Structure of Pistil and Megasporogenesis Explained

The pistil, also called the gynoecium, is the female reproductive part of a flower. It consists of the stigma that receives pollen, the style through which the pollen tube grows, and the ovary that contains ovules. Megasporogenesis is the process by which a diploid megaspore mother cell inside the ovule undergoes meiosis to form haploid megaspores, one of which develops into the embryo sac.

This process is central to seed and fruit formation. Without a functional pistil and embryo sac, fertilisation cannot happen and the plant cannot produce seeds, no matter how much pollen is available.

Key Concepts

Stigma: The receptive top of the pistil where pollen lands.

Style: The slender middle part connecting stigma to ovary.

Ovary: The swollen base containing one or more ovules, which become seeds after fertilisation.

Ovule: Structure inside ovary containing megaspore mother cell, nucellus, and integuments with a micropyle opening.

Megasporogenesis: Formation of haploid megaspores from a diploid megaspore mother cell by meiosis.

Embryo sac: The female gametophyte, usually 7 celled and 8 nucleate in flowering plants.

How It Works

1. Inside each ovule, a single megaspore mother cell differentiates in the nucellus.
2. This diploid cell undergoes meiosis to produce four haploid megaspores in a line.
3. Three megaspores usually degenerate. The functional megaspore survives.
4. The functional megaspore undergoes three mitotic divisions without immediate wall formation in some stages.
5. The mature embryo sac has an egg cell, two synergids, three antipodal cells, and a central cell with two polar nuclei.
6. This embryo sac is ready to participate in double fertilisation.

Worked Example

A megaspore mother cell has 2n equals 36 chromosomes. After meiosis, each megaspore has n equals 18 chromosomes. Only 1 out of 4 megaspores survives. Survival percentage is 1 divided by 4 multiplied by 100 equals 25 percent. If an ovary has 20 ovules and each makes one functional megaspore, the flower produces 20 embryo sacs. If 15 are fertilised, fruit will potentially contain 15 seeds, assuming each fertilised ovule matures.

Common Misconceptions

• The ovary becomes the seed. The ovary becomes the fruit. The ovule becomes the seed.
• All four megaspores form embryo sacs. Usually only one is functional and the others degenerate.
• The egg is produced directly by meiosis. Meiosis produces a megaspore, which then divides mitotically to form the embryo sac containing the egg.

Real-World Applications

Breeders examine pistil receptivity to time artificial pollination for hybrids. Ovule culture is used to rescue embryos in wide crosses. Seedless fruits often result from failure in ovule or embryo sac development. Understanding megasporogenesis helps improve fruit set in orchards through proper nutrition and pollination management.

The pistil receives pollen and houses the ovule where megasporogenesis creates the female gametophyte. This precise sequence makes fertilisation and seed formation possible in flowering plants.