Pollen Grain, Diagram, Structure, Types, Function and Formation

Pollen Grain: A pollen grain is a cluster of microspores that aids in plant fertilization, also known as a microspore. These grains are found in the male parts of flowering plants and are produced by male structures, specifically the anther in angiosperms and the microsporangia in gymnosperms. The structure of a pollen grain is key to its function, which is to transport male gametes to female reproductive structures, thereby facilitating fertilization.

Cross-pollination involves a variety of biological agents, including water, wind, and animals (primarily birds and insects), which help transport pollen grains to the stigma. Pollinators are the animals that help with this process. Palynology, or the study of pollen grains, is important for understanding human allergies, and animal diets, and analyzing archaeological shipwrecks. Pollen grains help most plants produce seeds and fruits.

Pollen grains include flower pollen from roses and hibiscus, as well as non-flowering pollen grains from cycads. The article below discusses the structure and function of pollen grains from the chapter Sexual Reproduction in Flowering Plants class 12 for the NEET Exam.

What Are Pollen Grains?

Pollen grains, or small structures found in flowers, contain the androecium, a flower's male reproductive organ. The pollen grain contains cytoplasm, a tube cell that develops into a pollen tube, and a generative cell that produces sperm nuclei. Pollen grains vary in size and shape. Depending on the species, their size can range between three and two hundred micrometers. They may be bean-shaped, spherical, ovule-shaped, triangular, disc-shaped, or have a smooth to spiky texture.

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Pollen Grain Diagram

The structure of a pollen grain is illustrated below:

Pollen Grain Types

The types of pollen grains are as follows:

1. Formation: Pollen grains are produced through various processes.
2. Classification: Pollen grains can be categorized based on the orientation of their furrows in relation to the original tetrads from the microspores.
3. Sulcate: This type of pollen grain has a furrow that runs across its middle. A single furrow is termed monosulcate; two furrows are bisulcate, and more than two furrows are polysulcate.
4. Colpate: These pollen grains have furrows that run in places other than the middle of the outer face. If two or more furrows are fused at the ends, they are syncopate; if they are not fused, they are polysorbate.
5. Tricolpate: Some pollen grains are tricolpate, meaning they have three colpi, or they possess a shape similar to that of tricolpate evolutionary-derived pollen.

Pre Fertilisation - Structures And Events

Pollen Grain Structure

Pollen grains are complex microscopic structures that vary greatly in size and shape. The size of pollen grains varies greatly between plant species. Pollen grains typically measure between three and two hundred micrometers (microns).

Pollen grains can take a variety of shapes, including round, ovule, triangular, disc-like, and bean-shaped. These grains can also differ in texture, ranging from smooth to spiky. Pollen grains are naturally white, but this can vary depending on the plant species. Some pollen grains may be yellow, orange, cream, or another colour.

Pollen grains are structurally divided into three parts:

1. Inner Cell: This area is filled with living cytoplasm. However, it degrades rapidly during the fossilization process.
2. Intine: The intine is the cell wall's inner layer, which is primarily made up of cellulose and pectin. The intine, like the cytoplasm, degrades quickly during fossilization.
3. Exine: The exine, or outer layer of the cell wall, is primarily composed of sporopollenin. Sporopollenin is among the most resistant substances found in nature. It is an N-free polymeric substance from the diverse class of organic compounds known as terpenes.

Pollen Grain Structure is an important topic for the NEET Exam from the chapter on sexual reproduction in flowering plants from class 12 biology.

Sexual Reproduction in Flowering Plants MCQ

Pollen Grain Formation

Pollen, the male gametophyte of seed plants, is produced in specialized structures known as microsporangia. Pollen is produced in gymnosperms such as conifers, whereas angiosperms produce it in flower anthers. Based on the species, pollen grains vary in shape, size, and surface characteristics. Pollen grains from pines, firs, and spruces have wings for wind dispersal, while forget-me-nots have the smallest, measuring 2.5-5 µm in diameter. Corn pollen grains are large, measuring 90-100 µm.

Female Gametophyte

Microsporogenesis and Microgametogenesis

Microsporogenesis is the process of producing microspores from sporogenous cells. Cytokinesis occurs after both meiotic divisions in successive microsporogenesis, whereas in simultaneous microsporogenesis, meiotic division occurs first. Microgametogenesis occurs when mature microspores develop into mature male gametophytes (pollen grains) that contain the male gametes (sperm cells) required for plant reproduction.

In angiosperms, the anther goes through a complex development process. Initially, it appears undifferentiated, but four groups of sporogenous cells form within the anther as the flower develops. These cells undergo meiotic division to produce haploid microspores, which develop into pollen grains. The pollen grains are surrounded by layers of sterile cells that contribute to the structure of the pollen sac. Some cells differentiate into nutritive cells, providing essential nutrients for the developing pollen grains.

Pollen Grain Functions

Pollen grains have the following functions:

1. Pollen grains, which appear as inconspicuous yellow particles, play an important role in plants' reproductive cycle. They contain male gametes and act as an important link in plant reproduction.
2. Gymnosperms, which produce male cones, and angiosperms, which produce flowers, both require pollen grains for reproduction.
3. It facilitates the transfer of male gametes (male reproductive cells) to the ovule (female reproductive cells) in the embryonic sac, allowing the egg to be fertilized and develop into a seed.
4. It also contributes significantly to the maintenance of genetic diversity in plant populations.
5. Pollen grains are highly nutritious, so pollen tablets are used as food supplements.

Pollen Viability

Pollen viability is the capacity of pollen grains to deliver male gametes to the embryo sac successfully. For fertilisation, pollen grains must land on the stigma before losing their viability.

The duration of pollen viability varies significantly and is influenced by factors such as temperature and humidity. In certain cereals like rice and wheat, pollen grains lose viability within 30 minutes of release. However, pollen grains can remain viable for months in some plants of the Leguminoseae, Rosaceae, and Solanaceae families.

Pollen as a Nutritional Source

Pollen grains are highly nutritious, and pollen products, such as tablets and syrups, are available in the market. It has been suggested that consuming pollen can enhance the performance of athletes and racehorses.

Pollen Banks

Pollen banks are specialised facilities in flowering plants that collect, preserve, and utilise pollen for various purposes, including research, breeding, and conservation. Pollen grains can be stored for extended periods, even years, in liquid nitrogen (-196°C). Similar to seed banks, these stored pollen grains serve as valuable genetic resources for crop breeding programmes.

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Pollen Allergy

Pollen grains from various plant species can trigger severe allergies and bronchial afflictions in susceptible individuals, leading to chronic respiratory disorders such as asthma and bronchitis. Parthenium or carrot grass, which was introduced to India as a contaminant in imported wheat, has become widespread and is a common cause of pollen allergy.

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