8 March 2019
Ants, constipation, murder and the seeds of Ricinus communis
Revolving around a humble seed, we cover nearly everything from seed morphology and digestive problems to the beautiful smell of racing car engines and James Bond-style murder.
Known as 'castor beans', the seeds of Ricinus communis, a member of the spurge family (Euphorbiaceae), are among the most infamous seeds in the world. This blog is about their beauty, morphology and notoriety.
Being the only species of the genus, Ricinus communis is a pretty unique plant, and this holds true for many reasons, as you will discover. Botanically, it belongs to the spurge family (Euphorbiaceae) which means it’s closest relatives include economically important plants such as the Para rubber tree (Hevea brasiliensis) and cassava (Manihot esculenta) as well as popular ornamentals such as poinsettia (Euphorbia pulcherrima) and crown-of-thorns (Euphorbia milii).
Originally native to East and Northeast Africa and the Middle East, the castor oil plant (Ricinus communis) has spread throughout the tropics and has even become an invasive pest in places like Hawaii, South Africa, Australia and the Galapagos Islands. A fast growing tropical shrub that can reach a height of up to 5 m or more, it cannot survive our cold winters. Nevertheless, because of its striking large leaves and bold architectural ‘Gestalt’, Ricinus is a popular garden ornamental in temperate climates. It even comes in a range of different garden cultivars, for example ‘New Zealand Purple’ with dark red leaves.
Easily beating competition from the rather unspectacular wind-pollinated flowers, the most beautiful, interesting, useful and scariest part of the castor oil plant are its seeds, also called ‘castor beans’ (although they have nothing to do with real beans). They come in sets of three enclosed in soft-spiny capsules. The fruits add to the decorative looks of the plant but they eventually dry up as they ripen and explode to scatter their highly characteristic seeds. A distinctive feature of Ricinus communis, its seeds are smooth, shiny and mottled in various shades ranging from white over beige, brown and maroon to grey and black. Just as every zebra has a unique pattern of stripes, no two castor beans ever share the exact same seed coat pattern. The mottling provides the seeds with camouflage against mice and other seed eating animals as they lie on the ground after their explosive expulsion from the fruit. If lucky, they only need their camouflage for a short while as a rescue team of little helpers should soon be on the way...
The seeds’ shape is rather similar to a tick (e.g. Ixodes spp.) and with 'ricinus' being the Latin word for 'tick', it is pretty obvious where the castor oil plant got its scientific name from. Resembling the small capitulum (comprising the head and mouth parts) at the front end of a tick, castor beans carry a yellowish-white nodule that acts as an elaiosome (‘oil body’) and attracts ants for dispersal. The ants, irresistibly drawn to the fatty nutritious elaiosome, carry the seeds into their underground nests. Here, rather than devouring the precious morsel themselves, they feed it to their larvae. After the elaiosome has been removed, the ants usually abandon the still viable seeds inside the nest or discard them outside the nest on the rich soil of the colony’s refuse piles. In doing so they provide a vital service to the castor oil plant: they help disperse its seeds in order for them to find a suitable place for germination.
The castor oil plant is not the only plant to call on ants to help with the dispersal of its seeds. Lots of other plants have seeds equipped with elaiosomes that pursue the same strategy. This includes many of the castor oil plant’s close relatives in the Euphorbiaceae family as well as other species from across the plant kingdom such as members of the legume family (most notably Australian wattles, i.e. Acacia spp., Leguminosae), milkworts (Polygala spp., Polygalaceae), snowdrops (Galanthus nivalis, Amaryllidaceae), violets (Viola spp., Violaceae), greater celandine (Chelidonium majus, Papaveraceae), and some cacti (Cactaceae).
