Today's lab will deal with
propagation by specialized vegetative structures (modified stems and modified
roots). During this lab the specialized vegetative structures that we will be
studying are bulbs, corms, tubers, tuberous roots, rhizomes, and pseudobulbs.
A.
Specialized
Reproductive Structures
B.
Bulbs
C.
Corms
D.
Tubers
E.
Tuberous
roots
F.
Rhizomes
G.
Pseudobulbs
H.
Laboratory
Exercise Outline
I.
Laboratory
Questions
Certain herbaceous perennial
plants possess modified plant parts in which food storage tissue is coupled
with protective tissue and meristematic tissues to produce a specialized
reproductive "structure". The
purpose of such specialized "structures" are to carry the plant over
a dormant (environmentally unsuitable) period in a protected condition, and to
provide a means of vegetative (asexual) reproduction. Vegetative propagation is a separation of detachable plant
structures or by division of the plant into sections. The major types of specialized stems and
roots utilized in asexual propagation are: bulbs, corms, tubers, tuberous
roots, rhizomes, and pseudobulbs.
The specialized vegetative
structures that we will be studying during this lab can be divided into specialized
stems or specialized roots.
Specialized (modified) stems Specialized (modified) roots
Bulbs (more than just a modified stem) Tuberous roots
Corms
Tubers (& tuberous stems)
Rhizomes
Pseudobulbs
Plants producing these specialized vegetative structures can be
propagated in two ways:
1. Sexual (seed) - except for a few cases of specific
plants, sexual propagation is not
widely used except by breeders.
a. Cultivars of plants having specialized vegetative structures do not usually come true from seed.
b. Time required to
get plant to blooming or marketable size.
2. Asexual - for
most species is of great importance commercially
a. Separation
- production of naturally detachable structures, e.g. bulb offsets.
b. Division - cutting plant parts into sections (e.g. tuber -
white or Irish potato)
1. Tunicate
(laminate) bulb - e.g. onion, tulip, daffodil, amaryllis
a. Tunic - Outer bulb scales:
dry and membranous. The tunic provides protection from drying and mechanical
injury.
b. Bulb scales -the fleshy scales are in continuous, concentric layers or lamina so that the structure is more or less solid.
c. Cross section of the daffodil bulb.
1. Consists of bulb scales:
morphologically are the continuous sheathing leaf bases.
2. The outer scales are
generally fleshy and contain reserve food materials.
3. Scales toward the center function less as storage organs and are
more leaf-like.
d. Longitudinal section of the daffodil bulb.
2. Non-tunicate (scaly) bulbs - e.g. lily
a. These bulbs do not posses the enveloping dry covering.
b. The scales are separate and attached to the basal plate.
In general, the non-tunicate bulbs are easily
damaged and must be handled more carefully than tunicate bulbs; they must be
kept continually moist because they are injured by drying. Root primordia are present at the time of
harvesting. They do not elongate until
planted under proper conditions.
Thickened contractile roots are produced in many species of
scaly bulbs that shorten to pull the bulb to a given level in the soil. This is a very important mechanism for
survival.
3. Propagation of bulbous species -
a. Sexual (seed) - as mentioned earlier except for a few cases where
used primarily by breeders.
b. Asexual (vegetative) methods
1. Offsets (lateral)
bulblets - daffodil, tulip
2. Bulblet formation on
intact stems.
a. Underground stems
bulblets - lily
b. Aerial stem
bulblets (bulbils) - tiger lily
c. Cuttings- Instead of roots
and shoots forming on the cuttings as would occur in other plants, bulblets
form in the leaf axils and then produce roots and small shoots while still
attached to the cutting.
1.
Stem
cuttings – lilies
2.
Leaf-bud
cuttings – lilies
3.
Leaf
cuttings - grape hyacinths
d. Bulblet formation on scales (scaling) - lily
1. Propagation of Easter lily by scaling
Easter lilies can be propagated by a procedure known
as scaling in which individual bulb scales are separated from the mother bulb
and placed in growing conditions so that bulblets form at the base of each
scale. One to three bulblets will
development from each scale. This
method is particularly useful for rapidly building up stocks of a new cultivar
or to establish pathogen-free stock. Almost any lily species can be propagated
by scaling.
