Plant Systematics and Taxonomic Keys

A dichotomous key presents a series of numbered couplets. Each couplet offers two contrasting statements about a morphological character. The user examines the specimen and decides which statement applies, then follows the lead to either the next couplet or a named identification. Done properly, the process is reliable and reproducible; done carelessly, the same key from the same specimen can produce different results in different hands.

1. Start with a fresh, representative specimen
   A key designed for intact adult leaves will not work on a juvenile leaf, a damaged fragment, or a different organ. Confirm that the specimen type matches what the key was designed to key out.
2. Read both leads of each couplet before choosing
   The commonest error is selecting the first lead that seems to match without reading the alternative. Sometimes the second lead is a closer fit, or the contrast between the two clarifies a character that was ambiguous at first reading.
3. Record every step
   Document the couplet number chosen at each step. In a forensic context the identification must be auditable: a second expert should be able to follow the same path through the key and arrive at the same result, or identify where and why the paths diverged.
4. Verify the result against a full description
   Reaching the end of a key gives a name, not a confirmed identification. Always check the full species description in the flora or monograph and confirm that the specimen matches on all listed characters, not just the ones the key asked about.
5. Cross-reference against a herbarium specimen
   For any identification that matters to a case, compare the unknown against a named herbarium voucher, either physically or via a digitised image. This step is what separates a provisional identification from one that will hold up under cross-examination.

No single flora covers the world. A forensic botanist working on plant evidence from multiple geographic contexts needs to know which references cover which regions. The most important resources for routine forensic work include:

• Stace's New Flora of the British Isles: the standard reference for temperate British plant identification, used in the majority of UK forensic botanical cases. Current edition (4th, 2019) includes all established non-native species.
• Flora of North America (FNA): multi-volume comprehensive treatment, with many volumes freely available online. The reference for North American forensic casework.
• Plants of the World Online (POWO, Royal Botanic Gardens Kew): a curated online database of accepted names and synonyms for all known vascular plants. Essential for confirming current nomenclature when a key or older flora uses an outdated name.
• Flora of India (volumes by Botanical Survey of India): the standard reference for the Indian subcontinent, used for cases involving plant material from India or the wider South Asian region.
• Global Plants (JSTOR): digitised herbarium specimens from over 300 herbaria worldwide, searchable by taxon. Allows direct comparison of an unknown specimen against high-resolution images of named vouchers.

Forensic botany encounters certain families repeatedly because they are ecologically abundant, toxicologically important, legally regulated, or produce material that transfers and persists well. A working knowledge of these families shortens identification time and focuses analytical effort.

Herbaria are the ground truth of plant taxonomy. They hold pressed, dried, and mounted plant specimens collected over centuries, each identified, named, and associated with collection locality data. Type specimens, the actual specimens on which species were originally described, are held in herbaria and consulted when a disputed identification must be resolved.

For routine forensic work the analyst does not need to visit a herbarium in person. Digital access has transformed what is available remotely. JSTOR Global Plants holds images of nearly 3 million specimens from major herbaria. POWO at Kew provides authoritative nomenclature. iNaturalist and regional databases hold millions of georeferenced photographs that can help check whether a proposed identification is ecologically plausible in the location under investigation.

Physical herbarium visits remain important for critical identifications where high-resolution images are insufficient, or where access to the type specimen is needed. A disputed identification of a toxicologically critical species, for example hemlock versus a non-toxic Apiaceae, might require direct comparison with a named herbarium specimen to satisfy a court.

Morphological identification has clear limits in forensic work. Degraded, charred, or macerated material may retain too few characters for any key to resolve. Juvenile or out-of-season material may lack the reproductive structures that keys rely on. Some groups are taxonomically difficult, with species that look nearly identical yet are ecologically or toxicologically distinct.

• Scenario 1: Charred material. Fire destroys most diagnostic surface features. Anatomical identification from wood charcoal can reach genus level using cell-wall patterns, but species-level morphological ID is usually impossible. Molecular methods are also compromised by heat-damaged DNA.
• Scenario 2: Sterile vegetation only. Keys for flowering plants require flowers or fruit. Sterile vegetative fragments can often be taken to family by venation, trichomes, and anatomy, but genus or species level may require molecular work.
• Scenario 3: Taxonomically difficult groups. Brambles (Rubus), willowherbs (Epilobium), and many grass species hybridise extensively and are morphologically variable. A species-level morphological identification in these groups requires specialist expertise and may still need molecular confirmation.
• Scenario 4: Material in stomach contents or enteric material. Digestion removes most surface features. Cuticle analysis and starch grain morphology can take identification to family level; molecular methods (DNA barcoding) from the digested material may be needed for more precision.

The documentation of a taxonomic identification in a forensic report must be sufficient for another expert to assess and, if necessary, challenge it. This means recording the key or keys used (with edition and date), each couplet choice at each step, the characters that were assessed, and any ambiguities or conflicts encountered. It means naming the reference herbarium specimens or floras consulted, and stating clearly at which taxonomic level the identification was made and why it was not taken further.

A properly documented identification has the same structure as a forensic chemistry analysis: a chain of reasoning from the raw material to the named conclusion, with every step recorded and every method specified. An identification that says 'the plant was identified as Conium maculatum (hemlock)' without the supporting methodology behind it is inadequate as forensic evidence, however accurate it might be.