Some plants adapt the underground stems modification system in which stem that derived from stem tissue exists under the soil surface. The bulb is also a modified stem.
It is a highly condensed disc-like stem its upper surface a terminal bud and fleshy scale leaves are present. Adventitious roots arise from the lower region of the bulb.
The bulb is either tunicated (cover present) or non-tunicated (cover absent). A tunicated bulb is covered by a sheath of dry scale leaves, e.g., onion and garlic.
Non-tunicated bulb lacks the tunic of dry scale leaves; it is called the imbricated naked bulb, e.g. Lily.
