Botany

Soil, plant fragments, and pollen, maybe in trace amounts, are often left behind at the scene of a crime. Most people entering a house will bring in some soil or mud from outside. Even if they take off their shoes, their clothing may contain tiny smears of mud where they have been splashed or come into contact with a surface. Tools like shovels might also contain significant traces of mud. An expert in botany, the science of plants, can often help unravel the identity and significance of such trace evidence. Soil and mud, in particular, are often present in footprints or tire tracks and can help link a suspect to the scene of a crime. The pattern of mud on clothing can also be significant.

Soil is a mixture of mineral, plant, and animal matter that is often characteristic of a particular area and may reveal something about a suspect's movements. Often soil also contains some man-made products such as glass or paint. The forensic scientist is interested in the patterning of soil and mud staining and how it might relate to the circumstances of a crime. For instance, if an assault takes place out-of-doors, then the mud staining of a suspect's clothes could naturally be revealing.

The visual and chemical analysis of a soil trace can often link it to a particular geographical region. This, in turn, can help to track the movements of a suspect if he or she has traveled to the area where the crime was committed. If a body has been moved for burial, then soil or plant material in a vehicle could be important.

The forensic botanist, first with the naked eye, looks at any soil or mud and assesses its color and texture. Microscopic examination reveals more about the content of the soil and the range of particle sizes it contains. Large samples might be sieved to separate them into different portions, depending upon particle sizes. Then these can be further examined to give more information.

Chemical analysis using advanced techniques like atomic absorption spectroscopy will give the mineral composition of a soil sample, such as chalk or clay, which is often characteristic of the area it came from. The acidity of the soil is also measured, as this varies greatly with place of origin. Thermal analysis of the soil, heating it in an oven till it decomposes, is also often characteristic of its origin. There may be dramatic color change or the soil may absorb heat in a characteristic way.

Suspects and victims also, often unknowingly, carry various items of plant debris on their bodies and clothes such as flower petals, seeds, and pollen. These are often native to a specific area. For instance, if pine needles are found around a victim who seems to have perished in an area where there are no evergreens, it may tell the investigators something important and specific about the suspect and his or her movements. The botanist can investigate what species carries these particular needles and so help link the perpetrator to a specific source.

Pollen grains are tiny and are not usually noticed by those involved in a crime. Pollen is often found almost everywhere—in hair, on surfaces, and on paper. If pollen is found on the envelope of a threatening letter or a ransom note, for instance, it may provide a valuable link to the suspect. There are pollen databases which can show the investigators where a particular pollen sample may have come from.

When a body is left out in the open or in a shallow grave, plant debris, including leaves and needles, may cover the remains. Analysis of this growth can often help establish the time and season of death and burial.

In one British case from 1887, a 15-year-old boy was found drowned in a ditch. Footprints led down the bank of the ditch. Sand grains were found on the suspect's trousers and matched to the ditch. Mud on the clothing of the suspect's daughter, who turned out to be an accomplice, was examined microscopically. Hairs from the seeds of the groundsel plant were found. This mud matched samples taken from the part of the ditch where the body was found, but not mud found from other areas. If botany helped solve a case so long ago, it can be even more powerful today with modern analytical techniques.