Pollination in Plants

Pollination can be defined as the process by which mature pollen grains are transferred from the anthers of one flower, to the stigma of the same or another flower, hence making fertilization occur. It is a required process in reproduction in plants.


[Picture credit: Brocken Inaglory]

Types of Pollination

There are two main types of pollination: self pollination and cross pollination

1) Self Pollination: Self pollination is the type of pollination that takes place in the same plant. In other words, self pollination is the process by which mature pollen grains are transferred from the anthers of one flower, to the stigma of the same or another flower, of the same plant. In other words. Only one parent plant is required 
in self pollination.

Conditions Necessary for Self Pollination

The conditions necessary for self pollination are: homogamy and cleistogamy.

a) Homogamy: Homogamy can be defined as the process by which the anther and stigma of a bisexual flower ripen simultaneously. When this occurs, the following ways are means by which self pollination can occur:

1. The mature pollen grains may be blown by wind and be shed on the mature stigma below it.
2. The mature pollen grain may be transferred by insects to the stigma of the flower.
3. The mature stigma may push out of the corolla tube. At this time, they brush against the anthers, and during this process, pollen grains are collected. This is called self pollination. 
4. If it happens that the filaments are longer than the stigma, the filaments may bend to be in contact with the mature stigmas. 
5. If the styles are longer than the filaments, self pollination may occur. Possibly, the styles may also bend to make the stigmas be in contact with the anther.

b) Cleistogamy: This is the phenomenon in which ripe pollen grains are deposited on the stigma, and the stigma also becomes ripened simultaneously. It is common in bisexual flowers which never open, i.e closed flowers.

Benefits/Merits/Advantages of Self Pollination

The following are some of the benefits of self pollination:

1. It ensures pollination, especially in bisexual flowers.
2. It usually does not waste pollen grains.

Demerits/Disadvantages of Self Pollination

The following are some of the disadvantages of self pollination:

1. Weak offspring may be produced in the event of repeated self pollination.
2. The offspring produced are usually less adapted to the environment.

2) Cross Pollination: Cross pollination is the type of pollination that takes place in plants of same or closely related species. In other words, cross pollination is the process by which mature pollen grains are transferred from the anthers of a flower, to the stigma of a flower of another plant of the same or closely related species. Two parent plants are required in cross pollination.

Conditions Necessary for Cross Pollination
The conditions necessary for cross pollination are: unisexuality, dichogamy, self-sterility.

a) Unisexuality: This is the condition by which a plant has only one sex flower. That is, the plant only has male flower or female flower, and not both. Plants with this character are dioecious plants. 

b) Dichogamy: This is the process by which the sigmas and anthers of a bisexual flower becomes ripened separately. It can happen it two ways: protoandry and protogyny.

c) Self-sterility: This is the ability of a plant to sterilize itself. Presence of pollen on the stigmas is harmful to the development of the plant. It may cause them to wither and die. Nevertheless, fertilization can also take place by the presence of pollen grains. For instance, in the case of passion flowers and tea.

Advantages/Benefits/Merits of Cross Pollination.
The following are the advantages of cross pollination: 

1. The offspring produced through cross pollination are healthier than those of self pollination.
2. Offspring are more adapted to the environment.
3. New varieties with good features are produced. 
4. Viable seeds are produced.

Disadvantages/Demerits of Cross Pollination

The following are the disadvantages of cross pollination:

1. Pollen grains may waste, especially if pollinated by wind.
2. External agents like insects and wind are relied upon, and they may not be present at the right time.

Differences Between Self Pollination and Cross Pollination

Self Pollination	Cross Pollination
One parent is required	Two parents are required
New varieties are not assured	New varieties are assured
It only occur in bisexual plants	It occurs in unisexual and bisexual plants
Less pollen grains are wasted	More pollen grains are wasted
External agent may not be required	External agents are required

Agents of Pollination
Agents of pollination are also known as pollinators. They are means through which mature pollen grains are transferred from the anthers, and to the stigma of flowers. They aid pollination. They are usually required for pollination to take place. Common agents of pollination include:

• wind
• water
• insects
• man
• bats
• birds
• snails

But there are two major agents of pollination: 

1. wind
2. insects.

Features of Flowers Pollinated By Wind

Flowers pollinated by wind are called anemophilous flowers. The following are their features:

1. Scent is absent
2. Nectar is absent
3. Flowers are dull colored
4. Sepals/petals are small and inconspicuus 
5.  Pollen grains are produced in large quantity
6. Stigma is long, sticky and has large surface area
7. Pollen grains are small, smooth, light and not sticky

Examples of flowers pollinated by wind are maize, rice, wheat, e.t.c

Features of Flowers Pollinated by Insects

Flowers pollinated by insects are called entomophilous flowers. The following are their features:

1. Scent is present
2. Nectar is present
3. Flowers are brightly colored
4. Sepals/petals are shaped and arranged
5. Stigma is flat with sticky surface
6. Pollen grains are rough , sticky and few

Examples of flowers pollinated by insects are cowpea, pride of Barbados, and Hibiscus, e.t.c

Differences Between Flowers Pollinated By Wind And Flowers Pollinated By Insects 

Flowers Pollinated By Wind	Flowers Pollinated By Insects
Scent is absent	Scent is present
Nectar is absent	Nectar is present
Flowers are dull colored	Flowers are brightly colored
Pollen grains are produced in large quantity	Pollen grains are produced in small quantity
Flowers are held above leaves	Flowers may not be held above leaves

Categories: Biology, Reproduction, Reproductive Behavior