The common onion has one or more leafless flower stalks that reach a height of 75–180 cm (2.5–6 feet), terminating in a spherical cluster of small greenish whiteflowers. Some flower clusters producebulbils, tiny secondary bulbs that can be used to asexuallypropagate new plants. The concentric leaf bases of the developing plant swell to form the underground edible bulb. Most commerciallycultivated onions are grown from the plant’s small black seeds, which are sown directly in the field, but onions may also be grown from small bulbs or from transplants. Onions are very hardy and can survive in a wide range of growing conditions. The bulbs vary in size, shape, colour, and pungency, though warmer climates generally produce onions with a milder, sweeter flavour than do other climates. The onion’s characteristic pungency results from thesulfur-richvolatile oil it contains; the release of this oil during peeling or chopping brings tears to the eyes.