ASEXUAL REPRODUCTION
The methods of asexual reproduction include binary fission, budding, fragmentation, spore formation and vegetative reproduction.
Binary fission:
This is where an organism divides into two parts with each part then carrying out life processes independently.
It occurs in protozoa like amoeba and trypanasomes and in yeast and algae.
The nucleus divides first followed by the cytoplasm just like in the process of cell division called mitosis.
If the organism is motile (has the power of movement) the two daughter cells separate. If not, then they adhere to each other so that a colony is formed.
Fragmentation:
This is the breaking up of an organism into two or more fragments capable on separate existence.
Unlike in binary fission, there are no special cells involved.
Like in binary fission there is no identity of parent.
It occurs in aquatic annelids for example earthworms.
Budding:
In budding, an organism develops a bud which grows into a new organism.
Unlike in binary fission where the resulting organisms are the same size, in budding the result is a larger mother organism and a smaller daughter organism.
Unlike in binary fission and fragmentation, in budding the parent retains its identity.
Budding occurs in yeast and hydra.
The process of budding in the hydra:
- Some cells divide.
- Differentiation takes place.
- A bud develops.
- The bud grows bigger.
- The bud grows tentacles.
- The joint between the mother organism and the bud constricts.
- The bud breaks off forming a daughter organism.
SPORE FORMATION (SPORULATION):
This is the formation of a resting cell which can develop into an individual organism.
It has the advantages that the spores can withstand unfavourable conditions like dryness, heat, cold and unfavourable chemical environments.
Then when the conditions are favourable, the spores can grow into normal organisms.
Asexual spore formation is common in moulds and yeasts.
The process of spore formation:
Cell division results into small cells.
The small cells remain inside the mother cell.
The cytoplasm of these mother cells dries out.
Each small cell forms an extra-tough wall.
The mother cell is called a spore case.
The small inside cells are called spores.
The mother cell bursts and spores scatter.
During favourable conditions, the spores germinate to form new individuals.
Vegetative reproduction:
This is the development of offspring from part of the vegetative body of a plant.
Annuals survive the dry season as seeds because the rest of the vegetative body dries off after flowering and seeding; so they survive for one season. Examples include maize, cotton, black jack.
Biennials have a life cycle lasting two seasons storing food in one season and then flowering in the next for example onions, cabbages.
At times a plant may repeat its life cycle several times in one season. Such plants are called euphemerals for example agerantum.
Perennials grow from year to year. There are two types of perennials:
Woody perennials: These are shrubs and trees that persist from year to year.
Trees have one main stem whereas shrubs have many stems of equal sizes. Some are evergreen i.e. they shed their leaves throughout the year for example mango, coffee. Some are decidous i.e. they shed all leaves in the dry season for example silk cotton tree (Ceiba), Chlorophora excelsa (Mvule).
Herbacious perennials: These have no wood but persist from year to year but with reduced growth above the ground. They have perennating organs which enable them to store food during the dry season for use in the next season.
The perennating organs include bulbs, rhizomes, stolons, stem tubers or corms.
BULBS:
These are condensed shoots with fleshy leaves for example onions.
Characteristics of bulbs:
- The stem is short and is below the ground.
- They have short internodes with overlapping leaves.
- Their outer leaves are scaly and dry.
- The inner leaves are fleshy and thick.
Life cycle of an onion:
- Adventitious roots develop.
- A terminal bud appears.
- The fleshy leaves supply the food for growth.
- The fleshy leaves thus shrivel.
- The foliage leaves appear and manufacture food.
- The manufactured food enables the lateral bud to develop.
- The lateral bud forms a new bulb.
- Rain or watering will induce flowers to appear.
- Since the life cycle lasts two seasons, the onion is a biennial.
Rhizomes:
This is where there is an underground stem which continues to grow horizontally. Unlike in bulbs, the old part of the stem does not die away.
Examples include ginger and cana lily which are swollen rhizomes that store food. Others are spear grass and couch grass which do not store food.
