This killer, and ultimate, edition of Students of Ethnobotany comes from the mysterious E. Green.
One of the draws of botanical research, in my opinion, is the chance that you will discover something extraordinary, some intricate mechanism, adaptation, or chemical that has implications able to expand the scope of human knowledge. However, there are times when less information can be more exciting than a thorough chemical or physiological analysis. These are the instances where investigation into the mystery surrounding a plant actually reveals more about humans than the plant in question.
Take, for example, the Calabar bean, Physostigma venenosum (family Fabaceae). The plant is named for a region in Nigeria where its use was particularly ominous. The bean contains a significant amount of toxic alkaloids, which made it ideal for the practice of “trial by ordeal”. A person convicted of a crime (murder, witchcraft etc.) would be forced to consume the bean. If their body rejected the bean and they survived, it was viewed as a sign of their innocence. If they did not… they must have been guilty, and their sentencing was conveniently already taken care of. This may seem to be pure nonsense to our modern ears, but what is interesting is the fact that many people were surviving. Somehow, certain people were more susceptible to the poison than others. Could there be some link to innocence? What property of the bean enables this?
The plant, although impressive in size (reaching up to 50 feet tall) and striking in appearance (bright red/purple flowers), has a chemical composition that effects every person the same way. The major alkaloid, physostigmine, acts on the nervous system to stimulate muscles continuously, nearly always resulting in the consumer’s death. In all likelihood, the bean itself has nothing to do with the variation in results. Other plants with different chemical composition have been used to similar effect in ordeal trials (eg. the nut from Cerbera odollam, the bark from Eurythrophleum guineense). The suspect for the results could be the manner in which people consume the bean. If eaten with confidence (as in the case of someone convinced of their innocence), the high concentration of toxins would cause the stomach to reject the bean, and a life-saving vomit would occur. If someone was nervous, and stalled the consumption with small bites or delayed swallowing, the toxin would have more time to be absorbed into the bloodstream, and death would thus occur.
This is one of the cases where the “ethno” is more exciting than the “botany” in ethnobotany. But despite the lack of mystery, the “Killer Bean of Calabar” certainly lives up to its name.
Heitz, R.F. 2007. AJO History of Ophthalmology Series: The Ordeal Bean of Calabar. American Journal of Ophthalmology. 44(6): 900. DOI: 10.1016/S0002-9394(07)00941-5.
Spinney, L. 2003. The Killer Bean of Calabar. New Scientist. 178(2401): 48. http://www.lexisnexis.com/hottopics/lnacademic/?verb=sr&csi=158275&sr=HLEAD(The+killer+bean+of+Calabar.)+and+date+is+June+28%2C+2003
Stewart, A. 2009. Wicked Plants. Pp. 18-19. Algonquin Books of Chapel Hill: Chapel Hill, NC.
Zhao, B., Moochhala, S.M. and Tham, S. 2004. Biologically Active Components of Physostigma venenosum. Journal of Chromatography. 812(1/2): 183-192. DOI: 10.1016/j.jchromb.2004.08.031.