Plant Kingdom - Angiosperms (brief)

Angiosperms are flowering plants where the seeds are enclosed within fruits, unlike Gymnosperms where seeds are naked.

They are classified into two main groups: Dicotyledons (having two cotyledons, reticulate venation, and tetramerous or pentamerous flowers) and Monocotyledons (having one cotyledon, parallel venation, and trimerous flowers).

The male sex organ is the stamen, consisting of a slender filament and an anther. Pollen grains are produced in the anther following meiosis ($n$).

The female sex organ is the pistil or carpel, consisting of an ovary at the base, a long style, and a stigma. The ovary contains ovules.

Each ovule has a megaspore mother cell that undergoes meiosis to form four megaspores ($n$), three of which degenerate, and one develops into the embryo sac.

The embryo sac is a $7$-celled and $8$-nucleate structure consisting of one egg cell, two synergids, three antipodal cells, and one central cell with two polar nuclei.

Double Fertilization is a unique event in Angiosperms: Syngamy (fusion of one male gamete with the egg to form a $2n$ zygote) and Triple Fusion (fusion of the second male gamete with the two polar nuclei to form a $3n$ Primary Endosperm Nucleus).

Post-fertilization, the zygote develops into an embryo, the $PEN$ develops into the endosperm (nutritive tissue), ovules develop into seeds, and the ovary develops into a fruit.