What is Reproduction
This is the process by which new generations of offspring are produced in a population. Reproduction involves the transmission of genetic material from one generation to the next ensuring that the species survive over long periods of time even though individual members of the species die. Some members of the species will die before they reach the reproductive age due to factors like diseases, predation etc so that the species will only survive if each generation produces more offspring than the parental generation. Population size therefore varies according to the balance between rate of reproduction and mortality rate of the individuals.
Types of reproduction
- Asexual reproduction.
This is where new individuals are produced from a single parent without production of gametes. It usually results in production of genetically identical offspring. However, genetic variation may arise as a result of random mutations among the individuals.
- Sexual reproduction.
This involves the fusion of two gametes to form a zygote which develops into a new organism. Sexual reproduction forms the basis of genetic variation and provides the raw material for natural selection hence evolution of new species.
A Sexual Reproduction
Asexual reproduction allows production of new individuals from a single parent and does not involve formation of any gametes or sex cells. The offspring are produced by mitosis and are identical and are called clones. Members of a clone are always identical genetically but differ only when random mutation occurs.
Types of asexual reproduction
This occurs in simple protists e.g. amoeba, bacteria etc. it involves the division of the cell into two or more daughter cells identical to the parent cell.
In a bacterium, the cell divides into two identical daughter cells, a process called binary fission.
In plasmodium, the nucleus of the cell divides repeatedly by mitosis and each daughter nucleus breaks away with a small portion of the cytoplasm, a process called multiple fission. The splitting process during multiple fission is called schizogony and the cells produced are called schizonts.
Multiple fission produces as many as 1000 daughter cells from a single schizont during the parasites asexual cycle immediately they enter the liver cell. Each of this then enter the red blood cell and produce a further 24 daughter cells and this increases the infection rate rapidly.
When conditions are favorable fission results into rapid population growth.
Binary fission in a bacterium. (Diagram)Ref BS pg 15
This involves the formation of spores. Spores are microscopic reproductive cells produced in large numbers by cell division by the parent organism. They are very light and easily dispersed by air current, water as well animals. They are capable of germinating very fast if conditions are favorable.
Spores are produced by some bacteria, protists, fungi and many lower plants like mosses and ferns.
In fungi, the body structure consists of a mass of threads of fine tubes called hyphae and the whole mass of the hyphae is called a mycelium. At the tips of the hypha, spores are formed either enclosed in a spherical structure called sporangium or are free. Cytoplasmic streaming causes the nuclei to cluster at the tip. The tip swells as more nuclei and cytoplasm flow in the sporangium. The sporangium burst releasing spores which are later dispersed by wind. The spores germinate when conditions are favorable giving rise to a new hypha.
This is where new individuals are produced as an outgrowth (bud) of the parent plant which increases in size and eventually drops off to live an independent life. This occurs in yeast cells, hydra, certain flat worms as well as lower animals.
This is where an organism may break into one or more pieces which grow into a new individual.
Fragmentation depends on the ability of an organism to regenerate itself into a new individual.
It occurs in fungi, filamentous algae (spirogyra), sponges etc.
- Vegetative propagation.
This is the most common form of asexual reproduction in plants where part of the plant body grow and develop into a new plant. This may occur naturally or artificially.
Natural vegetative propagation
This is where a new individual develops on the parent plant and later detaches itself from the differentiated part. These plants have specialized parts which store food used for survival during unfavorable conditions called perennating organs e.g. bulbs, rhizome, tubers etc.
Food manufactured during photosynthesis in the aerial green leaves are translocated via conducting tissues to the base of the plant where they are changed into starch and stored in the perennating organ. In this state the plant remain dormant but as conditions become favorable, the stored starch is converted to sugar which is supplied to the young buds which grow and sprout while the older aerial shoot begin to die.
Types of perennating organs
- Bulb e.g. an onion.
This is a short swollen stem capable of vegetative propagation. It is covered with brown papery scaly leaves which protect the inside fleshy foliage leaves containing stored food as glucose and vitamin C. in the axil of these leaves are buds which grow and sprout to form a shoot which produces a new bud at the end of the growing season. They have adventitious roots that arise from the reduced stem which anchors the shoot in the soil.
