50

SPECIALIZED VEGETATIVE STRUCTURES

PlSc 300 LAB 7

REFERENCE: Text 551 – 558; 569 – 576; 577 – 584; 587.

OBJECTIVES:

1. Learn to identify various types of specialized structures, their parts, and their methods of

propagation.

2. Practice some of the more specialized propagation techniques (most special structures can be

propagated by simple separation or division).

INTRODUCTION: Some herbaceous perennial plants possess modified parts which serve both

as food storage and for vegetative reproduction. When the plant can be multiplied by naturally

detachable parts (e.g., cormels), the process is called separation; when plant parts must be cut

(e.g., tubers), the process is called division.

Bulbs

A bulb is a specialized, underground organ consisting of a short, fleshy, usually vertical, stem

axis (the basal plate) bearing a growing point or flower primordium at the apex. Most of the

bulb consists of thick, fleshy, modified leaves (either lamina or scales). Tunicate bulbs have

outer modified leaves, which are dry and paper thin. Non-tunicate or scaly bulbs lack this

protective (papery) covering and are more easily damaged. Meristems (lateral buds) develop

between the scales and stem axis to form bulblets, known as offsets when grown to mature size.

For lilies, bulblets which form in leaf axes on the underground or aerial portions of the stem are

called stem bulblets and bulbils, respectively.

51

Tunicate, spring flowering bulbs include the tulip, daffodil, hyacinth, bulbous iris and many

other species. All but the hyacinth are commercially propagated by offsets. Hyacinth is

propagated by various methods of basal cuttage. Tulip and bulbous iris mother bulbs

disintegrate after flowering leaving only offsets behind, whereas daffodil bulbs flower year after

year continually making new offsets.

Non-tunicate, summer flowering lilies (many species exist) can be propagated by bulbils,

stem bulblets, offsets, stem or leaf-bud cuttings or scaling depending on the species and speed of

multiplication desired.

Corms

A corm is a swollen base of the main stem of a plant, enclosed by dry scalelike leaves called

the tunic. The bulk of the corm is solid food storage tissue. Growth and flowering uses up food

(starch) in the original corm, and it disintegrates. A new corm develops as the product of

photosynthesis after flowering. In addition, between the old and new corms, small cormels

develop which then are separated and grown one or two years to reach flowering size. Examples

are gladiolus and crocus.

Stolons

These specialized, horizontal, lateral shoots grow on or just below the surface of the soil.

Mint is an example.

Tubers

Tubers are structures that develop below ground when food materials accumulate in the

subapical portions of stolons. Freshly harvested tubers are dormant for 6 to 8 weeks; after this

time, eyes (lateral buds) will grow out (even in storage). Tubers are propagated by division.

Each ―seed‖ (cut piece of the tuber) should contain a bud (eye), and weigh between one and 2 oz

52

(28 to 56 g). Before planting, ―seed‖ should be dried for 2 to 3 days so that suberization can seal

off the cut surfaces and protect against decay.

Runners

Runners are specialized stems developing from leaf axils at the crown. Runners grow

horizontally along the ground forming new plants at particular nodes. Many plants that form

runners are rosette type plants, e.g., strawberry and airplane (or spider) plant. Propagation

occurs naturally, after which daughter plants can be transplanted. Runner is strictly a

horticultural term; botanically they are simply classified as stolons.

Rhizomes

These types of structures make up the main axis of plants, unlike stolons which were only

lateral shoots. They are specialized stems which grow horizontally on or just below the ground

surface. Rhizomatous plants include ginger, bamboo, sugar cane, banana, iris (rhizomatous),

lily-of-the-valley, low bush blueberry, many orchids and many grasses. Rhizomes can be

pachymorph (e.g., ginger) which are thickened and determinate, or leptomorph (e.g., lily-of-the-

valley) which are slender, indeterminate and spreading. Intermediate types are called

mesomorphs. Propagation is normally by division at the beginning or end of the growth period.

Culms (upright growing shoots usually on bamboos) are cut off at the point of attachment to the

rhizome and transplanted to a new location. Many leptomorph rhizomes can be cut into pieces;

each piece will produce adventitious roots, and shoots will develop from buds which exist at

each node.

Tuberous Roots and Shoots

Thickened underground structures containing stored food and are excluded from all of the

above categories, can be tuberous roots (dahlia), fleshy roots (sweet potato), or enlarged stem

53

tissue (tuberous shoots - tuberous begonia, cyclamen, and gloxinia). Propagation is by

adventitious shoots or division. In the former, fleshy roots are placed in warm, moist, sand until

slips (new adventitious shoots) are produced. As the slips grow, additional sand can be piled

around them until they are well rooted and can be transplanted into the field. Cutting the fleshy

root in half, and/or heating it, will increase slip production. In the case of division, a shoot bud

must be included with each tuberous root or root section. Stem, leaf, or leaf-bud cuttings will

also successfully propagate many of these plants.

