Planting Clues: How Plants Solve Crimes 
by David J. Gibson.
Oxford, 237 pp., £18.99, August 2022, 978 0 19 886860 6
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The old​ Palais de Justice in Lyon is one of the finest examples of neoclassical architecture in France. Its entrance hall is flanked with marble columns, and winged lions prowl the architraves.Between 1845 and 1995, it housed the major courts for the surrounding region. From 1912, access to the world’s first official police forensic laboratory was gained by entering through a back door and climbing three flights of creaking stairs to the attic. Edmond Locard, who founded the laboratory, is credited with the idea that ‘every contact leaves a trace’ (modern forensic scientists refer to this as Locard’s exchange principle). What Locard himself wrote was more careful. He observed that committing a violent crime requires acting with intensity: the desperate struggle, the fatal blow, the frantic exit. And the universe isn’t all made of gleaming marble. We are surrounded by a messy, sticky world of atomistic matter. A criminal can’t avoid leaving some trace of their presence – and taking something away with them.

In Pietr the Latvian (1931), Inspector Maigret’s first outing, Georges Simenon cited Locard and the ‘amazing tools’ that the new study of forensics had given the police, ‘the principle of the trace and so on’. As David Gibson recounts in Planting Clues, Locard was also a keen botanist. One of the scores of cases he included in his textbooks described a man who had been found murdered in the countryside outside Lyon. A group of suspects was rounded up. Inspecting one of their coats, Locard noticed a single wispy dandelion seed caught in the fabric. At first he thought it was from the common dandelion, Taraxacum officinale. At that point, a lesser botanist might have rested. But Locard knew that the Taraxacum genus contains many different plants. In Britain alone there are more than 239 extant ‘microspecies’. On closer inspection, Locard realised the seed was from a rare species, the same sort that he had seen – aha! – growing in a clump next to the corpse. This ‘seemingly minute detail’, he declared with a flourish, had solved the case. The bloodstain on the suspect’s coat was also something of a clue, but Locard chose to elide it in his description of the ‘solution’. He was adhering to the conventions not of science but of literature: a mystery ought to be resolved by an insignificant detail in plain sight.

To the average person, dust just looks like dust. But look closer and dust turns out to be many magnitudes more varied than dandelions. Locard conducted a ten-year investigation of les poussières organiques. A good deal of the organic component of dust is made up of pollen and spores from plants and fungi, called ‘palynomorphs’. Palynology is the field devoted to this botanical diversity. Plant species have different shapes of pollen depending on their particular environment and means of dispersal. Pollen that travels by insect has characteristic ‘hooks and spines’ to help secure it; pollen that’s dispersed by wind tends to be smooth and spherical. Pine pollen has two ‘air bladders’ to help it stay aloft (it looks a bit like Mickey Mouse). A colleague who works on plants showed me the pollen of thale cress, Arabidopsis thaliana, a small weed that thrives in unloved patches. (It’s a common ‘model organism’ for plant genetics.) Under the microscope, what had been a speck of yellow dust became a miniature cantaloupe with a wrinkled skin.

The complexity of palynomorph shapes confers on a skilled palynologist seemingly supernatural abilities. Every location has its own collection of palynomorphs, determined by its mixture of plants and fungi. One of the applications of palynology is the reconstruction of past landscapes. Pollen is small but hardy, each grain an armoured vehicle for delivering a gametophyte from the stamen of one plant to the stigma of another, sometimes across large distances. Its outer layer, known as the exine, is made of sporopollenin, among the toughest of all organic materials. Archaeologists have found a 100-million-year-old fossilised bee with pollen still stuck to its legs.By classifying the palynomorphs they find in sediment and soil samples, palynologists can describe the phantom plants that shed them hundreds or thousands of years earlier.

The same techniques used to reconstruct a Roman farm can be applied in forensics. Gibson cites the palynologist Patricia Wiltshire as one of the pioneers of the ‘picture of place’ approach. As Wiltshire recounts in her memoir, Traces (2019), she worked as an environmental archaeologist at UCL for sixteen years without ever considering forensics. Then one day she got a call from the police: a man had been found dead in a ditch by a field in Hertfordshire. Officers had arrested a group of suspects and impounded a car, but wanted a way to prove that the car had been at the field. Someone suggested pollen. The police told Wiltshire they’d been given her name by a contact at Kew Gardens. ‘They weren’t able to help us … but they said you could.’

