Asexual reproduction occurs in flowering plants in the following two ways:
A. Vegetative reproduction and
B. Agamospermy.
A. Vegetative Reproduction:
The formation of a new plant from the vegetative parts of parent plant is called vegetative reproduction. Vegetative parts of the plant like stem, root or leaves are involved in this type of reproduction. The offspring so produced is genetically identical to parent and is considered as a clone.
Vegetative reproduction is more useful in plants which fail to flower naturally, e.g., banana and pineapple or those plants which give less viable seeds e.g. potato, grass, etc.
There are two ways in which vegetative reproduction takes place:
1. Natural vegetative reproduction and
2. Anthropogenic/artificial vegetative reproduction.
1. Natural Vegetative Reproduction:
a. By Roots:
Plants showing vegetative reproduction, develop adventitious buds in roots which are generally absent in roots. These buds give rise to new plants under favourable conditions e.g. Guava, Murraya, Albizzia lebbele, etc.
Generally, it has been found that roots that show vegetative reproduction are tuberous and store food, e.g., Dahlia (Fig. 1), Asparagus, etc.
b. By Stem:
Some aerial stems such as runner and suckers and almost all underground stems that show vegetative reproduction are corm, tuber, rhizome and bulb.
i. Tuber:
Such a stem is swollen and stores food. The scale leaves are highly reduced. It has depressions called eyes. Each eye has one or more buds from which new plants arise, e.g., Solanum tuberosum potato crop (Fig. 2a) is produced by this method and not by seeds.
ii. Rhizome:
Rhizomes have distinct nodes, internodes and axillary buds. They are fleshy due to storage of food. When the season is appropriate, the buds sprout to give rise to new branches which later detach to form a new plant e.g. ginger, etc.
iii. Bulbs:
In case of garlic, bulb has numerous buds and each bud can be separated and sown. Each bud gives rise to a new plant.
iv. Runner:
These are weak aerial and sub-aerial stems. Adventitious roots are produced at the nodes which separate to give rise to new plant, e.g. Cynodon (grass), Oxalis, Hydrocotyle.
v. Sucker:
In mint and Chrysanthemum, underground stem is divided into nodes and internodes. An aerial shoot arises from the nodal region. When the inter-nodal region decays, each aerial branch separates and forms a new plant.
vi. Corm:
This is an underground stem with scale leaves and buds. When the condition are favourable the buds give rise to new plant, e.g. Colocasia (Fig. 2b) Gladiolus, etc.
c. By Leaves:
The leaf of Bryophyllum (Fig 5a) has adventitious buds at its notches. Under favourable conditions these buds give rise to new plantlets. Other plants showing this kind of reproduction are Lilium, Begonia, Adiantum caudatum (walking fern), etc.
d. By Bulbils:
These are fleshy buds that store food. They are modified aerial buds arising in leaf axils. These bulbs are called bulbils. After detaching from the mother plant they germinate to give rise to new plants e.g. pineapple (Fig. 5b), Agave Americana, etc.
2. Artificial/Anthropogenic Vegetative Reproduction:
This method is used by horticulturists to multiply the plants of interest at a faster rate and generating plants identical to parent plant. This method of propagation is used for food as well as ornamental plants.
This method has two benefits:
a. Hybrid having good qualities of both the plants can be created.
b. It is a faster method where large number of off-springs are produced at a time.
Artificial vegetative reproduction can be done by any of the following methods:
i. Cutting:
In this method a portion of root, stem or leaf can be used. In case of stem the node of lower part is dipped in any one hormone – IAA (Indole acetic acid), IBA (Indole butyric acid) or NAA (Naphthelene acetic acid). These hormones induce root production. After dipping it in hormone this portion is sown in soil.
