Feature

A Grain of Evidence

Invisible pollen grains can be enough to sort out the guilty and the innocent.

Dallas Mildenhall

New Zealand is one of the few places in the world where forensic palynology is undertaken, almost as a matter of course.

What is forensic palynology? Forensic pertains to the courts and the law and palynology is the study of spores and pollen grains. These are produced by non-flowering (spores) and flowering and con-roducing plants (pollen) as a means of perpetuating the species.

Different plants produce pollen or spores of different shapes and sizes and in different quantities. Dispersal is mainly by wind, water or animals. Plants whose pollen is dispersed by wind produce vast amounts of usually small pollen grains. Those plants that use a more specialised form of dispersal usually produce fewer pollen grains, but these pollen grains are often larger and more ornamented and easier to identify. Pollen grains usually average about 30-60 microns in size -- a micron being 1,000th of a millimetre -- but can be as small as three to four microns or as large as 200 microns. They can therefore get anywhere, and this is of assistance to the forensic palynologist.

Pollen samples collected from one locality can contain over 100 different pollen and spore types and these can be characteristic of both time and place. Because pollen grains and spores readily adhere to almost any surface, they can be found anywhere.

For example, pollen grains can be found in the ink on banknotes and in the banknotes themselves, which can be of assistance, especially if the banknotes are forged. Pollen and spores found adhering to the enamel paint of a plank of wood stolen along with a tuatara from a museum was found to be identical in types and percentages to the spores and pollen from the cage from which the items were stolen.

Just one type of spore or pollen grain can tell us whether something at the scene of a crime can be connected with a suspect or whether something in connection with a suspect has been at the scene of a crime. Two examples are given of how just one species of pollen grain can help in solving a crime.

Opium pollen grain, 40µm (A human hair is 25-100µm wide)

Caught by Rope and Rosaceae

A rope used to tie up the hands, knees and feet of a woman during a robbery and assault in Hillsborough, Auckland, contained abundant pollen of the family Rosaceae (family of plants including the cherry). Rosaceae pollen is often found but very rarely in percentages as high as those found on these three separate pieces of rope -- and never for the particular genus that was identified.

The main suspect lived a migatory life and several of the suspect's previous locations held similar pieces of rope to the ones used in this crime. Only one rope had the same pollen profile -- the rope taken from the suspect's then current address. This provided strong evidence of a connection between the suspect and the scene and he was arrested, charged and convicted of the crime.

Damned by a Daisy

A woman accused a man of a sexual assault outside her home. The man denied ever being near the scene. The police took control samples from the scene and the clothing of the suspect and victim.

All samples contained abundant pollen of a particular shrub genus of daisy that is not very common in New Zealand. The amount of pollen on the suspect was so great that it could only have got on the clothes as a result of prolonged contact with the shrub or the ground immediately beneath the shrub.

The combination of other pollen and spore types indicated that it was the dirt from around the particular shrub in front of the victim's house that was on the suspect's clothing. The man was arrested and convicted of sexually violating the woman.

Forensic palynology has many other applications. It can also be used to source drugs and other illegal importations to their country of origin, determine the age of antiques, identify the plants bees collect pollen from, determine the food sources of native bats and prehistoric peoples, and disprove alibis.

There is more than a grain of truth in the saying (Locard's Principle) that a criminal leaves something behind at the scene of the crime and also takes something away with them, but often what they take away cannot be seen with the naked eye.

A daisy pollen grain, 28µm

Dr Dallas Mildenhall is a palynologist with the Institute of Geological and Nuclear Sciences Ltd.