Pollen Grains: Its Importance in Plant Reproduction

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In the springtime, when flowers and trees start to bloom, many individuals suffer from red, itchy eyes and sneezing due to allergies. These seasonal allergies are caused by pollen, the yellow coating on cars, leaves, and other outdoor structures often seen during the spring and summer. Pollen grains are fine or coarse powders consisting of microgametophytes and producing male gametes or sperm cells. Palynology is the biology branch that mainly studies pollen and its properties.

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Pollen grains form the base for the plant reproduction system. To gain more insight read below.

What are Pollen Grains?

Pollen grains are microscopic structures that bear androecium – a flower’s male reproductive organ. The interior section of the pollen grain comprises cytoplasm along with the tube cell. Cytoplasm converts into a pollen tube, and the generative cell releases the sperm nuclei. Pollen represents a cell containing the male gametes associated with reproduction in flowering and cone-bearing plants.

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During the spring and summer, plants release millions of pollen cells, or grains, including the male reproductive gametes. Plant embryo formation occurs when the gametes within the pollen fertilize an ovule or egg cell. This embryo represents the seed or fruit from which a new plant can develop and grow.

Structure of Pollen Grains

Pollen grains are microscopic structures varying in size and design. The size of pollen grains usually varies along with the species. However, the exact size of pollen grain ranges between three and two hundred micrometers or microns. The shape of the pollen grain is round, triangular discs, or a bean shape with a smooth to spiky texture. The natural color of pollen grains is white, which may also vary depending on the plant species. Some are yellow in color, orange or cream, and so on.

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Pollen grains consist of three parts:

  1. The inside of the cell, filled with living cytoplasm, deteriorates rapidly during fossilization.
  2. Intine is the inner layer of the cell wall comprising pectin and cellulose. Just like cytoplasm, the intine also degrades rapidly during fossilization.
  3. The exine is the outer layer of the cell wall and mainly consists of sporopollenin. It is one of the most resistant substances. It is an N-free polymeric substance belonging to a diverse class of organic compounds called terpenes.

What Does Pollen Contain?

As mentioned above, each pollen grain contains the male gametes needed for reproduction in flowering and cone-bearing plants. Each pollen grain possesses a unique shape and structure depending on the specific plant species. You can use these features to help determine the plant species associated with different types of pollen.

The shape of Pollen Grains

Pollen cells can be classified into one of four shapes: oval, sphere, triangle, and disk-shaped. The pollen cell can be puffy, flattened, or elongated within these categories. Although pollen grains, when clumped together, can appear visible to the naked eye as a yellow coating on the exterior surfaces of lawn furniture, cars, or other vegetation, each pollen grain is tiny, measuring anywhere from 10 to 100μm in size, depending on the plant species.

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Miniature plants have smaller pollen grains. On the other hand, the larger sizes typically represent trees or shrubs. The majority of pollen grains are yellow. However, some plants possess pollen that can be red, white, or orange.

Exterior of Pollen Grains

The external surface of a pollen grain, or pollen cell, consists of an exterior layer referred to as the exine. The exine is formed from a fibrous tissue known as sporopollenin. This tissue is exceptionally durable and capable of withstanding various environmental conditions. The exine is often studded with numerous pores or openings referred to as apertures.

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You can use the apertures’ number, size, and shape to help identify the plant species associated with different pollen types. The exine is also characterized by different surface textures, such as smooth, net-like, parallel striations, bumps, and irregular patterns. As with differences in aperture types, You can also use the surface texture of pollen grains to identify the species associated with a specific pollen type.

In some pollen, an internal cell wall referred to as the intine is present. Like other plant cells, the intine is made of cellulose, which contributes to the durability of each pollen grain.

Interior of Pollen Grains

So, what does pollen contain? Inside each pollen grain, there are a variety of structures that help to facilitate male gamete growth and function. Aside from the standard cytoplasm content of the pollen cell, each pollen grain contains a tube cell. The tube cell matures into the pollen tube when the pollen has been deposited onto the ovule of a flower. Within the pollen cell, a generative cell divides to form the two male gametes, sperm nuclei, needed for plant fertilization.

