Many plants in the genus Allium are known by the common name onion but, used without qualifiers, it usually refers to Allium cepa. Onions (usually but not exclusively the bulbs) are edible and possess a distinctive, strong flavour and pungent odour which is mellowed and sweetened by cooking. The bulb comprises fleshy layers of modified leaves, surrounded by papery outer layers. Used worldwide for culinary purposes, an array of onion varieties is available like the Salad Onions also known as Yellow or White Onions and the multipying kind known as Shallots. Tissue from onions is frequently used in school science laboratories because they have particularly large cells which are readily observed under a microscope, even at low magnifications.