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Geophytes
Bulbs
Bulbs are a primary example of a
group of plants called geophytes
that produce specialized underground 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 (see Chapter 15).
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Geophytes
Bulbs
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.
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Geophytes
Bulbs
There are two types of bulbs - Tunicate or Non-tunicate.
Non-tunicate
Tunicate
Lily
Hyacinth
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Geophytes
Bulbs
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.
Flowering onion
Daffodil
Tulip
Hyacinth
Iris
An assortment of popular
tunicate bulbs.
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Geophytes
Bulbs
Tulip is a common example of a
tunicate bulb.
Bulb with papery
tunic removed.
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 (red arrow)
developing from the basal plate
at the bottom of the bulb.
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Geophytes
Bulbs
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.
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Geophytes
Bulbs
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
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Geophytes
Bulbs
Fibrous
adventitious
roots will form
along the
emerging stem.
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.
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Fleshy
contractile roots.
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