Pollen Grains

​Pollen grains are minute structures of varying size and shape that contain the androecium, the male reproductive organ of a flower. Pollen grains are also called microgametophytes. The formation of pollen grains occurs through the process of microsporogenesis and consists of a protective outer layer called the exine, which contains the male gametes. The function of pollen grains is to transport male gametes to the female reproductive organs for fertilization and seed production.

What are Pollen Grains?

A pollen grain is a microscopic structure produced by the male reproductive organs of flowering plants, known as anthers. It aid in the fertilization and sexual reproduction of flowering plants. They usually appear as fine dust and are transported by various means like water, air, and insects to the female structure where fertilization takes place.

When it reaches the stigma, it germinates to form a pollen tube that transports sperm to the ovule for fertilization. Pollen grain are of different shape and sizes depending on the species to which it belongs. The scientific study of living and fossil pollen grains is known as palynology.

Structure of Pollen Grain

Pollen grains are mostly spherical in shape and measure about 25-50 micrometers in diameter. It has two layered walls, the hard outer layer, called exine, is made up of sporopollenin. It protects the gametophyte during pollination. The exine layer has a prominent aperture called a germ pore. The thin inner layer called intine is made up of cellulose and pectin. It consists of a cytoplasm in the center surrounded by a plasma membrane. At the time of shedding, pollen grains may have two cells (a vegetative cell and a generative cell) or three cells (a vegetative cell and two male gametes). The vegetative cell has a large irregular nucleus. It is large in size as compared to generative cells and has an abundant food reserve. The generative cell is spindle-shaped with dense cytoplasm and a nucleus that floats in the cytoplasm of a vegetative cell.

Diagram of Pollen Grain

The diagram of the structure of pollen grain is shown below.

Formation of Pollen Grain

The formation of pollen grains begins with a process called microsporogenesis. In gymnosperm, pollen grain is formed in the microsporangia of the male cone. In angiosperm, it is produced in the anthers of flowering plants. The process of formation of pollen grain is as follows:

• After the anther develops, the sporogenous tissue undergoes meiotic division to form microspore tetrad( arranged in clusters of four).
• Each cell of sporogenous tissue is a potential pollen or microspore mother cell. it is formed through mitosis.
• The pollen mother cells undergo mitotic division and produces four haploid cells.
• Three of these cells form the vegetative cells of the pollen grain, while the fourth becomes the generative cell.
• As anther develops and dehydrates, the microspores separate from each other and develop into pollen grains. Within each microsporangium, lakhs of pollen grains are formed that are released with the dehiscence of anther.

Function of Pollen Grains

Pollen grain may appear as insignificant yellow dust, but it contains male gametes and forms an important link in the reproductive cycle of plants.

• The pollen grain is essential for reproduction in both gymnosperms, plants that produce male cones, and also in angiosperms, flower-producing plants.
• It transfers the male gametes (male reproductive cells) to the ovule (female reproductive cells) in the embryonic sac, for fertilization of egg that develops into seed.
• Helps in maintaining genetic diversity within the plant population.
• Pollen grains are rich in nutrients. Pollen tablets are used as food supplements.

Types of Pollen Grains

Pollen grains can be classified on the bases of size, shape, orientation of the furrow, and surface texture.

On the basis of Shape

Pollen grains are mostly ovoid, spherical, triangular, or disc-shaped.

On the basis of Size

The size of the pollen grains varies from 6 micrometers to 100 micrometers in diameter. Most pollen grains are 10- 70 micrometers.

On the Basis of the Orientation of Furrow

Orientation of furrow about the original tetrads from the microspores distinguishes the pollen into two types: Sulcate and colpate. In the sulcate, a furrow runs across the center. Sulcate can be further divided into monosulcate (one sulcus), bisulcate( two sulcus), or polysulcate (more than one sulcus). In Colpate, a furrow runs from other than the center. Colpate can further be divided into syncopated ( two or more fused ends) and polysorbate ( does not have fused ends).

On the basis of Surface Texture

They can be classified as:

• Psilate: smooth surface texture
• Reticulate: net-like surface texture
• Striated: rough parallel patterns
• Rugulate: irregular pattern
• Verrucate: bumps on the surface

FAQs on Pollen Grain

1. Define Pollen Grain.

A pollen grain is a microscopic, male reproductive structure produced by flowering plants. It contains the male gametes necessary for fertilization and transport it to female reproductive organs for plant reproduction.

2. What is the Structure of the Pollen Grain?

A pollen grain consists of a tough outer layer called the exine, composed of sporopollenin. It protects the male gametes within. The inner part of the pollen grain contains the male reproductive cells, including a generative cell that divides to form sperm cells.

3. What is the Function of Pollen?

The function of pollen is to transport the male gametes (sperm cells) from the male reproductive organs of a flowering plant to the female reproductive organs for fertilization, enabling the plant’s reproduction.

4. What are The Type of Pollen Grains?

Pollen grains are classified into two main types: monocolpate, with a single furrow or pore in their outer layer, and tricolpate, which have three furrows or pores. This helps in identifying and categorizing various plant species based on their pollen characteristics.

5. What is the Formation of a Pollen Grain?

Pollen grain formation take place through the process of microsporogenesis in the anthers. The diploid microsporocytes undergo meiosis to produce haploid microspores which undergoes mitotic divisions. These microspores develop into mature pollen grains containing generative cells and protective outer layers.