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Asexual Reproduction in Multicellular Organism

New individuals are produced from a single parent without the act of fusion of gametes. The new individuals so formed are generally identical to the parent plant, as only mitotic divisions take place during their development. Regeneration of new plants from portions of vegetative organs is very common and is called vegetative propagation. For this purpose, organs like the root, stem, leaf, flower etc are variously modified.

This type of reproduction is very common in angiosperms and is well exploited by man. Vegetative reproduction results in the multiplication of plants and therefore also called as vegetative multiplication.

The methods of vegetative propagation may be classified into 2 types.

  • Natural vegetative propagation
  • Artificial vegetative propagation
 

Natural Vegetative Propagation

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In this method of vegetative propagation, a part of the plant, which may be stem, root, leaf or flower, gets detached from the body of the mother plant. The detached part, under suitable environmental conditions develops in to a new independent plant.

Among seed plants, vegetative propagation by natural methods is very common. A vegetative part reproducing by this method should possess

a) Sufficient food for early growth of new plant

b) Growing bud

The common structures that take part in natural vegetative propagation are roots, stem, leaves and buds.

Vegetative Propagation by Roots

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Roots of are some plants like raddish, carrot, Dahlia, Tapioca are tuberous and when planted in specially prepared soil, develop adventitious buds which grow into leafy shoots called slips. The young slips are detached from the parental plant and grown separately. In woody plants such as Albizzialebbek, Dalbergia sisoo, some of the roots running just below the ground level develop adventitious buds at intervals. The buds grow out in the form of shoots which produce the new plants.

Vegetative Propagation by Stem

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Aerial weak stems like runners and stolons, when they touch the ground, give off adventitious roots. When the connection with the parent plant is broken, the portion with the newly struck roots develops into an independent plant.

Some examples for propagation by stem are from

stolons - Vallisneria, strawberry

offsets - Eichhornia

rhizome - banana, ginger

bulbs - Alluim cepa, orion

corns - colacasia

tuber - potato

example for adventitious roots
A runner of the mint plant bearing numerous adventitious roots and a shoot at each node

A-Bulb of onion, B-L. S. of bulb; C-Corm of Crocus

 examples for propagation by stem sprouted potato tuber
A sprouted potato tuber showing the development of many plants

Vegetative Propagation by Leaves

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Leaves are not a common means of vegetative propagation in nature. But, in species of Bryophyllum, plantlets are produced from the notches of the margin of intact leaves, while the latter are still attached to the parent plant. In other species of Bryophyllum, the leaves must be detached or injured before plantlets arise.

 vegetative propagation by  leaves

Activity

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Take 3 pots with soil. Place an entire leaf of Bryophyllum in one pot, a piece of a leaf in another pot and an injured leaf in another pot. Observe the conditions under which the propagation occurs.

Other examples are Begonia, Kalanchoe Streptocarpus and Saintpaulia.

Vegetative reproduction from reproductive organs

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Flowers are primarily concerned with sexual reproduction. But, in some plants such as Agave, the flower bud becomes modified into specialised structures called the bulbils. The bulbils are fleshy on account of storage of food in the floral leaves. The bulbils drop from the parent plant on to the ground, give out adventitious roots and develop into new plants.

structure of bulbils

Artificial Vegetative Propagation

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Certain flowering plants have the capacity to develop a part of their somatic body into a new independent plant. In artificial vegetative propagation, such plants are identified and special techniques are applied to obtain new independent plant.

Horticulture have used the various methods of vegetative propagation adopted by plants in nature. These practices constitute the artificial means of vegetative propagation.

These methods are meant for
  • combining the good qualities of two different varieties.
  • propagating the desirable variety of plants economically, with least attention and in comparatively shorter time

Some common and important methods of artificial vegetative propagation are

Cutting

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It is a process in which a vegetative portion from a plant is taken and is rooted in the soil to form a new plant. The portion used is called a cutting.

Stem cuttings are most commonly used for this purpose. The factors to be considered for cutting are optimal length with a few nodes and internodes, diameter of the cutting, age of the plant and the season. Application of hormones leads to the quick formation of adventitious roots. For example sugarcane, roses, croton hibiscus, citrus plants, bougainvillea are grown by stem cutting.

