How a Flower Makes Pollen and an Embryo Sac

Before a seed can form, a flower must make male and female gametophytes in two hidden places — the anther and the ovule. Each begins with a diploid cell dividing by meiosis.

This part covers the parts of a flower, the microsporangium and microsporogenesis, the pollen grain, and the ovule with megasporogenesis.

A typical angiosperm flower has four whorls — calyx, corolla, androecium and gynoecium — and the androecium, made of stamens, is where male gametophytes (pollen grains) develop, while the gynoecium, made of pistils, holds the ovules in which female gametophytes (embryo sacs) develop.

The whorls:

- Calyx — sepals that protect the bud
- Corolla — petals that often attract pollinators
- Androecium — stamens, each a slender filament topped by an anther
- Gynoecium — one or more pistils, each with a stigma that receives pollen, a style and an ovary containing ovules

Anther structure. A typical anther is bilobed, each lobe having two theca, so it is dithecous; it is also tetrasporangiate, with four microsporangia at its corners.

An everyday example. Pulling apart a hibiscus flower shows all four whorls clearly, with the stamens fused into a tube around the style.

The substance. The anther and the ovule are sporangia, not gametophytes — the gametophytes develop inside them.

A microsporangium is surrounded by four wall layers — epidermis, endothecium, middle layers and tapetum — enclosing sporogenous tissue, whose microspore mother cells each divide by meiosis to form a tetrad of four haploid microspores that become pollen grains.

Wall layers, from outside to inside:

- Epidermis — protection
- Endothecium — protection and help in dehiscence, the splitting of the anther to release pollen
- Middle layers — also help in dehiscence
- Tapetum — the innermost layer; its cells have dense cytoplasm, often more than one nucleus, and nourish the developing pollen

Microsporogenesis:

- Each cell can act as a diploid microspore mother cell, or pollen mother cell
- Meiosis produces a cluster of four haploid cells — the microspore tetrad
- As the anther matures and dries, the microspores separate and develop into pollen grains

An everyday example. The yellow dust left on your fingers after touching a lily's stamens is pollen released when the anther wall splits.

The substance. Tapetum cells are often multinucleate, an unusual feature linked to their role in feeding the developing pollen.

A pollen grain has a tough sporopollenin exine with germ pores and a thin intine, holds a vegetative and a generative cell, and is shed 2-celled or 3-celled; it stays viable for minutes to months and can trigger allergies.

Wall layers:

- Exine — made of sporopollenin, one of the most resistant organic materials, unharmed by heat, strong acids or alkalis
- Germ pores — gaps in the exine where sporopollenin is absent; the pollen tube grows out through one
- Intine — thin and continuous, made of cellulose and pectin

Cells inside:

- Vegetative cell — large, with plenty of food reserve and an irregular nucleus
- Generative cell — small and spindle-shaped, floating in the vegetative cell's cytoplasm
- Shedding stage — in most flowering plants pollen is shed 2-celled; in the rest the generative cell first divides into two male gametes, so pollen is shed 3-celled

Viability. Rice and wheat pollen lose viability within about 30 minutes of release, while pollen of some members of Rosaceae, Leguminosae and Solanaceae stays viable for months. Pollen can be stored in liquid nitrogen at $-196^{\circ }$C in pollen banks for crop breeding.

Allergies and nutrition. Pollen of plants such as Parthenium, or carrot grass, causes chronic respiratory disorders including asthma and bronchitis.

An everyday example. Carrot grass on roadsides and wasteland releases pollen that worsens hay fever and asthma for many people.

The substance. Short viability limits plant breeding — pollen that dies in half an hour must reach the stigma almost at once.

An ovule is attached to the placenta by its funicle and has protective integuments with a small opening, the micropyle, surrounding the nucellus; one megaspore mother cell in the nucellus divides by meiosis into four megaspores, one survives, and three rounds of nuclear division turn it into the 7-celled, 8-nucleate embryo sac.

Parts of the ovule:

- Funicle — the stalk joining the ovule to the placenta
- Hilum — where the body of the ovule meets the funicle
- Integuments — one or two protective envelopes
- Micropyle — the small opening at the tip of the integuments
- Chalaza — the base of the ovule, opposite the micropyle
- Nucellus — the mass of cells within the integuments, holding food reserves and the embryo sac

Megasporogenesis. A single megaspore mother cell near the micropyle divides by meiosis into four megaspores. In most flowering plants three degenerate and one becomes the functional megaspore — monosporic development.

Forming the embryo sac:

- The functional megaspore's nucleus divides by mitosis, and the two nuclei move to opposite poles
- Two further mitotic divisions give 8 nuclei, four at each end
- Walls form around six: three at the micropylar end — two synergids and an egg cell, together the egg apparatus — and three antipodals at the chalazal end
- The remaining two polar nuclei lie in the large central cell

An everyday example. Each pea in a pod grows from an ovule that once held its own embryo sac.

The substance. The embryo sac is the female gametophyte, while the ovule as a whole is the megasporangium.