for large parts of South Asia, Africa, and Southeast Asia. Where reference data are sparse, the forensic palynologist must qualify the significance of a match and present confidence intervals on any frequency estimates.

The interpretation of botanical evidence should follow a likelihood ratio framework where possible: comparing the probability of the evidence if the prosecution hypothesis is correct against the probability if the defence hypothesis is correct. This is consistent with the approach recommended by the European Network of Forensic Science Institutes (ENFSI) and by the Forensic Science Regulator in England and Wales. Expert witnesses who present botanical evidence without a structured probabilistic framework risk overstating its significance, which courts in the UK, Australia, and Canada have identified as a ground for excluding or reducing the weight of expert testimony.

For detailed methodology, casework examples, and expert witness standards in botanical evidence, see the forensic botany subject. For the role of botanical material in wildlife crime, see wildlife forensics.