2. Twin-scaling bulbs
Twin-scaling is a method for rapid multiplication of
certain bulbous plants. The procedure
involves cutting a bulb into many pieces, each with a portion of the basal
plate attached.
3. Basal cuttage
The hyacinth bulb continues to increase in size each
year, but because the number of offsets produced is small, scooping or scoring
is used.
a. Scooping
The entire basal plate is scooped out using a
special curve-bladed scalpel, a round bowled spoon, or a small-bladed
knife. Adventitious bulblets develop
from the base of the exposed bulb scales (actually bulblets develop from callus
which forms at the bases of the exposed scales). Depth of cutting should be
deep enough to destroy the main shoot.
b. Scoring
Two or three straight knife cuts are made across the
base of the bulb each deep enough to go through the basal plate and the growing
point. Growing points in the axils of
the bulb scales begin growing into bulblets.
The depth of each cut should be sufficient to kill
the main flower bud at the apex of the basal plate and if done properly the
cuts should reach into the bulb at its widest point.
4. Tissue culture - lilies, hyacinth, etc.
1.
Dry
leaf bases which persist at each of the nodes and enclose the corm are known as
the tunic. The tunic protects it
against injury and water loss.
2. At the apex of the corm is a terminal shoot which will develop into
the leaves and flowering shoot.
3. Two types
of root systems are produced by the corm:
a. A fibrous system developing from the base of the mother corm.
b. Enlarged, fleshy contractile roots developing from the base of the
new corm - these roots maintain the corm at a particular depth in the soil,
they are very important for survival.
Examples: Irish (white) potato Solanum tuberosum
Jerusalem
artichoke Helianthus tuberosus
Caladium Caladium hortulanum
1. A
tuber has all the parts of a typical stem:
a. The "eyes" are present in regular order over the surface
and represent nodes, each consisting of one more small axillary buds subtended
by a leaf scar.
b. The arrangement of the nodes is a spiral, beginning with the
terminal bud on the end opposite the scar resulting from the point of
attachment to the stolon.
c. The terminal bud is located at the apical end of the tuber oriented
farthest (distally) from the crown of the plant.
2. Propagation of tuberous
species
a. Sexual (seed) - except for a few species sexual propagation is used
primarily by plant breeders
b. Asexual (vegetative)
1. Plant whole
tuber (separation)
2. Division - cut tuber into pieces - each containing one or more buds "eyes" - the pieces of the tuber used for propagation are commonly referred to as seed. Also, the large tubers used for this purpose are also referred to as seed.
1. Certain species of herbaceous perennials produce thickened
underground structures which contain large amounts of stored food.
2. In certain plants such as sweet potatoes (Ipomoea batatas) and
Dahlia sp. these thickened structures are true tuberous roots with external and
internal structures of roots.
3. Comparison of tubers and tuberous roots
a. Tuberous roots lack nodes and internodes.
b. Buds are present only on the crown or stem (proximal) end of
tuberous roots.
c. Fibrous roots are commonly produced only on the opposite (distal)
end of tuberous roots.
d. Polarity of a tuberous root is the reverse of that of the true tuber.
Note: Root cuttings form roots at the distal and
shoots at the proximal end.
4. Propagation of plants producing tuberous
roots
a. Sexual (seed) - as mentioned earlier, except for a few species is
used primarily by breeders.
Seed is widely used for
propagation of dwarf (miniature) dahlias.
b. Asexual (vegetative)
1. Adventitious shoots - e.g. slips on sweet
potatoes
2. Division - used experimentally with the sweet
potato and also used on dahlias.
Dahlias - division is used; this involves dividing
the crown so that each section bears a short bud.
3. Stem, leaf or leaf-bud cuttings - cuttings
develop tuberous roots at their bases.
4. Tissue culture
1.
Rhizome
- a specialized stem structure in which the main axis of the plant grows
horizontally at or just below the ground level.
2.
A
number of economically important plants such as bamboo, sugar cane,
banana, and many grasses, as well as a number of
ornamentals are rhizomatous in nature.
Rhizomatous ornamentals - Iris, Lily-of-the-valley,
Sansevieria (snake plant) and the Canna.
