Asexual is the type of reproduction in which new organisms are produced from a single parent without the production of gametes. The offspring called the clones are the same in all respects to each other and to the parent organisms that produced them.
General Forms of Asexual Reproduction
There are four major forms or methods of asexual reproduction. These are binary fission, budding, spore formation and vegetative reproduction.
1) Binary fission: Binary fission is a process of asexual reproduction in unicellular organism in which a single cell (parent) divides into two equal parts by mitosis to produce two new daughter cells. Examples are found in protozoa like Amoeba, Paramecium and Bacteria.
2) Budding: Budding is a form of asexual reproduction in which the parent organism forms an outgrowth (bud) which then grows into a new organism. The new organism later separates from the parent organism. This occurs mainly in yeast and in Hydra.
3) Spore formation: Spores are asexual reproductive units formed by some lower organisms especially fungi, e.g. Rhizopus, which produces numerous spores in sporangia that are borne at the ends of upright hyphae called sporangiosphores. When the sporangia become mature, they burst and the spores are dispersed by the wind. If a spore falls on a suitable substance, it germinates and gives rise to a new mycelium.
4) Vegetative Reproduction: Vegetative reproduction is the production of new individuals from the vegetative parts of plants. The vegetative parts of plants which are involved in this method of reproduction are stems, roots and leaves.
Types of Vegetative Reproduction
There are two types of vegetative reproduction. These are natural vegetative reproduction and artificial vegetative reproduction.
Natural Vegetative Reproduction
This involves the use of natural or vegetative parts of plants to reproduce and get new individuals. Such parts include stems, leaves and roots.
Forms of natural vegetative reproduction are:
- Use of modified underground stem: Some underground stems such as rhizomes, e.g. cana lily, ginger, sucker, e.g. banana, plantain and pineapple, corm, e.g. cocoyam and bulb, e.g. onion is one of many forms of natural vegetative reproduction.
- Use of fleshy leaves: This involves the use of some fleshy leaves of some plants to produce new individuals, e.g. Bryophyllum.
Artificial Vegetative Reproduction
This involves the use of intelligence by man to grow new plants from cut portion of the vegetative body of an older parent plant. Artificial vegetative propagation can be carried out by cutting, layering, budding, grafting and marcotting.
a) Cutting: When parts of a plant are cut into portions and planted to produce new plants, such cut portions are called cuttings. Cutting may come from stems or leaves depending on the type of plant being propagated. Plant whose stems produce roots readily are propagated by stem cuttings. Such stem must have at least two nodes close to the end which will be inserted into the ground. Examples of plants propagated by stem cuttings are cassava, hibiscus, sugar cane, cocoa, cola and sweet potato.
b) Layering: A branch or shoot growing near to the ground is bent over so that one or two of the nodes could touch the ground. A slit is made on the node, and the branch pegged to secure it in position (Fig. 5.22). The node is covered with soil. Adventitious roots grow out from the node.
When the roots are well established, the stem is severed from the parent plant and transplanted to the field to become a new plant. Examples of plant propagated by layering are tomato, cocoa, cola and coffee.
c) Budding: Budding is a skilled technique used when plants do not grow easily from cuttings. A bud is cut from a matured branch. Attached to the bud is a flat, shield-shaped piece of stem. The stem piece and bud is called the scion.
An inverted T-shaped cut is made in the bark of another tree of a desirable characteristics called the stock and the edges of the cut are turned back to expose the cambium. The scion is inserted into the stock and then the two are bound into position. The joint is made water-proof, e.g. by polythylene tape leaving the actual bud exposed. If the bud unites successfully with the stock, it begins to develop into a new shoot after about three weeks.
The top shoot above the union is cut off while all branches below the scion are carefully removed. When fully developed, the budded plant is carefully transplanted to its permanent site. Plants which are propagated by budding include orange, cocoa and rubber.
d) Grafting: Grafting is closely related to budding and is a method of artificial vegetative propagation which involves the application of a whole shoot or stem (the scion) into another plant (the stock). Grafting brings into close contact the vascular cambia of both the scion and the stock.
As in budding, both the scion and the stock must be closely related to avoid incompatibility which could result in the breaking of the scion from the stock at a later date. The basic grafting operation is similar to that of budding. Types of grafting include the cleft, saddle, side and whip and the tongue.
Plants easily propagated by grafting is mainly the citrus, e.g. orange, grape and tangerine.
e) Marcotting: In marcotting, the roots grow from the part of a branch at which a ring of tissue has been removed (Fig. 5.25). This branch can then grow into a new plant. Plants usually propagated by marcotting include garden shrubs, mango and lemon.
Advantages of Vegetative Propagation
- Young plant uses food reserves of the parent as it becomes easily established.
- Growth in young plant is rapid since there is no resting period.
- Only one parent is needed.
- iv Desirable characters are retained.
- Offspring are identical to parent.
- Offspring mature more rapidly.
- Plants are less susceptible to adverse weather condition.
- Plants that do not produce seeds can only be propagated vegetatively.
Disadvantages of Vegetative Propagation
- No new varieties of species are produced.
- There is no mixing of characters.
- Offspring and parents compete for
- light and nutrients since many individuals occupy a limited area of land.
- It reduces resistance to disease.
- It also reduces resistance to changes in climate.
- Colonisation of new localities is unlikely since offspring are always produced close to parent plant.
- Disease of parents can easily be transmitted to offspring.
- Undesirable characters are easily transmitted to offspring.