Pollen Grains: Process, Fertilization, Pollination, Types

Pollen can be defined as a bristly or fine powder that consists of microgametophytes and mass produces sperm cells or male gametes. The branch of biology that focuses on the study of pollen and its properties is called Palynology. Pollen grains are transferred by wind, water, air, birds, insects, animals to the female plants for the objective of fertilization. In this article, we will explore the type, structure, process of transfer, and sample questions for pollen grains.

Keyterms: Pollen, Palynology, Pollen grain, Microgametophytes, Sperm cells, Male gametes, Fertilization, Plants, Seed, Cytoplasm

Read Also: Reproduction

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Pollen Grains Definition and Structure

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Pollen grains refer to a powdery substance which is basically the male micro gametophytes of seeded plants, responsible for producing the male gametes.

Structure of pollen grains

Pollen grains have microscopic frameworks differing in their structure and size. The size of pollen grains ranges from species to species. The accurate size of a pollen grain ranges from two to three hundred micrometers. The pollen grain might be ovule, triangular, circle, disc, or bean-shaped having a smooth to spiky texture. The original color of pollen grains is white; however, this may vary depending on the plant species. Many may also be orange, cream, or yellow in color.

Structure of Pollen Grains

The pollen grain comprises mainly of 3 parts:

1. The thin inner layer goes by the name of intine, it is composed of cellulose and pectin. This layer degenerates briskly during fossilization.
2. The harder outer layer is called exine, it is made up of sporopollenin and is highly resistant. It belongs to the diverse organic class called terpenes.
3. The inside of the cell, filled with living cytoplasm, degrades rapidly during fossilization.

The exine has germ pores, which are distinct apertures that do not have sporopollenin present in them. After maturing, the pollen grains have two cells named the vegetative and generative cells.

Check out detailed notes for Sexual Reproduction in Flowering Plants. 

Read More About Sexual Reproduction in Flowering Plants

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Formation of Pollen Grains

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The formation of a pollen grain from a microspore involves the following changes-

• Augmentation of the microscope.
• Development of a large vacuole.
• Microspore nucleus replaced to an eccentric position against microspore wall.
• The nucleus undergoes first pollen mitosis.
• A small generative cell and a big vegetative cell are formed.
• The generative cell removes itself from the pollen grain wall.
• The generative cell is enveloped by the vegetative cell.
• A structure is formed having two cells between each other called a ‘cell within a cell’.
• The generative cell further goes through mitosis.
• Two sperm cells are formed within the cytoplasm of the vegetative cell.

Formation of Pollen Grain

Important Notes on Asexual reproduction

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Pollination 

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Transfer of pollen grains from anther to the stigma is termed as pollination. The same occurs with the help of pollinating agents like wind, air, water, birds, insects, etc.

Types of pollination

1. Autogamy: Transportation of pollen grains from anther to the stigma of the same flower. Observed in plants that generate Cleistogamous and Chasmogamous flowers. Example- Polygala, Viola
2. Xenogamy: The transportation of pollen grains from anther to the stigma of a different flower. It facilitates cross-pollination functionally as well as genetically.
3. Geitonogamy- Transportation of pollen grains from the anther to the stigma of a different flower belonging to the same plant. It facilitates cross-pollination functionally but self-pollination genetically.

Process of Pollination

1. Pollinating agents: Agents which help in transferring the pollen grains from the anther to the stigma of the same/different plant are termed as pollinating agents. Pollinating agents can be divided into two types-

• Biotic- Biotic agents are living organisms that take the role of pollinating agents such as insects and birds.
• Abiotic agents- Abiotic agents are non-living resources that act as agents such as wind, air, water, or wherein pollination occurs as a matter of chance.

Process of Pollination

The process of pollination takes place through various mediums such as:

2. Pollination by wind- Wind is known to carry pollen grain from anther to stigma. The same is very commonly observed in grasses. Some characteristics of the plant that is pollinated by wind are the following-

• Much exposed stamens;
• Light pollen grains;
• Large feather-like stigma;
• Non-sticky texture pollen grain

3. Pollination by insects- The most seen biotic pollinating agents are bees. Apart from insects several animals like bats, lizards, and rats also aid in the process of pollination. Some of the components that attract insects are the bright, attractive color of the flowers, nectar as well as fragrance they emit. In some rare cases, the plant and the insect display a reciprocal relationship. For example- The Moth-Yucca relationship; the insect moth facilitates in pollinating the Yucca flower.
4. Pollination by water- This is a rare method of pollination that can occur in a few ways:

• Female flowers navigate to the water surface with the help of a stalk wherein the male gametes were previously released. E.g. Vallisneria
• Male gametes are released inside water wherein flowers are present. E.g. Seagrass
• Water acts as a channel for gamete transportation in lower plants. E.g. bryophytes

Pollination in Plants

Read More: Apomixis

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Fertilization

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• The pollen tube dispatches the pollen grains to the ovary post pollination.
• Followed by which, one of the several male gametes unifies with the ovule/female gamete, and the other combines with the polar nuclei.
• The unification of a gamete with eggs produces a zygote, which gradually develops into an embryo.
• When the second gamete unifies with the polar nuclei, the endosperm nucleus is formed.
• It supplies nutrients to the embryo.
• The ovules grow into seeds after fertilization while the ovary grows into a fruit.

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Things to Remember

• Pollen grains refer to a powdery substance which is basically the male micro gametophytes of seeded plants, responsible for producing the male gametes.
• The pollen grain comprises mainly 3 parts: intine, exine, and the inside of the cell filled with cytoplasm.
• Pollination refers to the transfer of pollen grains from anther to the stigma of the flower.
• There are three types of pollination: Autogamy, Xenogamy, and Geitonogamy.
• The original color of pollen grains is white, however, it may seem yellow in various species.
• Sexual Reproduction in Flowering plants is an important topic of the CBSE Class 12 Board Examination. Objective or long answer types of questions are framed from this section for 2 to 5 marks. The entire unit of reproduction has a weightage of 14 marks in the board examination.

Read More: Parthenocarpy