Offsets

This term can represent lateral shoots which develop from the base of the main stem, lateral

shoots which arise on the stem of monocots (date palm), or lateral shoots arising from rhizomes

(banana). Propagation is by division.

Crowns

Crowns are also propagated by division. Many herbaceous perennial plants and some woody

shrubs have many shoots clumped together at the crown, which can be divided.

PROCEDURES AND RESULTS:

A. Display. A collection of various specialized structures will be available. Become very

familiar with these and be able to identify their parts and methods of propagation as you may

see them again on the lab practical. You are not required to sketch them, but it might be

helpful to do so on the blank page that follows.

B. Basal cuttage of hyacinth bulbs. This technique works best with fresh bulbs in late summer.

The bulbs we have for spring semester were for early fall sale and are by now of low vigor

and quite susceptible to rotting. Hands must be kept clean, tools sterile and cut surfaces must

54

be rinsed with a dilute bleach solution. Cut bulbs

need to suberize or they will rot. They are usually

left one week in open air at room temperature before

being placed in a warm, dark environment

conducive to bulblet formation.

Each person is to take one hyacinth bulb and

complete one of the scoring procedures shown in

the diagram. Get together with members in your

group, and each member should try a different type

of scoring. For the lab report, tell which type of

basal cuttage induced the most bulblet formation.

Why? Keep in mind that you may have to identify basal cuttage on the lab practical.

C. Scaling of non-tunicate bulbs.

1. Each person should complete scaling propagation individually.

2. Remove 15 scales from one lily bulb.

3. Place five in a small plastic bag with moist (not wet!) vermiculite.

4. Break the other five scales in half, placing the upper (distal) and lower (basal) halves in

two more plastic bags of vermiculite. Be sure to label the bags distal and proximal

(basal).

5. With the remaining five scales, you can test one of the following variables to determine

its effect on bulblet formation.

a. apply Hormex #1 or RooTone to cut or broken surface of scale

b. plant scales in an inverted position (plant in a pot with moistened vermiculite)

55

c. plant scales in a horizontal position (plant in a pot with moistened vermiculite)

d. cut the scales in half rather than breaking them

e. your own idea

6. Place the baggies that contain the bulb scales on the shelf and examine them periodically

for bulblet formation.

7. After the bulblets are well developed, you should pot them up (don’t detach them from

the mother scale the first year) so that their green leaves can grow out. Plant the new

bulblets just below the soil line. Record your data below, and explain your results. How

did your own idea affect bulblet formation?

Scale Treatment Ave. No. of Bulblets per Scale

Entire

Distal halves

Proximal (basal) halves

Notes:

D. Sweet potato slip production. Sweet potatoes are fleshy roots, which can produce

adventitious shoots in a manner analogous to the root cuttings we worked with in Lab 4.

Although shoot initials are preformed (present in sweet potatoes at the time they are dug

56

from the field), polarity can affect how many grow out. Division, increases shoot production.

Complete this procedure as a group. Cut the fleshy root in half (make a radial cut

through the ―round‖ part of the root).

Bury two halves of sweet potato in perlite in a 6-inch pot. The surface (peel) of the

potato must be at least 1.5 to 2 inches below the medium surface so the base of the new slips

will be able to root. Label and place in the greenhouse. To minimize chances of rotting, do

not water the medium until after two days. Only add water when the medium feels dry.

Comment briefly on slip formation and numbers from each half. Did the slips root?

E. White potato ―seed‖ propagation. Tubers are actually propagated by stem cuttings. They are

chopped up, and any stem piece with a lateral bud will grow a new shoot which roots as it

comes up through the soil. If sweet potato pieces are root cuttings, then white potato ―seeds‖

are cane cuttings (remember those?).

Complete this procedure as a group. Chop a tuber into 1 to 2 oz. Pieces. You may share

a tuber with other groups. Plant* several pieces in a 6 inch pot of sunshine medium. Again,

they must be at least 1.5 to 2 inches below the soil surface so that the new shoots can root as

they grow out. Label, and place in the greenhouse. Water thoroughly two days after

planting. Water again only when the potting medium is dry.

Comment briefly on plant formation. Did new shoots root?

* A good procedure is to let cut surfaces suberize for several days before planting. Sweet potato slips are

transplanted, but white potato ―seed‖ is sown directly into the field.

57

Sketches

(optional—not for grading)

c:\plsc300\lab7 specialized vegetative structures-12.doc