Pollen Grain: Structure, Development, and Viability

In the previous topic, we traced how sporogenous tissue inside the microsporangium undergoes meiosis to produce microspore tetrads, and how those tetrads eventually break apart into individual pollen grains. But what does a pollen grain actually look like once it is free? What is it made of on the inside? And once released from the anther, how long does it stay alive? This topic answers all of those questions, taking you from the outer shell of a pollen grain right down to the cells it carries within.

A World of Shapes and Patterns: External Appearance

Fig 1.4: Scanning electron micrographs of pollen grains from different species

If you were to touch the opened anthers of a Hibiscus flower, a yellowish powdery residue would stick to your fingers. These are pollen grains. Sprinkle a few on a glass slide with a drop of water, place them under a microscope, and the view is remarkable. Pollen grains from different species come in an astonishing range of sizes, shapes, colours, and surface patterns. Some have spiny surfaces, others are smooth and rounded, and still others display intricate net-like (reticulate) sculpturing.

Despite this diversity, most pollen grains share a common basic form: they are roughly spherical and measure about 25 to 50 micrometres in diameter. That is small enough that individual grains are invisible to the naked eye, which is why they appear as a fine powder when many of them collect together.

The Two-Layered Wall: Armour on the Outside, Flexibility on the Inside

Every pollen grain is enclosed by a wall made of two distinct layers. These layers differ sharply in their chemistry, their structure, and the jobs they perform.

Exine: The Indestructible Outer Shield

The outer layer is called the exine (the hard, sculptured outer wall of a pollen grain). It is built from a substance called sporopollenin (one of the most chemically resistant organic materials known in nature).

What makes sporopollenin so special? Three properties set it apart:

• Heat resistance — it can tolerate extremely high temperatures without breaking down
• Chemical resistance — it stands up to strong acids and strong alkalis alike
• Biological resistance — no enzyme capable of degrading sporopollenin has been discovered so far

This triple resistance is the reason pollen grains are commonly found as well-preserved fossils. Long after the rest of the plant has decayed, the sporopollenin shell of the pollen grain survives, sometimes for millions of years. Palaeobotanists (scientists who study ancient plants) rely heavily on fossil pollen to reconstruct what vegetation looked like in the distant past.

The exine is not a uniform, sealed layer. It has clearly defined openings called germ pores (apertures in the exine where sporopollenin is absent). These pores are essential for reproduction: when the time comes for the pollen grain to germinate on the stigma, the pollen tube grows outward through one of these germ pores because the tough sporopollenin layer cannot be penetrated anywhere else.

Why is the exine hard in the first place? Because the pollen grain faces a hostile journey. Once it leaves the anther, it may be blown by wind, carried on the body of an insect, or exposed to rain, sunlight, and temperature swings before it reaches a stigma. The sporopollenin exine acts as a suit of armour, protecting the delicate cells inside from all of these threats. The beautiful patterns and designs visible on the exine surface vary from species to species and serve as a reliable identification feature under the microscope.

Intine: The Soft Inner Layer

Beneath the exine lies the intine (the thin, continuous inner wall of the pollen grain). Unlike the exine, the intine is made of cellulose and pectin, materials that are soft and flexible. It forms a smooth, unbroken layer all around the pollen grain’s contents.

This softness is not a weakness. It serves a purpose. When the pollen grain germinates, the intine needs to stretch and push outward through the germ pore to form the pollen tube. A rigid material could not do this, but the elastic cellulose-pectin intine can extend considerably as the tube grows toward the ovule.

Just inside the intine, the cytoplasm of the pollen grain is enclosed by a plasma membrane, completing the set of boundaries that separate the living contents from the outside world.

Inside the Pollen Grain: Two Cells, Two Roles

Fig 1.5: (a) Enlarged view of a pollen grain; (b) Stages of a microspore maturing into a pollen grain

When a pollen grain reaches maturity, it contains two cells, each with a distinct role in the reproductive process.

