Pistil Megasporogenesis Embryo Sac

1.2.2 The Pistil, Megasporangium (Ovule), and Embryo Sac

The female reproductive whorl of a flower is the gynoecium. It is made of one or more carpels (also called pistils). A gynoecium with a single carpel is monocarpellary; one with many carpels is multicarpellary. When many carpels fuse into a single structure (as in papaya), the gynoecium is called syncarpous; when they remain free (as in lotus or rose), it is called apocarpous.

Each pistil has three regions:

• Stigma — the sticky, often feathery tip that receives pollen.
• Style — the elongated slender stalk between stigma and ovary; the pollen tube traverses it.
• Ovary — the swollen basal region that contains one or more ovules attached to a cushion-like placenta.

Structure of an Ovule (Megasporangium)

An ovule is a small structure attached to the placenta by a stalk called the funicle. The point where the funicle joins the body of the ovule is the hilum. The body itself has:

• Integuments — one or two protective layers that envelop the ovule, leaving a small opening called the micropyle at one end.
• Chalaza — the basal region opposite the micropyle.
• Nucellus — the mass of parenchymatous cells inside the integuments; the embryo sac differentiates here.

Megasporogenesis

Inside the nucellus, one cell differentiates into the megaspore mother cell (MMC). Its large size, dense cytoplasm and prominent nucleus make it easy to spot. The MMC undergoes meiosis, yielding four haploid megaspores. This process is called megasporogenesis.

In most flowering plants, three of the four megaspores degenerate and only one remains functional. This lone survivor develops into the female gametophyte. Because just one megaspore participates, the development is called monosporic — a pattern found in the vast majority of angiosperms.

Female Gametophyte — The Embryo Sac

The functional megaspore is the first cell of the female gametophyte. Its nucleus undergoes three free-nuclear mitotic divisions in sequence:

• Division 1 → 2 nuclei (one moves to each pole of the cell)
• Division 2 → 4 nuclei (2 at each pole)
• Division 3 → 8 nuclei (4 at each pole)

Cell walls now form around six of these eight nuclei:

• At the micropylar end — three cells form the egg apparatus: one central egg cell flanked by two synergids.
• At the chalazal end — three cells called the antipodals.
• The two remaining nuclei (one from each pole) migrate to the centre and sit in a large uncelled region — the central cell with two polar nuclei (these may later fuse to form a secondary nucleus, 2n).

So the mature embryo sac of the Polygonum type is described as 7-celled and 8-nucleate — seven cells (3 antipodals, 1 central, 2 synergids, 1 egg) but eight nuclei (because the central cell has two).

1.3 Pollination

Pollen sits on the anther and egg waits inside the ovule — they need to be brought together. Pollination is the transfer of pollen grains from the anther to the stigma of a pistil. It is nature's matchmaking service.

Types of Pollination (by source of pollen)

• Autogamy (self-pollination): transfer within the same flower. Requires synchrony in the maturation of stigma and anthers, and their close proximity. In cleistogamous flowers, the flower never opens at all, guaranteeing autogamy (e.g., Viola, Oxalis, Commelina).
• Geitonogamy: transfer between two flowers on the same plant. Although functionally cross-pollination (an agent carries the pollen), genetically it is similar to autogamy because both flowers are on the same individual.
• Xenogamy (cross-pollination): transfer between flowers on different plants of the same species. This is the only type that brings genetically different pollen to the stigma.

Agents of Pollination

(a) Abiotic Agents

Anemophily (wind): Pollen is light, dry, non-sticky and produced in enormous quantity. Stigmas are often feathery to trap airborne grains. Common in grasses, maize (where the "silks" hanging from the cob are actually stigmas) and many trees.

Hydrophily (water): Rare, found in a few aquatic species. In Vallisneria, male flowers detach and float on the water surface to reach stigmas of female flowers. In seagrasses such as Zostera, pollen is released inside water and travels in streams to stigmas; the grains are often protected by a mucilaginous covering.

(b) Biotic Agents (animals)

• Entomophily (insects) — by far the commonest. Bees, butterflies, moths, wasps and flies visit flowers that are large, colourful, scented, and offer nectar or edible pollen. Examples: roses, jasmine, sunflower, mustard.
• Ornithophily (birds) — sunbirds and hummingbirds pollinate tubular, often red flowers such as Bignonia and Hibiscus.
• Chiropterophily (bats) — tropical, night-blooming flowers with large, pale blossoms and strong fruity odours (e.g., Adansonia/baobab, Kigelia).

Outbreeding Devices

Continuous self-pollination over generations leads to inbreeding depression — a steady loss of vigour and adaptability. Many angiosperms have therefore evolved outbreeding devices to discourage autogamy:

• Temporal separation — pollen is released before the stigma becomes receptive, or the stigma matures before the anthers.
• Spatial separation — the anthers and stigma are placed at different heights so pollen cannot drop onto the stigma of the same flower.
• Self-incompatibility — a genetic mechanism that rejects pollen from the same plant, preventing self-fertilisation even if the pollen lands on a compatible stigma.
• Unisexual flowers — separate male and female flowers (monoecious if on the same plant, dioecious if on different plants, e.g., papaya) rule out at least autogamy.

Pollen–Pistil Interaction

When pollen reaches the stigma, the pistil does not passively accept it — it conducts a chemical dialogue. If the pollen is compatible, it takes up water, germinates and puts out a pollen tube. The tube grows through the tissues of the stigma and style (the generative cell, if still undivided, divides within the tube to produce the two male gametes), enters the ovule through the micropyle and finally pushes into the embryo sac — releasing the two male gametes into one synergid.

Artificial Hybridisation

Plant breeders deliberately cross two desired varieties to combine their traits. The steps:

1. Emasculation — removing anthers from a bud of the female parent before they dehisce, so no self-pollination occurs.
2. Bagging — covering the emasculated flower with a butter-paper bag to keep unwanted pollen out.
3. Controlled dusting — once the stigma is receptive, dusting it with pollen from the chosen male parent.
4. Re-bagging until fruit sets.

For plants that bear unisexual flowers or have self-incompatibility, emasculation is unnecessary — just bagging and dusting do the job.