ANATOMICAL BASIS OF ASEXUAL PROPAGATION

ANATOMICAL BASIS OF GRAFT UNION FORMATION

(Figures 11-5 and 11-6 from Hartmann, Kester and Davies 1990)

Top view of cleft graft

Stage 1 - Callus formation in region of cambium.

Parenchyma cells in young xylem and phloem around the cambium of both the stock and scion divide and produce undifferentiated callus (parenchyma) cells.

Enlarged view showing callus production from parenchyma cells in the young xylem near the cambium.

Stage 2 - Callus bridge formation.

The callus from the stock and scion proliferate, fill the gap between the scion and stock, and intermingle to form a callus bridge.

Stage 3 - Differentiation of new cambium.

Parenchyma cells, in the callus between the cambium of the stock and scion, differentiate into cambium cells, thus uniting the cambium of the stock with the cambium of the scion.

Stage 4 - Formation of secondary xylem and phloem from new cambium.

The new cambium produces secondary xylem on the inside and secondary phloem on the outside, which form a continuum with the xylem/phloem of the stock and scion. This vascular connection allows translocation between the stock and scion.

ANATOMICAL BASIS AND TIME COURSE OF BUD UNION FORMATION

(Fig. 11-11 and p. 319 from Hartmann, Kester and Davies 1990)

TIME COURSE OF BUD UNION FORMATION

(from Hartmann, Kester and Davies 1990, Mendel Palest. J. Bot. 1(2):13-46)

EVENT TIME AFTER BUDDING

1. First cell division 24 hours

2. First callus bridge 5 days

3. Differentiation of cambium in callus

a. In the callus of the bark flap 10 days

b. In the callus of the bud piece 15 days

4. First occurrence of secondary xylem formation
a. In the callus of the bark flap 15 days

b. In the callus of the bud piece 20 days

5. Lignification of the callus is complete
a. In the bark flap 25-30 days

b. In the bud piece 30-45 days

LATERAL ROOT INITITATION AND FORMATION

(Fig. 17.12, 17.13 and Plate 15 from Esau 1965)

Lateral roots have an endogenous origin in the pericycle.

A group of pericycle cells opposite the xylem poles undergo periclinal and anticlinal divisions (see A and B to left).

Cell divisions continue to form a root primordia. The endodermis divides by anticlinal divisions to form a protective covering over the young root primordia (see C to left).

Lateral root primordia continue to develop and grow out through the cortex and ruptures through the epidermis. It has the appearance of a fully developed root by the time it emerges.

By the time the lateral root emerges is has developed all the tissue regions of a an apical root.

Parenchyma cell between the vascular cylinder of the parent root and the developing lateral root differentiate into vascular tissue thereby connecting both (see B and C to left).

The endodermis forms a layer over the developing lateral root, but usually is shed upon emergence. By this time the root has a well developed root cap (see A to left).

ORIGIN OF ADVENTITIOUS ROOTS

PLANT	ORGAN	ORIGIN	REFERENCE
Phaseolus mungo (Mung Bean)	hypocotyl	parenchyma bordering phloem	Anzai, 1975
Ficus pumila (Creeping Fig)	stem cutting	phloem ray parenchyma (in adult stem, few in callus)	Davies 1982
Hedera helix (English Ivy)	stem cutting	phloem ray parenchyma (in adult stem, few in callus)	Girouard 1967
Vaccinium corymbosum (High Bush Blueberry)	stem cutting	phloem ray parenchyma	Mahlstede & Watson 1952
Rosa dilecta (Rose)	stem cutting	phloem ray parenchyma	Strangler 1955
Dianthus caryophyllus (Carnation)	stem cutting	pericycle parenchyma just outside primary phloem	Strangler 1955
Pinus radiata (Monterey Pine)	stem cutting	parenchyma of inner cortex	Smith and Thorpe 1975
Tamarix aphylla (Tamarisk)	stem cutting	parenchyma cells of lenticel	Ginzburg 1967
Chrysanthemum morifolium (Mum)	stem cutting	interfascicular region at flank of vascular bundle	Strangler 1975
Begonia semperflorens (Begonia)	stem cutting	-usually interfascicular cambium; -sometimes fascicular cambium at edge of vascular bundle	Smith 1936
Rubus (Black or Red Raspberry)	stem cutting stem cutting	along side vascular bundle, esp. near leaf or branch traces interfascicular parenchyma near vascular cambium	Sudds 1935 Wu and Overcash 1971
Carya illinoensis (Pecan)	stem cutting	newly formed phloem and cortex callus	Brutsch et al 1977
Pinus radiata (Monterey Pine)	stem layer	cambium, phloem and cortex callus	Cameron and Thomson 1969

PHASES OF ADVENTITIOUS ROOT/SHOOT FORMATION

(from Anzai 1975, Dore 1965, Argles 1959)

1a) Physiological Inception

No visual change.

