Children love a gory story. So during a forensic biology event staged as part of National Science Week, pupils from London schools set to with gusto analysing a jar labelled “Victor’s stomach contents”, the last meal of a murder victim.

The Plant Detectives event took place at the Chelsea Physic Garden, in London. While the eight-year-olds asked “is it true, Miss?” when they saw the chalked outline of the body in the greenhouse, the blase 13-year-olds accepted our scenario, “Murder Most Murky”, for what it was - a dramatisation on which to hang science investigations and some logic. But all the pupils examined the evidence with concentration and determination.

The mixture from Victor’s stomach looked disgusting, but was actually a harmless concoction of cornflour with added goodies such as rice and tomato seeds which the children could pick out and identify. Litmus paper tests were used to identify the fatal toxin.

The first step at each session for 30 children was to outline the details of the crime. They were introduced to Victor, and to the four suspects - conveniently called Arthur, Beryl, Charlie and Denise. The group then set off on a walk around the garden to set the scene and look for clues.

The children were next shown the spot in the greenhouse where the body had lain. This was their opportunity to make observations - the daffodils that shed pollen on to the suspect’s coat which could be picked off with Sellotape in time-honoured police fashion, the tree fern which had lost hairs, brushed off on to the victims clothes as his body was dragged past, the trampled soil in the greenhouse.

The tests were laid in a ring of five experiments, with the amateur sleuths allowed 15 minutes at each station. Pupils worked in groups of six, in pairs or in threes according to the equipment available and their own expertise.

Pollen and hairs from the suspects’ and victim’s clothing were stuck down on microscope slides. They were then compared with prepared slides of known pollen and fibres such as wool.

The difference in primary and secondary skills in focusing microscopes was apparent, but the younger ones found using this equipment a real thrill.

Soil from suspects’ shoes was compared with the greenhouse soil, using a pH test for acidity. The exercise demanded deft handling of messy materials and care in keeping the specimens separate.

Then there were the stains on the four suspects’ clothing. Was it blood from the victim’s scratched skin? Some was fruit juice, but some of the stains were meat juice. Plastic gloves were provided for handling the strong chemical indicator - prepared by a technician.

During the two-hour session, pupils got to grips with fair testing and safety procedures, learning how to make observations and note differences and similarities in specimens. They also gained experience in handling apparatus, using chemicals, making connections, sorting relevant from irrelevant results and drawing conclusions. One school even brought its OFSTED inspector.

Each student was given a workbook (two folded, double-sided A3 sheets) including instructions for the five experiments. Each also had a results table and some background notes on the people in the story. At the end each group was asked to name the murderer.

Many reached the right conclusion. But, as in a real jury, some bowed to peer pressure and accused the wrong person. A secret ballot may have been more effective. It was an interesting exercise in getting pupils to work as a team.

The game also revealed some division along sexual lines. A high proportion of primary school boys spoke for a mixed-sex group when there was no adult direction. But the girls were often best at the practical part of the exercise and their thinking led them to the right answer more often than the boys. I could hear girls muttering “it was Beryl”, while the boys were convinced Arthur did it, because he had blood on his clothes.

Our other scenario, “What a Nasty Pain”, involved looking at pieces of plants found in the “vomit” of some children who had become ill after playing in the Physic Garden and eating some plants as part of a game. The garden does have poisonous flora, and the young forensic botanists had to separate the four specimens into solid and liquid parts, identify the plants eaten and determine which, if any, would cause illness. Experts at Kew Gardens and London’s Guy’s Hospital poisons unit provided background information to ensure authenticity.

This exercise, too, was arranged as a circus of five separate investigations. Pupils had to examine patterns on leaf epidermis, compare berries and seeds and test for poison (with simple litmus paper colour changes).

This was the second time around for the week-long event, which originally ran at the Royal Botanic Garden, Edinburgh. With adaptations for the change of venue, and some minor improvements to the investigations it looks set to be a favourite for Science Week. But it could be run as a one-day session at a school. Last year, Bacon’s City Technology College, London, ran a version for its sixth-form students.

With the role play at the start, the walk round the garden to look at the harmful plants growing, and then the “laboratory” exercise in the classroom, it was a scenario that could be developed by any primary school. None of the plants used was exotic, and the materials and equipment are cheap. Ivy is ubiquitous, foxglove is common, jam jars are free, and a few magnifying glasses and small quantities of laboratory glassware will not break the bank. Microscopes can be borrowed from a local secondary school if needed. The children loved it.

In spite of the hours of preparation and the number of people involved - two event leaders, one technician and up to three volunteer helpers - once the kit is set up and the written materials prepared, the sessions can run and run. It is a fascinating and fun way to introduce plant biology to younger pupils and could be used to motivate “switched off” older pupils or challenge high flyers - depending on how it is presented at each stage. It is well worth the effort.

After all, when young people are enjoying what you have provided for them, it gives you a real buzz. And what better way to celebrate the Chelsea garden’s newly-built education centre?

Plant Detectives was a collaborative venture between the education department of the Chelsea Physic Garden and Action for Biology in Education For details of the Chelsea Physic Garden programme for schools and teachers telephone 0171 352 5646 ext 23 For information about Action for Biology in Education send an A4 stamped addressed envelope co The Education Officer, Chelsea Physic Garden, Royal Hospital Road, London SW3 3HS