Longitudinal section of an onion bulb (ref diagram BS)
- Corm. E.g. yam
This is a short vertical underground stem. A corm is a more or less spherical body containing heavy deposit of food materials stored in the swollen base of the stem. It has scale leaves but no fleshy foliage leaves like in bulbs. The nodes bear axiliary buds which may develop into new shoots in the next growing season. Food is manufactured by photosynthetic green leaves which are then carried down to the buds which grow into daughter corms. They have adventitious roots which grow from the short swollen stem.
Diagram of a corm (ref diagram BS)
- Rhizome e.g. ginger, canalilly, spear grass etc.
This is a horizontal under ground stem which bears green foliage leaves and scale leaves, buds and adventitious roots. In the axil of the dry scale leaves are buds that sprout to produce new lateral rhizomes or vertical shoots.
Rhizome of canalilly (ref diagram BS)
These are underground storage organs formed from a stem or root swollen with food and capable of perennation. Tubers survive for only one year and shrivel as their storage contents are used up during the growing season. New tubers are made at the end of the growing season but don’t arise from old tubers.
Stem tubers e.g. potato
These are stem structures produced at the tips of thin rhizomes. The stem structure is revealed by the presence of the axillary buds in the axil of the scale leaves. Each bud may grow into new plant during the next growing season.
Root tubers e.g. cassava
These are swollen adventitious roots. Being roots they lack scale leaves and buds although they act as storage organs. New shoots develop from the buds which are on the parent plant.
Tap roots may also become swollen with food storing tissues e.g. in carrots. Together with buds at the base of the old stem just above the tap root, they form organs of perennation.
5. Stolons and runners.
A stolon is a creeping horizontally growing stem that grows along the ground surface. It has adventitious roots growing from the nodes. The stem grow diagonally upwards then bends to the ground putting roots out at the tip and producing a bud which grows into a new daughter plant e.g. blackberry, black currant etc.
A runner bear scale leaves with axillary buds which give rise to adventitious roots and new plants. A number of stems usually radiate from the parent plant and new plants develop along the runner where it puts down adventitious roots. The old runner then decays once the new plants are produced e.g. straw berry, creeping buttercup etc.
Artificial vegetative propagation
These involve the separation of a portion from the mother plant and then growing it independently. This is mainly used in agriculture and horticulture. These include;
A cutting is a portion of a stem or root which when detached from the parent plant is capable of developing roots under favorable conditions and growing into a new plant e.g. stem cuttings of sugar cane, cassava etc.
Grafting involves transplanting of a portion of one plant called scion or donor onto another plant of the same genus called stock or recipient. The advantage of this is that certain beneficial traits of the two plants grafted are combined and also allows rapid multiplication. It mainly used in the propagation of citrus fruits, roses, hibiscus etc.
This is used in plants that produce runners such as straw berries. The runners are pegged out or layered around the parent plant until they produce roots and then cut to detach them from the parent plant so that they can grow independently.
These are young buds removed from the parent plant and allowed to grow independently e.g. pineapple, banana etc.
Advantages of asexual reproduction
- It results in maintenance of favorable characters since the offspring are genetically identical.
- The offspring have greater chances of survival and mature faster since the daughter organisms remain attached to the parent plants which usually have large food reserves.
- It leads to rapid rate of multiplication and hence rapid increase in population.
- It does not require much specialization like formation of reproductive organs in sexual reproduction.
- Only one parent is involved therefore does not require external agents like pollinators etc since every single individual is self sufficient.
- Enables rapid spread and dispersal of organisms to a wide area e.g. the spores produced is light and easily dispersed by air currents.
Disadvantages of asexual reproduction
- Does not promote genetic variation hence maintenance of undesirable characters in a population.
- It results into quick colonization of new areas which may result into over crowding, competition and exhaustion of nutrients since the organisms tend to colonize a limited area.
- Certain undesirable characteristics carried by the parent organisms are transmitted from parents to the daughter offspring hence the offspring are more prone to pest and diseases.
Differences between asexual and sexual reproduction.