The Natural History Museum’s herbarium is estimated to contain 5.2 million individual sheets. While I worked at the museum, I looked after about 620,000 specimens collected from Britain and Ireland, which of course is a lot more than the estimated 4,800 species found growing wild on these islands. Unlike Noah, museum curators are not content with two representatives of each species, we like lots. The main reason for this is that species vary, often a great deal. Plants are especially variable and having a reference collection to examine to is very useful in understanding natural variation within a plant species. This variation is caused by a huge range of factors relating to the species’ evolutionary history. In some cases, herbariums can hold hundreds or even thousands of specimens from each species. The scale of the museum’s collections is driven by one of its key roles – to protect and conserve the scientific and cultural heritage of natural history specimens. If the collections are of value and the museum has the resources (an increasingly doubtful question these days), then they will be taken in. This plenitude of specimens is incredibly useful for an ever-widening range of scientific uses, including climate-change research. Museum specimens contain locked-in past environmental information about how our world is changing. They are also invaluable for forensics.
I once received a request to examine some material in an arson case. The suspect was arrested on suspicion of attempting to burn down a building. Having made the attempt, the suspect made their getaway. Unfortunately for them, about ten minutes later they were stopped by a police officer who was aware of the incident. The officer decided the person was behaving suspiciously and arrested them. They were taken to a police station where the officer did something unusual (or as far as I am aware, it’s unusual). The suspect was made to stand on plastic sheeting and their clothing was careful examined and brushed down. The debris that fell to the floor, as well as that retrieved from the suspect’s clothing, was retained as an exhibit and sent to me. The amount recovered was tiny, two leaves and one flower, each no more than 5 millimetres long.
The police also visited the crime scene and retrieved another exhibit; some dried vegetation pulled from the property’s fencing that had been used to start the fire and this too was sent to me. Each exhibit was sent in separate sealed bags. Because the plant fragments retrieved from the suspect were so tiny, I examined them under a dissecting microscope and quickly confirmed that they were a match and from the same type of plant. The leaves were small, needle-shaped and their margins were slightly folded back on themselves, with lots of glands all over their surface. This is an unusual form for plants growing wild in Britain and straight away led me to suspect that the plant originally came from a warmer part of the world with an arid or Mediterranean climate. The flowers were also distinctive and did not resemble any widespread British wild plant. After some musing, I had a reasonable idea what the plant was and which part of the world it originally came from.
I was in a rush. It’s fair to say I was always in a rush. People have the idea that the life of museum curators is sedate. Not so. Most curators are expert at multitasking, and typically we have several curation projects and groups of volunteers on the go at any one time. I’d also be attempting to write or contribute to papers, draft grant applications, answer email enquiries from all over the world, contribute to the latest gallery development, attend fundraising jamborees with billionaires and do the odd interview for the television. Lots of fun, most of the time!
The impending sense of rush-related sweaty palms was exacerbated by the fact that I knew I had to leave the comfort of my relatively small collection of 620,000 specimens and head into the Natural History Museum’s General Herbarium which runs to several million. The General Herbarium covers the whole world minus Britain and Ireland. It’s an amazing collection. Highlights in the museum’s botany collections include the first set of plant specimens collected from Australia by Sir Joseph Banks and Daniel Solander when they accompanied Captain James Cook on his voyage to Australia in 1768 and the plants that Carl Linnaeus, the ‘father of taxonomy’, studied in his youth in Holland.
The herbarium is cavernous and, even after being at the museum for so long, I still had moments when I couldn’t remember where things were. I needed to get an answer to the police as soon as possible, and so I pounced on a retired colleague who continued to work voluntarily on the collections. She also had a labyrinthine knowledge of the herbarium. The Natural History Museum’s collection enchant many and it is not unusual for retirees to continue their labours, sometimes for decades. I asked my colleague to do me a favour and see if she could track down a specimen that matched the exhibits I was working on. About 45 minutes later she was at my desk with a match (about two or three hours faster than I expected the task to take). She was carefully holding a specimen in front of her (there are stringent rules on the handling of specimens; 300-year-old dead dried plants can easily be ruined). One glance told me that she’d got it − I knew she would, she’s brilliant but self-deprecating. My retired colleague does not have a doctorate and hails from an era when the museum was very hierarchical, those with qualifications were considered superior.
To be absolutely certain, I examined the museum specimen under the microscope. It was certainly the same plant, or very closely related. I noted down the collection details and returned the specimen to its place in the herbarium. Then, I returned to my desk to do a little research on the plant. It is not a plant that would be found wild in Britain. The plant hailed from the drier parts of South East Asia and Australia; it is occasionally grown here in botanic gardens but not outside, because it’s too cold in most places here. Further searching led me to learn that it is sometimes used to make brooms and light brushwood fencing. The last piece of information is very satisfying. The exhibit seized at the building originated from brushwood fencing. I wrote up my observations and my conclusion, which was that the most likely source of the leaf and flower fragments taken from the suspects clothing was from the crime scene.