This close association of seeds and ants is called ‘myrmecochory’, which literally means ‘ant dispersal' (Greek: myrmêx, myrmêkos= ant; khôreô/chorein = to move, to disperse, to wander). Myrmecochory is a beautiful example of how plants and animals have co-evolved to form mutually beneficial relationships: the ant is given a nutritious and reliable source of food while the plant's seeds are dispersed far enough to reduce competition between the seedlings and their parent. By burying the seeds a short distance below the soil surface, the ants not only hide the seeds from predators like mice and other rodents, they also prevent them from being destroyed by fire. The latter may explain the important role ants play as seed dispersers in the dry habitats of Australia and Africa which are swept by seasonal wild fires.
Being a seed morphologist, I have to perform an ‘autopsy’ on the seeds of Ricinus to see what’s inside. Although castor beans may vary in size depending on their provenance, they are typically around 12 mm long. Underneath the brittle seed coat lies a soft, oily nutritious tissue called ‘endosperm’ into which the embryo (i.e. the baby plant) with its short axis and large but very thin and flat cotyledons (i.e. the first pair of leaves) is embedded.
The oil that is stored in the endosperm of castor beans is meant to provide the embryo with energy during its germination. However, at least 6,000 years ago in ancient Egypt, humans had already discovered the usefulness of castor seed oil and took Ricinus communis into cultivation. Since these ancient times until the present day, ‘castor oil’ is used as a lubricating laxative to relief constipation. Taken in moderation, castor oil does just that but an overdose will inflict explosive diarrhoea. Accompanied by painful cramps, the latter can last for hours, causing unpredictable involuntary bowel movements of considerable magnitude even during sleep. Most people know about the effects of castor oil which is why it makes a very funny joke when Tom the cat (as in the cartoon ‘Tom and Jerry’) is threatened with a bottle of castor oil by his owner Nancy, should he not stay in his bed. As a child, I found this hilarious. [I still find it hilariously funny!]
In real life overdosing on castor oil is no laughing matter. For example, force-feeding castor oil to prisoners was used as a means of torture by the Nationalists under the leadership of General Francisco Franco during the Spanish Civil War (1936-1939). Likewise, in Benito Mussolini’s fascist Italy (1930-1943) the paramilitary Blackshirts used castor oil for the very same purpose to deal with opponents of the regime. The severe diarrhoea brought on by the ingestion of large amounts of castor oil led to dehydration which could ultimately cause death.
Castor oil is a very valuable oil that is produced on an industrial scale (about 300,000-500,000 tons per year), most of it in India, China and Brazil. Apart from effecting a powerful acceleration of bowel movement, coincidence has it that castor oil also makes a great lubricant in jet and racing car engines. The brand name ‘Castrol’ proves that point and the high-performance engine oil ‘Castrol R40’ is even famed for the beautiful smell it bestows on car races. But the industrial applications of castor oil and its derivatives go far beyond lubrication. In fact, its versatility is almost boundless. Castor oil is used in the manufacturing of paints, dyes, adhesives, inks, soaps, cosmetics, chocolate, hydraulic brake fluids, plastics, waxes, varnishes, sealants and synthetic resins. A product of the latter kind has recently been used to seal the leaking rain gutters of the Millennium Seed Bank. In accordance with Kew’s environmental commitment, the original stainless steel gutters were first lined with a flexible reinforcement fleece made from recycled plastic bottles. The fleece was then saturated with a solvent-free water-proofing resin made from sustainably produced castor oil.
I don’t have a morbid obsession with death but the story of the castor bean would be incomplete without revealing the pretty seed’s most lethal property. As if its potential to inflict death by violent diarrhoea was not enough, the castor bean contains yet another, much more potent substance, the tiniest amount of which can kill man, beast and bug. Notoriously famous for all the wrong reasons, ‘ricin’, as the toxin is called, ranks among the most poisonous substances found in nature, alongside abrin which is found in the seeds of the crab’s eye (Abrus precatorius, Leguminosae). Both ricin and abrin are glycoproteins (= proteins coupled with sugars) that kill individual animal cells by blocking their ribosomes. The latter are the cells' protein 'factories'. By knocking them out, ricin and abrin act like a spanner thrown into the works of the cell. Cell death leads to tissue necrosis which in turn can lead to organ failure and ultimately death.
