Onions are flowering plants that use insect pollination to produce viable seeds. Different types of bees are the main pollinators of onions. Commercial production of onions usually harvests before flowering, however, and instead exclusively uses daughter bulbs to reproduce cultivated onions.
Onions are bulbs of the Allium genus, which they share with garlic, leeks and chives. Scallions, meanwhile, are actually members of the onion species at different stages of development. Shallots are also members of the same species, but are a different variety.
The bulbs of onions are actually modified leaves wrapped around each other. The outer leaves dry out, creating a tough protection for the moist, living leaves beneath. The green leaves that photosynthesize spring from buds in the center of the bulb. The stem is just a flat disc of tissue at the bottom of the bulb from which the roots branch out.
Onions are the most widely used in their group, with garlic at a distant second place at about 10 percent of the production of onions. The white, yellow and red varieties of onion are just different varieties of the same species. Because of their method of cultivation, the vast majority of onions of any one variety grown today are genetically identical — clones.