Allium cepa L. (Amaryllidaceae)



Onion cv Sturon growing in Oxford Botanic Garden. Proliferating inflorescence of Allium cepa from Curtis’s Botanical Magazine (1812; t.1469).

Allium is a large, monocotyledonous genus of over 700 species of mostly perennial bulbs; the majority are found in the northern Hemisphere. Previously classified in its own family, Alliaceae, Allium is now part of the family Amaryllidaceae. While many Allium species, such as Allium circinnatum from Crete, have very narrow distributions and are rare, others provide familiar foods and are economically important on a global scale. Leeks, garlic and chives are all Allium species but the onion is arguably the most famous.

The long history of onion cultivation has obscured the onion's relationships with its wild relatives, which may be Central Asian. There is archaeological evidence of onion use in the Dead Sea area over 5,000 years ago. By the time Pliny the Elder described onions in the first century AD, six distinct varieties were known. Onions are depicted in Egyptian funerary art and have been found in the heads and thoraxes of Egyptian mummies.

In the modern world, the onion is a staple ingredient of cooking in many cultures and 80 million tonnes are produced annually. From the 1830s until about 1950, itinerant Breton onion traders, nicknamed 'Onion Johnnies', provided the English with a stereotypical image of the French that many a Parisian would find utterly baffling.

Sulphur compounds, such as dipropyldisulphide, are responsible for the onion's distinctive savoury taste while the enzymatic breakdown of damaged onion cells releases a pungent volatile gas (syn-propanethial S-oxide) which famously irritates the eyes.

Onions are credited with many medicinal properties and are known to prevent thrombosis and to contain both antifungal and antibacterial agents. In the 1530s, French military surgeon Ambroise Paré investigated the widespread folk-belief onions were useful in the treatment of burns. Paré deliberately treated one-half of a soldier's burned face with crushed onion paste and the other with a more typical remedy; he observed there were no blisters or scars on the onion-treated side compared to the other. This may well be the earliest recorded controlled comparison of different treatments for the same medical condition.

Allium cepa is a biennial plant, flowering in its second growing season. Plants grown from seed are stimulated to form bulbs by shortening day lengths after midsummer. The swollen, overlapping leaf bases provide a store of energy for the overwintering plant. Bulb development can be halted at an early stage with heat treatment to produce the 'sets' that are now popular with home gardeners.

Further reading

Block E 2010. Garlic and other alliums: the lore and the science. Royal Society of Chemistry.

Brewster JL 2008. Onions and other vegetable alliums. CABI.

Donaldson IML 2004. Ambroise Paré's accounts of new methods for treating gunshot wounds and burns. JLL Bulletin: Commentaries on the history of treatment evaluation James Lind Library.

James Penny