Bulbs are a primary example of a group of plants called geophytes that produce specialized under-ground structures that function as storage organs and permit the plant to survive adverse conditions.
Geophytes include bulbs, corms, tubers, tuberous roots, tuberous stems, and rhizomes.
These are often lumped together as flowering bulbs by gardeners, but each of these geophytes has a distinctive anatomy that impacts its propagation.
A bulb is a specialized underground organ consisting of a short, fleshy, stem axis (basal plate), bearing at its apex a growing point or a flower primordium enclosed by thick fleshy scales.
Bulbs are mostly produced by monocots. Most of the bulb consists of bud scales that act as reserve food storage while the bulb is dormant.
In the center of the bulb is either a vegetative or flower meristem.
There are two types of bulbs - Tunicate or Non-tunicate.
Tunicate (laminate) bulbs have an outer layer of bulb scales that are dry and papery.
This covering provides protection from drying and mechanical injury to the bulb.
The inner bulb scales are fleshy and give the bulb its solid feel.
An assortment of popular tunicate bulbs.
Tulip is a common example of a tunicate bulb.
It has a papery outer layer covering the storage scales tightly appressed to each other.
The main stem with the leaves and flower emerges through the center of the bulb.
Note the offsets (arrow) developing from the basal plate at the bottom of the bulb.
Non-tunicate or scaly bulbs lack the papery tunicate found in tunicate bulbs like tulip.
The non-tunicate bulb consists of separate "scales" attached at the basal plate.
In general, non-tunicate bulbs are easily damage and must be handled more carefully than tunicate bulbs.
Scales are modified leaves and these can be removed and handled as leaf cuttings where they form new bulbs at the cut surface of the scale.
Lily bulbs display two types of root systems. Adventitious roots develop from the new stem and act to absorb water and nutrients.
Contractile roots develop from the base of the bulb. Contractile roots function to readjust the depth of the bulb in the soil.
By shrinking and expanding the contractile roots pull the bulb to the proper depth in the soil.