A fleshy underground perennating organ formed by many members of the Liliaceae and Amaryllidaceae. It is a highly modified shoot, the bulk of which is made up of colourless swollen scale leaves or leaf bases. The central apical bud contains the immature foliage leaves, the future flower, and rudimentary adventitious roots at its base. It is surrounded by numerous layers of fleshy scales, which may be complete modified leaves or the leaf bases of previous years' foliage leaves. The first sign of growth is the rapid elongation of the adventitious roots. When these have established themselves in the soil the apical bud sprouts and the foliage leaves and inflorescence emerge, growing at the expense of the food reserves in the scale leaves. In some bulbs, e.g. tulip, the inflorescence develops from the apex of the bud and further apical growth is consequently prevented. Photosynthates from the foliage leaves are passed down to one or more lateral buds in the axils of the scale leaves. This is an example of sympodial growth. In other bulbs, e.g. daffodil, the inflorescence develops in the axil of one of the foliage leaves and the apex of the bud remains within the bulb. It persists from year to year, each year giving rise to foliage leaves and a lateral inflorescence. Propagation may be effected by the expansion of a lateral bud in the axil of the outermost scale leaf. Bulbs such as daffodil show monopodial rather than sympodial growth.
The food reserves of the bulb may be starch (as in tulip) or sugars (as in onion). The outermost leaves do not contain food but are thin, brown, and scaly and protect the bulb. Compare corm