SSS 2: Pollination in plants
POLLINATION
Pollination is the transfer of mature pollen grains from the anthers of one flower to the mature stigma of the same or another flower of the same or closely related species.
Pollen grains contain the male gamete and are present in the anthers of the flower.
The process of sexual reproduction in plants consists of several chronological steps:
- Production of flowers
- Pollination
- Fertilization (produces seeds)
- Formation of fruits
Agents of pollination
Agents of pollination are also called pollinators. They are organisms and other means by which pollen grains are transferred from the anthers to the stigma of flowers.
Examples of agents of pollination include
1. insects e.g. Butterfly, moths, bees, etc
2. wind
3. water
4. other animals such as snails, birds, bats goats, man, etc.
Types of pollination
Pollination is of two types:
1. Self-pollination
2. Cross-pollination
Self-pollination
Self-pollination is the transfer of mature pollen grains from the anthers of a flower to the mature stigma of the same flower or another flower on the same plant.
This type of pollination is common in hermaphrodite or dioecious plants which contain both male and female parts on the same flower. Only one parent plant is involved e.g. Pea, cotton, tomatoes, wheat, barley, oats, rice, potatoes, apricots, peaches, etc
Advantages of self-pollination
1. Only a few pollen grains are capable of pollinating the bloom
2. The race’s purity is preserved
3. Pollen grains are not wasted when plants self-pollinate
4. Pollination failure is less likely
5. It is the best way of enduring pollination, especially in bisexual flowers.
Disadvantages of self-pollination
1. Self-pollination that continues from generation to generation leads to the production of weak offspring
2. The breed’s weaker or faulty characteristics cannot be eradicated
3. It produces no new varieties
4. The offspring produced are less adapted to the environment over some time
Conditions(Devices) which aid self-pollination
Self-pollination is aided in nature by one of the following flower adaptations or conditions.
1. Homogamy: Homogamy is the ripening of the anthers and stigmas of a bisexual flower at the same time. Pollination and fertilization occur in a opened flower bud
2. Cleistogamy: Cleistogamy is a condition in which ripe pollen grains are deposited on the stigma which becomes ripened at the same time. Pollination and fertilization occur in unopened flower bud. It ensures self pollination and prevents cross pollination.
Cross-pollination
Cross-pollination is the transfer of mature pollen grains from the anthers of a flower to the mature stigma of a flower of another plant of the same or closely related species. e.g. Hibiscus, pride of Barbados, flamboyant (flame of the forest), Rose, Morning Glory, sunflowers, etc
Advantages of cross-pollination
1. It leads to the production of healthier offspring than self- pollination
2. It produces viable seeds
3. Offsprings are more adapted to the environmental conditions
4. It leads to the formation of new varieties with good characteristics.
Disadvantages of cross-pollination
1. It depends on external agents such as wind and insects whose presence cannot be guaranteed
2. It may lead to wastege of pollen grains, especially wind pollinated.
Conditions(Devices) which aid cross-pollination
Cross-pollination is aided in nature by one of the following flower adaptations or conditions
1. Dichogamy: Dichogamy is the ripening of the anthers and stigmas of a bisexual flower at different time.
Dichogamy occurs in two ways:
(a) Protandry: This is a condition in which the anthers of a flower mature earlier than the stigmas of the same flower or other flower on the same plant. The mature pollen grains are only useful to other flowers with matured stigmas to received them.
(b) Protogyny: This is a condition in which the stigmas of a flower mature earlier than it's own pollen grains. The mature stigmas can only receive pollen grains from flowers of other plants.
2. Unisexuality: This is the situation whereby some plants bear only male or female flowers and not both on the same plant e.g. Pawpaw. Such plants are called dioecious plants. Cross pollination may occur under this condition.
Note: Monoecious plants has both male and female flowers on the same plant
3. Self-sterility: This is a situation whereby some plants make themselves sterile. Examples include Tea, Tobacco, Potato, Crucifers etc
Differences between self-pollination and cross-pollination
| Self-Pollination | Cross-Pollination |
| 1.Transfer pollen grains from the anther to the stigma of the same flower. | 1.Transfer pollen grains from the anther to the stigma of a different flower. |
| 2. This process can take place in the same flower or a different flower of the same plant. | 2. This process can take place between two flowers present on different plants. |
| 3. Self-pollination increases genetic uniformity and decreases genetic variation. | 3. Cross-pollination decreases genetic uniformity and increases genetic variation. |
| 4. Produces limited amounts of pollen grains. | 4. Produces large amounts of pollen grains. |
| 5. In self-pollination, both the stigma and anther simultaneously mature | 5. In cross-pollination, both the stigma and anther mature at different times. |
| 6. Transfers a limited number of pollens. | 6. Transfers large numbers of pollen. |
| 7. This process is carried out even when the flowers are closed. | 7. For cross-pollination to happen, the flower should be open. |
| 8. No need for pollinators to transfer pollen grains. | 8. Require pollinators to transfer pollen grains. |
| 9. Pollen grains are transferred directly to a flower’s stigma. 10. It causes inbreeding | 9. Pollen grains are carried via wind, insects, animals, water, etc. 10. It causes out breeding |
Major agents of pollination
The major agents of pollination are wind and insects.
Characteristics of insect-pollinated flowers
Insect-pollinated flowers are also called ENTOMOPHILOUS FLOWERS.
1. They have large conspicuous petals or sepals
2. Flowers are usually bright coloured
3. They posses scent
4. They contain nectar
5. Pollen grains are rough, sticky and relatively few
6. The stigma is flat with a small sticky surface area to enable it receive pollen grains
7. Petals are shaped and arranged to enable visiting insect become dusted with pollen grains.
Examples of insect-pollinated flowers: hibiscus, pride of barbados, crotalaria, copper etc.
Characteristics of wind-pollinated flowers
Wind-pollinated flowers are also called ANEMOPHILOUS FLOWERS.
1. They have small inconspicuous petals or sepals
2. Flowers are usually dull coloured
3. There is aabsence scent
4. There is absence of nectar
5. Large quantities of pollen grains are produced
6. Pollen grains are small, smooth, light and not sticky
7. The stigma is elongated with a large sticky surface area to enable it receive pollen grains
8. Anthers are attached to the flower in such a way that they readily swing in the air and release the pollen grains.
Examples of wind-pollinated flowers are maize, guinea grass, rice, millet, wheat etc
ASSIGNMENT
1. State ten (10) differences between insect pollinated flowers and wind-pollinated flowers
2. Describe the structure and process of pollination by:
(a) wind of a maize flower
(b) insect of a pride of barbados flower
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