A peanut flower pushes its fertilised ovary into the soil to ripen
After a peanut flower is fertilised, a stalk at the base of its ovary lengthens into a thin peg that grows down into the ground. The pod then develops underground, a habit called geocarpy. It is rare among legumes, which is why Linnaeus named the species hypogaea, Greek for under the earth.
Despite the name, a peanut is not a nut in the botanical sense but a legume, a member of the pea family, like beans. The plant is a bushy annual 30 to 50 centimetres tall, with yellowish-orange flowers and leaves that fold up at night. Pods run 3 to 7 centimetres and usually hold one to four seeds. Like most legumes, it houses nitrogen-fixing bacteria in its root nodules, which enriches soil and makes it a useful rotation crop.
The cultivated peanut began as an accident. Two wild South American species, probably A. duranensis and A. ipaensis, crossed to form a sterile hybrid; a spontaneous doubling of its chromosomes then made it fertile again. Genetic studies suggest this happened only once, producing a wild peanut still found in parts of north-western Argentina or south-eastern Bolivia, from which farmers selected the domestic crop. Domestication made the plant more compact, with larger seeds, and thousands of local varieties followed.
The oldest pods found so far, from Peru, are about 7,600 years old, though cultivation probably began earlier in moister areas where remains decay. The Moche people depicted peanuts in their art, and Spanish conquistadors found them for sale in the market of Tenochtitlan, known by the Nahuatl name tlālcacahuatl, the source of the Spanish word cacahuate. Spain, especially Valencia, brought the crop into Europe in the 19th century, and traders then spread it around the tropics. In West Africa it largely displaced the Bambara groundnut, another legume that ripens underground.
Nothing goes to waste. Seeds become oil, butter, sauces and sweets; plant tops become silage; the pressed oilcake feeds animals or fertilises fields; and peanut oil turns up in paint, varnish and furniture polish. English alone has given it names from groundnut to monkey nut and goober, a borrowing from Kikongo.
Source: Peanut