Every one of us who uses environmental information in forensics is either directly or indirectly dependent upon the millions of specimens and collections housed in our natural history collections. Profiling soil types is dependent upon the knowledge accrued by several centuries of soil scientists examining and collecting soil samples from all over the country. These archives of soils are essential tools for understanding the diversity of soils we have and where they originate from. Searching the Soilscapes map of the Cranfield Soil and AgriFood Institute, I learn that the soil from my mother’s village is ‘Soilscape 7: Freely draining slightly acid but base-rich soils’ and that this soil type covers 3.1 per cent of England and Wales. Other resources like the UK Soil Observatory provide more detailed technical information. Using information resources such as these, an investigator can examine soil collected from the possessions of a suspect and provide important evidence about where they have been.
After my first ever crime scene, my entomology colleagues at the museum carefully examined the larval and pupal samples I collected. The larvae and pupae were most likely compared to the collections amassed during their decades of working in entomology and forensics. Much of their work involves understanding how fast insect larvae develop. Insect larvae feeding on bodies, or pupae found nearby can be used as a means of estimating the post-mortem interval − basically, how long someone has been dead. Using pig carcasses, they did dozens of research experiments looking at how the insects develop under different environmental conditions. Ideally, all research should be published as a paper in a peer-reviewed journal. Peer review is the process of checking that the work has been undertaken thoroughly and that the paper’s conclusions accurately reflect the observations. To support their work, scientists such as my entomology colleagues will keep voucher specimens collected during the study. These will then have been stored alongside the other 80 million or so specimens that the Natural History Museum looks after. The preserved museum specimens are an essential part of the information support
ing the published scientific work.
Seeds and fruit are another type of plant material often isolated from exhibits seized at a crime scene or from a suspect. Like hairs, seeds and fruit are very variable. Many are usually readily identifiable if they are not too badly damaged. What is the difference between a seed and a fruit? Botanically speaking, a fruit is the structure that bears the seed or seeds. It is not the same as the culinary use of the word; to us botanists, fruit can either be hard or soft, dry or succulent, edible or poisonous. Fruit structure is immensely variable. The main reason for this is that the fruit of many plants play a key role in the dispersal of the seed into the environment, which is vital for a plant’s survival. Like young humans, plants need to get away from the parents.
Some fruit types may be of limited value in the forensic environment. Those that are dispersed by the wind, such as the fruits of birch or maples and sycamore (Acer spp.), are highly mobile and may be found a considerable distance from their parent plant. Some of the most useful are those that are only dispersed a short distance or have evolved to be spread by animals. Many wild plants have fruit that have evolved to catch onto the fur or feathers of passing animals and birds. These often attach to the clothing of suspects or victims. Plants such as burdock (Arctium), goosegrass or cleavers (Galium aparine), and wood avens are examples of common plants that are often attached to our clothing after a walk in the woods. Plants such as this are very good at linking a suspect to a specific type of habitat. Even when found in soil samples, the remains of seeds and pieces of plant tissue can be very helpful in determining what sort of habitat the suspect has been in. The wild plant communities found growing on the margins of arable fields are very different from those growing in permanent grassland. Differences such as these can potentially help a botanist confirm where a crime took place or link a suspect to a crime scene or a victim. Knowing the identity of plants, understanding where they grow and how they reproduce is key to being able to do this.
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The Curious Microscopic World
Fear of dying is embedded within all of us. Some of us learn to find ways to manage that fear. Sadly, others are haunted by it. For many people, the way of dying that most frightens them is drowning. When I was around eight years old, one of my friends nearly drowned in our local village pond. We were mucking around on the ice and she fell through. I can still clearly see her face peering up at us through the ice. We were paralysed with fear and fascination. Luckily, she pulled herself back to the hole she had fallen through, and we were able to help her climb out.
I have read a few accounts of near drownings and many of them describe the panic and the pain. I can relate to this. Oxygen deprivation hurts. Like many people, I have asthma. A bad asthma attack is really painful. Homicide by drowning appears to be rare and is usually only accomplished if the assailant is more powerful than their victim. Not surprisingly, people fight back if they are pushed underwater. Occasionally, a victim will be drugged or knocked unconscious before being placed in the water.
Rather surprisingly, there are several forms of drowning. ‘Dry’ downing occurs when, after being submerged, the person attempts to breathe but the muscles of the larynx contract and the airway is blocked. The person dies of oxygen deprivation, but no water reaches their lungs. This appears to be relatively uncommon. More frequently people die by ‘wet’ drowning in which water enters the lungs. In cases such as this, there is no spasm of the larynx muscles and water is free to pass down the airway into the lungs. ‘Secondary’ downing occurs after removal from water and may occur within half an hour or after several days. Basically, the lungs are so damaged and their ability to allow oxygen to pass into the blood stream is so impaired that the person dies. Lastly, some people die owing to ‘immersion syndrome’. In cases such as this the heart stops because of a neurological shock. This sometimes occurs when people dive into still, deep lakes that have a layer of cold water beneath the uppermost warm layer. It is usually possible to determine what type of drowning the person suffered during post-mortem examination.