Life cycle of rhizome:
- Food manufactured by the foliage leaves goes to the rhizome.
- The rhizome grows horizontally.
- Terminal buds appear.
- They produce new leafy shoots with flowers.
- Adventitious roots grow from the nodes of the rhizome.
STOLONS (RUNNERS):
These are stems which creep above the ground for example sweet potato, wandering jew (Commelina bengalensis).
In stolons, adventitious roots develop at the nodes where leaves develop above the ground.
Stem tubers:
These grow from the main aerial stem into the ground then swell with food for example Irish potatoes, yams.
N.B: The carrot is not a stem tuber but a swollen taproot.
Corms
This is a swollen underground stem which seasonally produces new corms which grow on top of the old corm for example coco yam.
Unlike rhizomes, corms do not grow horizontally.
Life cycle of the corm:
Food is used up in the old corm.
A new corm is formed on top of the old one.
The new corm develops contractile roots.
The contractile roots dip their tips into the ground.
The contractile roots then contract to pull the new corm down so that it does not grow above the ground.
N.B: Because cassava is a root tuber, vegetative reproduction cannot result out of the root.
Advantages of vegetative reproduction over sexual reproduction by seeds:
1. The good characteristics of the plant are preserved because there are no new varieties got; instead a clone similar to the parent is got.
2. In reproduction from the seed cross-pollination could result in poor varieties.
3. The offspring grow in conditions which are already favourable to the parent.
4. In reproduction from seeds some seeds could be dispersed to unfavourable conditions.
5. Water is absorbed from a wide area by the parent plant.
6. In reproduction from seeds water is absorbed from only around the seed.
7. The offsprings get food from the parent's abundant reserves.
8. In reproduction from seeds food reserves are only cotyledons or endosperm.
9. There is fast reproduction unlike in seed reproduction which is slow.
10. Inheritable diseases are not attainable.
Disadvantages of vegetative reproduction over sexual reproduction by seed
There are no new varieties got, so poor characteristics can be maintained.
In reproduction from seeds new and more successful varieties can be got.
Colonization of new areas is very slow.
In reproduction from seeds, seed dispersal can scatter seeds to new colonies.
Vegetative reproduction presents a serious problem to farmers during weed control for example spear grass, couch grass.
To control weeds that reproduce by seeds, hoe them down before flowering stage.
Parthenogenesis:
This is the development of an egg without fertilization.
Examples:
In the honey bee all the male bees (drones) develop from unfertilized eggs; yet the workers and the queens develop from fertilized eggs.
In the water flea, the male sex is so minor that it tending to elimination. The females produces eggs which develop into more females without fertilization.
Artificial parthenogenesis:
This is the development of an egg without fertilization after inducing it artificially by treatment in inorganic salt solutions, acids, or with osmotic or electric shock.
Artificial parthenogenesis has been tried successfully in sea urchins, frogs and in 1939 in rabbits!
CLONING: This is an artificial method of culturing non-reproductive cells to form a new offspring called a clone which resembles the original organism in all aspects. It has been tried successfully in sheep but efforts to try it out in people are facing resistance from religious and conservative circles.
VEGETATIVE PROPAGATION:
This is the dividing of plant parts like rhizomes and tubers by farmers to make separate plants.
It can be done by cutting for example cassava.
It can be done by suckers for example bananas.
At times vegetative propagation is the only way of growing a crop plant for example pineapples, bananas.
Yams and sisal have swollen aerial tubers called bulbils which can drop off and develop into new plants.
Grafting and budding:
This is when a bud or a shoot from one plant is inserted or fixed under the bark of another closely related variety so that the cambium layers of both are in contact.
The rooted portion is called the stock and is chosen from plants that have good rooting systems and are resistant to diseases. The bud or shoot portion is called the scion and is chosen from plants with good fruiting traits.
If bud is used this is strictly called budding and it has the advantages over grafting in that it enables more plants to be developed on each stock. Budding and grafting are widely used in the horticulture of passion fruits and oranges.