O Level Revision : Integrated Science - Reproduction in Plants

Sexual reproduction

Plant flower

• The flower is the reproductive organ of a plant.
• The flower is modified (has a special structure) to increase the chances of fertilisation.
• Fertilisation is the union or fusion of the male sex cell with the female sex cell to form a zygote.
• The male sex cell is the pollen grain and the female sex cell is the ovule.
• The male sex cell (pollen grain) must be transported to the ovary in order for fertilisation to take place.
• Pollen grains are transferred from the anther to the stigma and this process is called pollination.
• The agents of pollination are either insects or wind.

Definition:  Pollination is the transfer of pollen grains from the anther to the stigma.

Adaption of a flower to mode of pollination

Stages leading to fertilisation

Pollination and fertilisation

1. Pollen grains are transferred from the anther to the stigma (pollination).
2. Pollen grains germinate and pollen tube grows down the style.
3. Pollen tube enters ovary.
4. Male nucleus moves down pollen tube into ovule.
5. In the ovule the male and female nuclei fuse to form a zygote (fertilisation).

Definition: Fertilisation is the fusion of male and female sex cells nuclei to form a zygote.

After fertilisation:

1. Stamens, style, stigma and petal fall off.
2. Fertilised ovule develops into a seed.
3. Ovary develops into a fruit.
4. The ovary wall becomes the fruit skin (pericarp).

Germination

• Seeds formed as a result of fertilisation under suitable conditions may germinate.
• The seed consists of a testa (seed coat) which encloses an embryo.
• The embryo is made up of a plumule, a radical and one or two cotyledons. In some cases the embryo also contains an endosperm.
• The embryo develops into the seedling, the radicle develops into roots and the plumule develops into the shoot.
• Seeds that have one cotyledon are called monocotyledons e.g. maize seed.
• Seeds that have two cotyledons are called dicotyledons e.g. bean seed.

Percentage germination is calculated as follows:

Percentage germination =     Number of seeds germinated ×  100

Original number of seeds

Dicotyledonous seed e.g. bean                             Monocotyledonous seed e.g. maize

Internal structure of a seed

Factors affecting germination

The following set-ups were placed under the conditions shown:

Set-up A:

Bean seeds placed on moist cotton wool in a flask and left in a refrigerator.

Conditions present: water, oxygen and low temperature.

Set-up B:

Bean seeds placed on moist cotton wool and left at room temperature. Conditions present: water, oxygen and suitable temperature.

Set-up C:

Bean seeds placed on dry cotton wool and left at room temperature. Conditions present: no water, oxygen and suitable temperature.

Set-up D:

Bean seeds placed on moist cotton wool in a flask and pyrogalic acid is placed near top of closed flask

and left at room temperature.

Conditions present: water, no oxygen and suitable temperature.

Results:

The highest percentage germination is obtained where water, oxygen and warmth are present.

Conclusion:

Factors which promote germination are water, oxygen and suitable temperature (warmth).

Vegetative reproduction

• Vegetative reproductionis the production ofnew individuals from vegetative structures ofa single parent.
• Vegetative structures include rhizomes, tubers and cuttings

1. Rhizomes

• Stem grows horizontally above the ground.
• New plants grow from buds along the stem.
• Many plants are established quickly over a wide area thus rate of propagation is high.
• An example of a plant which reproduces by means of rhizomes is grass.

2. Tubers

• Buds on stem that are in contact with the soil swell forming a tuber.
• The tubers are special structures for storage of food (food reserves).
• Many tubers are formed by one plant.
• Each tuber has buds which can grow into new plants.
• An example of a plant which reproduces by means of tubers is the Irish potato.

3. Cuttings

• Propagation by means of cuttings is artificial.
• The cut end of the plant is put into moist soil.
• Roots grow into soil from the part of the cutting that is in the ground. The aerial part of the plant continues to grow and produce leaves.

• One parent plant can be used to produce many new plants.
• Examples of plants which reproduce by means of cuttings are sugar cane & sweet potato.

►     Other vegetative strctures or methods are stolons, grafting, budding, layering etc.

Advantages of vegetative reproduction

1. New plants obtain food reserve from parent plants ensuring high chances of survival for offspring.
2. Offsprings are produced in the favorable conditions where parents are already surviving.
3. There is no genetic variation (this is an advantage in agriculture since a desired characteristic can be preserved).
4. There is high number of offspring produced from a single vegetative structure.
5. Plants mature earlier for example when grafting is used in fruit trees.
6. Enable propagation of seedless plants like banana.

Disadvantages of vegetative reproduction

1. No genetic variation (young plants are susceptible to the same pests and diseases that attacked their parents).
2. The off springs are overcrowded hence compete for nutrients, light and water between themselves and possibly with the parent.