Bulb is the underground stem which is conical in shape and gives rise to adventitious roots towards lower side. It produces fleshy scale leaves surrounding an apical bud. It produces the leafless stem, the scape, in favourable season. In this case, food is stored up in the fleshy scale leaves.
There are two types of bulbs based on the arrangement of fleshy scale leaves: tunicated bulb and imbricate bulb. The former possesses a number of concentric scale leaves scale leaves as found in onion (Allium cepa) and in the latter the scale leaves are not concentric but are arranged loosely overlapping one another only at the margins, e.g., garlic (Allium sativum). In garlic, the fleshy scale leaves are separate and said to be cloves being arranged in naked bulbs.
It is more or less rounded in shape, a condensed vertical root soack with a large apical bud. It is often somewhat flattened from top to bottom, e.g., Amorphophallus campanulatus. It has nodes, internodes and scale leaves with numerous buds and adventitious roots on its body.
It stores up the food material. At the time of development of the apical bud it gradually consumes the food material within the corm for which corm shrivels up.