Hi, welcome to this hall as we study about the pollination In Plants, types and agents of pollination in biology.
Table Of Contents
- Pollination in plants
- Types of pollination
- Agents of pollination
- Features of pollinated flower
- Advantages of cross-pollination
- Differences between self pollination and cross pollination
- Characteristics of wind pollinated flowers.
- Differences between insect pollinated flower and wind pollinated flower.
- Revision Questions
Pollination In Plants
Definition: Pollination is defined as the transfer of mature pollen grains from The anthers of one flower to the mature stigma of the same flower or another flower of the same plant or closely related species. Pollination is the first step which leads to the eventual coming together of the male and female gametes for the sake of fertilisation.
Types Of Pollination
There are two types of pollination in plants. These are self-pollination and cross-pollination.
- Self pollination
- Cross pollination
Self pollination: Self-pollination is the transfer of mature pollen grains from anther of a flower to the stigma of the same flower or to that of another flower of the same plant, e.g. Pea, cotton, tomato. For self-pollination, only one parent plant is involved.
Cross pollination: 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 or closely related species, e.g. morning glory, hibiscus, pride of Barbados. For cross-pollination to occur, two parent plants are involved.
Agents Of Pollination
Agents of pollination also called pollinators are organisms and other means by which pollen grains are transferred from the hunters to the stigma of flowers. Agents of pollination In plants includes:
- Insects
- Wind
- Water
- Snail
- Birds
- Bats
- Man
Common insects involved in pollination are butterfly, moth and bees.
Features Of Pollinated Flowers
Conditions or devices which aid self-pollination. Some plants have condition or devices which aid safe pollination to take place. These conditions are homogamy and cleistogamy.
- Homogamy
- Cleistogamy
Homogamy: Homogamy refers to the ripening of the anthers and stigmas of a bisexual flower at the same time.
Cleistogamy: Cleistogamy is defined as a condition in which ripe pollen grains are deposited on the stigma which becomes ripened at the same time. This situation usually occurs among closed flowers, that is, bisexual flowers which never open at all.
Advantages Of Self-pollination
- It is a sure way of ensuring pollination, especially in bisexual flowers.
- It may not waste pollen grains
Disadvantages Of Self-pollination
- It leads to the production of weak offspring as a result of continuous or repeated self-pollination.
- The offspring or individuals produced are less adapted to the environment.
Conditions or devices which aid cross-pollination. Some plants may have conditions or devices which may aid cross-pollination to take place. These are:
- Dichotomy
- Unisexuality
- Self-sterility
Dichotomy: Dichogamy refers to the ripening of the anthers and stigmas of a bisexual flower at different times. Dichogamy occurs in two ways. This are protandry and protogyny.
Unisexuality: Unisexuality is a situation in which some plant bear only male or female flowers and not both on the same plant. E.g. pawpaw, such plant are said to be dioecious plants.
Self-sterility: Self-sterility refers to situation in which some plants make themselves sterile. The presence of pollen on their stigmas is injurious to further development of the plants. For example they may wither and die. However, when pulling grains come from other plants, fertilization can take place in such plants. Examples are found in The Passion flowers and tea.
Advantages Of Cross Pollination
- Cross-pollination leads to the production of healthier offspring than self-pollination.
- It also produces viable seeds.
- Ofspring or individuals produced are more adapted to the environmental conditions.
- It also leads to the formation of new varieties with good characteristics.
Disadvantages Of Cross Pollination
- It relies on external agents such as wind and insects whose presence at the right time cannot be guaranteed.
- It may lead to wastage of pollen grains especially pollination by wind.
Differences between self-pollination and cross-pollination
Self Pollination
- Self-pollination takes place only bisexual plants.
- Only one parents is involved.
- Pollination may occur without an external agent
- It does not ensure new varieties.
- Pollen grains are effectively utilized.
Cross Pollination
- Cross-pollination takes place in both in unisexual and bisexual plants.
- Two parents are involved.
- This requires external agents, e.g, insects and wind.
- It results in the formation of new varieties.
- Much of the pollen grains are wasted.
Major Agents Of Pollination
Two major agents of pollination in plants are wind and insects.
- Wind
- Insects
Characteristics of insect pollinated flowers
- They have large conspicuous petals/sepals.
- Flowers are usually brightly coloured.
- They possess scent.
- Nectar is also present.
- Pollen grains are rough, sticky and relatively.
- The stigma is flat with sticky surface to enable it receive pollen grains.
- Petals are shaped and arranged to enable visiting insects become dusted with pollen grains.
Examples of insect pollinated flowers are:
- Hibiscus
- Delonix
- Cowpea
- Crotolaria
- Pride of Barbados.
Process of pollination by insects, (e.g. pride of Barbados)
The insects that normally pollinate pride of Barbados are the swallow tail butterfly and bees. When the insect land on the standard petal it uncoils its proboscis and insert it through the furrow in the standard petal that leads to the nectar. During this process, the hairy body or wings of the butterfly or bee touches the pollen grains and this stick to the body or wings of the insect. When the insect visits another pride of Barbados flower, the pollen grains on its body or wings may touch the stigma of this flower thereby bringing about cross-pollination.
Characteristics of wind pollinated flowers also known as anemophilous flowers
- They have small, inconspicuous petals/sepals.
- Flowers are usually dull coloured
- There is absence of scent.
- Large quantity of pollen grains are produced.
- Pollen grains are small, smooth, light and not sticky.
- Stigma is elongated and sticky with large surface area.
- 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.
Structure Of The Maize Flower
- Apedicel
- Two glumes (outer bracts) and two palea (inner bracts)
- Three stamens
Differences between in self-pollinated flower and wind pollinated flower
Insect Pollinated Flower
- Flowers are usually large and conspicuous
- Flowers are usually brightly coloured.
- There is presence of scent.
- Nectares are present.
- Pollen grains are rough, sticky and relatively few.
- Anthers may or may not be enclosed by the petals.
- Flowers may or may not be held above the leaves.
- Stigma is flat or lobed with sticky surfaces for easy adherenc of pollen grains.
- The shape and floral pairs are such that they enable insects get dusted with pollen grains during visiting.
Wind Pollinated Flowers
- Flowers are usually small and inconspicuous.
- Flowers are usually dull coloured.
- There is absence of scent.
- Nectares are absent.
- Pollen grains are light, smooth and very numerous.
- Filaments are long so that anthers hang outside the flowers.
- Flowers are carried above the leaves where they are exposed to the wind.
- Stigma is large and featherly hanging outside the flowers providing large easy trapping of pollen grains.
- There is particular adapting shape as flowers are small and exposed.
Process Of Pollination By Wind, e.g. in Maize
The male flower ripen before the female flowers on the same plants. This favours cross-pollination. The wind carries pollen grains of one maize plant to another plant with ripe female flowers. If the pollen falls on ripe stigma, cross pollination takes place.
Revision Questions
- (a) What is pollination (b) explain the two types of pollination (c) explain the conditions which favor self-pollination.
- (a) explain in details the conditions which aid cross-pollination (b) state the advantages and disadvantages of cross-pollination.
- Explain the process of pollination by insects of a named flower. State two major agents of pollination in plants.
- (a) state six characteristics each of a wind and incense pollinated flower (b) state five differences between insect pollinated flower and wind pollinated flower.