Pretty little liars
In my last blog I debunked some sadists of the plant world and now it is time, as promised, to expose the fraudsters of the plant kingdom. Hang on a moment, you may think! How could plants commit fraud when they can’t think? Well, it seems that evolution has done the ‘thinking’ for them.
Take the pretty but dangerous crab’s eye or rosary pea (Abrus precatorius), a member of the legume family (Leguminosae/Fabaceae) that is found throughout the tropics. When its fruits open, they reveal some very attractive looking shiny bright red-and-black seeds. The gaudy display catches the attention of birds. After all birds have very similar colour vision to humans (even better, in fact - they can see part of the UV spectrum!). In ‘dispersal sign language’, the colour ‘red’ is generally understood to advertise something juicy and edible, such as a sweet berry or stone fruit (think cherry!), for example.
Dispersing seeds of Abrus precatorius (Image: Gwilym Lewis)
The lollipop effect
By the way, this colour scheme already works with very small children. As many parents can testify, children clearly understand the message sent out by a red lollipop from an early age, and – worryingly dangerous - also from any red fruit they encounter in the garden. So to birds, the shiny round seeds of the crab’s eye represent the equivalent of sweet lollipops. But in reality these seeds are very hard and dry and have nothing edible to offer. Birds hungry or curious enough to pick up one of the seeds may carry it for a short distance but discard it as soon as they discover the scam. Those birds that are not so clever might even swallow the seeds and as long as they haven’t got a strong gizzard, like the specialised seed eaters, the seeds pass through the bird’s gut without harming it.
Bright red seeds of Abrus precatorius (Image: Wolfgang Stuppy)
Pretty but dangerous
Whilst the seeds of the crab’s eye are useless to birds, they are treasures to makers of botanical jewellery. In many tropical countries, seeds are strung together into beautiful necklaces, earrings and bracelets.
Botanical jewellery made from the seeds of Abrus precatorius (Image: Wolfgang Stuppy)
However, handling some seeds is a very dangerous business and those of the crab’s eye are the most dangerous of all. Despite their great beauty, the seeds of Abrus precatorius contain abrin, one of the strongest plant poisons known. With an estimated lethal dose for humans of between 0.1 and 1 microgram per kilogram of body weight (experiments to determine the precise dosage are difficult to conduct!), less than 0.003 grams could kill a child. Fortunately, the seeds have a very hard seed coat and as long as they are not damaged, they are harmless, even if swallowed (don’t try this at home!). However, the manufacture of botanical jewellery can become a hazardous occupation if holes have to be drilled to thread seeds together. The dust from crab’s eye seeds can cause blindness on contact with the eyes, and inhalation or contact with open wounds has even worse consequences.
What a way to go!
Symptoms of poisoning appear only hours or perhaps days after contamination; they include nausea, vomiting, severe abdominal pain, diarrhoea and a burning sensation in the throat. Later, drowsiness and ulcer-like lesions in the mouth and oesophagus, convulsions and shock finally lead to coma and death. Not a nice way to go! For adventurous types of the kind that indulge in fugu (Japanese for puffer fish), there is a simple way to detoxify the perilous seeds. At temperatures above 65°C, abrin breaks down so that the seeds are even allegedly edible after boiling (and don't try this at home either!).
The crab’s eye is not the only cheater
Apart from the crab’s eye, there are other examples of fraudulent seeds, most of which are also members of the legume family. Because of their hardness and shiny colours, all of them star prominently in botanical jewellery. Among the favourites are the pure red seeds of coral trees (members of the genus Erythrina) and others such as the red bead tree (Adenanthera pavonina) from south-east Asia and Australia, and the Texas mescal bean (Dermatophyllum secundiflorum), a native of the south-western United States and Mexico (see my previous blog on the Texas mountain laurel).
Image left: Flowers of Erythrina caffra; image right: Seeds of Erythrina herbacea (Images: Wolfgang Stuppy)
Even more eagerly sought are bicoloured red-and-black seeds like those of Abrus precatorius and others. Apart from the crab’s eye, such seeds can occur in certain species of Ormosia and Rhynchosia from Central and South America as well as in coral trees (Erythrina spp.), such as the Malagasy Erythrina madagascariensis.
Botanical jewellery made from Ormosia spp. (Images: Wolfgang Stuppy)
Seeds of Ormosia coccinea (photo by Wolfgang Stuppy)
One last thing... some people like to toy with their necklaces by putting them into their mouth. If you happen to be an aficionado of botanical jewellery, this might not be a good idea!
All above images by Wolfgang Stuppy are copyright Board of Trustees, Royal Botanic Gardens, Kew