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Can you use pollen to catch criminals?

Pollen is handy stuff. Apart from the plant fertilization process it is intended for, it has a wide variety of other uses: Fossilized pollen has been found in rocks dating back to the Carboniferous period proving the existence of primitive plants; bogs and lake sediments store fossilized pollen that can show vegetational succession from the last ice ages through to the present day; honey manufacturers use pollen trapped in their produce to test the provenance of suspect batches; doctors use it for allergy testing; and of course there’s modern day crime-fighting.

Imagine you are a thief. You’ve just stolen something precious and jumped into your car and made your getaway. After some time you dump the car and run across a field and lose the police that are following you. You get home, and to cover your back you call the police and report your car stolen. Now, modern-day police have an arsenal of techniques to use at their disposal to help trace the last movements of people or objects, but in this case they have a hunch that you are lying and they seize your shoes. The soil on the sole of your shoes is all they need to be able to extract pollen and match it to the pollen in the soil of the field that they lost you in. Clever, huh?

Since the 1950s the study of pollen — called palynology — has been used to solve forensic cases in criminal courts. The assemblage of pollen found at a particular location can be so specific that it can be enough to convict on that basis alone. It doesn’t need to be preserved in soil to be useful either. The outer shell of a pollen grain is very robust. And it gets everywhere: in dust, trapped in the weave of fabric, in your hair, under your nails, and even in your excrement!

It seems such a simple process, but there are some caveats to consider. Not all flowers produce the same amount of pollen. Different flowers shed different quantities of pollen depending on their dispersal method. A murder scene in woodland will offer a similar pollen assemblage across a large area, but finding single pollen grains of an insect pollinated bluebell (for example) can be significant. A murder case in Leeds, England, was partially solved by the pollen trapped in the victim's hair revealing a completely different location to the woodland that the body was discovered in. The mixture of garden weeds and cultivars led police to the perpetrator who was found guilty and sentenced to two life sentences.

The preservation of pollen can also be a hindrance, although in most cases good preservation is an advantage! Often a case can involve ‘unravelling’ several different years of pollen stratigraphy, and if a depth of soil is involved then it can be several millennia. In a case in New Zealand, a tenant farmer was incorrectly convicted of drugs offences when cannabis pollen was found in the farm outbuildings. It later transpired that the pollen was residual from the previous occupier of the farm and the tenant was acquitted.

Pollen can also travel long distances. It’s a handy feature for those plants relying on wind as their dispersal mechanism, but it can be confusing for palynologists when a particular species turns up hundreds or even thousands of kilometres from the nearest plants. Ambrosia pollen from plants growing on the east coast of the United States is often found in western Scotland.

Then there’s plant maturity: it takes 15 years, sometimes more, for certain tree species to reach sexual maturity. For those years they are not producing pollen. In one case of a missing body, police seized the garden tools of a suspect. The pollen from the soil on the spades sent police looking for the victim’s grave in mature pine and birch forests. The body was later discovered by dog walkers in a plantation of immature hemlock. The trees were not mature enough to produce pollen so they left no trace.

Crime shows on TV don’t really do this branch of forensic science justice. Machines and flashy tools aren’t used in palynology. Old fashioned microscopy and a skilled palynologist are all it might take to catch a crook.


Acknowledgments
This essay was inspired by a blog post by the author from March 2014; uglosbioscience.wordpress.com. References for the cases cited in the essay can be obtained by contacting the author at jwebb@glos.ac.uk.

Leonardo was a geophysicist

Heterogeneity + sparse sampling = uncertainty

Heterogeneity + sparse sampling = uncertainty

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