Calabar bean

Physostigma venenosum

A perennial woody climber, calabar bean has a twining vine that can extend more than 50 feet long. The three leaves are large and pinnate. Flowers are purple. Fruits are dark brown pods containing two to three brownish black, kidney-shaped seeds about an inch long.

The line between a deadly poison and a beneficial medication is often very fine. Exactly how fine is illustrated with terrible clarity by the fruit of a twining woody vine native to the area of Africa once known as calabar. The calabar vine roots in the riverbanks and sends its stem climbing 50 to 60 feet high among the trees. Long clusters of its big purple flowers hang in spring like bloated wisteria blossoms. After the flowers have fallen, 6-inch-long, brownish-yellow seedpods develop, and when they ripen they split, revealing two or three fat maroon seeds called calabar beans.

Until the past century or so, the calabar bean served the people of the region as a sort of botanical judge, jury, and executioner. Someone accused of a serious crime was given a drink made from the powdered beans. If he regurgitated the concoction, he was presumed innocent; if it killed him, he was guilty. It was a simple and direct system of justice, but when the British colonized the area, they disapproved. Hoping to end the practice of trial by ordeal, they banned the cultivation of calabar beans. The vines grew wild, however, and so the measure had little effect. In the mid-19th century they outlawed the practice altogether.

The potency of the beans remained a mystery. The beans contain several alkaloids, the key one being physostigmine, a powerful stimulant of muscular contractions. It kills by contracting the heart, diaphragm, and pulmonary muscles to the point of rigid paralysis. Because of this property, the medication has proved useful in reversing the action of such poisonous substances as atropine and curare, which kill by relaxing the same muscles. Doctors have also used physostigmine and its synthetic counterpart, neostigmine, to stimulate intestinal muscles after the shock of surgery and to counteract the symptoms of the serious muscle-weakening disease myasthenia gravis. Today, the medication's major applications are in the field of ophthalmology: it is a miotic, causing the pupil of the eye to contract; and it helps to reverse the buildup of pressure inside the eye that can lead to blindness in the disease glaucoma.

Other names

• Ordeal Bean

Parts used

Seed.

Uses

Infamous in Africa, the poisonous seeds of the calabar bean were once used as an "ordeal poison" to identify a witch or a person possessed by evil spirits. Made to ingest several beans, the accused was proclaimed innocent only if he or she regurgitated the beans and survived. Western settlers captured by native people and forced to undergo this ordeal soon learned that swallowing the seeds whole prevented the poisons release. Although outlawed in Africa, the calabar bean ordeal is still practiced in some tribal rituals. Chemicals derived from the bean have been used in agricultural insecticides and chemical warfare nerve gases. A variant was used in the Persian Gulf war.

Calabar beans principal constituent is the toxic alkaloid physostigmine, which helps prolong the activity of acetylcholine, a neurotransmitter that stimulates gastric secretion, contracts smooth muscles, and dilates blood vessels. Chiefly employed to treat the eye, physostigmine is used to contract the pupil and drain excess fluid, which helps prevent optic nerve damage and loss of vision symptomatic of glaucoma. It is also used to reverse the central nervous system toxicity of anticholinergic medications and to stimulate the intestinal tract. Calabar bean is currently being studied for the treatment of Alzheimer's disease.

Habitat & cultivation

Calabar bean is indigenous to western Africa in an area around Nigeria once known as Calabar. Calabar bean is also cultivated in India and parts of South America.

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