The Vegetative Cell: Storehouse and Support

The vegetative cell (the larger of the two cells inside the pollen grain) takes up most of the pollen grain’s interior. It is packed with abundant food reserves that will fuel the growth of the pollen tube later. Its nucleus is large and irregularly shaped, a sign of high metabolic activity.

The Generative Cell: Carrier of Male Gametes

The generative cell (the smaller cell that floats within the cytoplasm of the vegetative cell) has a very different appearance. It is spindle-shaped, with dense cytoplasm and its own nucleus. Its position is distinctive: rather than sitting beside the vegetative cell, it floats inside it, creating a cell-within-a-cell arrangement.

The generative cell has one critical job. It divides by mitosis (simple cell division that produces two identical daughter cells) to give rise to the two male gametes (sperm cells) needed for fertilisation. The timing of this division, however, varies across species.

When Is the Pollen Grain Shed: 2-Celled or 3-Celled?

In over 60 per cent of angiosperms, pollen grains are released from the anther while they still contain only two cells: the vegetative cell and the undivided generative cell. This is the 2-celled stage. In these species, the generative cell divides to form two male gametes only after the pollen grain lands on a stigma and begins to germinate.

In the remaining species (roughly 40 per cent), the generative cell divides before the pollen grain is shed. So by the time the pollen leaves the anther, it already contains three cells: the vegetative cell and two male gametes. This is the 3-celled stage.

Whether the pollen is shed at the 2-celled or 3-celled stage does not change the end result. Either way, two male gametes are present by the time fertilisation takes place. The only difference is the timing of the mitotic division of the generative cell.

Pollen and Human Health: Allergies

Not everyone appreciates pollen. For some people, pollen grains are a serious health hazard. The pollen of many species triggers severe allergic reactions and bronchial problems, leading to chronic respiratory disorders such as asthma and bronchitis.

One particularly troublesome plant is Parthenium hysterophorus, commonly known as carrot grass or Congress grass. This invasive weed entered India accidentally as a contaminant mixed with imported wheat. Since its introduction, it has spread across the country and become extremely common. Its pollen is a major trigger of pollen allergy for large numbers of people.

Pollen as a Nutritional Supplement

Fig 1.6: Commercial pollen products

On a more positive note, pollen grains are rich in nutrients. In western countries, this has led to a growing market for pollen-based food supplements. These are available as tablets, syrups, and other formulations. Proponents claim that regular pollen consumption can boost stamina and performance. There are even claims that pollen supplements improve the performance of athletes and race horses, although such benefits remain a subject of ongoing study.

How Long Do Pollen Grains Stay Viable?

Once pollen grains are released from the anther, they must reach a compatible stigma while they are still alive and functional. But how long do they remain viable? The answer varies enormously from one species to another, and it depends partly on the temperature and humidity of the surrounding environment.

At one extreme, cereals like rice and wheat have pollen that loses viability within just 30 minutes of being shed. These species must achieve pollination very quickly, or the pollen is wasted. At the other extreme, members of families like Rosaceae (the rose family), Leguminoseae (the legume family), and Solanaceae (the nightshade family) produce pollen that can stay viable for months.

Pollen Banks: Long-Term Preservation

Just as animal semen and sperm can be frozen and stored for artificial insemination, pollen grains of many plant species can be preserved for years by storing them in liquid nitrogen at $-196°C$. Collections of cryogenically stored pollen are called pollen banks (facilities that store pollen at ultra-low temperatures for use in future breeding). They work on the same principle as seed banks: by keeping the material at extremely low temperatures, all metabolic activity stops and the pollen remains viable indefinitely.

Pollen banks are a valuable resource for crop breeding programmes. If a plant breeder wants to cross two varieties that flower in different seasons or grow in different regions, stored pollen from one variety can be shipped and applied to the stigma of the other at the right time, removing the constraint of needing both plants to flower simultaneously.