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Plant 155


Dioscorea species (Dioscoreaceae)

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Yam



The genus commemorates Dioscorides, a first-century CE Greek physician and botanist. There are over 600 species in the genus, spread throughout the tropics and subtropics. They are perennials with tuberous roots or caudices (swollen stem bases), climbing stems and small, yellow-green flowers. Most species are dioecious, having separate male and female plants, although a few are monoecious, with male and female flowers on the same plant.

Dioscorea should not be confused with the yams of North America, where Ipomoea batatas is also known as the yam or sweet potato. Several Dioscorea species are grown as agricultural crops for their underground tubers. Dioscorea species are significant subsistence crops in south west Asia, West Africa and Central and South America. The tubers of some species can be up to 1.5 metres in length and care needs to be taken when harvesting them since damage reduces storage life. Dioscorea bulbifera, also known as the 'air potato', has the tubers (bulbils) at the base of the leaves as well as underground. Both types of tuber are harvested. The bulbils drop off and easily start another plant, and so Dioscorea bulbifera comes with a word of warning, as it can be a serious invasive plant and for this reason it is banned in Florida.

Before consumption, the tubers must go through a process of detoxification, by means of soaking, boiling, fermenting or roasting, otherwise many species can be extremely poisonous due to the presence of alkaloids. A few species have been used for arrow poison, murder, suicide and even fishing bait.

In 1943 it was discovered that Dioscorea mexicana, one of the caudiform types of Dioscorea, contained the steroid precursor diosgenin which could be converted into steroids such as progesterone, which made the production of the contraceptive pill affordable.

South African Dioscorea elephantipes is another caudiform yam. The woody, deeply fissured caudex can grow to one metre tall, giving rise to the common name of 'Elephant's Foot'. Six-metre long stems arise from the caudex when in growth. The caudex enables the plant to survive periods of drought in the rocky desert areas where it grows. This species is grown in the Arid House at Oxford Botanic Garden, where watering is much reduced over winter, but it has a little way to go before reaching its full potential. An unnamed species grows in the Palm House, scrambling over other plants, but dying back in the autumn.

Further reading

Huxley A 1999. The new Royal Horticultural Society dictionary of gardening. Groves Dictionaries Inc.

Llamas, AK 2003. Tropical flowering plants: A guide to identification and cultivation. Timber Press.

Oakeley HF 2012. Doctors in the medicinal garden. Plants named after physicians. Gutenburg Press.


Lucinda Lachlin