Formation of Pollen Grain

  • The male reproductive whorl of the flower is called androecium. An individual member of the androecium is called the stamen. The stamen consists of filament, connective, and anther.
  • The anther is generally dithecous (having two lobes) and tetrasporongiate. Each monothecous anther contains two pollen sacs.
  • The pollen grains are produced in the pollen sacs.
  • Anther Sporogenous cell forms sporogenous tissue. This tissue produces microspores mother cell or pollen mother cell.
  • Each microspore mother cell divides meiotically to form a tetrad of haploid microspores (pollen grains).
  • The formation of pollen grains from the microspore mother cell is termed microgametogenesis.
  • Microspores’ mother cell (MMC) undergoes the first mitotic division to produce more gigantic, naked vegetative and small, thin-walled generative cells. 
  • The vegetative cell is rich in food and has an irregularly shaped nucleus. 
  • The abundant cell floats in the cytoplasm of the vegetative cell.
  • The second mitotic division is concerned with generative cells only and gives rise to two non-motile male gametes. 
  • The mitotic division of abundant cells occurs either in the pollen grain or the pollen tube. In most angiosperms, the pollen grains are shed from the anther at this two-celled stage.
  • After the pollen grains reach the stigma, they germinate to form pollen tubes, which begin the fertilization process.

Plant Fertilization and Pollen Grains

A plant is nature’s gift since they provide us with food, oxygen, shelter, clothing, etc. Like all other living things, plants respire, grow, develop, excrete and reproduce. All higher plants reproduce by fertilization. Ralph B. Strasburger discovered fertilization in flowering plants in the year 1884.

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Fertilization is a process of sexual reproduction that occurs after pollination and germination. It isthe fusion of the male gametes with the female gametes. The fusion leads to the formation of a diploid zygote. It is a physicochemical process that occurs after the pollination of the carpel. The complete procedure takes place in the zygote to develop into a seed.

In fertilization, flowers play a significant role as they are the reproductive structures of angiosperms (flowering plants). Plant fertilization occurs when gametes in haploid conditions fuse to produce a diploid zygote.

Fertilization Process

Post pollination of the carpel, the pollen grain germinates. It grows into the stye by creating the pathway for the pollen grain to move down to the ovary. Consequently, the pollen tube opens into the ovule through the micropyle and bursts into the embryo sac. Here, the male nucleus unites with the nucleus of an egg inside the ovule, forming a diploid zygote. Subsequently, the zygote later swells up and develops into a fruit.

The fertilization process can be classified into three types based on the entry of the pollen tube into the ovule.

Porogamy

It is the standard type of fertilization carried out in all flowering plants. In this type of fertilization, the pollen tube enters the ovule through the micropyle.

Chalazogamy

Chalazogamy is carried out on all Casuarina species of plants. In this condition, the pollen tube enters the ovule through the pollen tube.

Monogamy

In this type of fertilization, the pollen tube enters the ovule through its middle part or the integuments of the ovule. Monogamy occurs in all Cucurbit plants, such as ridge gourds, pumpkin, bitter gourds, and other gourd plants.

Pollen Grains and Double Fertilization

Double fertilization features the fusion of a female gametophyte with two male gametes. In double fertilization, one sperm cell fuses with the egg-producing zygote, and the other sperm cell fuses with the two polar nuclei to make the endosperm. All angiosperm plants undergo a double fertilization process.

Bottom Line

The pollen grain is a microscopic, non-motile, haploid, unicellular body with a single nucleus. present on anther. Consequently, they give rise to the male gametophyte of a seed plant. The pollen grain is a microscopic, haploid, non-motile, unicellular body with a single nucleus. You can often find Pollen in fossils because sporopollenin in the outer membrane remains as fossils. You can identify the plant species by the fact that the surface mesh, radial pattern, and shape are different for each pollen. 

Read related topics like plant seeds, plant life cycle, reproduction in plants, types of plants, and more.

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