A bulb is a specialized form of underground bud containing, in an undeveloped condition, the rudiments of all parts of the complete flowering plant which will develop from it. These include the flower bud and foliage leaves, which are mounted on a thick flat stem bearing the beginnings of the roots on its lower surface.
In addition, the bulb contains large stores of food, either in special swollen scale leaves or in the swollen bases of the previous season’s foliage leaves. The contents are surrounded by a protective, scaly, brown tunic, formed from the exhausted leaf bases or scale leaves of a previous year.
How the Food Supply Is Stored
Such a structure may easily be seen in the cut half of a Daffodil or Narcissus bulb, where the food storage is mainly carried out by swollen leaf bases. In the Tulip bulb the food is stored in special scale leaves only; these never come above the soil and the foliage leaves grow directly on the flowering stem. A mixture of both kinds of storage organs is to be seen in Lily and Hyacinth bulbs.
Bulb Leaves Should Not Be Cut While Green
In general, the bulb, when planted, first grows a crop of roots. Then the leaves and flowers develop, its large stores of food making this early development possible. After flowering, the leaves elongate considerably and make more foodstuffs, which are conserved in their bases or in the scale leaves already referred to. Thus to cut off the leaves before they complete this process and wither naturally is to rob the new bulb of much of its food. The common white Madonna Lily, Lilium candidum, is one of the few hardy bulbs that retains some of its green foliage leaves throughout the winter.
Many other plants generally classed as bulbs, and referred to as such for convenience in planting instructions, are strictly not bulbs at all. The Crocus, Gladiolus, and Montbretia, for example, are quite solid inside, and are technically termed Corms.