Care is taken that a few nodes remain above the soil. After a few days a complete plant is formed. Cutting is generally done during spring season e.g. Sugarcane, roses, citrus, duranta, grapes, bougainvillea, carnation, etc. In plants like Bryophyllum, Sansevieria et cetera leaf cutting is used to give rise to new plant.
ii. Grafting:
Grafting is done in dicot plants because of presence of cambium. This is possible in closely related plants. In grafting, root system of one plant and shoot system of other plant are used. Former is called stock and later scion. Stock provides a strong root system which has high capacity of water and mineral absorption and is disease resistant.
Scion provides a shoot system which can bear healthy fruits and seeds. So a grafted plant has all the desired qualities. Care should be taken to remove all the shoots arising from the stock as their presence will not permit the growth of scion. The cut surface is held together tightly by wrapping. Grafting is used in plants such as mango, citrus, apple, guava, rubber plant, etc. (Fig. 6).
Grafting can be done by any of the following methods:
a. Wedge Grafting:
A ‘V’ shaped notch is cut in stock and wedge shaped notch in scion.
b. Tongue Grafting:
Both stock and scion are cut obliquely and then wrapped.
c. Side Grafting:
A wedge cut is made in scion and this is inserted in lateral slit of.
d. Crown Grafting:
Many scions are grafted on single stock. For such a grafting, diameter of stock should be much more than of scion.
iii. Layering:
This can be done with the plants having weak stem. In this kind of propagation, roots arise in stem while still on parent plant. Such a stem is called layer. Layering can be artificially induced by bending the stem on the ground and covering it with soil. Leaves touching the soil are removed. After some time adventitious roots arise underground. Now the stem can be separated from parent plant.
There are two types of layering, mound layering and air layering (Fig. 7):
a. Mound Layering:
It is successful with plants having weak and soft stem. Part of stem is defoliated and a small injury is made. This portion is pegged in soil. Roots arise from the injured portion. Injury can be made in the form of ringing, notching or tonguing, e.g. grapevine, jasmine, strawberry.
b. Air Layering:
Mound layering is not possible with plants having woody stem. In such plants, the method used is called air layering (Gootee). A ring of bark is removed in air layering. This is called girdling. Part of bark which is girdled is covered with grafting clay (combination of one part cow dung, two parts clay, some hay, cotton, root promoting hormone and water).
It is then covered with polythene. The portion covered with polythene is called gootee. Roots develop in a time period of two months. This portion is cut and sowed in soil. Air layering is done in bougainvillea, citrus, guava, litchi, etc.
iv. Micro-Propagation:
This is the latest technique used by horticulturists. In this technique (Fig. 8) few cells from the meristematic part, called as explant are taken from the parent plant. These cells are then grown under aseptic conditions in the growing medium. The undifferentiated mass of cells so formed is called callus.
This callus is then divided in small portions in large number of petriplates containing nutrient medium. This nutrient medium has growth hormones (cytokinin and auxin) which promote growth of shoot tip and root tip. By this method large number of high yielding and disease resistant plants can be generated in a short period of time.
This method can also be used to generate haploid plants by culturing pollen grains. This technique is also used for somatic cell hybridisation. Certain plants where micro-propagation is used are orchids, carnation, dahlia, Gladiolus, etc.
Significance of Vegetative Propagation:
a. Hybrid and superior plants can be generated.
b. Large number of plants in short duration can be prepared.
c. Characters present in parent plant can be preserved.
d. It is a less expensive method.
e. This is the only method of reproduction in those plants which do not produce viable seeds.
f. It is an easier method to get rid of pathogens.
B. Agamospermy:
This is also known as apomixes. In this there is no meiosis or fusion of gametes. Plants showing apomixes are called apomict.
In such plants embryo develops from:
a. Nucellus or integument (Adventitious embryo).
b. Diploid egg cell develops partheno-genetically from diploid megaspore mother cell (Recurrent apomixes).
c. Formation of sporophyte from diploid gametophyte but gametes are not formed (Apogamy).
d. Formation of diploid gametophyte from sporophyte without the formation of spores (Apospory).
e. Formation of embryo from diploid egg. This is called parthenogenesis. This is possible in angiosperms as the egg is diploid (Non-recurrent agamospermy).
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