Pollination in Plants – SS2 Biology Lesson Note (Third Term)

Pollination is the transfer of mature pollen grains from the anther of one flower to the mature stigma of the same or another flower of the same plant or another plant of closely related species. Pollination usually precedes fertilization. There are two types of pollination: self-pollination and cross-pollination.

Behavioural objectives

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

1. Define pollination.
2. Distinguish between self-pollination and cross-pollination.
3. State the advantages and disadvantages of each type of pollination.
4. List the features of self-pollinated and cross-pollinated flowers.
5. Name the agents of pollination.
6. Describe the characteristics of insect-pollinated and wind-pollinated flowers.

What is self-pollination?

Self-pollination is the transfer of mature pollen grains from the anther of a flower to the stigma of the same flower or to that of another flower of the same plant, e.g. tomato and pear. It therefore involves only one parent plant, i.e. bisexual flowers or monoecious plants.

Advantages of self-pollination

1. It ensures effective pollination in bisexual flowers.
2. It avoids the wastage of pollen grains.

Disadvantages of self-pollination

1. Continuous self-pollination produces weak offspring.
2. The offspring are less adaptive to the environment.

Features of self-pollinated flowers

Features favouring self-pollination include:

1. Homogamy: the ripening of the anther and stigma of a bisexual flower at the same time.
2. Cleistogamy: the flower stays closed, so pollen grains are deposited on the stigma inside the unopened flower.

What is cross-pollination?

Cross-pollination is the transfer of mature pollen grains from the anther of a flower to the stigma of a flower on another plant of the same or closely related species, e.g. hibiscus and pawpaw. It therefore involves two parent plants, i.e. unisexual flowers or dioecious plants. Cross-pollination depends on external agents such as wind and insects.

Advantages of cross-pollination

1. It produces healthier offspring.
2. It produces viable seeds.
3. The offspring are more adapted to the environment.
4. New varieties with good characteristics are formed.

Disadvantages of cross-pollination

1. It depends on external agents, e.g. wind and insects.
2. It leads to wastage of pollen grains, especially in wind pollination.

Features of cross-pollinated flowers

1. Dioecious flowers: male and female flowers occur on separate plants, e.g. pawpaw.
2. Dichogamy: male and female parts mature at different times. It can be of two kinds:
   • Protandry: the anther matures or ripens before the stigma, e.g. sunflower, okra and cotton.
   • Protogyny: the stigma matures before the anther, e.g. palms and figs.
3. Brightly coloured petals to attract insects, e.g. hibiscus.
4. A sweet smell to attract insects, e.g. rose flower.
5. Unisexual flowers.
6. Self-incompatibility.
7. Position of anthers and stigmas.

What are the agents of pollination?

Agents of pollination (pollinators) are the organisms and physical factors that help in the transfer of pollen grains from the anther to the stigma of flowers. The organisms include insects, birds, snails, bats and man. The physical factors are wind and water.

The two major agents of pollination are insects and wind. Flowers pollinated by each of them have certain features, shown in the table below.

Insect-pollinated and wind-pollinated flowers compared

Evaluation

1. Define pollination.
2. State two advantages and two disadvantages of self-pollination.
3. Explain the terms protandry and protogyny, giving one example of each.
4. List four characteristics of insect-pollinated flowers.
5. List four characteristics of wind-pollinated flowers.

Assignment

1. Differentiate between self-pollination and cross-pollination.
2. State four features of cross-pollinated flowers.
3. Name four agents of pollination.