Sexual Reproduction in Flowering Plants - Flower Structure

A flower is the reproductive unit in Angiosperms, often described as a modified shoot. It consists of four whorls: Calyx, Corolla, Androecium, and Gynoecium attached to the thalamus.

The Androecium consists of stamens. A typical stamen has a long filament and a bilobed, dithecous anther. Each lobe contains two microsporangia, making it tetrasporangiate.

The microsporangium wall consists of four layers: the epidermis, endothecium, middle layers, and the Tapetum. The tapetum nourishes the developing pollen grains.

Microsporogenesis: The process of formation of microspores from a Pollen Mother Cell ($PMC$) through meiosis. Each $PMC$ ($2n$) produces a microspore tetrad ($n$).

Pollen Grain: Represents the male gametophyte. It has a two-layered wall: the outer Exine (made of sporopollenin, $C_{90}H_{142}O_{27}$) and the inner Intine (cellulose and pectin).

The Gynoecium consists of the pistil (stigma, style, and ovary). The megasporangium, or ovule, is attached to the placenta by a funicle. The main body of the ovule is the nucellus ($2n$).

Megasporogenesis: The Megaspore Mother Cell ($MMC$) undergoes meiosis to form four megaspores. In most plants, three degenerate, and one functional megaspore develops into the female gametophyte (monosporic development).

The mature Embryo Sac is typically $7$-celled and $8$-nucleate. It contains the egg apparatus ($1$ egg cell + $2$ synergids), $3$ antipodals, and $1$ central cell with $2$ polar nuclei.