Plant of the week: Calabar Bean

Name: Calabar Bean (Physostigma venenosum)

Otherwise known as: Ordeal Nut, Chopnut

Habitat: A vigorous climbing perennial of the Leguminoseae family attaining a height of 15m in rich, moist soil in tropical West Africa. It closely resembles the scarlet runner with pinnately trifoliate leaves and pendulous racemes of purple flowers that produce a pod of about 16cm that contains three chocolate-coloured beans. The plant has no odour but the beans are fatal to mammals.

What does it do: Calabar contains a number of alkaloids such as physostigmine, calabarine, eseridine and eseramine, all of which are highly toxic. The plant first came to prominence when the British colonized that area of Africa known as the Calabar coast, now modern Nigeria, and discovered that the native tribes used the beans in an African equivalent of the medieval ducking stool. Those accused of witchcraft and suspected of crime would be forced to consume the bean; if they vomited it up within the hour they were deemed innocent, if they succumbed, they were guilty; very few survived. The British viewed this as a barbaric practice, particularly as the tribes were in the habit of making captured white settlers consume the bean, with similar results, but there are still instances of the practice continuing in rural Nigeria.

There is evidence that Galenus, physician to Marcus Aurelius, knew of the bean and some European herbalists used it to treat chronic constipation, epilepsy and cholera with varying results.

Physostigmine is known to prolong the neurotransmitters and dilate blood vessels and slow the heartbeat as well as preventing damage to the optic nerve. Modern medicine uses the alkaloids to treat cases of Myasthenia Gravis, a rare autoimmune disease, spastic colon, and to depress the effect on the spinal chord of tetanus; eye surgeons use it to treat glaucoma and corneal ulcers.

It is rumoured that the Iraqis manufactured a chemical weapon from Calabar in their war with Iran.

 

Next dangerous plant       Gelsemium

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