Nature

Onion (Állium cépa)

The onion is a monocot bulbous perennial. It is the most widely cultivated species of the genus Allium, which includes other important species such as garlic. Allium species are among the oldest cultivated crops. Diverse representations in Egyptian artifacts dating to 2700 B.C suggest that onions had been cultivated and in wide use by that time. The present species, A. cepa, is known only from cultivation, but appears to have been domesticated from wild ancestors in the Central Asian mountains. Numerous cultivars have been developed for size, form, color, storability, resistance to pests and pathogens, and climatic adaptations. Cultivars are divided into the Common Onion Group, which contains most of the economically important varieties and the Aggregatum Group, which includes shallots and potato onions, and typically produce clusters of small bulbs.
Onions are widely used on cooking in nearly all regions of the world, and have been used in diverse cultures and rituals throughout history.

Onions produce various sulfur-containing compounds probably for defense against fungi and insects, that, together with their breakdown products, produce their distinctive odor, flavor, and lachrymatory (tear-stimulating) properties. Throughout history, onions have been used in folk medicine for purposes ranging from treating wounds and stomach ailments to treating infertility. Scientific and pharmacological studies since World War II have found evidence that onions or their derived compounds have antimicrobial and antifungal properties, and may also be of benefit in preventing or treating heart disease and atherosclerosis, diabetes, cancer, and possibly asthma.

Despite their benefits to humans, onions are toxic to cattle, cats, and dogs, and, to a lesser extent, sheep and goats. Consumption by these animals of large amounts of onion may lead to anemia and impaired oxygen transport.

Global production of onions in 2008 was second only to tomatoes among horticultural crops: more than 73 million metric tons harvested from 3.6 million hectares. China alone produced more than 20 million metric tons; other leading producers were India, Australia, the United States, Pakistan, and Turkey. Onions have large cells visible under low magnification, so onion tissue is often used in high school science laboratories for learning about microscope use and cell structure.