I am quite often asked about drownings. I don’t usually work on these cases but a former colleague at the Natural History Museum does. He is a diatomist, a specialist in the biology of the group of organisms known as diatoms. These microscopic organisms can be either single celled or live in many-celled colonies. Diatoms are algae, which means they use sunlight energy but don’t have complex tissues like most land plants. Algae are confusing and complex. The algae that most people are familiar with, especially the green ‘pond scum’, are not related to diatoms but are more closely linked to land plants. Curiously, some types of ‘pond scum’ are not even algae: they are flowering plants (Lemna spp.) and some of their closest living relatives are lords-and-ladies (Arum spp.) and the enormous Swiss cheese plant (Monstera deliciosa). On the other hand, diatoms belong to a diverse lineage of microorganisms that even many biologists have never heard of. Their nearest relative that you’ll be familiar with are brown seaweeds like bladderwrack (Fucus spp.), the seaweeds that make popping sounds when you walk on it. There is much to marvel at about diatoms. They have been around for a long time, since at least the Triassic Period (250-200 million years ago) and are sometimes so abundant that they form their own geological feature, diatomaceous earth, which is made up of the fossilised remains of diatom skeletons. Diatomaceous earth is commercially extracted because it can be used in the filtration of swimming pools or fish tanks, as a fine mild abrasive for toothpaste and metal polishes, as an insecticide, or to make nitroglycerin more stable and prevent unwanted explosions. What makes diatoms so remarkable is silica. Unusually amongst living organisms, diatom skeletons are made of silica. In most animals, the skeleton is calcium based; in plants the equivalent to the skeleton is largely made up of cellulose or lignin. Relatively few organism groups have skeletons made of silica, and in most instances, they are microscopic; an exception are some sponges that have silica ‘spicules’ that act as part of their skeleton. The silica is the key to why diatoms are useful in crime-scene work.
One surprising aspect of drowning is that as a person attempts to breathe, the pressure inside the lungs is enough to push the diatom cells in the water through the lung membranes and into the blood stream. The cells then circulate around the body and finally lodge in the major organs as the blood circulation fails. The presence of diatoms in the organs is potentially good evidence of some types of drowning (but not ‘dry’ drowning, for example). Collecting a sample of diatoms from a body is not straightforward and this is where one of the properties of silica becomes important. Silica is tough and heat resistant, a lot more so than human tissue. During the post-mortem examination, a small piece of tissue, usually liver or bone marrow, will be removed to be sampled. The tissue is then ‘ashed’ by being heated sufficiently to burn the tissue but leave the silica shell of the diatoms intact. There are other means of extracting the diatoms from the tissue, for example digesting in nitric acid, but ‘ashing’ is the most frequently used technique in this country. The prepared ash can then be examined under a high-powered microscope. Diatom cells are fairly small, most are between 10 and 80 micrometres across. Some can be up to 200 micrometres or rarely over a millimetre. A micrometre is one thousandth of a millimetre. Only the smaller types of diatoms, those that are 60 micrometres or less, are able to pass through into the blood stream.
Diatoms are diverse. Scientists don’t know how many species of diatom there are, and estimates vary between 20,000 and 2 million! Diatoms are usually restricted to either fresh or marine waters. In Britain and Ireland there are about 2,500 known species of freshwater diatom. Many of these species are widespread, while others are restricted in distribution or specialists of certain habitat types. The key to identifying diatoms is their silica. The siliceous skeleton of diatoms originates from their cell walls, which are called frustules. Not only are frustules incredibly beautiful, they are varied in shape and ornamentation. This variation is key to identifying which type
of diatom is present in a sample and establishing what type of water body the diatoms originated from. The water of swimming pools or bath water will have very different diatom communities in them compared to an isolated pond in a wood or a canal. So, the presence of diatoms in a dead person’s organs can help confirm that they drowned and in what type of water body this happened. The key to confirming that someone died in a particular stretch of water is getting a sample from that water body, quickly! Diatom populations are highly dynamic: small shifts in water temperature, nutrient availability or light levels will change the abundance of the diatoms and can change the species composition. If an investigating team delays sampling by even a few days, the diatom community may be very different, and any comparison is likely to be pointless. For diatom-based evidence to be of value, the retrieval of environmental samples containing diatoms should be overseen by a scientist who is experienced in working with them.
I’m a great fan of pre-Victorian scientists, because they managed to achieve incredible discoveries about our world, often with only the most basic of equipment. In the mid-17th century a Dutchman, Antonie van Leeuwenhoek, built his own microscope and documented the microscopical world of fungi, insects, plants and single-celled organisms that he called animalcules. His pioneering work revolutionised biology and was internationally applauded. In England, his work was championed by the Royal Society. Founded in 1660, the Royal Society was devoted to ‘Improving Natural Knowledge’. In 1703, several decades after Leeuwenhoek’s discoveries, the Royal Society published an anonymous work originating from ‘a gentleman in the country’. This described for the first time a previously unknown microscopic organism, collected growing on algae ‘from the shallow side of a pond’ that was made up of ‘many pretty branches, compos’d of rectangular oblongs and exact squares that were joyn’d together’. The gentleman was describing Tabellaria, a diatom. This unknown man’s observations exemplify the curious minds of the era. They remind me that, when attending a crime scene, where possible, explore all aspects of the environment.
Murder Most Florid Page 13
