Flowers range from the barely noticeable to the exquisite to the outlandish. Few would notice the flowers on stems of grass, but everyone will notice an orchid. Regardless of aesthetics, flowers serve a single biological function.

Evolution

Flowering plants, called angiosperms, evolved from gymnosperms which are plants that produce seeds without flowers. Angiosperms are the latest evolutionary step in plant life.

Reproduction

Flowers serve one purpose; to manufacture seeds. Seeds are how flowering plants reproduce. Each seed contains a tiny plant capable of growing into an adult.

Stigma

The stigma sits atop the ovary where the seeds will develop. This is where pollen must contact the flower in order to fertilize it.

Stamen

The anthers ride high on filaments and surround the stigma. Anthers produce pollen which will fertilize the ovules within the flower ovary. The anthers and filaments make up the stamen.

Perfect Flowers

Flowers which have both male (stamen) and female (stigma) parts are called perfect flowers. Those that have only one part or the other are called imperfect.

Sexual Reproduction

The fertilization of ovum by pollen grains is sexual reproduction. Many flowering plants must be fertilized by another plant in order to produce seed in which case they are called self-sterile. Being able to fertilize itself is called self-fertilizing.

Source:

Biology Pages: Plants

More Information:

Wayne's World: Diversity of Flowering Plants