Root cutting from lemon, tamarind, etc also develop roots and shoots when planted in a moist soil.

Layering

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In this technique roots are induced on a stem before it is detached from the parent plant. In layering, the lower branch close to the ground is bent down and covered with moist soil. After some days roots develop from the branch, which can be cut and grown independently. The 2 important methods of layering are mound layering and air layering.

Mound Layering

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In this process, the lower stem branch of a plant is defoliated partially. It is then, bent close to the ground, pegged and covered with moist soil in such a way that its growing tip remains above the soil surface. After a few days, the covered portion on the stem branch develops the adventitious roots. The branch with the roots is then detached from the parent plant and grows into an independent plant. This technique is practised in Jasmine, strawberry, goose berry

representation of mound layering

Air Layering

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In plants where the stem cannot be bent to the ground, the stem is girdled or given an slit at an upward angle. Then, it is covered with moist moss or cotton and wrapped in a polythene sheet to preserve the moisture. After a few weeks, adventitious roots arise from the injured part. The branch along with the roots is then separated from a parent plant and planted to grow into a new plant.

Grafting

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The process of joining together parts of two different plants of closely related varieties in such a manner that they live as one plant is called grafting. This is usually practiced among dicotyledonous plants, especially fruit trees. Of the 2 plants, one is rooted in the soil and is known as the stock.

grafting

Vegetative Reproduction by Grafting

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The other part consists of a small shoot bearing one or more buds and is known as the scion. The union of the stock and the scion is carried out with the help of cambium. The stock supplies the water and minerals to the scion whereas the scion forms the upper portion and supplies the organic food to the root. Rubber, apple, pear, citrus, mango and guava are routinely propagated by grafting.

Grafting may be of the following types like approach grafting, bud grafting, tongue grafting, crown grafting and wedge grafting.

Gootee

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In this method, a healthy and woody branch is selected and the bark is sliced in a ring form of about 3-5 cm in length. A thick plaster of grafting clay (plaster formed of clay, cow dung, finely cut hay and water) to wrapped up with rag and tied on to the debarked portion. Care is taken to keep the gootee moist. In about 2-3 months, the roots emerge from the gootee. The gootee is now cut below the plaster for propagation. Lemon, orange, guava and litchi are usually propagated by using this method.

Micro Propagation or Propagation by Plant Tissue Culture Technique

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This method is actually an applied aspect of the tissue culture technique.

For micro propagation, a small amount of tissue from a suitable part of the parent plant is excised and grown on a nutrient medium under aseptic conditions. The tissue then develop into an undifferentiated mass of cells called callus. A small portion of the callus tissue is transferred to another suitable nutrient medium where they develop and differentiate into small plantlets. These plantlets on being transplanted in pots or soil, develop into mature plants.

Advantages

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  • an indefinite number of plants can be raised from a small mass of parental tissue.
  • It is useful for obtaining virus free healthy plants.
  • An unlimited number of plants are produced within a relatively short time.

Importance of Vegetative Propagation - Advantages

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a) It makes possible propagation of plants that have lost their capacity to produce seeds. E.g., banana, rose

b) Plants like Bermuda grass (Cynodon dactyoln) which produce only a small quantity of seeds are propagated vegetatively.

c) It is a more rapid, easier and cheaper method.

d) The plants produced will have the same characters and the hereditary potentials as the parent plant. The beneficial characters will be preserved in the neat generation.

e) Most of the ornamental plants are propagated by vegetative reproduction.

f) Grafting enables the physical and physiological joining of 2 separate individuals for the best economic advantage.

g) It is a method to get rid of pathogen from any part of plant.

Vegetative propagation is very important for plants with reduced power of sexual reproduction, long dormant period of seed or poor viability. Under these conditions this method is the only way of propagation of the plant.

Disadvantages

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a) Neither can good qualities be introduced nor can bad characters be eliminated.

b) Subsequent generations show a general fall in vigour and vitality.

c) Adaptability, to changed environment decreases due to the absence of variations.

d) Overcrowding and severe competition among the daughter plants results in low yield.



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