Lab experiments with human cell cultures have shown that penetration of just a single molecule of ricin into the cytoplasm of a cell is lethal. This explains the extreme toxicity of ricin. It is understandably difficult to carry out scientific experiments in order to establish the exact dose that is required to kill a human being. However, according to a report by the Federation of American Scientists entitled ‘Ricin: Technical Background and Potential Role in Terrorism’, the lethal dosage of ricin is as low as 3 to 5 micrograms per kilogram body weight if inhaled or injected. Uptake of ricin through the gastrointestinal tract is less effective so the lethal dose for ingestion is higher, something around 20-30 mg per kilogram body weight. In absolute terms this means that between 3-8 (some suggest up to 20) seeds chewed and swallowed are needed to kill a human.
Reports about the toxicity of castor beans vary widely as to the number of seeds required to induce death. According to a 2004 report of the Centre for Food Security and Public Health at Iowa State University, among domestic livestock, horses are the most sensitive to ricin and can die from the ingestion of 4-7 seeds. Chickens and ducks are much more resistant and can allegedly take up to 80 seeds before they are knocked out. However, following the death of several thousand wild ducks due to castor bean poisoning in the Texas panhandle between 1969-1971, a scientific study from 1981 found that the LD50 (i.e. the dose that kills 50% of the ‘contestants’ participating in the experiment) for mallards (Anas platyrhynchos) appears to be just 3-4 seeds. Likewise, other sources claim that it takes at least 60 seeds to kill a horse. These wildly differing results could at least partly be due to variations in the quantity of ricin present in various strains of castor beans.
Probably the most famous murder victim to be killed with ricin was Georgi Markov, a dissident Bulgarian writer who, at the time of his death in 1978, lived in London working for the BBC World Service. A prominent critic of the then communist regime in Bulgaria, Markov was assassinated in true James Bond-style. Whilst waiting at a bus stop on Waterloo Bridge, he suddenly felt a sharp sting in his right thigh and noticed a man speedily walking away holding an umbrella. Three days later Markov died at a hospital, aged just 49. A thorough autopsy revealed that Markov was shot with a tiny (1.7 mm) platinum pellet that had an x-shaped cavity filled with ricin. The sophisticated gun needed to launch the tiny pellet was built into the umbrella carried by the assassin, very much like one of ‘Q’s gadgets. Sadly, Markov must have died a very painful death. A lethal amount of ricin injected directly into the bloodstream causes severe internal bleeding followed by organ failure and death after 3-5 days of suffering.
Ardent consumers of castor oil need not worry about ricin. It is only soluble in water and not in oil. This means that the toxin will remain in the press cake and not cross over into the oil during the extraction process. By the way, ‘castor’ is Latin for ‘beaver’. Not that Ricinus communis has got anything to do with beavers. The name is a reminder of ‘castoreum’, an oily secretion from the beaver’s abdominal ‘castor sacs’. Combined with the beaver’s urine the animal uses castoreum to mark its territory. Since ancient Greece the aromatic smelling castoreum was used both as an aphrodisiac (perfume) and medicinally, for example, as a laxative. In the mid eighteenth century the oil from the seeds of Ricinus was (re-)discovered to be a much better laxative than castoreum. Thenceforth, the name was transferred, purely for reasons of pharmaceutical marketing. Today castoreum (‘beaver juice’) is still used in perfumes, cigarettes, candies and ice cream. Yum!
I want to thank my colleague John Adams who provided me the inspiration for this blog. Thanks to the industrial qualities of castor oil, as John pointed out to me, we will no longer have to put buckets under the few leaks in the corridors of the Millennium Seed Bank when it’s raining. Three other colleagues also very kindly contributed: Gemma Toothill took on the perilous task of cutting open and photographing the seeds of Ricinus communis, Mark Nesbitt specifically photographed some of the castor oil bottles in our Economic Botany Collections, and Andrew McRobb lent me some of his Ricinus photos.