Ricinus communis L. (Euphorbiaceae)


Castor oil plant

Ferdinand Bauer's watercolour of Ricinus communis, prepared for Sibthorp and Smith's 'Flora Graeca' (Sherard MS245, f.34). Male and female flowers of Ricinus communis growing in Oxford Botanic Garden.

A familiar sight in the summer as an 'accent plant' in municipal bedding displays around the country, Ricinus communis, with its striking red-tinted leaves, is also a major economic crop and a killer.

The castor oil plant is one of the few major crops to have an origin in Africa. It has been cultivated for at least 6,000 years; the Egyptians found its seed oil useful in lamps. It is still the seed oil for which this plant is grown commercially today. The seeds, which are about the size of the nail on your little finger, are shiny and dark brown, often with exquisite marbled markings on them. They resemble ticks, for which the plant gets its botanical name, Ricinus, whilst the species name means 'common'. The seeds contain valuable oil and a killer protein.

The killer protein is ricin, and a pellet of this substance was the 'bullet' that killed Georgi Markov, a Bulgarian political dissident, living and working in London in 1978. A miniscule pellet containing ricin was fired into his leg from an umbrella, whilst he was waiting for a bus on his way to work. Ricin is one of the most toxic substances known to man, less than one milligram is needed to kill an adult person. Thankfully, the process of extracting castor oil from the seeds 'denatures' the protein, i.e., its complex three-dimensional structure is unravelled, rendering it harmless.

More than one million tonnes of castor oil are produced each year, for use in applications as diverse as brake fluids, laxatives, and wood and leather preservatives. However, the oil also plays a part in modern medicine, especially in the form of its derivative Cremophor EL. This oil is an excellent solvent; it will dissolve or solubilise almost anything. When scientists were searching for a substance to enable them to formulate the anti-cancer drug paclitaxel, the only solution they found was to use Cremophor EL. Any paclitaxel marketed as Taxol is formulated using this super-solvent. However, it is not without its own toxicity and many patients being treated with this formulation have to be pre-medicated against their response to the Cremophor EL. There are now other less toxic formulations of paclitaxel available. However, if it was not for the castor oil plant, paclitaxel would never have made it through the clinical trials and been licensed for use; thousands of lives may have been lost to cancer.

Further reading

Emsley, J. (2008) Molecules of murder: criminal molecules and classic cases. RSC Publishing, London.

Alison Foster