BIOLOGY LESSON NOTE ON POLLINATION IN PLANT

 TOPIC: POLLINATION IN PLANTS

Pollination is the transfer of pollen grains from a mature anther to a mature stigma. In most flowering plants, external agents are required for pollination to occur. Pollination brings about fertilization and the production of fruits.

Types of Pollination

There are two types of pollination;

1. Self pollination is the transfer of mature pollen grains from the anther to the stigma of the same flower or another flower on the same plant.

2. Cross pollination is the transfer of mature pollen grains from the anther of a flower to the stigma of a flower of another plant of the same species. Cross pollination brings the male gametes and egg cells of two different plants together.

Features of Self Pollinated Flowers

1. Self pollination occurs when the anther and stigma of bisexual flowers mature at the same time. This condition is referred to as homogamy.

2. Self pollination occurs only in bisexual flowers and in unisexual flowers of monoecius plant. The anthers are located above the stigma so that when the anthers open, the stigma will be covered with pollen grains that fall from above.

3. Flowers of some plants e.g.groundnut never open fully or never open at all because the corolla encloses both the stamens and carpels. Thus, the corolla brings the anthers and stigmas together. This condition is known as cleistogamy.

4. The filaments of anthers may bend over and the anthers are brought near the stigmas. As the anthers burst, the pollen grains are discharged directly on the stigma. Example is found in four o' clock plant ( mirabilis)

Features of Cross Pollinated Flowers

1. Cross pollination occurs in dioecious plants since the male and female flowers are borne on separate plants.

2. Cross pollination occurs in monoecious plants if the male and female flowers ripen( mature) at different times.

3. Male and female parts are in one flower but ripen at different times e.g. crotalaria. This condition is known as dichogamy. If the female part ripen before the male, the flower is said to be protogynous but if the male part ripens before the female, the flower is said to be protandrous.

Advantages of Self Pollination

1. There is no wastage of pollen grains because they get to the stigma easily.

2. Genetic variation between the parents and offsprings is small, I.e. the offsprings resemble the parents.

3. Offsprings produced are many since many seeds are fertilized.4. Chances of fertilization is high because the distance covered by pollen grains is small.

Disadvantages of Self Pollination

1. The offsprings cannot withstand adverse and varied environmental conditions.

2. The offsprings are unable to compete with other plants that developed from varied seeds.

Advantages of Cross Pollination

1. It produces offsprings that can compete with other plants in times of scarcity of food and water.

2. The varied seeds are resistant to infections and diseases.

3. There is variation in the seeds and the offsprings produced.

4. The seeds and offsprings can survive adverse environmental conditions.

Disadvantages of Cross Pollination

1. Some pollen grains are wasted during transit without getting to the ovule.

2. Cross pollination depends solely on external agents like wind and insects for pollination to occur.

Agents of Pollination

Agents of pollination or pollinators are those organisms or media which aids the transfer of pollen grains from the mature anther to the mature stigma. The common agents of pollination are

1. Wind 
2. Insects.
3. Water
4. Other animals like birds, squirrels, bats, snails, etc.

1. Wind pollination: Most species of flowering plants are pollinated by wind, the most familiar being grasses such as cereal plants, sedges and several palms. Wind pollination is a highly wasteful process.

This is because plants produce millions of pollen grains of which only a small proportion land on receptive stigma.

Features of Wind Pollinated Flowers

Wind Pollinated flowers are known as anemophilous flowers. They exhibit certain characteristics which

include;

i. The flowers are small and inconspicuous but are borne in large inflorescences well above the leaves.

ii. The flowers are unisexual with male and female flowers occurring on the same plants or on separate plants.

iii. The sepals and petals, if present are pale or green in colour.

iv. The flower structure is usually simple with one or more floral parts absent.

v. The flowers are not scented and nectaries are absent.

vi. The stamens hang outside the flower when mature. The anthers are large and produce large quantities of pollen grains.

vii. The pollen grains are small, light and dry with smooth walls.

viii. The stigmas hang outside the flower, borne on long flexible styles. They are feathery or branched thereby providing a large surface area for trapping the pollen grains.

2. Insect Pollination: Insects visit flowers to obtain nectar and pollen grains for food.

These insects are attracted to the flowers by the colours and scents. In the process of reaching their source of food, insects bring about pollination. The common insects that bring about pollination are butterflies, bees, wasps, beetles, moths and ants.

Features of insect Pollinated Flowers

Insect Pollinated flowers are known as entomophilous flowers.They exhibit certain

characteristics which include;

I. The flowers are large and conspicuous e.g. clitoria, stenolobium stans, flamboyant). If each flower is small e.g. ixora and salvia, it occurs in a large and conspicuous inflorescence. In some cases, tiny florets are grouped into a conspicuous head e.g sunflower

II. The flowers are bisexual I.e. have both stamens and carpels in one flower.

III. The flowers are scented and produce nectar from nectarines. Scents help insects to locate their food source. It is important in guiding nocturnal insects.

IV. The stamens lie within the flowers. The anthers are small and firmly attached at their bases or fused along their backs to have contacts with visiting insects. The pollen grains are large and sticky which enables them to adhere firmly to the bodies of insects.

V. The stigmas are enclosed within the flowers. They are small and rigid with sticky surfaces so that pollen grains can adhere to them.

VI. The flowers usually show structural modifications specially suited for their

associated insect pollinators. The aims of the modifications are to get pollinators become dusted with pollen grains and also come in contact with the stigmas while feeding on the nectar and pollen.

EVALUATION: Students should answer the following questions;

1. Define pollination.

2. Briefly explain two types of pollination.

3. State four features of insect Pollinated flowers.

4. Tabulate four differences between wind and insect pollinated flowers.