Reproduction – SS1 Biology Lesson Note (First Term)

Reproduction is the ability of an organism to give rise to new individuals of the same species to ensure continuity of life.

Behavioural objectives

By the end of the lesson, students should be able to:

• define reproduction and name its two types;
• state the differences between asexual and sexual reproduction;
• describe binary fission, budding, spore formation and fragmentation, giving an example of each;
• explain vegetative propagation and name the organs of natural vegetative propagation with examples;
• list the types of artificial vegetative propagation.

What are the types of reproduction?

There are two types of reproduction:

1. Asexual reproduction
2. Sexual reproduction

Asexual reproduction

Asexual reproduction is the process whereby an organism produces offspring by itself, so only one parent is present. No gametes are involved, so there is no fusion of nuclei. The cells that give rise to the offspring usually divide by mitosis. The offspring are identical to the parent in all respects and are called clones.

Sexual reproduction

Sexual reproduction involves two parents and the fusion of the male and female gametes to form a zygote. The offspring show new variation. The sex cells (gametes) are produced by meiotic cell division. After fertilisation, the new individual grows and produces new cells by mitosis.

What are the forms of asexual reproduction?

1. Binary fission

Fission is the simplest form of asexual reproduction. A single organism divides into two complete organisms, each identical to the other and to the parent. Fission is common among unicellular organisms such as bacteria, many protists and some algae.

2. Budding

The parent organism develops an outgrowth, which forms the new individual organism. The buds break off from the parent without causing any injury and live an independent life. Budding is common in yeast and hydra.

3. Spore formation

Spores are DNA-containing capsules that can sprout into new organisms. Unlike most seeds, spores are produced without the sexual union of gametes. When dispersed, each spore can develop into a new organism. Spores are common in lower organisms, especially fungi such as Rhizopus and Penicillium.

4. Fragmentation

A part of the parent organism breaks off and develops into a new independent organism. This type of reproduction is also called regeneration. Fragmentation is common in Spirogyra and coelenterates.

5. Vegetative propagation

Vegetative propagation occurs in higher plants. A new plant grows from any portion of an old one other than the seeds. There are two methods of vegetative propagation: natural and artificial.

Natural vegetative propagation

Natural vegetative propagation uses vegetative parts such as stems, leaves, roots or buds. The part involved must have a store of food. It is sometimes able to act as a perennating organ, that is, an organ that enables the plant to survive from one growing season to the next. The organs of vegetative propagation include:

• Bulbil: axillary buds growing from the veins of leaves, e.g. Bryophyllum and Begonia.
• Runners or stolons: stems that creep horizontally on the soil surface. Buds and adventitious roots develop from the nodes of the parent plant, e.g. sweet potato and grass.
• Rhizomes: underground horizontal stems. They have scaly leaves which cover lateral buds at the nodes. The lateral buds grow into new aerial shoots, e.g. ginger and canna lily.
• Corms: underground stems which grow vertically in the soil. Buds develop from the axils of scale leaves, grow upward and form leaves and flowers, e.g. cocoyam.
• Stem tubers: underground stems which have swollen tips. Axillary buds on the tubers give rise to new aerial shoots, e.g. yam and sweet potato.
• Suckers: short underground horizontal branches, e.g. banana and pineapple.
• Bulb: an underground condensed shoot with a compressed stem and scaly leaves, e.g. onion and garlic.

Artificial vegetative propagation

Artificial propagation is the use of parts of the parent plant to multiply the plants. Budding, grafting, layering, cutting and marcotting are types of artificial propagation.

Evaluation

1. Define reproduction.
2. State two differences between asexual and sexual reproduction.
3. Name the form of asexual reproduction that is common in yeast and hydra.
4. What is a perennating organ?
5. Name the organ of vegetative propagation found in ginger and give one other example.

Assignment

1. Bacteria multiply rapidly by
   A. fragmentation
   B. binary fission
   C. spore formation
   D. budding
2. The type of reproduction that is common to both Hydra and Yeast is
   A. conjugation
   B. binary fission
   C. grafting
   D. budding
3. During asexual reproduction in Paramecium, how many times does the zygote divide to produce eight nuclei?
   A. 1
   B. 2
   C. 3
   D. 4
4. Vegetative propagation is described as asexual reproduction because
   A. reproductive organs are not involved
   B. many new plants are produced
   C. there is no exchange of genetic materials
   D. new individuals are not formed
5. Which of these organs of vegetative propagation is not a perennating organ?
   A. Rhizome
   B. Sucker
   C. Bulbil
   D. Bulb
6. List four forms of asexual reproduction, giving one example of an organism that exhibits each.
7. List four common natural vegetative parts in plants, giving one example of a plant in each case.
8. Describe five types of artificial vegetative propagation.