Entails internal physiological changes that are a prerequisite for the subsequent developmental changes. Probably involves hormonal changes, gene deregulation, etc. This is likely the stage that traditional hormonal treatments (rooting powders) have their effect.

b) De-differentiation

Cytoplasm becomes dense and nucleus enlarges.

Partially differentiated cells, usually parenchyma, become less specialized and de-differentiate to a meristematic state that is capable of cell division.

2) Inception or Initiation

From first cell division to formation of a primordia.

Begins with the first cell division, the formation of a group of unorganized and non-differentiated cells, then the formation into a root or shoot primordia.

3) Differentiation

Emerging primordia develops apical zonation and vascular differentiation, and formation of lateral leaf primordia if it is an emerging adventitious shoot.

The emerging organ takes on the morphological characteristics of either a root or a shoot.

4) Elongation

The emerging organ elongates through the parent plant body towards the exterior.

5) Maturation

The organ emerges and continues growth and development characteristic of a typical root or shoot for that particular plant. For all intents and purposes, it behaves like a primary shoot apex or root apex.

Preformed or Latent Root Initials

Some plants have preformed or latent adventitious roots in their stems. Examples are pothos ivy (Epipremnum) and willow (Salix). They develop normally as the stem develops, but their development is usually stopped at Stage 2 or 3. In some plants, such as pothos, they may actually emerge from the stem, but do not develop any further unless the stem touches a substrate or is cut-off the parent plant.

Latent Buds

Many plants have latent buds (shoot primordia) under the bark of their stems. These are the lateral shoots that emerge immediately after pruning a stem, even very large trunks. They will often emerge as suckers over the life of the plant. Most of these are not of adventitious origin, but are latent buds formed when the stem was young and elongate enough each year to keep pace with xylem rings. They occur just under the bark.

ANATOMICAL PHASES OF ADVENTITIOUS ROOT FORMATION IN STEM CUTTINGS OF Hedera helix. (from Girouard. 1967, CJB 45-:1883)

Early Phase 2 - Inception

First anticlinal division in phloem ray at day 6. ic=interfascicular cambium, p=phloem

Late Phase 2 - Inception

Continued cell divisions to create a region of cell division at day 8. ic=interfascicular cambium, p=phloem

Phase 3 - Differentiation

Root primordia organized, with epidermal/root cap covering, and elongated cells at the base indicating differentiation of vascular tissue, at day 10.

Phase 4 - Elongation

Fully organized root apex emerging from parent root at day 12.

ANATOMICAL PHASES OF ADVENTITIOUS ROOT FORMATION IN AIR LAYER OF PINE (from Cameron and Thomson, BG 130:242 1969)

Initial Cut

Exposing xylem (x), cambium (c), phloem (ph), and cortex (co).

Late Phase 2 - Inception

Meristematic region (cm) forms in callus (ca).

Phase 3 - Differentiation

Xylem forms in callus (cx) behind callus meristem (cm), tn=tracheid nest

Phase 4 - Elongation

Continued development of root primordia (rp) and elongation through callus.

ORIGIN OF ADVENTITIOUS SHOOTS

PLANT

ORGAN

ORIGIN

REFERENCE

Saintpaula ionantha

(African Violet)

leaf petiole

leaf lamina

epidermal cells

epidermal cells, but sub-epidermal cells may also contribute

Naylor and Johnson 1937

Sedum stahii

(Sedum)

leaf petiole

or lamina

petiole epidermis and sub-epidermal parenchyma and peripheral callus

Yarbrough 1936

Begonia xhiemalis

(Rieger Begonia)

leaf

petiole

epidermal and sub-epidermal parenchyma

Mikkelson and Sink 1978

Lilium longiflorum

(Easter Lily)

bulb

scale

epidermal and sub-epidermal parenchyma

Walker 1940

Malus sylvestrus

(Apple)

root

ray parenchyma of secondary phloem at border of phloem and cortex

Siegler and Bownan 1939

Convolvus

(Bind weed)

root

pericycle opposite primary xylem poles (i.e. same origin as lateral roots)

Torrey 1958

Tamarix pentandra

(Saltcedar)

root

pericycle near vascular cambium

Wilkinson 1966

ANATOMICAL PHASES OF ADVENTITIOUS SHOOT FORMATION ON LEAF PETIOLE CUTTINGS OF AFRICAN VIOLET (from Naylor and Johnson, AJB 24:673 1937)

View of cut end of petiole showing the endogenous origin of adventitious roots near the vascular bundles, and the superficial origin of adventitious shoots in the epidermis