Three general types of rhizomes are found:
a. Pachymorph - e.g. rhizomatous Iris and Ginger - the rhizome
is fleshy, thick and shortened in relation to length. Plants of this nature grow in a clump.
It is determinate; each clump terminates in a
flowering stalk, growth only continuing from lateral branches.
b. Leptomorph - e.g.
Lily-of-the-valley - The rhizome is slender with long internodes. It is indeterminate; it grows continuously
in length from the terminal apex and from lateral branch rhizomes. This type does not produce a clump but
spreads extensively over an area.
Indeterminate plant - main axis vegetative, in which flowers form in axillary
buds, e.g. cucumber.
Determinate plant -
main axis terminates in a flower stalk, e.g. sweet corn.
c. Mesomorph
- intermediate between 1 and 2. Some
bamboo species.
3. Propagation of rhizomatous species
A. Sexual
(seed)
B. Asexual
(vegetative)
1. Division (clumps and rhizomes)
a. Clumps
Pachymorph - individual sections (or
culms) are cut off at the point of attachment to the rhizome, the top is
cut-back and the piece with the cut-back top is transplanted.
Leptomorph
- individual offshoot is removed.
b.
Rhizome - cut in sections, being sure each piece has at least one lateral bud
or eye - is essentially a stem cutting.
c.
Culm
(pronounced kulm)- in large rhizome-bearing plants such as bamboos, the
aerial shoot or culm may be used as a cutting.
These may be whole culm cuttings in which the entire shoot or culm is laid
horizontally in a trench. New branches
arise at the nodes. Or, a stem cutting of 3-4 node sections may be planted
vertically in the ground.
C. Tissue culture
1. Pseudobulb - a specialized storage structure, produced by many
orchid species, consisting of an enlarged fleshy section of the stem made up of
one to several nodes - In general, the appearance of the pseudobulb varies with
the orchid species. The differences can
sometimes be used to identify species. Pseudobulbs arise during the growing
season on upright growth which develops laterally or terminally from the
horizontal rhizome. Leaves and flowers from either at the terminal end or at
the base of the pseudobulb depending on the species. During the growth period
they accumulate stored food materials and water and assist in surviving dormant
periods.
2. Propagation
of orchids by pseudobulbs
a. Offshoots - used in propagation of Dendrobruim species.
The pseudobulb is long and jointed being made up of
many nodes. Offshoots develop at these nodes and from the base of these
offshoots roots can develop. The rooted offshoots are cut back from the parent
plant and potted.
b. Division - many important species of orchids, including Cattleya,
Laelia, Miltonia, and Odontoglossum may be propagated by dividing
the rhizome into sections.
Division is accomplished during the dormant season,
and preferably just before at the beginning of a new period of growth.
The rhizome is divided into sections to include 4 to
5 pseudobulbs in the new section, leaving the old rhizome section with a number
of old pseudobulbs or back bulbs from which the leaves have abscised.
The section is potted when growth begins from the
bases of the pseudobulbs, and at the nodes.
Removal of the new section of rhizome from the old part stimulates new
growth or "back breaks" to occur from the old part of the
rhizome. These new growths grow for a
season and can be removed the following spring.
c. Back bulbs and green bulbs.
Back bulbs - those
without foliage.
Green bulbs - those with
foliage.
Back bulbs are
commercially used to propagate clones of Cymbidium.
The back bulbs are removed from the plant, the cut
surface is painted with a grafting compound and they are placed in a rooting
medium for a new shoot to develop. When
a rooted shoot has developed it is removed and potted. This back bulb can again be propagated and a
second shoot developed from it. Green bulbs (those with foliage) can also be
used in propagation. Treatment with IBA either by soaking or by painting with a
paste has been shown to be beneficial.
Exercise 1. Practice scooping and scoring on the
hyacinth bulbs provided by your lab instructor.
a. Scooping:
The entire basal plate is scooped out using a
special curve-bladed scalpel, a round bowled spoon, or a small-bladed
knife. Adventitious bulblets develop
from the base of the exposed bulb scales (actually bulblets develop from callus
which forms at the bases of the exposed scales). Depth of cutting should be
deep enough to destroy the main shoot.
b. Scoring
These straight knife cuts are made across the base
of the bulb each deep enough to go through the basal plate and the growing point. Growing points in the axils of the bulb
scales growing into bulblets.
