Reproduction - Sexual Reproduction in Flowering Plants

A flower is a modified shoot where the shoot apical meristem changes to floral meristem. The four whorls are Calyx, Corolla, Androecium, and Gynoecium.

The Microsporangium (Pollen sac) consists of four wall layers: the epidermis, endothecium, middle layers, and the tapetum which provides nourishment to the developing pollen grains.

Microsporogenesis is the process of formation of microspores from a Pollen Mother Cell ($PMC$) through meiosis. Each $PMC$ produces four microspores ($n$).

The Pollen grain represents the male gametophyte. It has a two-layered wall: the outer thick Exine made of sporopollenin (highly resistant) and the inner thin Intine made of cellulose and pectin.

Megasporogenesis is the formation of megaspores from the Megaspore Mother Cell ($MMC$). In most flowering plants, only one megaspore remains functional while the other three degenerate.

The female gametophyte (Embryo Sac) is typically 7-celled and 8-nucleate. It contains the egg apparatus (two synergids and one egg cell), three antipodals, and one large central cell with two polar nuclei.

Pollination types include Autogamy (same flower), Geitonogamy (different flower, same plant), and Xenogamy (different plant).

Double Fertilization is a unique event in Angiosperms where one male gamete ($n$) fuses with the egg ($n$) to form a zygote ($2n$), and the second male gamete ($n$) fuses with two polar nuclei ($2n$) to form the Primary Endosperm Nucleus ($3n$).

The $PEN$ develops into the endosperm, which provides nutrition to the developing embryo. Endosperm can be Nuclear, Cellular, or Helobial.