Structure of Pollen Grain: Definition, Structure, Formation, Types, Functions, Description, Characteristics

What Are Pollen Grain?

A pollen grain is, hence, the microstructure that bears the male gametes, otherwise known as the sperm cells, of plants capable of forming seeds. This hard outer coat protects genetic material being transported from the male anther to the female stigma during fertilization. It is the major means of pollination in plants, providing for fertilization by the transfer of male genetic material to the female ovule and hence giving rise to the seeds and fruits, their genetic variety, and propagation.

Detailed Structure Of Pollen Grain

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 very 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.

Cellular Components Of Pollen Grain

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.

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