Structure and Development of Male Gametophyte

The study of external morphology of mature pollen grain is called palynology.

The protective wall or covering of a pollen grain is called the sporoderm.

The region where the exine is absent and through which the pollen tube emerges is called a germ pore.

A yellowish, sticky, oily substance covering the exine of insect-pollinated pollen grains is called pollenkitt.

A pollen grain represents the male gametophyte of angiosperms (flowering plants). It is the microscopic structure that carries the male genetic material necessary for fertilisation.

The Two-Layered Wall

A pollen grain has a prominent two-layered wall:

Sporopollenin - The Armour of the Pollen

Sporopollenin is the chief component of the exine. It is an oxidative polymer of carotenoids and/or carotenoid esters.

Key Properties:

• Biologically the most resistant organic substance known in the plant kingdom
• Can withstand high temperatures and strong acids and alkalis
• No enzyme that degrades sporopollenin is known to exist
• Provides resistance to both physical and biological decomposition
• Because of sporopollenin, pollen grains are found well-preserved as fossils (palynofossils)

Germ Pores:

• Regions in the exine where sporopollenin is absent are called Germ Pores
• At germination, the intine protrudes through the germ pore to form the pollen tube
• In Dicots → typically 3 germ pores (triporate)
• In Monocots → typically 1 germ pore (monoporate)

Pollenkitt:

In insect-pollinated flowers, the exine is covered with a yellowish, viscous, sticky substance called pollenkitt.

Functions:

• Sticks to the body of insects → aids pollination
• Acts as an insect attractant
• Protects pollen grains against ultraviolet (UV) radiation

Internal Structure: The Two Cells of a Mature Pollen Grain

The pollen grain marks the beginning of the male gametophyte. Its full development involves two successive mitotic divisions:

Stage 1: First Mitotic Division (Inside the Anther)

Microspore → first mitotic division → two unequal cells:

1. Larger Vegetative Cell: rich in food; has a large, irregular nucleus
2. Smaller Generative Cell: initially attached to the intine; later detaches and floats freely in vegetative cytoplasm

This is the 2-celled stage (Bicellular stage)

• More than 60% of angiosperms shed pollen grains at this 2-celled stage

Stage 2: Second Mitotic Division

The generative cell divides mitotically → forms two non-motile male gametes

This second division may occur:

• Inside the pollen grain (before shedding) → 3-celled / tricellular stage
• Inside the pollen tube (after germination on stigma)

Development of male gametophyte

Pollen Allergy:

• Pollen causes severe allergies and bronchial afflictions → rhinitis, asthma, bronchitis
• Most common during spring when most plants flower
• Key species: Parthenium hysterophorus (carrot grass / gajar ghaas) — came to India as a contaminant with imported wheat; now ubiquitous; major cause of pollen allergy

In Parthenium grows widely along Indian roadsides, railway tracks, and wastelands. Its pollen triggers severe asthma, hay fever, and skin rashes in millions of people annually.

Pollen Products:

• Rich in: Carbohydrates, Proteins, and Fats
• Available as: tablets, syrups
• Uses: improve health, enhance athletic performance, food for race horses, cosmetics

• Pollen grains act as male gametophytes and are usually spherical, measuring about 25–50 μm in diameter.
• Each pollen grain has a two-layered wall (sporoderm) consisting of a thick outer exine and a thin inner intine.
• The exine contains sporopollenin, which makes pollen grains highly resistant and helps in their fossil preservation.
• Germ pores are present where the exine is absent; dicots usually have three pores, while monocots have one pore.
• The intine protrudes through the germ pore during germination to form the pollen tube.
• In insect-pollinated plants, pollen grains are coated with pollenkitt, which aids in insect attraction and pollination.
• Pollen viability varies widely, lasting minutes in cereals and months in some dicots; pollen can be preserved at –196°C in pollen banks.
• After release, the pollen grain divides into a vegetative cell and a generative cell, and the generative cell later forms two male gametes.