Wiltshire accepted, though she thought the chances of succeeding were ‘infinitesimally small’. After scraping dirt from every location she could think of in the car, she treated the samples as she would those from an archaeological dig: first powerful acids to dissolve the soil and rock – pollen is indifferent to these ‘vicious treatments’ – then dyes to stain and jelly to embed the remnants. Examining the slides under her microscope, she classified the palynomorphs, grain by grain. As she built up the species composition of various locations in the car – the brake pedal, the floor mat, the back seat – she realised she was looking at a ‘typical archaeological assemblage’: the margin of an arable field. Pollen grains from hedgerow plants including hawthorn, blackthorn and bramble were intermingled with weeds such as black nightshade, white dead-nettle and goosefoot. This was already enough for the police. But Wiltshire didn’t stop there. When she visited the field, she looked at the hedge and realised ‘this was not just one hedgerow, but many smaller ones, each stretch distinct.’ She insisted on trying to guess the precise location where the body had been discovered, and wasn’t surprised when she got it right: she had seen it all before under the microscope.

In​ 1827, the Scottish botanist Robert Brown undertook an investigation into the pollen of Clarkia pulchella, a straggly plant with slender pale purple petals that splay out like chicken feet. Placing the pollen grains in water, he saw through the microscope that they were ‘very evidently in motion’. Their jittering wasn’t, he thought, caused by currents in the water but ‘belonged to the particle itself’. Brown was intrigued by the possibility that he might be seeing evidence of some vital life-force. But he disproved his own hypothesis by repeating his observations with pollen from dead plants, which displayed the same dancing motion. Though the inquiry had started from a botanical angle, it widened to become a search for ‘active molecules’. Brown began to discern the dances of smaller and smaller particles. He left botany behind and looked at inorganic materials: dust, soot, a piece of window-glass – even, for some reason, a fragment of the Sphinx. All of them contained tiny molecules that came alive under the microscope, moving through the water erratically.

Some scientists argued that atoms were the hidden cause of this mysterious motion. The suggestion wasn’t new: Lucretius made a similar extrapolation in On the Nature of Things, appealing to the dancing of dust motes in a sunbeam to argue for an atomist worldview (in A.E. Stallings’s translation: ‘Such turmoil means that there are secret motions, out of sight, that lie concealed in matter … you may be sure this starts with atoms’). But this inference, whether Lucretian or Brownian, remained a qualitative insight. It said nothing about the real size of the invisible atoms. Even at the turn of the 20th century, a young Albert Einstein was frustrated that atomic theory remained more of a ‘visualising symbol’ than a concrete set of facts.He wanted to ‘guarantee as much as possible the existence of atoms’. In 1905, he outlined a theory for the thermal diffusion of small but visible spheres in liquid, showing that atomic jostling from the surrounding water would, indeed, lead to movements sufficiently large ‘that they can be easily observed in a microscope’. He gave a simple mathematical formula that related the wanderings of pollen to the size of a water molecule.

Pollen is halfway in scale between its own atomic structure and the world we experience – a pollen grain is about 100,000 times smaller than a person, atoms about 100,000 times smaller again. Pollen grains are among the smallest specks of visible matter: if you’re reading this in the print edition of the LRB, at least five Clarkia pollen grains would stretch across the full stop at the end of this sentence. It is also particularly insidious. On a clear day, the earth’s electric field increases in strength by about one volt for every vertical centimetre. A plant, earthed by its roots into the ground, extends upwards into increasingly positively charged air. This electrical imbalance redistributes charge within the plant, pulling negative charge into its upper surfaces: sharp edges such as the pollen-covered anthers have the highest charge densities. A bee, by contrast, is a fuzzy ball, crackling with positive charge built up by friction as it flies through the air. So, as a bee approaches a flower – even before it lands – pollen grains can make Lucretian leaps across the void and cling to its body. (Anyone who has tried to remove lint from a fuzzy jumper will have experienced the same ‘static cling’.)

