Allium cepa is the common onion, although it is usually thought of as a vegetable. Onions are perennials that are cultivated for food worldwide. There are many varieties. Bulbing onions have cylindrical, hollow leaves and an enlarged bulb that develops at ground level. The green stems and leaves can reach 3 ft (1 m) in height. The roots come off the bottom of the bulb. The flowers are produced in the second growing season (following a required "rest" period) in a rounded umbel (cluster with all flower stems originating from the same point) on a stalk 2-4 ft (0.6-1.2 m) tall. The umbels, about 2 in (5 cm) in diameter and consisting of many small purplish flowers, are quite showy. There are two main kinds of onions, based on the day length required for bulb formation. Short-day varieties start forming an enlarged bulb when days are 12 or 13 hours long; long-day varieties don't form a bulb until days are 14-16 hours long. For both types, bulb enlargement is arrested during hot, freezing or dry weather.
708 Allium cepa Cepa Group - Floridata, http://www.floridata.com/Plants/Liliaceae/Allium%20cepa%20Cepa%20Group/708 (accessed January 24, 2017).
Onion | DermNet New Zealand, http://www.dermnetnz.org/topics/onion/ (accessed January 24, 2017).