How a Single Pollen Grain Carries What a Flower Needs to Make a Seed

Before a mango or a grain of rice can form, the flower must make pollen in its anthers, build a tiny embryo sac inside each ovule, and get pollen onto the stigma.

This lesson covers pollen and the microsporangium, the ovule and embryo sac, pollination and its adaptations, and artificial hybridisation.

A typical anther is bilobed with four microsporangia, whose walls enclose sporogenous tissue that divides by meiosis into tetrads of haploid microspores, each maturing into a two-layered pollen grain.

Microsporangium wall, from outside in:

- Epidermis, endothecium and middle layers — protect the anther and help it split open
- Tapetum — the innermost layer, which nourishes the developing pollen

Microsporogenesis. Each pollen mother cell divides by meiosis into a tetrad of four haploid microspores, which separate and develop into pollen grains.

Structure of a pollen grain:

- Exine — a hard outer wall of sporopollenin, one of the most resistant organic materials known, with gaps called germ pores
- Intine — a thin inner wall of cellulose and pectin
- A mature grain has a large vegetative cell and a small generative cell, which divides into two male gametes

Economic importance:

- Pollen of plants such as Parthenium causes allergies and bronchial problems
- Pollen can be stored in liquid nitrogen at -196 °C in pollen banks for use in crop breeding

An everyday example. Sneezing fits when carrot grass (Parthenium) is in flower in many Indian towns are an allergic reaction to its pollen.

The substance. Sporopollenin explains why pollen survives as fossils — it withstands high temperatures and strong acids and alkalis, and no known enzyme breaks it down.

An ovule is attached to the placenta by a stalk called the funicle and contains a nucellus wrapped in integuments, and inside it one megaspore divides by three rounds of mitosis to form a seven-celled, eight-nucleate embryo sac — the female gametophyte.

Structure of the ovule:

- Funicle — stalk joining the ovule to the placenta, meeting it at the hilum
- Integuments — one or two protective layers, leaving an opening called the micropyle
- Chalaza — the basal part, opposite the micropyle
- Nucellus — cells with reserve food, enclosing the embryo sac

Megasporogenesis. A megaspore mother cell near the micropyle divides by meiosis into four megaspores; in most flowering plants, three degenerate and one stays functional.

Development of the embryo sac:

- The functional megaspore's nucleus divides by mitosis into 2, then 4, then 8 free nuclei, four at each end
- Cell walls form, organising the embryo sac
- Micropylar end — the egg apparatus: one egg cell and two synergids with a filiform apparatus that guides the pollen tube
- Chalazal end — three antipodal cells
- Centre — a large central cell with two polar nuclei

An everyday example. Each pea in a fresh pod began as an ovule, and the tiny stalk joining it to the pod wall is its funicle.

The substance. Only one of the four megaspores usually survives — which is why this pattern is called monosporic development.

Pollination is the transfer of pollen to a stigma — autogamy within the same flower, geitonogamy between flowers of the same plant, and xenogamy between different plants — carried by wind, water or animals, with flowers shaped to suit their agent.

Types of pollination:

- Autogamy — within the same flower; cleistogamous flowers, which never open, as in Commelina and Viola, are always self-pollinated
- Geitonogamy — another flower on the same plant; it needs an agent, but genetically it is like self-pollination
- Xenogamy — a flower on a different plant; it brings genetically different pollen

Wind-pollinated flowers:

- Light, non-sticky pollen produced in large amounts
- Well-exposed stamens and large, feathery stigmas
- Examples: grasses and maize, whose tassels are the stigmas and styles waving in the wind

Water-pollinated flowers:

- Found in only a few plants, such as Vallisneria, Hydrilla and Zostera
- Pollen is carried by water currents and is often protected by a mucilaginous covering

Insect-pollinated flowers:

- Large, colourful, fragrant flowers rich in nectar
- Examples: Salvia, sunflower and mango

An everyday example. Beekeepers place hives in flowering mustard fields in Rajasthan and Punjab, because bees carry pollen between the flowers.

The substance. Water hyacinth and water lily grow in water but are pollinated by insects or wind — living in water does not mean water pollination.

In artificial hybridisation, a breeder removes the anthers of the chosen female parent before they shed pollen, covers the flower with a bag, and later dusts its stigma with pollen from the chosen male parent, so only the desired cross takes place.

Steps:

- Emasculation — removing the anthers from a bisexual flower bud with forceps before they burst
- Bagging — covering the emasculated flower with a butter-paper bag to keep out unwanted pollen
- Pollination — when the stigma becomes receptive, dusting it with pollen collected from the male parent
- Rebagging — covering it again so the fruit develops without contamination

Unisexual flowers. If the female parent bears only female flowers, emasculation is not needed — the female flower buds are simply bagged before they open.

Significance:

- Combines desirable traits, such as high yield and disease resistance, from two parents

An everyday example. Hybrid tomato and maize seed sold to Indian farmers is produced by controlled crosses between selected parent lines.

The substance. Emasculation must happen before the anthers mature — if pollen has already been shed, the flower may pollinate itself and the cross is spoiled.