Structure of Pollen Grain: Definition, Structure, Formation, Types, Functions, Description, Characteristics
Pollen grains are small, powdery structures bearing male reproductive cells in seed plants. Pollen is made in anthers of flowering plants or cones in gymnosperms. They move from one place to another through pollination via different agents like wind and water. Pollen is protected by a strong outer covering called sporopollenin. It is part of class 12 Sexual Reproduction in Flowering Plants.
This Story also Contains
- What Are Pollen Grains?
- Structure Of Pollen Grain
- Cellular Components Of Pollen Grain
- Difference Between Angiosperm and Gymnosperm Pollen
- Recommended video on "Structure of Pollen Grain"
.jpg)
In biology, the study of pollen is called palynology, which is useful in archaeology, genetic diversity, forensic science (crime scene geographical linkage). Pollen grains affect human health and cause pollen allergies. The pollen grains are produced and released by dithecous anthers having four pollen sacs. This article includes Pollen Grain Structure, its cellular contents and the Difference Between Angiospem and Gymnosperm Pollen.
What Are Pollen Grains?
A pollen grain is the microstructure that bears the male gametes, otherwise known as the sperm cells of plants, which are capable of forming seeds. A bilobed anther has two parts, and each part makes and stores pollen grains. The hard outer coat of pollen protects the genetic material being transported from the male anther to the female stigma. Microsporogenesis is the process by which pollen grains are formed from microspore mother cells through stages of meiosis inside the pollen sac. The pollen transfer process is called pollination in plants to the female ovule, providing it for fertilisation and forming the seeds and fruits.
Also Read:
Structure Of Pollen Grain
The structure of a pollen grain is made up of two main layers—exine and intine. These layers protect the pollen and help in pollination and fertilisation. Understanding the exine and intine layers is important for learning how pollen survives and grows during reproduction. The detailed structure of the pollen grain is described below-
Exine (Outer Layer)
Composition and Function
Exine is the outer, harder layer of the pollen grain and consists of sporopollenin—one of the most resilient organic compounds known. The exine acts to protect the pollen during its travels from the anther to the stigma from damage by UV radiation, drying out, and microbial assault.
Role in Protection
The exine plays a vital role in the maintenance of the integrity of a pollen grain vis-à-vis adverse environmental conditions. Its solidity, hence, secures the safe transportation of the genetic material.
Patterns and Apertures
The exine surface may be very elaborate. It may develop specific patterns, like reticulate or net-like, spinate or spiny, or psilate (smooth), to facilitate identification. The apertures are the thinner regions of exine and typically involve structures called colpi, which are long furrows, and pores that are the entry point for the pollen tube when germination takes place.
Intine (Inner Layer)
Composition and Function
The intine is the inner layer of any pollen grain. Since the intine is made up of a mixture of mostly cellulose and pectin, the intine is pretty loose compared to the exine; thus, it can grow, expand, and stretch throughout the formation process of a pollen tube.
Flexibility and Growth Role
The intine, responsible for supporting the physiological activity of a pollen grain, participates in hydration and consequent germination of the latter upon its fall on the stigma. The elasticity in the intine is quite important in the germination process, as the pollen tube has to penetrate through these exine apertures to fertilise the ovule.
Differences Between Exine and Intine
While exine protects the spore/pollen externally and imparts typical characteristics for identification purposes, intine participates in internal physiological functions during fertilisation in plants.
Cellular Components Of Pollen Grain
The pollen grain has two main cells: the generative cell and the vegetative cell. These cellular components help in pollen tube formation and play a key role in fertilisation in flowering plants. The cellular components of the pollen grain are listed below-
Generative Cell
The generative cell is the main cell in the pollen grain. It undergoes mitotic division to produce two sperm cells for fertilisation.
Structure and Function
The generative cell is small with the male genetic material. It travels within the pollen tube to reach the ovule.
Process of Cell Division
During the pollen tube growth, the generative cell undergoes division that leads to the formation of two sperm cells. This step is significant in angiosperms while performing double fertilisation.
Vegetative Cell
It is the larger cell of the pollen grain that contains the generative cell.
Structure and Function
The vegetative cell provides the controlling factor for pollen tube growth and the direction towards the ovule via the style.
Role in Pollen Tube Formation
The vegetative cell's nucleus guides the extension of the pollen tube and also helps in delivering the sperm cells into the ovule.
Difference Between Angiosperm and Gymnosperm Pollen
Pollen grains are produced in both angiosperms—the flowering plants and gymnosperms—the cone-bearing plants. Although their main function is to carry the male gametophyte for fertilisation but they differ in origin, structure, and pollination method. Some of the differences between angiosperm and gymnosperm pollen are given below:
Feature | Angiosperm Pollen | Gymnosperm Pollen |
Site of Production | Produced in anthers (part of the stamens) | Produced in microsporangia of male cones |
Structure | Usually smaller, with smooth or varied surface | Larger, often winged for wind pollination |
Pollination Method | Insects (entomophily), animals, wind, and water | Mainly wind (anemophilous) |
Pollen Tube Formation | Forms after reaching the stigma | Forms even before pollination (in some cases) |
Fertilisation Type | Double fertilisation (unique to angiosperms) | Single fertilisation |
Germination Site | On the stigma of a flower | Near the ovule inside the female cone |
Adaptations | Sticky or spiny for animal pollination | Lightweight and winged for air travel |
Other Useful Resources:
Recommended video on "Structure of Pollen Grain"
Frequently Asked Questions (FAQs)
Sporopollenin protects the exine layer from chemical and biological degradation. This hard layer will protect the pollen grain from UV dehydrating conditions and microbial attack, ensuring the safe transport of genetic material from another to the stigma.
In this process, the generative cell goes through a division to result in two sperm that later participate in fertilisation. The vegetative cell, in turn, provides growth that will eventually elongate to form a pollen tube tip carrying the sperm cells toward the ovule.
Microsporogenesis is the process by which pollen grains develop within the anther. It initiates with the diploid microsporocytes. These undergo meiosis to produce four haploid microspores. Every microspore undergoes mitosis to form one vegetative cell and one generative cell, hence forming a pollen grain. This pollen grain grows to have a hard exine and finally gets released from the anther for pollination.
When a pollen grain falls on a compatible stigma, it rehydrates itself with water and other nutrients that finally germinate. The intine layer swells up and finally leads to the development of the pollen tube with the aid of the vegetative cell through the exine apertures. From signals of the vegetative cell, the pollen tube grows toward the ovule down the style. After this, it gets divided by the generative cell into two sperm cells. The pollen travels down the pollen tube to reach the ovule for fertilisation.
Two layers compose a pollen grain: exine, the outer, and intine, the inner. The exine is resistant, and tough chemically, whereas the intine is flexible and concerned with growth while germinating. There are two kinds of cells inside a pollen grain: a generative cell and a vegetative cell.