Vegetative Reproduction in Angiosperm

It is a process in which a part of the plant body gets detached and develops in to a new independent plant.

In lower plants it occurs through binary fission, budding, fragmentation, gemmae, resting buds, soredia (in lichens) etc.

In higher plants, any part of the body maybe capable of vegetative propagation. Many plants produce modified stems, roots, and leaves, specially for natural vegetative propagation. At the same time, man too has developed various methods of artificial vegetative propagation for many useful plants.

The methods of vegetative propagation may be classified into the following two types:

(1) Natural vegetative propagation

(2) Artificial vegetative propagation

1. Natural Vegetative Reproduction:

Different plant parts are variously modified for vegetative propagation. The common structures that take part in vegetative propagation are root, stem, leaves and buds.

These are briefly described below:

(i) Vegetative Propagation by Roots:

The ordinary roots in many plants, such as Dalbergia sisso, Populus, Guava, Murraya,Albizia lebbek develop adventitious buds which grow to form new plants. Root tubers with adventitious buds occur in sweet potato (Fig. 10.1), Tapioca, Yam, Dahlia (Fig. 10.2) and Asparagus.

When placed in the soil, the buds present on the tuberous roots grow in to leafy shoots called slips. Slips develop adventitious roots at their base. Slips are detached and planted to form new plants. Many garden plants, such as Phlox and Dahlia, are propagated from roots which have been separated from the parent plant and cut into pieces.

(ii) Vegetative Propagation by Stems:

This is one of the most common and prevalent methods of vegetative propagation. Different plant parts, such as bulbs, runners, rhizomes, corms, tubers, offset etc. help the plant to multiply under favourable conditions.

(a) Bulb. It is a modified shoot that has a very short stem and apical and axillary buds. Some of these grow to form shoots, e.g., Allium (onion), Allium sativum (garlic) (Fig. 10.3 & 10.4), Tulip (Tulipa), Jonquil (Narcisus) etc.

(b) Runners:

These are creeping stems which produce roots at nodes. Runners break at places and each piece develops into an independent plant, e.g., Cynodon (doob grass), Oxalis etc. (Fig. 10.5 & 10.6).

(c) Rhizomes:

These are underground, horizontally growing stems. They have prominent nodes, internodes and axillary buds. Aerial branches sprout from the axillary buds which get separated from the rhizome and form new plants, e.g. Zinger (Fig. 10.7).

(d) Corm:

It is, in fact, a condensed rhizome that grows in vertical direction. The axillary buds, present in the axil of scale leaves, produce daughter corms which later on form new independent plants, e.g., Crocus (Saffron), Colocasia (Taro) Arisaema etc. (Figs. 10.8 & 10.9).

(e) Tuber:

It is a modification of underground stem. The “eyes” or buds present on the tuber form new independent plants. Potato (Solarium tuberosum) is the most common example (Figs. 10.10 & 10.11).

(J) Offset:

It looks like a modification of runner, in being more or less thickened, prostrate branch with a tuft of leaves at the apex, e.g., Pistia (water lettuce), Eichornia (water hyacinth). They develop from the base of an old shoot or crown and after growing horizontally for some distance give rise to new crowns. They may break and form many independent plants (Fig. 10.12).

(g) Aerial Shoots:

A stem segment of Opuntia and other cacti develops into a new plant after falling on the soil (Fig. 10.13). A similar segment of sugarcane with at least one node is used in agriculture to produce new plant.

(iii) Vegetative Propagation by Leaves:

Leaves of a number of plants possess adventitious buds for vegetative propagation, e.g., leaf tips of walking fern (Adiantum caudatum), marginal notches in Kalanchoe and Bryophyllum (Fig. 10.14). In Bryophyllum, the marginal buds sprout while the leaf is attached to plant. In some other plants, the buds develop only when the leaf is injured or detached and fall on the moist soil, e.g., Begonia, Saintpaulia, and Streptocarpus.

(iv) Vegetative Propagation by Bulbils:

They are fleshy buds which develop into new plants after falling on the soil, e.g. Agave, Oxalis, Pineapple (Ananas camosus), Dioscorea, Lily, Chlorophyturn etc. Some of them are modified floral buds, e.g., Agave. In Dioscorea, they develop in the axils of leaves while in Oxalis they occur above the base of fleshy root. (Fig. 10.15).

2. Artificial Methods of Vegetative Propagation:

In some plants, where vegetative reproduction by natural means is difficult to occur, special techniques can be used. We know that plant cells are more totipotent than animal cells. So they can be forced to reproduce vegetatively. Thus all the techniques or methods which are carried out by human beings to produce plants vegetatively, are called artificial methods, these include stem cuttings, layering, root cuttings, grafting, gootee & micro-propagation (by tissue culture method)

1. Cutting:

(a) It is a simple method, in which a suitable part of stem or root (about 20 to 30 cm long) is cut and it is planted in the soil, along with some nutrients. This cut part soon develops new roots and develops into a new plant.