The depth of each cut should be sufficient to kill
the main flower bud at the apex of the basal plate and if done properly the
cuts should reach into the bulb at its widest point.
AFTER TREATMENT:
a. dust bulbs with hydrated lime/fungicide mixture. Wear gloves and
apply this mixture outdoors. Do not handle bulbs without gloves after
treatment.
b. Callus bulbs at about 21°C (70°F) for a few days to a few weeks in dry sand
or soil in open trays cut side down.
c. After callusing, the bulbs are incubated (up side down) in trays or
flats in dark or diffuse light at 70°F which is increased to 85
to 90°F (29.5-32°C) over a 2 week period and
held at high humidity (85% for 2 ½ to 3 months).
d. The mother bulbs are planted (right-side up) about 4 inches deep in
nursery beds in the fall. The next
spring bulblets produce leaves profusely.
Normally, the mother bulb disintegrates during the summer. Annual digging and replanting of the graded
bulbs is required until reach flowering size.
Exercise 2. Propagation of Easter lily by scaling
Easter lilies can be propagated by a procedure known
as scaling in which individual bulb scales are separated from the mother bulb
and placed in growing conditions so that bulblets form at the base of each
scale. One to three bulblets will
develop from each scale. This method is
particularly useful for rapidly building up stocks of a new cultivar or to
establish pathogen-free stock. Almost
any lily species can be propagated by scaling. There are several modifications
of this procedure. However, each
student is instructed to do the following.
Treatment:
a. Carefully break off healthy outer scales from an Easter lily bulb.
b. Soak in fungicide (Benlate) for 30 min. (1 oz. per 1 ½ gallon
water). Place scales in cheesecloth and wear gloves!
c. Space the scales well apart in slightly damp vermiculite in a
plastic bag.
d. Seal the bag and store it at a constant temperature of 20 -22°C (68 -72°F) for 6-8 weeks.
e. After storage for 6-8 weeks, the scales will be removed and various
data taken. These data will be recorded
on the attached sheet and the sheet turned in.
f. After the bulblets develop on the scales, they may need at least 8
weeks of chilling at 2 -4°C (35 -40°F) to overcome dormancy. This can be accomplished by placing the bags
in a refrigerator or potting the bulblets and storing them in a cold
frame. After the chilling requirement
has been satisfied, the bulblets are ready for planting outdoors.
Exercise 3. Twin scaling (Easter Lilies or
Daffodils)
Treatment:
a. Cut off the "nose" of the bulb and then cut downwards
through the bulb to make 4 or 6 segments.
Placing each segment on its side, slice through the base plate to
separate scales in pairs.
b. Benlate soak for 30 min. Place in cheesecloth and wear gloves!
c. Mix segments in moist vermiculite.
d. Store in sealed polyethylene bags.
e. Normal bulbs increase by a factor of 1.6 per year, so that it takes
about 15 years to produce 1,000 bulbs from one individual, with twin-scaling on
2 occasions the period could be reduced to 8 years.
Exercise 4. Division of Rhizomes
Many rhizomatous species are propagated by division
of the rhizomes. You will have an opportunity to divide several rhizomatous
species and plant the propagules in 1 quart pots. The procedure involves
cutting the rhizome into small pieces, each containing several buds and then
planting these divisions.
I. Laboratory Questions - Unit 6
1. Outline the steps necessary for successful propagation of hyacinths
by scooping or scoring (following the actual scooping or scoring operation) (5
points).
2. Why do
daffodil and tulip bulbs generally cost less than hyacinth bulbs? (3 points)
3. With the exception of plant breeders why are tulip cultivars
propagated commercially using asexual (vegetative) techniques? (3 points)
4. Describe how an individual could determine the proximal and distal
ends of an Irish pot ato. (3 points)
5. When propagating orchids by pseudobulbs, how does one distinguish
between a "back bulb" and a "green bulb"? (3 points)
6. Examine, sketch and label an external view and longitudinal section
of a tunicate bulb (5 points) and a corm (5 points) using the plant material
provided by your lab instructor. These
sketches will be handed in with the lab questions and graded.