Pollen is difficult to dislodge, burrowing down into the weave of fabric and insinuating itself into crevices. Inside our noses are delicate curled plates of bone known as the nasal turbinates, each covered in sticky soft mucosal tissue. Each time we draw breath, dust in the air is trapped here. As a result, the turbinates of a corpse contain a record of a person’s last breaths. To read that record, you have to first turn the corpse face-down, cut a hole through the cribriform plate of the skull, and then flush warm medicated shampoo through the nasal passage, washing out any lurking palynomorphs. Wiltshire says that she has worked on cases where the presence of a single pollen grain changed the reconstruction of someone’s last moments.

Palynology has many other forensic applications. Analysis of pollen from the mass graves at Srebrenica helped investigators to prove that paramilitary groups had exhumed and moved bodies between sites in an attempt to cover up the murders they had carried out. On a smaller scale, it can be used to narrow down the search for a missing body: the composition of pollen on the killer’s shoes can be compared with known plant distributions. Given enough samples, a palynologist can conclude not only that you were present at the scene of a crime, but that you knelt on the grass with your left knee, not your right, or that your shoulder made contact with a lichen-covered wall.

Gibson’s book includes a wide range of examples of ‘forensic botany’. One of the earliest is the Lindbergh kidnapping of 1932, where the wood grain in remnants of the homemade wooden ladder used to abduct the twenty-month-old Charles Lindbergh Jr was compared with that of floorboards in the suspect’s attic. Another, from the 1980s, is a murder case in which the classification of plant fragments in the victim’s stomach matched the salad bar at Wendy’s (red beans, cabbage, onion, lettuce and green peppers), giving the location of her final meal and linking her to the main suspect, whom staff remembered dining there on the evening of the murder.

Gibson makes clear that taxonomic classification is at the heart of all forensic applications of botany, from Locard’s dandelion to Wiltshire’s palynomorphs. Yet in a taxonomical sense, ‘botanists’ are an increasingly rare breed. The botany department where Gibson studied as an undergraduate no longer exists; Southern Illinois University, where he now teaches, no longer has a plant sciences school distinct from general biology. In Britain, the last remaining botany undergraduates finished their courses in 2013. No UK university offers botany or mycology as a stand-alone undergraduate degree. Some still offer a BSc in plant sciences, but as one botanist pointed out in 2016, undergraduates can complete these degrees without being able to identify a single British wild flower. If botanists are rare, palynologists are rarer still. Classifying pollen requires a level of expertise that most botanists don’t have. There are just so many small things to distinguish from one another. For crimes involving water, for example, one might want to classify diatoms – unicellular algae that build delicate silica lace around themselves. A colleague with a PhD in environmental forensics tells me that classifying diatoms can require not only a microscope but a German dictionary too, since many of the textbooks for rare species were written in the 19th century by industrious Germans.

Does it matter? Sequencing the DNA of plants enables us to analyse their relationships without having to memorise Latin binomials. Those relationships can be useful in criminal cases since they are more fine-grained even than botanists’ taxonomies. As Gibson points out, DNA ‘fingerprinting’ doesn’t apply only to humans; it has been used, among other things, to identify strains of cannabis. Yet although DNA has revolutionised forensics, it can’t solve every problem. At one point in the 2000s, the German police were offering a €300,000 reward for information about the ‘Phantom of Heilbronn’, a woman whose DNA had been found at more than forty crime scenes over fifteen years. It eventually transpired that the DNA belonged to someone who worked in the factory that made the cotton swabs used to take samples.

Britain no longer has a Forensic Science Service; it was closed in 2010, an early casualty of austerity. Scientists at the service went to work for private companies which now compete for tenders, each undercutting the next. The effects have been wide-ranging, but Gibson notes that it means forensic botany is now ‘in the hands of a few, albeit very competent, individuals’. The problem is that the pool of existing experts was created by an education system that no longer exists, and their specialisms have never been commercially viable. Some have seen a marked decline. Wiltshire’s husband, the forensic mycologist David Hawksworth, told a parliamentary committee in 2019 that over the previous decade the number of investigations seeking out the expertise of his subfield had fallen from ‘perhaps five or six cases per year … to one or two or zero’.

Forensic science begins with the crime scene. Even the most brilliant forensic botanist in the world needs samples to be carefully collected, yet as Gibson makes clear, in many countries it’s unusual for crime scene investigators to have any botanical training. In a coda he refers to ‘plant blindness’, a supposed cognitive bias humans have against plants. I think he’s right, but I’m not sure it’s fair to apply this to fields such as palynology, where the object of study is almost invisible – after all, that’s what makes it so valuable.

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