(b) Root cutting are commonly used in plants like lemon, tamarind, Blackberry and raspberry etc. Stem cuttings are very common in plants like Rose, Croton, Sugarcane, Tapioca, China rose, Bougainvillea, Lemon, Coffee and Grape etc.

(c) Leaf cutting also used to produce new offspring, in plants like Sansevieria.

2. Layering:

(a) It is one of the most common methods of artificial vegetative reproduction in plants.

(b) In this method, a twig (branch) of a plant is bent down, below the level of soil. This bent part is called layer.

(c) A small incision is made in this layer (bent portion). Now the portion is covered with soil. Moisture is given at regular intervals.

(d) Soon this covered portion develops new roots and become separated (or can be cut) from main body, giving rise to new plant. This plant then can be shifted to some new location.

(e) Layering is common in plants like-jasmine, Strawberry, Grapevine and Cherry etc.

In tip layering, the tip of the current season’s shoot is bent into the soil by digging a sloping hole. Soon the bent part develops roots. The tip also comes out as vertical shoot. The rooted shoot is separated, e.g., Blackberry, Raspberry.

In serpentine layering, the basal branch is pegged down in the soil at several places to form a number of new plants from a single branch, e.g., Clematis. In mound layering, the basal part of a lower branch is bent down and covered with soil. The branch tip is kept outside the soil, e.g., Currant, Gooseberry.

Trench Layering consists of pegging a branch or young plant (e.g., Walnut) in horizontal position in a trench. The horizontal shoot begins to develop vertical shoots. As soon as the shoots come out, their bases are covered to hasten rooting.

3. Grafting:

(a) It is the technique of joining parts of two different plants to from a composite plant.

(b) It can be done efficiently in those plants, which are closely related and have vascular cambium.

(c) One plant, which has a strong root system, is selected as stock or stump (basal part). The branch of other plant (which is to be grafted) is selected as scion. Scion is usually selected from plants which have desired of superior characters.

(d) The shoot of the stock is cut 20-30 cm above the ground. Leaves and buds are removed from this part.

(e) Now, complementary cuts are made in stock and scion, so that scion can be fitted exactly in the grooves of stock.

(f) After this fitting, the area is tied tightly with the help of a tough thread and then it I is covered with grafting wax, to avoid any infection.

(g) Grafting is carried out commonly in plants like Mango, Guava, Apple, Rubber plant, Citrus and Pear etc.

Types of grafting:

On the basis of method of uniting two parts, grafting can be of following types:

(i) Tongue grafting :

In this case the stock and scion have almost same diameter. They are given oblique or sloping cuts. A small notch is given to ensure perfect fixing of scion into stock groove.

(ii) Wedge grafting:

In this case also, the stock and scion have same diameter. But a ‘V shaped notch is given the stock while scion is cut like a wedge.

(iii) Crown grafting:

In this case stock has a lager diameter than scion. Many scions are selected and all of them are grafted on a single stock.

(iv) Side grafting:

In this case, lateral or side cuts are made in stock. One scion is fitted in each lateral cut of stock.

4. Gootee:

a. It is also called air layering. It is commonly employed for the propagation of litchi, lemon, guava and orange etc.

b. In this method a healthy, leaf bearing branch of the main plant is selected.

c. A ring of bark is removed (for a distance of 2-5 cm) from the basal part of this branch.

d. The open part is covered with moist grafting clay (2 parts clay, 1 part cow dung, some fine cut way, moss or cotton and water). The graft is enriched with a root-promoting chemical.

e. This area is then wrapped with a polythene paper to prevent desiccation and infection.

f. This area develops small roots 1-3 months.

g. The branch is cut down is planted to a new location.

Advantages of vegetative reproduction:

(a) It is easiest method of reproduction in plants.

(b) Since the offspring and parent both have the same genotype and same characters, this type of reproduction helps to preserve the useful characters of the parental plant.

(c) It is a quick method of multiplication.

(d) It is a very helpful method of reproduction in those plants, which are sexually weak or have long dormant period of seeds.

(e) Vegetative reproduction helps in cloning and micro propagation of plants. Which in turn helps in standing a uniform population of plants.

(f) This type of reproduction helps to remove common infections, through pruning, micro-grafting, and micro-propagation.

(g) Methods like grafting helps in getting economically important plants, which have useful character of two different individuals.

(h) In seedless plants such as Banana, sugarcane, seedless grapes etc., it is predominant method of reproduction.

Disadvantages of Vegetative Reproduction:

1. Good qualities cannot be introduced nor bad characters eliminated in plants multiplied through vegetative propagation.

2. Disease contacted by a parent spreads to all the daughter plants.

3. Vegetative organs useful for propagation cannot be preserved for long.

4. Vegetative propagates are not so efficiently protected as the seeds are. They get easily decayed and are prone to various viral, fungal and bacterial diseases

5. The plants may show degeneration due to absence of sexual stimulus and variation.

6. Variability is absent. So, adaptability to changed environment decreases.

7. There is no mechanism for dispersal. Vegetative multiplication causes overcrowding and hence severe competition which can damage most of the plants.