Key Points

The flower is the reproductive unit in angiosperms. It contains the male reproductive whorl called the androecium (stamens) and the female reproductive whorl called the gynoecium (pistils or carpels).

The stamen consists of a filament and an anther. The anther is typically bilobed and contains four microsporangia (pollen sacs), which produce pollen grains after meiosis.

This is the process of formation of microspores from a pollen mother cell (PMC) through meiotic divisions. Each diploid ($2n$) PMC produces a tetrad of four haploid ($n$) microspores.

The pollen grain (male gametophyte) has a two-layered wall. The hard outer layer is the exine, made of sporopollenin, and the thin inner layer is the intine, made of cellulose and pectin.

The pistil has three parts: stigma (receives pollen), style, and ovary. The ovary contains ovules (megasporangium), inside which the female gametophyte (embryo sac) develops.

This is the process of formation of megaspores from the megaspore mother cell (MMC). The diploid ($2n$) MMC undergoes meiosis to form four haploid ($n$) megaspores, of which usually one remains functional.

A mature embryo sac is typically 7-celled and 8-nucleate. It consists of an egg apparatus (one egg cell and two synergids), three antipodal cells, and a central cell with two polar nuclei.

Pollination is the transfer of pollen from anther to stigma. It includes autogamy (self-pollination within the same flower), geitonogamy (between flowers of the same plant), and xenogamy (between flowers of different plants).

Pollination is achieved by abiotic agents like wind and water, or biotic agents like insects, birds, and bats. Biotic pollination is more common.

To prevent self-pollination and promote cross-pollination, plants have devices like non-synchronization of pollen release and stigma receptivity, different positioning of anthers and stigma, and self-incompatibility.

This is a dynamic process involving the recognition of compatible pollen by the pistil. If compatible, the pistil accepts the pollen and promotes post-pollination events; if incompatible, it rejects the pollen.

A unique event in angiosperms involving two fusions. One male gamete fuses with the egg cell (syngamy) to form the zygote, and the other male gamete fuses with the two polar nuclei (triple fusion) to form the primary endosperm nucleus.

Syngamy results in a diploid zygote ($2n$). Triple fusion results in a triploid Primary Endosperm Nucleus (PEN) ($3n$).

After fertilization, the zygote develops into an embryo, the PEN develops into the endosperm (nutritive tissue), the ovules mature into seeds, and the ovary develops into a fruit.

The endosperm provides nutrition to the developing embryo. Seeds can be albuminous (endosperm persists, e.g., castor) or non-albuminous (endosperm is consumed, e.g., pea).

A seed is a fertilized ovule, containing an embryo and food reserves, enclosed in a protective seed coat. A fruit is a mature ovary, with its wall forming the pericarp.

Apomixis is the production of seeds without fertilization, a form of asexual reproduction. Polyembryony is the occurrence of more than one embryo in a seed, as seen in citrus.

This is the development of fruit without fertilization, resulting in seedless fruits. It occurs naturally in fruits like bananas and can be induced by growth hormones.